Latest / Boring Science / What Exists Past the Final Edge of Everything We Know

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What Exists Past the Final Edge of Everything We Know · Boring Science

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  Speaker 1: Now hold that thought, because it gets stranger. There is
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  a distance beyond which no light has ever had time
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  to reach us at all, not because nothing is there,
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  because the universe is only thirteen point eight billion years old,
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  and light only moves so fast, and some regions of
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  space have simply never had enough time to send us
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  a signal. We call the boundary of that region the
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  edge of the observable universe. It sits about forty six
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  billion light years out. In every direction past that line,
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  our instruments go dark. Our telescopes see nothing, not empty space, nothing.
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  The information has not arrived yet, and some of it
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  never will. Most people hear that and picture an edge,
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  a wall, a place where the universe stops and something
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  else begins. That instinct is wrong, and understanding why it
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  is wrong is the whole point of what follows. The
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  observable universe is not the universe. It is a bubble,
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  a lit room inside a house. We cannot see the
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  walls of everything we have ever mapped, every galaxy, every
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  black hole, every signal we have ever pulled out of
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  the sky fits inside that bubble. And the bubble is
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  almost certainly surrounded by something incomprehensibly larger, something we cannot measure,
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  cannot reach, and in some cases cannot even theorize about cleanly.
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  What fills the dark beyond that edge is one of
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  the genuinely open questions in all of science. Here is
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  what makes this specific question different from most. When physicists
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  say they do not know something, they usually mean the
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  data is incomplete, the instruments are not sensitive enough, the
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  math is still being worked out. Give it time, They say,
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  this is not quite that. The boundary of the observable
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  universe is not a technology problem. It is a geometry problem.
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  It is a speed of light problem. There are regions
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  of space that are receding from us faster than any
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  signal could ever cross the gap, regions whose secrets are
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  not hidden behind better telescopes, but behind the fundamental physics
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  of how space expands. We are not going to close
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  that gap by building bigger instruments. The gap is closing us.
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  The question is not whether something is out there past
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  the edge of everything we can see. The question is what.
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  The observable universe has a radius of about forty six
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  billion light years, and almost nothing about that number is intuitive.
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  Start with the age the universe is roughly thirteen point
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  eight billion years old. That is how long it has
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  been since the Big Bang, So you might expect the
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  farthest light we can possibly receive to have traveled thirteen
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  point eight billion light years to reach us one light
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  year per year. Simple math, clean answer, Except that is
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  not how it works. The universe has been expanding the
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  entire time that light was traveling through it. Space itself
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  was stretching. A photon that left some distant region thirteen
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  point eight billion years ago did not travel through static space.
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  It traveled through space that was pulling apart underneath it,
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  and the source of that photon has been moving away
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  the whole time. By the time the light finally arrives here,
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  the object that sent it is now roughly forty six
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  billion light years away, not thirteen point eight forty six.
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  The light caught up, but the source did not wait around.
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  This confuses people, and it should. It seems to suggest
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  that something moved forty six billion light years in thirteen
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  point eight billion years, which would mean it was traveling
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  faster than light. It was not. The object itself never
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  moved faster than light through space. Space simply expanded, carrying
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  the object with it, the way dots on the surface
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  of a balloon get farther apart when you inflate it,
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  without any dot moving across the surface. There is no
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  speed limit on how fast space can expand the light
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  speed limit applies to objects moving through space, not to
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  space itself stretching. So when you hear forty six billion
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  light years, that is not a contradiction. That is expansion
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  doing what expansion does. Here is the concrete picture. The
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  observable universe is a sphere. You are at the center.
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  Every direction you look, the sphere extends outward about forty
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  six billion light years before hitting the boundary, the point
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  beyond which light simply has not had time to reach
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  us yet. Everything inside that sphere is, in principle observable.
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  A telescope good enough could see it. Everything outside the
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  sphere is not hidden behind a wall or blocked by
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  anything physical. It just has not sent us any information yet,
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  because the universe is not old enough for that information
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  to have arrived. The boundary is a time problem. A
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  problem now here is the part that tends to stop
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  people cold. You are not special. There is nothing unique
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  about the fact that you are at the center of
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  your observable universe. Every single point in the cosmos is
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  at the center of its own observable sphere, equally large,
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  extending forty six billion light years in every direction from
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  wherever it sits. An observer in a galaxy ten billion
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  light years from here has their own sphere entirely centered
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  on themselves, and it contains billions of galaxies we will
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  never see. In every direction around that observer. Our spheres overlap.
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  There is a region we both can see, but their
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  sphere extends far past our horizon into territory completely invisible
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  to us, just as ours extends far past their horizon
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  into territory in visible to them. No location in the
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  universe has a privileged view. Every observer is surrounded by
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  the same kind of bubble. What changes is only what
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  is inside each particular bubble. Scale helps here, even if
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  scale eventually stops working the way your brain wants it to.
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  Our entire milky Way galaxy is about one hundred thousand
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  light years across the observable universe is roughly nine hundred
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  thirty thousand milky ways laid end to end in any
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  direction from where you are standing. The nearest large galaxy
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  to us. Andromeda sits about two point five million light
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  years away. Inside the observable universe, there are an estimated
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  two trillion galaxies, each containing hundreds of billions of stars,
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  spread across a volume so large that the number stops
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  behaving like a number and starts behaving like a symbol.
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  And all of that, every galaxy, every star, every atom
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  in that entire sphere sits inside the lit room. The
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  dark house around it is larger. Still. What sits at
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  the edge of the observable sphere is not a wall,
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  It is not darkness. Exactly what you are seeing when
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  you look toward the cosmic horizon is old light, light
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  from an era when the universe was dense and hot
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  and opaque, before the first stars existed. The farthest back
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  our instruments can see. The boundary where light finally stops
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  reaching us corresponds to roughly three hundred eighty thousand years
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  after the Big Bang, when the universe cooled enough for
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  atoms to form and for light to travel freely for
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  the first time. Before that moment, the universe was like
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  a fog. Photons could not get through. So even if
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  we built a perfect telescope with unlimited sensitivity. There is
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  a wall of fog at that distance, and behind the
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  fog nothing visible. But the fog is not the edge
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  of space. It is just the edge of what light
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  can show us. Space continues past the fog. The Big
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  Bang happened everywhere. The fog is thick in every direction
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  at the same distance, which means the universe, whatever it
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  looks like beyond it goes on. The keyword in everything
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  here is time. Every layer of the observable universe you
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  look at is a layer of the past. A galaxy
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  five hundred million light years away looks the way it
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  looked five hundred million years ago. A galaxy ten billion
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  light years away looks the way it looked ten billion
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  years ago. When you reach the cosmic horizon, you are
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  looking at the universe as it was nearly thirteen point
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  eight billion years ago. The boundary is not a place,
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  It is a time stamp. And because the universe is
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  the same age everywhere, the Big Bang happened at the
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  same time everywhere, every observer in the cosmos hits that
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  same time stamp at the same distance from themselves. The
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  edge of the observable universe is not a wall. You
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  could fly toward and eventually reach it is a time limit,
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  and it follows you wherever you go. To picture what
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  the edge actually is, you have to resist the instinct
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  to imagine it as a surface, a line, or a
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  place where something stops. That instinct is almost impossible to shake.
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  Every boundary you have ever encountered in your life has
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  been a physic thing, a wall, a shoreline, a fence.
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  The edge of the observable universe is none of those.
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  There is no barrier out there, no wall, no sign.
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  If you could travel toward it at any speed you wanted,
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  you would pass through it and feel nothing, because there
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  is nothing there to feel. What looks like an edge
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  from where we stand is entirely a consequence of two
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  facts that have nothing to do with space running out.
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  Light travels at a fixed speed. The universe has a
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  finite age. Put those two things together and you get
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  a horizon, not because space ends, but because time did.
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  Think about what that actually means. The Big Bang happened
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  roughly thirteen point eight billion years ago. In the time
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  since then, light traveling toward us from any direction has
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  had thirteen point eight billion years to make the trip.
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  That sets a hard limit. On how far back we
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  can see. Any object that was too far away when
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  the universe began for its light to have reached us
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  yet is invisible to us, not because it does not exist,
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  but because the signal has not arrived. The observable universe
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  is not the full universe. It is the portion of
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  the universe whose light has had time to reach us
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  so far. The edge is a deadline, not a destination.
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  Now Here is where it gets strange that exact same
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  logic applies to every other location in the universe. Equally,
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  there is nothing special about where we are sitting. An
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  alien civilization living in a galaxy forty billion light years
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  from here would have their own observed sphere centered on themselves,
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  with a radius of roughly forty six billion light years
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  in every direction. Their bubble would be just as large
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  as ours, and here is the part worth sitting with
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  hours and theirs would have no overlap. The two observable
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  spheres would be entirely separate regions of space, with no
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  single galaxy appearing in both. Two civilizations, each surrounded by
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  an enormous observable universe, each completely blind to what the
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  other can see. There is no master map there is
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  no privileged viewpoint from which you can see it all.
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  Every observer gets their own window, and no two windows
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  look out on exactly the same view. This also means
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  the edge is not a fixed feature of the universe.
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  It is fixed to you. It follows you. If you
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  move to a different galaxy, your observable sphere would move
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  with you. Centered on your new location, The region behind
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  you would slowly pass out of your view, and a
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  new region ahead of you would inch into it. The
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  horizon is not a place in space. It is a
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  radius around wherever you happen to be, set by the
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  speed of light and the age of the universe recalculated
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  at every position. It was also smaller in the past.
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  When the universe was one billion years old, an observer
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  anywhere in it could only see light that had been
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  traveling for one billion years. The observable universe at that
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  moment was dramatically smaller than it is today. Every day
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  that passes more light from further away has had time
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  to reach us, and the horizon expands slightly. You might
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  expect this to be purely good news. More universe revealed
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  over time, but expansion complicates that story in ways that
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  come up later. The simple version is that the horizon
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  is growing, but not fast enough to outrun certain parts
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  of what lies beyond it. What the horizon is not,
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  and this is worth being direct about, is evidence that
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  space stops. Nothing in the physics of the observable universe
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  suggests that matter, energy, or space itself ceases to exist
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  at the boundary of what we can detect. The universe
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  did not draw a circle around us. We drew the
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  circle ourselves by measuring the speed of light and counting
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  the years since the beginning. The boundary is ours. It
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  is a feature of our situation, not a feature of reality.
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  The disturbing implication of all this takes a moment to
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  settle in. When most people use the word universe, they
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  mean everything, all of space, all of matter, everything that exists.
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  But in practice, what scientists can actually study, measure, test,
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  and map is only the observable universe, our bubble, our
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  forty six billion light year sphere. Everything said about the
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  composition of the universe, the distribution of galaxies, the large
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  scale structure of space, is said on the basis of
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  that bubble alone. The observable universe is the only universe
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  we have access to. And the observable universe is almost
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  certainly a small fraction of the actual universe, whatever that
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  turns out to mean. Calling the observable universe the universe
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  is like assuming the town you grew up in is
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  the only town that exists because you have never been
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  able to leave it. The horizon is a limit on information,
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  not a limit on existence. Out past the farthest thing
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  we have ever detected, past the oldest light we have
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  ever measured, space continues on in all directions, filled with
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  whatever it is filled with, completely indifferent to the fact
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  that we cannot see it. That matter does not disappear
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  when it crosses our horizon. Those galaxies do not stop
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  existing when their light takes too long to reach us.
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  The universe did not end at the boundary of our
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  observable sphere. We just ran out of time to see
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  what came next. In every direction in the sky, coming
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  from the farthest distances any instrument has ever probed, there
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  is a faint glow, not light from a star, not
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  light from a galaxy, something older than either of those things.
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  It comes from everywhere, at once, uniform and ancient, and
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  it has been traveling towards you since before the Earth existed.
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  Astronomers call it the cosmic microwave background, and it is
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  the closest thing we have to a photograph of the
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  early universe. More than that, it is a message, And
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  once you know what it says, the question of what
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  lies past the edge starts to look a little less unanswerable.
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  To understand what the CMB actually is, you have to
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  go back to the first three hundred eighty thousand years
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  after the Big Bang. The universe at that point was
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  not dark and empty the way it is today. It
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  was a dense, ferociously hot plasma, something like the interior
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  of a start, wretched across all of space. Photons, the
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  particles that carry light, were constantly colliding with electrons and
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  getting scattered. They couldn't travel in straight lines. The universe
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  was opaque, like trying to see through a wall of
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  fog so thick that light simply bounced around inside it
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  without going anywhere. Then, as the universe expanded and cooled,
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  something changed. The temperature dropped enough that electrons could finally
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  bind to protons and form neutral hydrogen atoms. When that happened,
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  the fog lifted for the first time. Photons could travel
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  freely without being scattered. Light escaped the universe became transparent.
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  That moment is called recombination, and the light that was
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  released at that moment is still traveling. It is what
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  we detect as CMB. What the CMB looks like when
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  you map it across the full sky is one of
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  the most striking images in science. It is almost perfectly
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  uniform in every direction. The temperature of that ancient radiation
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  reads at about two point seven degrees above absolute zero.
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  Turn the telescope north south east west. It doesn't matter.
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  The number barely changes. For the universe to look that
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  consistent from every angle we can check, the conditions at
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  the time of recombination had to be nearly identical across
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  enormous distances, distances larger than anything that could have been
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  in contact with each other through ordinary cause and effect.
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  That uniformity is not something you get by accident. It
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  is telling you something deep about the structure of space itself.
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  Here is where it connects to the quest driving this
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  whole conversation. The CMB is coming to us from roughly
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  the edge of what we can observe. The region that
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  emitted this light sits about forty six billion light years
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  away in every direction, right at the limit of our
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  particle horizon. When we say the CMB looks the same
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  in every direction, what we are really saying is that
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  the universe, as far out as our instruments can possibly reach,
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  has the same properties everywhere we look. Same temperature, same density,
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  same basic conditions. There is no direction in which the
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  CMB suddenly looks different, No quadrant of the sky where
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  the signal breaks down, no hint that we are approaching
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  some boundary where things start to behave differently. The edge
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  of our observable universe looks in every every direction exactly
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  like everywhere else. The implication is hard to overstate. If
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  the universe is uniform right up to the edge of
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  what we can see, there is no particular reason to
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  expect it to become something radically different just past that edge.
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  The most straightforward reading of the CMB is that the
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  universe continues beyond our horizon with the same general character
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  it has on this side. More hydrogen, more galaxies, more
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  structure built from the same raw material, not necessarily identical,
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  but governed by the same physics, shaped by the same forces,
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  distributed with the same rough density. Now the CMB is
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  not perfectly uniform. That part matters too. When you amplify
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  the tiny temperature differences in the map variations at the
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  level of about one part in one hundred thousand, a
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  pattern emerges. Faint ripples, slight hot spots, and cool spots
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  scattered across the sky. These fluctuations were not random noise.
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  They were the imprints of pressure waves moving through the
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  early universe before recombination, sound waves, in a sense, frozen
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  into the structure of the plasma at the moment the
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  fog lifted. What happened to those fluctuations after recombination is
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  the entire history of cosmic structure. Regions slightly denser than
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  average had a little more gravitational pull. They drew in
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  more matter. Over hundreds of millions of years. Those regions
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  became the seeds of the first galaxies. The large scale
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  structure of the universe you see today, the filaments of galaxies,
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  the walls and the voids between them. All of it
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  traces back to those tiny temperature ripples in the CMB.
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  You are looking at the blueprint of everything that came after.
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  What that means for the question at hand, is this,
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  if the same fluctuations exist in the unobservable universe beyond
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  our horizon, and the uniformity of the CMB strongly suggests
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  they do. Then the same process of structure formation played
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  out over there. Gravity did the same work. Gas clouds, collapsed, stars,
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  ignited galaxies assembled themselves around dark matter scaffolding the same
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  way they did here. The details would differ because the
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  specific pattern of fluctuations would differ from place to place,
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  the way two snowstorms produce different snowflakes from the same
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  physical rules, but the broad bored strokes would be familiar.
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  The CMB is the last thing we can directly observe
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  from close to the edge. Past the surface where it
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  was emitted, Our instruments go dark, but that surface gives
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  us one final extraordinary data point about what exists in
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  the territory we cannot reach. It says, same rules, same conditions,
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  same universe. Whatever is past the edge, the CMB is
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  telling us it almost certainly looks a great deal like
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  what we can already see. Here is a number worth
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  sitting with for a moment. Of all the matter and
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  energy in the observable universe, the kind you can actually
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  see makes up roughly five percent, not fifty percent, not
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  twenty percent. Five Every star you have ever looked at,
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  every galaxy, the Hubble telescope has ever photo every planet,
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  every asteroid, every cloud of gas, every atom in your body.
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  All of it combined adds up to about one twentieth
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  of what the universe actually contains. The other ninety five
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  percent is something else, entirely two somethings, in fact, that
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  have almost nothing in common except that we cannot see
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  either of them, and we do not fully understand either
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  of them. Astronomers call them dark matter and dark energy,
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  and together they are the invisible majority that runs everything.
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  Dark matter came first in terms of discovery. The clues
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  showed up in the nineteen thirties when a Swiss astronomer
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  named Fritzwicky was studying a cluster of galaxies and notice
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  something wrong with the math. The galaxies in the cluster
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  were moving too fast. If you add added up all
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  the visible mass, every star, every gas cloud, the gravity
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  it produced was not nearly strong enough to hold the
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  cluster together at those speeds. By any calculation, those galaxies
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  should have flown apart long ago. They hadn't. Something was
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  holding them, something with mass that he couldn't see. Zwicky
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  called it dunkla MATTERI dark matter. People mostly ignored him
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  for decades. Then, in the nineteen seventies, an astronomer named VERA.
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  Rubin ran into the same problem, studying individual galaxies, and
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  this time the evidence was hard to dismiss. Stars at
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  the outer edges of galaxies were orbiting the galactic center
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  just as fast as stars near the middle. That should
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  not happen if the visible mass were all there was.
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  Outer stars should be orbiting much more slowly, the way
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  the outer planets of our Solar System orbit the Sun
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  more slowly than the inner ones. The fact that they
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  weren't meant there was invisible mass spread throughout and around
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  every galaxy, a vast halo of something with gravity but
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  no light. What is dark matter exactly? The honest answer
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  is that nobody knows. It does not emit light, It
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  does not absorb light. It does not interact with ordinary
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  matter except through gravity. Every particle detector built to catch
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  it has come up empty. Every candidate particle that theorists
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  proposed has been ruled out or remains unconfirmed. It passes
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  through walls, through planets, through you, without leaving a trace.
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  There are roughly twenty seven percent worth of it filling
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  the observable universe, threading through every galaxy and galaxy cluster,
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  forming the invisible scaffolding on which the visible universe is built.
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  The large scale structure of the cosmos, the great web
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  of filaments and walls and voids that shows up in
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  galaxy surveys, is not primarily a structure of stars and gas.
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  It is a structure oh a dark matter. The galaxies
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  are just the lit parts, like lights strung along a
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  frame you cannot see. The second invisible component is stranger. Still,
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  dark energy makes up about sixty eight percent of the
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  universe's total content, which means it is the single largest
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  ingredient by a wide margin. It was discovered in nineteen
• 29:49
  ninety eight when two independent teams of astronomers were studying
• 29:54
  distant supernovae to measure how fast the universe was expanding.
• 30:00
  The expectation was that gravity would be slowing the expansion
• 30:04
  down over time, that after the initial blast of the
• 30:08
  Big Bang, things would gradually be pulling back toward each other.
• 30:13
  What they found instead was the opposite. The expansion was
• 30:18
  not slowing, it was speeding up. The universe was not coasting.
• 30:24
  It was accelerating, pushed by something acting against gravity, something
• 30:30
  filling space itself and growing stronger as space grew larger.
• 30:35
  Both teams got the Nobel Prize in twenty eleven. The
• 30:39
  force driving this acceleration got a name, dark energy. What
• 30:44
  it actually is remains completely unknown. Here is what makes
• 30:49
  dark energy so unsettling in the context of everything we
• 30:53
  have been discussing. Dark matter at least behaves like matter.
• 30:59
  It has mass, It clumps, it pulls things together. You
• 31:04
  can map it, at least indirectly, by watching how gravity
• 31:08
  bends light around it. Dark energy doesn't do any of that.
• 31:13
  It is not a particle. It is not a field
• 31:16
  in any conventional sense. The leading candidate is something called
• 31:21
  the cosmological constant, a term Einstein originally added to his
• 31:26
  equations of general relativity and then famously removed, calling it
• 31:31
  his greatest blunder. It turns out he may have been
• 31:35
  right the first time. The cosmological constant is essentially an
• 31:41
  energy density built into the fabric of empty space. Every
• 31:46
  cubic centimeter of vacuum everywhere in the universe contains a
• 31:51
  tiny amount of this energy, and because space is expanding
• 31:57
  because there is always more space. The total amount of
• 32:01
  this energy keeps growing. The more it grows, the faster
• 32:06
  it pushes space apart. When physicists try to calculate what
• 32:10
  the cosmological constant should be from quantum field theory, they
• 32:15
  get a number roughly ten to the power of one
• 32:18
  hundred twenty times larger than what we actually observe. That
• 32:23
  is the worst prediction in the history of physics by
• 32:26
  an extraordinary margin. Something is canceling almost all of it out,
• 32:32
  leaving just a tiny residual that drives the accelerating expansion.
• 32:38
  Nobody knows what that cancelation mechanism is. Nobody knows why
• 32:43
  the residual is the particular value it is. This is
• 32:47
  not a minor gap in knowledge. It is a foundational mystery,
• 32:52
  sitting at the center of our best theory of the universe.
• 32:55
  What does any of this have to do with what
• 32:58
  lies past the edge of everything we can see? More
• 33:02
  than it might seem, dark matter shapes the structure of
• 33:06
  the universe on every scale we can observe. If it
• 33:10
  extends beyond the cosmic horizon, and there is no reason
• 33:14
  to think it stops there, then whatever structure exists in
• 33:18
  the unobservable universe is shaped by the same invisible scaffolding
• 33:23
  we see here. The web of filaments and voids continues,
• 33:29
  built on dark matter's skeleton in regions we will never
• 33:33
  directly probe dark energy, though, does something more directly relevant
• 33:38
  to the question of the edge itself. Because dark energy
• 33:42
  is accelerating the expansion of space, it is actively determining
• 33:46
  how much of the universe will ever be visible to us.
• 33:50
  It is the engine pulling galaxies past the horizon, placing
• 33:54
  them permanently beyond reach. The faster space expans, the more
• 34:00
  rapidly the observable universe is being carved away from the
• 34:04
  rest of existence. Understanding what lies past the edge requires
• 34:09
  understanding dark energy, and we are very far from that.
• 34:15
  Dark energy, which we cannot detect, cannot explain, and can
• 34:20
  only infer from the motion of things moving away from us,
• 34:24
  is the dominant force shaping everything that exists, including everything
• 34:30
  we will never see. Space has a shape, and the
• 34:33
  shape turns out to matter enormously For what exists beyond
• 34:37
  the horizon. That might sound strange. Space feels like nothing,
• 34:44
  like the absence of something rather than a thing in itself.
• 34:48
  But emptiness has geometry, and that geometry has consequences that
• 34:54
  reach all the way to the question of whether the
• 34:56
  universe ever ends. There are three options. Three ways space
• 35:02
  itself can be curved at the largest scales. The first
• 35:07
  is positive curvature. Picture the surface of a sphere. On
• 35:12
  a sphere, if you start walking in any direction and
• 35:15
  you never stop, you eventually come back to where you started.
• 35:20
  There is no edge, but there is a limit. The
• 35:24
  sphere is finite and boundless. At the same time, A
• 35:28
  positively curved universe would work the same way. Travel far
• 35:32
  enough in one direction, and in principle, you arrive back
• 35:37
  where you launched from. This kind of universe has no wall,
• 35:41
  no drop off point, but it has a size you
• 35:45
  could at least theoretically measure it. The second option is
• 35:50
  negative curvature. Picture the shape of a saddle or a
• 35:55
  mountain pass, curving upward in one direction and downward in another.
• 36:00
  A negatively curved universe bends the other way. Parallel lines
• 36:05
  spread apart, triangles have interior angles that add up to
• 36:09
  less than one hundred eighty degrees. This universe is infinite, open,
• 36:15
  with no centre and no edge, expanding outward in every
• 36:19
  direction forever. The third option is flat, no curvature at all.
• 36:25
  Parallel lines stay parallel forever, no matter how far they go.
• 36:30
  Draw a triangle anywhere in flat space, and its angles
• 36:34
  some to exactly one hundred eighty degrees. Flat space is
• 36:38
  the geometry you learned in school. Euclidean clean the universe
• 36:44
  as a perfect grid, and like negative curvature, a flat
• 36:48
  universe has no edge. It goes on forever. For most
• 36:53
  of the twentieth century, physicists had no clean way to
• 36:58
  measure which of these three descriptions fit the universe we
• 37:01
  actually live in. The math allowed all three. Then, starting
• 37:07
  in the late nineteen eighties, something changed. Instruments got sensitive
• 37:13
  enough to read the cosmic microwave background in real detail,
• 37:18
  and the CMB turned out to be carrying the answer.
• 37:22
  Here is how the measurement works. The CMB has tiny
• 37:27
  temperature fluctuations patches that are slightly warmer or slightly cooler
• 37:33
  than average. Those patches have a characteristic size set by
• 37:37
  physics in the very early universe. Physicists can calculate from
• 37:43
  first principles how large those patches should appear in the sky,
• 37:48
  but how large they appear to us depends on the
• 37:51
  curvature of the space the light has traveled through. If
• 37:55
  space is positively curved, the patches look bigger than they should.
• 38:00
  If space is negatively curved, they look smaller. If space
• 38:06
  is flat, they look exactly the size the equations predict.
• 38:11
  In the early nineteen nineties, experiments began measuring those patch sizes.
• 38:17
  By the late nineteen nineties, two balloon born observatories, one
• 38:22
  called Boomerang and one called Maxima, gathered enough data to
• 38:26
  give a real answer. The patches were the right size.
• 38:31
  Space was flat, not approximately flat, not flat within a
• 38:36
  generous margin, flat to the precision limit of the instruments.
• 38:41
  Later missions, including the WMAP satellite and then the Plank satellite,
• 38:48
  push that measurement further, same answer flat. This is the
• 38:54
  kind of result that looks simple and is actually devastating
• 38:58
  in its implications, because a perfectly flat universe cannot be
• 39:03
  finite the geometry will not allow it. On a sphere,
• 39:08
  curvature closes the space back on itself. That is what
• 39:13
  allows the sphere to be finite without having a boundary.
• 39:17
  Take that curvature away, and the mechanism that closes the
• 39:21
  space is gone. There is nothing to bring you home.
• 39:26
  You walk in one direction forever, and forever is what
• 39:31
  you get. Some physicists push back here and point out
• 39:34
  that a flat universe could still be finite if it
• 39:38
  has a strange, non trivial topology, a shape that wraps
• 39:42
  back on itself the way the surface of a doughnut does,
• 39:46
  even without any detectable curvature. That is technically true. Scientists
• 39:52
  have actually looked for signatures of this in the CMB,
• 39:57
  searching for repeated patterns that would appear if the universe
• 40:01
  looped back. Nothing has turned up. The universe, as far
• 40:06
  as any measurement has ever reached, shows no sign of
• 40:11
  wrapping around. It just goes. So what does flatness actually
• 40:16
  mean for the question this whole series is circling. It
• 40:20
  means the most natural reading of what we measure is
• 40:24
  a universe that extends infinitely in all directions, not very large,
• 40:31
  not almost infinite, infinite every direction you could choose. Space
• 40:37
  continues without end. The observable universe, that forty six billion
• 40:43
  light year sphere centered on wherever you happen to be standing,
• 40:47
  is not the universe. It is a region of the universe,
• 40:52
  a region bounded not by any wall or physical limit,
• 40:56
  but by the amount of time light has had to
• 40:59
  travel since the Big Bang past that boundary. By everything
• 41:04
  the geometry is telling us, space keeps going. There is
• 41:09
  something almost deliberately understated about the way this result got announced.
• 41:16
  A pair of balloon experiments in Antarctica and a handful
• 41:20
  of collaborating physicists produced measurements of temperature fluctuations in ancient
• 41:26
  light and concluded from the angles of those fluctuations that
• 41:32
  the universe is probably infinite. The headlines moved on quickly.
• 41:39
  The implications did not move on. They are still sitting there,
• 41:43
  enormous and mostly unexamined. Every time someone looks up at
• 41:48
  a night sky and wonders what's out there past the
• 41:51
  edge of everything we can see, the geometry says there
• 41:56
  is no edge. It says the space you can observe
• 42:01
  continues the same way the space you can observe does,
• 42:05
  with nothing to stop it. The data does not confirm
• 42:09
  a wall. The data does not confirm a boundary. A
• 42:14
  perfectly flat universe, infinite in all directions, with no curvature
• 42:19
  to ever bring you home. That is what the data says.
• 42:23
  We live in Galaxies are not sitting still in space
• 42:27
  waiting for their light to reach us. They are moving,
• 42:31
  and the movement is not slowing down. In the early
• 42:35
  nineteen twenties, an astronomer named Edwin Hubble was measuring the
• 42:39
  distances to faint smudges of light scattered across the sky.
• 42:45
  Most people assumed those smudges were clouds of gas inside
• 42:48
  our own galaxy. Hubble proved they were entire separate galaxies,
• 42:54
  each containing hundreds of billions of stars sitting at distance
• 42:59
  es is so enormous that the numbers barely meant anything.
• 43:05
  That discovery alone would have made him one of the
• 43:08
  most important scientists who ever lived. But then he noticed
• 43:13
  something stranger. Every single one of those galaxies, in every
• 43:19
  direction he looked, was moving away from us. Not just
• 43:24
  a few of them, not most of them, all of them.
• 43:28
  The immediate instinct is to assume this means we are
• 43:31
  at the center of some giant explosion, everything fleeing outward
• 43:36
  from a single point where we happen to be standing.
• 43:40
  That is the wrong picture. The right picture is harder
• 43:44
  to build in your head, but it is worth the effort.
• 43:49
  Imagine a loaf of raisin bread rising in an oven.
• 43:52
  Every raisin moves away from every other raisin as the
• 43:56
  doe expands. No raisin is at the center. No raisin
• 44:01
  is special from any single raisin. All the others appear
• 44:06
  to be retreating. The galaxies are the raisins. Space itself
• 44:11
  is the dough. This is not a metaphor that breaks
• 44:14
  down at the edges. It is actually how the physics works.
• 44:19
  The galaxies are not traveling through space away from us.
• 44:23
  The space between us and them is stretching. The distinction
• 44:27
  sounds like wordplay, but it has a consequence that stops
• 44:31
  most people cold when they first hear it, because it
• 44:35
  is space itself expanding, rather than objects moving through space.
• 44:40
  The recession speed of a distant galaxy is not limited
• 44:45
  by the speed of light. Objects cannot move through space
• 44:49
  faster than light, but space has no such speed limit.
• 44:55
  Space can stretch as fast as it wants, and at
• 44:59
  large enough distances, it stretches fast enough that galaxies are
• 45:04
  moving away from us faster than light could ever travel.
• 45:08
  There are galaxies we can see right now that are
• 45:10
  already receding from us at twice the speed of light.
• 45:15
  This is not a violation of any law of physics.
• 45:18
  It is a consequence of one, and it has a
• 45:22
  deeply strange implication that takes a moment to fully settle in.
• 45:26
  In the nineteen nineties, two independent teams of astronomers were
• 45:31
  trying to measure how quickly the expansion of the universe
• 45:35
  was slowing down. Everyone assumed it was slowing down. Gravity
• 45:41
  pulls things together. The Big Bang had sent everything flying outward,
• 45:46
  and gravity should have been gradually putting the brakes on.
• 45:51
  The two teams were simply trying to measure how hard
• 45:54
  those brakes were applied. They used a particular type of
• 45:58
  exploding star as a measuring tool, a type of supernova
• 46:03
  that always explodes with the same intrinsic brightness, making it
• 46:08
  possible to calculate exact distances across billions of light years.
• 46:14
  When the results came back, both teams found the same answer.
• 46:18
  The expansion was not slowing down. It was speeding up.
• 46:23
  The universe was not coasting to a gradual stop. It
• 46:27
  was exceller atting. The discovery earned the Nobel Prize in
• 46:33
  Physics in twenty eleven, and it introduced a concept that
• 46:38
  physicists still cannot fully explain. Dark energy, the name given
• 46:44
  to whatever force is driving the acceleration. What this acceleration
• 46:49
  means for the edge of everything is the part that
• 46:52
  deserves your attention. When Hubble first mapped those retreating galaxies,
• 46:58
  the situation was uncomfortable, but not hopeless. A sufficiently patient
• 47:03
  civilization might eventually reach distant galaxies if the expansion were slowing.
• 47:10
  With an accelerating expansion, that option closes permanently. The farther
• 47:16
  away a galaxy is the faster it recedes. Push that
• 47:21
  logic out far enough, and you reach galaxies so distant
• 47:25
  that the space between us and them is expanding faster
• 47:29
  than light. Any photon those galaxies emit today, aimed directly
• 47:34
  at us will spend its entire existence swimming upstream against
• 47:39
  a current of expanding space that is simply too strong.
• 47:44
  The photon will never arrive, not in a billion years,
• 47:49
  not ever. Here is where it gets genuinely strange. Some
• 47:54
  of the galaxies currently visible in our sky, galaxies whose
• 47:59
  ancient light is reaching our telescopes right now, have already
• 48:03
  crossed that threshold. The light arriving tonight left those galaxies
• 48:09
  billions of years ago, when they were close enough that
• 48:13
  their light could still make the journey. Since then, the
• 48:18
  expansion has carried those same galaxies so far away that
• 48:23
  any new light they emit today will never reach us.
• 48:27
  We are receiving the last message they will ever send us,
• 48:32
  and we do not know it is the last one.
• 48:35
  The channel is open while we watch, and it will
• 48:38
  simply go dark, not with a dramatic cutoff, but with
• 48:43
  a gradual fade, as the most distant signals get stretched
• 48:47
  by expansion into wavelengths too long to carry usable information.
• 48:52
  This is the acceleration's sharpest edge. A universe expanding at
• 48:56
  a constant rate would be strange enough. A universe expanding
• 49:01
  at an increasing rate is one that is actively, continuously
• 49:06
  and permanently reducing the amount of itself that any observer
• 49:10
  will ever be able to interact with. Right now, the
• 49:14
  observable universe contains roughly two trillion galaxies over cosmic time.
• 49:21
  As the acceleration continues and more galaxies cross the event horizon,
• 49:26
  the number accessible to any future civilization here will fall.
• 49:32
  The galaxies of the local group, the Milky Way, Andromeda,
• 49:36
  and a handful of smaller companions are gravitationally bound tightly
• 49:41
  enough that expansion will not pull them apart. Everything beyond
• 49:46
  that group is on a long enough timeline leaving for good.
• 49:51
  The universe is not just large, It is not just expanding.
• 49:55
  It is running from itself, pulling its own far reaches,
• 50:00
  permanently out of reach of any observer sitting anywhere within it,
• 50:05
  including here, including now. Every second, the universe is putting
• 50:11
  more of itself permanently beyond our reach, and nobody knows
• 50:16
  when or whether that process ever stops. Astronomers draw two
• 50:22
  different circles around the observable universe, and understanding why they
• 50:27
  are different circles is one of the stranger exercises in
• 50:31
  modern cosmology. The first circle is called the particle horizon.
• 50:38
  It marks the farthest distance from which light has had
• 50:41
  time to reach us since the Big Bang thirteen point
• 50:45
  eight billion years ago. That sounds simple enough. Light travels
• 50:50
  at a fixed speed. The universe has a fixed age,
• 50:55
  so there is a fixed distance beyond which no signal
• 50:58
  has had time to arrive. The particle horizon sits at
• 51:03
  roughly forty six billion light years in every direction, and
• 51:07
  everything inside it is at least in principle, observable. That
• 51:14
  is the circle most people picture when they imagine the
• 51:17
  edge of the universe. It is the lit room. It
• 51:21
  is the boundary of the known. The second circle is stranger.
• 51:27
  It is called the cosmic event horizon, and it does
• 51:30
  not describe what we can see now. It describes what
• 51:35
  we will ever be able to see given infinite time.
• 51:40
  Those two things sound like they should be the same.
• 51:44
  They are not, and the reason they are not is
• 51:47
  the same thing covered in the previous section. The expansion
• 51:52
  of the universe is accelerating. Because space is not just expanding,
• 51:57
  but expanding faster and faster. There is a distance beyond
• 52:02
  which any light emitted today will never reach us. The
• 52:06
  space between here and there will stretch too quickly. The
• 52:10
  signal will run and run and never close. The gap.
• 52:15
  That boundary is the cosmic event horizon, and it sits
• 52:19
  at a much closer distance than the particle horizon, roughly
• 52:23
  sixteen billion light years, not forty six. Sit with that
• 52:29
  for a moment. The particle horizon tells you about a
• 52:33
  sphere forty six billion light years across. The event horizon
• 52:39
  closes off everything beyond about sixteen billion light years permanently.
• 52:45
  Anything sitting in the gap between those two circles, and
• 52:49
  that gap contains an enormous amount of universe is technically
• 52:55
  visible right now because old light from it has already
• 52:59
  reached us. But new light from those regions emitted today
• 53:04
  will never arrive. Those galaxies are already gone in a
• 53:08
  meaningful sense. We can see their past, we will never
• 53:13
  see their future. This is not an abstraction. Astronomers have
• 53:19
  identified thousands of galaxies currently sitting inside our particle horizon,
• 53:25
  but outside our event horizon. We are watching them right now.
• 53:30
  Hubble and its successors have photographed them. They are real,
• 53:35
  mapped and catalogued. What those photographs show is ancient light,
• 53:42
  those galaxies as they were billions of years ago, when
• 53:46
  the universe was young enough that the light could still
• 53:50
  make the crossing. The galaxies themselves are still out there,
• 53:55
  still evolving, still exploding and merging and forming new stars.
• 54:01
  None of that will ever reach us. The connection is severed.
• 54:06
  The line went dead, and we did not notice because
• 54:10
  the last message took so long to arrive. Now Here
• 54:14
  is where it gets genuinely unsettling. Because expansion is accelerating,
• 54:20
  Even galaxies currently inside our event horizon will eventually cross
• 54:24
  it over time, as dark energy continues pushing space apart
• 54:30
  at an increasing rate. Galaxies that are visible today will
• 54:35
  drift across that sixteen billion light year boundary. Their light
• 54:39
  will stretch, wavelengths will redshift from visible to infrared to
• 54:45
  radio waves to wavelengths so long they carry almost no energy,
• 54:51
  then nothing. The signal does not stop sharply, it fades
• 54:56
  over cosmic time scales into silence, but the end point
• 55:02
  is the same. Those galaxies will go dark to us,
• 55:06
  not because they stopped shining, but because the space between
• 55:10
  us and them will eventually be expanding too fast for
• 55:13
  any light to bridge. This means the observable universe in
• 55:18
  a specific and important sense, is shrinking. The particle horizon
• 55:24
  keeps growing. Light from farther regions keeps arriving slowly, expanding
• 55:30
  our view, but the event horizon is drawing inward, cutting
• 55:35
  off more and more of the cosmos from any future communication.
• 55:39
  In the very far future, tens of billions of years
• 55:43
  from now, the galaxies beyond our local group will have
• 55:47
  redshifted into complete unobservability. Astronomers living in that era, if
• 55:53
  any exist, will look out at a night sky and
• 55:57
  see almost nothing, not because the universe emptied out, because
• 56:01
  they are sealed inside a shrinking bubble of visibility surrounded
• 56:06
  by a cosmos. They have no instrument to detect and
• 56:10
  no way to reach. The parallel to a black hole's
• 56:13
  event horizon is not casual, It is exact in a
• 56:17
  structural sense. An event horizon around a black hole marks
• 56:22
  the point past which nothing, no matter, no light, no
• 56:27
  signal of any kind can escape outward. Cross it going
• 56:31
  the wrong direction, and information about the outside world is
• 56:36
  permanently lost to you. The cosmic event horizon works the
• 56:40
  same way in reverse. Cross it going outward, and your
• 56:45
  signals never come back. Sit on this side of it,
• 56:49
  and no signal from the other side will ever arrive.
• 56:54
  The physics is different in its details, the consequence is identical.
• 57:00
  You are on one side of a one way barrier,
• 57:03
  and the universe enforces that barrier without exception. There is
• 57:08
  no engineering solution to this. No telescope built in any
• 57:13
  future century will see past the event horizon. A civilization
• 57:18
  a trillion years more advanced than ours would face the
• 57:22
  same wall. The barrier is not about sensitivity or resolution
• 57:27
  or funding. It is written into the geometry of accelerating space.
• 57:33
  Information requires a carrier. Every carrier travels at or below
• 57:38
  the speed of light. If the space between two points
• 57:42
  expands faster than light can cross it, no carrier gets through.
• 57:47
  The math closes the door and removes the handle. Beyond
• 57:52
  the event horizon, information about what exists there is not
• 57:57
  merely hard to get. It is gone permanently and by law.
• 58:03
  The most conservative, least dramatic answer to what lies past
• 58:07
  the edge of the observable universe is also, in its
• 58:11
  own way, the most unsettling one. No exotic physics, no
• 58:16
  parallel dimensions, no fire and collapse, just more of this. Galaxies,
• 58:22
  filaments of matter, vast empty voids, and the same rules
• 58:27
  running quietly in all of it. The region just past
• 58:31
  your cosmic horizon almost certainly looks like the region just
• 58:35
  inside it, Clusters of galaxies pulling on each other, supernovae
• 58:41
  going off in slow motion, stars burning through hydrogen at
• 58:46
  the same rate they burn through it. Here the universe
• 58:51
  just continuing, as if the boundary you cannot see past
• 58:55
  means nothing, because to the universe it doesn't. This. This idea
• 59:00
  has a name. Astronomers call it the cosmological principle, and
• 59:06
  it is one of the foundational assumptions of modern science.
• 59:10
  The principle says that on large enough scales, the universe
• 59:15
  looks the same in every direction and from every location.
• 59:20
  Not identical in detail. Your neighbourhood looks different from mine,
• 59:24
  but statistically uniform. Smooth the map out far enough, and
• 59:29
  no region outweighs any other, no center, no edge, no
• 59:34
  special spot. When you look at galaxy surveys, the largest
• 59:39
  map's humanity has ever assembled, this is what you find.
• 59:44
  Zoom out far enough, and the lumpy web of matter
• 59:48
  flattens into something almost perfectly. Even. The cosmological principle is
• 59:54
  not an article of faith. It is a reading of
• 59:57
  the data, and the CMB backs it up in the
• 1:00:01
  strongest possible terms that ancient radiation coming from every direction
• 1:00:07
  in the sky is uniform to better than one part
• 1:00:10
  in one hundred thousand. Wherever you look, the early universe
• 1:00:15
  had the same temperature, the same density, the same character.
• 1:00:21
  The differences are there tiny ripples that would eventually grow
• 1:00:25
  into galaxies, but the baseline is the same everywhere we
• 1:00:30
  can check, every direction we can probe points to a
• 1:00:35
  universe that was already statistically uniform before the first star ignited.
• 1:00:42
  If the universe was uniform inside the horizon in every direction.
• 1:00:47
  The most natural conclusion is that it was uniform outside
• 1:00:50
  it too, not guaranteed, but natural. So what does that
• 1:00:56
  mean for the region just past the edge of what
• 1:00:59
  you can see. It means there are probably galaxies out there,
• 1:01:03
  galaxy clusters, filaments hundreds of millions of light years long,
• 1:01:09
  threaded through with dark matter, lined with glowing points that
• 1:01:13
  are suns you will never see. It means there are
• 1:01:16
  almost certainly planets, dust clouds cooling into new solar systems,
• 1:01:23
  dyeing stars, expanding into red giants, and then going dark.
• 1:01:28
  The whole machinery of the universe running on the same
• 1:01:32
  physics in the same patterns past a horizon that is
• 1:01:37
  entirely invisible to you and entirely irrelevant to what happens
• 1:01:41
  on the other side of it. Here is where things
• 1:01:44
  get genuinely strange. If space is infinite, and the geometry
• 1:01:50
  we measured strongly suggests it is, then the amount of
• 1:01:54
  matter past your horizon is not just large, It is infinite.
• 1:02:00
  Infinite matter governed by a finite set of physical laws
• 1:02:04
  distributed at roughly the same density as the observable universe,
• 1:02:09
  arranging itself into every configuration. Those laws permit and that
• 1:02:14
  is where the unsettling part begins. There are only so
• 1:02:18
  many ways to arrange matter. Atoms can only be stacked
• 1:02:23
  in so many configurations. A region of space containing roughly
• 1:02:28
  the same number of particles as your observable universe has
• 1:02:32
  an enormous but finite number of possible arrangements. Think of
• 1:02:37
  it like a very long string of coin flips. Heads
• 1:02:41
  and tails can only combine in so many distinct sequences
• 1:02:45
  before they start to repeat. With an infinite number of
• 1:02:48
  coin flips, every finite sequence must occur not once, but
• 1:02:54
  infinitely often. The same logic applies to space. In an
• 1:02:59
  infinite at universe with the same density of matter as ours,
• 1:03:04
  every possible arrangement of particles repeats, including the arrangement that
• 1:03:09
  produced our solar system, including the arrangement that produced Earth,
• 1:03:15
  including the arrangement that produced you reading this in this moment,
• 1:03:21
  With exactly this particular configuration of memories and neurons, and
• 1:03:26
  uncertainty about where the universe ends, physicists have actually tried
• 1:03:33
  to put a number on how far away the nearest
• 1:03:36
  copy of your observable universe would statistically be. The estimate
• 1:03:42
  that shows up most often is staggering. The nearest region
• 1:03:45
  of space with a matter configuration essentially identical to our
• 1:03:50
  entire observable universe would be expected to sit somewhere around
• 1:03:55
  ten to the power of ten to the power of
• 1:03:58
  one hundred eighteen meters away. Write that out and you
• 1:04:03
  would fill more paper than exists. It is not a
• 1:04:07
  number that connects to any human intuition, but the logic
• 1:04:10
  behind it is straightforward. Finite number of configurations infinite space
• 1:04:18
  repetition is not just possible, it is mandatory. What that
• 1:04:23
  means in practice is harder to absorb. Somewhere past the
• 1:04:27
  horizon with enough distance, there is a planet that formed
• 1:04:32
  the same way Earth did, oceans, continents, life that evolved
• 1:04:38
  under a familiar sun. And if you follow that reasoning
• 1:04:42
  far enough, somewhere there is a version of this exact
• 1:04:46
  moment playing out with all the same particles in all
• 1:04:50
  the same positions. Write down to the particular thought you
• 1:04:54
  are having right now. The physics does not forbid it
• 1:04:58
  in an infinite, flat universe. It demands it. This is
• 1:05:03
  not a claim about souls or identity, or any philosophical
• 1:05:07
  question about what makes you you. It is a claim
• 1:05:10
  about matter, about the finite number of ways things can
• 1:05:14
  be arranged about what infinity actually forces on a universe
• 1:05:19
  that takes it seriously. The copy of you that exists
• 1:05:24
  past the cosmic horizon is not you in any meaningful sense.
• 1:05:29
  It is just an arrangement of atoms that matches yours.
• 1:05:34
  But that matching is not a coincidence or a miracle.
• 1:05:37
  It is arithmetic. Infinite space with the same rules and
• 1:05:41
  the same matter density as ours does not just allow
• 1:05:45
  for repeated structures, It mathematically requires them. There is a
• 1:05:51
  word that physicists use with extraordinary casualness given what it
• 1:05:55
  actually means. Infinite, not very large, not larger than we
• 1:06:01
  can measure, not so big that the difference stops mattering.
• 1:06:06
  Infinite without end, a thing that goes on not for
• 1:06:10
  a long time, but forever, not across a great distance,
• 1:06:14
  but across every distance, without stopping, without thinning, without approaching
• 1:06:20
  anything you could call a boundary. When a physicist writes
• 1:06:25
  that word in an equation, it slides past almost without comment.
• 1:06:31
  When you actually try to hold what it means as
• 1:06:34
  a physical claim about real space, something in the mind
• 1:06:38
  quietly refuses to cooperate. That resistance is worth taking seriously.
• 1:06:44
  Human intuition was built for a world of plains and
• 1:06:47
  hills and horizons, for distances you can walk and volumes
• 1:06:51
  you can fill. Infinity is not an experience. It is
• 1:06:57
  a conclusion, and the path to that can conclusion runs
• 1:07:01
  through some of the more disorienting territory in modern cosmology.
• 1:07:06
  To understand what exists past the final edge of the
• 1:07:09
  observable universe, you have to first sit with the genuinely
• 1:07:13
  strange possibility that space has no final edge at all,
• 1:07:18
  not because we haven't reached it yet, but because there
• 1:07:21
  is nothing to reach. Here is a way to picture
• 1:07:24
  what is being claimed. Imagine you are standing on an
• 1:07:28
  enormous flat plane. Fog sits at the horizon in every direction,
• 1:07:34
  maybe a mile out, maybe two. You can see clearly
• 1:07:38
  inside that circle, beyond it nothing. Now, the fog is
• 1:07:43
  not a wall. The plane does not stop where your
• 1:07:47
  vision does. If you walk toward the fog, you would
• 1:07:51
  pass through it and find more plane stretching out ahead.
• 1:07:56
  The fog follows you, Your circle of visibility travels with
• 1:08:00
  you wherever you go, and the plane keeps going. That
• 1:08:05
  is roughly the situation with the observable universe. The horizon
• 1:08:10
  is the fog. The forty six billion light year sphere
• 1:08:15
  is the circle you can see clearly, and the question
• 1:08:19
  is whether the plane ever ends. The measurements say it
• 1:08:23
  probably does not. When satellites map the cosmic microwave background
• 1:08:29
  with enough precision to detect its geometry, what they found
• 1:08:33
  was a universe that is spatially flat, to the limits
• 1:08:37
  of what any instrument could resolve. Flat geometry is significant.
• 1:08:43
  A positively curved space, like the surface of a sphere,
• 1:08:46
  is finite. Walk far enough in one direction, and you
• 1:08:50
  come back to where you started. A flat space has
• 1:08:54
  no such trick. Parallel lines stay parallel forever, triangles always
• 1:09:00
  add up to one hundred eighty degrees. There is no
• 1:09:04
  curvature pulling you gently back toward home. If space is
• 1:09:08
  genuinely flat, it extends outward without limit in every direction,
• 1:09:14
  and the observable universe sits inside it, the way a
• 1:09:18
  single grain of sand sits inside a desert with no border.
• 1:09:23
  That comparison is not poetic exaggeration. It may actually under
• 1:09:28
  sell the situation. The observable universe is forty six billion
• 1:09:33
  light years in radius. That sounds enormous, because by every
• 1:09:38
  human standard it is. But if space is infinite, then
• 1:09:42
  the ratio of the observable universe to all of space
• 1:09:46
  is not a small number. It is zero. Mathematically, any
• 1:09:52
  finite region divided by infinity is zero. The forty six
• 1:09:57
  billion light years we can see is not even a
• 1:10:00
  rounding error. It is nothing in the strictest sense of
• 1:10:04
  the proportion. Physicists who work on this tend not to
• 1:10:09
  dwell on that fact, probably because there is nothing useful
• 1:10:13
  to do with it, but it deserves to be said plainly.
• 1:10:17
  Not everyone is convinced that actual physical infinity is real,
• 1:10:22
  rather than a symptom of incomplete models. This is a
• 1:10:26
  legitimate debate. Some physicists argue that infinity appearing in an
• 1:10:32
  equation is usually a sign that the equation has broken down,
• 1:10:37
  that you have pushed a good model past where it applies.
• 1:10:42
  On this view, a truly infinite universe is not a discovery,
• 1:10:47
  but a placeholder, something that math produces when we don't
• 1:10:52
  yet know enough to constrain the answer properly. The concern
• 1:10:56
  is reasonable. Physics has a history of infinities that turned
• 1:11:00
  out to be problems to be solved, not features of reality.
• 1:11:05
  Whether cosmic spatial infinity falls into that category is genuinely unresolved.
• 1:11:12
  What nearly everyone does agree on is this, even if
• 1:11:16
  space is finite, it is almost certainly far larger than
• 1:11:21
  the ninety three billion light year bubble. We can observe.
• 1:11:25
  The CMB measurements constrain the overall curvature of space tightly
• 1:11:31
  enough that if space does curve back on itself, the
• 1:11:35
  full circumference of that curve must be enormously larger than
• 1:11:39
  our observable sphere. Some estimates put the lower bound at
• 1:11:44
  hundreds of times larger, others put it much higher. The
• 1:11:50
  observable universe being a small fraction of total space is
• 1:11:54
  not a fringe position. It is the mainstream one. So
• 1:11:58
  when someone says the universe is ninety three billion light
• 1:12:02
  years across, what they mean, with appropriate honesty is that
• 1:12:07
  the observable universe is ninety three billion light years across.
• 1:12:13
  The two phrases are not the same thing, and treating
• 1:12:16
  them as equivalent is an error so common it has
• 1:12:20
  stopped feeling like one. The observable universe is the part
• 1:12:25
  with a postcode. It is the part the laws of
• 1:12:28
  physics allow us to examine total space. The actual universe
• 1:12:34
  sits outside those laws. In the sense that we cannot
• 1:12:38
  reach it, cannot measure it, cannot probe it with any
• 1:12:42
  instrument currently imaginable. And here is the thing that should
• 1:12:47
  make this feel strange rather than comfortable. Every time you
• 1:12:52
  have heard someone describe the scale of the cosmos, every
• 1:12:56
  time a documentary has walked you through the hierarchy from Earth,
• 1:13:00
  Earth to Solar System, to galaxy to local group to
• 1:13:04
  the observable universe, the story has been stopping at the fog.
• 1:13:10
  The plane keeps going. The fog is just where we
• 1:13:14
  lose the light. When astronomers talk about the universe as
• 1:13:19
  if it were the same thing as the observable universe,
• 1:13:22
  they are making an assumption so enormous it barely registers.
• 1:13:27
  And it is almost certainly wrong. To understand what might
• 1:13:32
  exist past the final edge of everything we know, you
• 1:13:36
  have to go back to a moment that happened before
• 1:13:39
  anything we can observe, a moment in the first ten
• 1:13:43
  to the minus thirty six seconds of existence. That number
• 1:13:48
  is not a typo. It is a fraction so small
• 1:13:51
  that writing it out in decimal form would require you
• 1:13:55
  to place thirty five zeros between the decimal point and
• 1:13:58
  the first significance digit. Nothing in human experience scales down
• 1:14:04
  to that. Nothing in biology, nothing in chemistry, nothing in
• 1:14:09
  ordinary physics gives you any intuition for a unit of
• 1:14:13
  time that's small. But something happened in that window, and
• 1:14:19
  what happened there shaped everything that came after, including the
• 1:14:24
  size of the observable universe, the flatness of space, and
• 1:14:28
  the question of whether our Big Bang was unique. The
• 1:14:33
  theory is called cosmic inflation. It was first proposed by
• 1:14:38
  physicist Alan Guth in nineteen eighty, and the core idea
• 1:14:42
  is almost violent in its simplicity. In that first, unimaginably
• 1:14:47
  brief window, the universe did not expand at the pace
• 1:14:50
  we observed today. It expanded exponentially, not fast exponentially. The
• 1:14:58
  difference matters. Fast growth adds, exponential growth multiplies over and
• 1:15:04
  over in vanishingly small increments of time. A region of
• 1:15:09
  space far smaller than a single proton balloon to roughly
• 1:15:13
  the size of a grapefruit in something like ten to
• 1:15:16
  the minus thirty two seconds. That sounds small, it is not.
• 1:15:23
  In terms of the factor by which space itself stretched,
• 1:15:27
  Inflation expanded the universe by a number that dwarfs anything
• 1:15:31
  else in cosmology. The universe. You can see tonight, stretching
• 1:15:37
  forty six billion light years in every direction, traces back
• 1:15:42
  to a patch of space during inflation that was smaller
• 1:15:45
  than an atom. So why do physicists take this seriously?
• 1:15:49
  Because inflation solves two problems that had no other clean solution.
• 1:15:55
  The first is called the horizon problem. Look in opposite
• 1:15:59
  directions in the scale as far as any telescope can reach,
• 1:16:03
  and the temperature of the universe in both directions is
• 1:16:07
  identical to a precision of one part in one hundred thousand.
• 1:16:12
  Those two regions of sky are so far apart that,
• 1:16:17
  at the expansion rate we observe today, they could never
• 1:16:21
  have been in contact with each other. They had no
• 1:16:24
  time to exchange heat, no time to equilibrate. They should
• 1:16:29
  not look the same they do. Inflation explains this by
• 1:16:34
  pointing out that before the expansion, those regions were packed
• 1:16:38
  together close enough to reach thermal equilibrium, and then inflation
• 1:16:43
  stretched them to opposite ends of the observable universe in
• 1:16:47
  a fraction of a second. The sameness is not a coincidence,
• 1:16:52
  it is a memory. The second problem is flatness. Space
• 1:16:57
  is geometrically flat, measured to extraordinary precision by instruments studying
• 1:17:03
  the cosmic microwave background. A universe that starts out with
• 1:17:08
  even a tiny curvature should, over thirteen point eight billion
• 1:17:12
  years of expansion have that curvature amplified dramatically. The fact
• 1:17:19
  that space is still flat means it started almost perfectly flat,
• 1:17:24
  which seems like an extraordinary coincidence unless inflation is true.
• 1:17:29
  Inflation naturally drives any initial curvature towards zero. Whatever shape
• 1:17:35
  the universe had before inflation, the exponential expansion would have
• 1:17:40
  stretched it so violently that any curvature would be smoothed away,
• 1:17:46
  the same way a small wrinkle on a balloon disappears
• 1:17:49
  as you inflate it. Inflation doesn't just explain flatness, it
• 1:17:54
  predicts it. Here is where things get stranger. Inflation requires
• 1:17:59
  a field physicists call it the inflaton field to drive
• 1:18:04
  the expansion. Think of it as a kind of energy
• 1:18:08
  stored in space itself, pushing outward with enormous pressure. For
• 1:18:14
  inflation to end, this field has to decay. It has
• 1:18:18
  to release its energy and convert it into the hot,
• 1:18:21
  dense soup of particles that became everything. We can observe
• 1:18:26
  that transition The moment inflation ends and the fireball begins
• 1:18:32
  is what we call the big bang. The heat, the light,
• 1:18:37
  the matter, all of it comes from the inflaton field
• 1:18:41
  finally letting go. But here is the question that changes everything.
• 1:18:47
  What if that field does not let go everywhere at once?
• 1:18:51
  Quantum mechanics, the physics of the very small, introduces uncertainty
• 1:18:56
  into every process. At the fundamental level, the inflaton field
• 1:19:02
  is a quantum field. It fluctuates. In some regions of space,
• 1:19:07
  those fluctuations push the field down toward decay, and inflation ends.
• 1:19:13
  In other regions, the fluctuations push the field back up
• 1:19:17
  and inflation keeps going. The regions where inflation ends become fireballs,
• 1:19:23
  big bangs, expanding universes. The regions where inflation continues just
• 1:19:29
  keep expanding faster than anything inside them can escape. And
• 1:19:35
  here is the key. The inflating regions expand so fast
• 1:19:39
  that they always outpace the regions where inflation stops. No
• 1:19:44
  matter how many pockets collapse see into big bangs, the
• 1:19:48
  background of still inflating space always grows faster. Inflation, once started,
• 1:19:55
  never fully ends. This is what physicists call eternal inflation.
• 1:20:00
  Our entire observable universe, every galaxy, every star, every atom
• 1:20:06
  of everything that has ever existed within the forty six
• 1:20:10
  billion light year sphere we have spent this series mapping
• 1:20:14
  is one of those pockets, one small region where the
• 1:20:18
  inflate on field happened to decay. The fireball that resulted
• 1:20:24
  was our big Bang. Everything you have ever seen, touched,
• 1:20:29
  or measured came from that single event of inflation ending
• 1:20:33
  in one particular patch of an eternally expanding background space.
• 1:20:39
  That background is still out there, It is still expanding.
• 1:20:44
  Other pockets are still forming, collapsing into their own fireballs,
• 1:20:49
  producing their own big bangs right now, in a region
• 1:20:53
  of space so far beyond our cosmic horizon that no
• 1:20:57
  signal from there will ever reach. As were we special?
• 1:21:02
  Was our big bang? The one that mattered? Eternal inflation
• 1:21:07
  gives a quiet, almost unsettling answer. There is nothing in
• 1:21:12
  the theory to suggest our pocket is different from any other.
• 1:21:16
  The same physics that produced our universe is producing others
• 1:21:21
  constantly in a background that has no edge and no clock.
• 1:21:26
  The question of what came before the Big Bang stops
• 1:21:29
  being unanswerable and starts being almost mundane. What came before
• 1:21:35
  was more inflation, and before that, more inflation, stretching back
• 1:21:41
  without a starting point in any direction. If eternal inflation
• 1:21:46
  is correct, our Big Bang was not the beginning of everything.
• 1:21:51
  It was one spark in a fire that has no
• 1:21:53
  beginning and no end. Were we observing a star's final
• 1:21:58
  moments as they unfolded? That was the question in twenty nineteen.
• 1:22:04
  Throughout history, astronomers have sought to understand the ultimate fates
• 1:22:09
  of stars. The observation of stellar death represents one of
• 1:22:14
  the most compelling frontiers in astrophysical research. When a massive
• 1:22:20
  star approaches the end of its life cycle, it undergoes
• 1:22:24
  dramatic transformations that can be detected from Earth using modern
• 1:22:29
  astronomical instruments. These final stages involve violent processes that reshape
• 1:22:36
  the star's structure and release enormous amounts of energy into space.
• 1:22:42
  In twenty nineteen, the astronomical community was particularly focused on
• 1:22:47
  several celestial objects that exhibited unusual behaviour. Advanced telescopes equipped
• 1:22:54
  with sophisticated detection systems allowed researchers to monitor stars in
• 1:22:59
  un unprecedented detail. The question of whether we were witnessing
• 1:23:05
  a star's death in real time captured the imagination of
• 1:23:09
  scientists and the public alike. This inquiry reflected humanity's deep
• 1:23:16
  curiosity about the cosmos and our place within it. Stars
• 1:23:20
  spend most of their lives in a relatively stable state,
• 1:23:24
  converting hydrogen into helium through nuclear fusion in their cause.
• 1:23:30
  This equilibrium can persist for billions of years, depending on
• 1:23:34
  the star's mass. However, once a star exhausts its nuclear fuel,
• 1:23:41
  its fate is sealed by the laws of physics. The
• 1:23:45
  more massive the star, the more violent and spectacular its
• 1:23:49
  final explosion tends to be. These cosmic fireworks represent some
• 1:23:55
  of the most energetic events in the universe. The US
• 1:24:00
  year twenty nineteen was significant for several reasons. In stellar astronomy.
• 1:24:06
  Multiple observatories were trained on candidate objects that showed signs
• 1:24:10
  of imminent stellar death. Researchers analyze data from radio telescopes,
• 1:24:17
  optical instruments, infrared detectors, and X ray satellites. This multi
• 1:24:23
  wavelength approach provided a comprehensive picture of what was happening
• 1:24:28
  in these distant stellar systems. The coordination between different observing
• 1:24:34
  facilities represented the modern state of collaborative astronomical research. One
• 1:24:42
  particularly intriguing case involved the behavior of a red supergiant
• 1:24:47
  star that had been acting unexpectedly. Red supergiants are evolved
• 1:24:53
  massive stars that have expanded enormously and are approaching the
• 1:24:58
  end of their lives. When such stars undergo sudden changes
• 1:25:03
  in brightness or emission patterns, astronomers take notice the possibility
• 1:25:10
  that we might be witnessing the final years or even
• 1:25:14
  months before a catastrophic explosion excited the astronomical community. Predicting
• 1:25:21
  the exact moment of a star's death remains extraordinarily difficult.
• 1:25:27
  Stars can exhibit erratic behavior in their final stages with
• 1:25:31
  sudden increases or decreases in luminosity. They can undergo dramatic
• 1:25:37
  changes in their chemical composition as nuclear fusion reactions accelerate.
• 1:25:44
  The Complexity of stellar physics means that even with our
• 1:25:47
  best models and observations, surprises still occur. This unpredictability is
• 1:25:55
  part of what makes stellar astronomy so fascinating and challenging.
• 1:26:00
  The mechanisms behind stellar death vary depending on the star's
• 1:26:04
  mass and composition. Low mass stars like our Sun will
• 1:26:08
  eventually shed their outer layers and leave behind a white dwarf.
• 1:26:14
  Intermediate mass stars might produce planetary nebulae, beautiful structures of
• 1:26:20
  expelled gas illuminated by the dying stars remaining core. High
• 1:26:26
  mass stars, however, face a more violent end. When a
• 1:26:31
  massive star runs out of fuel, its core can no
• 1:26:35
  longer support itself against the crushing force of gravity. The
• 1:26:39
  collapse of a massive star's core can happen with shocking rapidity.
• 1:26:44
  Within hours or even minutes, the iron core implodes catastrophically.
• 1:26:50
  This collapse rebounds in a shock wave that tears the
• 1:26:54
  star apart. In a supernova explosion, a single supernova can
• 1:27:00
  briefly outshine an entire galaxy containing billions of stars. The
• 1:27:05
  energy released is almost incomprehensible to the human mind. In
• 1:27:10
  twenty nineteen, several candidate objects were under intense scrutiny by
• 1:27:15
  the astronomical community. Researchers had identified stars that showed characteristics
• 1:27:22
  suggesting they were in the late stages of stellar evolution.
• 1:27:27
  Regular observations tracked any changes in these stars properties. Teams
• 1:27:33
  of scientists around the world coordinated their efforts to ensure
• 1:27:37
  continuous monitoring. The possibility of catching a supernova in the
• 1:27:42
  act of occurring drove this collaborative effort. Supernovae are rare
• 1:27:49
  events when observed from our perspective on Earth. In a
• 1:27:53
  typical large galaxy, a supernova occurs only once every hundred
• 1:27:58
  years or so. This rarity makes each potential supernova citing
• 1:28:04
  scientifically valuable. Catching a supernova early in its explosion, before
• 1:28:10
  it reaches peak brightness is especially important. Early observations provide
• 1:28:17
  crucial information about the explosion mechanism and the star's composition.
• 1:28:23
  The question posed in twenty nineteen reflected both excitement and uncertainty.
• 1:28:30
  Astronomers had strong suspicions about certain objects, but could not
• 1:28:34
  be entirely certain of what would happen next. Science progresses
• 1:28:39
  through careful observation, analysis and comparison with theoretical models. The
• 1:28:46
  twenty nineteen observations contributed to our understanding of stellar evolution,
• 1:28:53
  regardless of whether a supernova occurred during that specific year.
• 1:28:57
  Each data point adds to humanity collective knowledge of the universe.
• 1:29:03
  Technology has greatly enhanced our ability to detect and study
• 1:29:07
  distant stars. Modern spectrographs can analyze the light from stars
• 1:29:13
  and reveal their chemical composition. Photometric instruments can measure brightness
• 1:29:19
  changes with extreme precision. Infrared observations pierce through dust that
• 1:29:25
  blocks visible light. X ray and ultraviolet observations reveal the
• 1:29:31
  hottest regions around dyeing stars. This technological toolkit allows astronomers
• 1:29:38
  to ask increasingly sophisticated questions about stellar physics. The twenty
• 1:29:44
  nineteen investigations demonstrated the maturity of modern observational astronomy. Multiple
• 1:29:51
  teams competing and cooperating simultaneously brought rigor to the investigation.
• 1:29:58
  Peer review en shure or that observations and interpretations met
• 1:30:03
  scientific standards. The data was made available to the broader
• 1:30:08
  scientific community for analysis and reanalysis. This open approach to
• 1:30:14
  science accelerates discovery and validates findings. Whether we were truly
• 1:30:20
  witnessing a star's final moments in twenty nineteen remained an
• 1:30:25
  open question that would be answered only by time and
• 1:30:29
  continued observation. The search itself represented humanity's ongoing quest to
• 1:30:36
  understand the cosmos. String theory, the most ambitious attempt to
• 1:30:41
  unify all of physics into a single framework, has a
• 1:30:46
  problem that turned out to be either its greatest flaw
• 1:30:50
  or its most important prediction, depending on who you ask.
• 1:30:54
  The theory started as an attempt to do something that
• 1:30:57
  has defeated physicists for a century. Every other force in
• 1:31:01
  nature has been brought under a single mathematical roof except gravity.
• 1:31:07
  Quantum mechanics describes the very small with extraordinary precision. General
• 1:31:13
  relativity describes the very large with equal precision. When you
• 1:31:18
  try to make them talk to each other, the math
• 1:31:21
  breaks down. String theory proposed a way out. Instead of
• 1:31:26
  treating the fundamental constituents of reality as point particles, it
• 1:31:32
  treats them as tiny vibrating strings of energy. Different vibrational
• 1:31:38
  modes of the same string produce different particles. One vibration
• 1:31:43
  gives you an electron, another gives you a photon. A
• 1:31:48
  particular mode gives you the particle that carries gravity. On paper,
• 1:31:54
  this is elegant in practe. See it came with a
• 1:31:59
  condition nobody asked for. The equations only work if space
• 1:32:04
  has more than three dimensions, not four, not five, ten,
• 1:32:09
  or eleven, depending on the version, which means that if
• 1:32:14
  string theory is correct, there are six or seven spatial
• 1:32:17
  dimensions that you have never experienced and never will. Physicists
• 1:32:23
  call this an inconvenience. What it actually is, once you
• 1:32:28
  follow the implications out far enough, is a door into
• 1:32:32
  something almost too large to think about. Those extra dimensions
• 1:32:36
  have to go somewhere. They cannot just sit alongside the
• 1:32:40
  three we experience without being detectable. So the theory requires
• 1:32:45
  them to be compactified, which means curled up into shapes
• 1:32:49
  so small that no instrument we have or could build
• 1:32:53
  would ever resolve them. Think of a garden hose. Seen
• 1:32:57
  from a distance, it looks like a love a one
• 1:33:01
  dimensional object. Get close enough and it has thickness, curvature,
• 1:33:07
  a second dimension wrapped around the first. The extra dimensions
• 1:33:12
  of string theory work the same way. They are there.
• 1:33:17
  They are just small enough to be invisible. Here is
• 1:33:20
  where the problem enters. There is not one way to
• 1:33:24
  compactify six extra dimensions. There are many. The extra dimensions
• 1:33:30
  can be curled up into an enormous variety of geometric shapes,
• 1:33:35
  each with different topologies, different sizes, different configurations. Each distinct
• 1:33:42
  configuration produces a universe with different physical constants, a different
• 1:33:48
  value for the strength of gravity, a different mass for
• 1:33:52
  the electron, a different cosmological constant, a different everything. The
• 1:34:00
  number of geometrically distinct ways to compactify the extra dimensions
• 1:34:05
  has been estimated at roughly ten to the power of
• 1:34:08
  five hundred, not ten to the fifth ten to the
• 1:34:13
  five hundredth This is called the string landscape. Each point
• 1:34:18
  on that landscape is a possible set of physical laws,
• 1:34:22
  a possible universe. For a long time, physicists treated this
• 1:34:27
  as an embarrassment. A good theory of everything should predict
• 1:34:32
  one universe, the one we live in. A theory that
• 1:34:37
  produces ten to the five hundred equally valid universes looks
• 1:34:42
  less like a theory and more like a shrug. Some
• 1:34:46
  physicists still feel that way. Others began to suspect the
• 1:34:51
  landscape wasn't a flaw. It was the answer to a
• 1:34:54
  question they had been asking for decades. That question connects
• 1:34:59
  directly to what the last section left you with. If
• 1:35:03
  eternal inflation is correct, new bubble universes are constantly forming
• 1:35:08
  as pockets of the inflating background space collapse into their
• 1:35:12
  own big bangs. The question was always what physics each
• 1:35:17
  of those bubbles gets if the string landscape is real.
• 1:35:22
  The answer might be that each bubble lands on a
• 1:35:25
  different point in the landscape. Each bubble draws a different
• 1:35:30
  compactification of the extra dimensions, like pulling a different card
• 1:35:35
  from an incomprehensibly large deck. Each bubble gets different constants,
• 1:35:41
  different particle masses, a different cosmological constant, different rules the
• 1:35:48
  inflating background space keeps producing bubbles. Each bubble is a universe.
• 1:35:54
  Each universe runs a different experiment in physics. Some of
• 1:35:59
  those experiments produce constants that allow atoms to form, some don't.
• 1:36:04
  Some produce stars, Some produce nothing but diffuse hydrogen that
• 1:36:09
  never clumps. Some produce a cosmological constant so large that
• 1:36:14
  space tears itself apart before a single galaxy can condense.
• 1:36:19
  Most of the landscape, by most estimates, produces something hostile
• 1:36:24
  to complexity. A small fraction does not. This is where
• 1:36:29
  the reasoning gets circular, in a way that either satisfies
• 1:36:33
  you or unsettles you, depending on your disposition. The question
• 1:36:38
  of why our universe has the constances it has, why
• 1:36:42
  gravity is exactly the strength it is, why the cosmological
• 1:36:47
  constant is not a billion times larger, has no clean answer.
• 1:36:51
  Inside a single universe picture, the numbers look finely tuned suspiciously,
• 1:36:58
  so in a landscape picture, the question changes shape. You
• 1:37:03
  stop asking why the constants are what they are, and
• 1:37:07
  start asking which universes in the landscape produce observers capable
• 1:37:12
  of noticing. The answer is the ones with constants in
• 1:37:17
  the right range. We are in one of those, because
• 1:37:20
  we could not be in any other. This is the
• 1:37:24
  anthropic principle, and it is either a profound insight or
• 1:37:28
  a way of explaining ever a thing by explaining nothing.
• 1:37:33
  And physicists have been arguing about which of those it
• 1:37:36
  is since the nineteen eighties. What nobody disputes is the scale.
• 1:37:41
  If the string landscape exists, the catalog of possible universes
• 1:37:46
  is not a short list. It is not even a
• 1:37:49
  long list. Each configuration is geometrically distinct. Each produces a
• 1:37:55
  coherent set of physical laws. Each could, in principle, correspond
• 1:38:01
  to a bubble universe somewhere in the eternally inflating background.
• 1:38:06
  Ten to the five hundred is a number so large
• 1:38:09
  that describing it as astronomical would be an insult to astronomy.
• 1:38:14
  And if string theory is right, every single entry in
• 1:38:19
  that catalog is a real place. The universe appears to
• 1:38:24
  have been built with extraordinarily precise specifications, and explaining why
• 1:38:29
  those specifications are what they are turns out to be
• 1:38:33
  one of the hardest problems in all of science. The
• 1:38:36
  string landscape gives you ten to the five hundred possible universes,
• 1:38:42
  each with different physical constants. That number only matters if
• 1:38:47
  the constants actually make a difference, and they do. They
• 1:38:52
  make a difference so large, so decisive, that physicists have
• 1:38:57
  spent decades trying to figure out whether the values we
• 1:39:00
  measure are accidental, inevitable, or something else. Entirely. Start with gravity.
• 1:39:08
  The gravitational constant, the number that sets how strongly mass
• 1:39:12
  pulls on other mass, is extraordinarily weak compared to the
• 1:39:17
  other fundamental forces. It is roughly ten to the thirty
• 1:39:21
  six times weaker than electromagnetism. That sounds like a flaw,
• 1:39:27
  it is not. If gravity were even modestly stronger, stars
• 1:39:31
  would form, burn hot, and collapse in millions of years
• 1:39:36
  instead of billions. Life needs time billions of years of it.
• 1:39:42
  A universe with slightly stronger gravity is a universe that
• 1:39:46
  never gets around to biology. The window for the gravitational
• 1:39:51
  constant to land in a range that permits complexity is narrow,
• 1:39:55
  and we are sitting squarely inside it. Then there is
• 1:39:59
  the costsmological constant. This is the number that describes the
• 1:40:04
  energy density of empty space, the quantity tied directly to
• 1:40:09
  dark energy and the expansion rate of the universe. It
• 1:40:14
  is almost zero. Theoretical calculations of what it should be
• 1:40:19
  based on quantum field theory, suggest a value roughly ten
• 1:40:23
  to the one hundred twenty times larger than what we
• 1:40:27
  actually measure. That is not a small discrepancy. That is
• 1:40:32
  the largest mismatch between theory and observation in the history
• 1:40:37
  of physics. And here is what makes it strange. If
• 1:40:42
  the cosmological constant were even a fraction of what the
• 1:40:45
  equations naively predict, space would have expanded so violently in
• 1:40:51
  the early universe that matter never had the chance to
• 1:40:55
  clump together. No galaxies, no stars, no periodic table worth
• 1:41:02
  mentioning the cosmology col constant appears to have been dialed
• 1:41:07
  down to almost nothing, with a precision that has no
• 1:41:11
  physical explanation anyone can fully account for. The proton neutron
• 1:41:15
  mass difference is another one. A proton is slightly lighter
• 1:41:20
  than a neutron by about zero point one percent. That
• 1:41:25
  tiny gap matters enormously if the neutron were lighter than
• 1:41:29
  the proton instead of heavier, free protons would decay into neutrons,
• 1:41:35
  Hydrogen would be unstable, and the chemistry that underlies all
• 1:41:40
  known life simply would not exist. Change that mass difference
• 1:41:46
  by a small fraction in either direction, and the periodic
• 1:41:50
  table either collapses or never assembles properly. In the first place,
• 1:41:56
  the number has to land in a very specific range,
• 1:41:59
  and it does, you can keep going. The strong nuclear
• 1:42:04
  force which holds atomic nuclei together is precisely tuned to
• 1:42:08
  allow carbon to form inside stars. Carbon is the backbone
• 1:42:14
  of organic chemistry. If that force were a few percent weaker,
• 1:42:18
  carbon nuclei would not be stable. A few percent stronger,
• 1:42:23
  and hydrogen would fuse so readily in the early universe
• 1:42:28
  that it would all burn off before stars could sustain themselves.
• 1:42:33
  Fred Hoyle, the British astronomer, predicted an energy resonance in
• 1:42:38
  carbon nuclei before it was discovered, specifically because he reasoned
• 1:42:43
  that carbon had to be able to form in stellar interiors,
• 1:42:48
  and the only way that was possible was if such
• 1:42:51
  a resonance existed. He was right. The universe builds carbon,
• 1:42:57
  and the numbers are set just right to allow so
• 1:43:01
  what do you do with this? There are two broad answers,
• 1:43:05
  and neither is comfortable. The first is that the constants
• 1:43:09
  are what they are by design, that some intelligence, some
• 1:43:13
  agency outside the physical framework selected these values. This is
• 1:43:19
  an old answer, and it feels like an answer to
• 1:43:22
  many people, but it is not a scientific answer. It
• 1:43:27
  does not generate predictions you can test. It does not
• 1:43:31
  point toward any new physics. Science can acknowledge the question
• 1:43:36
  without being able to touch it, and this particular explanation
• 1:43:41
  lives permanently outside the boundaries of what experiments can reach.
• 1:43:45
  The second answer is the multiverse. If there are enough
• 1:43:50
  universes with enough variation in their physical constants, then some
• 1:43:55
  fraction of them will land on values that allow complexity. Observers,
• 1:44:01
  by definition, can only find themselves inside universes where observers
• 1:44:06
  are possible. This is the anthropic principle. It sounds almost
• 1:44:12
  trivial when you state it plainly. Of course, we are
• 1:44:16
  in a universe compatible with our existence, where else would
• 1:44:20
  we be. But the logic has real teeth. It means
• 1:44:25
  you do not need to explain why the constants are
• 1:44:27
  finely tuned. You only need enough universes with enough variation
• 1:44:33
  and selection takes care of the rest. The string landscape,
• 1:44:38
  with its ten to the five hundred vacua, provides exactly
• 1:44:43
  that kind of variation. Eternal inflation provides the mechanism to
• 1:44:48
  populate the m Put the two together, and the fine
• 1:44:52
  tuning problem dissolves, not because the numbers were chosen carefully,
• 1:44:57
  but because every combination was tried somewhere. The problem is
• 1:45:02
  that this explanation is almost impossible to test. You cannot
• 1:45:06
  reach the other universes. You cannot observe the ensemble. You
• 1:45:11
  are making an inference about an infinite population from a
• 1:45:15
  sample size of one. Some physicists find this deeply unsatisfying.
• 1:45:21
  A theory that explains everything by appealing to an unobservable
• 1:45:26
  infinity of cases, they argue, is not really explaining anything.
• 1:45:32
  Others think the logic is sound, and that demanding a
• 1:45:35
  testable mechanism for every inference is an over correction. The
• 1:45:40
  debate has not been settled. It is not close to
• 1:45:44
  being settled. What is settled is the observation itself. The
• 1:45:49
  constants are narrow, the windows are real. The universe sits
• 1:45:54
  inside those windows with a precision that demands some kind
• 1:45:59
  of accounting. Either the numbers are what they are for
• 1:46:02
  a reason we do not understand, or the numbers are
• 1:46:06
  what they are because in the vast ocean of universes
• 1:46:10
  with different numbers, this is the one we happen to occupy,
• 1:46:15
  and nobody has yet found a way to determine which
• 1:46:18
  of those answers is true. There is a second kind
• 1:46:21
  of multiverse that has nothing to do with inflation or
• 1:46:24
  string theory, and it comes from the strangest corner of physics,
• 1:46:29
  the behavior of particles smaller than atoms. Everything covered so
• 1:46:34
  far bubble universes, the string landscape, the fine tuning of constants.
• 1:46:41
  All of that emerges from cosmology and high energy physics,
• 1:46:46
  from questions about the large scale structure of space. This
• 1:46:51
  multiverse is different. It grows out of a problem that
• 1:46:55
  physicists have been arguing about since the nineteen twenties, and
• 1:47:00
  the problem sounds almost too simple to take seriously. When
• 1:47:05
  you measure a quantum particle, what actually happens that question
• 1:47:10
  turns out to be genuinely unresolved. Here is the situation.
• 1:47:16
  Quantum mechanics describes particles not as objects with definite positions
• 1:47:20
  and speeds, but as waves. Of probability. An electron doesn't
• 1:47:26
  have a location the way a baseball does. It has
• 1:47:29
  a spread of possible locations, each with an associated likelihood.
• 1:47:35
  The math governing that's spread is clean and precise. Physicists
• 1:47:39
  can calculate it exactly. The wave function, as it's called,
• 1:47:45
  evolves in a completely predictable way. Then something strange happens.
• 1:47:51
  The moment you measure the electron, the moment any detector
• 1:47:55
  interacts with it, the spread collapses. You don't get a
• 1:47:59
  cloud of probabilities. You get a single outcome, one position
• 1:48:05
  one reading. The wave function, which had been spread across possibilities,
• 1:48:11
  snaps to a single answer. This is the measurement problem.
• 1:48:16
  The math before measurement is beautiful and deterministic. The act
• 1:48:21
  of measurement itself is neither. Nobody knows why observation selects
• 1:48:26
  one outcome. Nobody knows what does the selecting. For decades,
• 1:48:32
  the standard answer was to simply not ask. You plug
• 1:48:36
  in the wave function, you calculate the probability distribution, You
• 1:48:42
  compare your results to experiment, and everything matches perfectly. Don't
• 1:48:48
  worry about what's really happening underneath. This is what physicists
• 1:48:53
  call the Copenhagen interpretation, and for a long time it
• 1:48:58
  was enough. Then in nineteen fifty seven, a graduate student
• 1:49:03
  at Princeton named Hugh Everett proposed a different approach. What
• 1:49:09
  if the wave function never collapses. What if, instead of
• 1:49:13
  one outcome being chosen and the others disappearing, every outcome
• 1:49:18
  actually happens. The measuring device doesn't force the electron into
• 1:49:23
  a single position. The measuring device, along with the electron,
• 1:49:28
  along with the physicist doing the measuring, along with the
• 1:49:31
  entire universe, splits into separate branches, one for each possible outcome.
• 1:49:39
  In one branch, the electron landed here. In another branch,
• 1:49:44
  it landed there. Both are equally real. Neither knows about
• 1:49:50
  the other. This is the many World's interpretation, and it
• 1:49:54
  solves the measurement problem completely. There is no collapse because
• 1:49:59
  there is nothing to collapse. The wave function always evolves deterministically,
• 1:50:05
  every possibility plays out. The reason you only experience one
• 1:50:10
  outcome is that you are inside one branch. The other
• 1:50:14
  branches are just as real and just as convinced that
• 1:50:19
  their outcome was the one that happened. The implications are
• 1:50:23
  not subtle. Every quantum event that has more than one
• 1:50:27
  possible outcome generates a branching. Every radioactive atom that might
• 1:50:33
  or might not decay at a given moment, every photon
• 1:50:37
  that might or might not be absorbed by a molecule,
• 1:50:40
  every unstable particle at every moment in the history of
• 1:50:44
  the cosmos. The number of branchings that have occurred since
• 1:50:49
  the Big Bang is so far beyond astronomical that the
• 1:50:53
  word astronomical becomes useless. You are not just one version
• 1:50:58
  of yourself navigating one version of history under many worlds.
• 1:51:05
  Every decision that had any quantum sensitivity, and most decisions
• 1:51:10
  do because neurons fire based on chemistry that depends on
• 1:51:14
  quantum processes, branches into separate histories. Somewhere in the structure,
• 1:51:22
  there is a version of you that made every choice differently.
• 1:51:26
  Physicists who take this seriously are not a fringe. A
• 1:51:31
  significant fraction of working physicists who think carefully about the
• 1:51:35
  foundations of quantum mechanics consider many worlds at minimum plausible,
• 1:51:41
  and some consider it the most coherent interpretation available. The
• 1:51:46
  reason is that it takes the mathematics of quantum mechanics
• 1:51:50
  at face value. It doesn't add any extra mechanism to
• 1:51:54
  explain collapse. It doesn't require any special role for observers
• 1:51:58
  or measurements. The wave function is real. It does what
• 1:52:03
  the equations say it does. Everything else follows. The cost
• 1:52:09
  is onto logical extravagance, an incomprehensibly large number of parallel histories,
• 1:52:15
  but the equations don't charge extra for that. Now Here
• 1:52:21
  is where many Worlds connects to everything else. This script
• 1:52:24
  has been building. The inflationary multiverse produces separate regions of
• 1:52:30
  space with potentially different physical laws. The string landscape predicts
• 1:52:36
  an enormous number of possible universes with different constants. These
• 1:52:42
  are multiverses of space. Many Worlds is a multiverse of history.
• 1:52:48
  They are completely independent ideas, arising from completely different areas
• 1:52:54
  of physics. If both are correct, you don't add them,
• 1:52:58
  you multiply them. Every bubble universe in the inflationary multiverse
• 1:53:04
  has been branching quantum mechanically since the moment it formed.
• 1:53:09
  Every entry in the string landscape catalog of possible universes
• 1:53:14
  exists in every quantum branch it has ever generated. The
• 1:53:19
  already incomprehensible number of universes in eternal inflation gets raised
• 1:53:25
  to a power that has no name. And here is
• 1:53:28
  the piece that matters most for the question. This whole
• 1:53:31
  series is asking the region beyond your cosmic horizon, that vast,
• 1:53:37
  unobservable expanse that has been the subject of every section
• 1:53:41
  before this one, has been quantum branching just as continuously
• 1:53:46
  as the corner you occupy. Every atom out there, every
• 1:53:51
  photon crossing some galaxy you will never see. Every event
• 1:53:55
  in every star too far away to ever signal you
• 1:53:59
  has been generating branches at the same rate as everything
• 1:54:03
  inside your observable sphere. The Many World's interpretation does not
• 1:54:09
  restrict its branching to the observable universe. The wave function
• 1:54:15
  of the universe is the wave function of the whole universe.
• 1:54:20
  If Many Worlds is right, there is not just one
• 1:54:24
  version of whatever exists past the cosmic horizon. There are
• 1:54:29
  infi in itly many, branching every instant in ways that
• 1:54:34
  nobody can observe or verify. There is a version of
• 1:54:37
  cosmology that challenges something even more fundamental than the question
• 1:54:42
  of what lies past the edge. It challenges the idea
• 1:54:46
  that the Big Bang was the beginning. Every model we
• 1:54:51
  have looked at so far accepts one thing without arguing
• 1:54:55
  about it. There was a moment roughly thirteen point eight
• 1:55:00
  billion years ago, when everything that exists exploded into being
• 1:55:05
  from an unimaginably hot, dense point. Eternal inflation puts that
• 1:55:10
  moment inside a larger framework. String theory fills in the
• 1:55:15
  physics around it. Many worlds branches off from it in
• 1:55:19
  every direction, but none of them seriously question whether it happened.
• 1:55:25
  The ecperotic model does, and it does so with math
• 1:55:30
  that a serious fraction of the physics community finds genuinely
• 1:55:34
  hard to dismiss. The original version of the idea came
• 1:55:38
  from a group of physicists working in the early two thousands.
• 1:55:42
  Their starting point was string theory's requirement that extra dimensions exist.
• 1:55:48
  In string theory, the three dimensions we move through are
• 1:55:52
  not all there is. There are additional spatial dimensions, and
• 1:55:57
  in some versions of the theory are three dimensional. Universe
• 1:56:01
  is not floating in empty higher dimensional space alone. It
• 1:56:06
  sits on a membrane. Physicists call these objects brains, and
• 1:56:11
  the idea is that our entire observable universe, every galaxy,
• 1:56:16
  every star, everything is the surface of one such membrane.
• 1:56:21
  Drifting through a higher dimensional space called the bulk. Another
• 1:56:26
  brain could exist nearby. You would never see it. Nothing
• 1:56:30
  from it could reach you, because the particles and forces
• 1:56:34
  you are built from are locked to your membrane's surface.
• 1:56:38
  Only gravity leaks through. Here is where things get strange.
• 1:56:44
  The two brains are not stationary. They drift, and occasionally,
• 1:56:49
  over enormous stretches of time, they drift toward each other.
• 1:56:54
  When they collide. The energy released is catastrophic. From the
• 1:56:59
  perspective of anyone living on either membrane, it looks exactly
• 1:57:03
  like a big bang. Space floods with energy, mataforms, the
• 1:57:09
  universe heats up and begins expanding. What we have always
• 1:57:14
  called the beginning of everything was in this picture just
• 1:57:19
  two membranes slapping together. The collision was not the start
• 1:57:23
  of time. It was an event inside a longer story.
• 1:57:27
  This was the ecporotic model, named after the ancient Greek
• 1:57:31
  word for conflagration. It had problems. The original version struggled
• 1:57:38
  to reproduce the specific pattern of temperature fluctuations we observe
• 1:57:42
  in the cosmic microwave background, the same fluctuations that inflation
• 1:57:47
  explains so cleanly. Physicists poked at it. Some of those
• 1:57:53
  problems got patched, others did not. The debate is still running,
• 1:57:59
  but the the idea it seeded survived and grew into
• 1:58:03
  something broader. Cyclic cosmology takes the brain collision picture and
• 1:58:08
  extends it into a repeating sequence. The brains collide, triggering
• 1:58:14
  what looks like a big bang, The universe expands over
• 1:58:19
  hundreds of billions of years, The expansion slows, reverses, and
• 1:58:25
  the brains drift apart, pulled back toward each other by
• 1:58:28
  a force described through a scale of field threading the bulk.
• 1:58:33
  Then they collide again, another big bang, another universe, the
• 1:58:38
  same brains, over and over, running through the same basic cycle,
• 1:58:44
  potentially without any starting point, no first collision, no original moment,
• 1:58:52
  just cycles stretching back without limit. The newer versions of
• 1:58:57
  this idea, developed primarily by Poor Steinhardt and Neil Turok,
• 1:59:02
  moved away from the brain language and described the same
• 1:59:06
  physics through the evolution of a scalar field alone, which
• 1:59:10
  made the model somewhat cleaner and easier to work with mathematically,
• 1:59:16
  but the core implication stayed the same. Before our big
• 1:59:21
  bang was a previous universe cooling and collapsing toward the
• 1:59:25
  conditions that triggered ours before that another, each cycle lasting
• 1:59:32
  something in the range of a trillion years before the bounce.
• 1:59:37
  What each previous cycle looked like is genuinely uncertain, and
• 1:59:41
  this is one of the harder questions the model has
• 1:59:44
  to answer. In some versions, almost no information survives the bounce.
• 1:59:51
  The extreme energies of the collision scramble everything. Each cycle
• 1:59:55
  starts fresh with only the most basic physical constant inherited
• 2:00:00
  from the scale of Field's properties. In other versions, subtle
• 2:00:05
  imprints of previous cycles might survive, encoded in the gravitational
• 2:00:10
  wave background, potentially detectable with sufficiently sensitive instruments. Nobody has
• 2:00:17
  found such a signal yet, the search is ongoing. There
• 2:00:21
  is a direct tension between cyclic models and eternal inflation,
• 2:00:27
  and it is worth being honest about. Eternal inflation is
• 2:00:31
  currently the more widely accepted framework, partly because it fits
• 2:00:36
  the CMB data more naturally, and partly because it has
• 2:00:40
  been developed in much greater mathematical detail over a longer
• 2:00:45
  period of time. Cyclic models are the challenger. Some physicists
• 2:00:52
  think they are on a dead end. Others think they
• 2:00:55
  are pointing at something real that the mainstream has been
• 2:00:59
  too quick to dismiss. The honest position is that neither
• 2:01:03
  model is proven. Both remains serious theoretical proposals what cyclic
• 2:01:10
  cosmology does. Regardless of which camp wins, the argument is
• 2:01:15
  force a specific and uncomfortable question. If the universe has
• 2:01:19
  been cycling forever, there is no creation event, There is
• 2:01:24
  no moment before which nothing existed. The question of what
• 2:01:29
  came before the Big Bang gets a clean, unsettling answer
• 2:01:34
  another universe just like this, one running through the same
• 2:01:38
  sequence of expansion, cooling, and collapse, and before that another
• 2:01:46
  The phrase beginning of time loses its meaning entirely. This
• 2:01:51
  is a different kind of edge than the spatial horizon
• 2:01:55
  we have spent most of this series examining. It is
• 2:01:58
  not a boundary in any direction you could point. It
• 2:02:02
  is a boundary in time, or rather a challenge to
• 2:02:05
  whether time has a boundary at all. A universe that
• 2:02:09
  has always existed, cycling through an infinite series of expansions
• 2:02:14
  and collapses carries its own kind of edge, not of space,
• 2:02:20
  but of time, and what exists past that edge is
• 2:02:25
  the same question asked in a different direction. Science has
• 2:02:30
  a hard problem with the question of what lies past
• 2:02:33
  the edge, and the problem is not that the answer
• 2:02:36
  is complicated, It is that the answer may be permanently inaccessible.
• 2:02:43
  Every theory covered so far, eternal inflation, bubble universes, the
• 2:02:48
  string landscape, cyclic cosmology shares one uncomfortable feature. None of
• 2:02:55
  them can be tested in the traditional sense. You cannot
• 2:02:59
  send a probe beyond the cosmic event horizon. You cannot
• 2:03:03
  wait for a signal to arrive from a neighboring bubble universe.
• 2:03:07
  The inflating space between here and there expands faster than
• 2:03:11
  any message could travel, which means the barrier is not
• 2:03:15
  a matter of patience or funding. It is physics. A
• 2:03:20
  civilization a trillion years more advanced than ours runs into
• 2:03:24
  the same wall. The horizon is not a technological problem,
• 2:03:30
  it is a geometric one, and geometry does not care
• 2:03:33
  about ingenuity. This creates a genuine crisis for the scientific project,
• 2:03:39
  and physicists are not shy about saying so. Science works
• 2:03:44
  by making predictions and then checking them against reality. You
• 2:03:49
  propose something, you test it. The universe tells you whether
• 2:03:53
  you were right. That loop is the whole engine. But
• 2:03:58
  when the thing you are proposing egs exists permanently beyond
• 2:04:02
  any possible observation. The loop breaks. There is no experiment
• 2:04:07
  to run, there is no data to collect. The universe
• 2:04:12
  simply will not answer, so the question becomes harder than
• 2:04:16
  it looks. Is a theory that cannot be tested? Still science,
• 2:04:22
  some of the most respected physicists alive say yes. Their
• 2:04:27
  argument goes like this, the multiverse is not a wild
• 2:04:31
  speculation invented from nowhere. It is a direct consequence of
• 2:04:36
  theories inflation, quantum mechanics, string theory that have been tested
• 2:04:43
  extensively in the parts of the universe we can reach.
• 2:04:48
  If you accept those theories, the multiverse follows almost automatically.
• 2:04:54
  Rejecting it would mean inventing a reason for inflation to
• 2:04:57
  stop exactly at the boundary of what we can can observe,
• 2:05:01
  which itself would require an explanation. The multiverse, they argue,
• 2:05:07
  is the honest prediction of accepted physics. Dismissing it because
• 2:05:11
  it is inconvenient is not a scientific instinct. It is
• 2:05:16
  an esthetic preference. The other side is just as serious.
• 2:05:22
  Physicists like Paul Steinhardt, who actually helped develop inflation, have
• 2:05:28
  argued that a framework producing an infinite number of unobservable
• 2:05:32
  universes with every possible set of physical laws, has effectively
• 2:05:38
  explained nothing. If your theory can produce any outcome, it
• 2:05:43
  cannot be falsified. And a theory that cannot be falsified
• 2:05:48
  is not a scientific theory in any meaningful sense. It
• 2:05:52
  is metaphysics wearing the clothes of equations. The word they
• 2:05:57
  use is not flattering. They call it a multiverse of
• 2:06:01
  the gaps. Both camps have a point. That is what
• 2:06:05
  makes this uncomfortable. What keeps the debate alive is a
• 2:06:10
  thin thread of hope, the possibility of indirect evidence, not proof,
• 2:06:16
  but fingerprints. The most discussed candidate is something called a
• 2:06:21
  bubble collision. If our universe is one bubble in an
• 2:06:25
  eternally inflating sea of bubbles, then in the very early
• 2:06:30
  moments of our existence, a neighboring bubble might have grazed ours,
• 2:06:36
  two expanding spheres, each a universe unto itself, brushing against
• 2:06:42
  each other at the edge. The physics of such a
• 2:06:45
  collision would have left a mark, a subtle imprint in
• 2:06:48
  the cosmic microwave background, a circular pattern of slightly different
• 2:06:53
  temperatures stamped into the oldest light we can detect. Cosmologists
• 2:06:59
  have looked. There are cold spots in the CMB that
• 2:07:03
  have attracted attention, one particularly large anomaly in the southern
• 2:07:08
  sky that some researchers have flagged as a possible candidate.
• 2:07:14
  So far, nothing has cleared the bar of statistical significance.
• 2:07:19
  The cold spots have mundane explanations that fit standard cosmology.
• 2:07:25
  The fingerprint, if it exists, is buried beneath the noise,
• 2:07:30
  and we may not have the resolution to pull it out.
• 2:07:33
  That is probably the best shot we have, and it
• 2:07:37
  is a long shot. There is something almost dizzying about
• 2:07:41
  sitting with this. Honestly. Humanity has built telescopes that can
• 2:07:45
  detect a single photon from a galaxy billions of light
• 2:07:49
  years away. We have mapped the structure of the early
• 2:07:53
  universe to a precision of one part in one hundred thousand.
• 2:07:57
  We crack the atom, decoded the genome, landed machines on
• 2:08:02
  other planets, and the question of what lies past the
• 2:08:07
  final edge of the observable universe might simply stay unanswered,
• 2:08:13
  not because we have not tried, not because the technology
• 2:08:17
  isn't there yet, because the universe physically forbids the answer
• 2:08:21
  from reaching us. Some physicists find this freeing the horizon
• 2:08:27
  is an honest acknowledgment of our situation. We are not
• 2:08:31
  at the center of everything. We are not at a
• 2:08:34
  special place. We are inside a bubble of visibility, surrounded
• 2:08:39
  by a much larger reality that is not obligated to
• 2:08:43
  be knowable to us. The laws of nature were not
• 2:08:47
  designed with our curiosity in mind. They were not designed
• 2:08:51
  at all. The fact that we can understand as much
• 2:08:55
  as we do from inside this small lit sphere is remarkable.
• 2:09:01
  The fact that the sphere has a wall is not
• 2:09:04
  a betrayal, It is just geometry. Others find it genuinely frustrating.
• 2:09:11
  The entire arc of science has been a story of
• 2:09:14
  pushing back the boundaries of the unknown, and the cosmic
• 2:09:18
  horizon is a boundary that does not push back. Every
• 2:09:22
  other wall has eventually yielded. This one is built from
• 2:09:26
  the speed of light and the age of the universe,
• 2:09:29
  and those are not negotiable parameters. What is clear is this,
• 2:09:34
  the theories are real theories, constructed by serious people using
• 2:09:39
  the best tools available. The evidence that inspired them. Inflation,
• 2:09:45
  the flatness of space, the uniformity of the CMB is
• 2:09:50
  real evidence the multiverse is not science fiction. It may
• 2:09:55
  also not be science in the strictest sense. It exists
• 2:10:00
  in the strange territory between and we do not yet
• 2:10:03
  have a word for that territory. That everybody agrees on
• 2:10:08
  the universe is not keeping secrets out of malice. It
• 2:10:12
  is keeping them out of geometry, and geometry does not negotiate.
• 2:10:18
  At some point in a conversation about what lies beyond
• 2:10:22
  the observable universe, Physics quietly hands the question off to philosophy.
• 2:10:28
  And the handoff is not a failure. It is an
• 2:10:31
  honest recognition of a boundary. Physics is extraordinarily good at
• 2:10:37
  telling you what it cannot do. That is actually one
• 2:10:40
  of its strengths. The uncertainty principle does not say we
• 2:10:45
  haven't built a precise enough instrument, yet, it says the
• 2:10:49
  precision you want is forbidden at a fundamental level by
• 2:10:54
  the structure of reality. The interior of a black hole
• 2:10:58
  past the event horizon is not merely hard to observe.
• 2:11:03
  Anything that falls in cannot send information back out, ever,
• 2:11:07
  by any means. The singularity at the Big Bang is
• 2:11:12
  not a moment we have failed to reconstruct, yet it
• 2:11:16
  is a place where the equations that describe the universe
• 2:11:19
  break down completely and return nonsense. Physics runs into these
• 2:11:24
  walls with some regularity. The cosmic horizon is one more
• 2:11:29
  wall of the same kind. Not a technological problem, not
• 2:11:33
  a funding problem, a law. What makes the horizon different
• 2:11:39
  from those other limits is the scale of what sits
• 2:11:42
  on the other side. The uncertainty principle governs the behavior
• 2:11:47
  of particles smaller than atoms. The black hole interior hides
• 2:11:52
  whatever happens inside a collapse star. These are bounded regions
• 2:11:56
  of ignorance. The horizon, though, is hiding something that is
• 2:12:01
  almost certainly larger than everything we can see, possibly infinitely larger.
• 2:12:07
  The unobservable universe is not a footnote to the observable one.
• 2:12:12
  It may be the main event, and we are the footnote.
• 2:12:17
  Here is what makes that genuinely strange. We have no
• 2:12:21
  way to characterize what we are missing. When you don't
• 2:12:25
  know the answer to a specific question, you can at
• 2:12:28
  least frame the question. You know what you're looking for.
• 2:12:33
  But the region beyond the horizon could contain structures, phenomena,
• 2:12:39
  or configurations of matter that have no analogue anywhere. In
• 2:12:44
  the forty six billion light years we can check. We
• 2:12:48
  wouldn't know to look for them. We couldn't frame a
• 2:12:52
  question about them. The shape of our ignorance is itself
• 2:12:55
  hidden from us. This matters in a way that goes
• 2:12:59
  deeper than curureuriosity. Every physical law we have was derived
• 2:13:04
  from observations made inside this bubble. Every equation, every constant,
• 2:13:10
  every framework, from quantum mechanics to general relativity was built
• 2:13:16
  by measuring things we could actually reach. That process has
• 2:13:22
  worked astonishingly well. The laws of physics we have derived
• 2:13:26
  here on Earth seem to hold in galaxies billions of
• 2:13:30
  light years away to the limits of our observation. That
• 2:13:35
  consistency is genuinely remarkable, but it is consistency within the
• 2:13:41
  observable universe. It tells us nothing definitively about whether those
• 2:13:47
  laws hold everywhere. There are physicists who take this concern
• 2:13:52
  seriously and others who dismiss it. The dismissers have a
• 2:13:57
  reasonable point. The CMB looks the same in every direction.
• 2:14:02
  The same spectral lines appear in the farthest galaxies we
• 2:14:05
  can detect. The evidence for uniformity is strong, but strong
• 2:14:10
  evidence within your observation window is not the same. As
• 2:14:14
  proof that nothing changes outside it. A scientist in a
• 2:14:18
  single city could observe that the boiling point of water
• 2:14:22
  is one hundred degrees celsius everywhere she checks, and she
• 2:14:27
  would be right about every data point she gathered, and
• 2:14:31
  still wrong about everest. There is a version of this
• 2:14:35
  problem that physicists call systematic bias. When you can only
• 2:14:41
  observe from one vantage point, your models will be shaped
• 2:14:45
  by that vantage point in ways you cannot fully account for.
• 2:14:50
  The observable universe is not a random sample of all
• 2:14:54
  possible universes. It is the one we find ourselves inside.
• 2:14:59
  The constance that govern its behavior are the constants that
• 2:15:03
  produce something complex enough to contain observers asking questions about constants.
• 2:15:10
  That circularity is uncomfortable and real, and it means our
• 2:15:15
  physical laws might describe a special case rather than the
• 2:15:19
  general rule. The uncertainty principle, the black hole interior, the
• 2:15:25
  Big Bang singularity. Each of those limits is a place
• 2:15:30
  where our best theories fail or stop. The horizon is different.
• 2:15:35
  Our theories don't fail there, they simply have no data
• 2:15:39
  to work with. That is, in some ways a quieter problem,
• 2:15:44
  and in other ways a more unsettling one. A theory
• 2:15:48
  that breaks down at least announces itself. A theory that
• 2:15:52
  has never been tested beyond a certain range doesn't announce anything.
• 2:15:57
  It just sits there, confident and un tested, describing the bubble.
• 2:16:03
  Some physicists argue, this is fine. The observable universe is
• 2:16:07
  what we can interact with, what we can affect and
• 2:16:11
  be affected by. Science is about the measurable, and the
• 2:16:16
  measurable ends at the horizon. Build your models on what
• 2:16:20
  you have. Others push back and argue that a theory
• 2:16:25
  of everything that only describes the everything we can see
• 2:16:30
  is not really a theory of everything. It is a
• 2:16:33
  very good local approximation, and confusing local approximations for universal
• 2:16:39
  laws is the oldest mistake in the history of science.
• 2:16:44
  People thought Newtonian mechanics was complete once too. None of
• 2:16:49
  this makes the physics we have wrong, It makes it bounded.
• 2:16:54
  That is a different thing. The equations still work here,
• 2:16:58
  the predictions still within the observable sphere, but the question
• 2:17:03
  of whether they are describing something universal or something parochial
• 2:17:09
  remains genuinely open, and it may be permanently open, given
• 2:17:14
  that the evidence needed to answer it is. On the
• 2:17:17
  other side of a horizon that no signal will ever cross.
• 2:17:23
  The physics we have is the physics of the observable universe,
• 2:17:27
  built entirely from observations made inside a forty six billion
• 2:17:32
  light year sphere, and we have no guarantee that it
• 2:17:36
  describes anything beyond it correctly. The three dimensions of space
• 2:17:42
  we move through length, width, height are the only spatial
• 2:17:48
  dimensions most humans will ever think about, and there is
• 2:17:52
  no particular reason to believe they are the only ones
• 2:17:56
  that exist. String theory, the framework physicist developed in attempting
• 2:18:01
  to unify gravity with quantum mechanics, requires extra spatial dimensions
• 2:18:06
  to work mathematically, not one extra dimension, not two, depending
• 2:18:13
  on the version of string theory you are working with,
• 2:18:16
  Somewhere between six and seven additional spatial dimensions have to
• 2:18:21
  be hiding somewhere. The question physicists have spent decades wrestling
• 2:18:26
  with is not whether those dimensions are there, It is
• 2:18:30
  where they went. The leading answer is that they are compactified,
• 2:18:36
  rolled up, curled into themselves at scales so small that
• 2:18:41
  no particle accelerator ever built, and none currently on the
• 2:18:45
  drawing board, could probe them. Imagine a garden hose lying
• 2:18:50
  on the ground from a distance. From far away, it
• 2:18:53
  looks like a line one dimensional. Get closer and you
• 2:18:58
  can see it has a second wrapping around its circumference.
• 2:19:03
  The extra dimension was always there, you just needed to
• 2:19:07
  be close enough to see it. The extra dimensions in
• 2:19:11
  string theory are thought to work the same way, except
• 2:19:15
  the scale at which they become visible is believed to
• 2:19:19
  be near the plank length roughly ten to the minus
• 2:19:23
  thirty five meters, which is so far below anything we
• 2:19:27
  can measure that it might as well be invisible forever.
• 2:19:32
  If those dimensions exist, they are not somewhere exotic. They
• 2:19:38
  are here right now, wrapped around every point in space,
• 2:19:44
  including the space inside your own body. What that means
• 2:19:49
  for what lies beyond the cosmic horizon is genuinely unknown.
• 2:19:54
  The extra dimensions might be compactified identically everywhere, uniform across
• 2:19:59
  the en tire infinite universe, the same tight little curl
• 2:20:03
  at every point, or they might not be. In the
• 2:20:08
  eternal inflation picture, different bubble universes settling into different vacuum
• 2:20:13
  states could end up with different compactifications, entirely different numbers
• 2:20:19
  of large dimensions, different shapes for the curled up ones,
• 2:20:24
  different physics. As a result, the region just outside our
• 2:20:29
  observable sphere might have three large spatial dimensions just like ours,
• 2:20:35
  or it might have four or two. We have no
• 2:20:39
  way of knowing, and no instrument that could tell us.
• 2:20:43
  There is a second stranger possibility that has nothing to
• 2:20:47
  do with extra dimensions. Space might have an unusual topology.
• 2:20:52
  Topology is not the same thing as geometry. Geometry describes
• 2:20:57
  the curvature and shape of space. Topology describes how it
• 2:21:01
  is connected. A flat piece of paper and the surface
• 2:21:05
  of a cylinder are locally identical. If you stood on
• 2:21:09
  either one and looked around, the geometry would seem the same,
• 2:21:14
  but they are globally different. On the cylinder, if you
• 2:21:19
  walk far enough in one direction, you come back to
• 2:21:23
  where you started. Physicists have asked seriously whether the universe
• 2:21:29
  might work this way, whether space folds back on itself,
• 2:21:35
  whether if you traveled in one direction for long enough,
• 2:21:38
  you would eventually re enter the observable universe from the
• 2:21:42
  opposite side. This is not a fringe idea. Several serious
• 2:21:47
  topology models have been studied. A three dimensional taurus, where
• 2:21:53
  space wraps in all three directions the way a flat
• 2:21:56
  video game screen wraps at the edges, a space shaped
• 2:22:00
  like a sockerball, a space with a more complex twist
• 2:22:04
  built into its connectivity. Each of these would leave a fingerprint.
• 2:22:10
  If the universe wraps back on itself at a scale
• 2:22:13
  smaller than the observable universe, the same structures would appear
• 2:22:18
  in multiple directions in the sky. The CMB, with its
• 2:22:23
  fine temperature fluctuations mapped in extraordinary detail by the WMAP
• 2:22:28
  and Plank satellites, would show repeated patterns matching circles echoes.
• 2:22:35
  Physicists looked for exactly these signatures. They ran careful searches
• 2:22:41
  through the CMB data for the telltale repetition that a
• 2:22:45
  compact topology would produce. Nothing turned up. If space does
• 2:22:50
  fold back on itself, the scale at which it does
• 2:22:53
  so is larger than the observable universe, which means we
• 2:22:58
  cannot detect it, which means we are back to not knowing.
• 2:23:03
  That result is worth sitting with. We check the entire
• 2:23:07
  sky with the most sensitive instruments ever built. We looked
• 2:23:12
  for the universe repeating itself inside our observable sphere and
• 2:23:17
  found no sign of it. And the honest conclusion from
• 2:23:21
  that is not that the topology is trivial. It is
• 2:23:25
  that if there is a repeating structure in space, the
• 2:23:29
  universe is simply too large for us to see it. Fold.
• 2:23:33
  The echo is there, we just cannot hear it. Remnants
• 2:23:39
  of earlier cosmic phases are a third category of strange
• 2:23:43
  geometry that might be lurking beyond the horizon. Some cosmological
• 2:23:48
  models allow for what are called domain walls, boundaries between
• 2:23:53
  regions where the universe settled into different vacuum states after inflation.
• 2:23:58
  These would be physical sir potentially enormous, separating zones of
• 2:24:04
  space with different properties. Inside our observable sphere, no such
• 2:24:10
  wall has been detected, but the absence of a wall
• 2:24:13
  inside forty six billion light years says nothing about whether
• 2:24:18
  one exists at fifty billion light years or five hundred
• 2:24:22
  billion or father. Put all of this together, and the
• 2:24:27
  picture of space beyond our horizon gets genuinely strange. We
• 2:24:32
  know the geometry is flat inside our sphere. We know
• 2:24:36
  there are three large spatial dimensions here. We know that
• 2:24:41
  apology appears trivially simple at scales we can probe beyond
• 2:24:46
  that the constraints evaporate the rules we have confirmed apply
• 2:24:51
  to the region we can observe, they are not guaranteed
• 2:24:55
  to hold in every direction forever. The universe we have
• 2:24:59
  mapped is a small, quiet, regular seeming patch, and the
• 2:25:04
  question of whether the larger structure is equally quiet, or
• 2:25:08
  curved or folded, or higher dimensional or threaded with domain
• 2:25:14
  walls from a phase transition we will never see. That
• 2:25:18
  question is open in a way that cannot be closed
• 2:25:21
  by any observation we are currently capable of making. What
• 2:25:26
  the universe looks like geometrically past the last reach of
• 2:25:30
  our instruments is an open question, and the range of
• 2:25:34
  possible answers includes things for which we do not yet
• 2:25:38
  have words. Step back from the theories and the numbers
• 2:25:42
  and the competing models, and there is a simpler way
• 2:25:45
  to say what we have been working through this entire time.
• 2:25:50
  Go outside on a clear night, pick any direction, look up.
• 2:25:56
  What you are seeing is a bubble, a sphere of
• 2:26:00
  light that managed to reach you before the universe got
• 2:26:03
  too big for it to finish the trip. Everything beyond
• 2:26:07
  that bubble exists. It is not hypothetical. It is not
• 2:26:12
  a gap in the math waiting to be filled in.
• 2:26:15
  It is just out there. Silent and unreachable, separated from
• 2:26:21
  your eyes by a speed limit that nothing in physics
• 2:26:24
  can break. The horizon you are staring past is not
• 2:26:28
  a wall someone built. It is a consequence. Light is
• 2:26:33
  fast but finite. The universe is old, but not infinitely old.
• 2:26:39
  And those two facts together draw a circle around every
• 2:26:42
  observer in existence. You happen to be one of those observers.
• 2:26:48
  So does every civilization that has ever existed or will
• 2:26:52
  ever exist anywhere in an infinite cosmos. Every single one
• 2:26:58
  of them is standing inside them their own bubble, looking
• 2:27:01
  out at their own horizon, unable to see past it.
• 2:27:06
  That is the thing that tends to get lost in
• 2:27:08
  conversations about the edge of the observable universe. People hear
• 2:27:14
  the phrase and picture something like the end of a map,
• 2:27:18
  a place marked with dragons, or a boundary beyond which
• 2:27:22
  the parchment runs out. But the horizon is not out
• 2:27:26
  there somewhere waiting to be reached. It moves with you.
• 2:27:31
  Take a spacecraft and fly toward the edge for a
• 2:27:34
  billion years at close to the speed of light, and
• 2:27:37
  when you stop, you will still be at the center
• 2:27:40
  of an observable sphere exactly the same size as the
• 2:27:44
  one you left, the horizon will have retreated in front
• 2:27:48
  of you the entire way. You cannot approach it, You
• 2:27:52
  cannot stand at it and peer over. It is not
• 2:27:57
  a location. It is a relationship between you and light,
• 2:28:01
  and it follows you everywhere you go. What the discovery
• 2:28:06
  of the cosmic horizon actually represents is not a failure
• 2:28:10
  that framing gets it backwards. It took centuries of careful observation,
• 2:28:16
  the invention of general relativity, the measurement of expansion, the
• 2:28:21
  detection of the cosmic microwave background, and the construction of
• 2:28:26
  satellites precise enough to map temperature fluctuations smaller than one
• 2:28:30
  hundred thousandth of a degree. All of that just to
• 2:28:34
  understand why we cannot see further. That is not a defeat.
• 2:28:40
  That is one of the deepest things science has ever
• 2:28:43
  worked out. We know the shape of our ignorance, we
• 2:28:47
  know its size, We know exactly why it exists, and
• 2:28:52
  can describe its geometry in equations that have been tested
• 2:28:56
  and confirmed repeatedly. Most limits on human knowledge are vague.
• 2:29:02
  This one is precise, and precision is a form of
• 2:29:05
  understanding the question. The whole script has been circling is
• 2:29:10
  what actually exists past that edge, and the answers stack
• 2:29:15
  up in layers. The first layer is the conservative one,
• 2:29:20
  more universe roughly like ours, running on the same physics,
• 2:29:25
  containing galaxies and stars and whatever happens to be orbiting them.
• 2:29:30
  The CMB says the universe is uniform at large scales,
• 2:29:35
  which means the region just beyond our horizon almost certainly
• 2:29:39
  looks like the region just inside it. Structures we recognize
• 2:29:45
  rules we already know. The second layer is stranger. If
• 2:29:51
  space is genuinely infinite, and the flatness measurement strongly suggest
• 2:29:56
  it is, then the matter out there recombined into every
• 2:30:01
  possible arrangement infinitely many times, including this one, including you
• 2:30:08
  somewhere in an infinite, flat universe. The configuration of particles
• 2:30:13
  that produced every moment of your life has repeated, will repeat,
• 2:30:18
  and is repeating right now, too far away to ever
• 2:30:21
  interact with this copy. That is not poetry. It is
• 2:30:26
  combinatorics applied to infinity. It has an unsettling mechanical quality
• 2:30:32
  that no amount of familiarity makes feel less strange. The
• 2:30:37
  third layer is where the physics gets speculative but not
• 2:30:40
  careless eternal inflation. If it is correct, means the space
• 2:30:46
  beyond our horizon is mostly still inflating, a churning background
• 2:30:52
  in which other pockets have stopped and collapsed into their
• 2:30:55
  own big bangs, other universes. Each word, the one potentially
• 2:31:01
  enormous by any measure, We have, each one potentially running
• 2:31:05
  on different physical constants, with different particle masses, different forces,
• 2:31:11
  different outcomes. The string landscape suggests there are something like
• 2:31:17
  ten to the five hundred versions of the underlying physics available,
• 2:31:22
  and eternal inflation may have sampled most of them. If
• 2:31:27
  any of this is right, what exists past the final
• 2:31:30
  edge is not just more of the same universe. It
• 2:31:34
  is a collection of outcomes so large and so varied
• 2:31:38
  that the word universe in the singular stops making sense.
• 2:31:44
  And then there is the layer that physics cannot reach
• 2:31:47
  at all. Beyond the cosmic event horizon. Information does not
• 2:31:52
  travel slowly. It does not travel not ever, not with
• 2:31:58
  infinite time, and in technology, the accelerating expansion of space
• 2:32:03
  has permanently severed the connection. Whatever is out there past
• 2:32:08
  that absolute boundary is not observable in principle, not just
• 2:32:13
  in practice. We built our entire picture of physical law
• 2:32:17
  from observations made inside this sphere, and we have no
• 2:32:21
  guarantee those laws hold beyond it. We cannot check the
• 2:32:27
  physics we use to reason about what might be out
• 2:32:30
  there was derived from data collected in here. That is
• 2:32:35
  a constraint serious enough that some physicists think the whole
• 2:32:39
  project of theorizing about regions beyond the event horizon is
• 2:32:43
  not science. Others disagree. Nobody has settled it. What is
• 2:32:50
  honest is this. The universe almost certainly extends far past
• 2:32:55
  everything we can see in every direction without an edge,
• 2:33:00
  may be populated by observers exactly like us, sealed inside
• 2:33:04
  their own horizons, looking out at their own unanswerable silence.
• 2:33:10
  They cannot contact us, We cannot contact them. Each bubble
• 2:33:15
  is its own complete epistemic universe surrounded by a reality
• 2:33:19
  it cannot access. The horizon does not mean nothing is there.
• 2:33:25
  It means the news will never arrive. The horizon is real,
• 2:33:31
  the silence beyond it is real, and somewhere on the
• 2:33:35
  other side of both, the universe keeps going.