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Before the Big Bang_ A Journey to the Beginning of Everything · Night Science Stories

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  Speaker 1: Good evening and welcome to The Sleepy Scientist. I'm very
• 0:03
  Speaker 1: glad you found your way here tonight. Before we wander
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  Speaker 1: Perhaps you're already tucked beneath the covers. Perhaps you're stretched
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  Speaker 1: out on the sofa with the lights turned low. Or
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  Speaker 1: of the house has finally gone still. Wherever tonight has
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  Speaker 1: found you, I hope you can settle in and leave
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  Speaker 1: everything else alone for a little while. And if you
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  Speaker 1: uploading episodes to Spotify. You'll find the link down in
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  Speaker 1: the description, so whenever your eyes have had enough, you
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  Speaker 1: spending these quiet evenings exploring the stranger corners of science,
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  Speaker 1: subscribing is a lovely way to stay along for the ride,
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  Speaker 1: and it helps the channel reach a few more curious
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  Speaker 1: night owls. And if you've been here before, Thank you
• 0:59
  Speaker 1: for coming back. It really does mean a lot. Tonight,
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  Speaker 1: we're going somewhere rather unusual. We're going to travel backwards
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  Speaker 1: through the history of the universe, past galaxies, past stars,
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  Speaker 1: past atoms, and eventually toward a point where even our
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  Speaker 1: familiar ideas of space and time begin to lose their footing.
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  Speaker 1: And then we'll try to go further. Because the Big
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  Speaker 1: Bang may describe the birth of the universe as we
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  Speaker 1: know it, but that leaves a remarkably difficult question behind.
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  Speaker 1: Was there anything before it? Or was the Big Bang
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  Speaker 1: not simply the origin of matter and space, but the
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  Speaker 1: origin of time itself? So, get comfortable, let the day
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  Speaker 1: fade gently into the background and let's begin our journey
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  Speaker 1: toward the beginning of everything. The phrase Big Bang has
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  Speaker 1: a habit of creating the wrong picture in our minds.
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  Speaker 1: It sounds like an explosion, something enormous detonating in a
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  Speaker 1: dark and empty universe. throwing matter outward in every direction.
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  Speaker 1: It is an understandable image, and certainly a dramatic one.
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  Speaker 1: But it is not what modern cosmology actually describes. The
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  Speaker 1: Big Bang was not an explosion that happened somewhere in space.
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  Speaker 1: As far as our best models can tell us, space
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  Speaker 1: itself was expanding. That distinction changes almost everything. If we
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  Speaker 1: look at the universe around us today, we find galaxies
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  Speaker 1: separated by enormous distances. On sufficiently large scales, those distances
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  Speaker 1: are increasing. The farther apart two distant regions of the
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  Speaker 1: universe are, the faster the space between them tends to grow.
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  Speaker 1: If we imagine running that expansion backwards, those same regions
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  Speaker 1: become progressively closer together. Keep running the clock backwards and
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  Speaker 1: the universe becomes denser. Go farther and it becomes hotter.
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  Speaker 1: Farther still, and conditions become so extreme that atoms cannot
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  Speaker 1: remain intact, then atomic nuclei cannot remain intact, and eventually
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  Speaker 1: even our familiar descriptions of matter begin to change. This
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  Speaker 1: is the central idea behind the Big Bang model. It
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  Speaker 1: tells us that the universe we observe today developed from
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  Speaker 1: an earlier state that was vastly hotter, denser, and more
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  Speaker 1: uniform than it is now. One of the people who
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  Speaker 1: first recognised where this idea was leading was Georges Lemaitre,
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  Speaker 1: a Belgian physicist, mathematician, and Catholic priest. During the 1920s,
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  Speaker 1: Lemaître studied Albert Einstein's general theory of relativity, the theory
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  Speaker 1: that describes gravity not simply as a force pulling objects together,
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  Speaker 1: but as the curvature of space and time produced by
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  Speaker 1: matter and energy. Einstein's equations allowed the geometry of the
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  Speaker 1: universe itself to change. Space did not have to remain static.
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  Speaker 1: Lemaître realised that a universe governed by these equations could expand,
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  Speaker 1: and in 1927 he published a model describing an expanding cosmos.
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  Speaker 1: He also connected that expansion with observations of distant galaxies,
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  Speaker 1: and derived a relationship between their distance and their apparent recession.
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  Speaker 1: This was an extraordinary idea at a time when even
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  Speaker 1: Einstein had initially preferred a universe that remained essentially unchanged
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  Speaker 1: on the larger scales. Observations soon made the expanding picture
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  Speaker 1: much harder to ignore. Edwin Hubble, working at Mount Wilson
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  Speaker 1: Observatory in California, studied galaxies beyond the Milky Way and
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  Speaker 1: combined measurements of their distances with information about their motion.
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  Speaker 1: In 1929, he published evidence showing that more distant galaxies
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  Speaker 1: generally appeared to be receding more rapidly than nearby ones.
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  Speaker 1: The relationship became one of the great clues of modern cosmology.
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  Speaker 1: It is tempting to picture Earth sitting somewhere near the
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  Speaker 1: centre while galaxies race away from us. but that would
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  Speaker 1: bring us straight back to the misleading explosion picture. There
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  Speaker 1: is no evidence that we occupy a special central position.
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  Speaker 1: Imagine instead a collection of points drawn on the surface
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  Speaker 1: of a balloon as the balloon is inflated. Every point
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  Speaker 1: becomes more distant from the others, yet none of the
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  Speaker 1: points on the surface is the centre of the expansion.
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  Speaker 1: The analogy is imperfect because the universe is not simply
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  Speaker 1: the two-dimensional surface of a balloon. and it does not
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  Speaker 1: need to be expanding into some larger surrounding space. But
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  Speaker 1: it captures one important idea. Expansion can occur everywhere at once.
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  Speaker 1: A distant observer in another galaxy would also see sufficiently
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  Speaker 1: remote galaxies generally moving away. From their perspective, they too
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  Speaker 1: might briefly be tempted to imagine themselves at the centre.
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  Speaker 1: The universe is rather democratic in that respect. Cosmologists describe
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  Speaker 1: this changing cosmic geometry using something called the scale factor.
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  Speaker 1: The scale factor is essentially a way of keeping track
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  Speaker 1: of the relative size of distances in the expanding universe.
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  Speaker 1: As the universe expands, the scale factor increases. If we
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  Speaker 1: mathematically trace cosmic history backwards, the scale factor becomes smaller.
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  Speaker 1: That does not mean galaxies themselves are simply shrinking into
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  Speaker 1: tiny versions of what they are today. Gravitationally bound systems
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  Speaker 1: such as galaxies, solar systems, planets and people do not
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  Speaker 1: expand along with the universe in this straightforward way. Cosmic
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  Speaker 1: expansion becomes important across immense distances where matter is not
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  Speaker 1: held together strongly enough to resist it. Running the scale
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  Speaker 1: factor backwards gives us a remarkable way of exploring the past.
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  Speaker 1: Galaxies move closer together, matter becomes more concentrated, radiation becomes
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  Speaker 1: more energetic. temperatures rise. Eventually the universe becomes so hot
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  Speaker 1: that the familiar structures surrounding us could not possibly exist.
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  Speaker 1: There were no planets, there were no stars. Go far
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  Speaker 1: enough back and there were not even neutral atoms. But
• 6:52
  Speaker 1: there is another distinction worth keeping in mind. The universe
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  Speaker 1: we can observe is not necessarily the same thing as
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  Speaker 1: the entire universe. Because light travels at a finite speed,
• 7:03
  Speaker 1: and the universe has a finite age in its present
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  Speaker 1: observable history, there is a limit to how far we
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  Speaker 1: can see. The region accessible to us is called the
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  Speaker 1: observable universe. There may be vastly more universe beyond it.
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  Speaker 1: The entire cosmos could be finite. It could be infinite.
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  Speaker 1: Current observations tell us that space is extremely close to
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  Speaker 1: geometrically flat on the largest measurable scales, but that alone
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  Speaker 1: does not determine its total size or global shape. So
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  Speaker 1: when we imagine the early universe becoming smaller, we must
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  Speaker 1: be careful with the language. We can confidently say that
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  Speaker 1: the distances between regions within our observable universe were once
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  Speaker 1: much smaller. We cannot simply conclude that the whole universe
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  Speaker 1: was once a tiny ball floating inside something else. And
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  Speaker 1: this brings us to perhaps the most important caution of all.
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  Speaker 1: If we keep running the equations backwards, classical general relativity
• 7:59
  Speaker 1: eventually drives us toward increasingly extreme conditions. In the simplest extrapolation,
• 8:05
  Speaker 1: density and curvature can grow without limit. It can be
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  Speaker 1: tempting to look at that mathematical result and declare that
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  Speaker 1: we have reached the literal beginning of everything, but an
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  Speaker 1: equation working beautifully over an enormous range does not guarantee
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  Speaker 1: that it will remain complete under every imaginable condition. At
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  Speaker 1: sufficiently early times, quantum physics is expected to become important
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  Speaker 1: alongside gravity. and we do not yet possess a fully
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  Speaker 1: tested theory that combines the two. So, the Big Bang
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  Speaker 1: model tells us something astonishingly powerful. The universe was once
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  Speaker 1: much hotter and denser than it is today, and the
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  Speaker 1: space between distant regions has been expanding for billions of years.
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  Speaker 1: What it does not yet tell us by itself is
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  Speaker 1: whether that expanding history began from an absolute first moment.
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  Speaker 1: The Big Bang may lead us towards the beginnings. but
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  Speaker 1: whether it takes us all the way to the beginning
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  Speaker 1: of everything is a much deeper question. If the universe
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  Speaker 1: really was once far hotter and denser than it is today,
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  Speaker 1: then that idea should have left evidence behind. And remarkably
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  Speaker 1: it did. The modern Hot Big Bang model rests on
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  Speaker 1: several independent lines of observation that all point towards the
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  Speaker 1: same conclusion. The universe is expanding. Space is filled with
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  Speaker 1: ancient radiation left over from a much hotter era. and
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  Speaker 1: the proportions of the lightest chemical elements match what should
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  Speaker 1: have been produced when the universe was only minutes old,
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  Speaker 1: each clue is impressive on its own. Together they form
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  Speaker 1: one of the strongest foundations in modern cosmology. The first
• 9:45
  Speaker 1: clue is the expansion itself. As we have already seen,
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  Speaker 1: distant galaxies are generally receding from one another as space expands.
• 9:54
  Speaker 1: If that expansion is reversed mathematically, then the universe becomes
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  Speaker 1: progressively dense And because radiation is stretched as space expands,
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  Speaker 1: reversing that process also means that radiation becomes more energetic.
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  Speaker 1: That tells us something important. The early universe should not
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  Speaker 1: merely have been denser, it should have been hotter. During
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  Speaker 1: the 1940s, physicists began exploring what such a hot beginning
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  Speaker 1: would actually imply. George Gamow was particularly interested in the
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  Speaker 1: possibility that nuclear reactions could have taken place throughout the
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  Speaker 1: young universe. Working with Ralph Alpha, and later with Robert Herman,
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  Speaker 1: he helped develop the idea that the early cosmos had
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  Speaker 1: once been filled with intense thermal radiation. If that were
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  Speaker 1: true … The expansion of the universe should have stretched
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  Speaker 1: the wavelengths of this radiation over billions of years, steadily
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  Speaker 1: reducing its temperature. The original fire would have faded, but
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  Speaker 1: it should not have vanished completely. A weak background glow
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  Speaker 1: ought still to fill the universe today. That prediction would
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  Speaker 1: eventually become one of the most extraordinary successful forecasts in cosmology.
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  Speaker 1: The radiation was discovered in 1965. although not quite in
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  Speaker 1: the way anyone had planned. Arno Penzias and Robert Wilson
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  Speaker 1: were working with a sensitive radio antenna at Bell Telephone
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  Speaker 1: Laboratories in New Jersey. They were trying to remove an
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  Speaker 1: annoying background signal that seemed to be coming from every direction.
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  Speaker 1: It did not matter where they pointed the antenna. The
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  Speaker 1: noise remained. They checked the equipment carefully. They investigated possible interference.
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  Speaker 1: They even dealt with pigeons that had taken an unfortunate
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  Speaker 1: interest in the antenna. The signal refused to disappear. The
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  Speaker 1: reason was that it was not coming from the antenna
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  Speaker 1: at all. It was coming from the universe. Penzias and
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  Speaker 1: Wilson had detected what is now called the cosmic microwave background,
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  Speaker 1: ancient radiation left over from the hot early cosmos. The
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  Speaker 1: cosmic microwave background surrounds us in every direction. Today, Its
• 12:04
  Speaker 1: temperature is only about 2.7 Kelvin, just a few degrees
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  Speaker 1: above absolute zero. That sounds extremely cold, because it is.
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  Speaker 1: But this faint microwave glow is the cooled remnant of
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  Speaker 1: radiation that was once vastly more energetic. To understand where
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  Speaker 1: it came from, we have to imagine the universe roughly 380,000
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  Speaker 1: years after the Big Bang. At that time, the universe
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  Speaker 1: was far hotter than it is now, with a temperature
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  Speaker 1: of around 3000 Kelvin. Matter existed largely as a plasma
• 12:38
  Speaker 1: containing free electrons and atomic nuclei. Photons could not travel
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  Speaker 1: very far before scattering from charged particles. The universe was
• 12:47
  Speaker 1: effectively foggy. As expansion continued, the temperature dropped far enough
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  Speaker 1: for electrons to become bound to nuclei, producing neutral atoms,
• 12:56
  Speaker 1: mainly hydrogen and helium. With far fewer free-charged particles available
• 13:01
  Speaker 1: to scatter light, photons were suddenly able to travel enormous distances.
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  Speaker 1: The universe became transparent. Those photons have been travelling through
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  Speaker 1: space ever since. As the universe expanded, their wavelength stretched
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  Speaker 1: with it, shifting the radiation from visible and infrared wavelengths
• 13:21
  Speaker 1: into the microwave part of the electromagnetic spectrum. That is
• 13:25
  Speaker 1: the radiation we detect today. and it has been measured
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  Speaker 1: with extraordinary precision. The National Aeronautics and Space Administration Cosmic
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  Speaker 1: Background Explorer mission, launched in 1989, showed that the spectrum
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  Speaker 1: of this radiation matches an almost perfect thermal blackbody curve.
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  Speaker 1: Its measurements were so precise that the result became one
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  Speaker 1: of the clearest pieces of evidence that the early universe
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  Speaker 1: really had been in a hot thermal state. The National
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  Speaker 1: Aeronautics and Space Administration Wilkinson Microwave Anisotropy Probe mapped tiny
• 14:02
  Speaker 1: variations in the temperature of the background radiation across the sky.
• 14:07
  Speaker 1: These differences are extremely small, typically only a few parts
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  Speaker 1: in 100,000. But those tiny variations are enormously important. They
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  Speaker 1: represent slight differences in density in the young universe, differences
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  Speaker 1: that gravity would later amplify into galaxies, clusters of galaxies
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  Speaker 1: and the vast cosmic web. The European Space Agency Planck
• 14:31
  Speaker 1: mission later produced even more detailed maps, allowing cosmologists to
• 14:35
  Speaker 1: measure the properties of the early universe with remarkable precision.
• 14:40
  Speaker 1: Yet the cosmic microwave background is not the only relic.
• 14:44
  Speaker 1: There is another piece of evidence that reaches even farther back.
• 14:48
  Speaker 1: During the first few minutes after the Big Bang, The
• 14:51
  Speaker 1: universe was hot enough for nuclear reactions to occur almost everywhere.
• 14:55
  Speaker 1: This period is known as Big Bang nucleosynthesis. At first,
• 14:59
  Speaker 1: temperatures were so extreme that atomic nuclei could not survive
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  Speaker 1: for long. But as the universe expanded and cooled, protons
• 15:07
  Speaker 1: and neutrons began combining. Hydrogen nuclei remained the most common.
• 15:12
  Speaker 1: Some protons and neutrons joined to form heavier nuclei, especially helium.
• 15:18
  Speaker 1: Small amounts of deuterium Helium-3 and lithium were also produced.
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  Speaker 1: The process did not continue indefinitely. The universe was expanding
• 15:28
  Speaker 1: and cooling very quickly, so after only a few minutes
• 15:31
  Speaker 1: the temperature and density became too low for widespread nuclear
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  Speaker 1: fusion to continue efficiently. The result was a universe dominated
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  Speaker 1: by hydrogen, with roughly a quarter of the ordinary matter
• 15:43
  Speaker 1: by mass in the form of helium, along with much
• 15:46
  Speaker 1: smaller amounts of other light nuclei. Those proportions can be
• 15:50
  Speaker 1: calculated from nuclear physics. They can also be measured by
• 15:54
  Speaker 1: observing ancient gas clouds, stars and other environments that preserve
• 16:00
  Speaker 1: relatively primitive chemical compositions. The broad agreement between prediction and
• 16:05
  Speaker 1: observation is striking. Expansion tells us that the universe was
• 16:09
  Speaker 1: denser in the past. The cosmic microwave background tells us
• 16:13
  Speaker 1: that it was hotter. The abundance of light elements tells
• 16:17
  Speaker 1: us that conditions during the first few minutes were hot
• 16:20
  Speaker 1: and dense enough for nuclear reactions to occur across the cosmos.
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  Speaker 1: These are separate pieces of evidence. They come from different observations,
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  Speaker 1: different areas of physics, and different periods of cosmic history.
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  Speaker 1: Yet they all point in the same direction. Long before
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  Speaker 1: stars formed, before galaxies existed, and before even neutral atoms
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  Speaker 1: could survive … The universe was filled with hot plasma
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  Speaker 1: and radiation. That part of the story is supported by
• 16:49
  Speaker 1: an enormous amount of evidence. But there is still an
• 16:53
  Speaker 1: important line we should not cross too quickly. These observations
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  Speaker 1: show that the universe passed through an extremely hot and
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  Speaker 1: dense early phase. They do not by themselves prove that
• 17:03
  Speaker 1: this was the absolute beginning of existence. The cosmic microwave
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  Speaker 1: background gives us a picture from hundreds of thousands of
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  Speaker 1: years after the Big Bang. The light elements carry information
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  Speaker 1: from the first few minutes. Particle physics can allow us
• 17:18
  Speaker 1: to push our reconstruction even farther backwards. But none of
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  Speaker 1: these observations yet tells us with certainty whether there was
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  Speaker 1: a first moment, whether something existed before the hot big bang,
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  Speaker 1: or whether the idea of a meaningful before eventually disappears altogether.
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  Speaker 1: What they do tell us is already remarkable enough. The
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  Speaker 1: early universe was not simply imagined. Its heat is still
• 17:42
  Speaker 1: glowing quietly around us. Looking out into space is, in
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  Speaker 1: a very literal sense, looking backwards in time. Light does
• 17:51
  Speaker 1: not arrive instantaneously. It moves extraordinarily quickly, travelling almost 300,000
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  Speaker 1: kilometres every second. But the universe is so large that
• 18:01
  Speaker 1: even light can spend years, millions of years or billions
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  Speaker 1: of years crossing it. The moon appears to us as
• 18:08
  Speaker 1: it was a little over a second ago. the Sun
• 18:11
  Speaker 1: appears roughly eight minutes in the past, and when astronomers
• 18:15
  Speaker 1: observe a galaxy billions of light-years away, they are seeing
• 18:19
  Speaker 1: light that began its journey billions of years before reaching Earth.
• 18:23
  Speaker 1: That makes the night sky something rather unusual. It is
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  Speaker 1: not simply a map of places, it is also a
• 18:30
  Speaker 1: map of times. The farther away astronomers look, the younger
• 18:36
  Speaker 1: the universe they are generally seeing. This becomes especially important
• 18:40
  Speaker 1: because the expansion of the universe stretches the wavelengths of
• 18:43
  Speaker 1: travelling light. As space grows, light travelling through that space
• 18:48
  Speaker 1: is stretched with it, shifting towards longer and redder wavelengths.
• 18:53
  Speaker 1: This effect is called cosmological redshift. A larger redshift generally
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  Speaker 1: means that the light has travelled through more cosmic expansion
• 19:01
  Speaker 1: and therefore began its journey at an earlier stage in
• 19:04
  Speaker 1: the history of the universe. With powerful telescopes, astronomers can
• 19:08
  Speaker 1: therefore look back to galaxies that existed when the cosmos
• 19:12
  Speaker 1: was only a small fraction of its present age. Reversing
• 19:16
  Speaker 1: our journey from today, the familiar universe gradually begins to disappear.
• 19:22
  Speaker 1: The grand spiral galaxies and enormous elliptical galaxies of the
• 19:26
  Speaker 1: modern cosmos give way to younger, less settled systems, travel
• 19:31
  Speaker 1: farther backwards, and even those vanish. The first generations of
• 19:35
  Speaker 1: stars appear perhaps a few hundred million years after the
• 19:38
  Speaker 1: Big Bang, although exactly when the earlier stars began shining
• 19:41
  Speaker 1: remains an active field of research. These first stars formed
• 19:45
  Speaker 1: from clouds made almost entirely of hydrogen and helium. There
• 19:50
  Speaker 1: had been no previous generations of stars to manufacture large
• 19:53
  Speaker 1: quantities of heavier elements such as carbon, oxygen, silicon or iron.
• 19:59
  Speaker 1: Before those stars existed came a period often called the
• 20:02
  Speaker 1: Cosmic Dark Ages. There were no stars illuminating space. There
• 20:06
  Speaker 1: were no mature galaxies. The universe contained large quantities of
• 20:11
  Speaker 1: neutral hydrogen and helium, together with dark matter, and gravity
• 20:16
  Speaker 1: was slowly amplifying the small density variations inherited from much
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  Speaker 1: earlier times. The universe was dark, but it was not
• 20:23
  Speaker 1: empty or inactive. The seeds of future structure were quietly growing,
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  Speaker 1: continue backwards to roughly 380,000 years after the Big Bang
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  Speaker 1: and the universe changes dramatically again. Temperatures rise to around 3,000 Kelvin,
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  Speaker 1: neutral atoms can no longer exist so comfortably. Going backwards
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  Speaker 1: through this period takes us across an event known as recombination.
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  Speaker 1: Despite the name, it was not really a case of
• 20:50
  Speaker 1: electrons recombining with nuclei after having previously been combined in
• 20:54
  Speaker 1: exactly the same way. The term survives largely through historical convention.
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  Speaker 1: As the universe cooled in the forward direction of time,
• 21:03
  Speaker 1: free electrons became bound to atomic nuclei, particularly protons. creating
• 21:09
  Speaker 1: neutral hydrogen atoms. Before recombination, free electrons were everywhere. Photons
• 21:15
  Speaker 1: constantly scattered from these charged particles and could not travel
• 21:20
  Speaker 1: very far in a straight line. The early universe was
• 21:23
  Speaker 1: therefore opaque, rather like trying to see through a brilliantly
• 21:27
  Speaker 1: glowing fog. Once neutral atoms formed, the situation changed. Photons
• 21:34
  Speaker 1: could travel much more freely. The light released from that
• 21:38
  Speaker 1: period has continued crossing the universe ever since, gradually stretched
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  Speaker 1: by cosmic expansion into the microwave wavelengths we now observe
• 21:48
  Speaker 1: as the cosmic microwave background. But if we keep rewinding,
• 21:53
  Speaker 1: that ancient glow disappears back into the hot plasma from
• 21:57
  Speaker 1: which it emerged. Temperatures climb. Matter becomes increasingly ionised. Electrons
• 22:04
  Speaker 1: separate from nuclei. radiation becomes more energetic, and the distinction
• 22:10
  Speaker 1: between the quiet darkness of modern intergalactic space and the
• 22:14
  Speaker 1: matter around us begins to vanish completely. Now move backwards
• 22:19
  Speaker 1: from hundreds of thousands of years to only a few
• 22:22
  Speaker 1: minutes after the Big Bang. The temperature rises into the
• 22:26
  Speaker 1: range where nuclear reactions can occur throughout space. This is
• 22:30
  Speaker 1: the era of Big Bang nucleosynthesis. Atomic nuclei can exist,
• 22:36
  Speaker 1: although complete atoms cannot. Protons and neutrons collide and combine,
• 22:42
  Speaker 1: producing deuterium and then helium, with smaller quantities of other
• 22:46
  Speaker 1: light nuclei. For only a short period, the entire universe
• 22:50
  Speaker 1: behaves like an immense nuclear environment. Yet even this does
• 22:54
  Speaker 1: not last long. In the forward direction, expansion quickly lowers
• 22:59
  Speaker 1: the temperature and density, until widespread nuclear reactions largely cease.
• 23:04
  Speaker 1: Going backwards, however, we enter conditions that become steadily more energetic. Soon,
• 23:10
  Speaker 1: even many atomic nuclei cannot survive. A few minutes become seconds.
• 23:16
  Speaker 1: Seconds become fractions of seconds. During approximately the first second,
• 23:21
  Speaker 1: the universe bears almost no resemblance to anything visible around
• 23:25
  Speaker 1: us today. It is an extraordinarily hot, dense mixture of
• 23:30
  Speaker 1: radiation and elementary particles. Electrons move through the plasma alongside
• 23:35
  Speaker 1: their antimatter counterparts, positrons. Neutrinos are present in enormous numbers.
• 23:42
  Speaker 1: Protons and neutrons are continually colliding. Photons carry immense energies.
• 23:49
  Speaker 1: Particles are being created and destroyed as collisions convert energy
• 23:53
  Speaker 1: into matter and matter back into energy. The temperature during
• 23:58
  Speaker 1: this early period is measured in billions of Kelvin. Under
• 24:02
  Speaker 1: such conditions, ordinary ideas such as solids, liquids, gases, planets,
• 24:08
  Speaker 1: or even atoms have become completely irrelevant. The universe is
• 24:12
  Speaker 1: not filled with objects in anything like the everyday sense.
• 24:16
  Speaker 1: It is better understood as a rapidly evolving sea of
• 24:18
  Speaker 1: interacting particles and radiation, and this is where travelling backwards
• 24:23
  Speaker 1: through cosmic history begins to change character. For most of
• 24:27
  Speaker 1: the history of the universe, astronomy gives us the clearest
• 24:30
  Speaker 1: picture– We observe stars, we observe galaxies, we detect distant
• 24:35
  Speaker 1: light and reconstruct how structures formed. But as we move
• 24:39
  Speaker 1: into the first seconds, there are no stars or galaxies
• 24:42
  Speaker 1: left to study. There are only particles, radiation, fundamental forces
• 24:47
  Speaker 1: and extraordinary energies. Astronomy gradually gives way to particle physics.
• 24:53
  Speaker 1: The early universe becomes a natural laboratory at energy scales
• 24:56
  Speaker 1: far beyond those encountered in ordinary life. And one second,
• 25:00
  Speaker 1: which sounds almost insignificant when compared with nearly 14 billion
• 25:04
  Speaker 1: years of cosmic history, turns out to contain an enormous
• 25:08
  Speaker 1: amount of physics. The first second was not a single
• 25:12
  Speaker 1: frozen moment waiting politely for the universe to begin properly.
• 25:16
  Speaker 1: Within fractions of that second, temperatures changed enormously. Particle populations changed.
• 25:24
  Speaker 1: Interactions that were common at one instant became impossible shortly afterwards.
• 25:29
  Speaker 1: Processes unfolded over thousands, millionths, billionths and still smaller fractions
• 25:34
  Speaker 1: of a second. The farther backwards we travel, the more
• 25:37
  Speaker 1: compressed the story becomes. Billions of years give way to millions.
• 25:42
  Speaker 1: Millions become thousands. Thousands become minutes. Minutes become seconds. And
• 25:47
  Speaker 1: eventually a fraction of a second becomes enough time for
• 25:50
  Speaker 1: the physical character of the universe to change completely. Rewinding
• 25:56
  Speaker 1: the cosmic clock therefore does more than take us towards
• 25:58
  Speaker 1: an earlier date. It strips away, one layer at a time,
• 26:02
  Speaker 1: almost everything familiar. Galaxies disappear, stars disappear, atoms disappear. Eventually
• 26:11
  Speaker 1: even nuclei cease to be permanent structures, and behind them
• 26:15
  Speaker 1: lies a universe governed increasingly by the behaviour of its
• 26:18
  Speaker 1: most fundamental particles and forces– where enormously important changes occur
• 26:24
  Speaker 1: in intervals of time far shorter than anything human experience
• 26:28
  Speaker 1: has prepared us to imagine, continue far enough backwards through
• 26:32
  Speaker 1: the first second, and even protons and neutrons begin to
• 26:36
  Speaker 1: disappear from the story. Today, these particles form the foundations
• 26:40
  Speaker 1: of atomic nuclei. Protons sit inside every atomic nucleus, while
• 26:45
  Speaker 1: neutrons accompany them in all but the simplest form of
• 26:48
  Speaker 1: hydrogen– They feel wonderfully permanent by everyday standards. A proton
• 26:53
  Speaker 1: can survive for an extraordinarily long time, perhaps indefinitely for
• 26:58
  Speaker 1: all we currently know. But there was an era when
• 27:01
  Speaker 1: even protons had not yet formed. Within roughly the first
• 27:05
  Speaker 1: millionth of a second after the Big Bang, the universe
• 27:08
  Speaker 1: was so hot that matter existed in a much more
• 27:11
  Speaker 1: fundamental state. Temperatures reached trillions of Kelvin, and particles collided
• 27:17
  Speaker 1: with energy so enormous that quarks could not settle comfortably
• 27:21
  Speaker 1: into the composite particles we recognise today. Instead, the universe
• 27:26
  Speaker 1: contained what physicists call a quark-gluon plasma. Quarks are elementary particles.
• 27:32
  Speaker 1: As far as experiments can currently determine, they do not
• 27:35
  Speaker 1: contain smaller components. Several varieties exist, but ordinary matter around
• 27:41
  Speaker 1: us is built primarily from up quarks and down quarks.
• 27:45
  Speaker 1: A proton contains two up quarks and one down quark.
• 27:49
  Speaker 1: A neutron contains one up quark and two down quarks.
• 27:53
  Speaker 1: Yet saying that a proton simply contains three quarks leaves
• 27:57
  Speaker 1: out an important part of the picture. The interior of
• 28:00
  Speaker 1: a proton is an intensely active quantum system, containing gluons
• 28:04
  Speaker 1: and continually fluctuating quark and antiquark contributions. The particles responsible
• 28:10
  Speaker 1: for binding quarks together are called gluons. Their behaviour is
• 28:14
  Speaker 1: described by a theory known as quantum chromodynamics. the part
• 28:18
  Speaker 1: of particle physics that describes the strong nuclear interaction. The
• 28:22
  Speaker 1: name colour appears within this theory, although it has nothing
• 28:26
  Speaker 1: whatsoever to do with visible colours such as red or blue.
• 28:30
  Speaker 1: Quarks possess a quantum property physicists call colour charge, and
• 28:34
  Speaker 1: gluons transmit the strong interaction between particles carrying this property.
• 28:39
  Speaker 1: Under ordinary conditions, the strong interaction has a rather unusual feature.
• 28:43
  Speaker 1: Quarks do not appear freely on their own. try to
• 28:46
  Speaker 1: separate two quarks, and the interaction between them does not
• 28:50
  Speaker 1: simply fade away in the manner we might expect from
• 28:52
  Speaker 1: familiar forces. Instead, the energy stored in the strong interaction
• 28:56
  Speaker 1: grows until it becomes favourable to create new particles. The
• 29:00
  Speaker 1: result is called confinement. Quarks remain locked inside composite particles
• 29:06
  Speaker 1: such as protons and neutrons. But at the tremendous temperatures
• 29:10
  Speaker 1: of the very early universe, conditions were different. Matter was
• 29:15
  Speaker 1: energetic enough that quarks and gluons formed a hot dense
• 29:18
  Speaker 1: state in which they were not confined inside individual protons
• 29:22
  Speaker 1: and neutrons in the familiar way. This was the quark-gluon plasma.
• 29:28
  Speaker 1: Calling it a plasma can make it sound rather similar
• 29:30
  Speaker 1: to the electrically charged plasma found inside stars, but the
• 29:34
  Speaker 1: two are quite different. A stellar plasma consists largely of
• 29:38
  Speaker 1: atomic nuclei and free electrons. A quark-gluon plasma exists at
• 29:44
  Speaker 1: a far more fundamental level, before protons and neutrons have
• 29:48
  Speaker 1: settled into their familiar identities. For a brief period, essentially
• 29:53
  Speaker 1: the entire observable universe would have been filled with this
• 29:56
  Speaker 1: extraordinary state of matter. As expansion continued, the universe cooled. Eventually,
• 30:02
  Speaker 1: the temperature dropped enough for the strong interaction to confine quarks.
• 30:07
  Speaker 1: Around a few millionths of a second after the beginning
• 30:10
  Speaker 1: of the Hot Big Bang expansion, quarks became bound into
• 30:13
  Speaker 1: composite particles called hadrons. Among those hadrons were protons and neutrons.
• 30:20
  Speaker 1: This transition transformed the character of the universe. The ingredients
• 30:24
  Speaker 1: required for future atomic nuclei had appeared although actual atoms
• 30:28
  Speaker 1: were still an unimaginably long way off. For most of
• 30:32
  Speaker 1: cosmic history, the quark-gluon plasma has not existed naturally across
• 30:37
  Speaker 1: enormous regions of space, but physicists have found a way
• 30:41
  Speaker 1: to produce tiny amounts of something remarkably similar. At the
• 30:44
  Speaker 1: European Organisation for Nuclear Research near Geneva, heavy atomic nuclei
• 30:49
  Speaker 1: can be accelerated to almost the speed of light and
• 30:53
  Speaker 1: collided inside the Large Hadron Collider. Brookhaven National Laboratory in
• 30:58
  Speaker 1: the United States performs related experiments using the relativistic heavy
• 31:03
  Speaker 1: ion collider. In these collisions, nuclei such as lead or
• 31:08
  Speaker 1: gold strike one another with tremendous energy. For an almost
• 31:13
  Speaker 1: absurdly short interval, the resulting temperatures can exceed trillions of Kelvin.
• 31:18
  Speaker 1: Under those conditions, protons and neutrons effectively dissolve into a
• 31:23
  Speaker 1: tiny droplet of quark-gluon plasma. The droplet does not last
• 31:28
  Speaker 1: long enough to keep in a jar, which is probably fortunate.
• 31:32
  Speaker 1: It expands, cools and rapidly turns back into ordinary particles.
• 31:37
  Speaker 1: Yet the particles emerging from these collisions allow physicists to
• 31:41
  Speaker 1: reconstruct some of its properties. One surprising discovery has been
• 31:46
  Speaker 1: that quark-gluon plasma behaves less like a loose gas of
• 31:49
  Speaker 1: independent particles and more like an extraordinarily low viscosity liquid.
• 31:55
  Speaker 1: Quarks and gluons interact strongly with one another, producing collective
• 31:59
  Speaker 1: behaviour that can be studied experimentally. These experiments do not
• 32:03
  Speaker 1: recreate the entire early universe, of course. They produce microscopic
• 32:08
  Speaker 1: regions lasting only tiny fractions of a second. But the
• 32:12
  Speaker 1: underlying physics is the same physics that should have operated
• 32:15
  Speaker 1: when the universe itself was filled with comparable temperatures and densities.
• 32:20
  Speaker 1: And we can travel backwards farther still. Before the Quark epoch,
• 32:25
  Speaker 1: temperatures become so high that another familiar distinction begins to disappear. Today,
• 32:31
  Speaker 1: electromagnetism and the weak nuclear interaction appear to be separate forces.
• 32:36
  Speaker 1: Electromagnetism governs electrically charged particles, producing phenomena ranging from magnets
• 32:42
  Speaker 1: and lightning to the chemistry holding atoms together. The weak
• 32:46
  Speaker 1: nuclear interaction is responsible for certain forms of radioactive decay,
• 32:50
  Speaker 1: and and plays an important role in processes involving particles
• 32:54
  Speaker 1: such as neutrinos. At sufficiently high energies, however, these two
• 32:59
  Speaker 1: interactions are understood as different expressions of a deeper unified
• 33:02
  Speaker 1: framework called electroweak theory. The development of this theory is
• 33:06
  Speaker 1: closely associated with Sheldon Glashow, Abdus Salam and Steven Weinberg.
• 33:12
  Speaker 1: Their work showed how electromagnetic and weak interactions can arise
• 33:16
  Speaker 1: from a single underlying electroweak structure. In the cool universe
• 33:21
  Speaker 1: around us today, the symmetry between them is broken. The
• 33:25
  Speaker 1: electromagnetic interaction is carried by massless photons, while the weak
• 33:30
  Speaker 1: interaction involves massive particles called the W bosons and the
• 33:33
  Speaker 1: Z boson. At extremely high temperatures, similar to those expected
• 33:39
  Speaker 1: in the early universe, that distinction changes. The electroweak symmetry
• 33:44
  Speaker 1: is restored, and the forces behave as parts of a
• 33:47
  Speaker 1: more unified description. This introduces one of the most fascinating
• 33:52
  Speaker 1: themes in our journey backwards. The hotter the universe becomes,
• 33:56
  Speaker 1: the more some distinctions that seem fundamental today begin to dissolve.
• 34:01
  Speaker 1: Particles that are separate at low energy can behave as
• 34:03
  Speaker 1: members of larger families. Forces that appear different can reveal
• 34:08
  Speaker 1: hidden relationships. The modern universe may therefore look complicated partly
• 34:12
  Speaker 1: because it has cooled. Some of its apparent variety emerged
• 34:16
  Speaker 1: as earlier symmetries were broken. and the universe settled into
• 34:20
  Speaker 1: lower energy states. Our confidence also begins to change as
• 34:24
  Speaker 1: we move deeper into this territory. The existence of quarks, gluons,
• 34:29
  Speaker 1: quantum chromodynamics, and electro-weak physics is supported by extensive experiments.
• 34:37
  Speaker 1: Particle accelerators have tested these theories with remarkable precision, but
• 34:41
  Speaker 1: applying them to the earliest universe requires reconstructing conditions we
• 34:45
  Speaker 1: cannot observe directly. The farther backwards we travel, the fewer
• 34:50
  Speaker 1: direct astronomical signals remain. Cosmology increasingly relies on the laws
• 34:55
  Speaker 1: of particle physics, laboratory experiments and mathematical extrapolation. That does
• 35:01
  Speaker 1: not make the reconstruction mere guesswork. It does mean that
• 35:04
  Speaker 1: the nature of the evidence is changing. We have travelled
• 35:07
  Speaker 1: from galaxies we can photograph, to ancient radiation we can detect,
• 35:12
  Speaker 1: to conditions we can partly reproduce in accelerators. And now,
• 35:17
  Speaker 1: As the temperature rises still farther, even those experiments begin
• 35:21
  Speaker 1: to fall short of the energies we're trying to understand.
• 35:24
  Speaker 1: The universe has already lost stars, atoms, nuclei, protons and neutrons.
• 35:30
  Speaker 1: What remains is a world of fundamental particles, quantum fields
• 35:34
  Speaker 1: and symmetries. And somewhere beyond it lies a regime where
• 35:38
  Speaker 1: even our most successful descriptions of particle physics will no
• 35:41
  Speaker 1: longer be enough. By the time we travel backwards to
• 35:45
  Speaker 1: less than one trillionth of a second after the beginning
• 35:48
  Speaker 1: of the hot big bang, the universe has become almost
• 35:51
  Speaker 1: unimaginably energetic. Temperatures are vastly higher than anything found naturally
• 35:57
  Speaker 1: in the modern cosmos. Particles collide with extraordinary force. Familiar
• 36:03
  Speaker 1: distinctions between matter, radiation and the fundamental interactions begin to blur.
• 36:09
  Speaker 1: And this is where our confidence starts to weaken. For
• 36:13
  Speaker 1: much of cosmic history, the laws of physics work beautifully.
• 36:17
  Speaker 1: We can describe planets using gravity. We can explain atoms
• 36:21
  Speaker 1: using quantum mechanics. We can understand radioactive decay, particle collisions, light, electricity,
• 36:28
  Speaker 1: magnetism and nuclear interactions using theories that have been tested
• 36:32
  Speaker 1: again and again. But the earliest universe forces these successful
• 36:37
  Speaker 1: ideas into the same place at the same time. That
• 36:41
  Speaker 1: turns out to be a problem. The modern theory describing
• 36:45
  Speaker 1: known elementary particles is called the standard model of particle physics.
• 36:49
  Speaker 1: It contains the basic ingredients from which ordinary matter is built.
• 36:54
  Speaker 1: There are quarks, which combine to form particles such as
• 36:57
  Speaker 1: protons and neutrons. There are leptons, including electrons and neutrinos.
• 37:03
  Speaker 1: There are particles that transmit interactions, including photons, gluons, and
• 37:09
  Speaker 1: the particles responsible for the weak nuclear interaction. The standard
• 37:14
  Speaker 1: model also includes the Higgs field and the Higgs boson,
• 37:18
  Speaker 1: which play a crucial role in explaining why several fundamental
• 37:21
  Speaker 1: particles possess mass. It is one of the most successful
• 37:26
  Speaker 1: theories humans have ever developed. Predictions made using the standard
• 37:30
  Speaker 1: model have been confirmed by particle accelerator experiments with astonishing precision.
• 37:37
  Speaker 1: It describes electromagnetism. It describes the weak nuclear interaction. And
• 37:43
  Speaker 1: through quantum chromodynamics, it describes the strong nuclear interaction. Three
• 37:49
  Speaker 1: of the four known fundamental interactions fit within this quantum framework.
• 37:55
  Speaker 1: One does not. Gravity. Gravity is instead described most successfully
• 38:01
  Speaker 1: by Albert Einstein's general theory of relativity. In general relativity,
• 38:06
  Speaker 1: gravity is not treated in the same way as the
• 38:08
  Speaker 1: forces of the standard model. Matter and energy influence the
• 38:12
  Speaker 1: geometry of spacetime itself. Spacetime bends, and objects follow paths
• 38:18
  Speaker 1: through that curved geometry. A planet orbiting a star can
• 38:23
  Speaker 1: therefore be understood not simply as something being tugged by
• 38:26
  Speaker 1: an invisible force, but as an object moving through curved spacetime.
• 38:32
  Speaker 1: The theory has passed an extraordinary range of tests. It
• 38:36
  Speaker 1: explains the motion of planets. It predicts gravitational time dilation.
• 38:41
  Speaker 1: It describes black holes. It predicted gravitational waves, which were
• 38:46
  Speaker 1: eventually detected directly. And on the larger scales, general relativity
• 38:51
  Speaker 1: provides the foundation of modern cosmology. There is nothing obviously
• 38:55
  Speaker 1: wrong with either theory. That is precisely what makes the
• 38:59
  Speaker 1: difficulty so interesting. The standard model works remarkably well in
• 39:04
  Speaker 1: the quantum world. General relativity works remarkably well in the
• 39:09
  Speaker 1: gravitational world, but they are built using very different ideas.
• 39:14
  Speaker 1: Quantum physics describes nature in terms of fields, probabilities, fluctuations
• 39:21
  Speaker 1: and quantist interactions. General relativity describes gravity as smooth spacetime geometry,
• 39:29
  Speaker 1: under ordinary conditions this mismatch causes little trouble. Inside an atom,
• 39:33
  Speaker 1: gravity is fantastically weak compared with the other interactions, so
• 39:38
  Speaker 1: it can usually be ignored. When describing a planet, star
• 39:42
  Speaker 1: or galaxy, quantum fluctuations in spacetime are usually irrelevant, so
• 39:47
  Speaker 1: general relativity works perfectly well. Nature politely keeps the two
• 39:52
  Speaker 1: problems apart. The very early universe did not offer us
• 39:55
  Speaker 1: that convenience. As we move farther backwards, the universe becomes
• 40:00
  Speaker 1: both denser and hotter. Eventually, enormous amounts of energy are
• 40:04
  Speaker 1: concentrated into extraordinarily small regions. Gravity becomes increasingly important because
• 40:11
  Speaker 1: energy and density curve space-time. Quantum physics also becomes increasingly
• 40:17
  Speaker 1: important because we are dealing with extremely small scales and
• 40:21
  Speaker 1: extremely energetic particles. We therefore reach a regime where we
• 40:26
  Speaker 1: need both theories simultaneously, and simply placing them beside one
• 40:30
  Speaker 1: another does not work. Attempts to treat gravity as an
• 40:35
  Speaker 1: ordinary quantum field in the same straightforward way as the
• 40:39
  Speaker 1: other interactions tend to produce uncontrollable, mathematical infinities at sufficiently
• 40:45
  Speaker 1: high energies. This does not mean that quantum gravity is impossible.
• 40:50
  Speaker 1: It means that our current descriptions are incomplete. Physicists, therefore,
• 40:55
  Speaker 1: search for a theory of quantum gravity. Such a theory
• 40:58
  Speaker 1: would provide a consistent description of spacetime when quantum effects
• 41:02
  Speaker 1: and strong gravity are both unavoidable. Several approaches have been proposed.
• 41:07
  Speaker 1: String theory attempts to replace point-like elementary particles with tiny
• 41:11
  Speaker 1: extended objects whose different modes of vibration appear as different particles.
• 41:17
  Speaker 1: including a particle with properties appropriate for quantum gravity. Loop
• 41:21
  Speaker 1: quantum gravity takes a different approach, attempting to describe the
• 41:25
  Speaker 1: geometry of spacetime itself as possessing a discrete quantum structure
• 41:30
  Speaker 1: at extremely small scales. There are other approaches as well,
• 41:35
  Speaker 1: and none has yet been confirmed experimentally as the final theory.
• 41:39
  Speaker 1: The difficulty is partly practical. The energies at which quantum
• 41:43
  Speaker 1: gravitational effects are expected to become obvious, are enormously beyond
• 41:48
  Speaker 1: those accessible in present-day laboratories. This brings us to a
• 41:52
  Speaker 1: collection of scales associated with the physicist Max Planck. The
• 41:56
  Speaker 1: Planck scale is not one single number. It includes characteristic
• 42:01
  Speaker 1: scales of length, time, mass, temperature and energy constructed from
• 42:07
  Speaker 1: fundamental constants of nature. These scales indicate the regime where
• 42:11
  Speaker 1: both quantum physics and gravity are expected to become simultaneously important.
• 42:16
  Speaker 1: The Planck length is roughly 1.6 times 10 to the
• 42:19
  Speaker 1: power of negative 35 metres. That is an astonishingly small distance.
• 42:25
  Speaker 1: A proton is already tiny, with a diameter of roughly
• 42:28
  Speaker 1: 10 to the power of negative 15 metres. The Planck
• 42:32
  Speaker 1: length is around 20 orders of magnitude smaller still. Another
• 42:37
  Speaker 1: important quantity is the Planck time. The Planck time is
• 42:40
  Speaker 1: approximately 5.4 times 10 to the power of negative 44 seconds.
• 42:46
  Speaker 1: It is often rounded loosely to around 10 to the
• 42:48
  Speaker 1: power of negative 43 seconds when discussing the earliest universe.
• 42:54
  Speaker 1: Writing such a number in ordinary decimal form would require
• 42:57
  Speaker 1: a very large collection of zeros before anything interesting happened.
• 43:01
  Speaker 1: It represents a duration so short that no everyday comparison
• 43:05
  Speaker 1: comes remotely close to making it intuitive. The important point
• 43:09
  Speaker 1: is not that physicists have measured a cosmic event occurring
• 43:12
  Speaker 1: exactly at the Planck time. They have not. The Planck
• 43:16
  Speaker 1: time is instead a theoretical scale. It marks a region
• 43:20
  Speaker 1: where our familiar separation between quantum mechanics and classical space-time
• 43:24
  Speaker 1: is expected to stop being reliable. Sometimes it is described
• 43:29
  Speaker 1: as the earliest meaningful moment in physics. That wording needs care.
• 43:34
  Speaker 1: It does not mean that nothing existed earlier. It does
• 43:37
  Speaker 1: not prove that time began there. And it certainly does
• 43:40
  Speaker 1: not mean that nature placed a small sign at the
• 43:42
  Speaker 1: Planck time politely informing the universe. conventional physics ended here.
• 43:47
  Speaker 1: It means that our current tested theories can no longer
• 43:50
  Speaker 1: be confidently extrapolated into earlier conditions without a more complete
• 43:54
  Speaker 1: description of quantum gravity. Before the Planck era, even concepts
• 44:00
  Speaker 1: that seem obvious may become uncertain. Distance may not behave
• 44:04
  Speaker 1: in a classical way. Time may not function as a
• 44:07
  Speaker 1: smooth continuous parameter. The geometry of spacetime may itself possess
• 44:12
  Speaker 1: quantum fluctuations, The distinction between a fixed background and the
• 44:17
  Speaker 1: particles moving through it may cease to make sense. These
• 44:20
  Speaker 1: possibilities come from theoretical attempts to combine gravity with quantum physics.
• 44:25
  Speaker 1: But they are possibilities, not direct observations. That difference matters enormously.
• 44:32
  Speaker 1: We have strong observational evidence for the universe thousands, millions
• 44:36
  Speaker 1: and billions of years after the Big Bang. We have
• 44:39
  Speaker 1: good theoretical confidence about many processes occurring during the first seconds.
• 44:43
  Speaker 1: particle accelerator experiments give us considerable confidence in physics reaching
• 44:48
  Speaker 1: much earlier epochs. But as we approach the Planck scale,
• 44:52
  Speaker 1: the bridge between experiment and cosmological reconstruction becomes increasingly fragile.
• 44:58
  Speaker 1: Beyond it, we enter territory where several theories may offer
• 45:01
  Speaker 1: different descriptions, and where no experiment has yet told us which,
• 45:05
  Speaker 1: if any, is correct. This is not a failure of science,
• 45:10
  Speaker 1: knowing exactly where knowledge becomes uncertain and is one of
• 45:13
  Speaker 1: science's great strengths. We have followed the universe backwards until
• 45:18
  Speaker 1: the two most successful pillars of modern physics are both
• 45:21
  Speaker 1: required and neither is sufficient on its own. General relativity
• 45:25
  Speaker 1: tells us that space-time itself must respond to the enormous density.
• 45:30
  Speaker 1: Quantum physics tells us that nature cannot simply be treated
• 45:33
  Speaker 1: as perfectly smooth and classical at arbitrarily small scales. Some
• 45:38
  Speaker 1: deeper theory must connect them. And until that theory is
• 45:41
  Speaker 1: found and tested, the earliest imaginable moments remain hidden behind
• 45:46
  Speaker 1: one of the most important boundaries in modern physics, not
• 45:49
  Speaker 1: necessarily a boundary in the universe itself, but a boundary
• 45:53
  Speaker 1: in what we currently know how to describe. If we
• 45:56
  Speaker 1: continue following the expansion of the universe backwards using classical
• 46:00
  Speaker 1: general relativity, something rather unsettling begins to happen. Distances shrink,
• 46:07
  Speaker 1: density rises. Temperatures climb, and the curvature of spacetime becomes
• 46:12
  Speaker 1: increasingly extreme. Push the equations far enough, and in the
• 46:17
  Speaker 1: simplest classical description these quantities can grow without limit. Eventually
• 46:22
  Speaker 1: the mathematics appears to lead towards what is commonly called
• 46:25
  Speaker 1: the Big Bang Singularity. This is often presented as an
• 46:30
  Speaker 1: infinitely small point containing the entire universe at infinite density.
• 46:35
  Speaker 1: That picture is memorable. it is also much more certain
• 46:38
  Speaker 1: than the science actually allows. In general relativity, a singularity
• 46:44
  Speaker 1: is not best understood as a physical object sitting somewhere
• 46:47
  Speaker 1: waiting to be discovered. More carefully, it describes a failure
• 46:51
  Speaker 1: in the spacetime predicted by the theory. One of the
• 46:55
  Speaker 1: important concepts here is geodesic incompleteness. A geodesic is essentially
• 47:00
  Speaker 1: the natural path followed through spacetime by an object moving
• 47:03
  Speaker 1: freely under gravity. or by light travelling through the universe.
• 47:08
  Speaker 1: If space-time is geodesically incomplete, there are paths that simply
• 47:12
  Speaker 1: cannot be extended indefinitely even though the traveller following them
• 47:15
  Speaker 1: has reached no ordinary boundary. The mathematical description ends. In
• 47:20
  Speaker 1: the cosmological case, following certain paths backwards through classical space-time
• 47:25
  Speaker 1: can bring us to a finite point beyond which the
• 47:28
  Speaker 1: theory provides no continuation. That is a much more precise
• 47:33
  Speaker 1: meaning of a singularity than simply imagining a tiny ball
• 47:37
  Speaker 1: containing infinite amounts of stuff. The modern understanding of these
• 47:42
  Speaker 1: singularities was transformed by the British mathematician and physicist Roger Penrose.
• 47:47
  Speaker 1: During the 1960s, Penrose developed powerful mathematical techniques for studying
• 47:53
  Speaker 1: gravitational collapse. Before this work, it was possible to wonder
• 47:57
  Speaker 1: whether singularities appeared only in unusually neat mathematical models. Perhaps
• 48:02
  Speaker 1: because those models assumed perfectly symmetrical stars or perfectly arranged matter.
• 48:07
  Speaker 1: Real stars, after all, are messy. They rotate. They contain irregularities.
• 48:13
  Speaker 1: They are rarely kind enough to satisfy the aesthetic preferences
• 48:16
  Speaker 1: of theoretical physicists. Penrose showed that the problem was much deeper.
• 48:22
  Speaker 1: Under broad physical conditions, sufficiently severe gravitational collapse could produce
• 48:27
  Speaker 1: incomplete spacetime even without requiring perfect symmetry. His original singularity
• 48:33
  Speaker 1: theorem was particularly important for understanding black holes. Stephen Hawking
• 48:38
  Speaker 1: then recognised that related mathematical reasoning could be turned towards
• 48:42
  Speaker 1: the universe as a whole. If gravity could force collapsing
• 48:45
  Speaker 1: matter towards a singular boundary, perhaps an expanding universe could
• 48:49
  Speaker 1: reveal something similar when its history was followed in reverse.
• 48:53
  Speaker 1: Hawking and Penrose developed a collection of results now known
• 48:56
  Speaker 1: as the Hawking-Penrose singularity theorems. These theorems showed that Under
• 49:02
  Speaker 1: certain assumptions about gravity, matter, causality, and the overall behaviour
• 49:07
  Speaker 1: of spacetime, singular behaviour is remarkably difficult to avoid within
• 49:12
  Speaker 1: classical general relativity. In a universe resembling the one described
• 49:17
  Speaker 1: by conventional cosmological models, running the clock backwards does not
• 49:22
  Speaker 1: necessarily allow spacetime to remain smooth forever. Eventually, the classical
• 49:27
  Speaker 1: description reaches an edge. This was an enormously important result,
• 49:32
  Speaker 1: but it is worth being very careful about what it
• 49:35
  Speaker 1: does and does not tell us. The singularity theorems do
• 49:39
  Speaker 1: not provide a photograph of the first moment. They do
• 49:42
  Speaker 1: not tell us that scientists have measured infinite density. They
• 49:46
  Speaker 1: do not prove that the universe literally existed as a
• 49:49
  Speaker 1: dimensionless point. And they do not describe what happened at
• 49:53
  Speaker 1: the singularity itself. What they demonstrate is that classical general relativity,
• 49:59
  Speaker 1: under the relevant conditions, cannot always provide a complete history,
• 50:04
  Speaker 1: extending arbitrarily far into the past. That difference matters because
• 50:09
  Speaker 1: infinities in physics deserve suspicion. An infinite answer can sometimes
• 50:15
  Speaker 1: represent a genuine property of a mathematical model. But it
• 50:18
  Speaker 1: can also be nature's rather unsubtle way of telling us
• 50:21
  Speaker 1: that we have taken the model somewhere it was never
• 50:23
  Speaker 1: designed to go. Physics has encountered similar situations before. Classical
• 50:29
  Speaker 1: ideas about electromagnetic radiation once produced what became known as
• 50:33
  Speaker 1: the ultraviolet catastrophe. When 19th century physics was applied to
• 50:38
  Speaker 1: thermal radiation, calculations predicted that an ideal hot object should
• 50:43
  Speaker 1: emit ever-increasing amounts of energy at shorter wavelengths, leading to
• 50:47
  Speaker 1: a nonsensical divergence. Real objects did nothing of the sort.
• 50:52
  Speaker 1: The solution did not come from insisting that nature must
• 50:55
  Speaker 1: somehow contain an infinite amount of ultraviolet radiation. It came
• 51:00
  Speaker 1: from changing the theory. Max Planck introduced the idea that
• 51:04
  Speaker 1: energy exchange occurs in discrete amounts, helping to open the
• 51:08
  Speaker 1: path towards quantum physics. Another example appears in classical electromagnetism
• 51:14
  Speaker 1: when an electron is treated as a perfectly point-like charged object.
• 51:20
  Speaker 1: Trying to calculate the energy stored in its own electric
• 51:22
  Speaker 1: field leads naturally towards troublesome infinities. Quantum field theory handles
• 51:28
  Speaker 1: particles and interactions in a much deeper way. although it
• 51:32
  Speaker 1: introduces mathematical complications of its own. The broader lesson is simple.
• 51:38
  Speaker 1: When a successful theory produces an infinity at the edge
• 51:41
  Speaker 1: of its domain, we should not automatically assume that the
• 51:45
  Speaker 1: infinity itself exists physically. Sometimes the theory is telling us
• 51:50
  Speaker 1: that something deeper is missing. That possibility becomes especially important
• 51:54
  Speaker 1: at the Big Bang Singularity. General relativity is a classical theory.
• 51:59
  Speaker 1: It treats spacetime as a smooth geometric structure. That approximation
• 52:04
  Speaker 1: works extraordinarily well for planets, stars, galaxies, gravitational waves, and
• 52:12
  Speaker 1: even the spacetime surrounding black holes. But following the early
• 52:16
  Speaker 1: universe backwards concentrates enormous energy into ever smaller scales. Eventually
• 52:23
  Speaker 1: we approach conditions where quantum effects cannot reasonably be ignored.
• 52:28
  Speaker 1: At scales near the Planck regime, the geometry of spacetime
• 52:31
  Speaker 1: itself may need a quantum description. And that is precisely
• 52:35
  Speaker 1: what ordinary general relativity does not provide. So if classical
• 52:40
  Speaker 1: equations tell us that density becomes infinite and spacetime curvature
• 52:44
  Speaker 1: becomes infinite, there are at least two very different ways
• 52:48
  Speaker 1: to interpret the result. One possibility is that these infinities
• 52:52
  Speaker 1: correspond in some sense to a genuine boundary of physical reality.
• 52:58
  Speaker 1: Another is that they appear because we are using classical
• 53:01
  Speaker 1: general relativity beyond the point where it remains valid. Without
• 53:05
  Speaker 1: a tested theory of quantum gravity, we cannot confidently choose
• 53:10
  Speaker 1: between those possibilities. Some proposed quantum gravity models remove the
• 53:14
  Speaker 1: classical singularity. Certain versions of loop quantum cosmology, for example,
• 53:20
  Speaker 1: replace it with a transition between contraction and expansion. Other
• 53:24
  Speaker 1: approaches suggest that ordinary notions of space and time change
• 53:28
  Speaker 1: so radically at extreme energies that the question of a
• 53:31
  Speaker 1: conventional singular point may no longer make sense. None of
• 53:35
  Speaker 1: these possibilities has yet been experimentally established as the correct
• 53:40
  Speaker 1: description of our universe. This means the familiar picture of
• 53:44
  Speaker 1: everything beginning at an infinitely dense point should be treated
• 53:48
  Speaker 1: with caution. It is useful as shorthand for what happens
• 53:52
  Speaker 1: when classical equations are extrapolated backwards. It should not be
• 53:57
  Speaker 1: mistaken for something astronomers have directly observed. We have never
• 54:01
  Speaker 1: measured infinite temperature. We have never measured infinite density. And
• 54:06
  Speaker 1: we have never observed the supposed first instant itself. What
• 54:10
  Speaker 1: we have is a remarkably successful theory of gravity which,
• 54:14
  Speaker 1: when pushed backwards towards increasingly extreme conditions, eventually predicts its
• 54:19
  Speaker 1: own inability to continue. There is something rather elegant about that.
• 54:24
  Speaker 1: General relativity can carry us astonishingly far. It describes the
• 54:29
  Speaker 1: expanding universe. It explains how matter shapes space-time. It allows
• 54:34
  Speaker 1: us to reconstruct much of cosmic history. And then, near
• 54:38
  Speaker 1: the place where we most want an answer, its classical
• 54:41
  Speaker 1: description runs out. The Big Bang singularity is therefore not
• 54:45
  Speaker 1: necessarily the confirmed object from which everything emerged. It is
• 54:50
  Speaker 1: better understood as a warning at the edge of our
• 54:52
  Speaker 1: current map. Beyond that boundary, gravity cannot remain purely classical.
• 54:57
  Speaker 1: Quantum physics must somehow enter the description. And until we
• 55:01
  Speaker 1: understand exactly how spacetime behaves, when both gravity and quantum
• 55:05
  Speaker 1: effects become unavoidable, the singularity marks not a place where
• 55:10
  Speaker 1: science has discovered the beginning of everything, but a place
• 55:13
  Speaker 1: where one of our greatest theories can no longer tell
• 55:16
  Speaker 1: us what happened. The question of what happened before the
• 55:19
  Speaker 1: Big Bang sounds perfectly natural. We're used to everything having
• 55:25
  Speaker 1: a before. Before sunrise there was night. Before a tree
• 55:30
  Speaker 1: there was a seed. Before a person was born there
• 55:33
  Speaker 1: were parents, grandparents, and an entire chain of earlier events
• 55:38
  Speaker 1: stretching backwards through history. Our minds are built around sequences.
• 55:43
  Speaker 1: One thing happens, then another. So when we hear that
• 55:46
  Speaker 1: the universe was once in an extremely hot and dense state,
• 55:50
  Speaker 1: it feels almost automatic to ask what came before it.
• 55:54
  Speaker 1: But physics introduces a rather strange possibility. There may not
• 55:58
  Speaker 1: have been a before, not because nothing happened during some
• 56:02
  Speaker 1: mysterious empty interval, but because ordinary time itself may not
• 56:06
  Speaker 1: have existed in the way we understand it. This idea
• 56:10
  Speaker 1: becomes easier to appreciate once we stop thinking of time
• 56:13
  Speaker 1: as a universal clock sitting outside the universe. That was
• 56:17
  Speaker 1: roughly how time was treated in classical Newtonian physics. Isaac
• 56:22
  Speaker 1: Newton imagined time as flowing uniformly everywhere, independently of matter,
• 56:27
  Speaker 1: motion or anything else. Whether the universe contained stars, planets
• 56:31
  Speaker 1: or nothing at all, the clock of time would continue
• 56:35
  Speaker 1: ticking in the background. Albert Einstein changed that picture profoundly.
• 56:40
  Speaker 1: In special relativity, space and time became deeply connected. In
• 56:45
  Speaker 1: general relativity, that connection became even more intimate. Space and
• 56:49
  Speaker 1: time are combined into a four-dimensional structure called space-time. Matter
• 56:54
  Speaker 1: and energy influence the geometry of space-time, and that geometry
• 56:58
  Speaker 1: influences the motion of matter and light. Time is therefore
• 57:02
  Speaker 1: not merely a backdrop against which the universe performs, it
• 57:06
  Speaker 1: is part of the universe's physical structure. This already creates
• 57:10
  Speaker 1: effects that would seem very strange if time were a
• 57:13
  Speaker 1: single universal rhythm. Two observers moving at different speeds can
• 57:18
  Speaker 1: measure different amounts of elapsed time between the same events.
• 57:22
  Speaker 1: A clock moving extremely quickly relative to another clock can
• 57:26
  Speaker 1: tick more slowly. Gravity also changes the passage of time.
• 57:30
  Speaker 1: A clock deeper inside a gravitational field ticks more slowly
• 57:35
  Speaker 1: than one located where gravity is weaker. These effects are
• 57:39
  Speaker 1: not philosophical curiosities. They have been measured. Atomic clocks placed
• 57:44
  Speaker 1: at different altitudes accumulate slightly different amounts of time because
• 57:48
  Speaker 1: the strength of Earth's gravitational field changes with height. Satellite
• 57:53
  Speaker 1: navigation systems have to account for relativistic differences in clock
• 57:57
  Speaker 1: rates in order to work accurately. So even in the
• 58:00
  Speaker 1: universe around us today, there is no single invisible cosmic
• 58:03
  Speaker 1: clock ticking identically everywhere. Time is woven into space-time. And
• 58:08
  Speaker 1: if spacetime itself changes radically as we approach the earliest universe,
• 58:13
  Speaker 1: then time may change with it. This is why some
• 58:16
  Speaker 1: physicists have questioned whether the phrase before the Big Bang
• 58:20
  Speaker 1: is meaningful at all. A famous analogy compares the question
• 58:24
  Speaker 1: with asking what lies north of the North Pole. On
• 58:27
  Speaker 1: the surface of Earth, you can travel north for thousands
• 58:31
  Speaker 1: of kilometres. Eventually, however, you reach the North Pole. Once
• 58:35
  Speaker 1: you're standing there, There is no direction on Earth's surface
• 58:38
  Speaker 1: that takes you farther north. You can certainly keep travelling,
• 58:42
  Speaker 1: but whichever direction you choose is now south. The coordinate
• 58:46
  Speaker 1: has reached its boundary. In the same way, if time
• 58:49
  Speaker 1: truly begins with the earliest state of the universe, asking
• 58:53
  Speaker 1: what happened before that state might be like asking for
• 58:55
  Speaker 1: a moment earlier than the first moment. The question assumes
• 58:59
  Speaker 1: the existence of something that the model does not contain.
• 59:02
  Speaker 1: It is a useful analogy, but it should not be
• 59:05
  Speaker 1: mistaken for proof. The Earth has been measured. The North
• 59:08
  Speaker 1: Pole is a well-defined location on a known surface. The
• 59:12
  Speaker 1: beginning of cosmic time is much less certain. We do
• 59:15
  Speaker 1: not yet possess a confirmed theory describing the universe all
• 59:19
  Speaker 1: the way through the regime where quantum gravity becomes important.
• 59:23
  Speaker 1: So the North Pole analogy tells us how a beginning
• 59:25
  Speaker 1: of time could work conceptually. It does not establish that
• 59:29
  Speaker 1: nature actually works that way. There is also an important
• 59:33
  Speaker 1: distinction between an event that happens in time and a
• 59:36
  Speaker 1: boundary of time itself. Imagine the birth of a star.
• 59:41
  Speaker 1: Before that star existed, there was a cloud of gas.
• 59:44
  Speaker 1: Before the cloud collapsed, there were earlier stages of galactic history.
• 59:48
  Speaker 1: The formation of the star is an event within an
• 59:52
  Speaker 1: already existing timeline. We can ask what happened one year
• 59:56
  Speaker 1: before it. a million years before it, or far earlier still.
• 1:00:02
  Speaker 1: But suppose the timeline itself has a first point. Then
• 1:00:06
  Speaker 1: that first point is different. It is not simply an
• 1:00:09
  Speaker 1: event that happened after another event. It is the boundary
• 1:00:13
  Speaker 1: beyond which the timeline does not extend. A finite past
• 1:00:17
  Speaker 1: does not automatically require an earlier physical moment any more
• 1:00:22
  Speaker 1: than the first page of a book requires page zero
• 1:00:24
  Speaker 1: to exist before it. That comparison has limits too, because
• 1:00:29
  Speaker 1: a book exists in an external world containing earlier events.
• 1:00:33
  Speaker 1: The universe, by definition, does not offer us such an
• 1:00:37
  Speaker 1: obvious outside reference frame. This is where the question becomes
• 1:00:41
  Speaker 1: especially subtle. If time began with the universe, then saying
• 1:00:46
  Speaker 1: that the universe was created at a particular moment can
• 1:00:49
  Speaker 1: be misleading. Creation normally suggests a transition from an earlier
• 1:00:53
  Speaker 1: state to a later one, But if there was no
• 1:00:56
  Speaker 1: earlier time, there was no waiting period beforehand, there was
• 1:01:00
  Speaker 1: no empty clock counting down, there was no endless darkness
• 1:01:03
  Speaker 1: that suddenly became the universe. Those images all quietly assume
• 1:01:08
  Speaker 1: that time was already present. Yet this is only one possibility.
• 1:01:13
  Speaker 1: Other cosmological models allow time to extend beyond what we
• 1:01:16
  Speaker 1: call the Big Bang. In bouncing cosmologies, for example, our
• 1:01:21
  Speaker 1: expanding universe may have been preceded by a contracting phase.
• 1:01:25
  Speaker 1: In that picture, there is a genuine earlier history. The
• 1:01:29
  Speaker 1: Big Bang is not the beginning of time, but a
• 1:01:31
  Speaker 1: transition between two cosmic states. Some cyclic models go further,
• 1:01:36
  Speaker 1: suggesting repeated eras of cosmic evolution. Other approaches to quantum
• 1:01:41
  Speaker 1: cosmology propose that ordinary time may emerge from a deeper
• 1:01:44
  Speaker 1: quantum description, in which the concept of time does not
• 1:01:48
  Speaker 1: operate in the familiar way. In these ideas, time may
• 1:01:52
  Speaker 1: neither simply begin nor simply extend smoothly into the past.
• 1:01:56
  Speaker 1: It may emerge. That possibility is particularly difficult to imagine
• 1:02:00
  Speaker 1: because human thought is saturated with time. We explain change
• 1:02:04
  Speaker 1: using before and after. We explain causes using earlier and later.
• 1:02:09
  Speaker 1: Even the phrase time emerged seems to invite the question
• 1:02:12
  Speaker 1: of when it emerged, which immediately uses the very concept
• 1:02:16
  Speaker 1: we are trying to explain. Physics is not always obliged
• 1:02:19
  Speaker 1: to respect the habits of human grammar. The deeper question
• 1:02:23
  Speaker 1: is therefore not simply whether there was something before the
• 1:02:25
  Speaker 1: Big Bang, it is whether the word before remains physically
• 1:02:29
  Speaker 1: meaningful as we approach the earliest possible state. General relativity
• 1:02:33
  Speaker 1: shows that time is part of spacetime. Quantum theory suggests
• 1:02:37
  Speaker 1: that familiar classical descriptions may fail at extremely small scales.
• 1:02:42
  Speaker 1: A complete theory of quantum gravity may eventually reveal whether
• 1:02:46
  Speaker 1: time had a true beginning. whether it continued through an
• 1:02:49
  Speaker 1: earlier universe, or whether it emerged from something more fundamental
• 1:02:54
  Speaker 1: that cannot be described using ordinary temporal language at all.
• 1:02:57
  Speaker 1: For now, science does not know which of these possibilities
• 1:03:01
  Speaker 1: is correct. The hot Big Bang model describes an early
• 1:03:05
  Speaker 1: universe that was extraordinarily hot and dense. It does not,
• 1:03:09
  Speaker 1: on its own, tell us whether the cosmic clock began there.
• 1:03:12
  Speaker 1: So the phrase before the Big Bang may ultimately turn
• 1:03:15
  Speaker 1: out to describe an earlier chapter of physical history. or
• 1:03:18
  Speaker 1: it may be asking for a direction that does not exist.
• 1:03:22
  Speaker 1: And that is what makes the question so difficult. Before
• 1:03:26
  Speaker 1: we can discover what happened before the beginning, we may
• 1:03:29
  Speaker 1: first have to discover whether there was ever such a
• 1:03:32
  Speaker 1: thing as before. If the hot big bang was not
• 1:03:35
  Speaker 1: necessarily the absolute beginning, then one of the most important
• 1:03:38
  Speaker 1: questions becomes what might have happened immediately before it. One
• 1:03:42
  Speaker 1: of the leading ideas in modern cosmology is called cosmic inflation.
• 1:03:47
  Speaker 1: Inflation proposes that, during an extremely early stage of cosmic history,
• 1:03:51
  Speaker 1: space underwent a brief period of extraordinarily rapid, accelerated expansion.
• 1:03:57
  Speaker 1: This was not simply the ordinary expansion of the universe
• 1:04:00
  Speaker 1: running slightly faster. The change would have been immense. A
• 1:04:04
  Speaker 1: region of space far smaller than anything we could see
• 1:04:07
  Speaker 1: directly today may have expanded by an enormous factor in
• 1:04:11
  Speaker 1: a tiny fraction of a second. The idea emerged partly
• 1:04:15
  Speaker 1: because the original Hot Big Bang model, Successful though it was,
• 1:04:18
  Speaker 1: left several puzzles behind. One of the physicists who brought
• 1:04:22
  Speaker 1: these puzzles together was Alan Guth. Around 1980, Guth developed
• 1:04:27
  Speaker 1: an early model of inflation while investigating problems connected with
• 1:04:30
  Speaker 1: particle physics and the young universe. His original version did
• 1:04:34
  Speaker 1: not provide a completely successful description of how inflation would end,
• 1:04:38
  Speaker 1: but the central insight proved extremely influential. Other physicists, including
• 1:04:44
  Speaker 1: André Linde, Andreas Albrecht and Paul Steinhardt, developed improved models
• 1:04:50
  Speaker 1: in which inflation could end more naturally and produce a
• 1:04:53
  Speaker 1: universe resembling the one we observe. The motivation for inflation
• 1:04:57
  Speaker 1: can be understood by beginning with a surprisingly simple observation.
• 1:05:01
  Speaker 1: The universe is remarkably uniform. Look at the cosmic microwave
• 1:05:05
  Speaker 1: background in opposite directions across the sky. And the radiation
• 1:05:09
  Speaker 1: has almost exactly the same temperature. The differences are tiny.
• 1:05:14
  Speaker 1: typically only a few parts in 100,000. That uniformity becomes
• 1:05:18
  Speaker 1: puzzling when we trace the history backwards using a simple,
• 1:05:22
  Speaker 1: hot Big Bang model without inflation. Two widely separated regions
• 1:05:27
  Speaker 1: of the cosmic microwave background can be so far apart that,
• 1:05:31
  Speaker 1: before the radiation was released, there would not have been
• 1:05:34
  Speaker 1: enough time for light or any other causal influence to
• 1:05:36
  Speaker 1: travel between them. In other words, those regions appear never
• 1:05:41
  Speaker 1: to have been in contact. and yet their temperatures match
• 1:05:44
  Speaker 1: with astonishing precision. Imagine placing two cups of water in
• 1:05:49
  Speaker 1: separate rooms that have never communicated with one another, and
• 1:05:53
  Speaker 1: discovering that both happen to have almost exactly the same temperature.
• 1:05:58
  Speaker 1: It is possible. But if the agreement is extraordinarily precise
• 1:06:02
  Speaker 1: across the entire cosmos, we naturally begin looking for an explanation.
• 1:06:07
  Speaker 1: This is known as the horizon problem. Inflation provides one.
• 1:06:12
  Speaker 1: Before the period of rapid expansion, the region that eventually
• 1:06:16
  Speaker 1: became our observable universe could have been much smaller. Parts
• 1:06:20
  Speaker 1: of it that are now separated by tens of billions
• 1:06:22
  Speaker 1: of light years could once have been close enough to interact.
• 1:06:26
  Speaker 1: Radiation and particles could exchange energy. Temperatures could even out.
• 1:06:31
  Speaker 1: The region could approach thermal equilibrium. Then inflation would stretch
• 1:06:35
  Speaker 1: that once connected patch of space to an enormous size.
• 1:06:39
  Speaker 1: Areas that began close together would be carried so far
• 1:06:42
  Speaker 1: apart that today they appear on opposite sides of the
• 1:06:45
  Speaker 1: observable universe. Their similar temperatures would no longer be mysterious.
• 1:06:50
  Speaker 1: They would match because long ago they really had been
• 1:06:53
  Speaker 1: in contact. Inflation also addresses another puzzle known as the
• 1:06:57
  Speaker 1: flatness problem. In general relativity, the geometry of the universe
• 1:07:01
  Speaker 1: can be positively curved, negatively curved or extremely close to flat.
• 1:07:07
  Speaker 1: Observations indicate that On very large scales, the universe is
• 1:07:12
  Speaker 1: remarkably close to spatially flat. Without inflation, obtaining such near
• 1:07:19
  Speaker 1: flatness today requires the early universe to have begun with
• 1:07:22
  Speaker 1: its density extraordinarily close to a particular value. Small deviations
• 1:07:27
  Speaker 1: would have grown over cosmic time. Inflation naturally drives observable
• 1:07:32
  Speaker 1: space towards flatness. A useful comparison is the surface of Earth.
• 1:07:37
  Speaker 1: Earth is curved. but if you examine a tiny patch
• 1:07:40
  Speaker 1: of ground, it looks nearly flat. Inflation takes a small
• 1:07:45
  Speaker 1: region of spacetime and stretches it enormously. Any initial curvature
• 1:07:50
  Speaker 1: within the observable portion becomes diluted until that region appears
• 1:07:53
  Speaker 1: extremely flat. There was also a third difficulty connected with
• 1:07:57
  Speaker 1: early attempts to combine cosmology with high-energy particle theories. Some
• 1:08:03
  Speaker 1: proposed theories predicted the production of exotic relics, including extremely
• 1:08:08
  Speaker 1: massive particles such as magnetic monopoles. A magnetic monopole would behave,
• 1:08:13
  Speaker 1: very loosely speaking, like an isolated magnetic north pole or
• 1:08:16
  Speaker 1: south pole, rather than the paired poles found in ordinary magnets.
• 1:08:21
  Speaker 1: Such particles have never been convincingly detected. If large numbers
• 1:08:26
  Speaker 1: had been produced in the early universe and survived, they
• 1:08:29
  Speaker 1: could have become cosmologically troublesome. Inflation offers a simple way
• 1:08:33
  Speaker 1: to reduce their abundance. Rapid expansion would spread any pre-existing
• 1:08:38
  Speaker 1: relics across an enormous volume, leaving so few within our
• 1:08:42
  Speaker 1: observable region that their absence would no longer be surprising.
• 1:08:47
  Speaker 1: The mechanism responsible for inflation is usually represented by a
• 1:08:51
  Speaker 1: high energy field. This hypothetical field is often called the
• 1:08:55
  Speaker 1: inflatant field, although no specific inflatant particle has ever been
• 1:09:00
  Speaker 1: experimentally identified. Under the right conditions the energy associated with
• 1:09:05
  Speaker 1: such a field could act in a way that produces
• 1:09:08
  Speaker 1: accelerated expansion. This is where ordinary intuition about gravity becomes
• 1:09:13
  Speaker 1: slightly unreliable. We normally imagine gravity pulling things together, But
• 1:09:18
  Speaker 1: in general relativity, sufficiently unusual forms of energy and pressure
• 1:09:22
  Speaker 1: can drive accelerated expansion of space. Inflation would have been
• 1:09:27
  Speaker 1: an extreme example. During the inflationary phase, distances between points
• 1:09:32
  Speaker 1: not gravitationally bound to one another could increase exponentially. This
• 1:09:36
  Speaker 1: does not mean particles were travelling through space faster than
• 1:09:40
  Speaker 1: light in violation of Albert Einstein's relativity. The restriction on
• 1:09:44
  Speaker 1: the speed of light applies locally to objects moving through spacetime.
• 1:09:48
  Speaker 1: General relativity allows the geometry of space itself to expand
• 1:09:52
  Speaker 1: in such a way that sufficiently distant regions separate faster
• 1:09:56
  Speaker 1: than light could cross the growing distance between them. Inflation
• 1:10:00
  Speaker 1: is usually thought to have lasted for an extraordinarily short interval.
• 1:10:05
  Speaker 1: Exactly when it began, how long it lasted, and at
• 1:10:08
  Speaker 1: what energy it occurred depend on the particular model. A
• 1:10:12
  Speaker 1: commonly discussed requirement is that the scale of the universe
• 1:10:16
  Speaker 1: increased by at least around 26 orders of magnitude during
• 1:10:21
  Speaker 1: the relevant inflationary period, although the precise amount depends on
• 1:10:25
  Speaker 1: the model and assumptions. Then inflation had to stop. That
• 1:10:29
  Speaker 1: ending is essential because the universe around us is clearly
• 1:10:32
  Speaker 1: not still dominated everywhere by the same inflationary state. As
• 1:10:37
  Speaker 1: inflation ended, the energy associated with the field driving it
• 1:10:43
  Speaker 1: would have been transferred into ordinary particles and radiation. This
• 1:10:46
  Speaker 1: process is generally called reheating. The name can sound a
• 1:10:50
  Speaker 1: little odd because the universe had just undergone tremendous expansion,
• 1:10:55
  Speaker 1: which we normally associate with cooling. But during inflation, much
• 1:10:59
  Speaker 1: of the relevant energy was stored in the inflationary field
• 1:11:02
  Speaker 1: rather than in a conventional hot bath of particles. When
• 1:11:06
  Speaker 1: inflation ended, that energy could be converted into particles. Those
• 1:11:11
  Speaker 1: particles interacted, produced more particles, and established a hot thermal state.
• 1:11:17
  Speaker 1: This is the point where the familiar hot Big Bang
• 1:11:19
  Speaker 1: picture begins. That distinction is extremely important. In modern inflationary cosmology,
• 1:11:26
  Speaker 1: the phrase Big Bang can be used in different ways.
• 1:11:30
  Speaker 1: Sometimes it refers broadly to the entire earliest history of
• 1:11:33
  Speaker 1: the universe. But more precisely, the hot Big Bang describes
• 1:11:38
  Speaker 1: the hot dense phase that followed inflation and reheating. If
• 1:11:42
  Speaker 1: inflation actually occurred, then the Hot Big Bang was not
• 1:11:46
  Speaker 1: necessarily the first physical state. It may have been the
• 1:11:49
  Speaker 1: aftermath of something earlier. In that sense, inflation pushes the
• 1:11:53
  Speaker 1: apparent beginning farther backwards. It does not necessarily remove the
• 1:11:57
  Speaker 1: need for a beginning altogether. Inflation has enjoyed enormous influence
• 1:12:02
  Speaker 1: because several of its broad predictions match observations impressively well.
• 1:12:07
  Speaker 1: It predicts a universe that is very close to spatially flat.
• 1:12:11
  Speaker 1: It provides a mechanism for explaining the large-scale uniformity of
• 1:12:15
  Speaker 1: the cosmic microwave background. And remarkably, quantum fluctuations during inflation
• 1:12:22
  Speaker 1: can be stretched to astronomical sizes, providing seeds from which
• 1:12:26
  Speaker 1: later cosmic structures can grow. That last point connects inflation
• 1:12:31
  Speaker 1: not merely with the smoothness of the universe, but with
• 1:12:34
  Speaker 1: the tiny irregularities that eventually became galaxies. Yet inflation is
• 1:12:40
  Speaker 1: not a completed story. Scientists do not know exactly what
• 1:12:44
  Speaker 1: physical field drove it. No inflaton has been directly detected.
• 1:12:49
  Speaker 1: The energy scale of inflation remains uncertain. There are many
• 1:12:52
  Speaker 1: different inflationary models, and observations have ruled out some possibilities
• 1:12:56
  Speaker 1: while leaving others viable. A clear detection of certain primordial
• 1:13:01
  Speaker 1: gravitational wave signatures could reveal much more about the process.
• 1:13:05
  Speaker 1: But such evidence has not yet provided a definitive measurement
• 1:13:09
  Speaker 1: of inflation's underlying mechanism. So inflation occupies an interesting position
• 1:13:14
  Speaker 1: in cosmology. It is more than a casual speculation. It
• 1:13:18
  Speaker 1: provides elegant solutions to several real problems and makes predictions
• 1:13:22
  Speaker 1: consistent with important observations. But the exact physics behind it
• 1:13:27
  Speaker 1: remains unresolved. And even if inflation occurred, another question immediately appears.
• 1:13:34
  Speaker 1: What was happening before inflation? Inflation may place an earlier
• 1:13:38
  Speaker 1: stage before the hot big bang. It may explain how
• 1:13:41
  Speaker 1: a small connected region became the enormous observable universe around us.
• 1:13:46
  Speaker 1: But rather than closing the question of cosmic origins, it
• 1:13:49
  Speaker 1: moves the mystery one level deeper. The beginning of the
• 1:13:53
  Speaker 1: hot universe may not have been the beginning of everything
• 1:13:56
  Speaker 1: at all. One of the strangest ideas in modern cosmology–
• 1:14:00
  Speaker 1: is that the enormous structure of the universe may have
• 1:14:03
  Speaker 1: begun with something unimaginably small. Galaxies. Clusters containing thousands of galaxies.
• 1:14:10
  Speaker 1: Vast filaments stretching across hundreds of millions of light years.
• 1:14:15
  Speaker 1: Even the immense pattern known as the cosmic web may
• 1:14:18
  Speaker 1: ultimately trace its origins back to tiny quantum fluctuations in
• 1:14:22
  Speaker 1: the earliest universe. To understand what that means, we have
• 1:14:25
  Speaker 1: to leave behind the familiar idea that empty space is
• 1:14:28
  Speaker 1: truly empty. In quantum field theory, the universe is filled
• 1:14:32
  Speaker 1: with fields. There is an electron field, there are quark fields,
• 1:14:37
  Speaker 1: there is an electromagnetic field. And in our best current
• 1:14:41
  Speaker 1: descriptions of nature, what we call particles, are excitations of
• 1:14:46
  Speaker 1: these underlying quantum fields. Even when no ordinary particles are present,
• 1:14:51
  Speaker 1: the fields themselves do not become perfectly still. Quantum mechanics
• 1:14:56
  Speaker 1: does not allow them to possess completely definite values everywhere
• 1:15:00
  Speaker 1: at once. Instead, they exhibit unavoidable fluctuations. These are not
• 1:15:06
  Speaker 1: simply disturbances caused by hidden particles rattling around in the background.
• 1:15:11
  Speaker 1: They are part of the quantum nature of the fields themselves.
• 1:15:15
  Speaker 1: This is one reason the quantum vacuum is very different
• 1:15:18
  Speaker 1: from the everyday meaning of emptiness. A vacuum in quantum
• 1:15:22
  Speaker 1: field theory is the lowest energy state available to a system.
• 1:15:26
  Speaker 1: It does not mean the complete absence of fields, laws
• 1:15:29
  Speaker 1: or physical structure, and it certainly does not mean philosophical nothingness.
• 1:15:35
  Speaker 1: That distinction becomes especially important when people hear phrases such
• 1:15:39
  Speaker 1: as quantum fluctuation and imagine entire universes casually popping out
• 1:15:45
  Speaker 1: of absolutely nothing. Quantum theory by itself does not say that.
• 1:15:50
  Speaker 1: A quantum fluctuation describes behaviour occurring within a defined quantum
• 1:15:56
  Speaker 1: system governed by physical laws. The truly remarkable part is
• 1:16:00
  Speaker 1: what inflation may have done to these tiny fluctuations. During
• 1:16:04
  Speaker 1: cosmic inflation, space is thought to have expanded at an
• 1:16:07
  Speaker 1: extraordinary rate. A microscopic region could be stretched to astronomical
• 1:16:12
  Speaker 1: dimensions in a tiny fraction of a second. The quantum
• 1:16:16
  Speaker 1: fields present during inflation would also have possessed fluctuations. At first,
• 1:16:21
  Speaker 1: these variations would have existed on extremely small scales. One
• 1:16:25
  Speaker 1: location might contain very slightly more energy than another. Another
• 1:16:29
  Speaker 1: region might contain very slightly less. Normally, quantum fluctuations on
• 1:16:34
  Speaker 1: such tiny scales would remain microscopic. Inflation changes that. As
• 1:16:40
  Speaker 1: space expands rapidly, the physical wavelengths associated with these fluctuations
• 1:16:45
  Speaker 1: are stretched. Tiny variations that once belonged to the quantum
• 1:16:49
  Speaker 1: world can be carried outward until they span enormous regions
• 1:16:53
  Speaker 1: of the universe. In effect, inflation can take microscopic quantum
• 1:16:58
  Speaker 1: irregularities and enlarge them into primordial differences in density. The
• 1:17:04
  Speaker 1: universe emerging from inflation would therefore be extremely smooth, but
• 1:17:09
  Speaker 1: not perfectly smooth. That slight imperfection is crucial. If every
• 1:17:14
  Speaker 1: region of the early universe had contained exactly the same density,
• 1:17:19
  Speaker 1: with absolutely no variation anywhere, gravity would have had very
• 1:17:23
  Speaker 1: little to work with. Matter would have remained distributed almost
• 1:17:27
  Speaker 1: perfectly evenly. There would be no slightly denser patches to
• 1:17:31
  Speaker 1: begin attracting extra material, no natural seeds from which galaxies
• 1:17:35
  Speaker 1: could grow. Instead, the early universe appears to have contained
• 1:17:39
  Speaker 1: tiny density variations. Some regions held a little more matter
• 1:17:44
  Speaker 1: and energy than average. Others held a little less. Gravity
• 1:17:48
  Speaker 1: gradually amplified those differences. A slightly denser region exerted a
• 1:17:52
  Speaker 1: slightly stronger gravitational pull. It attracted more matter. That made
• 1:17:57
  Speaker 1: it denser still. Over hundreds of millions and then billions
• 1:18:01
  Speaker 1: of years, this quiet process transformed minute primordial irregularities into
• 1:18:07
  Speaker 1: enormous cosmic structures. The denser regions eventually produced stars and galaxies.
• 1:18:13
  Speaker 1: Galaxies gathered into groups and clusters. Clusters and galaxies became
• 1:18:18
  Speaker 1: arranged along vast filaments. Between them emerged enormous under-dense regions
• 1:18:24
  Speaker 1: known as cosmic voids. The result is the cosmic web,
• 1:18:28
  Speaker 1: the large-scale structure of the universe that astronomers observe today.
• 1:18:32
  Speaker 1: In this sense, the universe around us may be a
• 1:18:35
  Speaker 1: greatly magnified record of quantum physics operating almost at the
• 1:18:39
  Speaker 1: beginning of cosmic history. And remarkably, traces of those original
• 1:18:44
  Speaker 1: variations can still be measured. The cosmic microwave background is
• 1:18:49
  Speaker 1: not perfectly uniform. Its average temperature today is roughly 2.7 Kelvin,
• 1:18:55
  Speaker 1: but careful measurements reveal tiny variations from one direction in
• 1:18:58
  Speaker 1: the sky to another. These differences are extremely small, generally
• 1:19:03
  Speaker 1: only tens of millionths of a Kelvin, yet they carry
• 1:19:06
  Speaker 1: an extraordinary amount of information. The National Aeronautics and Space
• 1:19:11
  Speaker 1: Administration Wilkinson Microwave Anisotropy Probe mapped these variations across the
• 1:19:15
  Speaker 1: sky with great precision. Later, the European Space Agency Planck
• 1:19:20
  Speaker 1: mission measured them in even greater detail. The resulting maps
• 1:19:24
  Speaker 1: show a mottled pattern of slightly warmer and slightly cooler regions.
• 1:19:28
  Speaker 1: These temperature variations are related to differences in density and
• 1:19:32
  Speaker 1: gravitational potential in the young universe. They provide a kind
• 1:19:36
  Speaker 1: of fossil record of the seeds from which later structure developed.
• 1:19:40
  Speaker 1: The overall statistical pattern of these fluctuations agrees remarkably well
• 1:19:45
  Speaker 1: with the broad expectations of simple inflationary models. The variations
• 1:19:49
  Speaker 1: are close to scale invariant, meaning that fluctuations occur across
• 1:19:53
  Speaker 1: a wide range of sizes with a particularly nearly balanced distribution.
• 1:19:58
  Speaker 1: They are also close to Gaussian, which means their statistical
• 1:20:01
  Speaker 1: properties are approximately those expected from many simple random quantum fluctuations.
• 1:20:08
  Speaker 1: Neither of these facts proves that inflation occurred exactly as
• 1:20:11
  Speaker 1: any particular model describes. There are many versions of inflation,
• 1:20:16
  Speaker 1: and important details remain unresolved. But the agreement between early
• 1:20:20
  Speaker 1: quantum fluctuation models and the observed structure of the cosmic
• 1:20:24
  Speaker 1: microwave background is one reason inflation remained such an influential idea.
• 1:20:31
  Speaker 1: It also creates one of the most extraordinary connections in
• 1:20:34
  Speaker 1: all of physics. Quantum mechanics is usually associated with the
• 1:20:37
  Speaker 1: very small– atoms, electrons, photons, subatomic particles. Cosmology deals with
• 1:20:44
  Speaker 1: the very large– galaxies, galaxy clusters, the observable universe. Inflation
• 1:20:51
  Speaker 1: provides a possible bridge between the two. A quantum fluctuation
• 1:20:54
  Speaker 1: that began on a microscopic scale could be stretched across
• 1:20:57
  Speaker 1: an enormous region of space. Gravity could then amplify its
• 1:21:01
  Speaker 1: effects for billions of years. Eventually, that fluctuation might influence
• 1:21:06
  Speaker 1: where thousands or millions of galaxies formed. In a very
• 1:21:09
  Speaker 1: real sense, the largest visible structures in the universe may
• 1:21:13
  Speaker 1: carry the fingerprints of processes that began at almost unimaginably
• 1:21:17
  Speaker 1: small scales. That idea naturally raises an even deeper question.
• 1:21:23
  Speaker 1: So far, quantum physics has been applied to fields existing
• 1:21:26
  Speaker 1: within spacetime. The field fluctuates. Space expands. The fluctuation is stretched.
• 1:21:32
  Speaker 1: But what happens when we push farther towards the earliest
• 1:21:35
  Speaker 1: possible state, where space-time itself may no longer behave classically?
• 1:21:39
  Speaker 1: Can space possess quantum properties? Can time fluctuate? Can the
• 1:21:44
  Speaker 1: geometry of the universe itself exist in a quantum state?
• 1:21:48
  Speaker 1: Questions like these belong to a broad area known as
• 1:21:51
  Speaker 1: quantum cosmology. Quantum cosmology attempts to apply the principles of
• 1:21:55
  Speaker 1: quantum theory not merely to particles and fields inside the universe,
• 1:21:59
  Speaker 1: but potentially to the universe as a whole. that is
• 1:22:02
  Speaker 1: a far more difficult task. Ordinary quantum mechanics often describes
• 1:22:06
  Speaker 1: a system within a larger environment. An atom can be
• 1:22:10
  Speaker 1: studied inside a laboratory. A particle can be measured using
• 1:22:13
  Speaker 1: an apparatus. But if the quantum system is the entire universe,
• 1:22:18
  Speaker 1: there is no obvious external laboratory sitting outside it. There
• 1:22:23
  Speaker 1: is no external clock or observer that can simply be assumed.
• 1:22:27
  Speaker 1: Even the concepts used to formulate quantum theory begin to
• 1:22:30
  Speaker 1: become difficult. And this brings the question of cosmic origins
• 1:22:34
  Speaker 1: into unfamiliar territory. Perhaps the earliest mystery is not simply
• 1:22:38
  Speaker 1: what kinds of particles existed before the Hot Big Bang.
• 1:22:42
  Speaker 1: Perhaps the deeper question is whether particles, fields, space and
• 1:22:46
  Speaker 1: time were already separate concepts at all. The same quantum
• 1:22:50
  Speaker 1: principles that may have planted the seeds of galaxies could
• 1:22:53
  Speaker 1: also become essential for understanding the origin of spacetime itself.
• 1:22:58
  Speaker 1: The tiny fluctuations written across the cosmic microwave background, therefore
• 1:23:02
  Speaker 1: do more than explain why the universe contains structure. They
• 1:23:06
  Speaker 1: hint at a profound connection between the quantum world and
• 1:23:09
  Speaker 1: the cosmos. Everything from the Milky Way to the largest
• 1:23:13
  Speaker 1: galaxy clusters may owe its existence to microscopic irregularities stretched
• 1:23:18
  Speaker 1: across the young universe, and if quantum physics can shape
• 1:23:22
  Speaker 1: the contents of spacetime on such enormous scales, it becomes
• 1:23:26
  Speaker 1: difficult to avoid an even stranger possibility. At the deepest
• 1:23:30
  Speaker 1: beginning of Space-time itself may have been quantum too. If
• 1:23:34
  Speaker 1: time itself may not extend indefinitely into the past, then
• 1:23:38
  Speaker 1: perhaps the beginning of the universe does not need to
• 1:23:41
  Speaker 1: be imagined as a sharp edge. That possibility lies at
• 1:23:45
  Speaker 1: the heart of one of the most famous ideas in
• 1:23:47
  Speaker 1: quantum cosmology. It is known as the no-boundary proposal. The
• 1:23:52
  Speaker 1: idea was developed by James Hartle and Stephen Hawking as
• 1:23:55
  Speaker 1: an attempt to describe the earliest universe without simply forcing
• 1:23:59
  Speaker 1: classical general relativity all the way back to a singular beginning.
• 1:24:03
  Speaker 1: The problem they were trying to address was a difficult one.
• 1:24:07
  Speaker 1: If classical spacetime is extrapolated backwards, the equations can lead
• 1:24:12
  Speaker 1: towards a boundary where density and curvature become extreme and
• 1:24:16
  Speaker 1: the usual description stops working. But if quantum physics becomes
• 1:24:20
  Speaker 1: essential near that regime, then perhaps the geometry of spacetime
• 1:24:24
  Speaker 1: itself should also be treated quantum mechanically. The no boundary
• 1:24:29
  Speaker 1: proposal takes that possibility seriously. In ordinary experience, time seems
• 1:24:34
  Speaker 1: very different from space. We can move forwards, backwards, left
• 1:24:39
  Speaker 1: or right through space. Time appears to offer only one direction.
• 1:24:44
  Speaker 1: But in the mathematical description used by Hartland Hawking, the
• 1:24:48
  Speaker 1: distinction between time and spatial dimensions can become much less
• 1:24:51
  Speaker 1: familiar near the earliest state of the universe. Instead of
• 1:24:55
  Speaker 1: imagining time running backwards, until it crashes into a hard beginning.
• 1:25:00
  Speaker 1: The geometry can become smooth. A familiar analogy is the
• 1:25:04
  Speaker 1: surface of Earth. Earth has a finite surface area. You
• 1:25:08
  Speaker 1: can travel north for thousands of kilometres until eventually you
• 1:25:11
  Speaker 1: reach the North Pole, but the North Pole is not
• 1:25:13
  Speaker 1: an edge where the surface suddenly stops. There is no
• 1:25:16
  Speaker 1: cliff beyond which the planet disappears. The surface remains smooth.
• 1:25:21
  Speaker 1: The coordinate we call north simply loses its ordinary meaning
• 1:25:24
  Speaker 1: at that point. Hartle and Hawking proposed that something vaguely
• 1:25:28
  Speaker 1: similar might happen with cosmic time. As the universe is
• 1:25:32
  Speaker 1: traced towards its earlier state, time may gradually behave more
• 1:25:36
  Speaker 1: like a spatial direction within the mathematical description. Instead of
• 1:25:40
  Speaker 1: ending at a sharp, singular boundary, the geometry could round
• 1:25:44
  Speaker 1: off smoothly. The universe could therefore be finite in its
• 1:25:48
  Speaker 1: past extent without possessing a conventional beginning edge. This is
• 1:25:52
  Speaker 1: where the phrase no boundary comes from. It does not
• 1:25:56
  Speaker 1: mean that the universe has no structure. It does not
• 1:25:59
  Speaker 1: mean that there are no limits to what can be described.
• 1:26:02
  Speaker 1: It means that the quantum description does not require an
• 1:26:05
  Speaker 1: ordinary temporal boundary that demands another region or earlier moment
• 1:26:10
  Speaker 1: beyond it. To formulate this idea, Hartley and Hawking used
• 1:26:15
  Speaker 1: a mathematical technique involving what is called imaginary time. The
• 1:26:20
  Speaker 1: phrase can sound as though physicists have simply invented a
• 1:26:23
  Speaker 1: fictional kind of time, and hoped nobody would notice. That
• 1:26:27
  Speaker 1: is not what imaginary means here. In mathematics, imaginary numbers
• 1:26:32
  Speaker 1: involve the square root of negative one. They are well-defined
• 1:26:37
  Speaker 1: mathematical objects and are used throughout physics, engineering, and quantum theory.
• 1:26:41
  Speaker 1: Imaginary time is a mathematical transformation in which the time
• 1:26:45
  Speaker 1: coordinate is treated differently in order to simplify the geometry
• 1:26:49
  Speaker 1: in the quantum calculations. Near the proposed beginning, This transformed
• 1:26:54
  Speaker 1: description can make spacetime behave more like a smooth, four-dimensional
• 1:26:58
  Speaker 1: geometry without the sharp distinction between one time direction and
• 1:27:03
  Speaker 1: three space directions that we experience today. It is important
• 1:27:07
  Speaker 1: not to picture imaginary time as a hidden clock physically
• 1:27:10
  Speaker 1: ticking somewhere beside ordinary time. It is better understood as
• 1:27:14
  Speaker 1: part of the mathematical machinery used to describe the quantum
• 1:27:18
  Speaker 1: state of the universe. In the no-boundary proposal, This allows
• 1:27:22
  Speaker 1: the earliest geometry to avoid the classical singularity. Rather than
• 1:27:27
  Speaker 1: beginning from a point where the theory produces infinite curvature,
• 1:27:31
  Speaker 1: the universe can emerge from a smooth quantum regime. At
• 1:27:35
  Speaker 1: larger scales, ordinary time then becomes recognisable. The universe enters
• 1:27:40
  Speaker 1: a more classical phase. Expansion proceeds. Matter and radiation evolve. Eventually,
• 1:27:47
  Speaker 1: the familiar hot Big Bang history takes over. From this perspective,
• 1:27:51
  Speaker 1: asking what happened before the beginning becomes difficult for a
• 1:27:54
  Speaker 1: very specific reason. The model does not contain an ordinary
• 1:27:58
  Speaker 1: earlier time. There is no previous Tuesday. There is no
• 1:28:02
  Speaker 1: empty era waiting around before the universe appeared. There is
• 1:28:05
  Speaker 1: no hidden cosmic clock ticking in darkness. The geometry simply
• 1:28:10
  Speaker 1: does not extend into a conventional before. That makes the
• 1:28:13
  Speaker 1: no-boundary proposal very different from theories in which an earlier
• 1:28:17
  Speaker 1: physical phase really does exist. In some inflationary models, inflation
• 1:28:22
  Speaker 1: itself may have preceded the hot big bang. If inflation
• 1:28:25
  Speaker 1: arose from an even earlier state, then there may be
• 1:28:28
  Speaker 1: a meaningful history before reheating. In bouncing cosmologies, the difference
• 1:28:33
  Speaker 1: is even clearer. A contracting universe may precede the expanding one.
• 1:28:38
  Speaker 1: Time continues through the transition. In those models, asking what
• 1:28:43
  Speaker 1: happened before the Big Bang is not meaningless at all.
• 1:28:46
  Speaker 1: There is something to investigate. The no-boundary proposal changes the
• 1:28:51
  Speaker 1: question more radically. Instead of supplying an earlier event, it
• 1:28:55
  Speaker 1: challenges the assumption that an earlier event is required That
• 1:28:59
  Speaker 1: does not mean the proposal has been proven. It remains
• 1:29:02
  Speaker 1: a theoretical idea. James Hartle and Stephen Hawking developed a
• 1:29:07
  Speaker 1: mathematically sophisticated framework, but the earliest quantum state of the
• 1:29:11
  Speaker 1: universe is far beyond anything that can currently be observed directly.
• 1:29:16
  Speaker 1: There are also technical debates about how the proposal should
• 1:29:19
  Speaker 1: be formulated, how its mathematical integral should be interpreted, and
• 1:29:23
  Speaker 1: what predictions it makes for inflation and the large-scale universe.
• 1:29:28
  Speaker 1: Different approaches to quantum cosmology do not all agree on
• 1:29:31
  Speaker 1: the same picture. That uncertainty matters. The no-boundary proposal is
• 1:29:36
  Speaker 1: not the accepted final answer to the origin of everything.
• 1:29:40
  Speaker 1: It is one possible way of showing that the beginning
• 1:29:42
  Speaker 1: may be very different from the simple image of a
• 1:29:45
  Speaker 1: universe suddenly appearing at a first tick of an already
• 1:29:48
  Speaker 1: existing clock. Its deepest suggestion is that the earliest universe
• 1:29:52
  Speaker 1: may not have had an edge in time at all.
• 1:29:55
  Speaker 1: Just as the surface of Earth can be finite without
• 1:29:58
  Speaker 1: ending at a boundary, cosmic history may in principle possess
• 1:30:02
  Speaker 1: a finite past without requiring a moment before the first moment.
• 1:30:07
  Speaker 1: If that picture is correct, then the question of what
• 1:30:09
  Speaker 1: came before the universe may not be unanswered. It may
• 1:30:13
  Speaker 1: be built on an assumption that fails at the deepest level.
• 1:30:17
  Speaker 1: Perhaps the beginning was not a wall. Perhaps spacetime simply
• 1:30:21
  Speaker 1: became smooth in a direction where our ordinary idea of
• 1:30:24
  Speaker 1: before no longer applies. Once spacetime itself enters the quantum story,
• 1:30:29
  Speaker 1: the idea of a beginning becomes much stranger than simply
• 1:30:32
  Speaker 1: asking what object existed first. One possibility explored by physicists
• 1:30:39
  Speaker 1: is that the universe could emerge through a genuinely quantum process.
• 1:30:44
  Speaker 1: Among the researchers most closely associated with this idea is
• 1:30:47
  Speaker 1: Alexander Vilenkin. who developed models in quantum cosmology in which
• 1:30:52
  Speaker 1: an expanding universe can emerge from a state with no
• 1:30:55
  Speaker 1: ordinary classical spacetime. This is sometimes described rather dramatically as
• 1:31:01
  Speaker 1: a universe appearing from nothing. That phrase needs considerable care.
• 1:31:06
  Speaker 1: Before dealing with the nothing, it helps to understand the
• 1:31:09
  Speaker 1: process that inspired the idea. Quantum tunnelling is a well-established
• 1:31:14
  Speaker 1: phenomenon in quantum mechanics. In classical physics, there are situations
• 1:31:18
  Speaker 1: in which a system simply cannot cross a barrier. Imagine
• 1:31:22
  Speaker 1: a ball sitting in a valley with a hill beside it.
• 1:31:25
  Speaker 1: If the ball does not possess enough energy to climb
• 1:31:27
  Speaker 1: over the hill, classical physics says it stays where it is.
• 1:31:31
  Speaker 1: Quantum mechanics behaves differently. A quantum object is described using
• 1:31:36
  Speaker 1: probabilities rather than a perfectly definite classical trajectory. Under suitable conditions,
• 1:31:43
  Speaker 1: there can be a non-zero probability of finding the object
• 1:31:46
  Speaker 1: on the other side of a barrier, even when it
• 1:31:48
  Speaker 1: did not possess enough classical energy to cross over the top.
• 1:31:52
  Speaker 1: The object has effectively tunnelled through. This is not merely theoretical.
• 1:31:57
  Speaker 1: Quantum tunnelling is involved in radioactive decay, modern electronics, scanning
• 1:32:03
  Speaker 1: tunnelling microscopes, and even the nuclear reactions helping stars to shine.
• 1:32:09
  Speaker 1: Vilenkin and other physicists explored whether something conceptually related might
• 1:32:13
  Speaker 1: apply to the universe itself. Instead of a particle tunnelling
• 1:32:18
  Speaker 1: through an ordinary spatial barrier, a quantum state describing a
• 1:32:22
  Speaker 1: universe might make a transition into an expanding classical spacetime.
• 1:32:27
  Speaker 1: The analogy must not be pushed too literally. There is
• 1:32:30
  Speaker 1: no suggestion that a tiny universe was sitting on one
• 1:32:33
  Speaker 1: side of a physical wall before suddenly popping through it.
• 1:32:38
  Speaker 1: The barrier exists within the mathematical description of the possible
• 1:32:42
  Speaker 1: states of the universe. The deeper idea is that quantum
• 1:32:45
  Speaker 1: theory permits transitions that have no equivalent in classical physics.
• 1:32:50
  Speaker 1: If spacetime itself is quantum near the deepest beginning, perhaps
• 1:32:55
  Speaker 1: the appearance of an expanding universe does not require an
• 1:32:58
  Speaker 1: earlier classical universe behaving according to familiar rules. This leads
• 1:33:02
  Speaker 1: directly to the troublesome word nothing. When physicists discuss a
• 1:33:08
  Speaker 1: universe emerging from nothing in this context, nothing usually does
• 1:33:12
  Speaker 1: not mean absolute philosophical nothingness. Absolute nothingness would mean no space,
• 1:33:18
  Speaker 1: no time, no matter, no energy, no quantum state, no
• 1:33:23
  Speaker 1: physical laws, no mathematical structure, no possibilities waiting quietly in
• 1:33:29
  Speaker 1: the background. Physics has no established experimental theory describing such
• 1:33:34
  Speaker 1: absolute nothingness. In quantum cosmology, the starting point may instead
• 1:33:40
  Speaker 1: be a condition without ordinary classical space and time. Yet
• 1:33:44
  Speaker 1: the mathematical framework, quantum principles, boundary conditions, or some notion
• 1:33:50
  Speaker 1: of possible states may still be part of the theory.
• 1:33:54
  Speaker 1: So saying that the universe came from nothing can conceal
• 1:33:57
  Speaker 1: more than it explains. A more careful statement is that
• 1:34:01
  Speaker 1: certain models explore whether a classical expanding universe could emerge
• 1:34:05
  Speaker 1: from a quantum state that does not contain classical spacetime
• 1:34:09
  Speaker 1: in the familiar sense. That is already strange enough. Trying
• 1:34:14
  Speaker 1: to apply quantum mechanics to the entire universe creates another difficulty.
• 1:34:20
  Speaker 1: Ordinary quantum experiments usually involve a system sitting inside something larger.
• 1:34:25
  Speaker 1: A physicist might study an electron inside a laboratory. The
• 1:34:30
  Speaker 1: electron is the system. There is measuring equipment around it.
• 1:34:34
  Speaker 1: There is an external clock. There is an environment. And
• 1:34:38
  Speaker 1: there is an observer who is not included as part
• 1:34:41
  Speaker 1: of the tiny mathematical system being studied. With the entire universe,
• 1:34:46
  Speaker 1: this arrangement becomes awkward. There is no obvious laboratory outside
• 1:34:50
  Speaker 1: the cosmos. There is no external observer standing beyond space-time
• 1:34:55
  Speaker 1: with a clipboard. There is no larger physical environment into
• 1:34:58
  Speaker 1: which the universe can simply be placed. Whatever quantum description
• 1:35:03
  Speaker 1: we construct must, in principle, include everything. Attempts to do
• 1:35:07
  Speaker 1: this helped produce one of the central mathematical frameworks of
• 1:35:10
  Speaker 1: quantum cosmology. John Archibald Wheeler and Bryce DeWitt were closely
• 1:35:15
  Speaker 1: associated with an approach that tries to apply quantum principles
• 1:35:19
  Speaker 1: to the geometry of the universe itself. The resulting framework
• 1:35:23
  Speaker 1: is often connected with what is known as the Wheeler-DeWitt equation.
• 1:35:28
  Speaker 1: Rather than describing the quantum state of one particle, Moving
• 1:35:32
  Speaker 1: through an already existing spacetime, the idea is to describe
• 1:35:36
  Speaker 1: a quantum state that includes the geometry of space along
• 1:35:39
  Speaker 1: with the matter it contains. This quantum state is sometimes
• 1:35:43
  Speaker 1: informally called the wave function of the universe. The phrase
• 1:35:47
  Speaker 1: sounds grand, but the central problem is quite precise. If
• 1:35:51
  Speaker 1: general relativity says that spacetime geometry is dynamical, and quantum
• 1:35:56
  Speaker 1: theory applies to fundamental physical systems, then perhaps geometry itself
• 1:36:01
  Speaker 1: should be included in the quantum description. Doing that produces
• 1:36:05
  Speaker 1: a particularly strange difficulty. Time seems to disappear. In ordinary
• 1:36:11
  Speaker 1: quantum mechanics, time plays a clear role. A quantum state
• 1:36:15
  Speaker 1: evolves as time passes. There is a clock in the
• 1:36:18
  Speaker 1: background against which that evolution is described. But general relativity
• 1:36:23
  Speaker 1: does not treat time as a fixed external background. Time
• 1:36:27
  Speaker 1: is part of space-time geometry. If the geometry itself is
• 1:36:31
  Speaker 1: being quantized, then using an independent external time variable becomes problematic.
• 1:36:36
  Speaker 1: In important versions of the Wheeler-DeWitt framework, the fundamental equation
• 1:36:41
  Speaker 1: does not contain time in the ordinary form expected from
• 1:36:44
  Speaker 1: familiar quantum mechanics. This is part of what physicists call
• 1:36:49
  Speaker 1: the problem of time in quantum gravity. At first glance,
• 1:36:53
  Speaker 1: that sounds disastrous. We are trying to describe the origin
• 1:36:56
  Speaker 1: and evolution of the universe. only to discover that time
• 1:37:00
  Speaker 1: has apparently wandered out of the equation. But perhaps that
• 1:37:03
  Speaker 1: is telling us something important. It may be that time
• 1:37:07
  Speaker 1: is not fundamental in the way everyday experience suggests. Perhaps
• 1:37:11
  Speaker 1: the deepest quantum description of reality contains relationships between physical
• 1:37:15
  Speaker 1: quantities without an external, universal clock. Under appropriate conditions, when
• 1:37:21
  Speaker 1: the universe becomes large and behaves approximately classically, a familiar
• 1:37:25
  Speaker 1: notion of time may emerge. One part of the universe
• 1:37:29
  Speaker 1: can effectively serve as a clock relative to another. Sequences
• 1:37:33
  Speaker 1: of events become meaningful. Space-time takes on its classical form.
• 1:37:38
  Speaker 1: The universe then appears to evolve through time in the
• 1:37:40
  Speaker 1: way we recognise. This possibility would profoundly alter the meaning
• 1:37:45
  Speaker 1: of a cosmic beginning. Instead of imagining a first moment
• 1:37:49
  Speaker 1: occurring at the start of a pre-existing timeline, the timeline
• 1:37:53
  Speaker 1: itself might arise from a deeper quantum state. There would
• 1:37:56
  Speaker 1: be no ordinary earlier second because seconds would not yet
• 1:37:59
  Speaker 1: be part of the fundamental description. Quantum creation proposals remain
• 1:38:04
  Speaker 1: attempts to solve an unfinished problem. There's no observation showing
• 1:38:08
  Speaker 1: that our universe definitely tunnelled into existence. There's no experiment
• 1:38:12
  Speaker 1: confirming Alexander Vilenkin's particular boundary proposal as the origin of
• 1:38:16
  Speaker 1: our cosmos. And physicists still disagree about the correct formulation
• 1:38:21
  Speaker 1: of quantum gravity and quantum cosmology. These models are serious
• 1:38:25
  Speaker 1: pieces of theoretical physics, but they are not eyewitness accounts
• 1:38:29
  Speaker 1: of creation. What they demonstrate is that once quantum mechanics
• 1:38:33
  Speaker 1: is applied to the universe itself, the range of possible
• 1:38:37
  Speaker 1: beginnings becomes much wider than classical intuition suggests. A universe
• 1:38:42
  Speaker 1: may not need to emerge from an earlier collection of stars, particles,
• 1:38:46
  Speaker 1: or even ordinary spacetime. It may emerge from a quantum
• 1:38:49
  Speaker 1: description in which those familiar ingredients have not yet acquired
• 1:38:53
  Speaker 1: their classical meaning. And if time itself is among the
• 1:38:56
  Speaker 1: things that emerge, then the deepest beginning may not have
• 1:39:00
  Speaker 1: occurred at some unimaginably early moment. The very idea of
• 1:39:04
  Speaker 1: a moment may have appeared with the universe. What if
• 1:39:07
  Speaker 1: the Big Bang was not the beginning at all? What if,
• 1:39:11
  Speaker 1: instead of emerging from an absolute first moment, our expanding
• 1:39:15
  Speaker 1: universe was preceded by something else entirely? Perhaps another universe,
• 1:39:21
  Speaker 1: not another universe somewhere else, but an earlier phase of
• 1:39:24
  Speaker 1: this one. This is the basic idea behind a family
• 1:39:28
  Speaker 1: of models known as the Big Bounce. In the simplest picture,
• 1:39:32
  Speaker 1: the universe does not begin from a classical singularity. Instead,
• 1:39:36
  Speaker 1: there is an earlier period in which the cosmos is contracting. Galaxies,
• 1:39:42
  Speaker 1: or whatever structures existed in that phase, move closer together.
• 1:39:46
  Speaker 1: The density rises. temperatures increase, the contraction continues towards conditions
• 1:39:53
  Speaker 1: of extraordinary compression. But rather than collapsing into an infinitely
• 1:39:58
  Speaker 1: dense singularity, where the equations cease to make sense, some
• 1:40:03
  Speaker 1: new physical effect intervenes. The contraction stops. The universe reaches
• 1:40:08
  Speaker 1: a minimum scale, and then expansion begins. The bounce replaces
• 1:40:13
  Speaker 1: a beginning. It turns the Big Bang from the first
• 1:40:17
  Speaker 1: instant of everything into a transition between two very different
• 1:40:21
  Speaker 1: cosmic phases. There is no single agreed Big Bounce theory.
• 1:40:26
  Speaker 1: That is important. The phrase describes a broad family of ideas,
• 1:40:31
  Speaker 1: and different models propose very different reasons why a collapsing
• 1:40:35
  Speaker 1: universe might avoid a singularity and begin expanding again. Some
• 1:40:40
  Speaker 1: introduce unusual forms of matter or energy. Others modify gravity
• 1:40:44
  Speaker 1: at extreme density. And some rely on quantum effects in
• 1:40:48
  Speaker 1: spacetime itself. One of the most studied approaches comes from
• 1:40:53
  Speaker 1: loop quantum gravity. Loop quantum gravity is an attempt to
• 1:40:57
  Speaker 1: construct a quantum theory of gravity without assuming that spacetime
• 1:41:01
  Speaker 1: remains perfectly smooth at every scale. Researchers including Abhay Ashtakar
• 1:41:07
  Speaker 1: helped develop the underlying theory, while Martin Bodewald and others
• 1:41:10
  Speaker 1: explored its implications for cosmology. When these ideas are applied
• 1:41:15
  Speaker 1: to highly simplified models of the universe, the result is
• 1:41:19
  Speaker 1: known as loop quantum cosmology. The central idea is that
• 1:41:23
  Speaker 1: geometry itself may possess quantum properties. In ordinary general relativity,
• 1:41:29
  Speaker 1: space can be treated as a smooth continuum. Distances can
• 1:41:33
  Speaker 1: be divided again and again without obvious limit. Loop quantum
• 1:41:37
  Speaker 1: gravity proposes a very different picture. At extremely small scales,
• 1:41:42
  Speaker 1: geometric quantities such as area and volume can take discrete
• 1:41:46
  Speaker 1: quantum values. In a loose sense, spacetime may have something
• 1:41:50
  Speaker 1: resembling an atomic structure of its own, although this should
• 1:41:54
  Speaker 1: not be pictured as space being made from tiny conventional particles.
• 1:41:58
  Speaker 1: The geometry itself is being quantised. Under ordinary conditions, these
• 1:42:03
  Speaker 1: quantum properties would be unimaginably small. and general relativity would
• 1:42:08
  Speaker 1: emerge as an excellent approximation. But close to the enormous
• 1:42:13
  Speaker 1: densities associated with the earliest universe, the corrections can become important.
• 1:42:19
  Speaker 1: In some models of loop quantum cosmology, they change the
• 1:42:23
  Speaker 1: behaviour of gravity dramatically. Classically, gravity tends to drive contraction
• 1:42:28
  Speaker 1: harder as matter becomes denser. The smaller the universe becomes,
• 1:42:33
  Speaker 1: the more severe the gravitational collapse appears. General relativity eventually
• 1:42:38
  Speaker 1: leads towards the singularity, but the quantum-corrected equations used in
• 1:42:42
  Speaker 1: certain loop quantum cosmology models can produce an effective repulsive
• 1:42:47
  Speaker 1: behaviour at extremely high density. The contraction slows, the density
• 1:42:53
  Speaker 1: reaches a maximum finite value rather than becoming infinite, and
• 1:42:58
  Speaker 1: instead of space-time ending, the universe begins to expand. A
• 1:43:03
  Speaker 1: quantum bounce has occurred. This is one of the reasons
• 1:43:06
  Speaker 1: the Big Bounce is so appealing. It gives a concrete
• 1:43:09
  Speaker 1: example of how quantum gravity might resolve the classical Big
• 1:43:13
  Speaker 1: Bang singularity. The infinity is removed. Space-time continues. There is
• 1:43:19
  Speaker 1: a before. But there is a crucial caution. A bounce
• 1:43:23
  Speaker 1: appearing in a mathematical model does not mean that our
• 1:43:26
  Speaker 1: universe definitely bounced. The versions of loop quantum cosmology that
• 1:43:31
  Speaker 1: are easiest to calculate usually rely on strong simplifications. The
• 1:43:36
  Speaker 1: universe may be treated as extremely homogeneous and symmetrical. Complicated
• 1:43:41
  Speaker 1: quantum fields and irregular structures may be reduced to more
• 1:43:45
  Speaker 1: manageable forms. These models are extremely useful for exploring possibilities.
• 1:43:50
  Speaker 1: They do not amount to an observation of a previous universe.
• 1:43:54
  Speaker 1: and loop quantum gravity itself has not yet been experimentally
• 1:43:58
  Speaker 1: confirmed as the correct theory of quantum gravity. So the
• 1:44:02
  Speaker 1: bounce remains a serious theoretical possibility, rather than an established
• 1:44:06
  Speaker 1: piece of cosmic history. Still, it changes the question of
• 1:44:10
  Speaker 1: the beginning in a fascinating way. If there really was
• 1:44:13
  Speaker 1: a contracting phase, then time did not begin with our
• 1:44:16
  Speaker 1: expanding universe. There were earlier physical events, a clock, at
• 1:44:21
  Speaker 1: least in some meaningful sense, keep running backwards through the bounce.
• 1:44:26
  Speaker 1: The Hot Big Bang would mark a transition from contraction
• 1:44:28
  Speaker 1: to expansion rather than the creation of spacetime from nothing.
• 1:44:34
  Speaker 1: That immediately raises another question. What was the universe like
• 1:44:37
  Speaker 1: before the bounce? One possibility is that the contracting phase was,
• 1:44:42
  Speaker 1: in some ways, a reversed version of an expanding universe.
• 1:44:46
  Speaker 1: Perhaps it once contained enormous structures spread across vast distances
• 1:44:52
  Speaker 1: As contraction proceeded, those structures might have been compressed and disrupted.
• 1:44:56
  Speaker 1: Radiation would become increasingly energetic. Matter would heat. Density fluctuations
• 1:45:02
  Speaker 1: could grow. But it is not as simple as taking
• 1:45:05
  Speaker 1: a film of our present universe and playing it backwards.
• 1:45:09
  Speaker 1: A contracting universe has its own dynamics. Small irregularities can
• 1:45:14
  Speaker 1: behave very differently during contraction, and some models struggle with
• 1:45:18
  Speaker 1: the possibility that anisotropies– meaning differences in expansion or contraction
• 1:45:23
  Speaker 1: along different directions, could grow large enough to dominate the
• 1:45:26
  Speaker 1: approach to the bounce. Any realistic model has to explain
• 1:45:30
  Speaker 1: how the universe could pass through this extreme state and
• 1:45:34
  Speaker 1: emerge looking as smooth and uniform as the cosmos we observe.
• 1:45:39
  Speaker 1: Then there is the question of memory. Could anything from
• 1:45:42
  Speaker 1: the previous phase survive? Not a planet or a galaxy
• 1:45:45
  Speaker 1: in any recognisable form. Conditions near the bounce would be
• 1:45:49
  Speaker 1: far too extreme for structures like those to remain intact.
• 1:45:54
  Speaker 1: The more interesting possibility is whether information could survive in
• 1:45:57
  Speaker 1: subtler ways. Perhaps patterns in primordial density fluctuations could carry
• 1:46:02
  Speaker 1: traces of the contracting phase. Perhaps a background of gravitational
• 1:46:06
  Speaker 1: waves could preserve signatures of events before the bounce. Perhaps
• 1:46:11
  Speaker 1: the statistical structure of the cosmic microwave background could contain
• 1:46:15
  Speaker 1: indirect evidence. These possibilities are actively studied in bouncing cosmologies.
• 1:46:21
  Speaker 1: Different models predict different signatures. Some produce characteristic patterns in
• 1:46:26
  Speaker 1: primordial fluctuations. Others alter the expected spectrum of gravitational waves.
• 1:46:32
  Speaker 1: The difficulty is that many of these predicted signals can
• 1:46:35
  Speaker 1: resemble effects produced by inflation or other early universe scenarios.
• 1:46:41
  Speaker 1: Even if an unusual feature were discovered, demonstrating that it
• 1:46:44
  Speaker 1: came specifically from a pre-bounce phase would be extremely challenging.
• 1:46:49
  Speaker 1: There is also a deeper conceptual problem. If information survives
• 1:46:53
  Speaker 1: the bounce, how much survives? Could physical conditions in the
• 1:46:57
  Speaker 1: contracting universe determine the state of the expanding one? Or
• 1:47:01
  Speaker 1: does the extreme quantum regime erase most of the details,
• 1:47:05
  Speaker 1: leaving only broad properties behind? The answer depends on the theory.
• 1:47:10
  Speaker 1: And until quantum gravity is better understood, the question remains open.
• 1:47:14
  Speaker 1: The Big Bounce therefore offers one of the clearest ways
• 1:47:17
  Speaker 1: to imagine a universe before our universe. It does not
• 1:47:20
  Speaker 1: require an external space. It does not need another cosmos
• 1:47:24
  Speaker 1: sitting beside ours. It simply extends the history of spacetime
• 1:47:28
  Speaker 1: backwards beyond the hot Big Bang. Contraction comes first. Density rises.
• 1:47:35
  Speaker 1: Quantum gravity becomes important. A bounce occurs. expansion follows. If
• 1:47:41
  Speaker 1: that picture is correct, then the Big Bang was not
• 1:47:44
  Speaker 1: the beginning of everything. It was the moment when the
• 1:47:47
  Speaker 1: direction of cosmic evolution changed. The universe did not appear
• 1:47:51
  Speaker 1: from an absolute first instant. It passed through one of
• 1:47:55
  Speaker 1: the most extreme transitions nature could possibly allow. And somewhere
• 1:48:00
  Speaker 1: on the other side of that transition may lie an
• 1:48:02
  Speaker 1: entire era of cosmic history that we have never seen.
• 1:48:06
  Speaker 1: The Big Bounce opens the door to an even more
• 1:48:09
  Speaker 1: extraordinary possibility. Perhaps the universe has not bounced only once.
• 1:48:14
  Speaker 1: Perhaps cosmic history is made of repeated eras, with expansion
• 1:48:18
  Speaker 1: and contraction forming part of a much larger pattern. This
• 1:48:23
  Speaker 1: broad family of ideas is known as cyclic cosmology. Instead
• 1:48:27
  Speaker 1: of imagining a universe with one unique beginning followed by
• 1:48:31
  Speaker 1: a single unbroken future, cyclic models ask whether something resembling
• 1:48:36
  Speaker 1: the Big Bang could happen again and again. The idea
• 1:48:39
  Speaker 1: is much older than modern quantum cosmology. Soon after physicists
• 1:48:44
  Speaker 1: began applying general relativity to the universe as a whole,
• 1:48:48
  Speaker 1: some models suggested that cosmic expansion might eventually slow down,
• 1:48:52
  Speaker 1: stop and reverse. Gravity would gradually pull everything back together.
• 1:48:58
  Speaker 1: Galaxies would approach one another. Density would rise. The universe
• 1:49:03
  Speaker 1: would contract towards an extremely hot state. Then, somehow, A
• 1:49:07
  Speaker 1: new expansion might begin. The universe would grow again, produce
• 1:49:12
  Speaker 1: new structures, and perhaps eventually repeat the entire process. These
• 1:49:17
  Speaker 1: early ideas became known as oscillating universe models. They were
• 1:49:21
  Speaker 1: appealing because they seemed to avoid an absolute beginning. Instead
• 1:49:26
  Speaker 1: of asking what happened before the Big Bang, one could
• 1:49:28
  Speaker 1: simply point to the previous cosmic cycle, and before that, another,
• 1:49:33
  Speaker 1: and perhaps another before that. but the simple picture quickly
• 1:49:38
  Speaker 1: encountered serious difficulties. One problem is that observations show the
• 1:49:43
  Speaker 1: expansion of our universe is accelerating rather than slowing towards
• 1:49:47
  Speaker 1: an obvious future collapse. Dark energy, or whatever physical mechanism
• 1:49:52
  Speaker 1: is responsible for this acceleration, makes a straightforward return to
• 1:49:56
  Speaker 1: contraction much harder to arrange. There is also the problem
• 1:50:00
  Speaker 1: of what happens at maximum compression. Classical general relativity tends
• 1:50:05
  Speaker 1: to produce a singularity rather than a gentle turnaround. A
• 1:50:10
  Speaker 1: successful cyclic universe therefore needs some mechanism capable of carrying
• 1:50:15
  Speaker 1: the cosmos safely from contraction back into expansion, and then
• 1:50:19
  Speaker 1: there is entropy. Entropy is often described as a measure
• 1:50:22
  Speaker 1: of disorder, although more precisely it measures how many microscopic
• 1:50:26
  Speaker 1: arrangements correspond to the large-scale state of a system. The
• 1:50:31
  Speaker 1: second law of thermodynamics tells us that In an isolated system,
• 1:50:35
  Speaker 1: total entropy tends to increase. Stars burn fuel, black holes form,
• 1:50:41
  Speaker 1: radiation spreads, useful concentrations of energy become less available. The
• 1:50:47
  Speaker 1: universe gradually moves towards states with greater entropy. That creates
• 1:50:51
  Speaker 1: an awkward question for a simple eternal cycle. If entropy
• 1:50:55
  Speaker 1: grows during each cosmic era, then every new cycle should
• 1:50:59
  Speaker 1: begin with more entropy than the one before it. These
• 1:51:02
  Speaker 1: cycles would not simply repeat identically. They could become progressively larger, longer,
• 1:51:08
  Speaker 1: or otherwise different. Tracing the sequence backwards might then still
• 1:51:12
  Speaker 1: lead towards a special beginning, rather than an infinite chain
• 1:51:15
  Speaker 1: of identical universes. Modern cyclic cosmology tries to build something
• 1:51:20
  Speaker 1: more sophisticated. Two physicists strongly associated with these newer approaches
• 1:51:25
  Speaker 1: are Paul Steinhardt and Neil Turok. Together with other researchers,
• 1:51:30
  Speaker 1: they developed cyclic models related to an idea known as
• 1:51:33
  Speaker 1: the ekpyrotic scenario. The word ekpyrotic comes from an ancient
• 1:51:37
  Speaker 1: Greek term associated with cosmic destruction by fire, although the
• 1:51:42
  Speaker 1: modern theory has very little to do with the ancient
• 1:51:44
  Speaker 1: philosophy beyond borrowing the name. The central physical idea is
• 1:51:48
  Speaker 1: that the universe can undergo a long period of slow
• 1:51:50
  Speaker 1: contraction before entering the hot expanding state. This contraction is
• 1:51:56
  Speaker 1: not merely the reverse of ordinary cosmic expansion. Under the
• 1:52:00
  Speaker 1: right conditions, an ekperotic phase can smooth the universe. Irregularities
• 1:52:05
  Speaker 1: can be suppressed. Spatial curvature can become less important. The
• 1:52:09
  Speaker 1: result can be a cosmos that emerges from the transition
• 1:52:12
  Speaker 1: extremely uniform and close to geometrically flat. These are some
• 1:52:17
  Speaker 1: of the same properties that cosmic inflation was designed to explain.
• 1:52:21
  Speaker 1: Inflation achieves them through extremely rapid expansions. Ekpirotic cosmology attempts
• 1:52:27
  Speaker 1: to achieve them through a particular form of slow contraction.
• 1:52:31
  Speaker 1: The two pictures therefore approach the early universe from almost
• 1:52:34
  Speaker 1: opposite directions. Some versions of the ekpirotic and cyclic models
• 1:52:39
  Speaker 1: were inspired by string theory. String theory proposes that the
• 1:52:43
  Speaker 1: fundamental ingredients of nature may be tiny extended objects rather
• 1:52:47
  Speaker 1: than perfectly point-like particles. Certain forms of the theory also
• 1:52:52
  Speaker 1: involve additional spatial dimensions beyond the three we experience directly.
• 1:52:57
  Speaker 1: Within these frameworks, physicists introduced objects called brains, which can
• 1:53:02
  Speaker 1: be thought of as higher dimensional structures existing within a
• 1:53:05
  Speaker 1: larger dimensional setting. In some early versions of the echoprotic scenario,
• 1:53:10
  Speaker 1: our observable universe was associated with one such brain. A
• 1:53:14
  Speaker 1: collision between brains could generate conditions resembling the hot Big Bang.
• 1:53:19
  Speaker 1: Later cyclic versions developed this into a repeated process in
• 1:53:23
  Speaker 1: which brains approach one another, interact, separate and eventually return.
• 1:53:29
  Speaker 1: It is a wonderfully ambitious idea. It is also highly theoretical.
• 1:53:34
  Speaker 1: String theory has not been experimentally established as the correct
• 1:53:37
  Speaker 1: description of nature, and additional dimensions or cosmological brains have
• 1:53:42
  Speaker 1: not been directly observed. So a brain collision should not
• 1:53:46
  Speaker 1: be treated as something physicists know occurred before our Big Bang.
• 1:53:51
  Speaker 1: It is one proposed mechanism within a much larger theoretical framework.
• 1:53:56
  Speaker 1: Modern cyclic models also have to confront the entropy problem
• 1:54:00
  Speaker 1: that troubled earlier oscillating universes. One proposed way around it
• 1:54:04
  Speaker 1: is to make each cycle enormously large and allow the
• 1:54:08
  Speaker 1: accelerated expansion between cycles to dilute matter, radiation, and entropy
• 1:54:14
  Speaker 1: density across an immense volume. The next contracting phase can
• 1:54:18
  Speaker 1: then begin in a region that is extremely smooth and sparse,
• 1:54:22
  Speaker 1: even though total entropy has not simply vanished. Different cyclic
• 1:54:27
  Speaker 1: theories handle this issue differently, and the question remains an
• 1:54:31
  Speaker 1: important area of debate. There are other technical challenges as well.
• 1:54:35
  Speaker 1: A viable model has to explain how a contracting universe
• 1:54:40
  Speaker 1: avoids dangerous instabilities It must provide a physically consistent transition
• 1:54:46
  Speaker 1: into expansion. It must produce the particular spectrum of primordial
• 1:54:50
  Speaker 1: density variations that observations reveal. And ideally, it should predict
• 1:54:55
  Speaker 1: something distinctive enough that astronomers can tell it apart from inflation.
• 1:55:00
  Speaker 1: That last requirement is particularly important. A theory of the
• 1:55:03
  Speaker 1: early universe becomes much more useful if it can produce
• 1:55:07
  Speaker 1: observational signatures that another theory does not. Researchers therefore study
• 1:55:12
  Speaker 1: possible differences in primordial gravitational waves, density fluctuations, and other
• 1:55:18
  Speaker 1: features that could preserve information from an earlier contracting phase.
• 1:55:22
  Speaker 1: So far, no observation has established that our universe passed
• 1:55:26
  Speaker 1: through an echoperotic contraction, or that cosmic history is genuinely cyclic.
• 1:55:32
  Speaker 1: But the idea reveals how dramatically our interpretation of the
• 1:55:35
  Speaker 1: Big Bang can change. In the conventional picture, The Big
• 1:55:39
  Speaker 1: Bang seems to sit near the beginning of the cosmic timeline.
• 1:55:42
  Speaker 1: In a cyclic universe, it becomes something more like a doorway.
• 1:55:47
  Speaker 1: There may have been an entire cosmic era on the
• 1:55:49
  Speaker 1: other side, and that era may itself have emerged from
• 1:55:53
  Speaker 1: another one before it. The history of everything would then
• 1:55:56
  Speaker 1: extend far beyond the 13.8 billion years of our current
• 1:56:01
  Speaker 1: expanding phase. Our Big Bang would not be the unique
• 1:56:04
  Speaker 1: moment when existence began. It would be one transition in
• 1:56:08
  Speaker 1: a sequence potentially stretching through unimaginable depths of time. Perhaps
• 1:56:13
  Speaker 1: the universe has a beginning. Perhaps it bounced once. Or
• 1:56:17
  Speaker 1: perhaps what we call the beginning has happened in some
• 1:56:19
  Speaker 1: form many times before. There is another way to imagine
• 1:56:23
  Speaker 1: a universe with no obvious first beginning. And it does
• 1:56:26
  Speaker 1: not require everything to collapse back together. In this picture,
• 1:56:30
  Speaker 1: the universe simply keeps expanding. Galaxies drift farther apart. Stars
• 1:56:36
  Speaker 1: burn out, black holes eventually dominate what remains, and over
• 1:56:41
  Speaker 1: unimaginably long stretches of time, the cosmos becomes colder, darker
• 1:56:47
  Speaker 1: and emptier. Yet somehow, in the mathematics, that remote future
• 1:56:53
  Speaker 1: can be connected to another Big Bang. This is the
• 1:56:56
  Speaker 1: central idea behind conformal cyclic cosmology, a theory developed by
• 1:57:01
  Speaker 1: the British mathematical physicist Roger Penrose. It is very different
• 1:57:06
  Speaker 1: from the traditional image of a cyclic universe. There is
• 1:57:10
  Speaker 1: no enormous reversal in which expansion stops and everything falls
• 1:57:14
  Speaker 1: back together. There is no cosmic contraction followed by a bounce. Instead,
• 1:57:20
  Speaker 1: one universe expands into an almost incomprehensibly distant future, and
• 1:57:25
  Speaker 1: that future is mathematically related to the beginning of another
• 1:57:28
  Speaker 1: cosmic era. Penrose calls each complete era an aeon. Our
• 1:57:34
  Speaker 1: observable universe would be one aeon. Before ours, there may
• 1:57:39
  Speaker 1: have been another. Before that, another still. And after the
• 1:57:44
  Speaker 1: remote future of our universe, another aeon might begin. The
• 1:57:49
  Speaker 1: strange part is understanding how the end of one universe
• 1:57:53
  Speaker 1: could possibly resemble the beginning of another. The key idea
• 1:57:56
  Speaker 1: involves scale. The universe around us is full of things
• 1:58:00
  Speaker 1: that provide natural rulers and clocks. An electron has a mass.
• 1:58:05
  Speaker 1: A proton has a mass. A planet has a size.
• 1:58:08
  Speaker 1: A star has a lifetime. Because massive particles exist, there
• 1:58:13
  Speaker 1: are physical ways to distinguish one distance from another and
• 1:58:16
  Speaker 1: one interval of time from another. A metre and a
• 1:58:19
  Speaker 1: billion kilometres are very different things because physical objects allow
• 1:58:23
  Speaker 1: those scales to be compared. But imagine the universe in
• 1:58:27
  Speaker 1: its extraordinarily distant future. Stars have long since stopped shining.
• 1:58:33
  Speaker 1: Galaxies have faded. Black holes have eventually evaporated. Matter has
• 1:58:38
  Speaker 1: become enormously diluted. In the idealised version of conformal cyclic cosmology,
• 1:58:45
  Speaker 1: what remains is effectively dominated by massless radiation and other
• 1:58:49
  Speaker 1: massless degrees of freedom. Massless particles behave differently from massive ones.
• 1:58:55
  Speaker 1: A photon, for example, has no rest frame. There is
• 1:58:59
  Speaker 1: no perspective in which a photon can simply sit still
• 1:59:02
  Speaker 1: while a clock ticks beside it. And the laws governing
• 1:59:05
  Speaker 1: massless particles possess a special mathematical property connected with scale.
• 1:59:10
  Speaker 1: If a universe contains only massless physics, absolute distances and
• 1:59:15
  Speaker 1: times can become much less physically meaningful. The geometry can
• 1:59:19
  Speaker 1: be rescaled without changing certain essential relationships. This is where
• 1:59:24
  Speaker 1: the word conformal becomes important. A conformal transformation changes the
• 1:59:29
  Speaker 1: scale of a geometry while preserving local angles and parts
• 1:59:32
  Speaker 1: of its causal structure. It can make something enormous appear
• 1:59:36
  Speaker 1: mathematically compact without simply claiming that the physical universe has
• 1:59:40
  Speaker 1: literally shrunk. Penrose uses this kind of mathematical transformation to
• 1:59:45
  Speaker 1: relate the unimaginably expanded future of one aeon to the
• 1:59:49
  Speaker 1: extremely compressed beginning of another. In ordinary physical terms, the
• 1:59:54
  Speaker 1: two states sound completely opposite. One is huge, cold, and
• 1:59:59
  Speaker 1: nearly empty. The other is associated with the extraordinarily hot
• 2:00:03
  Speaker 1: beginning of a new universe. But if absolute scale has
• 2:00:07
  Speaker 1: lost its physical significance, Penrose argues that the distinction between
• 2:00:11
  Speaker 1: infinitely large and extremely small can become less meaningful in
• 2:00:15
  Speaker 1: the conformal description. The remote future can then be mathematically
• 2:00:20
  Speaker 1: joined to a new beginning. There is no contraction in between.
• 2:00:24
  Speaker 1: The old universe does not collapse. Its infinite expansion is,
• 2:00:28
  Speaker 1: in a sense, reinterpreted through geometry. The end of one
• 2:00:33
  Speaker 1: aeon becomes the beginning of the next. This makes conformal,
• 2:00:37
  Speaker 1: cyclic cosmology very different from a big bounce. In a
• 2:00:41
  Speaker 1: bounce model, the scale of the universe decreases during a
• 2:00:44
  Speaker 1: real period of contraction. Density rises. Quantum gravity or another
• 2:00:49
  Speaker 1: mechanism eventually halts the collapse. Expansion then begins again. In
• 2:00:54
  Speaker 1: Penrose's picture, the universe continues expanding. There is no conventional turnaround.
• 2:01:00
  Speaker 1: The connection occurs because the far future and the next
• 2:01:03
  Speaker 1: Big Bang can be related through conformal geometry. Once scale
• 2:01:07
  Speaker 1: ceases to have the same physical meaning, it is an
• 2:01:11
  Speaker 1: elegant idea. It is also extremely ambitious. And if it
• 2:01:15
  Speaker 1: were true, there is an obvious question. Could anything from
• 2:01:19
  Speaker 1: a previous aeon leave a trace in ours? Penrose has
• 2:01:22
  Speaker 1: argued that the answer might be yes. One possibility involves
• 2:01:26
  Speaker 1: black holes. In an earlier aeon, enormous black holes could
• 2:01:31
  Speaker 1: have formed and eventually merged. Those mergers would release immense
• 2:01:35
  Speaker 1: amounts of energy in the form of gravitational radiation. In
• 2:01:39
  Speaker 1: conformal cyclic cosmology, Penrose and collaborators have suggested that certain
• 2:01:44
  Speaker 1: effects associated with events in a previous aeon could potentially
• 2:01:48
  Speaker 1: leave patterns in the cosmic microwave background of the next one.
• 2:01:53
  Speaker 1: Another proposed possibility involves what have been called Hawking points.
• 2:01:57
  Speaker 1: The idea is connected with the eventual evaporation of extremely
• 2:02:01
  Speaker 1: massive black holes in the previous aeon and the possibility
• 2:02:05
  Speaker 1: that their final energy could leave concentrated signatures after the
• 2:02:09
  Speaker 1: conformal transition. Researchers sympathetic to conformal cyclic cosmology have claimed
• 2:02:16
  Speaker 1: to find possible features of this kind in observations of
• 2:02:20
  Speaker 1: the cosmic microwave background. Those claims remain controversial. Other scientists
• 2:02:26
  Speaker 1: have argued that the apparent patterns are consistent with the
• 2:02:29
  Speaker 1: random fluctuations expected in standard cosmology. Statistical analyses have not
• 2:02:35
  Speaker 1: produced broad agreement that the cosmic microwave background contains evidence
• 2:02:39
  Speaker 1: of an earlier aeon. No accepted observation currently demonstrates that
• 2:02:44
  Speaker 1: another universe existed before ours in the way conformal cyclic
• 2:02:49
  Speaker 1: cosmology proposes. That distinction is essential. Conformal cyclic cosmology is
• 2:02:55
  Speaker 1: a mathematically developed hypothesis. It is not an established extension
• 2:02:59
  Speaker 1: of the standard cosmological model. Its assumptions about the remote future,
• 2:03:04
  Speaker 1: the role of mass, the treatment of quantum effects, and
• 2:03:07
  Speaker 1: the connection between successive aeons remain subjects of debate. Yet
• 2:03:12
  Speaker 1: even without observational confirmation, the idea reveals how strange the
• 2:03:16
  Speaker 1: question of a beginning can become. We usually imagine only
• 2:03:19
  Speaker 1: a few possibilities. Perhaps the universe began, Perhaps it bounced.
• 2:03:25
  Speaker 1: Perhaps it has always existed. Penrose offers something subtler. Perhaps
• 2:03:30
  Speaker 1: the universe can expand forever and still participate in a cycle.
• 2:03:36
  Speaker 1: Perhaps the infinite future of one cosmic age can, under
• 2:03:40
  Speaker 1: the right mathematical description, become indistinguishable in scale from the
• 2:03:44
  Speaker 1: beginning of another. There would be no first explosion followed
• 2:03:48
  Speaker 1: by everything else. There could instead be a succession of aeons,
• 2:03:52
  Speaker 1: each with its own stars, galaxies, black holes and long
• 2:03:56
  Speaker 1: decline into darkness. Our Big Bang would then not be
• 2:04:00
  Speaker 1: the absolute beginning. It would be the boundary between one
• 2:04:03
  Speaker 1: immense cosmic age and the next. Whether nature actually works
• 2:04:06
  Speaker 1: this way remains unknown, but conformal cyclic cosmology provides a
• 2:04:11
  Speaker 1: striking reminder that the phrase before the Big Bang does
• 2:04:15
  Speaker 1: not have only one possible meaning. There may have been
• 2:04:18
  Speaker 1: a contracting universe. there may have been a quantum state
• 2:04:21
  Speaker 1: without ordinary time. Or, in Roger Penrose's picture, there may
• 2:04:26
  Speaker 1: have been an entire previous aeon whose distant future became
• 2:04:30
  Speaker 1: our beginning. Inflation may solve several problems in the early universe,
• 2:04:34
  Speaker 1: but some versions of the idea lead to an even
• 2:04:37
  Speaker 1: stranger possibility. Inflation might not have ended everywhere. Instead, it
• 2:04:43
  Speaker 1: may continue in some regions while ending in others, producing
• 2:04:47
  Speaker 1: new regions of ordinary hot universe again and again, This
• 2:04:51
  Speaker 1: possibility is known as eternal inflation. Physicists, including Andrei Linder
• 2:04:56
  Speaker 1: and Alexander Vilenkin, helped develop versions of inflation in which
• 2:05:01
  Speaker 1: quantum effects make the process self-sustaining on sufficiently large scales.
• 2:05:07
  Speaker 1: The basic idea begins with the field thought to drive inflation.
• 2:05:11
  Speaker 1: During inflation, quantum fluctuations can cause that field to vary
• 2:05:16
  Speaker 1: slightly from place to place. In some regions, inflation ends,
• 2:05:21
  Speaker 1: The energy associated with the inflationary field is converted into
• 2:05:25
  Speaker 1: particles and radiation, creating a hot expanding region capable of
• 2:05:30
  Speaker 1: developing into stars, galaxies and perhaps eventually observers. Elsewhere, quantum
• 2:05:36
  Speaker 1: fluctuations can keep the field in a state that continues
• 2:05:40
  Speaker 1: driving inflation. Those regions keep expanding enormously. Because the inflating
• 2:05:45
  Speaker 1: volume can grow faster than regions leave the inflationary state,
• 2:05:49
  Speaker 1: Inflation may never end everywhere at once. Globally, the process continues. Locally,
• 2:05:55
  Speaker 1: it ends again and again. The resulting picture is sometimes
• 2:05:58
  Speaker 1: described using bubbles, a region where inflation stops can behave
• 2:06:02
  Speaker 1: rather like a bubble, forming within a larger inflating background.
• 2:06:07
  Speaker 1: More bubbles appear elsewhere. Some may possess physical conditions similar
• 2:06:11
  Speaker 1: to ours. Others, depending on the underlying theory, could potentially
• 2:06:15
  Speaker 1: evolve differently. but the image of ordinary soap bubbles floating
• 2:06:19
  Speaker 1: inside a familiar three-dimensional room should not be taken too literally.
• 2:06:24
  Speaker 1: The geometry involved is spacetime itself. These regions do not
• 2:06:29
  Speaker 1: necessarily sit as conventional spheres inside some larger pre-existing space
• 2:06:34
  Speaker 1: that could be viewed from outside. Eternal inflation naturally leads
• 2:06:38
  Speaker 1: into discussions of the multiverse. If inflation continually generates causally
• 2:06:43
  Speaker 1: separated regions, then our observable universe may be only one
• 2:06:47
  Speaker 1: region within a vastly larger structure. Different regions might have
• 2:06:51
  Speaker 1: different histories, and some theories even allow differences in low-energy
• 2:06:55
  Speaker 1: physical properties. This is a dramatic extension of cosmology. It
• 2:07:01
  Speaker 1: is also far harder to test directly than the ordinary
• 2:07:04
  Speaker 1: hot big bang model. We can observe evidence for an
• 2:07:07
  Speaker 1: early hot universe. We can measure the cosmic microwave background.
• 2:07:11
  Speaker 1: We can test primordial element abundances. but other inflationary regions,
• 2:07:16
  Speaker 1: if they exist, may lie permanently beyond our observable horizon.
• 2:07:20
  Speaker 1: That makes the broader multiverse interpretation much more speculative. Eternal
• 2:07:25
  Speaker 1: inflation creates another interesting asymmetry. It can continue indefinitely towards
• 2:07:30
  Speaker 1: the future. As time passes, new inflating volume can keep appearing,
• 2:07:36
  Speaker 1: with local regions continually exiting inflation while the overall process
• 2:07:40
  Speaker 1: carries on. That sounds as though it might provide a
• 2:07:44
  Speaker 1: universe with no beginning at all. But the mathematics is
• 2:07:48
  Speaker 1: not so simple. Arvind Borde, Alan Guth and Alexander Vilenkin
• 2:07:53
  Speaker 1: studied this question and developed what is now commonly called
• 2:07:57
  Speaker 1: the Borde-Guth-Vilenkin theorem. Their result applies to a broad class
• 2:08:02
  Speaker 1: of spacetimes that have, along certain past-directed trajectories, experienced sufficiently
• 2:08:08
  Speaker 1: positive average expansion. Under those conditions, those trajectories cannot be
• 2:08:13
  Speaker 1: extended arbitrarily far into the past. In technical language, the
• 2:08:18
  Speaker 1: spacetime is past and complete. This does not mean the
• 2:08:21
  Speaker 1: theorem proves that everything was created from nothing, it does
• 2:08:25
  Speaker 1: not tell us what existed at the boundary, it does
• 2:08:28
  Speaker 1: not identify a first event, and it does not provide
• 2:08:31
  Speaker 1: a complete theory of the beginning. What it shows is
• 2:08:34
  Speaker 1: that inflation itself cannot automatically be used to extend the
• 2:08:38
  Speaker 1: history of such a spacetime indefinitely backwards. Even if inflation
• 2:08:42
  Speaker 1: continues without end into the future, the same description may
• 2:08:46
  Speaker 1: still require something else in the past. that something could
• 2:08:50
  Speaker 1: be a quantum beginning. It could be a bouncing phase.
• 2:08:53
  Speaker 1: It could involve physics not yet understood. Or it could
• 2:08:56
  Speaker 1: require a completely different description of space-time. The theorem does
• 2:09:00
  Speaker 1: not choose between these possibilities. It simply warns us that
• 2:09:04
  Speaker 1: eternal inflation does not, by itself, remove the question of origins.
• 2:09:10
  Speaker 1: This creates a rather curious cosmic picture. Imagine an inflationary
• 2:09:14
  Speaker 1: process capable of continuing forever into the future. New regions
• 2:09:19
  Speaker 1: keep forming. Hot big bang phases occur again and again
• 2:09:23
  Speaker 1: in different places. Entire observable universes may emerge within a
• 2:09:28
  Speaker 1: much larger structure. And yet, when the mathematics is followed backwards,
• 2:09:34
  Speaker 1: the story may still refuse to extend forever using inflation alone.
• 2:09:38
  Speaker 1: A universe can therefore be future-eternal without being past-eternal. That
• 2:09:44
  Speaker 1: distinction matters. Infinity in one direction does not guarantee infinity
• 2:09:49
  Speaker 1: in the other. Eternal inflation may place our own hot
• 2:09:53
  Speaker 1: Big Bang inside an enormous and continuing cosmic process. It
• 2:09:57
  Speaker 1: may suggest that what we call our universe is only
• 2:10:00
  Speaker 1: one local region within something far larger. But it does
• 2:10:04
  Speaker 1: not necessarily answer the oldest question. Even after replacing a
• 2:10:08
  Speaker 1: single Big Bang with an immense background of continuing inflation,
• 2:10:12
  Speaker 1: we may still reach an edge where the theory stops
• 2:10:14
  Speaker 1: telling us what came before. The mystery has not disappeared.
• 2:10:18
  Speaker 1: It has simply been pushed farther away. After all the
• 2:10:22
  Speaker 1: possibilities surrounding the earliest universe, there is one question that
• 2:10:25
  Speaker 1: matters more than any elegant theory. Can we ever find
• 2:10:29
  Speaker 1: out which of them is actually true? Inflation, bouncing cosmology,
• 2:10:35
  Speaker 1: cyclic universes, quantum beginnings and other models can all be
• 2:10:40
  Speaker 1: described mathematically. But physics ultimately depends on observation. The difficulty
• 2:10:47
  Speaker 1: is that we cannot simply point a telescope at the
• 2:10:49
  Speaker 1: Big Bang. The oldest electromagnetic image available to us is
• 2:10:53
  Speaker 1: the Cosmic Microwave Background, released when the universe became transparent
• 2:10:58
  Speaker 1: roughly 380,000 years after the Hot Big Bang began. Anything
• 2:11:03
  Speaker 1: earlier is hidden behind an opaque plasma that ordinary light
• 2:11:07
  Speaker 1: could not travel through freely. Yet the Cosmic Microwave Background
• 2:11:11
  Speaker 1: may still preserve indirect traces of much earlier events, Its
• 2:11:15
  Speaker 1: temperature is extraordinarily uniform, but not perfectly so. Tiny variations
• 2:11:21
  Speaker 1: reveal primordial differences in density that later grew into galaxies
• 2:11:26
  Speaker 1: and clusters. The precise statistical pattern of those variations places
• 2:11:31
  Speaker 1: strong restrictions on theories of the early universe. Scientists also
• 2:11:36
  Speaker 1: study the polarisation of the cosmic microwave background, meaning the
• 2:11:40
  Speaker 1: preferred orientations carried by its electromagnetic waves. different physical processes
• 2:11:46
  Speaker 1: create different polarisation patterns. Measurements of those patterns can therefore
• 2:11:51
  Speaker 1: provide clues about what happened long before the radiation itself
• 2:11:54
  Speaker 1: was released. Observations also constrain the large-scale curvature of space.
• 2:12:00
  Speaker 1: Our observable universe appears extremely close to spatially flat, which
• 2:12:05
  Speaker 1: is consistent with many inflationary models. The detailed distribution of
• 2:12:10
  Speaker 1: primordial fluctuations provides further tests. A successful model cannot simply
• 2:12:15
  Speaker 1: produce a universe. It has to produce something remarkably similar
• 2:12:19
  Speaker 1: to ours. One especially exciting possibility involves primordial gravitational waves.
• 2:12:27
  Speaker 1: Gravitational waves are ripples in spacetime itself. Modern detectors have
• 2:12:32
  Speaker 1: observed gravitational waves created by violent events, such as collisions
• 2:12:37
  Speaker 1: between black holes and neutron stars. But extremely energetic processes
• 2:12:42
  Speaker 1: in the early universe, could have produced a much older
• 2:12:45
  Speaker 1: gravitational wave background. Inflation, for example, can generate primordial gravitational
• 2:12:52
  Speaker 1: waves in some models. These ancient ripples would not be
• 2:12:56
  Speaker 1: blocked by the hot plasma that prevents us from seeing
• 2:12:58
  Speaker 1: ordinary light from the first hundreds of thousands of years.
• 2:13:02
  Speaker 1: Their influence might instead appear indirectly in a distinctive swirling
• 2:13:06
  Speaker 1: pattern in the polarisation of the cosmic microwave background. Experiments
• 2:13:11
  Speaker 1: including the Background Imaging of Cosmic Extragalactic Polarisation Collaboration and
• 2:13:16
  Speaker 1: the Keck Array Collaboration have searched for this signal. The
• 2:13:20
  Speaker 1: task is extremely difficult because dust within the Milky Way
• 2:13:24
  Speaker 1: can produce polarised microwave emission that mimics some of the
• 2:13:28
  Speaker 1: desired patterns. So far there has been no confirmed detection
• 2:13:33
  Speaker 1: of a primordial gravitational wave signal that provides a direct
• 2:13:37
  Speaker 1: window onto the deepest beginnings. But gravitational waves are only
• 2:13:42
  Speaker 1: one possible messenger. Scientists also search for unusual patterns known
• 2:13:47
  Speaker 1: as non-Gaussianity in the primordial density fluctuations. Certain early universe
• 2:13:53
  Speaker 1: models predict deviations from the nearly Gaussian statistical pattern expected
• 2:13:58
  Speaker 1: in simpler scenarios. Other theories could produce cosmic defects, unusual
• 2:14:04
  Speaker 1: gravitational wave backgrounds, or distinctive correlations in the distribution of matter.
• 2:14:10
  Speaker 1: Bouncing cosmologies may leave different primordial signatures from conventional inflation.
• 2:14:16
  Speaker 1: Cyclic models may produce others. The challenge is finding a
• 2:14:20
  Speaker 1: prediction distinctive enough that nature cannot easily imitate it through
• 2:14:25
  Speaker 1: another mechanism. There may also be limits that no better
• 2:14:28
  Speaker 1: telescope can completely overcome. One is cosmic variance. We possess
• 2:14:34
  Speaker 1: only one observable universe. We cannot rerun cosmic history several
• 2:14:38
  Speaker 1: thousand times under slightly different conditions and compare the results.
• 2:14:43
  Speaker 1: At the largest scales, there are only a limited number
• 2:14:45
  Speaker 1: of independent regions available for measurement. That creates an unavoidable
• 2:14:50
  Speaker 1: statistical uncertainty. Our cosmic sample size is, rather inconveniently, one,
• 2:14:57
  Speaker 1: and the observable universe itself is finite. Regions beyond our
• 2:15:02
  Speaker 1: cosmic horizon may forever remain inaccessible, even if the entire
• 2:15:06
  Speaker 1: universe extends much farther. None of this means the earliest
• 2:15:10
  Speaker 1: universe is beyond science. Better observations can rule theories out.
• 2:15:16
  Speaker 1: Improved measurements of cosmic microwave background polarisation can restrict inflationary models.
• 2:15:23
  Speaker 1: Future gravitational wave observations may reveal backgrounds produced at energies
• 2:15:27
  Speaker 1: far beyond those achievable in laboratories. Measurements of large-scale structure
• 2:15:32
  Speaker 1: can test the primordial patterns which galaxies eventually formed. Science
• 2:15:38
  Speaker 1: often advances not by immediately identifying the correct answer, but
• 2:15:43
  Speaker 1: by steadily eliminating answers that nature refuses to support. That
• 2:15:48
  Speaker 1: leaves us with several very different levels of certainty. The hot,
• 2:15:52
  Speaker 1: dense early universe is supported by overwhelming observational evidence. Cosmic expansion,
• 2:15:58
  Speaker 1: primordial element abundances and the cosmic microwave background establish their
• 2:16:04
  Speaker 1: history extremely well. Inflation has substantial indirect support and explains
• 2:16:09
  Speaker 1: several observed features elegantly, but its exact mechanism remains unknown.
• 2:16:15
  Speaker 1: Bouncing universes, cyclic cosmologies, conformal cyclic cosmology, and quantum creation
• 2:16:21
  Speaker 1: proposals are mathematically serious possibilities, but none has been observationally
• 2:16:26
  Speaker 1: established as the history of our universe. and beyond them
• 2:16:30
  Speaker 1: lies the region where our current theories themselves become incomplete.
• 2:16:34
  Speaker 1: Perhaps a successful theory of quantum gravity will eventually connect
• 2:16:38
  Speaker 1: these clues into a single description. It may reveal that
• 2:16:42
  Speaker 1: spacetime emerged from a quantum state. It may show that
• 2:16:46
  Speaker 1: an earlier universe existed. It may replace the classical singularity
• 2:16:51
  Speaker 1: with a bounce. Or it may reveal something that none
• 2:16:54
  Speaker 1: of our current models has anticipated. There is even the
• 2:16:57
  Speaker 1: possibility that nature will force us to abandon the idea
• 2:17:01
  Speaker 1: of one absolute first moment altogether. The beginning of everything
• 2:17:05
  Speaker 1: may eventually become something science can describe, but discovering it
• 2:17:09
  Speaker 1: may require more than looking farther away. It may require
• 2:17:13
  Speaker 1: learning what space and time themselves really are. So, before
• 2:17:17
  Speaker 1: the Big Bang, what was there? After travelling as far
• 2:17:20
  Speaker 1: backwards as modern cosmology allows, the most accurate answer is
• 2:17:25
  Speaker 1: still wonderfully simple. We do not yet know. What science
• 2:17:28
  Speaker 1: can do is trace the observable universe backwards with extraordinary
• 2:17:32
  Speaker 1: confidence through billions of years of cosmic history, into an
• 2:17:36
  Speaker 1: era before stars, before galaxies, before atoms, and eventually into
• 2:17:40
  Speaker 1: conditions so hot and dense that the familiar structures of
• 2:17:44
  Speaker 1: matter could not exist. The evidence for this hot early
• 2:17:47
  Speaker 1: universe is exceptionally strong. Distant galaxies reveal that cosmic space
• 2:17:53
  Speaker 1: is expanding. The abundances of hydrogen, helium and other light
• 2:17:58
  Speaker 1: elements match the nuclear reactions expected during the first few minutes,
• 2:18:03
  Speaker 1: and the cosmic microwave background still surrounds us as the
• 2:18:07
  Speaker 1: cooled radiation left behind when the universe became transparent, roughly 380,000
• 2:18:15
  Speaker 1: years after the Hot Big Bang began. These observations leave
• 2:18:19
  Speaker 1: little doubt that the universe was once far hotter, denser
• 2:18:24
  Speaker 1: and more uniform than it is today. But that does
• 2:18:28
  Speaker 1: not automatically mean we have discovered the absolute beginning of everything.
• 2:18:33
  Speaker 1: The Hot Big Bang describes an early physical state, it
• 2:18:37
  Speaker 1: does not by itself explain why there was a universe
• 2:18:40
  Speaker 1: at all, whether something preceded that state, or whether the
• 2:18:44
  Speaker 1: idea of a preceding moment even makes sense. As we
• 2:18:48
  Speaker 1: trace the universe farther backwards, our theories eventually reach an
• 2:18:53
  Speaker 1: uncomfortable boundary. Albert Einstein's general theory of relativity describes gravity
• 2:19:00
  Speaker 1: and the geometry of spacetime with remarkable success. Quantum physics
• 2:19:05
  Speaker 1: describes matter and the other fundamental interactions with equally remarkable precision.
• 2:19:12
  Speaker 1: Yet at the enormous densities and tiny scales expected near
• 2:19:16
  Speaker 1: the Planck era, both become essential at once, and we
• 2:19:20
  Speaker 1: do not yet possess an experimentally confirmed theory that fully
• 2:19:24
  Speaker 1: combines them. Classical general relativity, extrapolated backwards, produces the Big
• 2:19:30
  Speaker 1: Bang singularity. Density and spacetime curvature can grow without limit,
• 2:19:36
  Speaker 1: But this should not be confused with direct evidence that
• 2:19:39
  Speaker 1: the entire universe, once existed, has a literally infinitely dense
• 2:19:44
  Speaker 1: physical point. A singularity is better understood as a warning
• 2:19:49
  Speaker 1: that the classical description has reached the edge of its usefulness.
• 2:19:53
  Speaker 1: The equations have carried us somewhere they can no longer
• 2:19:56
  Speaker 1: reliably describe. What replaces them remains unknown, and that uncertainty
• 2:20:02
  Speaker 1: leaves several possibilities open. perhaps time itself genuinely began. If
• 2:20:08
  Speaker 1: time is part of space-time rather than an independent clock
• 2:20:12
  Speaker 1: existing outside the universe, then there may simply be no
• 2:20:16
  Speaker 1: earlier moment. Asking what happened before the beginning could then
• 2:20:20
  Speaker 1: be like asking for a location beyond a boundary that
• 2:20:23
  Speaker 1: does not exist in that direction. Or perhaps the hot
• 2:20:26
  Speaker 1: big bang was preceded by cosmic inflation. In that picture,
• 2:20:31
  Speaker 1: The hot universe emerged after an earlier period of extraordinarily rapid,
• 2:20:36
  Speaker 1: accelerated expansion. The Big Bang would then be less like
• 2:20:40
  Speaker 1: the beginning of existence, and more like the beginning of
• 2:20:43
  Speaker 1: the hot phase we can reconstruct. Quantum cosmology offers stranger
• 2:20:48
  Speaker 1: possibilities still. The earliest universe may have been described by
• 2:20:52
  Speaker 1: a quantum state in which ordinary ideas of space and
• 2:20:55
  Speaker 1: time had not yet acquired their familiar meaning. The universe
• 2:20:59
  Speaker 1: might possess no classical temporal boundary. Time itself might have
• 2:21:03
  Speaker 1: emerged only when spacetime became sufficiently classical. Other models allow
• 2:21:08
  Speaker 1: a genuine physical history before our expansion. A contracting universe
• 2:21:13
  Speaker 1: may have approached an extreme density before quantum gravitational effects
• 2:21:16
  Speaker 1: produced a bounce. Cyclic cosmologies imagine repeated cosmic eras. Roger
• 2:21:22
  Speaker 1: Penrose's conformal cyclic cosmology goes further. suggesting that the unimaginably
• 2:21:28
  Speaker 1: distant future of one aeon could be mathematically connected to
• 2:21:31
  Speaker 1: the beginning of another. These possibilities are not equally established.
• 2:21:35
  Speaker 1: That distinction is essential. The hot big bang is supported
• 2:21:40
  Speaker 1: by extensive observation. Inflation has important indirect successes, but no
• 2:21:45
  Speaker 1: confirmed underlying mechanism. Bouncing universes, cyclic models, no boundary cosmology–
• 2:21:52
  Speaker 1: and quantum creation proposals remain theoretical attempts to extend physics
• 2:21:57
  Speaker 1: beyond regions we can currently test directly. Science does not
• 2:22:02
  Speaker 1: improve these ideas by pretending to know more than the
• 2:22:04
  Speaker 1: evidence allows. Their uncertainty is part of the story. And
• 2:22:09
  Speaker 1: perhaps the most remarkable part is how much we know
• 2:22:12
  Speaker 1: despite those limits. From one small planet orbiting an ordinary star,
• 2:22:16
  Speaker 1: humans have measured light that has travelled for billions of years.
• 2:22:20
  Speaker 1: We have mapped radiation released when the universe was still young.
• 2:22:24
  Speaker 1: We have reconstructed nuclear reactions from its first minutes. Particle
• 2:22:28
  Speaker 1: physics allows us to infer conditions from far earlier fractions
• 2:22:32
  Speaker 1: of a second. We can test mathematical models of events
• 2:22:35
  Speaker 1: occurring at areas no natural environment on Earth could ever approach.
• 2:22:39
  Speaker 1: The universe has left clues everywhere, but the final clue
• 2:22:43
  Speaker 1: remains hidden. Some future theory of quantum gravity may eventually
• 2:22:47
  Speaker 1: explain what happened where our present descriptions fail. It may
• 2:22:51
  Speaker 1: reveal an earlier universe. It may show that space-time emerged
• 2:22:55
  Speaker 1: from something deeper. It may demonstrate that there was a
• 2:22:58
  Speaker 1: true first moment. Or it may show that searching for
• 2:23:02
  Speaker 1: one unique beginning was never quite the right way to
• 2:23:04
  Speaker 1: think about reality. For now, the journey towards the beginning
• 2:23:08
  Speaker 1: of everything ends not with a final answer, but with
• 2:23:12
  Speaker 1: a boundary. Behind us lies almost 14 billion years of
• 2:23:17
  Speaker 1: reconstructable cosmic history. Ahead of us, looking backwards, is the
• 2:23:22
  Speaker 1: place where space, time, gravity and quantum physics become inseparable.
• 2:23:29
  Speaker 1: And somewhere within that unfinished piece of physics may be
• 2:23:32
  Speaker 1: the answer to the oldest question we can ask. Not
• 2:23:34
  Speaker 1: merely what happened at the beginning, but whether there was
• 2:23:37
  Speaker 1: ever a beginning at all.