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What Actually Happened During the Big Bang The Sleepy Scientist · Night Science Stories

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0:59
Speaker 2: Good evening and welcome to the Sleepy Scientist. It's lovely
1:02
Speaker 2: to have you here tonight. I'm always curious where these
1:05
Speaker 2: evenings end up, so if you feel like it, let
1:08
Speaker 2: me know in the comments where in the world you're
1:10
Speaker 2: listening from. Maybe a dark bedroom, maybe a kitchen table
1:13
Speaker 2: long after the dishes are done. Wherever you are, I
1:16
Speaker 2: hope you're warm and ready to let the day drift away.
1:19
Speaker 2: Close your eyes whenever you're ready. I'll carry on talking
1:22
Speaker 2: either way. And if you enjoy these slow wanderings through science,
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Speaker 2: a like and a subscribe help this little channel drift
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Speaker 2: a bit further out into the world. If you're already
1:32
Speaker 2: with us, thank you. Your company on these evenings means
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Speaker 2: a great deal. Now, I'll do my best not to
1:37
Speaker 2: make tonight's subject so interesting that it keeps you up.
1:41
Speaker 2: But if it does, at least you'll drift off knowing
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Speaker 2: that the Big Bang was not really a gigantic explosion
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Speaker 2: in the way most of us imagine it. There was
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Speaker 2: no centre where everything went off. no debris racing into
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Speaker 2: an empty cosmic room, and perhaps most strangely of all,
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Speaker 2: nowhere outside the universe for anything to explode into. So,
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Speaker 2: shall we begin? If the Big Bang wasn't an explosion,
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Speaker 2: what actually happened? The phrase Big Bang has always had
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Speaker 2: a slight problem built into it. It sounds dramatic. It
2:14
Speaker 2: sounds sudden. Most of all, it sounds like an explosion.
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Speaker 2: It is very easy to picture one. Somewhere in an
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Speaker 2: enormous black emptiness, perhaps unimaginably long ago, there is a
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Speaker 2: tiny point containing everything. Then, in an instant, it detonates.
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Speaker 2: Matter flies outward in every direction. Space fills with debris.
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Speaker 2: Galaxies eventually appear among the expanding fragments, and here we
2:43
Speaker 2: are sitting on one of them, looking back towards the
2:46
Speaker 2: place where it all began. It is a wonderfully clear picture.
2:51
Speaker 2: It is also wrong in almost every important respect. An
2:55
Speaker 2: ordinary explosion is something that happens at a location. Imagine
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Speaker 2: a firework high above a field. Before it explodes, there
3:03
Speaker 2: is already space around it. When it bursts, hot gas
3:07
Speaker 2: and glowing fragments race outward through that surrounding space. You
3:11
Speaker 2: could point towards the centre of the event. You could
3:15
Speaker 2: stand closer to it or farther away. And if you
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Speaker 2: could somehow freeze the scene... the pieces would form an
3:21
Speaker 2: expanding cloud around the place where the explosion occurred. The
3:26
Speaker 2: Big Bang does not work like that. Modern cosmology does
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Speaker 2: not describe the beginning of the observable universe as matter
3:34
Speaker 2: bursting outward from one special point into a vast, pre-existing emptiness. Instead,
3:40
Speaker 2: it describes an early universe that was extraordinarily hot and dense.
3:45
Speaker 2: followed by an ongoing expansion in which the large-scale distances
3:49
Speaker 2: between sufficiently separated regions increased with time. That distinction sounds
3:54
Speaker 2: small at first. In reality, it changes almost everything. The
3:59
Speaker 2: galaxies are not simply pieces of cosmic shrapnel thrown through
4:02
Speaker 2: otherwise motionless space. On the largest scales, the geometry of
4:07
Speaker 2: the universe itself evolves. Regions that are not held together
4:11
Speaker 2: strongly enough by gravity can become farther apart as the
4:14
Speaker 2: cosmic expansion continues. This is why astronomers often say that
4:19
Speaker 2: space itself is expanding. It is a useful phrase, although
4:24
Speaker 2: it should not be taken to mean that space is
4:26
Speaker 2: literally made of some rubbery substance being stretched by invisible hands.
4:31
Speaker 2: What matters is the measurable result. Two distant regions that
4:35
Speaker 2: take part in the general cosmic expansion can have more
4:39
Speaker 2: space between them later than they did earlier. And if
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Speaker 2: that sounds rather different from an explosion, that is because
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Speaker 2: it is. There is another consequence that is even more peculiar.
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Speaker 2: If the Big Bang had been an ordinary blast, we
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Speaker 2: should be able, at least in principle, to ask where
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Speaker 2: it happened. Somewhere ought to be the centre. Somewhere ought
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Speaker 2: to be the ancient cosmic crater from which everything else
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Speaker 2: has been travelling away. But there is no known location
5:06
Speaker 2: like that. The Milky Way is not racing away from
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Speaker 2: a central explosion point. Neither is the Andromeda Galaxy. Neither
5:14
Speaker 2: are the galaxy's billions of light-years from us. On sufficiently
5:19
Speaker 2: large scales, the expansion does not pick out one privileged
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Speaker 2: place as the origin. Instead, every region of the observable
5:27
Speaker 2: universe traces back to the same much hotter and denser
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Speaker 2: cosmic past. That includes the region you are sitting in now.
5:35
Speaker 2: Long before there was an Earth, before there was a Sun,
5:38
Speaker 2: and long before anything resembling the Milky Way existed, The
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Speaker 2: matter and energy associated with this part of the universe
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Speaker 2: were embedded in that early hot cosmic environment. So the
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Speaker 2: Big Bang did not happen at some unimaginably distant location
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Speaker 2: that we might one day travel towards. In the cosmological sense,
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Speaker 2: it happened here as well, and over there, and in
6:01
Speaker 2: the region now occupied by every distant galaxy we can observe.
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Speaker 2: This is one of those ideas that can sound almost
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Speaker 2: absurd until the picture changes in your mind. We're used
6:12
Speaker 2: to events happening somewhere. A volcano erupts on a mountain.
6:16
Speaker 2: A star explodes at a location in a galaxy. A
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Speaker 2: glass falls from a particular table, usually shortly after someone
6:25
Speaker 2: has insisted it is nowhere near the edge. But cosmic
6:28
Speaker 2: expansion is not an event occurring inside a large room.
6:32
Speaker 2: It is a change in the universe on the largest
6:35
Speaker 2: scales we can describe. According to the standard cosmological model,
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Speaker 2: The universe has been expanding from its hot, dense early
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Speaker 2: state for about 13.8 billion years. That number is sometimes
6:48
Speaker 2: described casually as the time since the Big Bang, which
6:51
Speaker 2: is reasonable enough, but it can accidentally bring the explosion
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Speaker 2: picture straight back. 13.8 billion years ago there was not,
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Speaker 2: as far as the model tells us, a bomb-like object
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Speaker 2: sitting in a dark arena and waiting to go off.
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Speaker 2: The age instead describes how far back we can trace
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Speaker 2: the history of cosmic expansion within our best supported model
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Speaker 2: of the universe. The farther backward we follow that history,
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Speaker 2: the smaller the characteristic distances become. Matter and radiation become denser.
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Speaker 2: Temperatures rise enormously. Eventually our familiar pictures of stars, atoms
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Speaker 2: and even ordinary atomic nuclei disappear entirely. but the universe
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Speaker 2: does not become a fireball located somewhere within space. The
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Speaker 2: early conditions belong to space throughout the observable region. That
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Speaker 2: is the first great correction hidden inside the phrase Big Bang.
7:47
Speaker 2: It was not matter exploding into space. It was an
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Speaker 2: early hot and dense universe undergoing expansion. Once that difference
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Speaker 2: settles in, several other strange questions appear naturally. If nothing
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Speaker 2: exploded outward from a centre … What exactly do physicists
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Speaker 2: mean when they say the universe expanded? If the early
8:08
Speaker 2: universe was everywhere, can we meaningfully ask where the Big
8:11
Speaker 2: Bang happened? And if the universe was not expanding into
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Speaker 2: some larger emptiness, what, if anything, was on the other side?
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Speaker 2: Those questions lead away from the image of a cosmic
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Speaker 2: explosion and towards something much stranger. Not a blast travelling
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Speaker 2: across the universe, but the universe itself changing with time.
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Speaker 2: The name itself has done a remarkable amount of damage.
8:35
Speaker 2: Big Bang sounds as though someone has already explained the
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Speaker 2: event before the explanation has even begun. Two short words
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Speaker 2: give us a flash, a blast, an expanding cloud of
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Speaker 2: debris and perhaps something nearby being knocked off a shelf.
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Speaker 2: It is vivid, memorable and almost impossible not to picture.
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Speaker 2: The curious part is that the phrase was not invented
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Speaker 2: by one of the scientists trying to persuade the world
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Speaker 2: that the universe began in a hot, dense state. It
9:07
Speaker 2: came from one of their most famous opponents. The British
9:11
Speaker 2: astronomer Fred Hoyle used the expression Big Bang during a
9:15
Speaker 2: British Broadcasting Corporation radio programme in 1949 while discussing competing
9:22
Speaker 2: ideas about the history of the universe. Cosmology at the
9:26
Speaker 2: time was still very much an argument in progress. Astronomers
9:30
Speaker 2: had evidence that the universe was expanding, but expansion did
9:34
Speaker 2: not automatically tell them whether the universe had a beginning.
9:38
Speaker 2: Hoyle strongly preferred a different possibility. Together with the Austrian-born
9:44
Speaker 2: physicist Hermann Bondi and the Austrian-born astronomer Thomas Gold, Hoyle
9:49
Speaker 2: became one of the leading advocates of the steady-state theory.
9:53
Speaker 2: the steady-state universe expanded just as observations indicated. Galaxies could
10:00
Speaker 2: move farther apart over time. But unlike the hot Big
10:03
Speaker 2: Bang picture, the universe did not have to become steadily
10:07
Speaker 2: emptier or trace back towards a dramatically denser beginning. Instead,
10:13
Speaker 2: new matter would be created as space expanded. The required
10:17
Speaker 2: amount was extraordinarily small, spread across the enormous volume of
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Speaker 2: the universe, only a tiny trickle of newly created matter
10:25
Speaker 2: would be necessary to maintain approximately the same average density
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Speaker 2: over cosmic time. New material could eventually form new stars
10:34
Speaker 2: and galaxies in the gaps created by expansion. The details
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Speaker 2: were unusual, but the goal was elegant. In the steady
10:41
Speaker 2: state model, the universe could look broadly similar not only
10:44
Speaker 2: in different places, but also at different times. There need
10:48
Speaker 2: not have been an era when everything was compressed into
10:51
Speaker 2: a much hotter and denser state. The cosmos could simply
10:55
Speaker 2: continue indefinitely into both the past and the future. This
10:59
Speaker 2: idea became closely associated with what was called the perfect
11:03
Speaker 2: cosmological principle. The ordinary cosmological principle says, roughly speaking, that
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Speaker 2: on sufficiently large scales the universe should look much the
11:13
Speaker 2: same from place to place and in different directions. The stronger,
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Speaker 2: steady-state version extended that similarity through time as well. Hoyle
11:23
Speaker 2: found this eternal picture attractive, and while discussing the alternative
11:27
Speaker 2: during his radio broadcast, he referred to the idea that
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Speaker 2: matter in the universe had been created in one great
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Speaker 2: event in the distant past as a big bang. The
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Speaker 2: name stuck with almost absurd efficiency. There is a popular
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Speaker 2: version of this story in which Hoyle deliberately invented the
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Speaker 2: expression as an insult, sneering at a theory he considered ridiculous.
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Speaker 2: The reality appears to be less tidy. Hoyle certainly disagreed
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Speaker 2: with the expanding hot early universe model. He defended the
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Speaker 2: steady state theory vigorously and remained sceptical of conventional Big
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Speaker 2: Bang cosmology long after many other astronomers had changed their minds.
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Speaker 2: but it is difficult to demonstrate that the phrase Big
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Speaker 2: Bang itself was intended simply as mockery. Hoyle later indicated
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Speaker 2: that he had wanted a vivid way to distinguish the
12:20
Speaker 2: competing cosmological pictures for a radio audience. Whatever tone he
12:25
Speaker 2: originally intended, the expression was wonderfully suited to broadcasting. Big
12:30
Speaker 2: Bang is rather easier to remember over breakfast than something
12:33
Speaker 2: like an evolving, relativistic cosmological model containing an early high-temperature phase. Unfortunately,
12:41
Speaker 2: memorable names sometimes bring their own baggage. The phrase naturally
12:46
Speaker 2: encourages us to picture a conventional explosion. Hoyle's wording sounds
12:50
Speaker 2: like an event happening at one place and one moment,
12:54
Speaker 2: followed by material racing away from the scene. That is
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Speaker 2: not what the modern theory says. And yet the name survived.
13:02
Speaker 2: During the following decades, the steady-state theory began running into
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Speaker 2: increasingly serious observational difficulties. Astronomers discovered that the distant universe
13:12
Speaker 2: did not look exactly like the nearby universe, meaning that
13:16
Speaker 2: the cosmic population had clearly changed over time. Then came
13:20
Speaker 2: an even more decisive discovery. In the 1960s, astronomers found
13:26
Speaker 2: the cosmic microwave background, a faint bath of radiation arriving
13:30
Speaker 2: from every direction in the sky. This was exactly the
13:34
Speaker 2: sort of relic expected if the universe had once been
13:37
Speaker 2: vastly hotter and denser. The steady-state theory could not explain
13:42
Speaker 2: the evidence nearly as naturally. Gradually, the Hot Big Bang
13:46
Speaker 2: model became the dominant cosmological framework. Fred Hoyle lost the
13:51
Speaker 2: larger argument, but the name he had helped popularise remained
13:55
Speaker 2: attached to the winning theory. There is something gently ironic
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Speaker 2: about that. One of the best-known labels in modern science
14:03
Speaker 2: came from a scientist who did not believe the model
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Speaker 2: it now describes. And the label itself suggests something the
14:10
Speaker 2: theory does not actually claim. Science is full of names
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Speaker 2: like this. Terms are often created before understanding is complete.
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Speaker 2: Sometimes they come from historical accidents. Sometimes they survive because
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Speaker 2: they are short, catchy, or simply too familiar to replace.
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Speaker 2: Once millions of people know a phrase, persuading everyone to
14:32
Speaker 2: start saying something more precise can be rather like trying
14:36
Speaker 2: to rename the moon. So Big Bang remained. It appears
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Speaker 2: in textbooks, documentaries, classrooms and conversations. Scientists use it themselves,
14:47
Speaker 2: fully aware that the words can be misleading if taken literally.
14:52
Speaker 2: The important thing is to separate the name from the physics.
14:56
Speaker 2: Big Bang does not mean a bomb going off in
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Speaker 2: empty space. It refers to a universe that was once
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Speaker 2: much hotter and denser than it is today, and which
15:04
Speaker 2: has been expanding and cooling over cosmic history. Fred Hoyle's
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Speaker 2: memorable phrase gave cosmology a wonderfully dramatic title. Understanding the
15:15
Speaker 2: universe requires quietly forgetting the picture that title seems to create.
15:20
Speaker 2: Long before anyone used the word Big Bang, physics had
15:24
Speaker 2: already begun to change what the universe could be. For centuries,
15:28
Speaker 2: space had mostly been treated as a kind of stage.
15:32
Speaker 2: Stars and planets moved across it, objects fell through it,
15:36
Speaker 2: and light travelled across it. but space itself was usually
15:40
Speaker 2: imagined as something fixed in the background. Then Albert Einstein
15:45
Speaker 2: changed the rules. In 1915, Einstein completed his general theory
15:50
Speaker 2: of relativity, a new description of gravity that replaced the
15:54
Speaker 2: older picture of gravity as a force acting invisibly across
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Speaker 2: empty space. In Einstein's theory, matter and energy affect the
16:03
Speaker 2: geometry of space-time. and that curved geometry in turn affects
16:07
Speaker 2: how matter and light move. A planet orbiting a star
16:11
Speaker 2: is not simply being tugged by an invisible rope. It
16:14
Speaker 2: is following the natural paths available through curved spacetime. That
16:19
Speaker 2: idea was already strange enough. But when physicists began applying
16:24
Speaker 2: general relativity to the entire universe, something even more surprising emerged.
16:30
Speaker 2: Spacetime did not necessarily have to remain static. The geometry
16:35
Speaker 2: of the universe could change. This was a deeply unfamiliar possibility.
16:41
Speaker 2: At the beginning of the 20th century, many scientists assumed
16:44
Speaker 2: that the universe, taken as a whole, should be eternal
16:48
Speaker 2: and essentially unchanging. Stars might be born and die. Galaxies
16:53
Speaker 2: might move. But the grand structure of the cosmos itself
16:58
Speaker 2: was expected to remain more or less the same. Einstein
17:02
Speaker 2: initially shared that preference. When he applied general relativity to cosmology,
17:07
Speaker 2: he found that a universe filled with matter did not
17:10
Speaker 2: naturally want to sit perfectly still. Gravity would tend to
17:15
Speaker 2: make such a universe evolve. To preserve a static cosmic model,
17:20
Speaker 2: Einstein introduced an additional term into his equations known as
17:23
Speaker 2: the cosmological constant. In that early context, it could act
17:28
Speaker 2: in a way that helped counterbalance gravitational attraction and allowed
17:32
Speaker 2: a static solution. it was mathematically permitted. Nature, however, was
17:37
Speaker 2: under no obligation to share Einstein's preference for a motionless cosmos.
17:42
Speaker 2: One of the first people to take the dynamical possibilities
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Speaker 2: of general relativity seriously was the Russian physicist and mathematician
17:50
Speaker 2: Alexander Friedman. In the early 1920s, Friedman explored Einstein's field
17:56
Speaker 2: equations without insisting that the universe remain static. He found
18:01
Speaker 2: mathematical solutions in which the scale of the universe changed
18:04
Speaker 2: with time. Some solutions described expansion. Others described contraction. The
18:12
Speaker 2: importance of this was enormous. Friedman was not imagining galaxies
18:17
Speaker 2: being fired outward from a central explosion. He was finding
18:21
Speaker 2: that the geometry describing the universe itself could evolve. Distances
18:26
Speaker 2: between widely separated locations could change. Because the underlying cosmological
18:31
Speaker 2: solution changed with time, Einstein initially resisted Friedmann's work and
18:37
Speaker 2: at first believed there was an error in his calculations.
18:41
Speaker 2: After Friedmann responded, Einstein acknowledged that the mathematics was correct.
18:46
Speaker 2: The idea of a changing universe had entered serious physics.
18:50
Speaker 2: A few years later, another scientist took the concept further.
18:55
Speaker 2: Georges Lemaitre was a Belgian physicist. astronomer and Catholic priest
19:00
Speaker 2: with an unusual combination of interests and an unusually important
19:05
Speaker 2: role in modern cosmology. In 1927, Le Maistre independently developed
19:11
Speaker 2: a model of an expanding universe using general relativity, but
19:15
Speaker 2: he went beyond simply finding a mathematical possibility. He also
19:20
Speaker 2: connected the theory with astronomical observations. Astronomers had begun measuring
19:25
Speaker 2: shifts in the light from distant nebulae, objects that would
19:29
Speaker 2: later be recognised as galaxies beyond the Milky Way. Many
19:33
Speaker 2: showed redshifts, meaning that features in their spectra appeared at
19:36
Speaker 2: longer wavelengths than expected. Lemaître interpreted these observations within an
19:42
Speaker 2: expanding cosmological model. In effect, he recognised that a changing
19:47
Speaker 2: universe could help explain why distant galaxies appeared to be receding.
19:51
Speaker 2: This was before the expanding universe became widely accepted. And crucially,
19:57
Speaker 2: Lemaitre did not picture the expansion as debris flying away
20:01
Speaker 2: from a cosmic blast site. The expansion was built into
20:04
Speaker 2: the geometry of the universe. That distinction matters enormously. If
20:10
Speaker 2: a grenade explodes in a field, the fragments move through space.
20:15
Speaker 2: If the universe expands in the cosmological sense, the large-scale
20:20
Speaker 2: distances between sufficiently separated regions can increase because spacetime itself
20:26
Speaker 2: is described by an evolving geometry. The two situations may
20:30
Speaker 2: look superficially similar if all we notice is that objects
20:34
Speaker 2: become farther apart. Physically, they are very different. Lemaitre later
20:39
Speaker 2: pushed his ideas further backwards in cosmic history. In the
20:43
Speaker 2: early 1930s, he proposed that the universe might have emerged
20:47
Speaker 2: from an extraordinarily compact initial state that he called the
20:51
Speaker 2: primeval atom. The phrase can sound rather literal today, as
20:55
Speaker 2: though Lemaitre imagined the entire universe as one enormous version
20:59
Speaker 2: of an ordinary atom. That was not really the idea.
21:04
Speaker 2: The primeval atom was an early attempt to think about
21:07
Speaker 2: a universe whose expansion could be traced backwards towards an
21:11
Speaker 2: extremely concentrated state. Modern Big Bang cosmology is far more
21:16
Speaker 2: developed than LeMaitre's original proposal. It includes nuclear physics, particle physics,
21:23
Speaker 2: the cosmic microwave background, the growth of structure, dark matter,
21:28
Speaker 2: dark energy and precise measurements of cosmic expansion. But the
21:33
Speaker 2: conceptual leap was already there. The universe might have a history.
21:38
Speaker 2: It might have been different in the past. Its geometry
21:41
Speaker 2: might evolve. And the farther back one looked, the denser
21:45
Speaker 2: the cosmic conditions might become. This represented a remarkable shift
21:50
Speaker 2: in thinking. Space was no longer merely an unchanging container
21:54
Speaker 2: in which the history of matter played out. Spacetime itself
21:58
Speaker 2: became part of the story. Einstein's own view gradually changed
22:03
Speaker 2: as astronomical evidence accumulated. the static universe he had originally
22:08
Speaker 2: preferred became increasingly difficult to defend, while models based on
22:12
Speaker 2: Friedman's and Lemaître' s work became more compelling. The cosmological constant
22:18
Speaker 2: itself did not disappear permanently. Much later, observations of accelerated
22:23
Speaker 2: cosmic expansion would give cosmologists a new reason to include
22:27
Speaker 2: a cosmological constant or something with similar effects in their models.
22:32
Speaker 2: Its role, however, was completely different from Einstein's original attempt
22:37
Speaker 2: to hold the universe still. That history is a useful
22:41
Speaker 2: reminder that equations can contain possibilities before anyone knows which
22:45
Speaker 2: possibility nature has chosen. General relativity allowed the universe to evolve.
22:51
Speaker 2: Friedman showed mathematically how it could expand or contract. Lemaître
22:56
Speaker 2: connected expansion with what astronomers were seeing in the sky.
23:00
Speaker 2: and began considering what an expanding universe might imply about
23:04
Speaker 2: its past. Together, these ideas changed the meaning of cosmic history.
23:10
Speaker 2: Instead of imagining matter moving around inside a permanently fixed arena,
23:15
Speaker 2: cosmologists were forced to consider an arena that could change
23:18
Speaker 2: along with everything inside it. And once space and time
23:22
Speaker 2: themselves became dynamic, The idea of the Big Bang could
23:26
Speaker 2: no longer be reduced to something exploding in the middle
23:28
Speaker 2: of an otherwise empty universe. The stage itself was part
23:33
Speaker 2: of the event. Long before anyone could take a photograph
23:36
Speaker 2: of a distant galaxy, astronomers had already begun noticing that
23:40
Speaker 2: something unusual was hidden in their light. One of the
23:44
Speaker 2: people responsible was an American astronomer named Vesto Slipher. In
23:49
Speaker 2: the early 20th century, Slipher worked at the Lowell Observatory
23:52
Speaker 2: in Arizona. studying objects then known as spiral nebulae. Through telescopes,
23:58
Speaker 2: they appeared as faint, swirling patches of light. At the time,
24:03
Speaker 2: astronomers were still arguing about what these objects actually were.
24:08
Speaker 2: Some suspected they were enormous collections of stars far beyond
24:12
Speaker 2: the Milky Way. Others thought they might be relatively nearby
24:15
Speaker 2: structures contained within our own galaxy. Slipher approached the problem
24:20
Speaker 2: by studying their spectra. When light is separated into its
24:24
Speaker 2: component wavelengths, particular atoms leave distinctive patterns of bright or
24:29
Speaker 2: dark lines. Those patterns act almost like fingerprints. If the
24:35
Speaker 2: source of the light is moving relative to the observer,
24:38
Speaker 2: the entire pattern can shift. A shift towards shorter wavelengths
24:42
Speaker 2: is called a blue shift. A shift towards longer wavelengths
24:46
Speaker 2: is called a red shift. Beginning in the 1910s, Slipher
24:51
Speaker 2: measured the spectra of several spiral nebulae and found something striking.
24:56
Speaker 2: Many of them showed substantial redshifts. Interpreted through the Doppler effect,
25:01
Speaker 2: these shifts suggested that the objects were receding from us,
25:04
Speaker 2: in some cases at hundreds or even more than a
25:07
Speaker 2: thousand kilometres per second. That was remarkably fast by astronomical
25:12
Speaker 2: standards at the time. But Slipher did not yet have
25:15
Speaker 2: the other half of the puzzle. He could measure how
25:18
Speaker 2: the spectra were shifted, but astronomers still needed reliable distances.
25:23
Speaker 2: That became possible through the work of Edwin Hubble and others.
25:27
Speaker 2: Using the enormous 100-inch Hooker telescope at Mount Wilson Observatory
25:31
Speaker 2: in California, Hubble identified individual variable stars within some spiral nebulae.
25:38
Speaker 2: Among them were Cepheid variable stars, whose changing brightness could
25:42
Speaker 2: be used to estimate distance. The result helped settle one
25:46
Speaker 2: of astronomy's great debates. The spiral nebulae were not small
25:50
Speaker 2: objects sitting inside the Milky Way. They were galaxies in
25:54
Speaker 2: their own right, located far beyond it. Suddenly, the known
25:58
Speaker 2: universe became enormously larger. Hubble then compared estimates of galaxy
26:04
Speaker 2: distances with measurements of their redshifts. Many of those velocity
26:08
Speaker 2: measurements came originally from Slipher's work, and later observations were
26:13
Speaker 2: extended with the help of astronomer Milton Humerson. who became
26:16
Speaker 2: extraordinarily skilled at obtaining spectra from faint galaxies. A pattern emerged.
26:24
Speaker 2: In 1929, Hubble published evidence that galaxies farther away generally
26:28
Speaker 2: showed larger recession velocities. Put very simply, the greater the distance,
26:33
Speaker 2: the faster the apparent recession. This relationship became one of
26:37
Speaker 2: the foundations of observational cosmology, but there is an important
26:41
Speaker 2: piece of history that was overlooked for many years. Two
26:44
Speaker 2: years before Hubble's famous publication, Georges Lemaître had already derived
26:49
Speaker 2: an expanding universe solution from general relativity, and used available
26:54
Speaker 2: astronomical measurements to estimate a relationship between galaxy distance and recession.
27:00
Speaker 2: Lemaître was not merely guessing that the universe might be expanding.
27:04
Speaker 2: He had connected the mathematics with observations. For that reason,
27:08
Speaker 2: the relationship is now formally known as the Hubble-Lemaître Law.
27:12
Speaker 2: Its basic idea is wonderfully simple. On sufficiently large scales,
27:17
Speaker 2: a galaxy that is twice as far away should, on average,
27:21
Speaker 2: have roughly twice the cosmological recession rate. A galaxy three
27:26
Speaker 2: times farther away should show roughly three times the recession.
27:30
Speaker 2: Real observations are messier than that perfect description because galaxies
27:34
Speaker 2: also possess their own local motions. But once astronomers examine
27:38
Speaker 2: large enough distances, the overall pattern becomes clear. The universe
27:43
Speaker 2: is expanding. There is, however, a very easy mistake to
27:47
Speaker 2: make at this point. If almost every distant galaxy appears
27:50
Speaker 2: to be moving away from us, it sounds as though
27:53
Speaker 2: the Milky Way must be sitting near the centre. That
27:56
Speaker 2: would be exactly what we might expect after an ordinary explosion.
28:00
Speaker 2: Stand near the blast site and fragments move outward in
28:04
Speaker 2: every direction. But cosmic expansion produces the same observation without
28:09
Speaker 2: giving us any special location. Imagine several galaxies distributed throughout
28:15
Speaker 2: an expanding universe. As the distances between them increase, an
28:19
Speaker 2: observer in one galaxy sees the others becoming farther away.
28:23
Speaker 2: Move the observer to a different galaxy and the same
28:26
Speaker 2: general pattern appears. From there, distant galaxies also seem to recede.
28:32
Speaker 2: No galaxy has to occupy the centre because the expansion
28:35
Speaker 2: is not occurring outward from one point within space. The
28:39
Speaker 2: distances throughout space are changing. There is another subtlety hidden
28:43
Speaker 2: in the word redshift. For relatively nearby objects, it can
28:47
Speaker 2: often be useful to think of redshift as a Doppler
28:50
Speaker 2: effect caused by motion through space. Cosmological redshift is broader
28:55
Speaker 2: than that simple picture. Light travelling through an expanding universe
28:59
Speaker 2: can itself have its wavelength stretched during the journey. Imagine
29:04
Speaker 2: a wave of light leaving a distant galaxy. While that
29:07
Speaker 2: light spends millions or billions of years crossing the cosmos,
29:11
Speaker 2: The scale of the universe changes. By the time the
29:15
Speaker 2: wave reaches us, the distance between successive peaks of that
29:18
Speaker 2: wave can be greater than when it began its journey.
29:22
Speaker 2: Its wavelength has increased. The light has become red-shifted. This
29:26
Speaker 2: does not mean ordinary motion is irrelevant. Galaxies can move
29:30
Speaker 2: locally through their surroundings as well. But on great enough scales,
29:35
Speaker 2: cosmological expansion dominates the pattern. The Andromeda galaxy process a
29:40
Speaker 2: useful exception. Andromeda is roughly 2.5 million light-years away, and
29:46
Speaker 2: is approaching the Milky Way rather than receding from it.
29:49
Speaker 2: That does not contradict cosmic expansion. The Milky Way and
29:54
Speaker 2: Andromeda belong to the same gravitationally bound collection of galaxies
29:58
Speaker 2: known as the Local Group. Their mutual gravity and local
30:01
Speaker 2: motions are strong enough to overwhelm the gentle effect of
30:05
Speaker 2: cosmic expansion across that relatively modest distance. Eventually, the two
30:10
Speaker 2: galaxies are expected to interact and merge. The universe can
30:14
Speaker 2: expand overall while individual galaxies still move towards one another.
30:19
Speaker 2: Solar systems remain bound. Galaxies remain bound. Clusters of galaxies
30:24
Speaker 2: can remain bound. cosmic expansion becomes clearest only when we
30:29
Speaker 2: step back far enough. And when astronomers did step back,
30:33
Speaker 2: they found something extraordinary. The galaxies were not behaving like
30:37
Speaker 2: fragments from a bomb centred on the Milky Way. Instead,
30:41
Speaker 2: the same broad relationship between distance and recession appeared throughout
30:46
Speaker 2: the observable cosmos. The farther away we looked, the stronger
30:51
Speaker 2: the signature of expansion became. What Slipher, Lemaitre, Hubble and
30:56
Speaker 2: Humason helped reveal was not simply that galaxies move. It
31:01
Speaker 2: was that the universe itself has a measurable history of expansion.
31:05
Speaker 2: And once that became clear, the idea of a single
31:09
Speaker 2: cosmic explosion point became harder and harder to maintain. Saying
31:13
Speaker 2: that the universe is expanding sounds simple enough until we
31:17
Speaker 2: ask what, exactly, is doing the expanding? Galaxies are not
31:22
Speaker 2: all sitting on some enormous invisible conveyor belt. Space is
31:26
Speaker 2: not being pushed outward from a central engine, and there
31:29
Speaker 2: is no cosmic wind carrying matter away from the place
31:32
Speaker 2: where everything began. The idea is subtler than that. On
31:37
Speaker 2: the larger scales, cosmologists describe the universe using a geometry
31:41
Speaker 2: that can change with time. Galaxies that are sufficiently far apart,
31:46
Speaker 2: and not gravitationally bound to one another, can become more
31:49
Speaker 2: widely separated as that geometry evolves. In other words, the
31:53
Speaker 2: distance between them can increase even if we do not
31:56
Speaker 2: imagine either galaxy firing its engines and travelling through space
32:00
Speaker 2: in the ordinary sense. This general movement is sometimes called
32:03
Speaker 2: the Hubble flow, or the cosmological flow. Galaxies that are
32:08
Speaker 2: approximately carried along with it are described as co-moving. That
32:13
Speaker 2: word is useful because it separates two different kinds of motion.
32:16
Speaker 2: A galaxy can have an ordinary local velocity through its surroundings.
32:21
Speaker 2: It can orbit inside a galaxy cluster, fall towards another galaxy,
32:26
Speaker 2: or move because of nearby gravitational attractions. At the same time,
32:31
Speaker 2: the enormous region containing those galaxies can participate in the
32:34
Speaker 2: overall cosmic expansion. Cosmologists therefore need a way to describe
32:39
Speaker 2: the expansion separately from all that local jostling. One of
32:44
Speaker 2: the most important tools for doing this is the cosmological
32:47
Speaker 2: scale factor. The scale factor is a mathematical quantity representing
32:52
Speaker 2: how the characteristic distances between widely separated commoving locations change
32:57
Speaker 2: over cosmic time. Imagine two idealised galaxies that are simply
33:02
Speaker 2: participating in the general expansion without large additional motions of
33:06
Speaker 2: their own. If the scale factor doubles, then the proper
33:10
Speaker 2: distance between those galaxies also doubles. If it becomes three
33:14
Speaker 2: times larger, their separation becomes three times larger. The galaxies
33:18
Speaker 2: themselves have not necessarily travelled through all of that intervening space,
33:23
Speaker 2: in the way a car travels down a road. Instead,
33:27
Speaker 2: the geometry defining the distance between their locations has changed.
33:32
Speaker 2: This is the reason people often say that space itself
33:34
Speaker 2: is stretching. It is a useful phrase. But like most
33:38
Speaker 2: phrases involving the entire universe, it can become dangerous if
33:42
Speaker 2: imagined too literally. Space is not known to be made
33:46
Speaker 2: from elastic fabric. There is no rubber-like substance between galaxies
33:50
Speaker 2: becoming thinner as it is pulled. Nothing has to grab
33:54
Speaker 2: the edges of the universe and tug. The phrase simply
33:57
Speaker 2: gives us an intuitive way to describe a mathematical fact.
34:01
Speaker 2: The large-scale geometry of the universe evolves, and as it does,
34:05
Speaker 2: the distances between suitably separated come-oving regions can increase. Once
34:11
Speaker 2: that idea is accepted, another common question appears almost immediately.
34:16
Speaker 2: If space is expanding, why are you not expanding with it?
34:19
Speaker 2: Why does a ruler remain the same length? Why does
34:22
Speaker 2: Earth not slowly grow larger? Why does the solar system
34:26
Speaker 2: not widen every year as the universe expands? The answer
34:30
Speaker 2: is that cosmic expansion does not overwhelm every other physical interaction.
34:35
Speaker 2: At small scales, other forces dominate. An atom is held
34:39
Speaker 2: together primarily by the electromagnetic interaction between its negatively charged
34:44
Speaker 2: electrons and positively charged nucleus. The electromagnetic forces involved are
34:49
Speaker 2: enormously stronger than the incredibly slight cosmological effect across something
34:54
Speaker 2: as small as an atom. So atoms do not simply
34:58
Speaker 2: swell with the universe. The same is true of your body.
35:02
Speaker 2: The atoms and molecules making up a person are bound
35:04
Speaker 2: by electromagnetic forces. skin, water, proteins and every other part
35:12
Speaker 2: of the body are maintained by interactions that completely dominate
35:15
Speaker 2: over cosmic expansion at human scales. There is no detectable
35:20
Speaker 2: tendency for your height to increase, because distant galaxies are receding.
35:25
Speaker 2: This is probably fortunate, particularly for anyone who has recently
35:29
Speaker 2: bought trousers. Gravity plays the same role on larger scales.
35:33
Speaker 2: Earth remains gravitationally bound to the Sun, that planets remain
35:37
Speaker 2: within the solar system. Stars remain bound within the Milky Way.
35:42
Speaker 2: The Milky Way and the Andromeda Galaxy are gravitationally interacting
35:46
Speaker 2: strongly enough that they are moving towards one another despite
35:49
Speaker 2: the overall expansion of the universe. Even many groups and
35:53
Speaker 2: clusters of galaxies remain bound together. Cosmic expansion becomes important
35:58
Speaker 2: when we examine sufficiently large distances where systems are no
36:02
Speaker 2: longer tightly held together by local gravity. then the general
36:07
Speaker 2: expansion of the universe becomes increasingly apparent. Cosmologists describe these
36:12
Speaker 2: enormous distances in more than one way. One useful idea
36:16
Speaker 2: is proper distance. Proper distance is, roughly speaking, the physical
36:22
Speaker 2: separation between two locations at a particular cosmic time. If
36:26
Speaker 2: two distant co-moving galaxies remain fixed in their co-moving coordinates
36:30
Speaker 2: while the universe expands, their proper distance increases. co-moving distance
36:36
Speaker 2: works differently. It is designed to factor out the expansion.
36:41
Speaker 2: Imagine drawing an enormous coordinate grid across the universe and
36:45
Speaker 2: allowing that grid to expand along with the general cosmic flow.
36:49
Speaker 2: A galaxy that simply participates in the expansion can remain
36:53
Speaker 2: at roughly the same co-moving coordinate even though its proper
36:57
Speaker 2: distance from another galaxy grows enormously. Cummoving distance, therefore, gives
37:03
Speaker 2: cosmologists a convenient way to label locations, while separating the
37:07
Speaker 2: overall expansion from ordinary local motion. It is a little
37:11
Speaker 2: like marking two raisins inside a loaf before the dough rises.
37:15
Speaker 2: The actual distance between the raisins increases as the dough expands,
37:20
Speaker 2: but each raisin can remain associated with the same place
37:23
Speaker 2: in a coordinate system that expands with the dough. The
37:26
Speaker 2: analogy is imperfect, because the universe is not a loaf.
37:30
Speaker 2: and does not need an oven, a baker, or a
37:33
Speaker 2: kitchen surrounding it. But the distinction between changing physical separation
37:39
Speaker 2: and fixed, commoving labels is extremely useful. Behind all of
37:44
Speaker 2: this sits the mathematics of general relativity. When Alexander Friedman
37:49
Speaker 2: applied Albert Einstein's equations to a universe that was broadly
37:53
Speaker 2: homogeneous and isotropic, he found equations describing how the cosmic
37:57
Speaker 2: scale factor could change with time These became known as
38:01
Speaker 2: the Friedmann equations. Their importance is difficult to exaggerate. They
38:06
Speaker 2: connect the expansion of the universe with what the universe contains.
38:10
Speaker 2: Matter affects the evolution. Radiation affects it. Spatial curvature can
38:16
Speaker 2: affect it. A cosmological constant or another component behaving in
38:21
Speaker 2: a similar way can affect it as well. The expansion
38:25
Speaker 2: rate is therefore not something added artificially to cosmology. It
38:29
Speaker 2: emerges from the relationship between space-time and its contents, and
38:34
Speaker 2: the universe does not have to expand at the same
38:37
Speaker 2: rate forever. At different periods in cosmic history, different forms
38:41
Speaker 2: of energy have dominated the behaviour of the expansion. Radiation
38:46
Speaker 2: was especially important in the very early universe. Matter later
38:50
Speaker 2: became dominant. In the comparatively recent cosmic past, the expansion
38:55
Speaker 2: began accelerating. a phenomenon associated with what cosmologists call dark energy.
39:01
Speaker 2: So cosmic expansion has a history of its own. That
39:05
Speaker 2: history is very different from what we would expect from
39:07
Speaker 2: debris thrown outward by an explosion. Fragments from a conventional
39:12
Speaker 2: blast move through pre-existing space. Their trajectories begin at a
39:16
Speaker 2: particular location. Their speeds are determined by forces acting during
39:21
Speaker 2: and after the explosion. Cosmological expansion is not built around
39:26
Speaker 2: a central launch point. It is described through an evolving
39:29
Speaker 2: scale factor applying throughout the large-scale universe. This is the
39:34
Speaker 2: conceptual reversal that makes the Big Bang so easy to misunderstand.
39:40
Speaker 2: The familiar picture starts with an object and then imagines
39:44
Speaker 2: its contents being hurled outward. Modern cosmology starts with space-time itself.
39:50
Speaker 2: Its geometry changes. As it changes, the separations between distant,
39:56
Speaker 2: unbound regions change with it. Galaxies can therefore grow farther
40:01
Speaker 2: apart without having been fired from one central location. There
40:05
Speaker 2: need not be an explosion site. There need not be
40:08
Speaker 2: a surrounding empty arena. There need not even be an
40:12
Speaker 2: outside into which everything is moving. The Big Bang was
40:16
Speaker 2: not a moment when matter was given enormous outward speeds
40:20
Speaker 2: and released into an already waiting universe. It was an
40:24
Speaker 2: early stage of the universe's own evolution, and what expanded
40:28
Speaker 2: was not a cloud inside space. It was the large-scale
40:31
Speaker 2: pattern of distances within the cosmos itself. If everything in
40:36
Speaker 2: the universe seems to be moving apart, there is an
40:39
Speaker 2: almost irresistible question to ask. Where is the centre? After all,
40:45
Speaker 2: That is how ordinary expansion usually works in everyday life.
40:49
Speaker 2: If an explosion sends fragments flying across a field, we
40:53
Speaker 2: can trace their paths backwards and find the place where
40:56
Speaker 2: the explosion occurred. If ripples spread across a pond, there
41:00
Speaker 2: is a point where the disturbance began. If people walk
41:04
Speaker 2: outward in every direction from a town square, the square
41:08
Speaker 2: sits rather conveniently in the middle. So when astronomers discovered
41:12
Speaker 2: that distant galaxies generally appear to be receding from us,
41:16
Speaker 2: it was natural to wonder whether somewhere in the universe
41:19
Speaker 2: there ought to be an equivalent cosmic starting point. Perhaps,
41:23
Speaker 2: rather suspiciously, it might even be somewhere near us. It
41:27
Speaker 2: is not. As far as modern cosmology can determine, there
41:31
Speaker 2: is no central galaxy from which the rest of the
41:35
Speaker 2: and no location in ordinary three-dimensional space that can be
41:39
Speaker 2: identified as the centre of the Big Bang. The reason
41:42
Speaker 2: becomes clearer when we introduce one of the basic ideas
41:46
Speaker 2: used in cosmology, known as the cosmological principle. On relatively
41:51
Speaker 2: small scales, the universe is obviously uneven. You're sitting somewhere
41:56
Speaker 2: on or near the surface of a planet. That planet
41:59
Speaker 2: circles a star. The star belongs to the Milky Way,
42:03
Speaker 2: which contains hundreds of billions of stars. Galaxies gather into groups,
42:09
Speaker 2: clusters and enormous filamentary structures separated by immense cosmic voids,
42:15
Speaker 2: so the universe is certainly not perfectly uniform everywhere we look.
42:20
Speaker 2: But when astronomers average over sufficiently enormous distances, a simpler
42:25
Speaker 2: pattern appears. The cosmological principle says that on these very
42:29
Speaker 2: large scales the universe is approximately homogeneous and isotropic. Homogeneous
42:35
Speaker 2: means that no sufficiently large region should be fundamentally privileged
42:39
Speaker 2: over another. The average properties of the cosmos should be
42:43
Speaker 2: broadly similar from place to place. Isotropic means that, again
42:47
Speaker 2: on sufficiently large scales, the universe should look broadly similar
42:52
Speaker 2: in every direction. Observations of the distribution of galaxies And, especially,
42:58
Speaker 2: the remarkable uniformity of the cosmic microwave background support this
43:02
Speaker 2: general picture. And once the universe is approximately homogeneous, a
43:08
Speaker 2: central point becomes unnecessary. Imagine a perfectly straight line containing
43:13
Speaker 2: a series of evenly spaced dots. Perhaps the dots begin
43:17
Speaker 2: one centimetre apart. Now imagine that every separation doubles. The
43:23
Speaker 2: first dot and the second dot are now two centimetres apart.
43:27
Speaker 2: The second and third are also two centimetres apart. Dots
43:31
Speaker 2: that originally had several spaces between them have separated by
43:35
Speaker 2: correspondingly larger amounts. Choose any one of those dots and
43:39
Speaker 2: pretend that you live there. From your perspective, every other
43:44
Speaker 2: dot is moving away. The nearest dots move away relatively slowly.
43:49
Speaker 2: Dots twice as far away separate from you twice as quickly.
43:52
Speaker 2: More distant dots show still greater recession. That sounds remarkably
43:57
Speaker 2: similar to the large-scale pattern astronomers observe among galaxies. But
44:02
Speaker 2: notice something important. Nothing in the arrangement identifies your dot
44:07
Speaker 2: as the centre. Move to another dot and repeat the measurement.
44:12
Speaker 2: From there the other dots also appear to be receding.
44:15
Speaker 2: Every observer sees the same basic pattern. The expansion has
44:19
Speaker 2: no preferred dot. A three-dimensional universe is more complicated than
44:24
Speaker 2: a row of marks on a page, of course, but
44:27
Speaker 2: the underlying idea remains the same. If distances between commuting
44:31
Speaker 2: locations increase everywhere in a homogeneous way, every observer taking
44:36
Speaker 2: part in that expansion can see distant regions moving away
44:40
Speaker 2: without any observer occupying a special central position. This is
44:46
Speaker 2: where the explosion picture fails particularly badly. In a conventional explosion,
44:51
Speaker 2: there really is a centre. Fragments close to the blast
44:54
Speaker 2: and fragments far from it occupy different positions relative to
44:59
Speaker 2: that centre. If you were riding on one fragment, careful
45:03
Speaker 2: measurements of all the other fragments could eventually reveal where
45:06
Speaker 2: the explosion had happened. Homogeneous cosmic expansion does not work
45:12
Speaker 2: that way. There is no corresponding location to recover. From
45:16
Speaker 2: the Milky Way, we look outward and see the large-scale
45:19
Speaker 2: universe surrounding us in every direction. This can create a
45:23
Speaker 2: peculiar impression that we are sitting in the middle of everything.
45:27
Speaker 2: But that impression is unavoidable wherever an observer happens to be.
45:32
Speaker 2: Imagine an astronomer in a distant galaxy several billion light-years away.
45:38
Speaker 2: Ignoring the complications introduced by looking back through cosmic time,
45:42
Speaker 2: that astronomer would also regard their own location, As the
45:46
Speaker 2: centre of what they can observe, their telescopes would point
45:50
Speaker 2: outward in every direction, their observable universe would surround them,
45:55
Speaker 2: and we would appear somewhere in it. This introduces an
46:00
Speaker 2: important distinction between the universe as a whole and the
46:03
Speaker 2: observable universe. The observable universe is the region from which
46:08
Speaker 2: signals have had enough time to reach a particular observer
46:11
Speaker 2: during cosmic history. Because light travels at a finite speed,
46:16
Speaker 2: and the universe has a finite age in the standard
46:19
Speaker 2: cosmological model, we cannot receive information from arbitrarily distant places.
46:25
Speaker 2: That naturally places us at the centre of our observable universe.
46:30
Speaker 2: But this is only an observer-dependent centre. It is not
46:34
Speaker 2: a physical centre of cosmic expansion. An observer in another
46:38
Speaker 2: galaxy has their own observable universe centred on them. These
46:42
Speaker 2: observable regions overlap enormously, if the observers are not too
46:47
Speaker 2: far apart, but their centres are different simply because the
46:50
Speaker 2: observers occupy different locations. There is nothing cosmically special about
46:55
Speaker 2: either one. This is rather like standing in fog. You
46:59
Speaker 2: appear to be at the centre of the visible patch
47:02
Speaker 2: of landscape because visibility extends a certain distance around you.
47:07
Speaker 2: Walk somewhere else and the visible patch moves with you.
47:11
Speaker 2: Nobody would conclude that the fog itself had been created
47:15
Speaker 2: at your original position, merely because you happened to be
47:18
Speaker 2: standing at the centre of what you could see. The
47:21
Speaker 2: Milky Way's position in the observable cosmos is similar in spirit.
47:26
Speaker 2: We see outward from here because here is where we
47:28
Speaker 2: happen to be. This absence of a cosmic centre should
47:32
Speaker 2: not be confused with saying that nothing in the universe
47:35
Speaker 2: has a centre. Earth certainly has one. The Sun has one.
47:41
Speaker 2: The Milky Way has a central region containing a supermassive
47:44
Speaker 2: black hole called Sagittarius A-star. Galaxy clusters can have identifiable
47:50
Speaker 2: centres defined in various useful ways. These are objects occupying
47:55
Speaker 2: regions within the universe, and objects can possess centres. The
48:00
Speaker 2: large-scale expansion of the universe is a different kind of thing.
48:04
Speaker 2: It is not an object sitting somewhere inside a larger space.
48:08
Speaker 2: so asking where its centre is may be asking for
48:10
Speaker 2: something the geometry simply does not contain. This also tells
48:15
Speaker 2: us something important about the Big Bang itself. If cosmic
48:19
Speaker 2: expansion has no preferred central location today, running the expansion
48:24
Speaker 2: backwards does not cause all galaxies to converge upon one
48:27
Speaker 2: particular spot somewhere beyond the Milky Way. Instead, the separations
48:33
Speaker 2: between co-moving regions become smaller everywhere. The early universe becomes
48:38
Speaker 2: denser everywhere. It becomes hotter everywhere. Every region we observe
48:44
Speaker 2: today traces its history back into those earlier conditions. The
48:48
Speaker 2: Big Bang, therefore, did not happen in the middle of
48:51
Speaker 2: the universe. It happened, in the cosmological sense, throughout the
48:55
Speaker 2: region that became the observable universe. The centre is not
48:59
Speaker 2: hidden somewhere among distant galaxies. It is not beyond the
49:03
Speaker 2: edge of our telescopes. And it is certainly not conveniently
49:06
Speaker 2: located near Earth. There simply is no known central point
49:10
Speaker 2: from which everything else was thrown outward. Every observer sees
49:14
Speaker 2: the cosmos from the middle of their own view. But
49:17
Speaker 2: no galaxy gets to claim the middle of the universe.
49:20
Speaker 2: universe itself. Cosmology has a habit of reaching for household
49:24
Speaker 2: objects when the universe becomes difficult to picture. One of
49:29
Speaker 2: the most famous examples is the balloon. Imagine taking an
49:33
Speaker 2: ordinary balloon before it has been inflated and drawing several
49:36
Speaker 2: small dots across its Each dot can represent a galaxy.
49:41
Speaker 2: The rubber surface represents space, at least for the purpose
49:44
Speaker 2: of the analogy. Now slowly inflate the balloon. As the
49:49
Speaker 2: surface expands, every dot becomes farther away from every other dot.
49:54
Speaker 2: A dot does not need to crawl across the rubber
49:56
Speaker 2: by itself. The distance grows because the surface between the
50:00
Speaker 2: dots is expanding. Choose one dot and pretend that you
50:04
Speaker 2: live there. Every other dot appears to move away from you.
50:08
Speaker 2: Nearby dots separate relatively slowly. Dots that are farther away
50:13
Speaker 2: tend to gain distance more quickly because there is more
50:16
Speaker 2: expanding surface between you and them. Move to another dot
50:20
Speaker 2: and repeat the experiment. Exactly the same thing happens. From
50:25
Speaker 2: its point of view, all the other dots also move away.
50:29
Speaker 2: There is no privileged dot on the surface from which
50:32
Speaker 2: the expansion begins. That is why the balloon analogy is
50:36
Speaker 2: so popular. It captures several important features of cosmic expansion
50:40
Speaker 2: with very little machinery. No dot is the centre. No
50:45
Speaker 2: dot has to be firing itself across the surface. And
50:49
Speaker 2: every observer sees something broadly similar. There is an especially
50:53
Speaker 2: useful trick hidden inside the example. Imagine creatures that can
50:58
Speaker 2: experience only the two-dimensional surface of the balloon. They can
51:02
Speaker 2: move left and right across it, or forward and backward
51:05
Speaker 2: across it, but they have no access to the third
51:08
Speaker 2: dimension outside the rubber. For them, the surface is their
51:12
Speaker 2: entire spatial world. As their universe expands, they can measure
51:18
Speaker 2: increasing distances between locations. They can observe other dots moving away.
51:23
Speaker 2: They can even discover mathematical rules describing the expansion. But
51:29
Speaker 2: nowhere on their two-dimensional universe would they find a centre.
51:33
Speaker 2: The centre of the physical balloon exists only in the
51:36
Speaker 2: extra dimension that we, standing outside the analogy, can see.
51:41
Speaker 2: And that is also where the analogy begins causing trouble.
51:45
Speaker 2: A real balloon does have a centre. It sits in
51:48
Speaker 2: ordinary three-dimensional space. It begins at one size, expands outward,
51:53
Speaker 2: and occupies a larger region of the room as air
51:56
Speaker 2: is blown into it. If we take those features literally
51:59
Speaker 2: … We immediately recreate several of the misconceptions we were
52:03
Speaker 2: trying to escape. We might imagine the universe as the
52:07
Speaker 2: skin of some gigantic balloon. Then we naturally wonder what
52:11
Speaker 2: exists inside it. We ask what surrounds it. We might
52:15
Speaker 2: picture the whole thing swelling into some larger emptiness. None
52:19
Speaker 2: of those conclusions is required by modern cosmology. The useful
52:24
Speaker 2: part of the balloon is the changing separation across its surface.
52:28
Speaker 2: The surrounding room is not part of the lesson, neither
52:31
Speaker 2: is the balloon centre. Cosmological space does not have to
52:36
Speaker 2: be embedded in a higher-dimensional room simply because a classroom
52:39
Speaker 2: balloon happens to be. The mathematics of general relativity allows
52:44
Speaker 2: the geometry of space to be described intrinsically. That means
52:48
Speaker 2: inhabitants can measure distances, curvature and expansion from within their
52:53
Speaker 2: own space. without requiring some ordinary external space for it
52:57
Speaker 2: to sit inside. This is one of those moments when
53:01
Speaker 2: an analogy works beautifully right up until it does not.
53:05
Speaker 2: Another favourite involves food. Imagine a lump of bread dough
53:09
Speaker 2: containing raisins. Before the dough rises, the raisins are relatively
53:14
Speaker 2: close together. As yeast produces gas and the dough expands,
53:19
Speaker 2: the raisins become more widely separated. Once again, the raisins
53:23
Speaker 2: do not have to propel themselves through the dough. They
53:26
Speaker 2: can remain embedded in roughly the same local region of
53:29
Speaker 2: the mixture while increasing amounts of dough appear between them.
53:33
Speaker 2: From the viewpoint of one raisin, the others become farther away.
53:36
Speaker 2: A raisin twice as distant has roughly twice as much
53:40
Speaker 2: expanding dough between it and our chosen raisin, so its
53:45
Speaker 2: separation can increase more quickly. The analogy resembles the large-scale
53:50
Speaker 2: behaviour captured by the Hubble-Lametta law. And unlike the balloon surface,
53:55
Speaker 2: the bread provides us with a three-dimensional picture, which can
53:59
Speaker 2: sometimes feel more intuitive. Unfortunately, bread also has a habit
54:04
Speaker 2: of becoming too literal. A loaf possesses an edge. It
54:08
Speaker 2: has a centre. It sits on a tray or inside
54:11
Speaker 2: an oven. It expands into the surrounding air. Its dough
54:17
Speaker 2: is made of ordinary matter, physically changing shape because of
54:21
Speaker 2: gas bubbles and chemical processes. The universe does not have
54:25
Speaker 2: to share any of those properties. If we ask which
54:28
Speaker 2: part of the kitchen represents the space outside the universe,
54:32
Speaker 2: the analogy has already been pushed farther than it was
54:35
Speaker 2: designed to go. The baker is not a cosmological constant.
54:40
Speaker 2: The oven is not a higher-dimensional container. And despite what
54:43
Speaker 2: an especially ambitious raisin might believe, The edge of the
54:47
Speaker 2: loaf is not evidence for the edge of reality. Analogies
54:51
Speaker 2: are useful because they preserve selected relationships while throwing away
54:54
Speaker 2: everything else. A map of a city is useful because
54:58
Speaker 2: the streets have approximately the correct arrangement. Nobody expects the
55:02
Speaker 2: paper to contain full-sized buildings. In the same way, the
55:07
Speaker 2: balloon and raisin bread pictures preserve one crucial idea. Distances
55:12
Speaker 2: between many locations can increase together without those locations being
55:17
Speaker 2: launched outward from a common explosion point. That is all
55:20
Speaker 2: they really need to do. Neither analogy tells us the
55:24
Speaker 2: exact global shape of the universe. Neither proves that the
55:28
Speaker 2: universe is finite. Neither tells us whether space has an edge.
55:32
Speaker 2: And neither demonstrates that our three-dimensional space must be expanding
55:36
Speaker 2: into some larger external realm. Modern observations indicate that the
55:41
Speaker 2: observable universe is extremely close to spatially flat on very
55:45
Speaker 2: large scales. But spatial flatness does not by itself tell
55:50
Speaker 2: us whether the entire universe is infinite or finite with
55:54
Speaker 2: a more complicated topology. Those are separate questions. The important
56:00
Speaker 2: point is that cosmic expansion can be described from within
56:03
Speaker 2: the universe itself. It does not require a surrounding void.
56:08
Speaker 2: When cosmologists say that distances increase as the scale factor grows,
56:12
Speaker 2: They are describing a relationship within spacetime. There is no
56:16
Speaker 2: need to add another ordinary space outside it, merely to
56:19
Speaker 2: provide somewhere for the universe to go. This is why
56:23
Speaker 2: the simplest question, what is the universe expanding into, may
56:28
Speaker 2: carry an assumption that does not belong in the physics.
56:32
Speaker 2: A balloon expands into a room, bread expands into an oven.
56:36
Speaker 2: The universe is not known to need the equivalent of either.
56:40
Speaker 2: Its large-scale geometry can change while remaining all the space
56:44
Speaker 2: there is in the model. So balloons and raisins remain
56:48
Speaker 2: useful companions, provided they are treated gently. They can help
56:52
Speaker 2: us picture increasing separation. They can show why every galaxy
56:57
Speaker 2: does not require a central starting point. And they can
57:00
Speaker 2: rescue us from the image of cosmic debris being fired
57:03
Speaker 2: through empty space. But eventually, both analogies have to be
57:08
Speaker 2: left behind. The universe is not a balloon, it is
57:12
Speaker 2: not a loaf of bread, and its expansion does not
57:15
Speaker 2: require anything outside it to make room. There is a
57:19
Speaker 2: sentence in cosmology that sounds almost deliberately confusing the first
57:24
Speaker 2: time you hear it. The Big Bang happened everywhere. Not
57:29
Speaker 2: everywhere in the sense that fragments from an explosion eventually
57:33
Speaker 2: travelled everywhere. Not everywhere because a blast expanded until it
57:37
Speaker 2: filled the universe. Everywhere, because every region of the observable
57:41
Speaker 2: universe traces its history back into the same, much hotter
57:45
Speaker 2: and denser, early cosmic state. That includes the region occupied
57:49
Speaker 2: by Earth. If we could somehow follow the matter, making
57:53
Speaker 2: up the solar system backwards through cosmic history, the familiar
57:57
Speaker 2: structure around us would gradually disappear. Earth would cease to exist.
58:02
Speaker 2: The planets would no longer be separate worlds. The Sun
58:05
Speaker 2: would not yet have formed. Go farther back, and the
58:10
Speaker 2: cloud of gas and dust from which the solar system
58:13
Speaker 2: eventually emerged would no longer exist as a distinct object.
58:17
Speaker 2: Continue backwards, and even the Milky Way would lose its
58:20
Speaker 2: familiar shape. Its stars would not yet have formed. The
58:24
Speaker 2: material that would one day become our galaxy would be
58:27
Speaker 2: spread through a much younger universe. Keep going, and conditions
58:32
Speaker 2: become hotter, denser, and increasingly unlike anything we experience today.
58:38
Speaker 2: Eventually we reach epochs before stars existed at all, before galaxies,
58:43
Speaker 2: before neutral atoms, and much earlier still, before ordinary atomic
58:48
Speaker 2: nuclei could remain intact. At no point during this backwards
58:52
Speaker 2: journey do we have to imagine travelling towards some distant
58:56
Speaker 2: cosmic location where the Big Bang occurred. We are not
58:59
Speaker 2: moving closer to an explosion site. We are moving backwards
59:03
Speaker 2: in time. That difference matters. The place where you are
59:07
Speaker 2: now is not 13.8 billion light years away from the
59:11
Speaker 2: Big Bang. In a meaningful cosmological sense, this place was
59:15
Speaker 2: involved in it. The region now occupied by your room,
59:18
Speaker 2: your town, Earth and the wider solar system was once
59:23
Speaker 2: part of that ancient hot and dense universe. Of course,
59:27
Speaker 2: the exact matter sitting around you today has travelled. mixed,
59:31
Speaker 2: collapsed into stars, been expelled from stars and rearranged many
59:36
Speaker 2: times over cosmic history. The point is not that one
59:40
Speaker 2: neat parcel of material stayed perfectly fixed for billions of years.
59:45
Speaker 2: The point is that the spatial region from which our
59:48
Speaker 2: part of the cosmos developed belongs to the same expanding
59:51
Speaker 2: history as everything else we can observe. Now imagine travelling
59:55
Speaker 2: billions of light years away to another galaxy. The same
59:59
Speaker 2: argument applies. Trace that galaxy backwards and it too becomes
1:00:04
Speaker 2: part of a denser and hotter cosmic environment. Move to
1:00:08
Speaker 2: another distant region and repeat the process. Again, the same
1:00:12
Speaker 2: thing happens. Every observable region leads backwards towards earlier conditions
1:00:18
Speaker 2: in which the characteristic distances separating matter were much smaller.
1:00:23
Speaker 2: There is no single location that gets special treatment. This
1:00:27
Speaker 2: is why saying that the Big Bang happened everywhere is
1:00:30
Speaker 2: more accurate than saying it happened somewhere. It also leads
1:00:34
Speaker 2: to another question that is almost impossible to resist. If
1:00:38
Speaker 2: the universe expanded, what did it expand into? Everyday experience
1:00:43
Speaker 2: makes that question feel completely reasonable. A balloon expands into
1:00:47
Speaker 2: the air around it. Smoke expands into a room. A
1:00:50
Speaker 2: cloud of gas expands into surrounding space. Something gets larger.
1:00:55
Speaker 2: so surely there must be somewhere larger for it to occupy.
1:00:59
Speaker 2: But standard cosmology does not require the universe to sit
1:01:03
Speaker 2: inside an external empty container. There does not have to
1:01:07
Speaker 2: be a greater ordinary space surrounding our own, waiting for
1:01:10
Speaker 2: the universe to grow into it. Cosmic expansion is described
1:01:15
Speaker 2: through changing distances within the universe itself. As the cosmological
1:01:20
Speaker 2: scale factor changes, widely separated co-moving regions can become farther apart.
1:01:26
Speaker 2: The model works without introducing an outside that does not
1:01:30
Speaker 2: prove that no larger structure of any kind could possibly exist.
1:01:35
Speaker 2: Some speculative theories consider additional dimensions, multiple universes or more
1:01:41
Speaker 2: elaborate cosmic structures. But none of those ideas is required
1:01:45
Speaker 2: simply to explain ordinary cosmic expansion. The standard model does
1:01:51
Speaker 2: not need a dark room surrounding the universe. And this
1:01:54
Speaker 2: is where another important distinction becomes useful. The observable universe
1:01:59
Speaker 2: is not necessarily the same thing as the entire universe.
1:02:03
Speaker 2: The observable universe is simply the region from which light
1:02:06
Speaker 2: or other signals have been able to reach us during
1:02:09
Speaker 2: cosmic history. Because the universe has a finite age and
1:02:13
Speaker 2: light travels at a finite speed, there is a limit
1:02:16
Speaker 2: to how far away we can presently receive information from
1:02:20
Speaker 2: That limit does not form a wall. There is no
1:02:23
Speaker 2: shell at the edge of the observable universe where space
1:02:26
Speaker 2: suddenly stops. If you could somehow travel there, you would
1:02:31
Speaker 2: not expect to find a sign saying that the universe
1:02:33
Speaker 2: has ended and please do not lean over the barrier.
1:02:37
Speaker 2: An observer in a distant galaxy has their own observable
1:02:41
Speaker 2: universe centred on their location. Some regions visible to them
1:02:45
Speaker 2: may lie beyond what is visible to us. Our observable
1:02:49
Speaker 2: universe is therefore an observer-dependent region, not necessarily the whole
1:02:54
Speaker 2: of reality. The complete universe may extend vastly farther. It
1:02:59
Speaker 2: may even be spatially infinite. At present, observations cannot tell
1:03:06
Speaker 2: us with certainty whether the total universe is infinite or finite.
1:03:10
Speaker 2: Measurements indicate that space is extremely close to geometrically flat
1:03:15
Speaker 2: on the larger scales we can observe. An infinite flat
1:03:18
Speaker 2: universe is one possibility, but certain finite geometries or topologies
1:03:23
Speaker 2: can also be compatible with a universe that appears locally
1:03:27
Speaker 2: and observationally very close to flat. And even if the
1:03:31
Speaker 2: universe were finite, finite does not automatically mean it must
1:03:35
Speaker 2: have an edge. The surface of Earth offers a simple comparison.
1:03:40
Speaker 2: Earth's surface has a finite area. You can travel across it.
1:03:45
Speaker 2: You can measure distances upon it. Yet if you keep
1:03:48
Speaker 2: walking in one direction, there is no point where the
1:03:51
Speaker 2: surface suddenly runs out and you tumble into some geographical nothingness.
1:03:57
Speaker 2: The surface is finite but unbounded. A three-dimensional universe could,
1:04:02
Speaker 2: in principle, possess an analogous kind of geometry. The comparison
1:04:07
Speaker 2: should not be taken as proof that our universe has
1:04:09
Speaker 2: that shape. It simply demonstrates that the words finite and
1:04:13
Speaker 2: edge do not mean the same thing. All of this
1:04:16
Speaker 2: is difficult to picture because human intuition evolved inside a
1:04:19
Speaker 2: world of objects contained within larger surroundings. A cup sits
1:04:24
Speaker 2: on a table. A table sits in a room. A
1:04:27
Speaker 2: room sits inside a building. Eventually our instincts expect everything
1:04:32
Speaker 2: to be inside something else. Cosmology may not give us
1:04:36
Speaker 2: that comfort. The universe, by definition in the usual cosmological sense,
1:04:42
Speaker 2: includes the space whose behaviour we're describing. So asking what
1:04:46
Speaker 2: ordinary space lies outside all space may not have a
1:04:50
Speaker 2: meaningful answer. And once that idea settles in, the Big
1:04:54
Speaker 2: Bang looks less and less like an explosion. There was
1:04:57
Speaker 2: no known central point. There was no shell of debris
1:05:01
Speaker 2: expanding away from one location. There was no required external
1:05:05
Speaker 2: void waiting to receive the expanding universe. The early hot
1:05:09
Speaker 2: and dense state existed throughout the region that developed into
1:05:12
Speaker 2: everything we can now observe. The place occupied by Earth
1:05:16
Speaker 2: was part of it. The place occupied by the most
1:05:19
Speaker 2: distant observable galaxy was part of it. Every direction we
1:05:23
Speaker 2: look carries us not towards the location of the Big Bang,
1:05:27
Speaker 2: but backwards through different parts of the same cosmic history.
1:05:31
Speaker 2: The Big Bang was not somewhere far away. In the
1:05:35
Speaker 2: deepest cosmological sense somewhere is the wrong idea. It happened everywhere.
1:05:42
Speaker 2: The second half of the name is almost as misleading
1:05:44
Speaker 2: as the first. If there was a big bang, surely
1:05:47
Speaker 2: there must have been a bang. That word immediately suggests sound,
1:05:52
Speaker 2: a violent crack, a deep boom, perhaps the loudest noise imaginable,
1:05:57
Speaker 2: travelling outward across the newborn universe. But ordinary sound does
1:06:02
Speaker 2: not work that way. Sound is a mechanical disturbance travelling
1:06:07
Speaker 2: through a material medium. When something explodes in Earth's atmosphere,
1:06:11
Speaker 2: rapidly expanding gas pushes against the surrounding air. Molecules are compressed,
1:06:17
Speaker 2: then displaced, creating a travelling pressure wave. When that pressure
1:06:22
Speaker 2: wave eventually reaches your ear, it causes your eardrum to vibrate.
1:06:27
Speaker 2: Your nervous system then turns those vibrations into the experience
1:06:31
Speaker 2: we call sound. The same basic physics works in water
1:06:34
Speaker 2: and in solids, although the details and speeds are different.
1:06:39
Speaker 2: What sound cannot do is propagate through completely empty space.
1:06:43
Speaker 2: There must be something capable of carrying the pressure disturbance.
1:06:47
Speaker 2: This is why films involving spaceships occasionally take a little
1:06:51
Speaker 2: artistic license. A spectacular explosion occurring in a near-perfect vacuum
1:06:56
Speaker 2: would not send an ordinary acoustic boom across empty space
1:07:00
Speaker 2: to another spacecraft. The Big Bang creates an even deeper
1:07:03
Speaker 2: problem for the familiar picture. There was no known atmosphere
1:07:07
Speaker 2: surrounding the universe. There was no external ocean, gas or
1:07:11
Speaker 2: other material medium waiting outside everything through which a gigantic
1:07:15
Speaker 2: cosmic sound could travel. And since modern cosmology does not
1:07:20
Speaker 2: describe the Big Bang as an explosion occurring at one location,
1:07:23
Speaker 2: there was no central detonation sending a pressure wave outward
1:07:27
Speaker 2: into a surrounding void in the first place. So if
1:07:30
Speaker 2: by bang we mean the sort of sound produced by
1:07:32
Speaker 2: an explosion on Earth, The universe did not begin with
1:07:36
Speaker 2: one enormous noise spreading away from a central point. But
1:07:40
Speaker 2: there is an important complication. It would also be misleading
1:07:43
Speaker 2: to say that the early universe was simply silent because
1:07:46
Speaker 2: sound like waves were impossible. The early universe contained plenty
1:07:50
Speaker 2: of material capable of supporting pressure disturbances. For hundreds of
1:07:56
Speaker 2: thousands of years after the earliest hot phases, ordinary matter
1:07:59
Speaker 2: existed largely as a hot ionised plasma. Electrons were not
1:08:03
Speaker 2: yet permanently attached to atomic nuclei. Photons interacted repeatedly with
1:08:09
Speaker 2: charged particles. Matter and radiation were tightly coupled together into
1:08:13
Speaker 2: a dense cosmic mixture. That mixture could support waves. Not
1:08:18
Speaker 2: sound travelling through an external atmosphere, but real pressure oscillations
1:08:23
Speaker 2: propagating through the material of the early universe itself. Gravity
1:08:28
Speaker 2: was constantly trying to pull slightly over-dense regions together. Where
1:08:33
Speaker 2: matter was a little more concentrated, gravitational attraction encouraged further compression.
1:08:38
Speaker 2: But the intense radiation mixed with that matter produced pressure
1:08:42
Speaker 2: resisting the collapse. The result was an enormous competition between
1:08:46
Speaker 2: gravity and pressure. Regions compressed. Pressure increased. The material pushed
1:08:52
Speaker 2: outward again. These disturbances travelled through the early plasma as
1:08:56
Speaker 2: acoustic waves. They are known today through a phenomenon called
1:09:00
Speaker 2: baryon acoustic oscillations. The word baryon refers to a class
1:09:05
Speaker 2: of particles that includes protons and neutrons, which make up
1:09:09
Speaker 2: most of the ordinary matter in atomic nuclei. Acoustic tells
1:09:13
Speaker 2: us that pressure waves are involved. An oscillation describes the
1:09:17
Speaker 2: repeated compressing and rarifying of the material. These waves did
1:09:21
Speaker 2: not originate from one explosive centre. They occurred throughout the
1:09:25
Speaker 2: early universe. Tiny differences in density, ultimately connected to primordial fluctuations,
1:09:32
Speaker 2: provided the initial unevenness. Gravity amplified those differences while pressure
1:09:38
Speaker 2: in the hot photon and matter fluid resisted compression. The
1:09:42
Speaker 2: result was a characteristic pattern of waves passing through the plasma.
1:09:47
Speaker 2: For an observer somehow placed within that early environment, The
1:09:51
Speaker 2: frequencies involved would not necessarily resemble anything a human ear
1:09:55
Speaker 2: would recognise as an ordinary audible noise. The physical meaning
1:10:00
Speaker 2: of sound is broader than human hearing. Humans detect only
1:10:04
Speaker 2: a limited range of pressure wave frequencies in air. A
1:10:08
Speaker 2: pressure oscillation can exist at frequencies far below or far
1:10:12
Speaker 2: above that range and still be governed by acoustic physics.
1:10:16
Speaker 2: So it is reasonable, with a proper qualification, to say
1:10:20
Speaker 2: that the early universe contained sound waves. It is not
1:10:24
Speaker 2: reasonable to imagine one enormous boom from the Big Bang.
1:10:28
Speaker 2: The distinction becomes especially interesting because the ancient acoustic waves
1:10:32
Speaker 2: did not vanish without leaving evidence. As the universe expanded
1:10:37
Speaker 2: and cooled, it eventually reached a stage roughly 380,000 years
1:10:42
Speaker 2: after the Hot Big Bang when electrons could remain bound
1:10:45
Speaker 2: to atomic nuclei. Neutral atoms formed in large numbers, Photons
1:10:50
Speaker 2: were then able to travel much more freely through space
1:10:53
Speaker 2: rather than continually scattering from charged particles. The close coupling
1:10:58
Speaker 2: between radiation and ordinary matter came to an end. The
1:11:02
Speaker 2: acoustic pattern effectively became frozen into the distribution of matter.
1:11:08
Speaker 2: Billions of years later, astronomers can still find its statistical imprint.
1:11:13
Speaker 2: Galaxies are not distributed completely randomly. When astronomers measure enormous
1:11:19
Speaker 2: numbers of them, there is a slightly enhanced probability of
1:11:22
Speaker 2: finding pairs separated by a particular characteristic distance. That preferred
1:11:27
Speaker 2: scale is a fossil of those ancient pressure waves. After
1:11:31
Speaker 2: cosmic expansion stretched it for billions of years, the characteristic
1:11:35
Speaker 2: scale is now roughly 500 million light-years in co-moving distance.
1:11:40
Speaker 2: The same underlying acoustic physics also left a striking signature
1:11:44
Speaker 2: in the cosmic microwave background. Tiny variations in the temperature
1:11:49
Speaker 2: of that ancient radiation form a pattern of peaks and
1:11:52
Speaker 2: troughs when analysed across different angular scales. Those acoustic peaks
1:11:57
Speaker 2: record the behaviour of the early photon and matter plasma.
1:12:01
Speaker 2: Some regions were compressed, others were rarefied. The timing of
1:12:06
Speaker 2: those oscillations depended on the contents and expansion history of
1:12:10
Speaker 2: the young universe. So one of the great ironies hidden
1:12:13
Speaker 2: inside the phrase Big Bang is that there really were
1:12:15
Speaker 2: acoustic phenomena in the early cosmos, just not the kind
1:12:19
Speaker 2: suggested by the name. There was no single noise racing
1:12:23
Speaker 2: away from the centre of creation. There was no external
1:12:27
Speaker 2: atmosphere carrying a shockwave across some pre-existing darkness. Instead, the
1:12:32
Speaker 2: early universe itself contained a hot interacting plasma, capable of
1:12:37
Speaker 2: supporting pressure waves throughout space. Those waves belong to the
1:12:41
Speaker 2: internal physics of the cosmos, and there is another difference
1:12:44
Speaker 2: between them and an ordinary explosion. A conventional blast often
1:12:49
Speaker 2: produces a shock front. This is a relatively sharp boundary
1:12:53
Speaker 2: separating violently disturbed material from the more undisturbed surroundings ahead
1:12:58
Speaker 2: of it. An expanding fireball can have an identifiable outer
1:13:02
Speaker 2: region moving through its environment. The Big Bang had no
1:13:07
Speaker 2: corresponding outer shock fronts sweeping across the universe. There was
1:13:11
Speaker 2: no boundary with hot Big Bang material on one side,
1:13:14
Speaker 2: an untouched empty space waiting on the other. The hot,
1:13:18
Speaker 2: dense conditions existed throughout the region that became the observable universe.
1:13:24
Speaker 2: Expansion and cooling happened throughout that region as well. So
1:13:28
Speaker 2: the word bang survives, mostly because Fred Hoyle happened upon
1:13:32
Speaker 2: a phrase too memorable to disappear. It gives us drama.
1:13:36
Speaker 2: It gives us a sound effect. It gives us an
1:13:38
Speaker 2: image of a blast. But the physics tells a calmer
1:13:41
Speaker 2: and much stranger story. No central detonation. No outward moving
1:13:46
Speaker 2: wall of debris. No thunder crossing an external void. There
1:13:50
Speaker 2: were pressure waves, certainly, but they were ripples within the
1:13:54
Speaker 2: young universe itself, and even those ancient ripples are still
1:13:58
Speaker 2: faintly written into the cosmos we see today. There is
1:14:02
Speaker 2: one consequence of cosmic expansion. that sounds, at first, as
1:14:07
Speaker 2: though it should be impossible. Some galaxies can recede from
1:14:10
Speaker 2: us faster than light. That statement tends to collide immediately
1:14:14
Speaker 2: with one of the best-known rules in modern physics. Albert
1:14:18
Speaker 2: Einstein's special theory of relativity tells us that the speed
1:14:21
Speaker 2: of light is the ultimate speed limit. Nothing carrying matter
1:14:25
Speaker 2: or information can simply accelerate through its local surroundings until
1:14:30
Speaker 2: it overtakes a beam of light. And that rule still holds.
1:14:35
Speaker 2: The apparent contradiction appears because two very different ideas are
1:14:38
Speaker 2: being described using the word speed. Special relativity places its
1:14:43
Speaker 2: restriction on local motion through spacetime. Imagine a spacecraft passing
1:14:48
Speaker 2: close to Earth. No matter how powerful its engines become,
1:14:52
Speaker 2: an observer beside it will never measure that spacecraft travelling
1:14:56
Speaker 2: locally through space faster than light. The same applies to particles.
1:15:01
Speaker 2: The same applies to signals. In any sufficiently small region
1:15:05
Speaker 2: where spacetime can be treated approximately as flat, light moves
1:15:10
Speaker 2: at the familiar universal speed, and massive objects cannot overtake it.
1:15:15
Speaker 2: Cosmological recession is different. When astronomers say that an extremely
1:15:20
Speaker 2: distant galaxy is receding faster than light, they are not
1:15:24
Speaker 2: necessarily saying that the galaxy is racing through its local
1:15:27
Speaker 2: patch of space at a forbidden velocity. They are describing
1:15:31
Speaker 2: how rapidly the proper distance between our location and that
1:15:35
Speaker 2: distant galaxy is changing as the large-scale geometry of the
1:15:39
Speaker 2: universe evolves. There can be billions of light-years of expanding
1:15:43
Speaker 2: space between us, and the total increase in that enormous
1:15:47
Speaker 2: distance can exceed the distance light travels during the same interval.
1:15:52
Speaker 2: Nothing nearby has broken Einstein's rule. This is one of
1:15:56
Speaker 2: the clearest demonstrations that cosmic expansion should not be imagined
1:16:00
Speaker 2: as ordinary motion through fixed space. The effect can be
1:16:05
Speaker 2: understood through the Hubble-Lemaître relationship. On sufficiently large scales, more
1:16:11
Speaker 2: distant co-moving galaxies generally have greater recession rates. The quantity
1:16:16
Speaker 2: describing the expansion at a particular cosmic time is called
1:16:19
Speaker 2: the Hubble parameter. At the present epoch, Its value tells
1:16:24
Speaker 2: us roughly how much additional recession is associated with each
1:16:28
Speaker 2: additional unit of cosmic distance. So, a relatively nearby galaxy
1:16:33
Speaker 2: participating in the general expansion has a modest recession rate.
1:16:37
Speaker 2: A galaxy twice as distant has roughly twice that recession rate.
1:16:41
Speaker 2: Continue far enough and eventually the calculated recession rate reaches
1:16:45
Speaker 2: the speed of light. Continue farther still and it exceeds it.
1:16:51
Speaker 2: The corresponding distance is associated with what cosmologists call the
1:16:54
Speaker 2: Hubble radius. At the present time, the Hubble radius is
1:16:58
Speaker 2: roughly 14 billion light years in proper distance, although its
1:17:03
Speaker 2: exact value depends on the measured expansion rate. Objects beyond
1:17:08
Speaker 2: that distance can have recession rates greater than the speed
1:17:11
Speaker 2: of light. That sounds as though their light should never
1:17:14
Speaker 2: be able to reach us. Strangely, that conclusion does not
1:17:18
Speaker 2: always follow. Imagine a photon emitted towards us by a
1:17:22
Speaker 2: galaxy beyond the Hubble radius. Locally, the photon still moves
1:17:26
Speaker 2: towards us at the speed of light. It is behaving
1:17:29
Speaker 2: perfectly normally. But at first, the space separating us from
1:17:33
Speaker 2: the photon may be increasing so rapidly that despite the
1:17:37
Speaker 2: photon's local motion in our direction, its proper distance from
1:17:40
Speaker 2: us can initially increase. It is swimming towards us while
1:17:45
Speaker 2: the cosmic current temporarily carries the surrounding geometry apart even faster.
1:17:50
Speaker 2: The important point is that the expansion rate of the
1:17:52
Speaker 2: universe changes over time. The Hubble parameter changes. The Hubble
1:17:57
Speaker 2: radius changes. So a photon that begins its journey in
1:18:00
Speaker 2: a region receding from us faster than light can sometimes
1:18:03
Speaker 2: later enter a region where the recession rate is below
1:18:07
Speaker 2: the speed of light. From then on, its proper distance
1:18:11
Speaker 2: from us can begin decreasing. Eventually the photon may arrive
1:18:15
Speaker 2: at Earth. In fact, some of the light we observe
1:18:18
Speaker 2: today was emitted by regions that were receding from us
1:18:21
Speaker 2: faster than light when those photons began travelling. There is
1:18:26
Speaker 2: no violation of relativity because the photons were always moving
1:18:29
Speaker 2: locally through spacetime at the speed of light. It was
1:18:33
Speaker 2: the enormous cosmological distance between source and observer that was evolving.
1:18:38
Speaker 2: This is also why the Hubble radius should not be
1:18:41
Speaker 2: confused with the permanent edge of what we can see.
1:18:44
Speaker 2: There are several different cosmological horizons, and their names describe
1:18:48
Speaker 2: different things. The Hubble radius marks the distance at which
1:18:52
Speaker 2: the recession rate associated with the Hubble flow equals the
1:18:56
Speaker 2: speed of light at a particular cosmic time. It is
1:18:59
Speaker 2: not necessarily an absolute observational boundary. The particle horizon is different.
1:19:05
Speaker 2: It describes, roughly speaking, the greatest present distance from which
1:19:10
Speaker 2: light or other causal signals could have reached us since
1:19:13
Speaker 2: the early universe. Because the universe has expanded while those
1:19:17
Speaker 2: signals travelled, the present proper distance to the particle horizon
1:19:22
Speaker 2: is much greater than 13.8 billion light years. It is
1:19:26
Speaker 2: roughly 46 billion light years. That does not mean the
1:19:30
Speaker 2: universe is 46 billion years old. It means that regions
1:19:34
Speaker 2: whose light has been travelling towards us for most of
1:19:37
Speaker 2: cosmic history, are now vastly farther away than they were
1:19:40
Speaker 2: when that light was emitted. Then there is the cosmic
1:19:43
Speaker 2: event horizon. This asks a different question. Instead of asking
1:19:48
Speaker 2: what signals have been able to reach us so far,
1:19:51
Speaker 2: it concerns which events occurring now or in the future
1:19:54
Speaker 2: can ever send signals that eventually reach us, given the
1:19:57
Speaker 2: future expansion of the universe. Because cosmic expansion is currently accelerating,
1:20:03
Speaker 2: There are sufficiently distant regions from which light emitted now
1:20:07
Speaker 2: will never reach us, even given unlimited time. Those regions
1:20:12
Speaker 2: lie beyond our cosmic event horizon. The distinction matters because
1:20:16
Speaker 2: faster-than-light recession by itself does not automatically mean permanent invisibility.
1:20:22
Speaker 2: A galaxy may be beyond the Hubble radius yet still
1:20:26
Speaker 2: lie within the event horizon. Under the right conditions, a
1:20:30
Speaker 2: photon emitted from there can eventually arrive. A still more
1:20:34
Speaker 2: distant galaxy may recede so rapidly and remain in a
1:20:38
Speaker 2: region affected by accelerated expansion in such a way that
1:20:42
Speaker 2: light emitted now never reaches us. Cosmological horizons are therefore
1:20:46
Speaker 2: subtler than a wall drawn at one fixed distance. They
1:20:50
Speaker 2: depend on the entire expansion history of the universe, and
1:20:54
Speaker 2: none of this allows us to build a faster-than-light spacecraft.
1:20:58
Speaker 2: A rocket cannot exploit cosmic expansion to pass a nearby
1:21:02
Speaker 2: beam of light. Locally, special relativity remains intact. The strange
1:21:07
Speaker 2: behaviour appears only when we compare objects separated by enormous
1:21:12
Speaker 2: cosmological distances and ask how their proper separation changes over time.
1:21:17
Speaker 2: That difference is another reason the explosion picture fails. If
1:21:22
Speaker 2: galaxies were simply pieces of debris flying through static space
1:21:25
Speaker 2: from one central blast, faster-than-light recession would create a genuine problem.
1:21:31
Speaker 2: Objects would have to be travelling through that space at
1:21:34
Speaker 2: forbidden speeds. But an expanding universe is not a collection
1:21:38
Speaker 2: of fragments crossing a fixed background. The background geometry is
1:21:42
Speaker 2: part of the story. Distances evolve. Horizons move. Light can
1:21:47
Speaker 2: spend billions of years travelling through a cosmos whose scale
1:21:51
Speaker 2: changes during the journey. So a galaxy can recede from
1:21:54
Speaker 2: us faster than light without locally breaking the speed limit.
1:21:58
Speaker 2: Einstein's rule survives perfectly well. It is our everyday idea
1:22:03
Speaker 2: of what distance and motion mean across an expanding universe
1:22:07
Speaker 2: that has to become a little more flexible. If we
1:22:10
Speaker 2: want to understand what the Big Bang really means, one
1:22:13
Speaker 2: of the simplest things we can do is imagine running
1:22:15
Speaker 2: the history of the universe backwards. Today galaxies are separated
1:22:19
Speaker 2: by enormous distances. Between the Milky Way and many of
1:22:23
Speaker 2: the galaxies visible through our telescopes, like millions or billions
1:22:28
Speaker 2: of light-years of space. Now reverse the expansion. As cosmic
1:22:32
Speaker 2: time runs backwards, those large-scale distances become smaller. Galaxies draw
1:22:38
Speaker 2: closer together, go sufficiently far back, and the galaxies themselves
1:22:42
Speaker 2: disappear because we reach an era before they had formed.
1:22:46
Speaker 2: Continue farther and the stars vanish as well. Matter becomes
1:22:50
Speaker 2: increasingly concentrated. Radiation becomes increasingly energetic. and the average temperature
1:22:57
Speaker 2: of the universe rises. This is the central picture behind
1:23:01
Speaker 2: the Hot Big Bang model. The early universe was not
1:23:05
Speaker 2: simply today's universe squeezed into a slightly smaller volume. Its
1:23:09
Speaker 2: physical conditions were radically different. As the universe expands, matter
1:23:14
Speaker 2: becomes more dilute because the same amount of material occupies
1:23:18
Speaker 2: increasingly large volumes. Radiation also changes. The wavelengths of photons
1:23:24
Speaker 2: are stretched by cosmic expansion, reducing their individual energies. Running
1:23:29
Speaker 2: that process backwards reverses both effects. Matter becomes denser. Photons
1:23:35
Speaker 2: become more energetic. Temperatures climb. Eventually we reach conditions so
1:23:40
Speaker 2: hot that atoms cannot survive. Continue farther and even atomic
1:23:44
Speaker 2: nuclei cannot remain assembled in the forms familiar today. Earlier still,
1:23:49
Speaker 2: Matter exists in states that can be described only using
1:23:53
Speaker 2: particle physics. This is a much better way to picture
1:23:56
Speaker 2: the early universe than imagining a tiny ball exploding into emptiness.
1:24:01
Speaker 2: The hot, dense conditions were not confined to one compact
1:24:05
Speaker 2: object surrounded by otherwise vacant space. They existed throughout the
1:24:10
Speaker 2: region that would eventually become our observable universe. This is
1:24:14
Speaker 2: why one familiar sentence requires a little caution. It is
1:24:18
Speaker 2: often said that the entire universe was once compressed into
1:24:22
Speaker 2: a single point. That description is memorable, but it says
1:24:25
Speaker 2: more than the evidence allows us to say with confidence.
1:24:29
Speaker 2: We can trace our observable universe backwards towards states of
1:24:33
Speaker 2: extraordinary density and temperature. But the observable universe is not
1:24:37
Speaker 2: necessarily the entire universe. The whole cosmos may extend vastly
1:24:42
Speaker 2: beyond the region we can observe. It may even be
1:24:46
Speaker 2: spatially infinite. And if the universe is infinite today, then
1:24:50
Speaker 2: under many standard cosmological descriptions, it could also have been
1:24:54
Speaker 2: infinite at earlier times, even while the distance between any
1:24:58
Speaker 2: two chosen co-moving locations was far smaller than it is now.
1:25:03
Speaker 2: An infinite universe can become denser without becoming finite. That
1:25:08
Speaker 2: is not especially friendly to everyday intuition, but infinity rarely
1:25:11
Speaker 2: seems worried about being friendly. So the statement that everything
1:25:16
Speaker 2: was once a point should not be treated as a
1:25:18
Speaker 2: literal photograph of the early cosmos. There is, however, a
1:25:22
Speaker 2: related idea that emerges from the mathematics of general relativity.
1:25:27
Speaker 2: If certain expanding cosmological solutions are extrapolated backwards far enough
1:25:32
Speaker 2: using classical general relativity alone, the cosmic scale factor can
1:25:37
Speaker 2: approach zero, densities can formally grow without bound, the curvature
1:25:43
Speaker 2: of spacetime can also diverge. This limiting behaviour is associated
1:25:48
Speaker 2: with what is called the Big Bang Singularity. The word
1:25:52
Speaker 2: singularity can make the idea sound like an actual tiny
1:25:55
Speaker 2: object that once existed somewhere. That is not the safest interpretation.
1:26:01
Speaker 2: A singularity in a physical theory often signals that the
1:26:04
Speaker 2: mathematical description has reached a regime in which it can
1:26:07
Speaker 2: no longer provide a complete account of nature. General relativity
1:26:12
Speaker 2: is an extraordinarily successful theory. It describes the motion of planets,
1:26:17
Speaker 2: the behaviour of black holes, gravitational waves and the large-scale
1:26:21
Speaker 2: evolution of the universe with remarkable precision. But general relativity
1:26:26
Speaker 2: is a classical theory. It does not include quantum mechanics
1:26:30
Speaker 2: in a complete description of spacetime itself. That becomes increasingly
1:26:35
Speaker 2: important when the universe is extrapolated towards conditions of extreme
1:26:39
Speaker 2: density and curvature. At some sufficiently early stage, quantum effects
1:26:44
Speaker 2: involving space-time are expected to become impossible to ignore. Physicists, therefore,
1:26:50
Speaker 2: do not generally treat the classical singularity as a directly
1:26:53
Speaker 2: observed object, or as established proof that the universe literally
1:26:57
Speaker 2: began as a point of infinite density. Instead, it marks
1:27:01
Speaker 2: a boundary beyond which our familiar classical description cannot simply
1:27:05
Speaker 2: be trusted. This leads to one of the largest unfinished
1:27:09
Speaker 2: problems in fundamental physics. We need a successful theory of
1:27:13
Speaker 2: quantum gravity. Quantum mechanics describes particles, atoms, and the microscopic
1:27:19
Speaker 2: world with extraordinary accuracy. General relativity describes gravity and spacetime
1:27:25
Speaker 2: extremely well on larger scales and in strong gravitational environments.
1:27:31
Speaker 2: But combining those two frameworks into one experimentally confirmed description
1:27:36
Speaker 2: remains an unresolved challenge. Several approaches have been developed. String
1:27:42
Speaker 2: theory seeks a framework in which fundamental entities are described
1:27:45
Speaker 2: in terms of extended strings rather than point-like particles, with
1:27:50
Speaker 2: gravity emerging naturally among their possible excitations. Loop quantum gravity
1:27:55
Speaker 2: takes a different route, attempting to quantise aspects of spacetime
1:28:00
Speaker 2: geometry itself. Other ideas explore emergent spacetime causal structures bouncing
1:28:06
Speaker 2: cosmologies and a variety of quantum cosmological models, none has
1:28:11
Speaker 2: yet become an experimentally established complete theory of the earliest universe.
1:28:16
Speaker 2: This means there is a limit to how confidently we
1:28:18
Speaker 2: should speak about the first imaginable instant. Fortunately, Big Bang
1:28:24
Speaker 2: cosmology does not depend upon knowing exactly what happened at
1:28:28
Speaker 2: that ultimate boundary. As we move forward from the hypothetical
1:28:32
Speaker 2: regime where quantum gravity would dominate, the physics eventually enters
1:28:37
Speaker 2: conditions we can describe with increasing confidence. The universe remains
1:28:42
Speaker 2: extraordinarily hot, but our established theories of particles and radiation
1:28:47
Speaker 2: become more useful. Later still, predictions can be tested against
1:28:52
Speaker 2: direct cosmological evidence. We can calculate how an expanding hot
1:28:57
Speaker 2: universe should cool. We can predict how light atomic nuclei
1:29:01
Speaker 2: should form during the first few minutes. We can predict
1:29:04
Speaker 2: that a hot early universe should leave behind a bath
1:29:08
Speaker 2: of ancient radiation. We can examine the cosmic microwave background.
1:29:13
Speaker 2: We can measure the abundances of light elements. We can
1:29:16
Speaker 2: map galaxies across enormous distances and compare their distribution with
1:29:21
Speaker 2: models of cosmic evolution. These are not guesses about an
1:29:25
Speaker 2: inaccessible beginning. They are observations of consequences that remain measurable today,
1:29:31
Speaker 2: and they provide strong evidence that the observable universe really
1:29:35
Speaker 2: did pass through a much hotter, denser and more uniform state.
1:29:41
Speaker 2: That distinction is one of the most important in cosmology.
1:29:45
Speaker 2: There is an enormous difference between saying that scientists have
1:29:49
Speaker 2: strong evidence for a hot early universe and saying that
1:29:52
Speaker 2: scientists know exactly how reality began. The first statement is
1:29:58
Speaker 2: well supported, the second is not. We know that cosmic
1:30:02
Speaker 2: expansion can be traced backwards towards increasingly extreme physical conditions.
1:30:08
Speaker 2: We know that the early universe was much hotter and denser.
1:30:12
Speaker 2: We know that matter and radiation behaved very differently from
1:30:15
Speaker 2: the way they do in the present cosmos. But when
1:30:18
Speaker 2: we push our equations towards the most extreme conceivable beginning,
1:30:23
Speaker 2: certainty decreases rather than increases. the classical equations eventually point
1:30:28
Speaker 2: towards a singularity. Quantum physics tells us that those equations
1:30:33
Speaker 2: are probably no longer sufficient there, and without a tested
1:30:37
Speaker 2: theory of quantum gravity the final step remains beyond established physics.
1:30:41
Speaker 2: So rewinding the universe does not reveal a tiny bomb
1:30:45
Speaker 2: sitting in an empty darkness waiting to explode. It reveals
1:30:49
Speaker 2: a cosmos becoming hotter, denser, and more compressed on observable scales. Then,
1:30:55
Speaker 2: at the very edge of what our established theories can
1:30:58
Speaker 2: reliably describe, the picture becomes uncertain. The Hot Big Bang
1:31:03
Speaker 2: is supported by an extraordinary body of evidence. The absolute beginning,
1:31:08
Speaker 2: if there was one in the simple sense we imagine,
1:31:11
Speaker 2: remains a different question entirely. If we keep running the
1:31:14
Speaker 2: history of the universe backwards, there is a point where
1:31:17
Speaker 2: the story becomes stranger still. Long before stars, before galaxies,
1:31:23
Speaker 2: before atoms … And perhaps before many of the particles
1:31:27
Speaker 2: we're familiar with today, cosmologists think the universe may have
1:31:31
Speaker 2: passed through an extraordinarily brief period of accelerated expansion. This
1:31:37
Speaker 2: idea is called cosmic inflation. Inflation is not simply ordinary
1:31:42
Speaker 2: expansion happening a little faster. It proposes that during an
1:31:46
Speaker 2: extremely early moment in cosmic history, the scale factor of
1:31:50
Speaker 2: the universe increased at an astonishing rate. The idea became
1:31:55
Speaker 2: especially influential around 1980 through the work of the American
1:31:59
Speaker 2: physicist Alan Guth. Guth was trying to understand several puzzling
1:32:04
Speaker 2: features of the early universe that were difficult to explain
1:32:07
Speaker 2: within a simple hot big bang picture. His first version
1:32:11
Speaker 2: of inflation was not the final model physicists use today.
1:32:15
Speaker 2: It had technical problems, but it introduced a powerful new possibility.
1:32:21
Speaker 2: Perhaps the early universe underwent a brief phase of accelerated
1:32:24
Speaker 2: expansion before settling into the slower expansion associated with the
1:32:29
Speaker 2: Hot Big Bang. Other physicists soon developed improved versions. Andre
1:32:35
Speaker 2: Linde introduced influential inflationary models of his own. Andreas Albrecht
1:32:40
Speaker 2: and Paul Steinhardt independently developed what became known as New Inflation,
1:32:46
Speaker 2: helping to solve problems in the original proposal. Over time,
1:32:51
Speaker 2: inflation stopped being one single model and became a broad
1:32:55
Speaker 2: family of ideas. Different versions proposed different fields, different energy
1:33:00
Speaker 2: scales and different detailed histories, but most share the same
1:33:04
Speaker 2: central feature. For a very short time, the expansion of
1:33:09
Speaker 2: the universe accelerated enormously. One reason this idea became so
1:33:14
Speaker 2: attractive involves the shape of space itself. Observations show that
1:33:19
Speaker 2: the observable universe is extremely close to spatially flat on
1:33:23
Speaker 2: very large scales. Spatial flatness does not mean that the
1:33:27
Speaker 2: universe is shaped like a sheet of paper. It means that,
1:33:31
Speaker 2: when averaged across immense distances, its geometry is very close
1:33:36
Speaker 2: to the familiar geometry taught in school. Parallel lines behave
1:33:40
Speaker 2: approximately as expected. Large triangles have angles adding to roughly
1:33:45
Speaker 2: 180 degrees. The overall curvature is extremely small. That is
1:33:50
Speaker 2: slightly puzzling. In a non-inflationary cosmology, even a small amount
1:33:56
Speaker 2: of early spatial curvature tends to become increasingly noticeable as
1:34:00
Speaker 2: the universe evolves. For the universe to appear so flat today,
1:34:04
Speaker 2: its early conditions would have needed to be extraordinarily close
1:34:08
Speaker 2: to flat already. Inflation offers a natural explanation. Imagine standing
1:34:14
Speaker 2: on the surface of Earth. The ground beneath you appears
1:34:17
Speaker 2: flat because the radius of the planet is enormous compared
1:34:21
Speaker 2: with the patch of surface you can see. Now imagine
1:34:24
Speaker 2: somehow making Earth vastly larger without changing your local patch
1:34:29
Speaker 2: very much. The visible surface would appear flatter still. Inflation
1:34:34
Speaker 2: works in a somewhat similar mathematical sense. A region of
1:34:38
Speaker 2: space that may initially have possessed noticeable curvature can be
1:34:42
Speaker 2: stretched to such enormous dimensions. that the small part accessible
1:34:46
Speaker 2: to us later appears almost perfectly flat. Inflation also addresses
1:34:52
Speaker 2: another puzzle known as the horizon problem. When astronomers study
1:34:56
Speaker 2: the cosmic microwave background, they find that radiation arriving from
1:35:01
Speaker 2: opposite directions in the sky has almost exactly the same temperature.
1:35:05
Speaker 2: The average temperature is roughly 2.7 Kelvin, and variations across
1:35:10
Speaker 2: the sky are only around one part in 100,000. That
1:35:14
Speaker 2: remarkable uniformity needs explaining. Without inflation, some widely separated regions
1:35:20
Speaker 2: of the early universe appear to have been too far
1:35:22
Speaker 2: apart to exchange light or any other causal signal before
1:35:26
Speaker 2: the cosmic microwave background was released. Yet their temperatures are
1:35:30
Speaker 2: nearly identical. It is a little like finding two sealed
1:35:34
Speaker 2: rooms on opposite sides of a city, discovering that nobody
1:35:37
Speaker 2: has ever travelled between them, and then measuring the air
1:35:40
Speaker 2: temperature in both to an astonishing level of precision. you
1:35:44
Speaker 2: would naturally wonder how they ended up so similar. Inflation
1:35:48
Speaker 2: provides a possible answer. Before the rapid expansion, the regions
1:35:53
Speaker 2: now separated by enormous cosmic distances may once have been
1:35:57
Speaker 2: close enough together to interact. They could exchange energy. They
1:36:01
Speaker 2: could approach similar temperatures. They could share broadly similar physical conditions.
1:36:06
Speaker 2: Then inflation stretched that tiny connected region to an immense size.
1:36:11
Speaker 2: Locations that were once causally connected became separated by distances
1:36:16
Speaker 2: so enormous that today they appear on opposite sides of
1:36:19
Speaker 2: the observable sky. Their similarity would therefore not be a coincidence.
1:36:25
Speaker 2: It would be a remnant of their common early history.
1:36:28
Speaker 2: Inflation may also help explain something even more important. The
1:36:32
Speaker 2: universe is not perfectly uniform. Galaxies exist. Galaxy clusters exist.
1:36:39
Speaker 2: Enormous filaments and voids form what astronomers call the cosmic web.
1:36:44
Speaker 2: All of that structure had to begin somewhere. In inflationary cosmology,
1:36:49
Speaker 2: microscopic quantum fluctuations may provide the initial seeds. Quantum fields
1:36:55
Speaker 2: do not remain perfectly featureless even in their lowest energy states.
1:36:59
Speaker 2: Tiny fluctuations naturally occur. During inflation, these microscopic variations could
1:37:05
Speaker 2: have been stretched to enormous physical scales. A region that
1:37:10
Speaker 2: began with slightly more energy or density than its surroundings
1:37:13
Speaker 2: could later become a slightly over-dense region in the young universe.
1:37:18
Speaker 2: Gravity would gradually amplify those differences. Over hundreds of millions
1:37:23
Speaker 2: and then billions of years, slightly denser regions would attract
1:37:27
Speaker 2: more matter. Gas would collect. Stars would form. Galaxies would grow.
1:37:34
Speaker 2: Clusters and filaments would emerge. In that sense, some of
1:37:37
Speaker 2: the largest structures in the observable universe may ultimately trace
1:37:42
Speaker 2: their origins to quantum fluctuations that began on unimaginably small scales.
1:37:48
Speaker 2: Measurements of the cosmic microwave background contain patterns broadly consistent
1:37:53
Speaker 2: with this inflationary picture. The primordial fluctuations appear close to
1:37:58
Speaker 2: the kind of nearly scale, invariant, approximately Gaussian variations many
1:38:04
Speaker 2: simple inflationary models predict. That agreement is one reason inflation
1:38:09
Speaker 2: remains so influential. But inflation should not be mistaken for
1:38:13
Speaker 2: ordinary matter travelling impossibly fast. There were no galaxies during inflation.
1:38:19
Speaker 2: There were no stars racing away from one another. There
1:38:22
Speaker 2: were no planets being carried through space at superluminal speeds.
1:38:27
Speaker 2: Inflation describes an extraordinary change in the scale factor of
1:38:31
Speaker 2: the universe. Distances between widely separated co-moving locations could increase
1:38:37
Speaker 2: faster than light without anything locally overtaking a beam of light.
1:38:42
Speaker 2: Just as with later cosmic expansion, special relativity remains valid locally.
1:38:48
Speaker 2: The geometry itself is evolving. This is another reason why
1:38:52
Speaker 2: the Big Bang cannot be understood as a conventional explosion. Inflation,
1:38:57
Speaker 2: if it occurred, would have been an expansion of space
1:39:00
Speaker 2: on a scale far beyond anything resembling fragments being thrown outward.
1:39:05
Speaker 2: And the end of inflation introduces an important piece of terminology.
1:39:10
Speaker 2: In many modern cosmological descriptions, inflation is treated as a
1:39:13
Speaker 2: phase that occurred before what is sometimes called the Hot
1:39:17
Speaker 2: Big Bang. During inflation, energy was stored in whatever field
1:39:21
Speaker 2: or mechanism drove the accelerated expansion. When inflation ended, that
1:39:26
Speaker 2: energy had to go somewhere. A process often called reheating
1:39:30
Speaker 2: converted much of it into particles and radiation. The universe
1:39:35
Speaker 2: became filled with an extremely hot mixture of matter and energy.
1:39:39
Speaker 2: That hot state then expanded and cooled. So when cosmologists
1:39:44
Speaker 2: use the phrase hot big bang … they may be
1:39:47
Speaker 2: referring specifically to the period after inflation rather than to
1:39:51
Speaker 2: inflation itself. This distinction is useful because it separates two
1:39:56
Speaker 2: ideas that are often casually blended together. Inflation is a
1:40:01
Speaker 2: proposed mechanism for a very early phase of accelerated expansion.
1:40:06
Speaker 2: The Hot Big Bang describes the subsequent evolution of a
1:40:09
Speaker 2: hot dense universe through cooling, particle formation, and eventually the
1:40:14
Speaker 2: appearance of atoms, stars and galaxies. And although inflation solves
1:40:19
Speaker 2: several important problems elegantly, it is not a finished story.
1:40:24
Speaker 2: Physicists still do not know what physical field drove inflation,
1:40:28
Speaker 2: assuming it occurred. They do not know its exact energy scale.
1:40:33
Speaker 2: Different inflationary models make different predictions. Some have been ruled
1:40:37
Speaker 2: out or constrained by observations, while many remain possible. No
1:40:42
Speaker 2: confirmed particle corresponding to an inflation-driving field has been discovered,
1:40:47
Speaker 2: and inflation does not necessarily tell us what happened at
1:40:51
Speaker 2: an absolute beginning. It may describe only one early phase
1:40:55
Speaker 2: of a larger history. Some models place inflation after another
1:40:59
Speaker 2: prior state. Others explore eternal inflation, quantum cosmology or bouncing scenarios.
1:41:06
Speaker 2: None of these has been experimentally established as the final answer.
1:41:11
Speaker 2: So inflation sits in an interesting position. It is strongly motivated.
1:41:16
Speaker 2: It explains several otherwise puzzling properties of the observable universe.
1:41:21
Speaker 2: Its predictions have important agreements with measurements of the cosmic
1:41:24
Speaker 2: microwave background. But its deepest physical mechanism remains unknown. What
1:41:31
Speaker 2: it gives us is not a complete answer to how
1:41:33
Speaker 2: everything began. It gives us a possible explanation for how
1:41:37
Speaker 2: a tiny early region could become an immense, smooth and
1:41:42
Speaker 2: nearly flat cosmos containing the small primordial variations from which
1:41:47
Speaker 2: everything we see eventually grew, not an explosion. Not matter
1:41:52
Speaker 2: blasting through an empty void, but an extraordinary change in
1:41:56
Speaker 2: the scale of the universe itself. When inflation ended, assuming
1:42:01
Speaker 2: inflation is indeed part of our cosmic history, the universe
1:42:05
Speaker 2: entered a very different phase. The rapid, accelerated expansion gave
1:42:11
Speaker 2: way to a hot, dense environment filled with radiation and
1:42:15
Speaker 2: elementary particles. There were no stars, no galaxies, no planets,
1:42:20
Speaker 2: no atoms, and certainly no stretches of apparently empty intergalactic
1:42:24
Speaker 2: space like the ones surrounding galaxies today. The early universe
1:42:30
Speaker 2: was instead an intensely energetic mixture in which particles were
1:42:34
Speaker 2: constantly moving, colliding. appearing and disappearing through high-energy interactions. It
1:42:41
Speaker 2: was less like a collection of objects scattered through space,
1:42:45
Speaker 2: and more like a hot cosmic plasma filling everything we
1:42:49
Speaker 2: could meaningfully call the observable universe. As expansion continued, one
1:42:55
Speaker 2: change became especially important. The universe cooled. This relationship between
1:43:01
Speaker 2: expansion and cooling is fundamental to the Hot Big Bang model.
1:43:05
Speaker 2: Radiation loses energy as the universe expands because its wavelength
1:43:09
Speaker 2: is stretched along with the changing scale of space. A
1:43:13
Speaker 2: photon with a shorter wavelength carries more energy than one
1:43:16
Speaker 2: with a longer wavelength. So as cosmic expansion stretches the
1:43:20
Speaker 2: wavelengths of radiation, the average energy carried by those photons falls.
1:43:26
Speaker 2: Temperature falls with it. Matter also becomes less densely packed
1:43:31
Speaker 2: as the same number of particles occupies an increasing volume.
1:43:35
Speaker 2: So cosmic history becomes, in large part, a story of
1:43:38
Speaker 2: changing temperature and density. At very early times, when temperatures
1:43:43
Speaker 2: were enormously high, the particles we know today did not
1:43:47
Speaker 2: behave as stable, isolated objects in the way they do now.
1:43:51
Speaker 2: The universe contained quarks and antiquarks. It contained gluons, the
1:43:56
Speaker 2: particles associated with the strong nuclear interaction. There were electrons
1:44:01
Speaker 2: and their antimatter partners, positrons. There were neutrinos and antineutrinos,
1:44:07
Speaker 2: and there were huge numbers of photons. These particles were
1:44:11
Speaker 2: not flying outward from a single point like sparks from
1:44:14
Speaker 2: an explosion. They existed throughout the hot early universe. The
1:44:19
Speaker 2: expansion did not carry them away from one central location.
1:44:23
Speaker 2: It reduced the overall density and temperature everywhere as the
1:44:26
Speaker 2: scale of the universe increased. Quarks are especially important in
1:44:30
Speaker 2: this story. Today, Quarks are not observed wandering freely through
1:44:35
Speaker 2: ordinary matter. They are confined inside composite particles such as
1:44:39
Speaker 2: protons and neutrons. A proton contains three valence quarks. A
1:44:44
Speaker 2: neutron also contains three. Those quarks are bound by the
1:44:48
Speaker 2: strong interaction, described by the theory known as quantum chromodynamics.
1:44:54
Speaker 2: In the extremely hot early universe, however, temperatures were high
1:44:58
Speaker 2: enough that ordinary protons and neutrons could not yet remain
1:45:01
Speaker 2: as stable bound structures. Quarks and gluons existed in a
1:45:06
Speaker 2: hot state often called a quark-gluon plasma. As expansion continued
1:45:11
Speaker 2: and the temperature fell, the universe passed through a transition
1:45:15
Speaker 2: where quarks became confined. Instead of moving through the plasma
1:45:20
Speaker 2: as effectively free constituents, they became locked into composite particles.
1:45:25
Speaker 2: Protons formed. Neutrons formed. other short-lived particles formed as well.
1:45:32
Speaker 2: The universe was beginning to acquire some of the building
1:45:35
Speaker 2: blocks that would eventually make familiar matter possible. But there
1:45:39
Speaker 2: was another enormous issue to resolve. Matter was not alone.
1:45:44
Speaker 2: The early universe also contained antimatter. For many particles, there
1:45:48
Speaker 2: exists an antimatter counterpart with the same mass but opposite
1:45:53
Speaker 2: electric charge or other reversed quantum properties. An electron has
1:45:57
Speaker 2: a positron, A proton has an antiproton. A quark has
1:46:02
Speaker 2: an antiquark. When matter and antimatter meet, they can annihilate.
1:46:07
Speaker 2: Their mass and energy are converted into other particles, often
1:46:10
Speaker 2: including photons. In the early universe, this process happened constantly.
1:46:16
Speaker 2: Particles and antiparticles were created in energetic collisions. They annihilated.
1:46:22
Speaker 2: They appeared again. At high enough temperatures … Energy could
1:46:26
Speaker 2: be converted back and forth between radiation and massive particles
1:46:31
Speaker 2: with extraordinary ease. As the universe cooled, however, creating heavy
1:46:36
Speaker 2: particle and antiparticle pairs became more difficult. Annihilation began to dominate.
1:46:43
Speaker 2: Most matter and antimatter disappeared together. And yet not quite
1:46:47
Speaker 2: all of the matter vanished. For reasons physicists still do
1:46:51
Speaker 2: not fully understand, there appears to have been a tiny
1:46:54
Speaker 2: excess of matter over antimatter. Perhaps for every enormous number
1:46:58
Speaker 2: of matter and antimatter particles, there was one extra matter
1:47:02
Speaker 2: particle left unmatched. The exact picture depends on how the
1:47:06
Speaker 2: process occurred, and that process remains uncertain. But without such
1:47:11
Speaker 2: an imbalance, the universe today would look very different. Nearly
1:47:16
Speaker 2: all matter and antimatter would have annihilated, leaving behind mostly radiation.
1:47:21
Speaker 2: There would be no stars, no planets, no chemistry. No
1:47:26
Speaker 2: sleeping physicists wondering where all the matter went. The observed
1:47:30
Speaker 2: dominance of matter is known as baryon asymmetry. A baryon
1:47:35
Speaker 2: is a composite particle made from three quarks, with protons
1:47:38
Speaker 2: and neutrons being the most familiar examples. The mystery is
1:47:42
Speaker 2: not merely that matter exists. The mystery is why the
1:47:46
Speaker 2: early universe produced slightly more matter than antimatter in the
1:47:49
Speaker 2: first place. Several mechanisms have been proposed. They generally require
1:47:54
Speaker 2: physical processes that distinguish matter from antimatter in subtle ways,
1:47:59
Speaker 2: along with conditions in the early universe that allow a
1:48:02
Speaker 2: small imbalance to develop. But no complete experimentally confirmed explanation
1:48:08
Speaker 2: has yet been established. This is one of the major
1:48:11
Speaker 2: unsolved problems connecting cosmology with particle physics. Another important transition
1:48:17
Speaker 2: involved neutrinos. Neutrinos interact extremely weakly with matter. Today, enormous
1:48:24
Speaker 2: numbers of them pass through Earth and through your body
1:48:27
Speaker 2: without leaving any noticeable trace. In the early universe, however,
1:48:33
Speaker 2: everything was so hot and dense that even neutrinos interacted
1:48:36
Speaker 2: often enough to remain part of the general thermal mixture.
1:48:40
Speaker 2: As expansion continued, the density dropped. Particles became farther apart. Eventually,
1:48:46
Speaker 2: neutrinos stopped interacting frequently enough to remain in thermal equilibrium
1:48:51
Speaker 2: with the rest of the cosmic plasma. They decoupled. From
1:48:55
Speaker 2: that point onward, they travelled largely independently through the expanding universe.
1:49:00
Speaker 2: This should have left behind a cosmic neutrino background, an
1:49:03
Speaker 2: ancient sea of neutrinos filling space today. It is an analogue,
1:49:08
Speaker 2: in a sense. of the much later cosmic microwave background.
1:49:13
Speaker 2: But neutrinos are vastly harder to detect because they interact
1:49:17
Speaker 2: so weakly. The cosmic neutrino background has not been directly
1:49:22
Speaker 2: imaged in the way microwave radiation has, though its expected
1:49:26
Speaker 2: effects are supported indirectly through cosmological measurements and the successful
1:49:31
Speaker 2: agreement between early universe physics and observations. By this stage,
1:49:35
Speaker 2: the pattern of cosmic history becomes easier to see. Temperature falls.
1:49:40
Speaker 2: Density falls. Interactions that were once common become rare. Certain
1:49:45
Speaker 2: particles stop remaining in equilibrium with others. Unstable combinations disappear.
1:49:51
Speaker 2: Stable particles survive. And the entire character of the universe
1:49:55
Speaker 2: changes because the physical rules favour different structures at different temperatures.
1:50:00
Speaker 2: This is a very different narrative from one involving explosive debris.
1:50:05
Speaker 2: An explosion usually invites us to focus on where the
1:50:08
Speaker 2: fragments are travelling. Early cosmology asks something else. What is
1:50:13
Speaker 2: the temperature? What particles can exist at that temperature? Which
1:50:17
Speaker 2: interactions happen frequently enough to maintain equilibrium? Which particles can
1:50:22
Speaker 2: remain bound together? Which reactions become impossible once the universe
1:50:27
Speaker 2: cools below a particular energy scale? Those are the questions
1:50:31
Speaker 2: that organise the early universe. Time and temperature are deeply connected.
1:50:36
Speaker 2: At earlier times, conditions were hotter. At later times, expansion
1:50:41
Speaker 2: allowed the universe to cool. And as the temperature dropped,
1:50:45
Speaker 2: new kinds of structure became possible. Quarks became confined into
1:50:49
Speaker 2: protons and neutrons. Matter survived its encounter with antimatter because
1:50:53
Speaker 2: of a tiny primordial imbalance. Neutrinos decoupled and began travelling freely.
1:50:59
Speaker 2: The cosmic mixture gradually became less exotic. But even then,
1:51:04
Speaker 2: there were still no ordinary atoms. The universe remained far
1:51:08
Speaker 2: too hot. The protons and neutrons that had survived were
1:51:12
Speaker 2: simply waiting, in a sense, for the temperature to fall
1:51:16
Speaker 2: far enough for the next transformation. For the first time,
1:51:20
Speaker 2: some of them would begin joining together, and within only
1:51:23
Speaker 2: a few minutes of cosmic history, the first light atomic
1:51:27
Speaker 2: nuclei would start to form. For the first few minutes
1:51:31
Speaker 2: of cosmic history, the universe became something rather like a
1:51:34
Speaker 2: nuclear laboratory without walls. It was still unimaginably hot, but
1:51:39
Speaker 2: it was cooling quickly. Protons and neutrons had already formed
1:51:43
Speaker 2: from the earlier quark-filled plasma. They were moving through an
1:51:48
Speaker 2: intense sea of photons, electrons, positrons, neutrinos and other particles.
1:51:53
Speaker 2: And as the temperature continued to fall, conditions briefly became
1:51:57
Speaker 2: suitable for some of those protons and neutrons to begin
1:52:00
Speaker 2: joining together. This period is known as Big Bang nuclear synthesis.
1:52:06
Speaker 2: Nuclear synthesis simply means the formation of atomic nuclei. And
1:52:10
Speaker 2: although the name Big Bang once again encourages thoughts of
1:52:14
Speaker 2: an explosion creating chemical debris, the actual process was much
1:52:18
Speaker 2: quieter in principle. Nuclear reactions were occurring throughout the hot
1:52:22
Speaker 2: early universe, as expansion steadily reduced its temperature and density.
1:52:27
Speaker 2: Several physicists were crucial in developing this picture. George Gamow
1:52:32
Speaker 2: was one of the strongest early advocates of a hot
1:52:34
Speaker 2: beginning to the universe and recognised that extreme early temperatures
1:52:38
Speaker 2: could allow nuclear reactions to occur. Ralph Alpher developed detailed
1:52:43
Speaker 2: calculations of how elements might form under those conditions. Robert
1:52:48
Speaker 2: Herman worked closely with Alpher and helped explore the thermal
1:52:51
Speaker 2: history of the expanding universe. Together, their work helped establish
1:52:56
Speaker 2: the idea that a hot early cosmos should leave behind
1:52:59
Speaker 2: measurable consequences. Those consequences included both the primordial abundances of
1:53:05
Speaker 2: light elements and a relic background of radiation left over
1:53:09
Speaker 2: from the early heat. In the first moments relevant to
1:53:13
Speaker 2: nuclear synthesis, however, there was a problem. The universe was
1:53:17
Speaker 2: too hot. That may sound like a strange obstacle to
1:53:21
Speaker 2: nuclear reactions. We often associate high temperatures with fusion, and
1:53:26
Speaker 2: stars certainly rely on enormous heat and pressure to push
1:53:29
Speaker 2: nuclei together. But there is such a thing as being
1:53:33
Speaker 2: too energetic. Whenever a proton and a neutron combined to
1:53:37
Speaker 2: form a nucleus of deuterium, energetic photons were plentiful enough
1:53:41
Speaker 2: to destroy it again. Deuterium is a heavier isotope of
1:53:45
Speaker 2: hydrogen containing one proton and one neutron. It is also
1:53:49
Speaker 2: an essential stepping stone towards building helium in the early universe.
1:53:54
Speaker 2: Without stable deuterium, the nuclear assembly line could barely get started.
1:53:59
Speaker 2: So for a time, protons and neutrons were present in abundance,
1:54:03
Speaker 2: but attempts to build larger nuclei were continually undone by
1:54:07
Speaker 2: energetic radiation. This delay is sometimes called the deuterium bottleneck.
1:54:13
Speaker 2: Expansion gradually solved the problem. As the universe grew, its
1:54:17
Speaker 2: radiation was stretched to longer wavelengths and its temperature fell. Eventually,
1:54:22
Speaker 2: the typical photon no longer carried enough energy to destroy
1:54:26
Speaker 2: newly formed deuterium as efficiently as before. Deuterium could survive
1:54:31
Speaker 2: in larger quantities. Once that happened, nuclear reactions accelerated. Deuterium
1:54:37
Speaker 2: nuclei could combine with protons or other deuterium nuclei. Intermediate
1:54:41
Speaker 2: nuclei such as helium-3 and tritium could form. Those reactions
1:54:46
Speaker 2: provided pathways towards helium-4, a particularly stable nucleus containing two
1:54:51
Speaker 2: protons and two neutrons. Helium-4 became the great success story
1:54:56
Speaker 2: of primordial nuclear synthesis. A significant fraction of the available
1:55:01
Speaker 2: neutrons ended up locked inside helium nuclei. By mass, the
1:55:05
Speaker 2: ordinary matter produced by Big Bang nuclear synthesis was roughly
1:55:09
Speaker 2: three-quarters hydrogen and about one-quarter helium. with only tiny amounts
1:55:14
Speaker 2: of other light nuclei. The exact fractions depend on how
1:55:18
Speaker 2: they are measured and on the detailed conditions of the
1:55:21
Speaker 2: early universe, but the broad prediction is remarkably robust. There
1:55:26
Speaker 2: were also small quantities of deuterium and helium-3. Tiny amounts
1:55:31
Speaker 2: of lithium were produced as well, including nuclei that would
1:55:34
Speaker 2: later contribute to lithium after radioactive transformations. The primordial lithium
1:55:40
Speaker 2: abundance is particularly interesting because observations do not agree perfectly
1:55:45
Speaker 2: with the simplest theoretical prediction. This discrepancy is known as
1:55:49
Speaker 2: the cosmological lithium problem and remains an active subject of research.
1:55:56
Speaker 2: The overall success of Big Bang nuclear synthesis, however, is striking,
1:56:00
Speaker 2: and all of this happened quickly. Within only the first
1:56:04
Speaker 2: several minutes, The universe had expanded and cooled enough that
1:56:07
Speaker 2: the main period of primordial nuclear reactions was effectively coming
1:56:12
Speaker 2: to an end. The density was falling. Particles encountered one
1:56:16
Speaker 2: another less frequently. Temperatures dropped below those required to sustain
1:56:21
Speaker 2: rapid fusion. The brief window for producing the primordial light
1:56:26
Speaker 2: elements began to close. The universe now contained many of
1:56:30
Speaker 2: the nuclei that would remain overwhelmingly dominant in ordinary matter
1:56:34
Speaker 2: for billions of years. But these were not yet atoms
1:56:38
Speaker 2: in the familiar sense. A hydrogen nucleus is simply a proton.
1:56:43
Speaker 2: A helium nucleus contains protons and neutrons. To make a
1:56:47
Speaker 2: neutral hydrogen or helium atom, electrons must remain bound around
1:56:51
Speaker 2: those nuclei. The early universe was still far too hot
1:56:55
Speaker 2: for that to happen on a large scale. Energetic photons
1:56:59
Speaker 2: could readily knock electrons away, so matter remained ionised. Nuclei
1:57:05
Speaker 2: moved through a plasma containing free electrons and an enormous
1:57:08
Speaker 2: bath of radiation. Hundreds of thousands of years would pass
1:57:12
Speaker 2: before the universe cooled enough for electrons to remain attached
1:57:16
Speaker 2: to nuclei in large numbers. That distinction matters because it
1:57:20
Speaker 2: separates two very different milestones. The first atomic nuclei formed
1:57:25
Speaker 2: within minutes. Neutral atoms appeared much later. Big Bang nucleosynthesis
1:57:31
Speaker 2: became one of the strongest tests of the hot early
1:57:34
Speaker 2: universe because it makes quantitative predictions. If the universe really
1:57:39
Speaker 2: passed through a hot, dense phase, nuclear physics tells us
1:57:43
Speaker 2: approximately how much deuterium, helium and other light nuclei should
1:57:47
Speaker 2: have been produced. Astronomers can then search for ancient, relatively
1:57:52
Speaker 2: unpressed material and compare those abundances with the calculations. deuterium
1:57:58
Speaker 2: is particularly useful. Stars generally destroy deuterium rather than producing
1:58:03
Speaker 2: large primordial quantities of it, so measurements in suitable environments
1:58:08
Speaker 2: can preserve valuable information about the early universe. Helium also
1:58:12
Speaker 2: provides an important test, especially when astronomers study regions containing
1:58:17
Speaker 2: relatively little material processed through generations of stars. The agreement
1:58:22
Speaker 2: between observations and the broad predictions of primordial nuclear synthesis
1:58:27
Speaker 2: is one of the major reasons cosmologists are confident that
1:58:31
Speaker 2: the universe really was once extremely hot and dense. It
1:58:35
Speaker 2: is difficult to explain these abundance patterns by imagining matter
1:58:39
Speaker 2: simply thrown outward from an explosion. The important variable was
1:58:43
Speaker 2: not distance from a blast centre. There was no privilege centre.
1:58:48
Speaker 2: The crucial quantities were temperature, density, expansion rate and nuclear
1:58:53
Speaker 2: reaction probabilities. The same physical conditions existed across the observable
1:58:59
Speaker 2: early universe, allowing similar nuclear processes to occur throughout it.
1:59:04
Speaker 2: And primordial nucleosynthesis did not create everything on the periodic table.
1:59:10
Speaker 2: It was mostly a light element factory. The early universe
1:59:13
Speaker 2: expanded too quickly and cooled too rapidly to build large
1:59:17
Speaker 2: quantities of heavy nuclei. Carbon, oxygen, silicon, iron and many
1:59:23
Speaker 2: other elements required a much longer cosmic story. Stars later
1:59:27
Speaker 2: fuse light nuclei into heavier ones in their interiors. Massive
1:59:32
Speaker 2: stars created increasingly heavy elements through successive stages of nuclear burning.
1:59:38
Speaker 2: Supernova explosions contributed to the production and dispersal of many elements.
1:59:44
Speaker 2: Neutron-star mergers and certain rare explosive environments became important sites
1:59:49
Speaker 2: for rapid neutron capture, producing some of the heaviest nuclei
1:59:54
Speaker 2: found in nature. The calcium in bones, the oxygen in
1:59:58
Speaker 2: the air, and the iron in blood belong largely to
2:00:01
Speaker 2: that later stellar history. But hydrogen, and much of the
2:00:06
Speaker 2: helium surrounding us, have a far older origin. Their nuclei
2:00:10
Speaker 2: were assembled when the universe was only minutes old. And
2:00:13
Speaker 2: once again, the evidence points not towards a bomb scattering
2:00:17
Speaker 2: material through empty space, but towards something far more unusual,
2:00:22
Speaker 2: an entire hot universe cooling together. its temperature falling, its
2:00:27
Speaker 2: density thinning, and for a brief window in its earliest minutes,
2:00:31
Speaker 2: conditions becoming just right for the first nuclei to remain assembled.
2:00:37
Speaker 2: For hundreds of thousands of years after the first atomic
2:00:40
Speaker 2: nuclei formed, the universe was still nothing like the transparent
2:00:44
Speaker 2: cosmos we see today. It was filled with a hot
2:00:47
Speaker 2: ionised plasma. Hydrogen and helium nuclei moved through a dense
2:00:52
Speaker 2: sea of free electrons. Photons were everywhere, but those photons
2:00:57
Speaker 2: could not travel very far before interacting with charged particles.
2:01:01
Speaker 2: A photon might move a short distance, scatter from an electron,
2:01:05
Speaker 2: change direction, then scatter again. And again. And again! The
2:01:12
Speaker 2: early universe was therefore opaque. Not dark, exactly. Quite the opposite.
2:01:18
Speaker 2: It was filled with radiation. but that radiation was trapped
2:01:21
Speaker 2: in a constantly interacting mixture of matter and light. A
2:01:26
Speaker 2: useful comparison is the interior of the Sun. Sunlight is
2:01:30
Speaker 2: produced deep inside the star, but photons do not simply
2:01:34
Speaker 2: travel in straight lines from the centre to the surface.
2:01:37
Speaker 2: They interact repeatedly with the surrounding material. The early universe
2:01:42
Speaker 2: behaved in a broadly similar way. Light existed everywhere. but
2:01:47
Speaker 2: it could not yet travel freely across enormous cosmic distances.
2:01:51
Speaker 2: Expansion gradually changed that. As the universe grew, the average
2:01:56
Speaker 2: temperature continued to fall. Eventually, around 380,000 years after the
2:02:03
Speaker 2: Hot Big Bang, conditions became cool enough for something important
2:02:07
Speaker 2: to happen. Electrons could remain bound to atomic nuclei. Hydrogen
2:02:12
Speaker 2: nuclei captured electrons and became neutral hydrogen atoms. Helium nuclei
2:02:17
Speaker 2: captured electrons as well. This period is usually called recombination.
2:02:23
Speaker 2: The name is slightly awkward because for most of these particles,
2:02:27
Speaker 2: there had never been an earlier era in which they
2:02:29
Speaker 2: were combined as neutral atoms. But the term became established,
2:02:34
Speaker 2: and cosmology, as we have already discovered, occasionally keeps names
2:02:38
Speaker 2: long after the names have stopped being perfectly helpful. The
2:02:41
Speaker 2: physical change was dramatic. Once large numbers of free electrons
2:02:46
Speaker 2: disappeared into neutral atoms, photons stopped scattering nearly as often.
2:02:51
Speaker 2: The universe became transparent. Radiation that had previously been trapped
2:02:56
Speaker 2: in the plasma could now travel enormous distances through space.
2:03:00
Speaker 2: Those photons are still travelling today. They surround us from
2:03:04
Speaker 2: every direction. We call this ancient radiation the cosmic microwave background.
2:03:10
Speaker 2: When those photons first began travelling freely, The universe was
2:03:14
Speaker 2: much hotter than it is now. Their wavelengths were far shorter,
2:03:18
Speaker 2: but cosmic expansion has stretched those wavelengths for more than
2:03:22
Speaker 2: 13 billion years. As the universe expanded, the radiation cooled.
2:03:28
Speaker 2: What began as much hotter thermal radiation has now been
2:03:31
Speaker 2: shifted into the microwave part of the electromagnetic spectrum. Today,
2:03:36
Speaker 2: the cosmic microwave background has an average temperature of about 2.7 Kelvin,
2:03:42
Speaker 2: That is only a few degrees above absolute zero. But
2:03:46
Speaker 2: despite being extremely cold, it fills the observable universe. Every
2:03:51
Speaker 2: cubic centimetre of space contains hundreds of these ancient photons.
2:03:56
Speaker 2: They pass through your room. They pass through Earth. They
2:03:59
Speaker 2: move through the space between galaxies. They are the cooled
2:04:03
Speaker 2: remains of radiation that last scattered when the universe was
2:04:07
Speaker 2: only a tiny fraction of its present age. Long before
2:04:10
Speaker 2: this radiation was actually detected, physicists had already realised that
2:04:15
Speaker 2: something like it should exist. George Gamow argued strongly for
2:04:19
Speaker 2: a hot early universe. Ralph Alpher and Robert Herman developed
2:04:23
Speaker 2: the idea further and recognised that if the universe had
2:04:27
Speaker 2: once been extremely hot and filled with radiation, then some
2:04:32
Speaker 2: cooled remnant of that radiation should still be present today.
2:04:36
Speaker 2: Expansion would not simply erase it. It would stretch its
2:04:40
Speaker 2: wavelengths and reduce its temperature. The prediction was conceptually powerful.
2:04:46
Speaker 2: If the universe had once passed through a hot, dense
2:04:49
Speaker 2: phase everywhere, then the sky today should still contain a
2:04:53
Speaker 2: faint bath of ancient radiation arriving from every direction. In
2:04:58
Speaker 2: the 1960s, that's essentially what two radio astronomers found. Arno
2:05:03
Speaker 2: Penzias and Robert Wilson were working at Bell Telephone Laboratories
2:05:07
Speaker 2: in New Jersey. with a large horn-shaped antenna designed for
2:05:10
Speaker 2: sensitive microwave observations. They encountered an irritating problem. There was
2:05:16
Speaker 2: a persistent background signal. It did not seem to come
2:05:19
Speaker 2: from any particular direction. It did not disappear when they
2:05:22
Speaker 2: changed where the antenna was pointing. They investigated possible instrumental causes.
2:05:29
Speaker 2: They considered interference. They checked the equipment carefully. There were
2:05:34
Speaker 2: even pigeons living around parts of the antenna. along with
2:05:37
Speaker 2: the sort of material pigeons tend to leave behind, which
2:05:41
Speaker 2: briefly became part of one of the more unusual equipment
2:05:44
Speaker 2: cleaning episodes in the history of cosmology. But the signal remained.
2:05:49
Speaker 2: It appeared to be coming from the sky itself. At
2:05:53
Speaker 2: roughly the same time, a group at Princeton University was
2:05:57
Speaker 2: preparing to search for relic radiation from the early universe.
2:06:02
Speaker 2: That group included Robert Dick, James Peebles, Peter Rolle, and
2:06:06
Speaker 2: David Wilkinson. When communication between the two groups finally connected
2:06:12
Speaker 2: the mysterious Bell Telephone Laboratory's signal with the theoretical expectation
2:06:17
Speaker 2: of cosmic background radiation, the importance became clear. The discovery
2:06:23
Speaker 2: was reported in 1965. The universe really did appear to
2:06:27
Speaker 2: contain a faint microwave glow arriving from every direction. This
2:06:32
Speaker 2: was extremely difficult for the steady-state theory to explain naturally,
2:06:37
Speaker 2: but it was exactly the kind of relic expected from
2:06:40
Speaker 2: a hot early universe, and its almost uniform presence across
2:06:44
Speaker 2: the sky carried another important lesson. The radiation was not
2:06:49
Speaker 2: streaming towards us from one distant point where an explosion
2:06:53
Speaker 2: had supposedly occurred. It surrounded us. Look in one direction,
2:06:57
Speaker 2: and it is there. Look in the opposite direction, and
2:07:00
Speaker 2: it is there too. The reason is simple. The hot
2:07:04
Speaker 2: plasma that produced the radiation filled the early observable universe.
2:07:09
Speaker 2: Every direction we look eventually carries our view back towards
2:07:12
Speaker 2: regions where photons last scattered from matter during recombination. We
2:07:18
Speaker 2: are not seeing a shell produced by an explosion moving
2:07:21
Speaker 2: outward from a centre. We are seeing ancient radiation arriving
2:07:25
Speaker 2: from different parts of the same early cosmic environment. The
2:07:29
Speaker 2: cosmic microwave background is remarkably uniform, but it is not
2:07:33
Speaker 2: perfectly uniform. Tiny temperature differences exist across the sky. They're
2:07:38
Speaker 2: extremely small, typically around one part in 100,000. Those variations
2:07:45
Speaker 2: are among the most important patterns in all of cosmology.
2:07:48
Speaker 2: They reflect tiny differences in density and temperature in the
2:07:51
Speaker 2: early universe. Slightly denser regions contained a little more matter
2:07:56
Speaker 2: than their surroundings. Over billions of years, gravity amplified those differences.
2:08:01
Speaker 2: Matter collected. Dark matter structures developed. Gas fell into gravitational wells.
2:08:07
Speaker 2: Stars formed. Galaxies appeared. Clusters and filaments grew. The immense
2:08:14
Speaker 2: cosmic web seen today ultimately developed from extremely small primordial irregularities.
2:08:21
Speaker 2: To study those irregularities, scientists needed increasingly precise maps. The
2:08:26
Speaker 2: National Aeronautics and Space Administration's Cosmic Background Explorer satellite provided
2:08:31
Speaker 2: decisive measurements of the microwave background's thermal spectrum and detected
2:08:36
Speaker 2: large-scale temperature variations. Later, The Wilkinson Microwave Anisotropy Probe produced
2:08:43
Speaker 2: a much sharper map of those fluctuations across the sky.
2:08:47
Speaker 2: Then the European Space Agency's Planck mission measured them with
2:08:50
Speaker 2: still greater precision. These observations allowed cosmologists to estimate fundamental
2:08:56
Speaker 2: properties of the universe. They helped constrain its age. They
2:09:00
Speaker 2: provided information about the amounts of ordinary matter and dark matter,
2:09:05
Speaker 2: They strengthened evidence that large-scale space is very close to
2:09:09
Speaker 2: geometrically flat. They revealed the characteristic acoustic patterns created by
2:09:15
Speaker 2: pressure waves in the early plasma. In effect, the cosmic
2:09:19
Speaker 2: microwave background became a photograph of the universe when it
2:09:23
Speaker 2: was only about 380,000 years old. Not a photograph in
2:09:28
Speaker 2: visible light, and not an image of objects as we
2:09:30
Speaker 2: normally understand them. but a map of ancient radiation carrying
2:09:35
Speaker 2: detailed information about the physical conditions of the early cosmos.
2:09:39
Speaker 2: And it does not stand alone. Cosmic expansion tells us
2:09:44
Speaker 2: that the universe was denser in the past. Primordial nucleosynthesis
2:09:49
Speaker 2: tells us that a hot early phase should have produced
2:09:52
Speaker 2: particular abundances of hydrogen, helium and other light nuclei. The
2:09:58
Speaker 2: cosmic microwave background tells us that the universe really was
2:10:02
Speaker 2: once filled with hot radiation that later cooled as space expanded.
2:10:07
Speaker 2: These are separate lines of evidence. They arise from different
2:10:11
Speaker 2: physical processes. They are measured in different ways. And yet
2:10:15
Speaker 2: they all point towards the same broad picture. A universe
2:10:20
Speaker 2: that was once much hotter and denser than it is now.
2:10:24
Speaker 2: Not debris from a central detonation. not matter racing away
2:10:28
Speaker 2: from one location through empty space, but an entire observable
2:10:33
Speaker 2: cosmos passing through a hot early phase, cooling as it
2:10:37
Speaker 2: expanded and leaving behind a faint glow that still surrounds
2:10:41
Speaker 2: us today. The oldest light we can see is not
2:10:45
Speaker 2: coming from the place where the Big Bang happened. It
2:10:48
Speaker 2: is coming from almost every direction, because the early universe
2:10:52
Speaker 2: was everywhere around the history of everything we now observe,
2:10:56
Speaker 2: The Big Bang theory is enormously successful, but it does
2:11:00
Speaker 2: not explain everything. That distinction is easy to lose because
2:11:04
Speaker 2: the phrase Big Bang is often used as though it
2:11:07
Speaker 2: were a complete scientific description of the ultimate beginning of reality.
2:11:11
Speaker 2: It can sound like a theory of how everything came
2:11:14
Speaker 2: from nothing, why the universe exists, what caused existence to begin,
2:11:19
Speaker 2: and what happened at the first moment of time. The
2:11:22
Speaker 2: standard Hot Big Bang model does not actually claim all
2:11:25
Speaker 2: of that. Its strongest and best-tested purpose is more specific.
2:11:30
Speaker 2: It describes how the observable universe evolved from an earlier
2:11:34
Speaker 2: state that was much hotter, denser and more uniform than
2:11:37
Speaker 2: the cosmos we see today. As cosmic expansion continued, temperatures fell,
2:11:43
Speaker 2: particles formed and interacted, protons and neutrons survived. Light atomic
2:11:49
Speaker 2: nuclei appeared during primordial nucleosynthesis. Hundreds of thousands of years later,
2:11:55
Speaker 2: neutral atoms formed and radiation began travelling freely. Gravity gradually
2:12:00
Speaker 2: amplified tiny variations in density until stars, galaxies, clusters and
2:12:07
Speaker 2: the enormous cosmic web emerged. That history is supported by
2:12:12
Speaker 2: several independent lines of evidence. The Hubble-Lemaître relationship shows that
2:12:17
Speaker 2: On sufficiently large scales, distant galaxies participate in cosmic expansion.
2:12:24
Speaker 2: The measured abundances of primordial hydrogen, helium and deuterium broadly
2:12:29
Speaker 2: match calculations of nuclear reactions in a hot early universe.
2:12:34
Speaker 2: The cosmic microwave background reveals an ancient bath of radiation
2:12:38
Speaker 2: that has cooled as the universe expanded. Its tiny temperature
2:12:42
Speaker 2: variations contain the imprint of the primordial irregularities which later
2:12:47
Speaker 2: cosmic structures developed. These observations fit together remarkably well. But
2:12:54
Speaker 2: none of them answers why there is a universe at all.
2:12:57
Speaker 2: The Hot Big Bang model does not tell us why
2:12:59
Speaker 2: existence exists rather than nothing. It does not identify an
2:13:03
Speaker 2: ultimate cause. And despite the language often used in popular descriptions,
2:13:09
Speaker 2: it does not establish through observation that physicists understand an
2:13:13
Speaker 2: absolute first instant. If the equations of classical general relativity
2:13:18
Speaker 2: are run backwards far enough in certain cosmological models, they
2:13:22
Speaker 2: lead towards what is called the Big Bang Singularity. Density
2:13:27
Speaker 2: and spacetime curvature can formally grow without limit, while the
2:13:31
Speaker 2: cosmological scale factor approaches zero. It is tempting to take
2:13:35
Speaker 2: this literally and imagine that physics has proved the entire
2:13:38
Speaker 2: universe began as an infinitely dense point. That conclusion goes
2:13:43
Speaker 2: beyond what the theory can safely tell us. A singularity
2:13:47
Speaker 2: is better understood as a warning that the mathematical description
2:13:50
Speaker 2: being used has reached the edge of its reliable territory.
2:13:55
Speaker 2: Albert Einstein's general theory of relativity is extraordinarily successful, but
2:14:00
Speaker 2: it is not a quantum theory. At sufficiently extreme energies
2:14:04
Speaker 2: and spacetime curvatures, quantum effects are expected to become essential.
2:14:10
Speaker 2: yet physics still lacks an experimentally confirmed complete theory of
2:14:14
Speaker 2: quantum gravity capable of describing those conditions, so the equations
2:14:20
Speaker 2: can be extrapolated towards a singularity. That does not mean
2:14:24
Speaker 2: anyone has observed one, nor does it establish that a
2:14:27
Speaker 2: literal point of infinite density physically existed. This uncertainty becomes
2:14:32
Speaker 2: especially important when we ask perhaps the most irresistible question
2:14:36
Speaker 2: of all. What happened before the Big Bang? At present,
2:14:41
Speaker 2: there is no experimentally established answer. Some cosmological models place
2:14:46
Speaker 2: a period of inflation before the conventional hot Big Bang phase.
2:14:51
Speaker 2: In those descriptions, the extremely hot mixture of particles and
2:14:54
Speaker 2: radiation appears after inflation ends, and energy is transferred into
2:14:59
Speaker 2: ordinary particle fields. But inflation itself does not necessarily identify
2:15:04
Speaker 2: an ultimate beginning. It may simply move the question farther back.
2:15:09
Speaker 2: Other ideas propose bouncing cosmologies, in which an earlier contracting
2:15:13
Speaker 2: phase is followed by expansion. Some approaches to quantum cosmology
2:15:18
Speaker 2: describe states in which ordinary ideas of time behave very
2:15:22
Speaker 2: differently from their familiar form. Other models investigate cycles, emergent
2:15:28
Speaker 2: universes or more complicated histories. These possibilities are scientifically interesting.
2:15:34
Speaker 2: They are not all equivalent. and none has yet been
2:15:37
Speaker 2: established as the confirmed history preceding the Hot Big Bang.
2:15:41
Speaker 2: Even the word before becomes surprisingly difficult. In ordinary life,
2:15:46
Speaker 2: every event seems to have an earlier moment. Breakfast comes
2:15:49
Speaker 2: after waking, Tuesday comes after Monday, a supernova occurs after
2:15:54
Speaker 2: the star that produces it has already lived for millions
2:15:57
Speaker 2: of years. So asking what happened before the early universe
2:16:00
Speaker 2: feels completely natural. But time is part of space-time. If
2:16:05
Speaker 2: the nature of space-time changes radically in an unknown quantum
2:16:09
Speaker 2: gravitational regime, our familiar concept of a universal sequence of
2:16:14
Speaker 2: earlier and later moments may no longer apply straightforwardly. That
2:16:18
Speaker 2: does not mean science has proved there was no before.
2:16:22
Speaker 2: It has not. It means we do not yet know
2:16:25
Speaker 2: whether the question can be extended indefinitely backwards, using the
2:16:29
Speaker 2: everyday meaning of time. There is uncertainty at that boundary.
2:16:33
Speaker 2: There is much less uncertainty about what the Big Bang
2:16:36
Speaker 2: was not. There is no known central detonation point somewhere
2:16:40
Speaker 2: in the cosmos. There is no requirement for an external
2:16:43
Speaker 2: empty void into which the universe exploded. There is no
2:16:47
Speaker 2: shell of galaxies moving away from a central blast site.
2:16:51
Speaker 2: There is no physical outer shock front separating expanding cosmic
2:16:54
Speaker 2: debris from untouched space. And there is no special galaxy
2:16:58
Speaker 2: sitting at the middle of everything. Those are features of
2:17:01
Speaker 2: the explosion picture. not of standard Big Bang cosmology. Cosmic
2:17:06
Speaker 2: expansion also did not finish once the early universe cooled.
2:17:10
Speaker 2: It continues today. Distant gravitationally unbound regions continue to participate
2:17:16
Speaker 2: in the changing large-scale geometry of the cosmos. That does
2:17:20
Speaker 2: not mean everything's expanding. Atoms remain bound by electromagnetism. Earth
2:17:26
Speaker 2: remains bound by its own gravity. The solar system does
2:17:30
Speaker 2: not simply swell with the scale factor. The stars of
2:17:34
Speaker 2: the Milky Way remain gravitationally connected, and many galaxy groups
2:17:39
Speaker 2: and clusters abound strongly enough that local gravity dominates over
2:17:43
Speaker 2: cosmological expansion. The modern universe is therefore deeply structured even
2:17:49
Speaker 2: while expansion continues between sufficiently separated regions. And that expansion
2:17:55
Speaker 2: contains another unresolved puzzle. Observations of distant exploding stars known
2:18:00
Speaker 2: as Type Ia supernovae in the late 1990s reveal that
2:18:05
Speaker 2: the expansion of the universe has been accelerating during comparatively
2:18:09
Speaker 2: recent cosmic history. Other cosmological measurements support the same broad conclusion.
2:18:16
Speaker 2: The unknown component associated with this accelerated expansion is called
2:18:21
Speaker 2: dark energy. Within the standard cosmological model, the simplest description
2:18:26
Speaker 2: uses Einstein's cosmological constant, but physicists still do not understand
2:18:32
Speaker 2: at a fundamental level. why the observed accelerated expansion has
2:18:36
Speaker 2: the magnitude it does, or whether dark energy is truly
2:18:39
Speaker 2: a cosmological constant or something more complicated. So even 13.8
2:18:45
Speaker 2: billion years into the story, cosmic expansion still contains unanswered questions.
2:18:52
Speaker 2: And that is perhaps the clearest way to understand the
2:18:54
Speaker 2: status of the Big Bang. There are things cosmology knows
2:18:58
Speaker 2: remarkably well. There are things it strongly suspects. and there
2:19:03
Speaker 2: are things it simply does not yet know. What observations
2:19:07
Speaker 2: establish with extraordinary confidence is that the observable universe was
2:19:12
Speaker 2: once vastly hotter, denser and more uniform than it is today.
2:19:17
Speaker 2: It expanded. It cooled. Particles, nuclei and atoms emerged as
2:19:23
Speaker 2: conditions changed. Tiny primordial variations grew into stars, galaxies and
2:19:29
Speaker 2: the large-scale structure surrounding us. That evolution has unfolded over
2:19:34
Speaker 2: approximately 13.8 billion years. What caused the deepest beginning, whether
2:19:40
Speaker 2: an absolute beginning occurred at all, what preceded the earliest
2:19:44
Speaker 2: well-understood phases, and why there is a universe in the
2:19:47
Speaker 2: first place remain open questions. The Big Bang Theory does
2:19:51
Speaker 2: not answer every question about existence. It does something more
2:19:55
Speaker 2: careful and scientifically more powerful. It tells us an extraordinary
2:20:00
Speaker 2: amount about how the universe we inhabit came to look
2:20:03
Speaker 2: the way it does. If we return one last time
2:20:06
Speaker 2: to the image that the words Big Bang so easily create,
2:20:10
Speaker 2: we can now see just how misleading it is. There
2:20:13
Speaker 2: was not a tiny object sitting in a vast black emptiness.
2:20:17
Speaker 2: There was not one central point where everything suddenly detonated.
2:20:21
Speaker 2: There was not a shell of matter racing outward into
2:20:24
Speaker 2: pre-existing space. and there was no cosmic blast front pushing
2:20:28
Speaker 2: into some untouched region beyond it. The scientifically supported picture
2:20:33
Speaker 2: is stranger, but also much more interesting. The observable universe
2:20:38
Speaker 2: was once far hotter, denser and more uniform than it
2:20:42
Speaker 2: is today. Over cosmic time, the large-scale distances between sufficiently
2:20:47
Speaker 2: separated co-moving regions increased. As the universe expanded, it cooled.
2:20:53
Speaker 2: Matter changed form. Radiation lost energy. New structures became possible.
2:20:59
Speaker 2: So the most direct answer is also the simplest. The
2:21:02
Speaker 2: Big Bang was not an explosion of matter from one
2:21:05
Speaker 2: point into empty space. It was the early expansion and
2:21:09
Speaker 2: cooling of the universe itself. The name has not exactly helped.
2:21:14
Speaker 2: Fred Hoyle's phrase Big Bang became one of the most
2:21:17
Speaker 2: memorable labels in all of science, even though Hoyle himself
2:21:21
Speaker 2: was arguing against the Hot Big Bang picture. There is
2:21:24
Speaker 2: a quiet irony in that. The name that survived suggests
2:21:28
Speaker 2: almost precisely the wrong mental image. Yet the physics that
2:21:32
Speaker 2: replaced that image was built gradually through theory and observation.
2:21:36
Speaker 2: Alexander Friedman showed that Albert Einstein's equations of general relativity
2:21:42
Speaker 2: allowed a dynamic universe whose scale could change with time.
2:21:46
Speaker 2: Georges Lemaître independently developed an expanding universe model and connected
2:21:51
Speaker 2: it with astronomical observations. Vesto Slipher measured the redshifts of
2:21:56
Speaker 2: distant spiral nebulae. Edwin Hubble helped establish their distances and
2:22:01
Speaker 2: revealed the broader relationship between distance and recession. Together, these
2:22:06
Speaker 2: ideas transformed the universe from a supposedly static backdrop into
2:22:11
Speaker 2: something with an evolving history. And once the universe itself
2:22:15
Speaker 2: could evolve, the idea of a central explosion became unnecessary.
2:22:20
Speaker 2: There is no known centre of cosmic expansion. The Milky
2:22:24
Speaker 2: Way is not sitting at a privileged location. Neither is
2:22:28
Speaker 2: any other galaxy. Every sufficiently large region participates in the
2:22:32
Speaker 2: same general expansion. The region now occupied by Earth was
2:22:36
Speaker 2: itself part of the hot early universe. So was the
2:22:40
Speaker 2: region now occupied by Andromeda. So were the regions that
2:22:43
Speaker 2: became galaxies billions of light-years away. The Big Bang was
2:22:48
Speaker 2: not somewhere else. In the cosmological sense, it happened everywhere.
2:22:52
Speaker 2: That is why the balloon and raisin bread analogies can
2:22:55
Speaker 2: be helpful, provided we remember where they fail. Dots on
2:22:59
Speaker 2: a balloon move farther apart as the surface expands. Raisins
2:23:03
Speaker 2: in rising dough separate as the dough between them grows.
2:23:07
Speaker 2: Neither analogy requires the dots or raisins to launch themselves
2:23:11
Speaker 2: away from a central blast. That is the useful lesson.
2:23:16
Speaker 2: But the universe does not need an external room, an oven.
2:23:20
Speaker 2: a surrounding atmosphere or some larger ordinary space into which
2:23:24
Speaker 2: it expands. Those belong to the analogy, not the cosmology.
2:23:29
Speaker 2: The same deeper logic explains another apparently impossible result. Very
2:23:34
Speaker 2: distant galaxies can have recession rates greater than the speed
2:23:37
Speaker 2: of light. That does not violate special relativity because those
2:23:42
Speaker 2: galaxies are not locally racing through nearby space faster than light.
2:23:47
Speaker 2: The proper distance between widely separated come-oving regions can increase
2:23:52
Speaker 2: faster than light, because the geometry between them is evolving. Locally,
2:23:57
Speaker 2: light still obeys the same speed limit it always does. Again,
2:24:01
Speaker 2: cosmic expansion behaves differently from debris flying through static space,
2:24:06
Speaker 2: and as the universe expanded its physical conditions changed dramatically.
2:24:12
Speaker 2: In the earliest hot phases we can describe with established physics,
2:24:16
Speaker 2: The cosmos contained an intense mixture of radiation and elementary particles.
2:24:21
Speaker 2: Quarks became confined into protons and neutrons. Matter survived its
2:24:26
Speaker 2: encounter with antimatter because of a small imbalance whose origin
2:24:30
Speaker 2: is still not fully understood. Within the first few minutes,
2:24:34
Speaker 2: protons and neutrons began forming the nuclei of hydrogen, helium
2:24:38
Speaker 2: and a few other light elements. Hundreds of thousands of
2:24:42
Speaker 2: years later, the universe cooled enough for electrons to remain
2:24:45
Speaker 2: bound to nuclei. Neutral atoms formed. Photons stopped scattering constantly
2:24:51
Speaker 2: from free electrons. And suddenly, on cosmic scales, the universe
2:24:57
Speaker 2: became transparent. Those ancient photons are still around us. Arno
2:25:02
Speaker 2: Penzias and Robert Wilson detected their microwave glow in the 1960s. Later,
2:25:08
Speaker 2: the cosmic background explorer The Wilkinson Microwave Anisotropy Probe and
2:25:13
Speaker 2: the Planck mission mapped that radiation with increasing precision. The
2:25:19
Speaker 2: cosmic microwave background now gives us one of our clearest
2:25:22
Speaker 2: observational windows into the young universe. Its tiny variations preserve
2:25:27
Speaker 2: information about the primordial differences in density that later grew
2:25:31
Speaker 2: into galaxies, clusters and the vast cosmic web. So the
2:25:36
Speaker 2: Big Bang model is not based on one clue. Cosmic
2:25:40
Speaker 2: expansion points backwards towards a denser past. Primordial element abundances
2:25:46
Speaker 2: match the predictions of a hot early phase. The cosmic
2:25:50
Speaker 2: microwave background reveals the relic radiation expected from that same history.
2:25:55
Speaker 2: Different observations, different physical processes, one coherent picture. That does
2:26:01
Speaker 2: not mean every question has been answered. Cosmic inflation remains
2:26:05
Speaker 2: strongly motivated but not fully understood. We do not know
2:26:09
Speaker 2: what physical mechanism drove it, assuming it occurred. We do
2:26:13
Speaker 2: not yet possess a confirmed theory of quantum gravity capable
2:26:17
Speaker 2: of describing the most extreme earliest regime. We do not
2:26:21
Speaker 2: know whether an absolute beginning occurred in the simple sense
2:26:25
Speaker 2: we usually imagine. And we do not know whether a
2:26:28
Speaker 2: meaningful earlier phase existed before the hot big bang. Those
2:26:31
Speaker 2: are genuine open questions, but they do not erase what
2:26:35
Speaker 2: the evidence already tells us. The observable universe really was
2:26:39
Speaker 2: much hotter, denser and more uniform in the past. It
2:26:44
Speaker 2: really has been expanding, it really has been cooling and
2:26:48
Speaker 2: that expansion has continued for approximately 13.8 billion years. So
2:26:53
Speaker 2: perhaps the best image to keep is not a fireball
2:26:56
Speaker 2: erupting into darkness. Picture instead an entire cosmos in an
2:27:01
Speaker 2: earlier hotter state. No centre, no surrounding void. No outer
2:27:06
Speaker 2: edge of debris, just distances changing throughout space, temperature falling
2:27:11
Speaker 2: as the universe grows, and simple particles gradually giving way
2:27:15
Speaker 2: to nuclei, atoms, stars, galaxies, planets, and eventually someone lying
2:27:21
Speaker 2: awake late at night wondering how all of it began.
2:27:26
Speaker 2: The Big Bang was not an explosion in space. It
2:27:29
Speaker 2: was the expansion of the universe. and in a very
2:27:32
Speaker 2: real sense that expansion is still happening all around us now.