Latest / Boring Science / How Do Galaxies Form Across Billions of Years

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How Do Galaxies Form Across Billions of Years · Boring Science

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  Speaker 1: For about 400 million years after the Big Bang, the
• 0:03
  Speaker 1: universe was dark. No stars. No galaxies. No light of
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  Speaker 1: any kind crossing the void. Just an enormous cooling ocean
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  Speaker 1: of hydrogen and helium gas, spreading outward in every direction,
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  Speaker 1: nearly perfectly smooth, nearly perfectly uniform, with nothing inside it
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  Speaker 1: that you would recognize as structure. That is the universe
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  Speaker 1: you came from. And the question of how it got
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  Speaker 1: from there to here, from featureless dark to the hundred
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  Speaker 1: billion galaxies visible in the deepest images ever taken by
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  Speaker 1: human telescopes, is one of the stranger stories in science.
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  Speaker 1: Here is why it matters personally. Every atom of calcium
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  Speaker 1: in your bones was forged inside a star, The iron
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  Speaker 1: in your blood was assembled in a stellar core and
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  Speaker 1: scattered by a supernova explosion billions of years before the
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  Speaker 1: solar system existed. The oxygen you are breathing right now
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  Speaker 1: was processed through multiple generations of massive stars, each one
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  Speaker 1: living and dying inside a galaxy, enriching the surrounding gas
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  Speaker 1: with heavier and heavier elements over billions of years. Without galaxies,
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  Speaker 1: none of that chemistry happens. Without that chemistry, there is
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  Speaker 1: no rocky planet, no ocean, no biology. The story of
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  Speaker 1: how galaxies formed is, in a very literal sense, the
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  Speaker 1: story of how the raw materials of everything you are
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  Speaker 1: came to exist. If you find comfort in these gentle
• 1:48
  Speaker 1: explorations of space and science, I'd be grateful if you'd
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  Speaker 1: consider subscribing and giving this video a like. So how
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  Speaker 1: does a smooth, dark, featureless universe build something like the
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  Speaker 1: Milky Way? That is not a simple question. For decades,
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  Speaker 1: astronomers assumed the broad outlines were understood. Gas collapsed under gravity,
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  Speaker 1: stars formed, galaxies assembled. Clean, sequential, manageable. What the evidence
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  Speaker 1: has actually revealed is something far stranger. There is an
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  Speaker 1: invisible substance permeating the universe, something that outweighs all the
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  Speaker 1: stars and gas and dust by a factor of five,
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  Speaker 1: something that does not emit light or absorb it, that
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  Speaker 1: has never been directly detected in a laboratory, and without
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  Speaker 1: which no galaxy would ever have formed at all. The
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  Speaker 1: structure of every galaxy you have ever seen in a
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  Speaker 1: photograph was built first in the dark. by something we
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  Speaker 1: cannot see. Then there are the first stars themselves, enormous
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  Speaker 1: objects unlike anything burning in the sky today, that lived
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  Speaker 1: for only a few million years before destroying themselves and
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  Speaker 1: permanently altering the chemistry of the universe. And the collisions,
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  Speaker 1: billions of years of galaxies crashing into each other and merging,
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  Speaker 1: building larger structures from smaller ones. The Milky Way itself
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  Speaker 1: assembled from dozens of smaller galaxies that no longer exist
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  Speaker 1: as separate things. And now, images from the James Webb
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  Speaker 1: Space Telescope showing galaxies in the early universe that are
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  Speaker 1: far too large and far too bright for our best
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  Speaker 1: models to explain. Galaxies that, by the numbers, should not
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  Speaker 1: be there. What you are about to see is not
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  Speaker 1: a settled history. parts of it are still being figured out.
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  Speaker 1: The answer starts before any star ever existed, and it
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  Speaker 1: starts in the dark. The Milky Way is roughly 100,000
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  Speaker 1: light years across. The number that gets used most often
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  Speaker 1: to describe it, 200 to 400 billion stars, is an estimate.
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  Speaker 1: This was because nobody has ever counted them. Nobody ever will.
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  Speaker 1: The best we can do is measure a region, Extrapolate.
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  Speaker 1: Accept that the uncertainty swallows a hundred billion stars without apology.
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  Speaker 1: That number is so large it stops being useful almost immediately.
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  Speaker 1: So here is a different way into it. If you
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  Speaker 1: tried to count every star in the Milky Way at
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  Speaker 1: a rate of one per second, the count would take
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  Speaker 1: longer than the current age of the universe. You would
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  Speaker 1: need to have started before the Big Bang, and you
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  Speaker 1: still wouldn't be done. But the stars are only part
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  Speaker 1: of what a galaxy is. In some ways, not even
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  Speaker 1: the most important part. Between the stars there is gas,
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  Speaker 1: enormous clouds of hydrogen and helium, some cold and dense
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  Speaker 1: and on the verge of collapsing into new stars, others
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  Speaker 1: hot and diffuse and still expanding outward from the explosions
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  Speaker 1: of stars that died millions of years ago. There is dust,
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  Speaker 1: tiny grains of carbon and silicate that scatter light and
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  Speaker 1: obscure whole regions of the sky from our view. That
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  Speaker 1: dust is not inert. It is the raw material of planets.
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  Speaker 1: The silicate in a grain of interstellar dust and the
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  Speaker 1: silicate in the rock under your feet are the same substance.
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  Speaker 1: Separated by billions of years and a brief stint inside
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  Speaker 1: a forming solar system, Then there is something you cannot
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  Speaker 1: see at all. Around the Milky Way, extending far beyond
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  Speaker 1: the visible edge of the galaxy's disk, is a halo
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  Speaker 1: of dark matter. It does not emit light. It does
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  Speaker 1: not absorb light. It does not interact with anything except
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  Speaker 1: through gravity. But it is there, and by mass, it
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  Speaker 1: is the dominant component of the galaxy. The stars and
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  Speaker 1: gas and dust that produce every photon of light you
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  Speaker 1: have ever seen from the night sky account for only
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  Speaker 1: a fraction of the Milky Way's total mass. The dark
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  Speaker 1: matter halo is several times more massive than all of
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  Speaker 1: that visible material combined. It extends roughly ten times farther
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  Speaker 1: out into space than the farthest stars. The galaxy you
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  Speaker 1: think you are looking at when you step outside on
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  Speaker 1: a clear night is a thin visible layer wrapped around
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  Speaker 1: something much larger and much stranger. How far away is
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  Speaker 1: the nearest thing like it? The Andromeda galaxy sits about 2.5
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  Speaker 1: million light years from here. That light crossing your eye
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  Speaker 1: right now left Andromeda when the earliest members of the
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  Speaker 1: genus Homo were first appearing on Earth. In between the
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  Speaker 1: two galaxies, there is not quite nothing, There are smaller
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  Speaker 1: satellite galaxies, streams of stripped stars, thin threads of gas,
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  Speaker 1: but the space is close enough to empty that the
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  Speaker 1: distance is genuinely hard to hold in mind. Two and
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  Speaker 1: a half million light years is the gap between the
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  Speaker 1: two largest members of our immediate neighborhood. The nearest galaxies
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  Speaker 1: beyond our local group are tens of millions of light
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  Speaker 1: years away. Pull back farther. The observable universe contains something
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  Speaker 1: on the order of two trillion galaxies. That estimate has
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  Speaker 1: shifted upwards significantly as telescopes improved, and it may shift again.
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  Speaker 1: Two trillion. Each one a gravitationally bound system containing billions
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  Speaker 1: to hundreds of billions of stars. Its own gas supply,
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  Speaker 1: its own dust, its own invisible dark matter halo. Some
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  Speaker 1: are larger than the Milky Way by a factor of ten.
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  Speaker 1: Some are smaller by a factor of a thousand. Dwarf
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  Speaker 1: galaxies exist that contain only a few million stars and
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  Speaker 1: barely qualify as galaxies at all by some definitions. Giant
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  Speaker 1: elliptical galaxies sit at the centers of galaxy clusters and
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  Speaker 1: contain more stars than there are grains of sand on
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  Speaker 1: every beach on Earth. The range is extraordinary. Here is
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  Speaker 1: where the common picture breaks down. Most people, if asked
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  Speaker 1: what a galaxy is, would say something like a big
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  Speaker 1: collection of stars spinning in space. That is not wrong, exactly,
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  Speaker 1: but it leaves out almost everything that matters. A galaxy
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  Speaker 1: is not a static object. It is a cycle. Gas
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  Speaker 1: collapses into stars. Stars burn for millions or billions of years.
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  Speaker 1: fusing lighter elements into heavier ones. Then they die, some quietly,
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  Speaker 1: shedding their outer layers into space, others violently, exploding and
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  Speaker 1: scattering material across thousands of light years. That expelled material
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  Speaker 1: mixes back into the surrounding gas, enriched with the heavy
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  Speaker 1: elements the star spent its life producing. New generations of
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  Speaker 1: stars form from that enriched gas. The cycle runs continuously,
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  Speaker 1: and it has been running in the Milky Way for
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  Speaker 1: almost 13 billion years. The galaxy is not a collection
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  Speaker 1: of objects so much as a process, a slow, enormous,
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  Speaker 1: ongoing process. chemical transformation powered by gravity and nuclear fire.
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  Speaker 1: That process does not run the same way in every galaxy.
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  Speaker 1: Spiral galaxies like the Milky Way have flat rotating disks
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  Speaker 1: threaded with lanes of gas and dust. With star formation
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  Speaker 1: still actively occurring, elliptical galaxies are rounder, redder, largely quiet.
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  Speaker 1: Their star formation shut off billions of years ago, their
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  Speaker 1: gas either consumed or expelled. Irregular galaxies have no organised
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  Speaker 1: structure at all, often because a collision or close encounter
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  Speaker 1: tore apart whatever order they once had. Dwarf spheroidals are
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  Speaker 1: nearly featureless balls of old stars, almost no gas remaining,
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  Speaker 1: ghosts of what were once more active systems. These are
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  Speaker 1: not just different shapes. They represent different life histories, different
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  Speaker 1: relationships between a galaxy and its environment, different outcomes from
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  Speaker 1: the same basic, starting material. That last point is the
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  Speaker 1: one worth sitting with. Every galaxy visible in any deep-field image,
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  Speaker 1: from the orderly grand spirals to the shapeless smears of
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  Speaker 1: irregular dwarfs, began from the same kind of raw material
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  Speaker 1: under the same physical laws. The universe did not start
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  Speaker 1: with spirals in one place and ellipticals in another. It
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  Speaker 1: started with nearly featureless expanding gas, with only the faintest
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  Speaker 1: wrinkles in density to break the uniformity. Everything you see
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  Speaker 1: in the night sky, the whole staggering catalogue of galactic shapes, sizes, colours, behaviours,
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  Speaker 1: is the result of those wrinkles being amplified by gravity
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  Speaker 1: over billions of years. In slightly different ways, in slightly
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  Speaker 1: different environments, producing wildly different outcomes. All of that diversity,
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  Speaker 1: every shape and size of galaxy visible eye, and the
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  Speaker 1: night sky, grew from the same featureless early universe. Tracing
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  Speaker 1: how it happened is one of the central projects of
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  Speaker 1: modern science. In the first 380,000 years after the Big Bang,
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  Speaker 1: light couldn't even travel freely through the universe. The entire
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  Speaker 1: cosmos was a plasma, a dense, superheated fog of electrons
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  Speaker 1: and protons so tightly packed that any photon trying to
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  Speaker 1: move through it immediately collided with something and scattered. Think
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  Speaker 1: of trying to see through a wall of white-hot steam.
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  Speaker 1: The light was there, but it went nowhere. It bounced.
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  Speaker 1: It ricocheted. It was trapped inside a universe that had
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  Speaker 1: not yet cooled enough to let it escape. Then, at
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  Speaker 1: roughly 380,000 years after the beginning, the temperature dropped to
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  Speaker 1: around 3,000 degrees. Electrons and protons combined into neutral hydrogen
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  Speaker 1: atoms for the first time. The fog cleared, and the
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  Speaker 1: light that had been trapped was suddenly free to travel
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  Speaker 1: in all directions across the cosmos. We can still detect
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  Speaker 1: that light today. It's called the cosmic microwave background, and
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  Speaker 1: it is the oldest light in the universe. But here
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  Speaker 1: is what happened next, Nothing. For a very long time,
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  Speaker 1: almost nothing at all. The universe expanded. It cooled. The
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  Speaker 1: hydrogen and helium gas that filled all of space thinned
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  Speaker 1: out gradually as the cosmos stretched in every direction. There
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  Speaker 1: were no stars producing ultraviolet radiation. There were no galaxies
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  Speaker 1: concentrating heat. There was no light source of any kind, anywhere.
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  Speaker 1: Astronomers call this period the Cosmic Dark Ages, and the
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  Speaker 1: name is not dramatic license. It is a literal description
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  Speaker 1: of what the universe was. Dark. Silent. Expanding. A vast,
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  Speaker 1: cooling ocean of gas with no internal illumination and no
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  Speaker 1: structure you would recognize. This phase lasted somewhere between 100
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  Speaker 1: and 400 million years. depending on where exactly you draw
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  Speaker 1: the line. On a cosmological scale, that is a blink.
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  Speaker 1: On any human scale, it is a number too large
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  Speaker 1: to sit comfortably in the mind. What was actually in
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  Speaker 1: this darkness? Mostly hydrogen, about 75% hydrogen, roughly 25% helium
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  Speaker 1: by mass, and trace amounts of lithium. That is almost
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  Speaker 1: the complete inventory of matter in the early universe. No carbon.
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  Speaker 1: No oxygen. No iron. None of the heavier elements that
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  Speaker 1: make up planets, or biology, or anything we would think
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  Speaker 1: of as interesting chemistry. Just the two simplest atoms ever assembled,
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  Speaker 1: cooling slowly inside an expanding universe that had no particular
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  Speaker 1: interest in putting them together into anything more complicated. Alongside
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  Speaker 1: this ordinary gas was something else. dark matter. Enormous quantities
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  Speaker 1: of it spread through space in ways that did not
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  Speaker 1: perfectly match the distribution of the hydrogen and helium. It
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  Speaker 1: didn't glow. It didn't interact with the gas in any
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  Speaker 1: obvious way. But it had gravity, and gravity is patient.
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  Speaker 1: Dark matter was already beginning to do the only thing
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  Speaker 1: it does. It was pulling. Now here is the question
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  Speaker 1: worth sitting with If the universe was this uniform, this featureless,
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  Speaker 1: how did anything ever form at all? A perfectly smooth
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  Speaker 1: universe would stay smooth forever. Gravity pulling equally in all
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  Speaker 1: directions doesn't build structure. It just holds everything in place.
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  Speaker 1: For galaxies to eventually exist, something had to break the symmetry.
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  Speaker 1: Something had to create a difference. Some regions slightly denser
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  Speaker 1: than others. some regions slightly emptier. And that something happened
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  Speaker 1: before any of this. It happened in the first fraction
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  Speaker 1: of the first second of existence, at scales so small
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  Speaker 1: they make atoms look enormous. Quantum mechanics says that at
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  Speaker 1: sufficiently small scales, the universe is fundamentally uncertain. Particle positions,
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  Speaker 1: energy levels, densities, None of these are perfectly fixed. They
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  Speaker 1: fluctuate randomly, constantly, at the tiniest scales. In the first
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  Speaker 1: moments after the Big Bang, the entire universe was compressed
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  Speaker 1: into a region smaller than any particle we know, and
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  Speaker 1: those quantum fluctuations were happening across it. Then inflation occurred,
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  Speaker 1: a period of extraordinarily rapid expansion in which the universe
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  Speaker 1: ballooned outward, by an enormous factor in a tiny fraction
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  Speaker 1: of a second. What inflation did to those quantum fluctuations
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  Speaker 1: was stretch them. Patterns that existed at subatomic scales were
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  Speaker 1: expanded to cosmic proportions. Random quantum noise became real, physical
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  Speaker 1: variations in the density of the early universe. Some regions
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  Speaker 1: wound up very slightly denser than average. Some regions wound
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  Speaker 1: up very slightly emptier Not by much. The differences were minuscule.
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  Speaker 1: Those differences showed up in the cosmic microwave background. When
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  Speaker 1: the fog cleared at 380,000 years and that ancient light
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  Speaker 1: was released, it carried an imprint of the density variations
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  Speaker 1: that existed at that moment. Different temperatures in slightly different
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  Speaker 1: directions across the sky. The maps we've made of the
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  Speaker 1: cosmic microwave background, most recently with the Planck satellite, show
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  Speaker 1: this directly. You're looking at a portrait of the infant universe,
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  Speaker 1: taken when it was less than one four hundredths of
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  Speaker 1: one percent of its current age. Every slight warm spot
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  Speaker 1: on that map is a region that was, at that moment,
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  Speaker 1: very slightly denser than its surroundings. Every slight cool spot
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  Speaker 1: is a region that was very slightly emptier. The differences
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  Speaker 1: are small enough that they require sensitive instruments to detect
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  Speaker 1: at all. But they are real, and they were the
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  Speaker 1: only thing the universe needed to get started. Then the
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  Speaker 1: Dark Ages settled in, and those variations just sat there. Cooling. Expanding. Waiting.
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  Speaker 1: This is the part that's easy to underestimate. Nothing dramatic
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  Speaker 1: was happening on human timescales. The universe wasn't building toward
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  Speaker 1: anything visible. Gas was cooling. Dark matter was very, very
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  Speaker 1: slowly drifting toward the denser regions. The over-dense patches were
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  Speaker 1: pulling in their neighbors, becoming fractionally denser still. It was
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  Speaker 1: a process playing out across hundreds of millions of years
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  Speaker 1: without a single photon to show for it. No explosion.
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  Speaker 1: No announcement. Just gravity. working silently on slightly uneven ground,
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  Speaker 1: in the dark. What had to happen before anything could ignite?
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  Speaker 1: The gas had to cool enough that pressure couldn't support
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  Speaker 1: it against gravity. The dark matter had to concentrate enough
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  Speaker 1: to create gravitational wells deep enough for ordinary gas to
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  Speaker 1: fall into. The density in certain regions had to cross
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  Speaker 1: a threshold, a point where collapse became inevitable rather than
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  Speaker 1: just possible. None of these conditions were met quickly. The
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  Speaker 1: universe had to wait. It had to cool further, expand further,
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  Speaker 1: let gravity accumulate its slow advantage. And inside all that waiting,
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  Speaker 1: those quantum wrinkles from the first instant of existence were
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  Speaker 1: being amplified, slowly, into the architecture of everything that would
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  Speaker 1: come next. Those wrinkles were imperceptibly small, a few parts
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  Speaker 1: in a hundred thousand, and they were going to build everything.
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  Speaker 1: In 1978, an astronomer named Vera Rubin published measurements of
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  Speaker 1: how fast stars at different distances from the center of
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  Speaker 1: a galaxy were moving, and the numbers made no sense.
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  Speaker 1: Here is what the numbers should have shown. When you
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  Speaker 1: move away from the center of a galaxy, the amount
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  Speaker 1: of visible matter pulling on you drops off. There are
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  Speaker 1: fewer stars, less gas, less of everything you can actually see.
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  Speaker 1: Basic physics says that the farther out you go, the
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  Speaker 1: slower things should be moving. Rubin's measurements broke it completely.
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  Speaker 1: The stars at the outer edges of the galaxies she
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  Speaker 1: studied were moving just as fast as the stars near
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  Speaker 1: the center. In some cases, faster. No slowdown, no drop-off,
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  Speaker 1: just a flat rotation curve stretching out to the edge
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  Speaker 1: of the detectable disk, as if there were no edge
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  Speaker 1: at all. Other astronomers looked at other galaxies and found
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  Speaker 1: the same thing. Whatever was holding those outer stars in
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  Speaker 1: their orbits, it was not the visible matter anyone had mapped.
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  Speaker 1: There were two possible responses to this finding. One was
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  Speaker 1: that our understanding of gravity was wrong at large scales.
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  Speaker 1: The other was that there was matter out there we
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  Speaker 1: couldn't see. For decades, a small group of physicists pushed
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  Speaker 1: on the gravity explanation, developing modifications to Newtonian dynamics that
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  Speaker 1: could reproduce some of the observations. But the modifications kept
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  Speaker 1: failing in new situations. The Invisible Matter Explanation uncomfortable as
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  Speaker 1: it was, kept working. It worked for rotation curves. It
• 23:10
  Speaker 1: worked for the bending of light around galaxy clusters. It
• 23:14
  Speaker 1: worked for the pattern of temperature fluctuations imprinted on the
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  Speaker 1: cosmic microwave background radiation. Every independent line of evidence pointed
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  Speaker 1: the same direction. Something was out there, and it was everywhere.
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  Speaker 1: Astronomers call it dark matter. which is a slightly misleading name.
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  Speaker 1: Dark implies that it absorbs light, or blocks it, the
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  Speaker 1: way a dark cloud crosses the sky. Dark matter doesn't
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  Speaker 1: do that. It doesn't interact with light at all. It
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  Speaker 1: doesn't absorb photons, doesn't reflect them, doesn't emit them. It
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  Speaker 1: passes through everything we know of without leaving a trace.
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  Speaker 1: The only thing it appears to do is gravity. It
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  Speaker 1: has mass That mass attracts other mass, and apart from
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  Speaker 1: that single interaction, it is as close to nothing as
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  Speaker 1: something can be while still existing. We know dark matter
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  Speaker 1: exists by what it does to the things around it.
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  Speaker 1: What it actually is remains genuinely unknown. Now go back
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  Speaker 1: further in time. Back to the cosmic dark ages described
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  Speaker 1: in the previous sections. when the universe was a nearly
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  Speaker 1: uniform soup of hydrogen, helium, and these invisible particles drifting
• 24:38
  Speaker 1: through a hot, expanding sea of radiation. In those early conditions,
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  Speaker 1: ordinary matter had a problem. It was coupled to the radiation,
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  Speaker 1: meaning it was constantly being pushed and pulled by the
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  Speaker 1: pressure of light itself. Ordinary gas couldn't simply fall toward
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  Speaker 1: denser regions and stay there, the radiation pressure kept pushing
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  Speaker 1: it back out. The density fluctuations in ordinary matter, those
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  Speaker 1: tiny quantum wrinkles, a few parts in a hundred thousand,
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  Speaker 1: were too small, and the radiation pressure fighting against them
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  Speaker 1: was too strong. If ordinary matter had been working alone,
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  Speaker 1: those fluctuations would have oscillated without growing, and the universe
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  Speaker 1: today would be smooth and empty. Dark matter changed everything.
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  Speaker 1: Because dark matter doesn't interact with radiation, it didn't have
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  Speaker 1: that problem. While ordinary gas was being buffeted back and
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  Speaker 1: forth by photon pressure, dark matter was quietly doing what
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  Speaker 1: gravity told it to do. It began falling towards slightly
• 25:48
  Speaker 1: denser regions. It began clumping. Slowly, over hundreds of millions
• 25:54
  Speaker 1: of years, those tiny over-densities in the dark matter grew.
• 25:59
  Speaker 1: The clumps attracted more dark matter. The clumps merged with
• 26:03
  Speaker 1: other clumps. The whole time, ordinary matter couldn't follow yet,
• 26:09
  Speaker 1: still pinned in place by radiation pressure. But the dark
• 26:13
  Speaker 1: matter kept building structure anyway, invisible and patient, constructing the
• 26:18
  Speaker 1: scaffolding before the visible universe had any idea what was coming. Then,
• 26:24
  Speaker 1: Around 380,000 years after the Big Bang, the universe cooled
• 26:29
  Speaker 1: enough for electrons and protons to combine into neutral hydrogen.
• 26:34
  Speaker 1: The radiation pressure dropped away almost instantly on a cosmic timescale.
• 26:41
  Speaker 1: Ordinary matter, freed from that constraint for the first time,
• 26:46
  Speaker 1: looked around and found that dark matter had already built
• 26:50
  Speaker 1: enormous gravitational wells across the universe. The gas didn't have
• 26:54
  Speaker 1: to start from scratch. The framework was already there. Think
• 26:59
  Speaker 1: of it like a bowl. The dark matter halo is
• 27:02
  Speaker 1: the bowl, dug deep and waiting, and ordinary gas is
• 27:07
  Speaker 1: sand being poured in from above. The gas follows the
• 27:10
  Speaker 1: contours of the bowl it cannot see. It falls in.
• 27:15
  Speaker 1: It settles. It concentrates. The deeper and larger the dark
• 27:20
  Speaker 1: matter well, the more gas falls in. and the more
• 27:24
  Speaker 1: mass accumulates in one place. What happens next, once enough
• 27:29
  Speaker 1: gas piles up in one of those wells, is the
• 27:32
  Speaker 1: beginning of everything we would eventually recognise as a galaxy.
• 27:37
  Speaker 1: But that step comes next. What matters here is that
• 27:42
  Speaker 1: the bowl had to exist first. The halos that dark
• 27:46
  Speaker 1: matter built were not flat disks. They were roughly spherical,
• 27:52
  Speaker 1: extending far beyond the regions where gas would eventually settle
• 27:56
  Speaker 1: and stars would eventually form. They were enormous. The dark
• 28:02
  Speaker 1: matter halo surrounding a galaxy like the Milky Way extends
• 28:06
  Speaker 1: hundreds of thousands of light years in every direction, dwarfing
• 28:11
  Speaker 1: the visible disk. The stars and spiral arms and glowing
• 28:16
  Speaker 1: nebulae that we can photograph represent a thin concentration at
• 28:21
  Speaker 1: the centre of something vastly larger that we cannot photograph
• 28:26
  Speaker 1: at all. When Rubin measured those flat rotation curves, she
• 28:31
  Speaker 1: was measuring the gravitational reach of structures that extend well
• 28:36
  Speaker 1: beyond where any telescope can see. The outer stars weren't
• 28:41
  Speaker 1: moving too fast for the visible galaxy. They were moving
• 28:45
  Speaker 1: at exactly the right speed for the full size of
• 28:48
  Speaker 1: the halo they were embedded in. This is worth sitting
• 28:52
  Speaker 1: with for a moment. Everything you have ever seen in
• 28:55
  Speaker 1: a picture of a galaxy, every spiral arm, every glowing
• 29:00
  Speaker 1: star cluster, every column of dust backlit by young blue stars,
• 29:06
  Speaker 1: is a relatively thin concentration of ordinary matter sitting inside
• 29:11
  Speaker 1: a much larger invisible sphere. It is a bright core
• 29:15
  Speaker 1: inside something much bigger. By mass, The dark matter halo
• 29:21
  Speaker 1: outweighs the stars and gas by a factor of roughly
• 29:24
  Speaker 1: 5 to 1 in a system like the Milky Way.
• 29:29
  Speaker 1: Some halos are even more extreme. The visible parts of
• 29:33
  Speaker 1: a galaxy, the parts we name and photograph and study,
• 29:38
  Speaker 1: are something like a small firefly. burning at the bottom
• 29:41
  Speaker 1: of a deep dark well. The well is what makes
• 29:44
  Speaker 1: the fire possible, but you can only see the fire,
• 29:49
  Speaker 1: and here is the harder implication. If this is true
• 29:53
  Speaker 1: for individual galaxies, it is true for the universe as
• 29:57
  Speaker 1: a whole. Ordinary matter protons, neutrons, electrons, everything that has
• 30:04
  Speaker 1: ever been touched or measured or looked at, makes up
• 30:09
  Speaker 1: roughly 5% of the total mass energy content of the universe.
• 30:14
  Speaker 1: Dark matter accounts for about 27%. The remainder is something
• 30:19
  Speaker 1: else entirely, the mysterious dark energy driving the universe's accelerating expansion,
• 30:26
  Speaker 1: which is a different problem for a different section. But
• 30:30
  Speaker 1: just the dark matter alone outweighs all the stars and
• 30:34
  Speaker 1: gas and planets and black holes and everything else we
• 30:37
  Speaker 1: have ever detected by more than five to one. The
• 30:42
  Speaker 1: universe we can see is a minority stake in something
• 30:45
  Speaker 1: we cannot. The stars and gas and dust that make
• 30:50
  Speaker 1: up everything we can see in a galaxy, in a
• 30:53
  Speaker 1: very real way, just the visible tip of a much
• 30:57
  Speaker 1: larger invisible structure. Gravity is patient. It does not need
• 31:02
  Speaker 1: to be strong. It does not need to act quickly.
• 31:06
  Speaker 1: It only needs time. And in the early universe, time
• 31:10
  Speaker 1: was the one thing available in extraordinary supply. The density
• 31:15
  Speaker 1: fluctuations left over from the Big Bang were not large.
• 31:20
  Speaker 1: A few parts in a hundred thousand. That is how
• 31:24
  Speaker 1: different the denser regions were from the emptier ones. By
• 31:28
  Speaker 1: any measure that matters to human intuition, the early universe
• 31:33
  Speaker 1: was almost perfectly smooth. But almost is not the same
• 31:38
  Speaker 1: as perfectly, and gravity knows the difference. Here is what
• 31:42
  Speaker 1: happens when you give gravity a slight advantage. A region
• 31:47
  Speaker 1: of space that is fractionally denser than its surroundings has
• 31:51
  Speaker 1: fractionally more gravitational pull. It draws in nearby matter. As
• 31:57
  Speaker 1: it draws in that matter, it becomes denser still. As
• 32:02
  Speaker 1: it becomes denser, its pull increases. The slightly emptier regions
• 32:08
  Speaker 1: around it lose matter to their denser neighbours, becoming emptier still,
• 32:13
  Speaker 1: their gravitational influence weakening. There is no mechanism to stop this.
• 32:19
  Speaker 1: There is no restoring force pushing matter back toward equilibrium.
• 32:24
  Speaker 1: The process runs in one direction only, and it runs indefinitely.
• 32:30
  Speaker 1: What starts as an almost imperceptible wrinkle in the density
• 32:34
  Speaker 1: of the universe, given enough time, becomes a wall of matter,
• 32:39
  Speaker 1: a filament, a collapsing halo. This is not a rapid
• 32:45
  Speaker 1: process by any hue, man-standard, but across hundreds of millions
• 32:50
  Speaker 1: of years, the math is relentless. Dark matter moved first.
• 32:55
  Speaker 1: This is the key fact. Ordinary gas in the early
• 32:59
  Speaker 1: universe was still coupled to radiation. It was being pushed
• 33:03
  Speaker 1: around by the intense pressure of photons, held back from
• 33:07
  Speaker 1: collapsing freely under gravity. Dark matter doesn't interact with light
• 33:12
  Speaker 1: at all. It feels no radiation pressure. While ordinary hydrogen
• 33:18
  Speaker 1: and helium gas was still being buffeted and smoothed out
• 33:21
  Speaker 1: by that photon pressure, Dark matter was already quietly clumping,
• 33:26
  Speaker 1: responding to gravity without any competing force to resist it.
• 33:30
  Speaker 1: The density fluctuations that ordinary matter couldn't yet act on,
• 33:35
  Speaker 1: dark matter was already amplifying. It had a head start
• 33:39
  Speaker 1: measured in tens of millions of years, and it used
• 33:42
  Speaker 1: that head start to build the scaffolding before the ordinary
• 33:45
  Speaker 1: matter even knew what was coming. What dark matter built
• 33:49
  Speaker 1: was halos. Roughly spherical concentrations of this invisible mass, growing
• 33:55
  Speaker 1: by pulling in more dark matter from their surroundings, growing
• 34:00
  Speaker 1: by merging with smaller neighboring halos, gradually accumulating mass the
• 34:05
  Speaker 1: way a snowball accumulates snow rolling downhill. These halos had
• 34:11
  Speaker 1: no sharp edges. There was no surface you could land on.
• 34:16
  Speaker 1: They were simply regions of elevated density, in an invisible
• 34:20
  Speaker 1: sea of dark matter, but their gravitational effect was real
• 34:25
  Speaker 1: and growing. Deep inside the densest halos, the gravitational potential
• 34:31
  Speaker 1: was becoming steep. Something falling into that potential well would
• 34:36
  Speaker 1: fall for a long time before it slowed down. Ordinary matter,
• 34:41
  Speaker 1: once it was finally free to move, was going to
• 34:45
  Speaker 1: fall a very long way. When the universe was roughly 380,000
• 34:50
  Speaker 1: years old, photons and ordinary matter decoupled. Light streamed away
• 34:56
  Speaker 1: in all directions. We see that light today as the
• 34:59
  Speaker 1: cosmic microwave background, and ordinary gas, no longer held smooth
• 35:05
  Speaker 1: by radiation pressure, was suddenly free to respond to gravity.
• 35:11
  Speaker 1: It looked around, so to speak. and found that the
• 35:14
  Speaker 1: dark matter halos had already been waiting. The gas began
• 35:19
  Speaker 1: to fall in. What happens to gas falling into a
• 35:22
  Speaker 1: gravitational well? It accelerates, and as it compresses, it heats up.
• 35:29
  Speaker 1: This is not a subtle effect. Gas falling into a
• 35:33
  Speaker 1: sufficiently deep potential well can heat to millions of degrees.
• 35:38
  Speaker 1: That heating creates pressure, and pressure resists further collapse. So
• 35:44
  Speaker 1: here is the problem. The ordinary matter that falls into
• 35:47
  Speaker 1: a dark matter halo needs to radiate away that heat
• 35:51
  Speaker 1: somehow before it can collapse further and begin forming stars.
• 35:57
  Speaker 1: If the gas can't cool, it just sits there, hot
• 36:01
  Speaker 1: and diffuse, unable to do anything interesting. This is why
• 36:05
  Speaker 1: not every dark matter halo produced a galaxy. A halo
• 36:09
  Speaker 1: had to be massive enough, and the gas inside it
• 36:13
  Speaker 1: had to be able to cool efficiently before anything recognizable
• 36:17
  Speaker 1: as a protogalaxy could form. The cooling is done by
• 36:21
  Speaker 1: atomic physics. Hydrogen atoms, heated by compression, emit photons as
• 36:28
  Speaker 1: electrons drop back to lower energy levels. Those photons carry
• 36:32
  Speaker 1: energy away from the gas. As the gas loses energy,
• 36:37
  Speaker 1: its temperature drops. As its temperature drops, Pressure support weakens,
• 36:43
  Speaker 1: and gravity, which has been waiting patiently the whole time,
• 36:47
  Speaker 1: begins pulling the gas inward again. The gas sinks deeper
• 36:52
  Speaker 1: into the dark matter halo, compresses more, heats again, radiates again. Slowly,
• 36:59
  Speaker 1: over millions of years, ordinary matter spirals inward through this cycle.
• 37:05
  Speaker 1: until the densest, coolest pockets are sitting deep in the
• 37:10
  Speaker 1: core of the halo compressed and cold enough that they
• 37:14
  Speaker 1: are finally approaching the conditions needed for something dramatic. The
• 37:19
  Speaker 1: smallest halos went through this process first simply because they
• 37:24
  Speaker 1: were the first to form. Gravity had amplified the smallest
• 37:29
  Speaker 1: density fluctuations earliest not because small scales were special but
• 37:35
  Speaker 1: because small-scale over-densities had a shorter distance to collapse. Those
• 37:40
  Speaker 1: early small halos weren't massive enough to contain enough gas
• 37:44
  Speaker 1: for large-scale star formation, but they were the seeds. Over time,
• 37:50
  Speaker 1: they merged with neighboring halos, and the merged structures pulled
• 37:54
  Speaker 1: in more gas, and the cycle of heating and cooling
• 37:58
  Speaker 1: played out on a larger and larger stage. The structure
• 38:02
  Speaker 1: built itself from the bottom up, each generation of halos
• 38:06
  Speaker 1: feeding into the next, the whole architecture becoming more complex
• 38:11
  Speaker 1: and more massive with every passing hundred million years. By
• 38:16
  Speaker 1: a few hundred million years after the Big Bang, there
• 38:20
  Speaker 1: were places in the universe where ordinary matter had been
• 38:24
  Speaker 1: falling into dark matter wells for long enough that something
• 38:28
  Speaker 1: was about to happen. The first stars were monsters, not
• 38:33
  Speaker 1: in any metaphorical sense. In a purely physical sense, the
• 38:38
  Speaker 1: first stars that ever ignited in the universe were objects
• 38:42
  Speaker 1: so massive they would dwarf anything alive in the sky today.
• 38:47
  Speaker 1: Current estimates place them at somewhere between 100 and 1000
• 38:52
  Speaker 1: times the mass of our sun. Some models push even higher.
• 38:58
  Speaker 1: To understand why they were so extreme, you have to
• 39:02
  Speaker 1: understand what they were made of, and more importantly, what
• 39:06
  Speaker 1: they were not made of. Every star forming today in
• 39:09
  Speaker 1: the galaxy forms inside a cloud of gas that contains
• 39:13
  Speaker 1: traces of carbon, oxygen, silicon, iron, and dozens of other
• 39:19
  Speaker 1: elements heavier than helium. These elements are called metals by astronomers,
• 39:25
  Speaker 1: which is a term that covers everything on the periodic
• 39:28
  Speaker 1: table past hydrogen. and helium, whether or not the element
• 39:32
  Speaker 1: is actually metallic. When a gas cloud collapses to form
• 39:37
  Speaker 1: a star today, those heavy elements play a quiet but
• 39:41
  Speaker 1: essential role. They radiate heat away. They act as a
• 39:47
  Speaker 1: cooling agent. allowing the collapsing gas to shed energy and
• 39:51
  Speaker 1: compress further without heating so dramatically that the pressure stops
• 39:56
  Speaker 1: the collapse. Metals make it easier to build small, stable stars.
• 40:02
  Speaker 1: Take the metals out, and the physics changes completely. The
• 40:06
  Speaker 1: first stars had no metals. The gas that collapsed to
• 40:11
  Speaker 1: form them was the gas left over from the Big
• 40:13
  Speaker 1: Bang itself, almost purely hydrogen and helium. with trace amounts
• 40:19
  Speaker 1: of lithium. There was nothing else. There had not been
• 40:23
  Speaker 1: enough time, not a single stellar generation, to produce anything heavier.
• 40:31
  Speaker 1: So when those first gas clouds began falling into dark
• 40:35
  Speaker 1: matter halos and collapsing under their own weight, there was
• 40:39
  Speaker 1: no cooling mechanism to keep temperatures manageable. The clouds could
• 40:45
  Speaker 1: not fragment efficiently, the way modern star-forming regions do. Instead
• 40:50
  Speaker 1: of breaking into dozens of smaller pieces, each collapsing into
• 40:55
  Speaker 1: a modest star, the early clouds stayed together. They collapsed
• 41:00
  Speaker 1: as single, enormous masses. The result was stars of staggering size.
• 41:06
  Speaker 1: Think about what a star a thousand times the mass
• 41:09
  Speaker 1: of our sun actually means. Our sun is so large
• 41:14
  Speaker 1: that a million Earths could fit inside it, A star
• 41:18
  Speaker 1: a thousand times more massive would contain, by volume, roughly
• 41:22
  Speaker 1: 10 to the 18th power Earths. The number is meaningless
• 41:27
  Speaker 1: to hold in your head. What matters is what that
• 41:31
  Speaker 1: mass does to the star's interior. Gravity scales with mass,
• 41:36
  Speaker 1: so a more massive star is under crushing internal pressure.
• 41:41
  Speaker 1: To balance that pressure, it has to burn hotter and faster.
• 41:45
  Speaker 1: Much faster. A star like our Sun can fuse hydrogen
• 41:50
  Speaker 1: for roughly 10 billion years. A star 10 times more
• 41:54
  Speaker 1: massive than our Sun burns through its fuel in perhaps
• 41:58
  Speaker 1: 20 million years. A star 100 times more massive might
• 42:03
  Speaker 1: last only a few million years. A star near the
• 42:07
  Speaker 1: upper end of what population 3 objects may have reached
• 42:11
  Speaker 1: would have burned through its fuel in perhaps a million
• 42:15
  Speaker 1: years or less. One million years is fast. On a
• 42:20
  Speaker 1: cosmic timeline where galaxies form over billions of years, a
• 42:25
  Speaker 1: million years is nearly instantaneous. These stars lit up, blazed
• 42:31
  Speaker 1: at temperatures and luminosities far beyond anything in the modern universe,
• 42:36
  Speaker 1: and then they were gone. But what happened when they
• 42:39
  Speaker 1: died is the part that changed everything. Stars this massive
• 42:45
  Speaker 1: don't just fade out. They don't quietly cool into white
• 42:49
  Speaker 1: dwarfs the way low-mass stars do. Stars above roughly eight
• 42:54
  Speaker 1: solar masses end in core collapse, a process where the
• 42:58
  Speaker 1: nuclear fuel runs out, the core falls in on itself
• 43:03
  Speaker 1: in a fraction of a second, and the outer layers
• 43:06
  Speaker 1: rebound in a supernova explosion. Stars at the upper end
• 43:10
  Speaker 1: of the Population 3 range may have undergone something even
• 43:14
  Speaker 1: more extreme. a pair instability supernova, where the core becomes
• 43:20
  Speaker 1: so hot that energy begins converting directly into electron-positron pairs
• 43:26
  Speaker 1: rather than supporting the star against gravity, triggering a total
• 43:30
  Speaker 1: collapse followed by a thermonuclear detonation that obliterates the star completely.
• 43:37
  Speaker 1: No remnant, just an explosion so powerful it briefly outshines
• 43:42
  Speaker 1: the galaxy it was born in. if a galaxy even
• 43:46
  Speaker 1: existed around it yet. And in that explosion, for the
• 43:50
  Speaker 1: first time in the history of the universe, heavy elements
• 43:54
  Speaker 1: were flung into space. Carbon, oxygen, nitrogen, neon, iron, silicon.
• 44:04
  Speaker 1: The whole machinery of stellar nucleosynthesis had been running inside
• 44:08
  Speaker 1: these stars for their brief lives. fusing heavier and heavier
• 44:13
  Speaker 1: nuclei in their cores, and the supernova scattered all of
• 44:17
  Speaker 1: it outward into the surrounding gas. The next generation of
• 44:22
  Speaker 1: gas clouds to collapse would form from slightly enriched material.
• 44:27
  Speaker 1: Not very enriched. The earliest population two stars that followed
• 44:33
  Speaker 1: still had extremely low metal content, a hundredth or a
• 44:37
  Speaker 1: thousandth of the metal abundance in our sun. But it
• 44:41
  Speaker 1: was something. The cooling efficiency of those clouds was slightly improved.
• 44:48
  Speaker 1: Stars could form at slightly smaller masses. The process had begun.
• 44:54
  Speaker 1: There is something else those first stars did that is
• 44:57
  Speaker 1: just as consequential. They produced extraordinary amounts of ultraviolet radiation.
• 45:03
  Speaker 1: Ultraviolet photons carry enough energy to strip electrons from hydrogen atoms.
• 45:09
  Speaker 1: a process called ionization. In the cosmic dark ages, hydrogen
• 45:15
  Speaker 1: throughout the universe was neutral, meaning every proton had an
• 45:19
  Speaker 1: electron paired with it. Neutral hydrogen is very good at
• 45:24
  Speaker 1: absorbing ultraviolet light, which is part of why the dark
• 45:28
  Speaker 1: ages were dark. But when the first massive stars switched on,
• 45:33
  Speaker 1: they began flooding their surroundings with ultraviolet photons. Bubbles of
• 45:39
  Speaker 1: ionized hydrogen formed around them. The electrons were knocked free.
• 45:44
  Speaker 1: As more stars appeared and more bubbles formed, those bubbles
• 45:49
  Speaker 1: began overlapping and merging, spreading outward through the universe. This
• 45:54
  Speaker 1: process is called reionization. Reionization took hundreds of millions of
• 46:00
  Speaker 1: years to complete. It was not a single event, but
• 46:04
  Speaker 1: a gradual phase transition. The universe going from mostly neutral
• 46:09
  Speaker 1: hydrogen to mostly ionized hydrogen. Were there enough stars forming
• 46:14
  Speaker 1: fast enough to drive it? Was it primarily the ultraviolet
• 46:19
  Speaker 1: output of the first massive stars? Or did the first
• 46:23
  Speaker 1: quasars and active galactic nuclei also contribute? Those questions are
• 46:29
  Speaker 1: still being worked out. What is clear is that the
• 46:33
  Speaker 1: universe we observe today Where light can travel freely between galaxies,
• 46:39
  Speaker 1: across billions of light years of nearly transparent intergalactic space,
• 46:45
  Speaker 1: is a universe that was made transparent by the radiation
• 46:50
  Speaker 1: of the first stars. Every atom in your body heavier
• 46:54
  Speaker 1: than helium was produced inside a star. The oxygen you
• 46:59
  Speaker 1: are breathing right now, the carbon in your cells, the
• 47:03
  Speaker 1: iron in your blood, All of it was forged in
• 47:06
  Speaker 1: stellar interiors and scattered by stellar explosions. The specific stars
• 47:13
  Speaker 1: that produced the very first batch of those elements were
• 47:16
  Speaker 1: Population III objects. Objects that no longer exist anywhere in
• 47:22
  Speaker 1: the observable universe, that lived and died before the Milky
• 47:26
  Speaker 1: Way existed, before the solar system existed, before Earth existed.
• 47:33
  Speaker 1: We have never directly observed one. When those first stars
• 47:37
  Speaker 1: died in supernova explosions, they scattered heavy elements into the
• 47:42
  Speaker 1: surrounding gas, and from that moment on, the universe was
• 47:47
  Speaker 1: chemically different. Permanently. The first things astronomers would recognise as
• 47:54
  Speaker 1: galaxies looked almost nothing like what that word calls to mind.
• 47:59
  Speaker 1: Picture what you imagine when someone says, Galaxies. A sweeping spiral, maybe.
• 48:05
  Speaker 1: Broad arms trailing outward from a bright core, studded with stars,
• 48:10
  Speaker 1: turning slowly against the dark. That image is so familiar,
• 48:15
  Speaker 1: it has become almost a logo for the universe itself.
• 48:19
  Speaker 1: Now set it aside completely, because the first galaxies bore
• 48:23
  Speaker 1: no resemblance to it whatsoever. They were small. They were chaotic.
• 48:29
  Speaker 1: They had no arms. No organised rotations. no clean structure
• 48:34
  Speaker 1: of any kind. If you could somehow travel back and
• 48:39
  Speaker 1: look at one of these objects directly, you might not
• 48:42
  Speaker 1: even call it a galaxy. You might call it a clump.
• 48:47
  Speaker 1: Here is the scale problem. The Milky Way contains somewhere
• 48:51
  Speaker 1: between 200 and 400 billion solar masses of stars. The
• 48:57
  Speaker 1: earliest galaxies, the ones that formed in the first few
• 49:00
  Speaker 1: hundred million years after the Big Bang, contained perhaps tens
• 49:05
  Speaker 1: of millions of solar masses. Not billions. Tens of millions.
• 49:11
  Speaker 1: That is a factor of roughly 10,000 times smaller. Imagine
• 49:16
  Speaker 1: shrinking a city down to the size of a single
• 49:19
  Speaker 1: neighborhood block. and you are still not capturing the difference.
• 49:24
  Speaker 1: These were not infant versions of what we see today.
• 49:28
  Speaker 1: They were something qualitatively different. Small, dense, turbulent concentrations of
• 49:35
  Speaker 1: gas and young stars. held together by dark matter halos,
• 49:40
  Speaker 1: just barely massive enough to do the job. And the
• 49:44
  Speaker 1: conditions inside them were nothing like the conditions inside galaxies today.
• 49:50
  Speaker 1: The gas that fell into those early halos was almost
• 49:53
  Speaker 1: entirely hydrogen and helium. No carbon, no oxygen, no iron.
• 50:00
  Speaker 1: Those elements had only just begun to appear, scattered by
• 50:04
  Speaker 1: the first supernovae. Without heavy elements, gas cannot cool efficiently.
• 50:11
  Speaker 1: It stays hot. Hot gas resists collapsing into stars. So
• 50:16
  Speaker 1: how did these early galaxies form stars at all? The
• 50:20
  Speaker 1: answer is that gravity doesn't particularly care about resistance. Gravity
• 50:26
  Speaker 1: just keeps pulling. The gas was dense enough, the halos
• 50:30
  Speaker 1: deep enough, and the pressure of gravity eventually won. But
• 50:35
  Speaker 1: the star formation that resulted was messy. There were no smooth,
• 50:41
  Speaker 1: settled disks. There were clumps, bright, compact knots of star
• 50:47
  Speaker 1: formation scattered through turbulent gas, each one blazing with massive
• 50:53
  Speaker 1: young stars burning through their fuel at a furious rate.
• 50:58
  Speaker 1: This is the thing that surprises people when they first
• 51:01
  Speaker 1: encounter it. Star formation rates in these early galaxies were
• 51:06
  Speaker 1: not low because conditions were harsh. In some cases, they
• 51:12
  Speaker 1: were actually quite high. Gas was abundant everywhere. Gravity was
• 51:18
  Speaker 1: actively pulling it together in multiple places at once. The
• 51:23
  Speaker 1: result was not one organized star-forming region, but many disconnected ones.
• 51:29
  Speaker 1: lighting up in different corners of the same small system,
• 51:33
  Speaker 1: with no central authority imposing any order. Were these galaxies
• 51:39
  Speaker 1: forming stars efficiently? Not by any modern definition. Were they
• 51:44
  Speaker 1: forming them anyway, relentlessly, chaotically? Yes. Gravity does not require
• 51:53
  Speaker 1: ideal conditions. It only requires mass. What made these early
• 51:58
  Speaker 1: proto-galaxies stranger still was how they related to each other. Today,
• 52:04
  Speaker 1: the Milky Way's nearest major neighbor is the Andromeda Galaxy,
• 52:09
  Speaker 1: roughly 2.5 million light-years away. The early universe was far
• 52:14
  Speaker 1: more crowded. Density was higher everywhere. Dark matter halos were
• 52:20
  Speaker 1: forming in close proximity, dragging their gas reservoirs with them.
• 52:25
  Speaker 1: and the proto-galaxies inside those halos were constantly within reach
• 52:29
  Speaker 1: of one another. They weren't isolated islands. They were bumping
• 52:34
  Speaker 1: into neighbors, trading gas, pulling material from each other with
• 52:38
  Speaker 1: tidal forces, merging before either one had fully settled into
• 52:43
  Speaker 1: anything stable. A galaxy would begin to take shape, and
• 52:48
  Speaker 1: before it had organized itself into anything coherent, another clump
• 52:53
  Speaker 1: of similar size would come in and scramble everything. Then
• 52:58
  Speaker 1: both would merge, dump more gas into the combined system,
• 53:03
  Speaker 1: and the cycle would begin again. There was no quiet period,
• 53:08
  Speaker 1: no time for things to settle. The first direct evidence
• 53:12
  Speaker 1: for what these early galaxies looked like came not from
• 53:16
  Speaker 1: a prediction, but from a photograph. In 1995, the Hubble
• 53:22
  Speaker 1: Space Telescope pointed at a tiny patch of sky near
• 53:26
  Speaker 1: the handle of the Big Dipper, a region chosen specifically
• 53:30
  Speaker 1: because it appeared to contain almost nothing, and held that
• 53:35
  Speaker 1: pointing for ten consecutive days, collecting light from the faintest
• 53:40
  Speaker 1: objects detectable. The result was the Hubble Deep Field, thousands
• 53:46
  Speaker 1: of galaxies packed into an area of sky roughly 1
• 53:51
  Speaker 1: 30th the diameter of the full moon. Some of those
• 53:55
  Speaker 1: galaxies dated back to 700 or 800 million years after
• 54:00
  Speaker 1: the Big Bang, which sounds like a long time, and
• 54:04
  Speaker 1: is actually, in the context of a 13.8 billion year universe,
• 54:10
  Speaker 1: extremely early. What did they look like? They were small.
• 54:15
  Speaker 1: They were bright rela, tiv to their size, blazing with
• 54:19
  Speaker 1: star formation, And they were irregular, strange asymmetric shapes, some
• 54:25
  Speaker 1: appearing as compact knots, some as elongated smears, some as
• 54:30
  Speaker 1: multiple bright clumps tethered together in what was clearly an
• 54:34
  Speaker 1: ongoing merger. Almost none of them had the clean, sweeping
• 54:39
  Speaker 1: appearance of a mature spiral. The universe at that age
• 54:43
  Speaker 1: looked nothing like the universe today. Subsequent deep-field observations pushed
• 54:49
  Speaker 1: further back. The Hubble Ultra Deep Field, released in 2004
• 54:54
  Speaker 1: from observations taken over nearly 12 days of exposure, captured
• 54:59
  Speaker 1: galaxies dating to roughly 700 million years after the Big
• 55:03
  Speaker 1: Bang and hinted at even earlier ones. The pattern held.
• 55:09
  Speaker 1: The earlier you looked, the smaller, brighter, and more chaotic
• 55:14
  Speaker 1: the galaxies appeared. The organized spirals and smooth ellipticals we
• 55:19
  Speaker 1: see in the nearby universe were nowhere in evidence. What
• 55:24
  Speaker 1: existed instead was a collection of building blocks, each one incomplete,
• 55:30
  Speaker 1: many of them in the process of merging with something else.
• 55:34
  Speaker 1: Then came a telescope that could see further still. The
• 55:38
  Speaker 1: James Webb Space Telescope, launched in late 2021, began returning
• 55:42
  Speaker 1: data in 2022 and immediately pushed observations back to within
• 55:49
  Speaker 1: a few hundred million years of the Big Bang itself.
• 55:53
  Speaker 1: What it found there will be covered in full later
• 55:56
  Speaker 1: in this series. But the previews alone were enough to
• 56:00
  Speaker 1: signal that the early universe was even more active, even
• 56:05
  Speaker 1: more populated with early structures than anyone had quite expected.
• 56:11
  Speaker 1: The full story of what James Webb revealed and why
• 56:14
  Speaker 1: it unsettled so many assumptions, deserves its own section. For now,
• 56:20
  Speaker 1: hold this image. Not the grand spiral you picture when
• 56:24
  Speaker 1: you hear the word galaxy. Something smaller. Something clumpy and
• 56:30
  Speaker 1: violent and unfinished, surrounded by other clumpy, violent, unfinished things,
• 56:36
  Speaker 1: all of them pulling on each other, and merging and
• 56:39
  Speaker 1: exploding and pulling together again. The universe in its first
• 56:44
  Speaker 1: billion years was not a collection of island galaxies quietly
• 56:49
  Speaker 1: spinning in the dark. It was a crowded, violent arena
• 56:54
  Speaker 1: of collisions and collapses, and the structures forming inside it
• 56:59
  Speaker 1: were being built and torn apart, almost simultaneously. The universe
• 57:06
  Speaker 1: builds galaxies the way cities grow, not by planning from
• 57:10
  Speaker 1: the top down, but by accumulating, piece by piece, from
• 57:16
  Speaker 1: smaller things that came before. That idea has a name.
• 57:21
  Speaker 1: Astronomers call it hierarchical clustering, and it is the dominant
• 57:25
  Speaker 1: framework for understanding how the structures we see in the
• 57:28
  Speaker 1: night sky actually came to exist. The logic is straightforward
• 57:34
  Speaker 1: once you hear it. Small dark matter halos formed first.
• 57:40
  Speaker 1: Those halos merged with neighboring halos, producing larger ones. The
• 57:45
  Speaker 1: larger halos merged with still larger ones. The gas and
• 57:49
  Speaker 1: proto-galaxies trapped inside followed the halos wherever gravity led them.
• 57:55
  Speaker 1: Over billions of years, this bottom-up process of accumulation built
• 58:00
  Speaker 1: the massive, complex galaxies we observe today. No blueprint required.
• 58:07
  Speaker 1: Just gravity, time, and an enormous number of small things
• 58:11
  Speaker 1: falling toward each other. This is not speculation in the
• 58:15
  Speaker 1: scientific sense. Decades of computer simulations, running the physics of
• 58:21
  Speaker 1: gravity and gas dynamics forward from the early universe, consistently
• 58:26
  Speaker 1: produce a universe that looks like the one we actually inhabit.
• 58:31
  Speaker 1: The right number of galaxies, roughly the right sizes, arranged
• 58:36
  Speaker 1: in the right large-scale patterns. When researchers strip dark matter
• 58:42
  Speaker 1: out of those simulations, the whole structure collapses. The galaxies
• 58:47
  Speaker 1: don't form on time, don't grow fast enough, don't cluster correctly.
• 58:53
  Speaker 1: Put dark matter back in, and the picture snaps into place.
• 58:57
  Speaker 1: What that tells you is that the hierarchical model isn't
• 59:01
  Speaker 1: just a reasonable guess. It's tracking something real. Here is
• 59:05
  Speaker 1: one of the cleanest pieces of evidence and it's sitting
• 59:09
  Speaker 1: right outside our own galaxy. The Milky Way is surrounded
• 59:13
  Speaker 1: by dozens of smaller satellite galaxies. The Large Magellanic Cloud,
• 59:19
  Speaker 1: the Small Magellanic Cloud, the Sagittarius Dwarf, the Sculptor Dwarf.
• 59:26
  Speaker 1: There are more than 50 of them, most of them
• 59:29
  Speaker 1: tiny and dim, scattered in a halo around us at
• 59:33
  Speaker 1: distances ranging from tens of thousands to a few hundred
• 59:37
  Speaker 1: thousand light years. These are not random neighbours. They are
• 59:43
  Speaker 1: surviving remnants of a process that has been feeding the
• 59:46
  Speaker 1: Milky Way for its entire life. Most of the small
• 59:51
  Speaker 1: galaxies that once surrounded our galaxy have already been absorbed.
• 59:56
  Speaker 1: What we see orbiting us now are the ones that
• 1:00:00
  Speaker 1: haven't fallen in yet. They're next. But what does a
• 1:00:04
  Speaker 1: galactic merger actually look like? Because the word collision suggests
• 1:00:10
  Speaker 1: something violent and instantaneous, and that picture is almost entirely wrong.
• 1:00:16
  Speaker 1: When two galaxies move toward each other under gravity, the
• 1:00:20
  Speaker 1: process unfolds over hundreds of millions of years. Stars in
• 1:00:25
  Speaker 1: both galaxies are spread across such enormous distances from each
• 1:00:30
  Speaker 1: other that even during a full merger, almost none of
• 1:00:34
  Speaker 1: them actually hit. The space between stars is so vast
• 1:00:39
  Speaker 1: that you could pass two entire galaxies through each other,
• 1:00:43
  Speaker 1: and the stellar collision rate would be effectively zero. If
• 1:00:48
  Speaker 1: you were living on a planet inside one of those galaxies,
• 1:00:52
  Speaker 1: You might not even notice the star's changing position much
• 1:00:55
  Speaker 1: at all, at least not on any timescale a civilization
• 1:00:59
  Speaker 1: could measure. The gas is a different story entirely. Gas
• 1:01:04
  Speaker 1: clouds are big, diffuse, and they fill large volumes of
• 1:01:08
  Speaker 1: space between the stars. When two galaxies make a close pass,
• 1:01:14
  Speaker 1: those gas clouds slam into each other at hundreds of
• 1:01:17
  Speaker 1: kilometers per second. The collisions are violent and compressive. Shockwaves
• 1:01:24
  Speaker 1: rip through the gas, compressing it suddenly, triggering a cascade
• 1:01:29
  Speaker 1: of star formation far more intense than anything happening in
• 1:01:33
  Speaker 1: a quiet, isolated galaxy. Astronomers call this a starburst, and
• 1:01:39
  Speaker 1: some merging galaxies produce new stars at rates 10 or
• 1:01:43
  Speaker 1: 20 times higher than normal. The light output of a
• 1:01:48
  Speaker 1: starburst galaxy during a major merger can exceed the combined
• 1:01:53
  Speaker 1: luminosity of billions of individual suns born all at once.
• 1:01:58
  Speaker 1: At the same time, the gravitational tides are reshaping everything.
• 1:02:03
  Speaker 1: As two galaxies swing past each other, the differential gravitational
• 1:02:08
  Speaker 1: pull across their structures stretches material outward into long, sweeping streams.
• 1:02:15
  Speaker 1: Stars get flung onto new orbits. Gas gets stripped away
• 1:02:19
  Speaker 1: from the outer regions and pulled into trails that can
• 1:02:23
  Speaker 1: extend for hundreds of thousands of light years. These tidal
• 1:02:28
  Speaker 1: tails are among the most spectacular structures in the universe,
• 1:02:33
  Speaker 1: and they are a direct signature of a merger in progress.
• 1:02:38
  Speaker 1: Images from the Hubble Space Telescope have captured dozens of them.
• 1:02:43
  Speaker 1: Pairs of galaxies connected by glowing bridges and streamers of
• 1:02:47
  Speaker 1: displaced stars. Systems in the middle of a gravitational dance
• 1:02:52
  Speaker 1: that started millions of years ago and won't resolve for
• 1:02:55
  Speaker 1: millions more. And here is the counterintuitive part about the timescales.
• 1:03:01
  Speaker 1: A merger that takes 500 million years to complete sounds slow.
• 1:03:07
  Speaker 1: In human terms, it is incomprehensibly slow. But in the
• 1:03:11
  Speaker 1: context of how long the universe has been running, 13.8
• 1:03:16
  Speaker 1: billion years, 500 million years, is a relatively brief episode.
• 1:03:23
  Speaker 1: Galaxies that began interacting a billion years ago may have
• 1:03:27
  Speaker 1: already merged and settled into something new. The universe is
• 1:03:32
  Speaker 1: old enough that this process has had time to run
• 1:03:36
  Speaker 1: through multiple cycles. Small things merged into medium things, medium
• 1:03:42
  Speaker 1: things merged into large things, and in some cases, large
• 1:03:46
  Speaker 1: things merged into still larger things. The end point of
• 1:03:51
  Speaker 1: that sequence is sitting in every galaxy cluster in the sky.
• 1:03:57
  Speaker 1: What determines how the merger turns out? Several things. The
• 1:04:02
  Speaker 1: mass ratio of the two galaxies involved matters enormously. When
• 1:04:07
  Speaker 1: a giant galaxy absorbs a tiny dwarf, the smaller system
• 1:04:11
  Speaker 1: gets disrupted and its stars get scattered into the larger
• 1:04:15
  Speaker 1: galaxy's halo, barely affecting the big galaxy's structure at all.
• 1:04:20
  Speaker 1: That kind of minor merger happens frequently and contributes steadily
• 1:04:25
  Speaker 1: to a galaxy's mass without fundamentally changing its shape. But
• 1:04:30
  Speaker 1: when two galaxies of comparable mass collide, what astronomers call
• 1:04:35
  Speaker 1: a major merger, The outcome is far more dramatic. Both
• 1:04:40
  Speaker 1: galaxies get disrupted. The organized rotating disk structure breaks down.
• 1:04:48
  Speaker 1: Stars that once moved in orderly circular orbits get thrown
• 1:04:52
  Speaker 1: into chaotic paths. The resulting system often looks nothing like
• 1:04:57
  Speaker 1: either of its precursors. This is why different merger histories
• 1:05:02
  Speaker 1: produce such different galaxies. A large galaxy that grew mostly
• 1:05:07
  Speaker 1: through minor mergers, absorbing small satellites over billions of years
• 1:05:12
  Speaker 1: while avoiding any comparably sized collision, can maintain its spiral
• 1:05:17
  Speaker 1: disk structure, as the Milky Way has largely done. A
• 1:05:21
  Speaker 1: galaxy that experienced one or two major mergers had its
• 1:05:26
  Speaker 1: disks scrambled and ended up as something rounder and less organised.
• 1:05:31
  Speaker 1: The diversity of galaxy shapes observable in any deep-field image
• 1:05:36
  Speaker 1: is not random. It is a record of different gravitational histories,
• 1:05:42
  Speaker 1: written in the arrangement of stars. How many mergers built
• 1:05:46
  Speaker 1: the Milky Way? Based on current estimates from simulations and
• 1:05:51
  Speaker 1: from the distribution of stars in the galactic halo, our
• 1:05:55
  Speaker 1: galaxy has absorbed somewhere between 100 and several hundred smaller
• 1:05:59
  Speaker 1: galaxies over its lifetimes. Most left only faint traces, thin
• 1:06:05
  Speaker 1: streams of stars, subtle chemical signatures in the halo population,
• 1:06:11
  Speaker 1: slight asymmetries in the outer disk. A few left marks
• 1:06:15
  Speaker 1: that are still clearly legible billions of years later. The
• 1:06:20
  Speaker 1: evidence is in the stars themselves, in their orbits and
• 1:06:24
  Speaker 1: their chemistry, and increasingly in the data from space telescopes
• 1:06:29
  Speaker 1: capable of measuring stellar motions. with extraordinary precision. Every large
• 1:06:35
  Speaker 1: galaxy in the universe today is, in some sense, a
• 1:06:40
  Speaker 1: fossil record of all the smaller structures that fell into
• 1:06:44
  Speaker 1: it and were absorbed over billions of years. At the
• 1:06:48
  Speaker 1: center of almost every large galaxy in the universe sits
• 1:06:51
  Speaker 1: a black hole so massive that the word massive stops
• 1:06:54
  Speaker 1: meaning anything useful. Start with the Milky Way because it
• 1:06:59
  Speaker 1: is the one you know. The black hole at the
• 1:07:02
  Speaker 1: center of our galaxy is called Sagittarius A-star, and it
• 1:07:06
  Speaker 1: contains roughly four million times the mass of the Sun.
• 1:07:11
  Speaker 1: Four million suns, compressed into a region smaller than our
• 1:07:15
  Speaker 1: solar system. That number is almost impossible to hold in
• 1:07:20
  Speaker 1: your mind, and yet, by the standards of what lives
• 1:07:24
  Speaker 1: at the centers of other galaxies, Sagittarius A-star is small.
• 1:07:30
  Speaker 1: It is modest. It is, in the context of what
• 1:07:34
  Speaker 1: astronomers have found elsewhere in the universe, practically ordinary. The
• 1:07:40
  Speaker 1: black holes sitting at the centres of active galaxies, the
• 1:07:44
  Speaker 1: ones powering the brightest objects ever detected, contain not millions
• 1:07:49
  Speaker 1: of solar masses, but billions. Some push past ten billions.
• 1:07:56
  Speaker 1: a few outliers appear to exceed 40 billion times the
• 1:08:01
  Speaker 1: mass of the Sun. These are not variations on the
• 1:08:05
  Speaker 1: same theme. These are objects in a different category entirely.
• 1:08:10
  Speaker 1: Here is where the story stops being merely astonishing and
• 1:08:13
  Speaker 1: starts being genuinely unsettling. When astronomers look at quasars, intensely
• 1:08:19
  Speaker 1: luminous objects powered by black holes in the act of feeding,
• 1:08:24
  Speaker 1: they can detect them at enormous distances, which means enormous
• 1:08:29
  Speaker 1: look-back times, which means they are seeing these objects as
• 1:08:33
  Speaker 1: they existed in the early universe. And what they find,
• 1:08:38
  Speaker 1: when they look far enough back, are quasars blazing with
• 1:08:42
  Speaker 1: the energy of black holes containing a billion or more
• 1:08:46
  Speaker 1: solar masses within the first billion years after the Big Bang.
• 1:08:51
  Speaker 1: Some appear even earlier, there is a black hole already
• 1:08:56
  Speaker 1: carrying over a billion solar masses at a time when
• 1:09:00
  Speaker 1: the universe was only 700 million years old. Stars had
• 1:09:05
  Speaker 1: only been forming for a few hundred million years before that.
• 1:09:10
  Speaker 1: The first galaxies were still small, chaotic things, barely a
• 1:09:15
  Speaker 1: few hundred million solar masses in total. And Inside some
• 1:09:20
  Speaker 1: of them, somehow, sat black holes already containing a billion
• 1:09:25
  Speaker 1: suns worth of mass. This should be almost impossible. The math,
• 1:09:30
  Speaker 1: when you run it, does not cooperate. To understand why
• 1:09:35
  Speaker 1: this is a problem, think about what a black hole
• 1:09:38
  Speaker 1: needs to grow. It needs to eat. Matter falls toward it,
• 1:09:44
  Speaker 1: forms a swirling disk of superheated gas, and some portion
• 1:09:48
  Speaker 1: of that gas crosses the event horizon and is consumed.
• 1:09:53
  Speaker 1: The rest is blasted away by the enormous energy released
• 1:09:57
  Speaker 1: in the process. Physicists have worked out a theoretical ceiling
• 1:10:02
  Speaker 1: on how quickly a black hole can feed, a rate
• 1:10:06
  Speaker 1: called the Eddington limit, set by the balance between the
• 1:10:10
  Speaker 1: inward pull of gravity and the outward pressure of radiation
• 1:10:14
  Speaker 1: produced by the infalling gas. feed any faster than this limit,
• 1:10:20
  Speaker 1: and your own energy output pushes your food supply away
• 1:10:24
  Speaker 1: before it can fall in. The physics is brutal, and
• 1:10:28
  Speaker 1: the ceiling is real. Starting from a seed black hole
• 1:10:33
  Speaker 1: of perhaps a hundred solar masses, the kind left be,
• 1:10:37
  Speaker 1: end by a massive star, growing to a billion solar
• 1:10:41
  Speaker 1: masses at the Eddington limit would take somewhere between 700
• 1:10:46
  Speaker 1: million and 800 million years of continuous, uninterrupted accretion. Were
• 1:10:54
  Speaker 1: the earliest black holes really doing that? For that long?
• 1:10:59
  Speaker 1: Without a single interruption? The universe in those early epochs
• 1:11:04
  Speaker 1: was not a calm place. Mergers were frequent, environments were chaotic,
• 1:11:10
  Speaker 1: gas supplies were irregular. The idea that any black hole
• 1:11:14
  Speaker 1: maintained maximum feeding continuously from birth to the point of
• 1:11:19
  Speaker 1: detection strains credibility. So the question is not just how
• 1:11:23
  Speaker 1: these objects got so large. The question is how they
• 1:11:26
  Speaker 1: got so large so fast. Three serious proposals exist, and
• 1:11:31
  Speaker 1: none of them has been proven. The first is called
• 1:11:34
  Speaker 1: direct collapse. In the very early universe, before the gas
• 1:11:40
  Speaker 1: had been enriched with heavy elements by the first generations
• 1:11:43
  Speaker 1: of dying stars, enormous clouds of hydrogen and helium could
• 1:11:48
  Speaker 1: potentially collapse under gravity without fragmenting into stars. Normally, when
• 1:11:55
  Speaker 1: a gas cloud collapses, it breaks apart into clumps that
• 1:12:00
  Speaker 1: each become individual stars. But if conditions are right, if
• 1:12:05
  Speaker 1: the gas is hot enough, If the radiation environment is
• 1:12:09
  Speaker 1: intense enough to prevent cooling, the whole cloud might collapse
• 1:12:14
  Speaker 1: directly into a single massive object, bypassing the star formation
• 1:12:19
  Speaker 1: stage entirely. This direct collapse black hole could be born
• 1:12:24
  Speaker 1: with a mass of tens of thousands to a hundred
• 1:12:27
  Speaker 1: thousand suns. Not a billion, but far larger than any
• 1:12:32
  Speaker 1: stellar remnant. A much bigger starting C, D, From there,
• 1:12:38
  Speaker 1: even conventional feeding rates start to produce the right numbers
• 1:12:42
  Speaker 1: by the time those early quasars appear. The second proposal
• 1:12:46
  Speaker 1: involves mergers. If the early universe produced dense clusters of
• 1:12:51
  Speaker 1: smaller black holes in close proximity to each other, those
• 1:12:55
  Speaker 1: black holes could merge rapidly, combining mass in a way
• 1:12:59
  Speaker 1: that sidesteps the feeding limit entirely. Each merger is nearly
• 1:13:05
  Speaker 1: instantaneous by cosmic standards. Merge enough of them quickly enough
• 1:13:11
  Speaker 1: and you can build a very large black hole without
• 1:13:13
  Speaker 1: the grinding patience of gas accretion. The problem is that
• 1:13:19
  Speaker 1: this requires an unusual environment. Extremely dense concentrations of stellar
• 1:13:25
  Speaker 1: mass black holes in a region small enough for repeated
• 1:13:29
  Speaker 1: mergers to occur before the cluster disperses. Whether the early
• 1:13:34
  Speaker 1: universe actually produced such environments in sufficient numbers to explain
• 1:13:40
  Speaker 1: all the billion solar mass objects we observe is not
• 1:13:44
  Speaker 1: yet clear. The third proposal is called super-Eddington accretion. What
• 1:13:50
  Speaker 1: if the Eddington limit is not actually a hard ceiling?
• 1:13:54
  Speaker 1: In certain geometries, under certain physical conditions, theoretical models suggest
• 1:14:00
  Speaker 1: that gas can fall inward faster than the standard limit allows,
• 1:14:05
  Speaker 1: with the excess radiation trapped inside the infalling material, rather
• 1:14:11
  Speaker 1: than pushing it back out. The black hole swallows more
• 1:14:15
  Speaker 1: than the limit should permit. Some simulations support this. Some
• 1:14:21
  Speaker 1: observations of bright X-ray sources in nearby galaxies hint that
• 1:14:26
  Speaker 1: it may already be happening. If early black holes regularly
• 1:14:31
  Speaker 1: exceeded the Eddington limit by a factor of several, the
• 1:14:35
  Speaker 1: growth problem becomes far less severe. But super-Eddington accretion has
• 1:14:41
  Speaker 1: never been directly confirmed at the scales that would be required,
• 1:14:46
  Speaker 1: and whether it could. Aldi operates steadily enough and long
• 1:14:50
  Speaker 1: enough to explain billion solar mass objects in the first
• 1:14:53
  Speaker 1: billion years is a genuinely open question. What makes this
• 1:14:58
  Speaker 1: more complicated is that these possibilities are not mutually exclusive.
• 1:15:04
  Speaker 1: The real answer might involve all three, in different proportions,
• 1:15:09
  Speaker 1: in different environments, at different times. Nature rarely uses one
• 1:15:15
  Speaker 1: mechanism where several are available, and the James Webb Space
• 1:15:20
  Speaker 1: Telescope has been making all of this worse in the
• 1:15:24
  Speaker 1: best possible way. Among the early galaxies JWST has detected,
• 1:15:31
  Speaker 1: galaxies that already shouldn't be as large as they are,
• 1:15:35
  Speaker 1: some appear to host central black holes that are disproportionately massive,
• 1:15:40
  Speaker 1: even relative to their host galaxies. In the modern universe,
• 1:15:45
  Speaker 1: there is a fairly tight relationship between how massive a
• 1:15:48
  Speaker 1: galaxy's central black hole is and how massive the galaxy
• 1:15:52
  Speaker 1: itself is. The black hole is typically a tiny fraction
• 1:15:57
  Speaker 1: of the total. In some of these very early systems,
• 1:16:01
  Speaker 1: that ratio looks different. The black holes appear to have
• 1:16:05
  Speaker 1: grown faster than the galaxies around them, or the galaxies
• 1:16:09
  Speaker 1: haven't had time to catch up, or something about the
• 1:16:13
  Speaker 1: early universe allowed black holes to assemble in ways the
• 1:16:16
  Speaker 1: standard picture doesn't account for. Every observation seems to add
• 1:16:22
  Speaker 1: complexity rather than remove it. Something built these objects in
• 1:16:27
  Speaker 1: the time available. Something grew a billion solar masses of
• 1:16:31
  Speaker 1: collapsed matter in a universe that was barely past its
• 1:16:35
  Speaker 1: own beginning. How those billion solar mass objects grew so
• 1:16:40
  Speaker 1: quickly is one of the problems in modern astrophysics that
• 1:16:44
  Speaker 1: nobody has cleanly solved. The biggest galaxies in the universe
• 1:16:50
  Speaker 1: are also, in a sense, the most dead. Take a
• 1:16:54
  Speaker 1: giant elliptical galaxy, one of the enormous featureless blobs of
• 1:16:59
  Speaker 1: stars that sit at the centres of galaxy clusters. These
• 1:17:03
  Speaker 1: objects can contain trillions of stars. They outmass the Milky
• 1:17:08
  Speaker 1: Way by factors of 10 or 20 or more. And
• 1:17:12
  Speaker 1: yet if you looked at the gas around them, you
• 1:17:14
  Speaker 1: would find it. The raw material is there, enormous hot
• 1:17:19
  Speaker 1: halos of gas surrounding these galaxies, extending millions of light
• 1:17:24
  Speaker 1: years in every direction, containing enough hydrogen to fuel star
• 1:17:29
  Speaker 1: formation for billions of years. By any reasonable expectation, these
• 1:17:35
  Speaker 1: galaxies should be churning out new stars constantly. They are not.
• 1:17:40
  Speaker 1: They stopped. In most cases, they stopped billions of years ago,
• 1:17:45
  Speaker 1: and they have been sitting quiet ever since. Meanwhile, smaller galaxies,
• 1:17:52
  Speaker 1: systems far less massive, far less spectacular, are still actively
• 1:17:57
  Speaker 1: forming stars right now. The universe's most powerful engines went
• 1:18:01
  Speaker 1: silent first. That is the paradox, and it demands an explanation.
• 1:18:07
  Speaker 1: The explanation that has survived the most scrutiny points directly
• 1:18:12
  Speaker 1: at what you just heard about in the previous section.
• 1:18:15
  Speaker 1: Supermassive black holes. Not their existence, but their behaviour when
• 1:18:21
  Speaker 1: they eat. When a black hole is actively feeding, it
• 1:18:26
  Speaker 1: is one of the most energetic things in the universe.
• 1:18:31
  Speaker 1: Gas falling toward the black hole does not drop straight in.
• 1:18:36
  Speaker 1: It spirals, compressing, heating, radiating. The material forms an accretion
• 1:18:43
  Speaker 1: disk that can glow brighter than an entire galaxy's worth
• 1:18:47
  Speaker 1: of stars combined, and a fraction of that infalling energy
• 1:18:51
  Speaker 1: does not vanish into the black hole. It escapes. It
• 1:18:56
  Speaker 1: escapes as radiation, as powerful winds of fast-moving particles blasting
• 1:19:02
  Speaker 1: outward from the disk, and in some cases, as jets.
• 1:19:07
  Speaker 1: Narrow columns of plasma accelerated to near the speed of light,
• 1:19:12
  Speaker 1: shooting out along the black hole's rotation axis and extending
• 1:19:17
  Speaker 1: sometimes for millions of light-years into the surrounding space. The
• 1:19:23
  Speaker 1: energy output is almost incomprehensible. A single actively feeding supermassive
• 1:19:30
  Speaker 1: black hole can inject more energy into its surrounding environment
• 1:19:35
  Speaker 1: in a million years than the Sun will produce across
• 1:19:39
  Speaker 1: its entire 10 billion year life. Now consider what that
• 1:19:43
  Speaker 1: energy does to the galaxy around it. The gas that
• 1:19:46
  Speaker 1: surrounds a massive galaxy is cold enough, in principle, to
• 1:19:51
  Speaker 1: eventually cool further, fall inward, and form stars. This is
• 1:19:56
  Speaker 1: how galaxies grow. Gas cools, collapses, fragments into clouds, and
• 1:20:03
  Speaker 1: those clouds become stellar nurseries. But the jets and winds
• 1:20:08
  Speaker 1: from an active black hole heat that gas. They pump
• 1:20:12
  Speaker 1: energy into it continuously. Gas that was slowly cooling and
• 1:20:17
  Speaker 1: falling inward gets blasted back outward, or heated to temperatures
• 1:20:22
  Speaker 1: where it simply cannot cool fast enough to collapse. The
• 1:20:27
  Speaker 1: cooling timescale, which might have been hundreds of millions of years,
• 1:20:32
  Speaker 1: suddenly stretches to longer than the age of the universe.
• 1:20:37
  Speaker 1: Star formation stalls. Then it stops. This is what astronomers
• 1:20:42
  Speaker 1: call AGN feedback, feedback from the active galactic nucleus, which
• 1:20:49
  Speaker 1: is a polite technical name for a black hole actively
• 1:20:52
  Speaker 1: tearing through its surroundings. But here is what makes this
• 1:20:56
  Speaker 1: more than just a dramatic story about black holes being destructive.
• 1:21:01
  Speaker 1: The feedback is not random. It is self-regulating. When a
• 1:21:06
  Speaker 1: black hole feeds aggressively, the energy it releases heats the
• 1:21:10
  Speaker 1: gas and pushes it away, cutting off the very supply
• 1:21:14
  Speaker 1: of gas the black hole was eating. The black hole
• 1:21:18
  Speaker 1: starves itself. The jets quiet down. The winds weaken. With
• 1:21:25
  Speaker 1: less energy being injected, the surrounding gas starts cooling again.
• 1:21:30
  Speaker 1: Some of it begins trickling back inward. The black hole
• 1:21:34
  Speaker 1: starts eating again. The cycle repeats. What you have is
• 1:21:39
  Speaker 1: a thermostat, a self-regulating feedback loop, where the black hole's
• 1:21:44
  Speaker 1: activity level is constantly adjusting in response to how much
• 1:21:49
  Speaker 1: gas is available, and the gas availability is constantly adjusting
• 1:21:55
  Speaker 1: in response to how active the black hole is. The
• 1:21:59
  Speaker 1: galaxy never gets too far in either direction. Too much
• 1:22:04
  Speaker 1: star formation triggers more black hole feeding, which suppresses further growth.
• 1:22:10
  Speaker 1: The system stabilizes, and it stabilizes at a very specific place.
• 1:22:16
  Speaker 1: The evidence for this picture comes from several directions. One
• 1:22:21
  Speaker 1: of the most striking pieces is a correlation that nobody
• 1:22:24
  Speaker 1: expected to find. When astronomers measured the masses of supermassive
• 1:22:30
  Speaker 1: black holes across many different galaxies and compared them to
• 1:22:34
  Speaker 1: the masses of the stellar bulges surrounding those black holes,
• 1:22:38
  Speaker 1: they found a tight relationship. Bigger bulge, bigger black hole,
• 1:22:44
  Speaker 1: with a ratio that holds across galaxies spanning orders of
• 1:22:48
  Speaker 1: magnitude in total mass. This is not what you would
• 1:22:52
  Speaker 1: expect if black holes and galaxies were simply growing independently
• 1:22:58
  Speaker 1: of each other. The black hole is a tiny fraction
• 1:23:02
  Speaker 1: of the galaxy's total mass. A black hole of a
• 1:23:06
  Speaker 1: billion solar masses sits inside a galaxy of hundreds of billions.
• 1:23:12
  Speaker 1: They shouldn't know about each other by rights, but they
• 1:23:16
  Speaker 1: clearly do. They grew together. something was regulating both of
• 1:23:21
  Speaker 1: them simultaneously. Another piece of evidence comes from looking at
• 1:23:25
  Speaker 1: massive galaxies at high redshift, far enough away that we
• 1:23:29
  Speaker 1: are seeing them as they appeared billions of years ago
• 1:23:33
  Speaker 1: when the universe was young. There is a period, roughly
• 1:23:37
  Speaker 1: 10 to 12 billion years ago, when the most massive
• 1:23:41
  Speaker 1: galaxies appear to have abruptly stopped forming stars. Before that point,
• 1:23:48
  Speaker 1: They were among the most productive star-forming systems in the universe.
• 1:23:53
  Speaker 1: After it, they went quiet. The timing coincides with periods
• 1:23:59
  Speaker 1: of intense black hole activity in the same systems. The
• 1:24:04
  Speaker 1: quenching was rapid. In cosmic terms, these galaxies went from
• 1:24:09
  Speaker 1: vibrant to dormant in what amounts to a few hundred
• 1:24:13
  Speaker 1: million years. Something with enormous energy turned them off. Nothing
• 1:24:19
  Speaker 1: else in the physics of galaxy formation produces that kind
• 1:24:22
  Speaker 1: of sudden, powerful suppression. Nothing except the black hole. The
• 1:24:27
  Speaker 1: picture that emerges from all of this is not a
• 1:24:31
  Speaker 1: simple one of black holes dominating their galaxies. It is
• 1:24:35
  Speaker 1: a picture of mutual regulation. The galaxy feeds gas to
• 1:24:40
  Speaker 1: the black hole. The black hole responds by heating and
• 1:24:44
  Speaker 1: dispersing the gas. Both systems end up shaped by the
• 1:24:48
  Speaker 1: same underlying gas supply, each responding to the other across
• 1:24:53
  Speaker 1: billions of years of coevolution. Take away the black hole,
• 1:24:58
  Speaker 1: and the gas would have kept cooling, kept collapsing, kept
• 1:25:03
  Speaker 1: forming stars. The galaxy would have grown far more massive
• 1:25:08
  Speaker 1: than it did. The presence of that black hole at
• 1:25:11
  Speaker 1: the centre fundamentally altered the outcome. The black hole at
• 1:25:17
  Speaker 1: the center of a galaxy is not just a passive passenger.
• 1:25:21
  Speaker 1: It is one of the main things that decided when
• 1:25:24
  Speaker 1: the galaxy stopped growing. The spiral galaxy is not as
• 1:25:28
  Speaker 1: stable as it looks. When you see an image of
• 1:25:31
  Speaker 1: a spiral galaxy, those sweeping arms, that flat luminous disk,
• 1:25:37
  Speaker 1: the soft glow of a billion stars arranged in something
• 1:25:41
  Speaker 1: almost too orderly to believe, it looks permanent. It looks
• 1:25:46
  Speaker 1: like something that was designed and placed. It wasn't. What
• 1:25:50
  Speaker 1: you're looking at is a temporary balance, a structure that
• 1:25:54
  Speaker 1: took billions of years of specific, careful history to produce,
• 1:25:59
  Speaker 1: and one that can be undone in a single catastrophic collision.
• 1:26:03
  Speaker 1: The fact that the Milky Way is a spiral is
• 1:26:06
  Speaker 1: not a coincidence. It is a record. It tells you
• 1:26:11
  Speaker 1: exactly what kind of life this galaxy has lived. To
• 1:26:15
  Speaker 1: understand why spirals exist at all, you have to start
• 1:26:18
  Speaker 1: with a concept called angular momentum. When gas begins falling
• 1:26:23
  Speaker 1: into a dark matter halo in the early universe, it
• 1:26:27
  Speaker 1: is never falling perfectly straight down. The gas is coming
• 1:26:32
  Speaker 1: from slightly different directions, with slightly different velocities, tugged by
• 1:26:37
  Speaker 1: nearby clumps of matter, jostled by turbulence. all of those
• 1:26:43
  Speaker 1: tiny sideways motions add up to a net rotation. As
• 1:26:48
  Speaker 1: the gas falls inward, that rotation speeds up. For the
• 1:26:52
  Speaker 1: same reason, a figure skater spins faster when she pulls
• 1:26:56
  Speaker 1: her arms in. Conservation of angular momentum means the gas
• 1:27:01
  Speaker 1: cannot simply collapse to a central point. It has to
• 1:27:05
  Speaker 1: go somewhere sideways. And so instead of forming a sphere,
• 1:27:10
  Speaker 1: the gas settles into a disk. Think of water draining
• 1:27:15
  Speaker 1: out of a bathtub. It spirals. It flatties, ends. It rotates.
• 1:27:22
  Speaker 1: In a galaxy, the process takes hundreds of millions of years,
• 1:27:26
  Speaker 1: and the disk never fully drains. But the physics behind
• 1:27:30
  Speaker 1: the shape is the same idea. Once that rotating disk
• 1:27:34
  Speaker 1: is established, stars begin forming inside it. The gas cools, fragments,
• 1:27:41
  Speaker 1: and collapses into stellar nurseries spread across the disk in
• 1:27:45
  Speaker 1: a wide, flat plane. And at that point, you have
• 1:27:49
  Speaker 1: something that looks, at least in silhouette, like a spiral galaxy.
• 1:27:54
  Speaker 1: But you don't have the arms yet. Here is where
• 1:27:57
  Speaker 1: it gets strange. Spiral arms are not fixed structures. The
• 1:28:03
  Speaker 1: stars inside a spiral arm are not the same stars
• 1:28:06
  Speaker 1: from one cycle to the next. they cannot be, because
• 1:28:11
  Speaker 1: the arm itself rotates around the galaxy at a different
• 1:28:15
  Speaker 1: speed than the individual stars do. Stars in the outer
• 1:28:20
  Speaker 1: parts of the disk orbit the galactic centre more slowly
• 1:28:24
  Speaker 1: than stars in the inner parts. If the arms were
• 1:28:28
  Speaker 1: rigid structures made of fixed stars, they would wind themselves
• 1:28:33
  Speaker 1: up tighter and tighter over billions of years until the
• 1:28:37
  Speaker 1: spiral pattern was completely erased. That doesn't happen. What astronomers
• 1:28:43
  Speaker 1: eventually worked out is that the arms are density waves.
• 1:28:48
  Speaker 1: Imagine a traffic jam on a highway. The individual cars
• 1:28:52
  Speaker 1: keep moving through it, slowing down, bunching up, then speeding
• 1:28:57
  Speaker 1: away on the other side. The jam itself sits in
• 1:29:01
  Speaker 1: roughly the same place, even as every car inside it
• 1:29:05
  Speaker 1: is different from one moment to the next. A spiral
• 1:29:09
  Speaker 1: arm works the same way. It's a region of slightly
• 1:29:13
  Speaker 1: elevated density slowly rotating around the galaxy, and as stars
• 1:29:18
  Speaker 1: and gas clouds pass through it, they slow down, compress,
• 1:29:22
  Speaker 1: pile up briefly, and then move on. The arm appears
• 1:29:26
  Speaker 1: bright because that compression is exactly the condition needed to
• 1:29:31
  Speaker 1: trigger star formation. New, massive, intensely luminous stars ignite inside
• 1:29:37
  Speaker 1: the arm. They burn fast. They die before they have
• 1:29:41
  Speaker 1: time to travel far from where they were born. So
• 1:29:45
  Speaker 1: the arm always glows, always looks full, even though the
• 1:29:50
  Speaker 1: specific stars lighting it up are constantly changing. This is
• 1:29:55
  Speaker 1: worth sitting with. The thing that makes a spiral galaxy beautiful,
• 1:30:00
  Speaker 1: those luminous trailing arms, is not a structure at all.
• 1:30:05
  Speaker 1: It's a wave. It's a pattern moving through material. The
• 1:30:10
  Speaker 1: stars you're seeing in those arms were mostly born there recently,
• 1:30:14
  Speaker 1: and most of them will be dead before the arm
• 1:30:16
  Speaker 1: has completed another full rotation. For that disk, and those arms,
• 1:30:22
  Speaker 1: to survive, the galaxy needs to avoid a specific kind
• 1:30:26
  Speaker 1: of violence. Minor mergers are fine. When a small dwarf
• 1:30:31
  Speaker 1: galaxy falls into a larger spiral, it mostly contributes gas
• 1:30:36
  Speaker 1: and stars to the outer regions of the disk. It
• 1:30:39
  Speaker 1: can actually thicken the disk slightly, add material to it,
• 1:30:44
  Speaker 1: stir up star formation in the outer arms. This is
• 1:30:48
  Speaker 1: not destruction. The Milky Way has been swallowing dwarf galaxies
• 1:30:53
  Speaker 1: for most of its existence. and it is still unmistakably
• 1:30:57
  Speaker 1: a spiral. Those minor collisions left scars, a thickened outer disk,
• 1:31:04
  Speaker 1: streams of stars in the halo, ripples in the structure.
• 1:31:09
  Speaker 1: But they didn't erase the fundamental organization. A major merger
• 1:31:14
  Speaker 1: is something different entirely. When two galaxies of comparable size collide,
• 1:31:21
  Speaker 1: the result is chaos. Tidal forces shred the outer regions
• 1:31:26
  Speaker 1: of both disks. The orderly orbital structure that took billions
• 1:31:31
  Speaker 1: of years to establish gets scrambled by the gravitational chaos
• 1:31:36
  Speaker 1: of two massive objects tearing at each other across hundreds
• 1:31:41
  Speaker 1: of thousands of light years. The gas clouds collide violently,
• 1:31:46
  Speaker 1: triggering enormous bursts of star formations. that consume most of
• 1:31:51
  Speaker 1: the available fuel in a relatively short period. After multiple
• 1:31:56
  Speaker 1: passes and final coalescence, what you're left with is not
• 1:32:01
  Speaker 1: a disk. It's a tangled mass of stars moving in random,
• 1:32:05
  Speaker 1: overlapping orbits, with no rotation axis, no plane, no arms.
• 1:32:12
  Speaker 1: The spiral is gone. What replaces it is something else,
• 1:32:16
  Speaker 1: which is a story for the next section. What this
• 1:32:20
  Speaker 1: means is that being a spiral galaxy is, in a
• 1:32:23
  Speaker 1: specific sense, a survival story. It means the galaxy grew,
• 1:32:29
  Speaker 1: because growth through minor mergers and gas accretion is how
• 1:32:34
  Speaker 1: disks build up their mass, but never suffered the kind
• 1:32:38
  Speaker 1: of head-on encounter that would have ended the disk entirely.
• 1:32:43
  Speaker 1: The Milky Way has had close calls. The Gaia Enceladus
• 1:32:48
  Speaker 1: Collision roughly 10 billion years ago, was significant enough to
• 1:32:53
  Speaker 1: scramble part of the early disk and leave a thick,
• 1:32:57
  Speaker 1: puffed-out stellar layer above and below the galactic plane. That
• 1:33:03
  Speaker 1: event left its mark, but it was not enough to
• 1:33:07
  Speaker 1: destroy the disk. The thin disk we live in today
• 1:33:11
  Speaker 1: formed afterward, in the relative quiet that followed, as gas
• 1:33:15
  Speaker 1: settled back into the rotational plane and resumed forming stars.
• 1:33:21
  Speaker 1: The spiral persisted. There is something almost improbable about that.
• 1:33:27
  Speaker 1: Every large spiral you can point to in the night
• 1:33:30
  Speaker 1: sky is a galaxy that threaded a very specific needle,
• 1:33:35
  Speaker 1: growing massive enough to be impressive while somehow avoiding the
• 1:33:39
  Speaker 1: catastrophic major mergers that would have erased it. The ordered
• 1:33:45
  Speaker 1: rotating disc of a spiral is the product of a
• 1:33:48
  Speaker 1: specific kind of history. Enough merging to grow, but not
• 1:33:54
  Speaker 1: so much that the whole structure got scrambled into something unrecognizable.
• 1:33:59
  Speaker 1: There is a class of galaxy that contains no spiral arms,
• 1:34:03
  Speaker 1: no active star formation, no cold gas, just hundreds of
• 1:34:08
  Speaker 1: billions of old red stars moving in orbits with no
• 1:34:12
  Speaker 1: particular organization, no disk, No rotation you could measure with
• 1:34:17
  Speaker 1: any confidence. Just a smooth, featureless blob of ancient starlight,
• 1:34:24
  Speaker 1: shaped roughly like a football or a sphere, hanging in
• 1:34:28
  Speaker 1: space like something that has given up. These are elliptical galaxies,
• 1:34:34
  Speaker 1: and they are, in a very specific sense, what happens
• 1:34:38
  Speaker 1: when the story of a spiral galaxy ends badly. To
• 1:34:43
  Speaker 1: understand how you get from a spinning structured disk to
• 1:34:47
  Speaker 1: a featureless elliptical, you have to watch what happens when
• 1:34:51
  Speaker 1: two spiral galaxies fall toward each other. Not a quick collision.
• 1:34:57
  Speaker 1: This plays out over hundreds of millions of years. The
• 1:35:01
  Speaker 1: first thing that happens on the first close pass is
• 1:35:05
  Speaker 1: that tidal forces reach across the gap between the two
• 1:35:08
  Speaker 1: galaxies and begin pulling material free. Stars and gas get
• 1:35:14
  Speaker 1: drawn out into long, sweeping tails, arcing away from the
• 1:35:18
  Speaker 1: main bodies into the space between them. You can see
• 1:35:23
  Speaker 1: this happening right now in real galaxies. The Antennae galaxies,
• 1:35:29
  Speaker 1: two spirals caught in the act of merging, have tidal
• 1:35:33
  Speaker 1: tails stretching hundreds of thousands of light-years in either direction.
• 1:35:39
  Speaker 1: They look like a pair of insects with their legs
• 1:35:42
  Speaker 1: tangled together. This is what the first pass does. After
• 1:35:47
  Speaker 1: that first encounter, the galaxies slow down, turn around, and
• 1:35:52
  Speaker 1: fall back toward each other. The second approach is worse.
• 1:35:57
  Speaker 1: The structure that was already compromised on the first pass
• 1:36:01
  Speaker 1: gets more severely disrupted. Spiral arms blur. The disk begins
• 1:36:07
  Speaker 1: to lose its coherence. Gas clouds throughout both galaxies are
• 1:36:12
  Speaker 1: slamming into each other at enormous velocities, and the compression
• 1:36:16
  Speaker 1: triggers star formation on a massive scale. Millions of new
• 1:36:21
  Speaker 1: stars ignite across a period that is brief by cosmic standards,
• 1:36:26
  Speaker 1: a few tens of millions of years, burning through enormous
• 1:36:30
  Speaker 1: quantities of gas. This is called a starburst, and it
• 1:36:35
  Speaker 1: is as violent as it sounds. By the third approach,
• 1:36:40
  Speaker 1: if there even is a third, the two galaxies are
• 1:36:44
  Speaker 1: already unrecognizable. What were once orderly rotating disks are now
• 1:36:50
  Speaker 1: a tangled mess of stars in scrambled orbits. The gas
• 1:36:55
  Speaker 1: that once filled the disks has been either consumed in
• 1:36:58
  Speaker 1: the starburst or heated and blown outward. And then, slowly,
• 1:37:05
  Speaker 1: over more hundreds of millions of years, the combined system settles.
• 1:37:10
  Speaker 1: Stars that were once moving in organized circles around a
• 1:37:14
  Speaker 1: disk are now moving in every direction, crossing each other's
• 1:37:18
  Speaker 1: paths in chaotic, overlapping orbits. The whole structure contracts slightly
• 1:37:25
  Speaker 1: under its own gravity and finds a new equilibrium, a smooth,
• 1:37:31
  Speaker 1: pressure-supported blob, an elliptical galaxy. What's left behind is a
• 1:37:37
  Speaker 1: stellar population that is almost entirely old. The starburst consumed
• 1:37:43
  Speaker 1: most of the cold gas. The merger heated the rest.
• 1:37:47
  Speaker 1: There is no raw material for a new generation of stars.
• 1:37:51
  Speaker 1: So the galaxy just sits there, slowly fading, as its
• 1:37:56
  Speaker 1: most massive stars burn out first, and only the small,
• 1:38:00
  Speaker 1: long-lived red stars remain. Astronomers call these galaxies Red and Dead.
• 1:38:07
  Speaker 1: The name is blunt, but it is accurate. The evidence
• 1:38:11
  Speaker 1: that massive ellipticals are merger products is not subtle. Nearly
• 1:38:16
  Speaker 1: all of them contain faint shells and ripples in their
• 1:38:19
  Speaker 1: outer stellar distributions, structures that persist for billions of years
• 1:38:25
  Speaker 1: after a merger event and would not exist in a
• 1:38:28
  Speaker 1: system that had always been a featureless blob. Many of
• 1:38:32
  Speaker 1: them contain two or more stellar populations with slightly different
• 1:38:36
  Speaker 1: ages and chemical compositions, reflecting the fact that they were
• 1:38:40
  Speaker 1: once separate galaxies with separate histories. Virtually every massive elliptical
• 1:38:47
  Speaker 1: galaxy also hosts a supermassive black hole, often an enormous one,
• 1:38:53
  Speaker 1: because major mergers tend to feed black holes as efficiently
• 1:38:57
  Speaker 1: as they feed star formation. In fact, estimates suggest that
• 1:39:03
  Speaker 1: nearly all galaxies with masses above a certain threshold have
• 1:39:07
  Speaker 1: undergone at least one major merger since the universe was
• 1:39:12
  Speaker 1: half its current age. It is not an exotic event,
• 1:39:17
  Speaker 1: It is the normal fate of large galaxies in a
• 1:39:21
  Speaker 1: dense enough environment. Here is the part that gets strange.
• 1:39:26
  Speaker 1: If major mergers produce ellipticals, and if large galaxies grew
• 1:39:31
  Speaker 1: through hierarchical clustering, small things falling into larger things across
• 1:39:37
  Speaker 1: billions of years, then the most massive ellipticals in the
• 1:39:41
  Speaker 1: universe are, almost by definition, the last things to finish forming.
• 1:39:48
  Speaker 1: The biggest galaxy in a galaxy cluster is not old
• 1:39:52
  Speaker 1: in the sense of having been assembled early. It is
• 1:39:55
  Speaker 1: old in the sense that it has been accumulating for
• 1:39:58
  Speaker 1: the entire history of the universe. Every collision adds more stars.
• 1:40:05
  Speaker 1: Every merger contributes another stellar population. These objects, called brightest
• 1:40:11
  Speaker 1: cluster galaxies, sit at the gravitational centers of enormous galaxy clusters.
• 1:40:18
  Speaker 1: and have grown by absorbing smaller galaxies repeatedly over cosmic history.
• 1:40:23
  Speaker 1: Some of them contain more than 10 trillion solar masses
• 1:40:26
  Speaker 1: in stars. That number is almost too large to hold
• 1:40:31
  Speaker 1: in your head. Our entire Milky Way contains a few
• 1:40:34
  Speaker 1: hundred billion. These things are 40, 50, 100 times larger.
• 1:40:42
  Speaker 1: Smaller galaxies that wander too close to the center of
• 1:40:45
  Speaker 1: a cluster do not survive the encounter intact. Tidal forces
• 1:40:51
  Speaker 1: strip their outer stars into the surrounding space, and what
• 1:40:55
  Speaker 1: remains spirals inward and merges with the central giant. This
• 1:41:01
  Speaker 1: process has been going on for billions of years and
• 1:41:04
  Speaker 1: is still happening now. Some of the largest ellipticals in
• 1:41:08
  Speaker 1: the universe are visibly surrounded by the disrupted remnants of
• 1:41:12
  Speaker 1: smaller galaxies that did not make it out. The most
• 1:41:16
  Speaker 1: massive galaxies in the universe today are, in almost every case,
• 1:41:21
  Speaker 1: the final merged products of collisions that played out over
• 1:41:25
  Speaker 1: billions of years, built last, from the most pieces, in
• 1:41:30
  Speaker 1: the most violent way. The Milky Way is 13 billion
• 1:41:35
  Speaker 1: years old, give or take a few hundred million, which
• 1:41:39
  Speaker 1: means it started forming when the universe was less than
• 1:41:42
  Speaker 1: a billion years old. Most people, when they think about
• 1:41:47
  Speaker 1: the galaxy they live in, picture the familiar image, a
• 1:41:51
  Speaker 1: flat spinning disc with graceful spiral arms, a glowing central bulge,
• 1:41:58
  Speaker 1: a quiet and orderly system. But it is incomplete in
• 1:42:03
  Speaker 1: a way that turns out to be deeply interesting, because
• 1:42:06
  Speaker 1: the Milky Way did not arrive at that orderly shape smoothly.
• 1:42:11
  Speaker 1: It got there through billions of years of collisions, absorptions,
• 1:42:15
  Speaker 1: and upheaval that astronomers are only now beginning to read
• 1:42:19
  Speaker 1: in full detail. The oldest parts of the galaxy are
• 1:42:23
  Speaker 1: not in the disk at all. They are in a
• 1:42:26
  Speaker 1: structure called the stellar halo, a diffuse, roughly spherical cloud
• 1:42:31
  Speaker 1: of stars that surrounds the entire disk and extends far
• 1:42:36
  Speaker 1: above and below it. Halo stars are old. Some of
• 1:42:41
  Speaker 1: them are nearly as old as the universe itself, with
• 1:42:44
  Speaker 1: ages pushing past 12 billion years. They contain almost no
• 1:42:49
  Speaker 1: heavy elements, iron, calcium, oxygen, because they formed before enough
• 1:42:56
  Speaker 1: stellar generations had lived and died to scatter those elements
• 1:43:00
  Speaker 1: through the surrounding gas. In the language astronomers use, they
• 1:43:05
  Speaker 1: are metal-poor, and that low metal content is a direct timestamp.
• 1:43:11
  Speaker 1: These are stars from before the galaxy had any real
• 1:43:14
  Speaker 1: history to speak of. Their orbits are the other telling thing.
• 1:43:19
  Speaker 1: Disc stars, like the Sun, move in roughly circular paths,
• 1:43:24
  Speaker 1: all going the same direction in the same flat plane.
• 1:43:28
  Speaker 1: Halo stars do not. They follow elongated, tilted, chaotic orbits
• 1:43:34
  Speaker 1: that take them plunging deep through the disk and out
• 1:43:37
  Speaker 1: the other side. That orbital chaos is a record of violence.
• 1:43:43
  Speaker 1: These stars were not born where they are now. They
• 1:43:47
  Speaker 1: were torn from smaller galaxies that fell into the young
• 1:43:50
  Speaker 1: Milky Way and were shredded apart. The halo is, in
• 1:43:55
  Speaker 1: a very real sense, a graveyard of galaxies the Milky
• 1:44:00
  Speaker 1: Way consumed. Younger than the halo, but still ancient, is
• 1:44:05
  Speaker 1: the thick disk. This is not a different galaxy, or
• 1:44:09
  Speaker 1: a separate structure. It is an older, puffier version of
• 1:44:13
  Speaker 1: the disk, sitting above and below the thin disk that
• 1:44:17
  Speaker 1: the Sun lives in, like a cushion on either side
• 1:44:20
  Speaker 1: of a table. Thick disk stars formed roughly 10 to
• 1:44:25
  Speaker 1: 12 billion years ago during a period when the Milky
• 1:44:29
  Speaker 1: Way was being actively disrupted by mergers and bombardment from
• 1:44:34
  Speaker 1: other systems. They contain more heavy elements than the Halo stars,
• 1:44:40
  Speaker 1: because by the time they formed, earlier supernovae had already
• 1:44:44
  Speaker 1: begun enriching the interstellar gas. But they are not as
• 1:44:49
  Speaker 1: metal-rich as younger stars, and their orbits are thicker and
• 1:44:54
  Speaker 1: more disordered, than the neat circular paths of stars in
• 1:44:58
  Speaker 1: the thin disk. The thick disk is a snapshot of
• 1:45:01
  Speaker 1: the Milky Way in middle adolescence, building up, accumulating material,
• 1:45:08
  Speaker 1: but still chaotic, still being knocked around. Something happened during
• 1:45:13
  Speaker 1: the thick disk's formation that stirred the galaxy significantly. Then
• 1:45:18
  Speaker 1: there is the thin disk. This is where the Sun lives,
• 1:45:23
  Speaker 1: where most of the galaxy's current star formation is happening
• 1:45:27
  Speaker 1: and where the spiral arms are most visible. The thin
• 1:45:31
  Speaker 1: disk formed later as the Milky Way finally settled into
• 1:45:35
  Speaker 1: something more stable. For a long time, the standard estimate
• 1:45:40
  Speaker 1: was that the thin disk began forming roughly 8 to
• 1:45:44
  Speaker 1: 10 billion years ago. But the key point is that
• 1:45:47
  Speaker 1: the thin disk represents a calmer epoch. Mergers were less
• 1:45:52
  Speaker 1: frequent gas was settling into orderly rotation. The galaxy was,
• 1:45:58
  Speaker 1: in the language of the field, quiescent, building steadily rather
• 1:46:03
  Speaker 1: than violently. Were there still disruptions? Yes. The thin disk
• 1:46:10
  Speaker 1: carries scars of its own, but the character of this
• 1:46:13
  Speaker 1: period was different from the thick disk era, and you
• 1:46:17
  Speaker 1: can see that difference in the stars themselves. They are
• 1:46:21
  Speaker 1: more metal-rich, more orderly, more circular in their orbits. So
• 1:46:26
  Speaker 1: you have three layers, each one the product of a
• 1:46:29
  Speaker 1: different phase in the same galaxy's history. The halo tells
• 1:46:34
  Speaker 1: you about the earliest mergers and the first chaotic assembly.
• 1:46:39
  Speaker 1: The thick disk tells you about a turbulent middle period
• 1:46:43
  Speaker 1: of heavy disruption. The thin disk tells you about the long,
• 1:46:48
  Speaker 1: relatively stable epoch that followed. The Milky Way did not
• 1:46:53
  Speaker 1: grow uniformly. It accumulated in waves, with different regions forming
• 1:46:59
  Speaker 1: at different times under different conditions, and then settling on
• 1:47:04
  Speaker 1: top of each other like geological strata. Reading those layers
• 1:47:09
  Speaker 1: requires measuring individual stars, their positions, their velocities, the precise
• 1:47:17
  Speaker 1: mixture of elements inside them. And for most of astronomy's history,
• 1:47:22
  Speaker 1: the data to do that at large scale simply did
• 1:47:25
  Speaker 1: not exist. That changed with a spacecraft called Gaia, launched
• 1:47:30
  Speaker 1: by the European Space Agency in 2013. Gaia's mission was
• 1:47:35
  Speaker 1: specific and extraordinary. Measure the positions and motions of stars
• 1:47:40
  Speaker 1: across the Milky Way with unprecedented accuracy. Not hundreds of stars,
• 1:47:46
  Speaker 1: not thousands, nearly two billion. For each one, Gaia recorded
• 1:47:52
  Speaker 1: precise coordinates, proper motions across the sky, and in many cases,
• 1:47:58
  Speaker 1: radial velocities, how fast the star was moving toward or
• 1:48:03
  Speaker 1: away from Earth. Combined with data on stellar chemistry from
• 1:48:08
  Speaker 1: ground-based surveys, this created something that had never existed before,
• 1:48:14
  Speaker 1: a three-dimensional kinematic map of a significant fraction of the
• 1:48:19
  Speaker 1: entire galaxy. The scale of the dataset is almost conceptually unwieldy.
• 1:48:26
  Speaker 1: Two billion stars. The number is large enough that the
• 1:48:31
  Speaker 1: data itself had to be released in stages, with the
• 1:48:35
  Speaker 1: astronomical community spending years extracting what was hid Then inside it,
• 1:48:41
  Speaker 1: before Gaia, the story of the Milky Way was told
• 1:48:44
  Speaker 1: in broad strokes. The halo formed early, from mergers. The
• 1:48:49
  Speaker 1: thick disk formed next, during a period of disruption. The
• 1:48:54
  Speaker 1: thin disk came last, in relative peace. That story was
• 1:48:59
  Speaker 1: not wrong, but it was rough. The details were missing.
• 1:49:03
  Speaker 1: Which mergers? When, exactly? How large were the incoming galaxies?
• 1:49:10
  Speaker 1: How did each event change the galaxy's structure and chemistry?
• 1:49:14
  Speaker 1: Those questions had answers that were either speculative or absent entirely.
• 1:49:21
  Speaker 1: After Gaia, specific merger events began to emerge from the
• 1:49:25
  Speaker 1: data with the kind of clarity that makes the story
• 1:49:29
  Speaker 1: feel real rather than theoretical. Stars whose orbits made no
• 1:49:34
  Speaker 1: sense as native Milky Way stars could be grouped together
• 1:49:39
  Speaker 1: and traced back to specific disrupted systems. Bursts of star
• 1:49:44
  Speaker 1: formation that had left chemical signatures in the thick disk
• 1:49:49
  Speaker 1: could be matched to specific disruption events at specific times.
• 1:49:55
  Speaker 1: The Milky Way's history went from a sketch to something
• 1:49:59
  Speaker 1: approaching a detailed record. What Gaia found, in short, was
• 1:50:04
  Speaker 1: that the galaxy's past was messier and more eventful than
• 1:50:08
  Speaker 1: the smooth narrative suggested. Merger events that had been hypothesized
• 1:50:14
  Speaker 1: were confirmed and dated. New ones were found. The boundary
• 1:50:20
  Speaker 1: between the thick disk and the thin disk turned out
• 1:50:23
  Speaker 1: to be less a clean transition and more a complicated overlap.
• 1:50:29
  Speaker 1: The halo turned out to contain at least two distinct
• 1:50:33
  Speaker 1: populations with different origins distinguishable by their orbital properties and
• 1:50:39
  Speaker 1: chemical compositions. Every time the data was pushed harder, another
• 1:50:46
  Speaker 1: layer of complexity appeared. The simple picture of the Milky
• 1:50:50
  Speaker 1: Way disk, halo, bulge, dun kept giving way to something
• 1:50:57
  Speaker 1: richer and stranger underneath. What Gaia found when it mapped
• 1:51:02
  Speaker 1: nearly two billion of those stars was a history far
• 1:51:06
  Speaker 1: more violent and surprising than the one that had been assumed.
• 1:51:11
  Speaker 1: Around 10 billion years ago, a smaller galaxy fell into
• 1:51:15
  Speaker 1: the Milky Way. Not a dwarf, not a wisp of
• 1:51:19
  Speaker 1: a satellite. Astronomers call it Gaia Enceladus, and at the
• 1:51:25
  Speaker 1: time of impact, it carried roughly one quarter of the
• 1:51:29
  Speaker 1: Milky Way's total mass. Put that in perspective... The Milky
• 1:51:34
  Speaker 1: Way at that point was already billions of stars deep,
• 1:51:38
  Speaker 1: already had a disk forming, already had a center, and
• 1:51:43
  Speaker 1: then something a quarter its size came in. That is
• 1:51:47
  Speaker 1: not a minor disturbance. That is a collision that would
• 1:51:50
  Speaker 1: have reshaped everything. The event didn't happen in an instant.
• 1:51:55
  Speaker 1: Nothing at galactic scales does. The whole process played out
• 1:52:00
  Speaker 1: over hundreds of millions of years. which sounds long until
• 1:52:03
  Speaker 1: you realize that is just a fraction of the time
• 1:52:06
  Speaker 1: that has passed since. First came close passes. Gaia Enceladus
• 1:52:12
  Speaker 1: swooped in, felt the Milky Way's gravity pulling at its stars,
• 1:52:17
  Speaker 1: and swung back out. But each pass bled energy away
• 1:52:21
  Speaker 1: from its orbit. The two galaxies were circling each other
• 1:52:25
  Speaker 1: like a slow drain, and the outcome was never in question.
• 1:52:29
  Speaker 1: With every approach, Tidal forces tore more stars loose. Trails
• 1:52:35
  Speaker 1: of stripped material stretched out behind the intruder. Eventually, the
• 1:52:40
  Speaker 1: last pass didn't end with a retreat. The two systems
• 1:52:44
  Speaker 1: fell into each other completely. Here is the remarkable thing.
• 1:52:49
  Speaker 1: Nobody watched this happen. No telescope pointed at our galaxy
• 1:52:54
  Speaker 1: 10 billion years ago. Everything we know about Gaia Enceladus
• 1:52:59
  Speaker 1: comes from evidence sitting in our sky right now, encoded
• 1:53:04
  Speaker 1: in the motions of stars we can measure today. The
• 1:53:08
  Speaker 1: Gaia Space Telescope is the reason any of this became clear.
• 1:53:12
  Speaker 1: Its mission was precise, measure the positions, velocities, and chemical
• 1:53:18
  Speaker 1: compositions of nearly 2 billion stars in the Milky Way.
• 1:53:23
  Speaker 1: When astronomers took that data and began sorting through the
• 1:53:27
  Speaker 1: halo stars, Those ancient stars orbiting far above and below
• 1:53:32
  Speaker 1: the galactic plane in no particular order, they found something
• 1:53:37
  Speaker 1: that didn't belong. A large population of stars with orbital
• 1:53:42
  Speaker 1: properties inconsistent with having formed in our galaxy. Their velocities
• 1:53:49
  Speaker 1: were wrong. Their orbital shapes were wrong. They were moving
• 1:53:54
  Speaker 1: on paths that made sense only if they had been
• 1:53:58
  Speaker 1: captured from somewhere else. And when researchers plotted their orbital
• 1:54:03
  Speaker 1: energies and angular momenta, those stars didn't scatter randomly. They clustered.
• 1:54:10
  Speaker 1: The distribution stretched into an elongated shape. Astronomers started calling
• 1:54:16
  Speaker 1: it the sausage. That name stuck. and the whole event
• 1:54:21
  Speaker 1: became known formally as the Gaia Enceladus Sausage merger. Those
• 1:54:26
  Speaker 1: sausage stars are the stripped remnants of a galaxy that
• 1:54:30
  Speaker 1: was torn apart 10 billion years ago. They've been orbiting
• 1:54:34
  Speaker 1: inside our galaxy ever since, distinguishable from Milky Way stars
• 1:54:40
  Speaker 1: only if you know exactly what to look for. The
• 1:54:43
  Speaker 1: fact that we can still pick them out, still separate
• 1:54:47
  Speaker 1: their orbits from native stars, is extraordinary. Ten billion years
• 1:54:53
  Speaker 1: of orbital evolution, and the fingerprints haven't washed out. But
• 1:54:58
  Speaker 1: Gaia Enceladus didn't just leave ghost stars behind. It physically
• 1:55:04
  Speaker 1: changed the Milky Way's structure. Before the collision, our galaxy
• 1:55:09
  Speaker 1: had a disk, a relatively thin, orderly rotating sheet of
• 1:55:14
  Speaker 1: stars and gas. When Gaia Enceladus came through, The gravitational
• 1:55:19
  Speaker 1: disruption hit that disk like a shockwave. Stars got knocked
• 1:55:24
  Speaker 1: onto higher, more chaotic orbits. The thin disk didn't collapse,
• 1:55:29
  Speaker 1: but it got puffed up, thickened. What astronomers now call
• 1:55:34
  Speaker 1: the thick disk, that older, broader layer of stars sitting
• 1:55:38
  Speaker 1: above and below the main galactic plane, carries the physical
• 1:55:42
  Speaker 1: scar of this event. Its stars are older. moving on
• 1:55:47
  Speaker 1: orbits more inclined than the younger thin disk stars below them.
• 1:55:52
  Speaker 1: The geometry of the thick disk is a record of
• 1:55:56
  Speaker 1: a violent encounter. Then the gas arrived. Gaia Enceladus brought
• 1:56:01
  Speaker 1: its own reservoir of gas with it, and when the
• 1:56:04
  Speaker 1: two galaxies merged, that gas slammed into the gas already
• 1:56:09
  Speaker 1: inside the Milky Way. The collision was not clean. Shockwaves
• 1:56:14
  Speaker 1: propagated through enormous clouds. Dense regions compressed further. And across
• 1:56:21
  Speaker 1: the galaxy, gas clouds started collapsing. What followed was one
• 1:56:27
  Speaker 1: of the most intense bursts of star formation in the
• 1:56:31
  Speaker 1: Milky Way's history. Researchers examining data from Gaia found a
• 1:56:36
  Speaker 1: sharp spike in stellar birth rates around 11 billion years ago,
• 1:56:41
  Speaker 1: right at the moment the merger was happening. That spike
• 1:56:45
  Speaker 1: didn't appear gradually. It appeared suddenly, and it was enormous.
• 1:56:51
  Speaker 1: Billions of new stars formed in a relatively short window,
• 1:56:56
  Speaker 1: all carrying the chemical signature of that particular moment in
• 1:56:59
  Speaker 1: galactic history. Chemistry is a time capsule. Stars record the
• 1:57:05
  Speaker 1: composition of the gas they formed from, and that composition
• 1:57:09
  Speaker 1: changes as each generation of stars lives, dies, and enriches
• 1:57:14
  Speaker 1: the surrounding medium with heavier elements. The thick disk stars
• 1:57:19
  Speaker 1: formed during the Gaia Enceladus burst have iron abundances that
• 1:57:24
  Speaker 1: jumped sharply. Some studies show a 30-fold increase in iron
• 1:57:29
  Speaker 1: content across the thick disk era. That's not a gradual drift.
• 1:57:36
  Speaker 1: That's the signature of massive rapid star formation and the
• 1:57:40
  Speaker 1: supernova explosions that followed. each one pumping heavy elements back
• 1:57:45
  Speaker 1: into the gas before the next generation ignited. The story
• 1:57:50
  Speaker 1: didn't stop there. The Milky Way, having digested Gaia Enceladus,
• 1:57:56
  Speaker 1: settled into a calmer phase. The thin disk grew. The
• 1:58:02
  Speaker 1: galaxy became, by cosmic standards, relatively quiet. But quiet is relative.
• 1:58:09
  Speaker 1: and a new intruder eventually appeared. The Sagittarius dwarf galaxy
• 1:58:15
  Speaker 1: has been falling through the Milky Way's disk repeatedly for
• 1:58:18
  Speaker 1: the last several billion years. Not merging, not yet, but
• 1:58:24
  Speaker 1: passing through on a decaying orbit that brings it back
• 1:58:27
  Speaker 1: around every few hundred million years. Each close pass distorts
• 1:58:33
  Speaker 1: the disk slightly. Each collision between the Sagittarius gas and
• 1:58:38
  Speaker 1: Milky Way gas compresses clouds and triggers new star formation.
• 1:58:45
  Speaker 1: Around six billion years ago, a noticeable dip in the
• 1:58:48
  Speaker 1: Milky Way's star formation rate was followed by a disruption
• 1:58:52
  Speaker 1: that lines up precisely with one of these Sagittarius passes.
• 1:58:58
  Speaker 1: The galaxy responded. New stars formed in the disturbance. Sagittarius
• 1:59:04
  Speaker 1: is still out there, being slowly shredded by tidal forces,
• 1:59:09
  Speaker 1: leaving a long stream of stars tracing its past orbits
• 1:59:13
  Speaker 1: around the galaxy. What all of this means is that
• 1:59:16
  Speaker 1: the Milky Way we live in today is not the
• 1:59:19
  Speaker 1: galaxy it started as. The thick disk came from a
• 1:59:23
  Speaker 1: collision The chemical richness of our stellar neighborhood came from
• 1:59:28
  Speaker 1: a burst of supernovae triggered by a merger. The subtle
• 1:59:32
  Speaker 1: distortions in the thin disk carry the mark of a
• 1:59:36
  Speaker 1: dwarf galaxy that is still in the process of being absorbed.
• 1:59:41
  Speaker 1: The history is written in the stars themselves, in their orbits,
• 1:59:46
  Speaker 1: in their compositions, in the way they move through the
• 1:59:50
  Speaker 1: sky right now. The stars formed in that burst of
• 1:59:54
  Speaker 1: activity are still here, still orbiting, their chemical compositions carrying
• 2:00:01
  Speaker 1: a permanent record of the collision that created them. If
• 2:00:05
  Speaker 1: you could somehow stand outside the observable universe and look
• 2:00:08
  Speaker 1: back at all the galaxies inside it, you would not
• 2:00:12
  Speaker 1: see a random scatter of points. What you would see
• 2:00:16
  Speaker 1: is something stranger, a vast, three-dimensional web stretching across billions
• 2:00:23
  Speaker 1: of light years, with long threads connecting dense knots and
• 2:00:28
  Speaker 1: enormous empty regions sitting between them. It looks, if you
• 2:00:34
  Speaker 1: see the computer renderings, almost biological, like a neuron, like
• 2:00:40
  Speaker 1: a sponge. Nothing about it looks like what you'd expect
• 2:00:44
  Speaker 1: from a universe that started as a featureless fog of gas.
• 2:00:48
  Speaker 1: This structure has a name. Astronomers call it the cosmic web,
• 2:00:53
  Speaker 1: and mapping it has taken decades of observation. The Sloan
• 2:00:57
  Speaker 1: Digital Sky Survey mapped the positions of hundreds of millions
• 2:01:01
  Speaker 1: of galaxies across a significant portion of the observable universe.
• 2:01:07
  Speaker 1: What came back was not noise. It was the same pattern,
• 2:01:12
  Speaker 1: over and over, at scale after scale. Long filaments of galaxies,
• 2:01:18
  Speaker 1: each filament hundreds of millions of light years long, dense
• 2:01:23
  Speaker 1: clusters sitting at the points where multiple filaments intersect, and
• 2:01:27
  Speaker 1: between them, vast voids, regions hundreds of millions of light
• 2:01:33
  Speaker 1: years across, where almost nothing lives. The universe is not
• 2:01:39
  Speaker 1: filled evenly with galaxies. Most of the volume of the
• 2:01:43
  Speaker 1: universe is nearly empty. The filaments are not coincidences. They
• 2:01:48
  Speaker 1: are the direct descendants of the same density fluctuations that
• 2:01:52
  Speaker 1: built individual galaxies, just operating on a far larger scale.
• 2:01:58
  Speaker 1: Wherever space was slightly denser than average in the first
• 2:02:02
  Speaker 1: moments after the Big Bang, gravity got to work. Over time,
• 2:02:07
  Speaker 1: slightly over-dense regions pulled in more matter, while under-dense regions
• 2:02:12
  Speaker 1: gave it up. On the scale of galaxy formation, This
• 2:02:17
  Speaker 1: process created dark matter halos. On scales a thousand times larger,
• 2:02:23
  Speaker 1: the same process carved the cosmic web. The over-dense regions
• 2:02:28
  Speaker 1: became filaments and clusters. The under-dense regions became voids. Same physics,
• 2:02:36
  Speaker 1: same starting conditions, completely different scales. The dark matter did
• 2:02:42
  Speaker 1: the heavy lifting here too. Because dark matter doesn't interact
• 2:02:46
  Speaker 1: with light, it wasn't slowed down by radiation pressure the
• 2:02:50
  Speaker 1: way ordinary gas was. It began clustering first, laying down
• 2:02:56
  Speaker 1: the scaffolding. Ordinary matter followed, falling along the dark matter structure,
• 2:03:03
  Speaker 1: flowing like water along channels that were already there. The
• 2:03:08
  Speaker 1: filaments you see in galaxy surveys today trace almost perfectly
• 2:03:14
  Speaker 1: The structure of the underlying dark matter web. You can't
• 2:03:18
  Speaker 1: see the dark matter directly, but you can see where
• 2:03:22
  Speaker 1: the galaxies went. And that tells you where the dark
• 2:03:25
  Speaker 1: matter must be. So the web emerged. But what happened
• 2:03:30
  Speaker 1: to the galaxies inside it? Here is something that took
• 2:03:34
  Speaker 1: astronomers a long time to appreciate. Where a galaxy lives
• 2:03:39
  Speaker 1: matters enormously for what it becomes. Two galaxies that started
• 2:03:44
  Speaker 1: out nearly identical can end up completely different depending on
• 2:03:49
  Speaker 1: whether they fell into a dense cluster or drifted into
• 2:03:52
  Speaker 1: a void. The environment reshapes them, sometimes violently. In galaxy clusters,
• 2:04:01
  Speaker 1: the environment is brutal. Clusters contain hundreds or thousands of
• 2:04:05
  Speaker 1: galaxies crammed into a region a few million light years across.
• 2:04:10
  Speaker 1: all orbiting inside a bath of superheated gas that fills
• 2:04:14
  Speaker 1: the space between them. That gas reaches temperatures of tens
• 2:04:19
  Speaker 1: of millions of degrees. When a galaxy falls through a
• 2:04:22
  Speaker 1: cluster at high speed, the hot gas strips away the
• 2:04:26
  Speaker 1: galaxy's own cold gas supply, like peeling the skin from
• 2:04:30
  Speaker 1: a piece of fruit. The process is called ram pressure stripping,
• 2:04:36
  Speaker 1: and it is efficient. Once a galaxy loses its cold gas,
• 2:04:41
  Speaker 1: it can't form new stars. It just keeps the ones
• 2:04:44
  Speaker 1: it has, which age and dim and redden over billions
• 2:04:48
  Speaker 1: of years. Cluster galaxies stop forming stars early. They are
• 2:04:54
  Speaker 1: overwhelmingly red and old and quiet. Galaxies along the filaments
• 2:04:59
  Speaker 1: have a different story. They receive a more gradual flow
• 2:05:03
  Speaker 1: of gas from the surrounding cosmic web, feeding star formation
• 2:05:07
  Speaker 1: at moderate rates over long timescales. They are often spirals,
• 2:05:13
  Speaker 1: still blue, still building new generations of stars. The Milky
• 2:05:18
  Speaker 1: Way sits in a relatively modest environment, not a dense cluster,
• 2:05:23
  Speaker 1: not a void. A small group of galaxies, loosely bound,
• 2:05:28
  Speaker 1: on a filament connecting larger structures. That modest environment is
• 2:05:34
  Speaker 1: part of why the Milky Way is still forming stars today.
• 2:05:38
  Speaker 1: Galaxies in the voids are almost untouched. They evolved slowly,
• 2:05:44
  Speaker 1: in isolation, receiving little of the disruption that cluster and
• 2:05:48
  Speaker 1: filament galaxies experienced. They tend to be smaller, bluer, less
• 2:05:54
  Speaker 1: chemically enriched. They are, in some ways, closer to what
• 2:05:59
  Speaker 1: the early universe looked like, because they've been left alone
• 2:06:03
  Speaker 1: to grow quietly, without the violent mergers that built the
• 2:06:07
  Speaker 1: big galaxies in denser regions. Environment, it turns out, is
• 2:06:13
  Speaker 1: one of the most powerful predictors of what a galaxy
• 2:06:16
  Speaker 1: looks like. More powerful, in some cases, than the galaxy's
• 2:06:21
  Speaker 1: initial mass or composition. Ask where a galaxy lives, and
• 2:06:27
  Speaker 1: you have already answered a lot of questions about what
• 2:06:30
  Speaker 1: it is. What makes the cosmic web remarkable is not
• 2:06:35
  Speaker 1: just that it exists, but that we predicted it before
• 2:06:38
  Speaker 1: we fully mapped it. Decades ago, cosmologists ran computer simulations
• 2:06:45
  Speaker 1: of a universe starting from nearly uniform initial conditions, letting
• 2:06:50
  Speaker 1: gravity evolve the density fluctuations forward in time. The simulations
• 2:06:57
  Speaker 1: produced a cosmic web, Then the surveys went out and
• 2:07:02
  Speaker 1: mapped the real universe. The match was striking, not perfect,
• 2:07:08
  Speaker 1: there are tensions, discrepancies being actively investigated, but the broad
• 2:07:14
  Speaker 1: architecture was there, exactly where the models said it would be.
• 2:07:19
  Speaker 1: The pattern of galaxies across the universe is, in a
• 2:07:23
  Speaker 1: very direct sense, a map of what the universe looked
• 2:07:27
  Speaker 1: like in its first fraction of a second. amplified by
• 2:07:31
  Speaker 1: gravity across 13 billion years. The James Webb Space Telescope
• 2:07:37
  Speaker 1: was not supposed to overturn our understanding of how the
• 2:07:41
  Speaker 1: early universe worked. It was supposed to confirm it. The
• 2:07:46
  Speaker 1: plan was straightforward. Build a telescope powerful enough to see
• 2:07:50
  Speaker 1: the first galaxies directly. Watch them appear on schedule, small
• 2:07:56
  Speaker 1: and faint and primitive, just as the models predicted. Decades
• 2:08:01
  Speaker 1: of cosmological simulations had produced a clear picture. The early
• 2:08:06
  Speaker 1: universe should be filled with tiny, irregular proto-galaxies, clumpy and chaotic,
• 2:08:12
  Speaker 1: slowly building toward the structured systems we see today. Nobody
• 2:08:18
  Speaker 1: expected a crisis. Nobody was braced for what actually came back.
• 2:08:24
  Speaker 1: JWST launched in December of 2021 and began returning science
• 2:08:29
  Speaker 1: data in the summer of 2022. Within months, the first
• 2:08:35
  Speaker 1: results were already strange. Astronomers started flagging objects in the
• 2:08:40
  Speaker 1: early data, galaxies sitting at extraordinary distances, seen as they existed,
• 2:08:47
  Speaker 1: when the universe was only a few hundred million years old.
• 2:08:51
  Speaker 1: That part was expected. The age was not the problem.
• 2:08:56
  Speaker 1: The problem was what those galaxies looked like. They were massive,
• 2:09:01
  Speaker 1: not small and faint and embryonic. Massive. Some of them
• 2:09:05
  Speaker 1: appeared to contain hundreds of billions of solar masses worth
• 2:09:09
  Speaker 1: of stars, assembled and organized, burning brightly, at a time
• 2:09:15
  Speaker 1: when nothing that size should have had any chance to exist.
• 2:09:20
  Speaker 1: To understand why this is a problem, you need to
• 2:09:23
  Speaker 1: understand what the models were actually saying. The hierarchical clustering model,
• 2:09:30
  Speaker 1: the dominant framework for galaxy formation for decades, makes a
• 2:09:34
  Speaker 1: specific prediction about the early universe. Small things form first.
• 2:09:41
  Speaker 1: Large things come later. assembled from the smaller things through
• 2:09:46
  Speaker 1: mergers and accretion across billions of years. A galaxy containing
• 2:09:52
  Speaker 1: several hundred billion solar masses of stars is not a
• 2:09:55
  Speaker 1: small thing. Under the Standard Model, building an object like
• 2:10:00
  Speaker 1: that takes time. It takes many mergers, each one playing
• 2:10:05
  Speaker 1: out over hundreds of millions of years. It takes dark
• 2:10:09
  Speaker 1: matter halos growing large enough to hold onto gas, and
• 2:10:13
  Speaker 1: that gas cooling enough to collapse, and those collapses producing stars,
• 2:10:18
  Speaker 1: generation after generation, accumulating mass across long stretches of cosmic history.
• 2:10:26
  Speaker 1: You do not get something that large in a few
• 2:10:28
  Speaker 1: hundred million years. The math doesn't work, or rather, it didn't.
• 2:10:35
  Speaker 1: What JWST was seeing were galaxies that appeared to have
• 2:10:39
  Speaker 1: skipped the line, Objects with stellar masses that, according to
• 2:10:44
  Speaker 1: every simulation run before 2022, should not have existed until
• 2:10:50
  Speaker 1: the universe was several billion years old, were sitting there
• 2:10:54
  Speaker 1: in the data, bright and fully formed, at redshifts corresponding
• 2:11:00
  Speaker 1: to a universe barely 500 million years old. Some researchers,
• 2:11:05
  Speaker 1: when they first ran the numbers, assumed they had made
• 2:11:09
  Speaker 1: an error, They checked the photometry, they checked the redshift estimates,
• 2:11:14
  Speaker 1: they checked the calibrations. The galaxies were real, the masses
• 2:11:19
  Speaker 1: were real, and nothing in the standard playbook explained them.
• 2:11:24
  Speaker 1: Here is how large the discrepancy was. Cosmological simulations of
• 2:11:30
  Speaker 1: the early universe, the same simulations that had successfully predicted
• 2:11:35
  Speaker 1: the cosmic web, the large-scale distribution of galaxies across space,
• 2:11:41
  Speaker 1: the statistics of galaxy clustering, predicted a specific number for
• 2:11:46
  Speaker 1: how much stellar mass should exist in the universe at
• 2:11:49
  Speaker 1: very high redshift. When researchers compared those predictions to the
• 2:11:54
  Speaker 1: JWST observations, some of the detected galaxies appeared to exceed
• 2:12:00
  Speaker 1: the total stellar mass budget the simulations allowed for their
• 2:12:04
  Speaker 1: entire observable region. not just above the predicted average, in
• 2:12:10
  Speaker 1: some cases, above the theoretical ceiling. That is not a
• 2:12:15
  Speaker 1: small discrepancy you paper over with a correction factor. That
• 2:12:20
  Speaker 1: is a model telling you something has gone wrong at
• 2:12:23
  Speaker 1: a fundamental level. Several explanations were put forward immediately. The
• 2:12:29
  Speaker 1: most conservative was measurement error. Estimating stellar masses from photometric
• 2:12:35
  Speaker 1: data at extreme redshift involves assumptions about stellar populations, dust content,
• 2:12:43
  Speaker 1: and star formation history, and those assumptions can compound. Some
• 2:12:48
  Speaker 1: of the most extreme mass estimates may have been inflated
• 2:12:51
  Speaker 1: by contamination from active galactic nuclei, black holes pumping out
• 2:12:57
  Speaker 1: radiation that mimics a stellar signal. Spectroscopic follow-up observations, which
• 2:13:03
  Speaker 1: give much more reliable data, have confirmed that some early
• 2:13:08
  Speaker 1: candidates were overestimated. But not all of them. The problem
• 2:13:13
  Speaker 1: did not go away when the conservative corrections were applied.
• 2:13:18
  Speaker 1: A significant number of genuinely anomalous galaxies remained. The more
• 2:13:24
  Speaker 1: interesting explanations involve the physics itself. One possibility is that
• 2:13:30
  Speaker 1: star formation efficiency in the early universe was dramatically higher
• 2:13:35
  Speaker 1: than the models assumed. In present-day galaxies, only a small
• 2:13:40
  Speaker 1: fraction of available gas actually collapses into stars. Feedback from
• 2:13:46
  Speaker 1: supernovae and black holes blows the rest away. But what if,
• 2:13:51
  Speaker 1: in the early universe, before that feedback had time to
• 2:13:55
  Speaker 1: establish itself, the conversion of gas into stars was almost
• 2:14:00
  Speaker 1: ruthlessly efficient. If even a modest increase in early star
• 2:14:05
  Speaker 1: formation efficiency is applied to the models, the mass problem
• 2:14:10
  Speaker 1: becomes significantly less severe. It doesn't fully disappear, but the
• 2:14:15
  Speaker 1: gap narrows considerably. Another possibility involves the initial density fluctuations themselves.
• 2:14:24
  Speaker 1: The models assume a specific statistical distribution of those fluctuations,
• 2:14:30
  Speaker 1: a distribution calibrated against measurements of the cosmic microwave background.
• 2:14:36
  Speaker 1: But what if that distribution had a longer tail than expected?
• 2:14:41
  Speaker 1: What if some regions of the early universe were significantly
• 2:14:46
  Speaker 1: over-dense relative to the average, producing halos massive enough to
• 2:14:51
  Speaker 1: host unexpectedly large galaxies much earlier than the standard distribution
• 2:14:57
  Speaker 1: would allow. This would not require changing the physics of
• 2:15:02
  Speaker 1: galaxy formation at all. It would just mean the early
• 2:15:06
  Speaker 1: universe was slightly lumpier than the best current measurements suggest.
• 2:15:11
  Speaker 1: A more radical proposal involves dark matter. The standard model
• 2:15:16
  Speaker 1: uses cold dark matter, meaning dark matter particles that were
• 2:15:21
  Speaker 1: moving slowly when structure began forming, which allows small-scale clumping
• 2:15:27
  Speaker 1: to proceed freely. Some researchers have asked whether a different
• 2:15:32
  Speaker 1: form of dark matter, one with slightly different properties at
• 2:15:36
  Speaker 1: small scales, might allow halos to grow faster in the
• 2:15:40
  Speaker 1: early universe, seeding the massive early galaxies JWST is finding.
• 2:15:47
  Speaker 1: This is speculative. There is no direct evidence for non-standard
• 2:15:52
  Speaker 1: dark matter behaviour. But the JWST results are, for the
• 2:15:58
  Speaker 1: first time, putting real observational pressure on the cold dark
• 2:16:03
  Speaker 1: matter assumption. What nobody is seriously arguing is that JWST
• 2:16:08
  Speaker 1: made the wrong observations. The instruments work. The data is real.
• 2:16:15
  Speaker 1: And the honest answer to the question of what is
• 2:16:18
  Speaker 1: causing this is that the field does not have a
• 2:16:21
  Speaker 1: clean consensus yet. Papers are being published every month. Simulations
• 2:16:27
  Speaker 1: are being revised. Old assumptions are being tested against new
• 2:16:32
  Speaker 1: data in ways that would have been impossible three years ago.
• 2:16:37
  Speaker 1: Some of the most respected cosmologists alive are publicly saying
• 2:16:42
  Speaker 1: that the early universe is more complicated than they thought.
• 2:16:47
  Speaker 1: That is not a comfortable thing to say after decades
• 2:16:51
  Speaker 1: of successful predictions. but it is the accurate thing to say.
• 2:16:57
  Speaker 1: What makes this genuinely exciting, not just unsettling, is what
• 2:17:02
  Speaker 1: it implies about how much is still left to understand.
• 2:17:06
  Speaker 1: The models that JWST is straining were not naive guesses.
• 2:17:12
  Speaker 1: They were sophisticated, well-tested frameworks built on decades of observations.
• 2:17:19
  Speaker 1: And a single telescope, in its first year of operation,
• 2:17:23
  Speaker 1: found objects that those frameworks struggle to explain. Whatever the
• 2:17:29
  Speaker 1: explanation turns out to be, the picture of early galaxy
• 2:17:33
  Speaker 1: formation that was taught in textbooks as recently as 2021
• 2:17:39
  Speaker 1: has been quietly abandoned. The iron in your blood was
• 2:17:43
  Speaker 1: made in a star that exploded before the solar system existed.
• 2:17:48
  Speaker 1: Not metaphorically, not approximately the actual iron atoms circulating through
• 2:17:54
  Speaker 1: your body right now were forged in the collapsing core
• 2:17:58
  Speaker 1: of a massive star ejected in a supernova explosion scattered
• 2:18:04
  Speaker 1: across interstellar space and eventually swept up into the cloud
• 2:18:08
  Speaker 1: of gas that became the Sun, the Earth, and everything
• 2:18:13
  Speaker 1: on it. That is not a poetic framing. That is
• 2:18:17
  Speaker 1: the literal chain of events. And it only happened because
• 2:18:21
  Speaker 1: galaxies spent billions of years building the conditions that made
• 2:18:25
  Speaker 1: it possible. The universe started with almost none of this.
• 2:18:30
  Speaker 1: After the Big Bang, the only elements in existence were hydrogen, helium,
• 2:18:34
  Speaker 1: and trace amounts of lithium. That is it. No carbon,
• 2:18:39
  Speaker 1: no oxygen, no iron, no calcium, no silicon. The entire
• 2:18:45
  Speaker 1: periodic table beyond those first few entries was missing, because
• 2:18:50
  Speaker 1: nothing had been built yet. The first generation of stars,
• 2:18:55
  Speaker 1: those enormous, short-lived monsters made entirely of hydrogen and helium,
• 2:19:01
  Speaker 1: were the first factories. They fused lighter elements into heavier
• 2:19:06
  Speaker 1: ones in their cores, and when they died in supernova explosions,
• 2:19:11
  Speaker 1: they scattered those elements into the surrounding gas. Carbon. Oxygen. Neon. Magnesium.
• 2:19:20
  Speaker 1: The universe's chemical inventory expanded in a single violent event,
• 2:19:25
  Speaker 1: repeated millions of times across millions of early galaxies. That
• 2:19:30
  Speaker 1: process never stopped. Each generation of stars inherited the chemical
• 2:19:35
  Speaker 1: leftovers of the generation before it, added more complexity, and
• 2:19:40
  Speaker 1: passed it forward. The gas that collapsed to form the
• 2:19:45
  Speaker 1: second generation of stars already contained trace amounts of carbon
• 2:19:49
  Speaker 1: and oxygen. That gas cooled more efficiently than the pure
• 2:19:54
  Speaker 1: hydrogen clouds of the first generation, which allowed smaller, longer-lived
• 2:20:00
  Speaker 1: stars to form. Those stars lived longer, enriched the gas
• 2:20:05
  Speaker 1: more slowly and steadily, and died in their own explosions.
• 2:20:11
  Speaker 1: Then came a third generation, and a fourth, each one
• 2:20:16
  Speaker 1: building on the last. Astronomers call this chemical enrichment, and
• 2:20:22
  Speaker 1: it is one of the most powerful tools available for
• 2:20:24
  Speaker 1: reading the history of a galaxy, because the chemical composition
• 2:20:29
  Speaker 1: of a star is essentially a timestamp. This is where
• 2:20:32
  Speaker 1: the Milky Way's history becomes readable in a specific and
• 2:20:36
  Speaker 1: striking way. The oldest stars in the galaxy are found
• 2:20:41
  Speaker 1: in the stellar halo, those metal-poor stars on chaotic, looping
• 2:20:46
  Speaker 1: orbits that formed in the earliest and most violent phase
• 2:20:50
  Speaker 1: of assembly. Their iron abundances are extremely low, sometimes hundreds
• 2:20:57
  Speaker 1: of times lower than the sun's. They are relics of
• 2:21:01
  Speaker 1: a time when the galaxy was young, when few supernovae
• 2:21:05
  Speaker 1: had yet occurred, and when the interstellar gas was still
• 2:21:09
  Speaker 1: nearly as pristine as the universe had left it. When
• 2:21:14
  Speaker 1: you measure the iron content of a halo star and
• 2:21:17
  Speaker 1: find it sitting at a fraction of a percent of
• 2:21:20
  Speaker 1: the sun's value, you are looking at something that formed
• 2:21:24
  Speaker 1: close to the beginning. The thick disk stars are younger,
• 2:21:28
  Speaker 1: They formed during the turbulent middle period of the Milky
• 2:21:32
  Speaker 1: Way's history, when mergers were still frequent and star formation
• 2:21:37
  Speaker 1: was running at a high rate. Their iron abundances are higher,
• 2:21:42
  Speaker 1: and here the data carries a very specific signature. Around
• 2:21:47
  Speaker 1: the time of the Gaia Enceladus collision, roughly 10 to
• 2:21:52
  Speaker 1: 11 billion years ago, something interesting happened to the chemical record.
• 2:21:58
  Speaker 1: the merger dumped an enormous quantity of gas into the
• 2:22:02
  Speaker 1: Milky Way. That gas triggered a burst of star formation
• 2:22:07
  Speaker 1: intense enough to show up in the abundance measurements of
• 2:22:11
  Speaker 1: stars that formed at that time. The iron content climbed steeply.
• 2:22:18
  Speaker 1: Thick disk stars at the end of that era are
• 2:22:21
  Speaker 1: dramatically more metal-rich than the ones at the beginning. The
• 2:22:25
  Speaker 1: collision is written into the chemistries as clearly as a
• 2:22:28
  Speaker 1: layer in geological rock. The thin disk stars, including the Sun,
• 2:22:35
  Speaker 1: formed later still from gas that had been enriched by
• 2:22:39
  Speaker 1: all of that history. The Sun is a relatively metal-rich
• 2:22:44
  Speaker 1: star for its age, which is part of why the
• 2:22:47
  Speaker 1: solar system ended up with enough heavy elements to build
• 2:22:50
  Speaker 1: rocky planets at all. Rocky planets are essentially compressed piles
• 2:22:57
  Speaker 1: of silicon, iron, magnesium, and oxygen, none of which existed
• 2:23:03
  Speaker 1: before stellar nucleosynthesis produced them. Without billions of years of
• 2:23:09
  Speaker 1: supernovae and galactic chemical enrichment, there is no material available
• 2:23:14
  Speaker 1: to build a planet like Earth. You cannot form a
• 2:23:18
  Speaker 1: terrestrial world from hydrogen and helium alone. Were the conditions
• 2:23:24
  Speaker 1: for life always going to emerge from this process? That
• 2:23:27
  Speaker 1: question is harder, but the chemistry of it is clear.
• 2:23:32
  Speaker 1: Carbon chains, nitrogen bonds, phosphorus, sulfur. The molecular building blocks
• 2:23:40
  Speaker 1: of biology require elements that only stars produce. The first
• 2:23:45
  Speaker 1: generation of stars could not have built a habitable planet
• 2:23:48
  Speaker 1: around them even in principle. The raw materials weren't there.
• 2:23:53
  Speaker 1: By the time a star like the Sun formed, Those
• 2:23:56
  Speaker 1: materials had been accumulating for nearly 9 billion years. That
• 2:24:02
  Speaker 1: gap is not incidental. It is the minimum amount of
• 2:24:06
  Speaker 1: time the universe needed to produce the right chemistry. This
• 2:24:10
  Speaker 1: is why the chemical history of galaxies is not a
• 2:24:13
  Speaker 1: footnote to the story of galaxy formation. It is the
• 2:24:17
  Speaker 1: part of the story that connects directly to whether anything
• 2:24:21
  Speaker 1: interesting could ever happen in the universe. Galaxy formation built
• 2:24:26
  Speaker 1: the furnaces, star formation lit them, supernovae distributed the output,
• 2:24:33
  Speaker 1: and then gravity swept it all back together into new clouds,
• 2:24:37
  Speaker 1: new stars, new planets. Every cycle enriched the raw material
• 2:24:43
  Speaker 1: a little more, nudging the universe incrementally toward a state
• 2:24:48
  Speaker 1: where complex chemistry was possible. The oldest stars in the
• 2:24:53
  Speaker 1: Milky Way's halo are still there, still orbiting, still carrying
• 2:24:58
  Speaker 1: in their spectra the record of a time before any
• 2:25:02
  Speaker 1: of this had happened. The Sun carries its own record,
• 2:25:06
  Speaker 1: and the planets that formed around it carry the record
• 2:25:10
  Speaker 1: of everything that came before them. Galaxy formation is not
• 2:25:15
  Speaker 1: an abstract cosmological process. It is the mechanism that that
• 2:25:20
  Speaker 1: built the raw materials of everything, and the billions of
• 2:25:24
  Speaker 1: years it took were not wasted. The Milky Way is
• 2:25:28
  Speaker 1: going to be destroyed. Not by a rogue star, not
• 2:25:33
  Speaker 1: by some catastrophic failure of physics, not by anything that
• 2:25:38
  Speaker 1: could have been prevented. The Milky Way is going to
• 2:25:42
  Speaker 1: be destroyed the same way countless galaxies before it were destroyed.
• 2:25:48
  Speaker 1: by another galaxy falling into it. Right now, 2.5 million
• 2:25:55
  Speaker 1: light years away, the Andromeda Galaxy is closing in. It
• 2:26:00
  Speaker 1: is already on its way. The two galaxies are approaching
• 2:26:04
  Speaker 1: each other at roughly 110 kilometers per second, and in
• 2:26:08
  Speaker 1: about 4 to 5 billion years, they will meet. What
• 2:26:13
  Speaker 1: happens after that is not a mystery. We have watched
• 2:26:17
  Speaker 1: it happen to other galaxies across the universe hundreds of times.
• 2:26:22
  Speaker 1: We know the sequence. We can run the simulations. The
• 2:26:26
  Speaker 1: outcome is not in doubt. What is worth sitting with
• 2:26:31
  Speaker 1: is what that outcome actually means for a galaxy that
• 2:26:35
  Speaker 1: has been building itself for 13 billion years. Start with
• 2:26:40
  Speaker 1: what the sky would look like, assuming anything is alive
• 2:26:44
  Speaker 1: on any planet to see it. Right now, Andromeda is
• 2:26:48
  Speaker 1: visible to the naked eye as a faint smudge in
• 2:26:51
  Speaker 1: the constellation of Perseus, a pale thumbprint in the darkness.
• 2:26:57
  Speaker 1: For the next several billion years, not much changes. Then, gradually,
• 2:27:04
  Speaker 1: that smudge begins to resolve. It grows. It brightens. The
• 2:27:10
  Speaker 1: outer spiral arms of Andromeda start to become visible as
• 2:27:14
  Speaker 1: something distinct from the background stars, a second structure rising
• 2:27:20
  Speaker 1: in the night sky alongside the band of the Milky
• 2:27:23
  Speaker 1: Way itself. There would be no single dramatic moment when
• 2:27:28
  Speaker 1: things started going wrong. The change would be slow beyond
• 2:27:33
  Speaker 1: any human experience of slowness, stretching across millions of years.
• 2:27:39
  Speaker 1: But on astronomical timescales, what comes next is sudden. The
• 2:27:45
  Speaker 1: first close pass would transform the sky entirely. Andromeda's disk
• 2:27:50
  Speaker 1: begins to distort as tidal forces pull at it. Our
• 2:27:54
  Speaker 1: own galaxy distorts in response. Long tails of stars pulled
• 2:27:59
  Speaker 1: free from both galaxies by the mutual gravitational stress, arc
• 2:28:04
  Speaker 1: out into intergalactic space. From any surviving planet, the sky
• 2:28:10
  Speaker 1: would now be occupied by a structure unlike anything the
• 2:28:13
  Speaker 1: galaxy had contained for billions of years. Two interacting systems,
• 2:28:19
  Speaker 1: both deformed, both shedding material, the whole scene lit up
• 2:28:25
  Speaker 1: by furious new star formations. triggered by colliding gas clouds.
• 2:28:31
  Speaker 1: When the gas clouds of two galaxies slam together at
• 2:28:34
  Speaker 1: high speed, the compression is violent enough to collapse enormous
• 2:28:38
  Speaker 1: quantities of material almost simultaneously. Stars ignite by the millions.
• 2:28:46
  Speaker 1: The rate of star formation during this phase could be
• 2:28:49
  Speaker 1: 10 or 20 times higher than anything. Gee, the Milky
• 2:28:53
  Speaker 1: Way experiences today. The sky would be bright with it,
• 2:28:57
  Speaker 1: Then the two galaxies would separate again. This is the
• 2:29:01
  Speaker 1: part that surprises people. A galactic merger is not a
• 2:29:05
  Speaker 1: single collision. The two galaxies swing past each other, pulled
• 2:29:11
  Speaker 1: back by gravity, swing past again, each pass slower than
• 2:29:15
  Speaker 1: the last. The process repeats across hundreds of millions of years.
• 2:29:21
  Speaker 1: Three passes, sometimes four. Each time, Both galaxies lose more
• 2:29:27
  Speaker 1: of their original structure. Each time, the gas supply for
• 2:29:32
  Speaker 1: new star formation depletes a little further. Each time, the
• 2:29:37
  Speaker 1: violence of the encounter heats more of the remaining gas
• 2:29:40
  Speaker 1: to temperatures where it cannot cool and collapse. Eventually, the
• 2:29:46
  Speaker 1: two systems do not separate again. They coalesce into a
• 2:29:51
  Speaker 1: single merged object. Here is something worth understanding about that merger.
• 2:29:57
  Speaker 1: Almost no stars will actually collide. The distances between individual
• 2:30:03
  Speaker 1: stars are so vast that two entire galaxies can pass
• 2:30:07
  Speaker 1: through each other with virtually no direct stellar impacts. Our
• 2:30:13
  Speaker 1: Sun is roughly 1.4 million kilometres wide. The nearest star
• 2:30:18
  Speaker 1: to the Sun is about 40 trillion kilometres away. In
• 2:30:23
  Speaker 1: a galaxy where that kind of spacing is typical, the
• 2:30:27
  Speaker 1: odds of any two stars hitting each other are negligibly small.
• 2:30:32
  Speaker 1: What changes for individual stars is not survival, but position.
• 2:30:38
  Speaker 1: Gravitational perturbations during the encounter can fling stars to entirely
• 2:30:43
  Speaker 1: different orbits. A star that spent its entire life in
• 2:30:47
  Speaker 1: the quiet outer disk could end up on a chaotic orbit,
• 2:30:52
  Speaker 1: that carries it thousands of light years out of the
• 2:30:55
  Speaker 1: galactic plane and back. The Sun's orbit could be fundamentally altered.
• 2:31:00
  Speaker 1: Whatever planetary system the Sun still has at that point,
• 2:31:05
  Speaker 1: and it will still have one, since planetary disruption from
• 2:31:08
  Speaker 1: a galactic merger is not particularly likely, could find itself
• 2:31:13
  Speaker 1: in a radically different position within the merged galaxy. The
• 2:31:18
  Speaker 1: end product of all of this has been given a name.
• 2:31:21
  Speaker 1: Some astronomers call it Milkomeda, others say Milkdromeda. Whatever you
• 2:31:27
  Speaker 1: call it, it will not look much like either of
• 2:31:30
  Speaker 1: its parent galaxies. The organized rotating disk of the Milky Way,
• 2:31:36
  Speaker 1: the spiral arms, the flat geometry that took billions of
• 2:31:40
  Speaker 1: years to build, all of it will be scrambled into
• 2:31:43
  Speaker 1: something rounder and less ordered. Stars that once moved in
• 2:31:48
  Speaker 1: calm circular orbits will move in elongated, tilted paths with
• 2:31:54
  Speaker 1: no common direction. No cold gas will remain to feed
• 2:31:58
  Speaker 1: new generations of stars. The result will be a large
• 2:32:02
  Speaker 1: elliptical galaxy, old and red and quiet, its star formation finished.
• 2:32:08
  Speaker 1: Everything described in the previous sections of this story, the
• 2:32:12
  Speaker 1: dark matter halo that seeded the Milky Way's formation, the
• 2:32:17
  Speaker 1: Gaia Enceladus collision that built the thick disk, the Sagittarius
• 2:32:22
  Speaker 1: dwarf galaxy spiralling in across billions of years. All of
• 2:32:27
  Speaker 1: it will have produced, in the end, raw material for
• 2:32:30
  Speaker 1: a merger that turns the whole structure into something fundamentally different.
• 2:32:36
  Speaker 1: This is not a tragedy in any cosmic sense. It
• 2:32:39
  Speaker 1: is the process. Every large elliptical galaxy in the universe
• 2:32:44
  Speaker 1: arrived at its current state through exactly this sequence of events.
• 2:32:49
  Speaker 1: Two disc galaxies, a slow approach, a violent encounter, a merger,
• 2:32:56
  Speaker 1: and then silence. The same thing has happened billions of
• 2:33:00
  Speaker 1: times at every scale across the history of the universe.
• 2:33:05
  Speaker 1: What makes this one different is only that it is ours,
• 2:33:09
  Speaker 1: and that the evidence for it is already sitting 2.5
• 2:33:13
  Speaker 1: million light-years away, closing in at 110 kilometers per second.
• 2:33:20
  Speaker 1: unremarkable against the sky. The collision with Andromeda is not
• 2:33:24
  Speaker 1: an ending specific to our galaxy. It is exactly the
• 2:33:28
  Speaker 1: same story that has played out for billions of galaxies
• 2:33:32
  Speaker 1: throughout cosmic history. And in that sense, the Milky Way
• 2:33:36
  Speaker 1: is simply arriving, a few billion years late, at the
• 2:33:41
  Speaker 1: same destination. Step back from all of it. The dark
• 2:33:45
  Speaker 1: matter halos, the merging spirals, the billion solar mass black
• 2:33:50
  Speaker 1: holes that appeared too early, and what you are left
• 2:33:54
  Speaker 1: with is a universe that built itself. No blueprint, no
• 2:33:59
  Speaker 1: single moment when galaxies switched on and the job was done.
• 2:34:04
  Speaker 1: Just gravity, time, and a set of initial conditions so
• 2:34:09
  Speaker 1: close to uniform that the differences between dense regions and
• 2:34:13
  Speaker 1: empty ones were smaller than anything you could measure with
• 2:34:17
  Speaker 1: a ruler. and from that, everything. The process took 13.8
• 2:34:23
  Speaker 1: billion years and it is still running. That is the
• 2:34:26
  Speaker 1: part that does not get enough attention. Galaxy formation is
• 2:34:30
  Speaker 1: not something that happened and then stopped, the way the
• 2:34:34
  Speaker 1: Big Bang happened and stopped. Galaxies are merging right now.
• 2:34:39
  Speaker 1: Stars are forming right now inside dark matter halos that
• 2:34:43
  Speaker 1: have been pulling in gas for billions of years. The
• 2:34:47
  Speaker 1: Sagittarius dwarf is being shredded by the Milky Way's gravity
• 2:34:51
  Speaker 1: at this moment. Andromeda is already on its way. The
• 2:34:55
  Speaker 1: universe is mid-sentence, not at a period. Direct observation confirms
• 2:35:01
  Speaker 1: this continuity. When astronomers look at the nearby universe, they
• 2:35:06
  Speaker 1: see mergers at every stage of completion, some systems locked
• 2:35:11
  Speaker 1: in the final approach, Others recently combined but still settling
• 2:35:16
  Speaker 1: into equilibrium, their dynamics still actively evolving. The timescale for
• 2:35:23
  Speaker 1: these collisions spans hundreds of millions of years. A merger
• 2:35:28
  Speaker 1: that began before dinosaurs walked the Earth might not be
• 2:35:31
  Speaker 1: finished today. This is not ancient history. It is the
• 2:35:36
  Speaker 1: ongoing state of the cosmos, visible in real time to
• 2:35:41
  Speaker 1: those who know where to look. What the science actually
• 2:35:45
  Speaker 1: shows is a process that was violent, inefficient, and deeply
• 2:35:50
  Speaker 1: strange at every stage. The first structures collapsed because dark matter,
• 2:35:56
  Speaker 1: something we still cannot identify, fell into clumps before ordinary
• 2:36:00
  Speaker 1: gas could. The first stars were probably so massive they
• 2:36:05
  Speaker 1: lived only a few million years. The first galaxies looked
• 2:36:09
  Speaker 1: nothing like galaxies, and then things collided. Again and again,
• 2:36:15
  Speaker 1: small halos merged into larger ones. Proto-galaxies cannibalized their neighbors.
• 2:36:21
  Speaker 1: Gas fell in, triggered star formation, got heated by supernovae
• 2:36:26
  Speaker 1: and black holes, and got expelled. Then fell back in.
• 2:36:31
  Speaker 1: The Milky Way alone has absorbed dozens of smaller systems.
• 2:36:37
  Speaker 1: Every large galaxy in the sky carries the same record
• 2:36:41
  Speaker 1: of accumulated wreckage. Here is the thing that should stop you.
• 2:36:45
  Speaker 1: Every atom that makes up the ground you are standing
• 2:36:48
  Speaker 1: on was processed through at least one stellar interior before
• 2:36:52
  Speaker 1: the solar system formed. The carbon, the oxygen, the iron,
• 2:36:58
  Speaker 1: forged in cores, scattered by explosions, recycled through generations of
• 2:37:04
  Speaker 1: stars across billions of years inside a galaxy that was
• 2:37:09
  Speaker 1: itself being assembled by collisions. You are not just observing
• 2:37:14
  Speaker 1: galaxy formation from the outside. You are a downstream product
• 2:37:18
  Speaker 1: of it. The planet exists because the Milky Way existed.
• 2:37:24
  Speaker 1: The Milky Way exists because a universe full of dark
• 2:37:28
  Speaker 1: matter and quantum fluctuations had 13 billion years to work.
• 2:37:34
  Speaker 1: And yet the picture is not finished. That is genuinely true,
• 2:37:40
  Speaker 1: not as a disclaimer, but as a scientific fact. The
• 2:37:44
  Speaker 1: early galaxies that JWST found, massive, bright, present far too
• 2:37:51
  Speaker 1: soon do not fit the models that had successfully described
• 2:37:55
  Speaker 1: everything else. Something about how star formation worked in the
• 2:37:59
  Speaker 1: early universe, or how dark matter clustered, or how black
• 2:38:04
  Speaker 1: holes seeded themselves, is not right in our current framework.
• 2:38:10
  Speaker 1: The question of what dark matter actually is remains completely open.
• 2:38:15
  Speaker 1: The mechanism that let billion solar mass black holes assemble
• 2:38:19
  Speaker 1: within the first billion years has no clean answer. These
• 2:38:24
  Speaker 1: are not minor loose ends. They are load-bearing mysteries sitting
• 2:38:29
  Speaker 1: at the center of the whole story. That is what
• 2:38:33
  Speaker 1: makes this worth paying attention to. Not just the scale
• 2:38:38
  Speaker 1: of it, though the scale is staggering. It is that
• 2:38:42
  Speaker 1: the universe left evidence everywhere. in the orbits of stars,
• 2:38:48
  Speaker 1: in the chemical composition of ancient halo populations, in the
• 2:38:52
  Speaker 1: shapes of galaxies, in the cosmic microwave background. Every observation
• 2:38:59
  Speaker 1: is a clue, and right now some of those clues
• 2:39:03
  Speaker 1: are pointing somewhere the models did not predict. The universe
• 2:39:08
  Speaker 1: has been assembling galaxies for 13 billion years, and it
• 2:39:12
  Speaker 1: has not finished yet.