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Islands have edges.

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Planets have edges.

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Even galaxies have edges.

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But what about the universe?

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As explorers,
as curious humans that we are,

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we're obsessed with boundaries
and limits.

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And we want to know,

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"does the whole thing,
the universe, have a limit?"

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Does the universe have an edge?
Well, the answer is yes and no.

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It depends on
what you mean by edge.

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The edge of what we can see?

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The edge of where we can go?

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Or the edge of reality itself?

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Looking out to the edge
of the universe

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is tremendously important
to understand

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our place
in the universe itself.

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We're talking
about our universe.

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We're talking about the thing
that we exist within.

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The most fundamental
thing there is,

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we're driven to understand it.

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There is always a desire to push
the knowledge to the edge.

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So, can we ever find

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the edge of the universe?

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captions paid for by
discovery communications

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2016, the Hubble space telescope

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turned toward
a dark patch of sky

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in the constellation Ursa Major.

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It captured an image
of an indistinct blob of light.

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The glow is from a distant
galaxy called GN-Z11...

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...the most distant galaxy
we've ever observed.

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But is this the edge
of the universe?

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The universe all around
us is filled with galaxies,

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so it's kind of natural to say,
"would there be a final galaxy?

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If you traveled far enough away,

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would you finally be at the very
last galaxy in the universe

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looking out into empty space?"

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That's a difficult
question to answer.

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Because there's a limit
to how far we can see.

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It all comes down
to the speed of light

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and the age of the universe.

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The key to understanding
the edge of the universe

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is that light travels
very, very fast,

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but not infinitely fast.

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It takes time for it
to get from one place

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in the universe to the other.

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You open the curtains,
light fills the room.

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It doesn't seem
to travel at all.

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But over the vast distances
of the universe,

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you actually notice
this travel time.

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Even the sun
93 million miles away,

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the light takes eight minutes
to get to us.

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When you look out at the stars,
we start to think of distance

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in terms of light-years
because it takes years

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for the light
to get from those stars to us.

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Then when you look at galaxies,
then you're talking about

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millions
or billions of light-years.

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When we look at the
light from galaxy GN-Z11,

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we're seeing light emitted
13.4 billion years ago.

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You can't really
even find a galaxy

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too much farther away than that

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because the universe
is only 13.8 billion years old,

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and it takes a certain amount of
time for galaxies to even form.

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So we're not going to find
too many more galaxies

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farther away than this.

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If things are far enough away,
there is no way that light

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can get to us
in the age of the universe.

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What this means
is there's a hard limit

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to the edge of the universe
that we can see,

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and this is set
by the age of the universe.

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GN-Z11 sparked into life

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early in the history
of the universe,

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just 400 million years
after the big bang.

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Before that, there were no stars
to send out light.

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If you look
in any direction at all,

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you get all the way back to
when there were no stars,

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no galaxies,
nothing but very, very hot gas,

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and that sort of forms
a shell around us.

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That outer shell is
the cosmic microwave background.

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It is the oldest light
in the universe,

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the echo of the birth
of the universe,

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the big bang.

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The edge of our universe,

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the very furthest thing
that we can see,

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is one of the earliest relics

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of the formation
of the universe itself.

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That is the cosmic microwave
background.

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We call this the edge
of our observable universe.

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So we have an observable
universe, but beyond that,

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even if there
are things out there,

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there's no way we can see them

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because the light just could
not have gotten to us by now.

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As the name states,
the observable universe

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is simply the part
of the universe we can see.

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We can think
of the observable universe

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sort of like a spotlight

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centered on wherever
you're standing right now

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and you can see to the edge of
your spotlight and not beyond.

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But if you move a little bit
to the left,

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a little bit to the right,

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your observable universe
actually moves with you.

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For someone living
in galaxy GN-Z11,

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a totally different part of the
universe would be observable.

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So that distant galaxy is at the
edge of our observable universe,

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and we are at the edge
of their observable universe.

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We have different spotlights.

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One of the wonderful things
to think about

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is that there are other spheres
around other galaxies,

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there are other aliens
looking up into the sky tonight

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wondering what the true
extent of the universe is.

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The true extent of our universe

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doesn't end with galaxy GN-Z11.

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But when astronomers use
the Hubble space telescope

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to accurately measure
the distance to GN-Z11,

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they find something shocking.

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It's 32 billion
light-years away,

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three times further
than thought possible.

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So if nothing can travel
faster than light

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and we measure the distance
to this galaxy,

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how can it be
32 billion light-years away?

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There hasn't been enough time

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in the history of the universe

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for light from GN-Z11
to reach us.

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There must be some mistake here.
Right?

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At this point, your brain

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is probably thinking
of leaping out

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of your skull
and running around screaming.

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Trust me, I know.
I'm an astronomer.

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I've been doing this
my whole life,

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and this stuff twists
my imagination up.

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It's really hard to grasp this.

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How do we see a galaxy that's
32 billion light-years away

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and only 13.4 billion years old?

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GN-Z11 is further away
than it should be

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because something strange
is going on with our universe.

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It's expanding.

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And if the universe
is expanding,

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then where does its edge lie

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and can we ever reach it?

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13.8 billion years ago,

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a speck of energy
burst into life.

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We call it the big bang...

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space and time
pushed out in all directions.

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Ever since, our universe
has expanded.

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But the way it's expanding
makes finding an edge

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a major challenge.

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The universe is expanding

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and expands according
to a very simple law

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that the farther away a galaxy
is from us,

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the faster it appears
to be receding away from us.

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The furthest galaxies
are moving at very high speeds.

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The most distant galaxy
we've ever spotted, GN-Z11,

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seems to have moved 32 billion
light-years away from us

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in just 13.4 billion years.

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That's faster than
the speed of light.

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We can measure the speeds
with which galaxies

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are moving away from us,

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and many, many galaxies
are moving away from us

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at speeds faster
than the speed of light.

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This sounds like
it's breaking the law, right?

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There's this idea
that you've all been told

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that relativity
says nothing goes faster

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than the speed of light.

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Okay, you've been lied to.

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Space itself can do
what it wants.

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It makes the rules,
it can break the rules.

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That rule applies to matter,
not to space itself.

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Space can expand
at whatever rate it wants.

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Simple way to think of this
expansion law is imagine

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standing on
an infinite rubber sheet

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that stretches all the way out
into the distance

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and you're standing
on the same place.

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You can mark it
with a little "X."

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Now, all the sheet expands
in every direction.

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So if it expands
by a factor of two,

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another galaxy that was, say,
one foot away from you

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is now two feet away from you
as we stretch the sheet,

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but another galaxy
was 10 feet away from you.

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Expand that by a factor of 2

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and now
it's 20 feet away from you.

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So in the same amount of time,
one galaxy moved one foot,

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where another galaxy
moved 10 feet.

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So the more stuff there is,

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the more elastic
between you and another galaxy,

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the more it seems
to expand away from you.

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Expansion means
our observable universe

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stretches for a colossal
46 billion light-years

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in all directions,
92 billion light-years across...

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...and getting bigger
by the second.

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This number is
so incomprehensibly large

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that it's difficult to wrap
your brain around.

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There are trillions of galaxies
within this volume.

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It's staggering.

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It's so much larger than
anything we're familiar with.

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If we were to travel

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to the edge
of the observable universe,

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we would enter even more
unfamiliar territory.

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Imagine we're in
an ultra-fast spaceship.

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We leave the solar system,
then the milky way.

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As we travel deeper
into intergalactic space,

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things start to get
really weird.

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For every million light-years
we go from the milky way,

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the galaxies move away from us

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at around 13 miles
per second faster.

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We have to accelerate
just to keep up.

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But the galaxies keep on moving,
always beyond our reach.

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Imagine you're a sprinter
on a racetrack.

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If you're running towards
the finish line,

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it may take you a few seconds
to cross it.

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But now imagine that that finish
line is moving away from you.

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If it's moving away from you at
the same speed you're running,

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you'll never reach it.

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And if it's moving faster
than the runner,

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then even faster runners
won't reach it.

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And that's sort of what we're
seeing here with the universe.

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Beyond a certain distance,

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galaxies are racing away from us
faster than the speed of light.

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It's a line called
the cosmic event horizon.

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And 97% of galaxies we see
in the observable universe

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00:13:05,500 --> 00:13:08,846
are beyond this line
and unreachable,

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including GN-Z11.

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They're sort of teasing us
to say, "look at me,

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what a nice piece
of real estate."

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But we know even if we started
going there now,

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we could never reach them.

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Anything that has
crossed the cosmic event horizon

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is out of our reach forever.

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But that's not the full picture

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because the expansion rate
of the universe is changing.

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A little over 20 years ago,
astronomers discovered

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that the current rate
of the universe's expansion

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is accelerating,
it's speeding up.

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Astronomers suspect
a mysterious force is at work...

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dark energy.

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Dark energy is what we think
is pushing the universe apart,

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causing this
accelerating expansion.

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And the origin and true
physical nature of dark energy

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is a big mystery.

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Thanks to dark energy,
more and more galaxies

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are crossing
the cosmic event horizon

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and leaving
the observable universe.

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These galaxies...

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Are lost to us forever.

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00:14:27,480 --> 00:14:30,296
There are galaxies
that we can see today

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00:14:30,320 --> 00:14:33,796
that in a few million years,
say, we won't be able to see

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00:14:33,820 --> 00:14:35,666
because the edge of
the observable universe

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has basically moved in closer
than that galaxy.

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That's going to happen
all the time.

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And in a trillion years
or something like that,

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00:14:43,400 --> 00:14:45,446
all these galaxies
that we see in our sky

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00:14:45,470 --> 00:14:46,976
will be completely invisible

251
00:14:47,000 --> 00:14:50,070
because they'll be beyond
the edge of the universe.

252
00:14:53,140 --> 00:14:55,926
So eventually, every last galaxy

253
00:14:55,950 --> 00:14:57,726
will be so far away from us

254
00:14:57,750 --> 00:15:01,326
that light cannot reach us
through that expanding space.

255
00:15:01,350 --> 00:15:04,096
It's almost as if you're driving
through a dark desert

256
00:15:04,120 --> 00:15:06,836
in your car
and the very, very last town

257
00:15:06,860 --> 00:15:08,696
that ever exists
has gone over the horizon

258
00:15:08,720 --> 00:15:10,990
and they'll never be
any light again.

259
00:15:14,360 --> 00:15:16,806
We can see less
and less of the universe

260
00:15:16,830 --> 00:15:18,976
as we go into the future.

261
00:15:19,000 --> 00:15:21,016
What a strange thought.

262
00:15:21,040 --> 00:15:25,210
So that means we should build
all the telescopes we can now.

263
00:15:27,240 --> 00:15:29,886
There's a limit
to the universe we can see,

264
00:15:29,910 --> 00:15:33,726
even with the most
advanced telescopes.

265
00:15:33,750 --> 00:15:36,226
But what lies beyond

266
00:15:36,250 --> 00:15:39,150
is one of the biggest
mysteries in astronomy.

267
00:15:41,260 --> 00:15:44,506
The greater universe
could be stranger

268
00:15:44,530 --> 00:15:46,630
than our wildest imagination.

269
00:15:49,000 --> 00:15:50,176
When you stand on the beach

270
00:15:50,200 --> 00:15:51,406
and you look at the horizon

271
00:15:51,430 --> 00:15:53,816
and you kind of think,
"oh, what beautiful lands

272
00:15:53,840 --> 00:15:55,276
are there beyond the horizon?

273
00:15:55,300 --> 00:15:58,586
Things I've never
imagined before."

274
00:15:58,610 --> 00:15:59,886
It's so natural.

275
00:15:59,910 --> 00:16:03,286
It's so human to ask,
"what lies beyond that?

276
00:16:03,310 --> 00:16:05,850
What is the true extent
of the universe?"

277
00:16:21,160 --> 00:16:23,146
The observable universe

278
00:16:23,170 --> 00:16:25,370
contains trillions of galaxies.

279
00:16:27,940 --> 00:16:31,140
It's about 92 billion
light-years across...

280
00:16:33,380 --> 00:16:34,886
...but astronomers believe

281
00:16:34,910 --> 00:16:37,780
this isn't the full extent
of the universe.

282
00:16:41,020 --> 00:16:44,266
What we don't know is how much
of the universe

283
00:16:44,290 --> 00:16:46,266
is our observable universe.

284
00:16:46,290 --> 00:16:49,306
It could be a tiny,
microscopic speck of this

285
00:16:49,330 --> 00:16:51,766
much more vast universe.

286
00:16:51,790 --> 00:16:53,836
We just don't know.

287
00:16:53,860 --> 00:16:56,106
We have no idea
how much stuff there is

288
00:16:56,130 --> 00:16:57,576
outside the observable universe,

289
00:16:57,600 --> 00:17:01,416
but because by definition, it's
outside the observable universe,

290
00:17:01,440 --> 00:17:04,216
we really don't know right now.

291
00:17:04,240 --> 00:17:06,286
So what is out there?

292
00:17:06,310 --> 00:17:10,686
One theory says that space
outside the observable universe

293
00:17:10,710 --> 00:17:16,126
is pretty much the same
as our own cosmic neighborhood.

294
00:17:16,150 --> 00:17:18,626
It's just more universe.
It's just like here.

295
00:17:18,650 --> 00:17:21,236
It's just far enough away
that we can't see it.

296
00:17:21,260 --> 00:17:24,706
So it's not like there's bizarre
places where time runs backwards

297
00:17:24,730 --> 00:17:27,090
or aliens have two heads,
well, yeah, maybe.

298
00:17:29,570 --> 00:17:30,946
But further out

299
00:17:30,970 --> 00:17:33,446
in the deepest parts
of the greater universe,

300
00:17:33,470 --> 00:17:35,400
all bets are off.

301
00:17:37,670 --> 00:17:40,556
We expect that as you go sort of
twice or three times

302
00:17:40,580 --> 00:17:42,056
beyond the observable universe,

303
00:17:42,080 --> 00:17:44,926
it's probably very similar
to the universe we inhabit.

304
00:17:44,950 --> 00:17:48,426
But if you go a thousand times
or a million times farther,

305
00:17:48,450 --> 00:17:50,150
who knows what you might see?

306
00:17:52,190 --> 00:17:55,220
It might be very, very different
if we go far enough away.

307
00:17:57,860 --> 00:17:59,776
Strangely, it all comes back

308
00:17:59,800 --> 00:18:02,206
to the expansion of the universe

309
00:18:02,230 --> 00:18:05,400
and one crucial detail
in that process.

310
00:18:08,070 --> 00:18:09,146
There was a brief moment

311
00:18:09,170 --> 00:18:11,286
in the very early history
of the universe

312
00:18:11,310 --> 00:18:15,316
where its expansion
accelerated hugely.

313
00:18:15,340 --> 00:18:17,386
This acceleration
is called inflation,

314
00:18:17,410 --> 00:18:20,826
and in a brief moment,
the universe itself expanded

315
00:18:20,850 --> 00:18:23,550
at multiple times
the speed of light.

316
00:18:25,820 --> 00:18:29,436
Inflation was a formative moment

317
00:18:29,460 --> 00:18:31,006
for our universe.

318
00:18:31,030 --> 00:18:32,476
By the time it stopped,

319
00:18:32,500 --> 00:18:35,530
the universe's basic
characteristics were set.

320
00:18:37,930 --> 00:18:39,846
There are these
fundamental constants

321
00:18:39,870 --> 00:18:42,586
that describe the phenomena
in our universe,

322
00:18:42,610 --> 00:18:46,510
the fundamentals of matter
and light and space-time.

323
00:18:48,140 --> 00:18:49,786
But some scientists think

324
00:18:49,810 --> 00:18:52,386
there could be regions
of the greater universe

325
00:18:52,410 --> 00:18:54,880
where inflation
has never stopped.

326
00:18:57,150 --> 00:19:02,236
The idea is the greater universe
is expanding at an insane speed,

327
00:19:02,260 --> 00:19:05,406
but here and there,
occasionally a little region

328
00:19:05,430 --> 00:19:10,746
will stop inflating and
just expand at the normal rate.

329
00:19:10,770 --> 00:19:12,746
Inflation can end somewhere

330
00:19:12,770 --> 00:19:15,016
and that gives rise
to the universe we live in,

331
00:19:15,040 --> 00:19:18,946
while inflation
continues somewhere else.

332
00:19:18,970 --> 00:19:21,056
Parts of the greater universe

333
00:19:21,080 --> 00:19:22,356
that continued to inflate

334
00:19:22,380 --> 00:19:25,650
would be left
with different laws of physics.

335
00:19:27,280 --> 00:19:29,326
This incredibly violent
inflation process

336
00:19:29,350 --> 00:19:33,366
is actually monkeyed with
the very fabric of space itself,

337
00:19:33,390 --> 00:19:35,266
so that a lot of the things
that we were taught

338
00:19:35,290 --> 00:19:39,976
that are laws of physics
are different there.

339
00:19:40,000 --> 00:19:41,276
So in an essence,

340
00:19:41,300 --> 00:19:43,176
inflation gives us
a very natural way

341
00:19:43,200 --> 00:19:46,076
to make this patchwork quilt of
different parts of the universe

342
00:19:46,100 --> 00:19:48,946
where things seem different.

343
00:19:48,970 --> 00:19:52,086
So what we could imagine
is a super large-scale structure

344
00:19:52,110 --> 00:19:55,656
where there's different regions
of the universe, domains,

345
00:19:55,680 --> 00:19:58,980
and each domain has different
local laws of physics.

346
00:20:01,950 --> 00:20:03,466
These different parts
of the universe

347
00:20:03,490 --> 00:20:07,550
are separated by frontiers
called domain walls.

348
00:20:11,360 --> 00:20:14,176
We have similar boundaries
on earth.

349
00:20:14,200 --> 00:20:17,006
Whenever you have something that
can be in many different states,

350
00:20:17,030 --> 00:20:19,876
you can end up with domain wall.

351
00:20:19,900 --> 00:20:25,516
If our fish swimming around
in the arctic near an iceberg,

352
00:20:25,540 --> 00:20:27,216
there will be a domain boundary

353
00:20:27,240 --> 00:20:29,486
between the water
being in the liquid state

354
00:20:29,510 --> 00:20:33,296
where I am and the solid state
inside the ice.

355
00:20:33,320 --> 00:20:36,866
So a domain wall is just a wall
between two domains.

356
00:20:36,890 --> 00:20:38,826
If it's water,
this could be ice,

357
00:20:38,850 --> 00:20:39,896
this could be liquid.

358
00:20:39,920 --> 00:20:41,336
If you're talking about space,

359
00:20:41,360 --> 00:20:43,906
this could be a kind of space
maybe you can live in.

360
00:20:43,930 --> 00:20:47,706
This could be a kind of space
where you don't want to be.

361
00:20:47,730 --> 00:20:51,606
Crossing a domain wall
would be very bad news

362
00:20:51,630 --> 00:20:54,376
for anyone who dared to try.

363
00:20:54,400 --> 00:20:57,016
Cross that domain wall
and the laws of physics

364
00:20:57,040 --> 00:20:59,316
could change dramatically.

365
00:20:59,340 --> 00:21:01,316
The number of dimensions
could change.

366
00:21:01,340 --> 00:21:03,926
If we were somehow able to
travel to places in the universe

367
00:21:03,950 --> 00:21:05,326
where the laws of physics
are different,

368
00:21:05,350 --> 00:21:07,696
we would die

369
00:21:07,720 --> 00:21:10,496
because all of the chemistry
going on in our bodies

370
00:21:10,520 --> 00:21:15,096
depends very, very sensitively
on the laws of physics.

371
00:21:15,120 --> 00:21:19,060
So you could just dissipate like
Thanos snap and you're gone.

372
00:21:21,060 --> 00:21:22,746
Domain walls
might be the closest

373
00:21:22,770 --> 00:21:25,400
we get to locating
an edge in the universe.

374
00:21:26,970 --> 00:21:29,316
Depends on how
you define the edge.

375
00:21:29,340 --> 00:21:32,686
If it is the realm where the
laws of our universe operate,

376
00:21:32,710 --> 00:21:36,640
then these domain walls are in
essence the age of the universe.

377
00:21:40,780 --> 00:21:42,720
But this is all just theory.

378
00:21:46,590 --> 00:21:48,396
If we ever really are
to work out

379
00:21:48,420 --> 00:21:50,866
what the true size
and shape of the universe is,

380
00:21:50,890 --> 00:21:54,536
we're going to have to look
for clues that are close to us.

381
00:21:54,560 --> 00:21:58,046
Clues that could
answer the ultimate question,

382
00:21:58,070 --> 00:22:01,946
how big is the rest
of the greater universe

383
00:22:01,970 --> 00:22:04,840
and could it go on forever?

384
00:22:20,420 --> 00:22:22,806
For tens of thousands of years,

385
00:22:22,830 --> 00:22:28,136
mankind has gazed in wonder
at the vastness of the cosmos,

386
00:22:28,160 --> 00:22:32,476
but just how extensive is it?

387
00:22:32,500 --> 00:22:34,176
If we could answer
that question,

388
00:22:34,200 --> 00:22:39,356
it might help us to understand
our place in the universe.

389
00:22:39,380 --> 00:22:41,686
One of the fundamental
questions in science

390
00:22:41,710 --> 00:22:44,986
is how big is the universe?

391
00:22:45,010 --> 00:22:47,726
To answer the question,
"how big is the universe?"

392
00:22:47,750 --> 00:22:51,066
We have to answer the question,
"what shape is the universe?"

393
00:22:51,090 --> 00:22:52,996
And by shape, I mean geometry.

394
00:22:53,020 --> 00:22:57,766
I mean, how is the universe
curved on its largest scales?

395
00:22:57,790 --> 00:23:00,176
If we are to discover
that the universe

396
00:23:00,200 --> 00:23:04,206
does have some
sort of geometric curvature,

397
00:23:04,230 --> 00:23:09,746
then this might imply that it
wraps around in on itself

398
00:23:09,770 --> 00:23:12,386
over incredibly large distances.

399
00:23:12,410 --> 00:23:16,126
And that if you could travel
in one direction long enough,

400
00:23:16,150 --> 00:23:18,126
you would end up
at your starting point.

401
00:23:18,150 --> 00:23:19,796
Another version is
that the universe

402
00:23:19,820 --> 00:23:22,626
is more like
an infinite flat plane.

403
00:23:22,650 --> 00:23:25,126
Okay?
No curvature at all.

404
00:23:25,150 --> 00:23:27,666
The further you travel,
well, the further you get

405
00:23:27,690 --> 00:23:30,560
and you never get back
to where you started.

406
00:23:32,800 --> 00:23:34,376
To work out the shape
of something,

407
00:23:34,400 --> 00:23:37,476
we would normally just step back
and take a look.

408
00:23:37,500 --> 00:23:42,746
But clearly moving outside
the universe is a nonstarter.

409
00:23:42,770 --> 00:23:44,146
You can't jump on a rocket

410
00:23:44,170 --> 00:23:47,386
and fly a thousand times larger
than our cosmic horizon

411
00:23:47,410 --> 00:23:49,216
and see what the shape
of the universe is.

412
00:23:49,240 --> 00:23:51,126
We just can't do that.

413
00:23:51,150 --> 00:23:54,156
Our human perspective
on the larger universe

414
00:23:54,180 --> 00:23:55,456
is so limited.

415
00:23:55,480 --> 00:23:57,596
So if we want to figure out
what the larger shape

416
00:23:57,620 --> 00:23:59,596
and scale of the universe is,

417
00:23:59,620 --> 00:24:01,960
we're going to have to be
very clever indeed.

418
00:24:04,790 --> 00:24:06,576
One way to be clever

419
00:24:06,600 --> 00:24:08,976
is to think of the geometry
of the universe

420
00:24:09,000 --> 00:24:11,746
in its simplest terms.

421
00:24:11,770 --> 00:24:14,076
When we talk about
the geometry of the universe,

422
00:24:14,100 --> 00:24:16,616
we really are talking
about geometry.

423
00:24:16,640 --> 00:24:19,786
In order to do geometry,
you have to take measures.

424
00:24:19,810 --> 00:24:21,986
You need a cosmic ruler
to do this,

425
00:24:22,010 --> 00:24:24,386
and it turns out
there's a great cosmic ruler

426
00:24:24,410 --> 00:24:27,250
known as baryon
acoustic oscillations.

427
00:24:29,690 --> 00:24:32,426
Baryonic acoustic
oscillations are ripples

428
00:24:32,450 --> 00:24:36,166
in the cosmic
microwave background,

429
00:24:36,190 --> 00:24:38,660
the oldest light
in the universe.

430
00:24:40,960 --> 00:24:42,676
As the universe expanded,

431
00:24:42,700 --> 00:24:47,300
these ripples were imprinted
in space in a uniform way.

432
00:24:48,770 --> 00:24:50,646
They provide a cosmic ruler

433
00:24:50,670 --> 00:24:53,856
to measure vast
distances over time,

434
00:24:53,880 --> 00:24:56,186
so we can gauge
if the universe expands

435
00:24:56,210 --> 00:24:59,910
in curved space
or over a flat plane.

436
00:25:02,820 --> 00:25:04,826
When we use these cosmic rulers

437
00:25:04,850 --> 00:25:07,096
to try to back out
the shape of the universe,

438
00:25:07,120 --> 00:25:09,336
we're sure
to a few percent accuracy

439
00:25:09,360 --> 00:25:12,936
that the universe is flat.

440
00:25:12,960 --> 00:25:15,076
If the universe is flat,

441
00:25:15,100 --> 00:25:17,846
we could set off
traveling into the cosmos

442
00:25:17,870 --> 00:25:20,570
and continue traveling forever.

443
00:25:22,670 --> 00:25:26,356
There may be no edge
to our universe

444
00:25:26,380 --> 00:25:30,926
because a flat universe
can be an infinite universe.

445
00:25:30,950 --> 00:25:32,956
Now we're thinking of
the universe as something

446
00:25:32,980 --> 00:25:34,826
that really does go on forever,

447
00:25:34,850 --> 00:25:38,866
that the stars and galaxies
never have an end,

448
00:25:38,890 --> 00:25:42,966
and how can something
truly infinite really exist?

449
00:25:42,990 --> 00:25:47,706
Infinity is weird
because it's a concept of,

450
00:25:47,730 --> 00:25:50,046
because it's endless.

451
00:25:50,070 --> 00:25:52,800
What does that mean? Who knows?
I don't know.

452
00:25:55,500 --> 00:25:59,046
Infinity is a concept
more than anything else.

453
00:25:59,070 --> 00:26:01,016
Our brains aren't
evolved for that.

454
00:26:01,040 --> 00:26:03,786
We evolved living in the plains.

455
00:26:03,810 --> 00:26:06,426
We were apes looking for food.

456
00:26:06,450 --> 00:26:09,426
We weren't evolved to think
about the universe

457
00:26:09,450 --> 00:26:12,166
and all of this stuff.

458
00:26:12,190 --> 00:26:14,566
I just can't stop
contemplating this stuff.

459
00:26:14,590 --> 00:26:17,466
The idea of infinity
and these large numbers

460
00:26:17,490 --> 00:26:19,506
and even the tininess
of everything.

461
00:26:19,530 --> 00:26:20,976
It's nuts.

462
00:26:21,000 --> 00:26:24,130
Yeah, thinking about infinity
makes my head hurt a little bit.

463
00:26:28,440 --> 00:26:31,586
An infinite universe
has profound implications

464
00:26:31,610 --> 00:26:34,310
for understanding our place
in the cosmos.

465
00:26:36,380 --> 00:26:39,310
It guarantees we're not alone.

466
00:26:40,920 --> 00:26:42,926
But if the universe is infinite,

467
00:26:42,950 --> 00:26:44,866
then there could be
an infinite number of galaxies

468
00:26:44,890 --> 00:26:49,296
that have planets with life,
an infinite number without life,

469
00:26:49,320 --> 00:26:51,636
then because life
did appear here on earth,

470
00:26:51,660 --> 00:26:53,076
it's physically possible,

471
00:26:53,100 --> 00:26:54,606
therefore,
it will definitely happen

472
00:26:54,630 --> 00:26:57,076
elsewhere in the universe.

473
00:26:57,100 --> 00:26:58,406
In a flat universe,

474
00:26:58,430 --> 00:27:03,386
alien life could come in
an infinite number of forms,

475
00:27:03,410 --> 00:27:07,616
but there is an altogether
stranger guarantee.

476
00:27:07,640 --> 00:27:10,726
If the universe has no edge,
this means that

477
00:27:10,750 --> 00:27:14,680
things that seem like they are
impossible become possible.

478
00:27:17,390 --> 00:27:20,236
Every possible arrangement
of matter,

479
00:27:20,260 --> 00:27:23,236
every possible history.

480
00:27:23,260 --> 00:27:27,706
A galaxy of a solar system
of a planet like earth

481
00:27:27,730 --> 00:27:28,976
is possible

482
00:27:29,000 --> 00:27:32,576
and is happening right now
in parallel to us

483
00:27:32,600 --> 00:27:35,070
somewhere over there.

484
00:27:36,610 --> 00:27:39,486
So that means that there
has to be another place

485
00:27:39,510 --> 00:27:42,956
that has a galaxy just like ours

486
00:27:42,980 --> 00:27:44,856
and it would have an earth
just like ours.

487
00:27:44,880 --> 00:27:49,696
It would have people who would
have another version of you,

488
00:27:49,720 --> 00:27:51,166
another version of me.

489
00:27:51,190 --> 00:27:55,366
It's 100% guaranteed that there
is another Max Tegmark out there

490
00:27:55,390 --> 00:27:58,790
having exactly this conversation
and in fact many of them.

491
00:28:00,860 --> 00:28:03,546
An infinite universe
full of Max Tegmarks

492
00:28:03,570 --> 00:28:05,606
may be a strange concept,

493
00:28:05,630 --> 00:28:08,146
but what's truly mind-bending
is understanding

494
00:28:08,170 --> 00:28:11,216
the physics of a flat universe.

495
00:28:11,240 --> 00:28:14,556
If the universe is infinite
and it's expanding,

496
00:28:14,580 --> 00:28:20,196
what's it expanding into
and what did it expand from?

497
00:28:20,220 --> 00:28:24,626
Was there ever an edge
to the universe?

498
00:28:24,650 --> 00:28:28,796
Fortunately, the answer is that
it doesn't make sense

499
00:28:28,820 --> 00:28:30,766
to ask that question.

500
00:28:30,790 --> 00:28:33,076
Everything is expanding,

501
00:28:33,100 --> 00:28:36,106
including the universe
that we exist within.

502
00:28:36,130 --> 00:28:39,076
So in fact, it's not expanding
into anything

503
00:28:39,100 --> 00:28:40,700
because it is everything.

504
00:28:45,670 --> 00:28:47,756
To help understand
what's going on

505
00:28:47,780 --> 00:28:49,356
in an infinite universe,

506
00:28:49,380 --> 00:28:51,610
we need to go back
to the big bang.

507
00:28:54,280 --> 00:28:57,596
We want to think of the big bang
as an explosion in space,

508
00:28:57,620 --> 00:28:59,326
like it happened some place,

509
00:28:59,350 --> 00:29:02,236
but there wasn't any place
before the big bang.

510
00:29:02,260 --> 00:29:05,506
Space existed inside
of the big bang itself.

511
00:29:05,530 --> 00:29:07,776
So it's not an explosion
in space,

512
00:29:07,800 --> 00:29:10,276
it's an explosion of space.

513
00:29:10,300 --> 00:29:12,346
We're sometimes told that
at the big bang,

514
00:29:12,370 --> 00:29:15,776
the universe started out very,
very small and then got big,

515
00:29:15,800 --> 00:29:19,716
but how can a finite point
become infinite?

516
00:29:19,740 --> 00:29:21,756
Well, if the universe
is infinite,

517
00:29:21,780 --> 00:29:25,526
then it was also infinite
at the big bang.

518
00:29:25,550 --> 00:29:27,826
This is a tough thing
to think about.

519
00:29:27,850 --> 00:29:29,296
Think about it this way.

520
00:29:29,320 --> 00:29:31,896
In an infinite universe,
the galaxies go on forever

521
00:29:31,920 --> 00:29:35,566
and now there's a great
distance between every galaxy.

522
00:29:35,590 --> 00:29:36,606
But once upon a time,

523
00:29:36,630 --> 00:29:38,106
the galaxies
were closer together,

524
00:29:38,130 --> 00:29:40,706
say half their current
distance apart,

525
00:29:40,730 --> 00:29:42,076
but they still went on forever.

526
00:29:42,100 --> 00:29:45,746
The universe was still infinite.

527
00:29:45,770 --> 00:29:47,176
In a flat universe,

528
00:29:47,200 --> 00:29:51,846
space was infinite
from the beginning.

529
00:29:51,870 --> 00:29:54,356
There was never
a single point in space

530
00:29:54,380 --> 00:29:56,986
where the big bang happened.

531
00:29:57,010 --> 00:30:00,596
It happened everywhere.

532
00:30:00,620 --> 00:30:05,596
An infinite universe offers
infinite possibilities

533
00:30:05,620 --> 00:30:08,050
but no edge to space.

534
00:30:11,190 --> 00:30:14,106
But there may be
another kind of edge,

535
00:30:14,130 --> 00:30:18,670
one that will only reveal itself
if the universe dies.

536
00:30:37,420 --> 00:30:42,336
We live in an infinite
and expanding universe.

537
00:30:42,360 --> 00:30:46,030
Space has no edge.
It goes on forever.

538
00:30:51,030 --> 00:30:55,916
But there could be a different
kind of edge to our universe,

539
00:30:55,940 --> 00:30:59,046
an edge of time.

540
00:30:59,070 --> 00:31:01,186
The universe seems to have begun

541
00:31:01,210 --> 00:31:03,256
13.8 billion years ago
in the past,

542
00:31:03,280 --> 00:31:06,356
so there's some inclination,
some impression,

543
00:31:06,380 --> 00:31:08,156
that it's finite in time.

544
00:31:08,180 --> 00:31:12,026
What we call the big bang is,
as far as we understand it,

545
00:31:12,050 --> 00:31:14,636
a beginning,
a start of the universe.

546
00:31:14,660 --> 00:31:16,906
The universe has a finite age.

547
00:31:16,930 --> 00:31:20,636
Now, does it have an edge
in the future?

548
00:31:20,660 --> 00:31:22,536
We used to think
that time would someday

549
00:31:22,560 --> 00:31:25,146
come to a catastrophic end,

550
00:31:25,170 --> 00:31:28,516
along with the planets,
galaxies,

551
00:31:28,540 --> 00:31:30,770
and all life in the universe.

552
00:31:33,440 --> 00:31:35,686
If we know there's a big bang,
if we know the universe started,

553
00:31:35,710 --> 00:31:38,526
it expanded and cooled,
it's very natural to wonder

554
00:31:38,550 --> 00:31:40,756
whether or not someday
the expansion will stop,

555
00:31:40,780 --> 00:31:44,020
reverse, and come back,
and that's a big crunch.

556
00:31:49,120 --> 00:31:50,336
In a big crunch,

557
00:31:50,360 --> 00:31:53,776
our expanding universe
would begin to contract.

558
00:31:53,800 --> 00:31:56,700
Stars and planets would smash
into each other.

559
00:31:58,870 --> 00:32:02,116
Galaxies would collide,

560
00:32:02,140 --> 00:32:05,386
and all of the life left
in space would be compressed

561
00:32:05,410 --> 00:32:08,940
with all the other matter
into a singularity.

562
00:32:17,590 --> 00:32:20,266
If this theory is true,
then the universe

563
00:32:20,290 --> 00:32:25,736
would have both a beginning
and an end of time.

564
00:32:25,760 --> 00:32:27,936
If we live in a universe
that will expand,

565
00:32:27,960 --> 00:32:30,876
stop expanding,
and then go back into a crunch,

566
00:32:30,900 --> 00:32:34,276
then it has,
in effect, two edges.

567
00:32:34,300 --> 00:32:38,646
But there's a much
stranger possibility.

568
00:32:38,670 --> 00:32:41,686
Perhaps the end is
but a beginning,

569
00:32:41,710 --> 00:32:43,786
where the universe
is a oscillating universe.

570
00:32:43,810 --> 00:32:45,686
It has a big bang
like beginning,

571
00:32:45,710 --> 00:32:47,426
it expands to a maximum size,

572
00:32:47,450 --> 00:32:49,426
and then goes back
into a big crunch

573
00:32:49,450 --> 00:32:51,780
and does that over and over.

574
00:32:53,520 --> 00:32:55,536
We could be residents
of a universe

575
00:32:55,560 --> 00:32:58,706
created from the ashes
of another,

576
00:32:58,730 --> 00:33:03,430
a single universe in a stream
of bouncing universes...

577
00:33:06,570 --> 00:33:11,446
...each full of galaxies,
planets, and life.

578
00:33:11,470 --> 00:33:14,986
But our most recent observations
of the universe suggest

579
00:33:15,010 --> 00:33:17,940
a big crunch isn't in the cards.

580
00:33:20,120 --> 00:33:23,580
Once again, dark energy is key.

581
00:33:26,250 --> 00:33:30,336
For a while we didn't know
if the expansion of the universe

582
00:33:30,360 --> 00:33:32,666
was going to slow, stop,

583
00:33:32,690 --> 00:33:35,306
and reverse itself
because of gravity.

584
00:33:35,330 --> 00:33:36,706
There are all these galaxies
in the universe

585
00:33:36,730 --> 00:33:38,846
and they're pulling on
each other by their gravity.

586
00:33:38,870 --> 00:33:40,516
And if the expansion
isn't fast enough,

587
00:33:40,540 --> 00:33:43,146
that gravity might be strong
enough to stop the expansion

588
00:33:43,170 --> 00:33:44,916
and re-collapse the universe.

589
00:33:44,940 --> 00:33:46,216
Now with dark energy,

590
00:33:46,240 --> 00:33:48,656
we know that there's no way
that can happen.

591
00:33:48,680 --> 00:33:51,186
The universe is going
to expand forever

592
00:33:51,210 --> 00:33:55,026
because dark energy is
pumping it full of acceleration.

593
00:33:55,050 --> 00:33:57,526
In order for there
to be a big crunch,

594
00:33:57,550 --> 00:34:01,136
our understanding of dark energy
would have to change a lot.

595
00:34:01,160 --> 00:34:03,736
That is, dark energy would have
to be extremely weird

596
00:34:03,760 --> 00:34:07,906
and turn off in some very funny
way for the universe

597
00:34:07,930 --> 00:34:10,930
to suddenly stop expanding
and re-collapse.

598
00:34:13,270 --> 00:34:14,876
Without a big crunch,

599
00:34:14,900 --> 00:34:19,246
there is no future edge to time.

600
00:34:19,270 --> 00:34:20,956
The universe is not
only expanding,

601
00:34:20,980 --> 00:34:22,686
but it's being driven
by dark energy

602
00:34:22,710 --> 00:34:26,056
to expand faster and faster
and the dark energy

603
00:34:26,080 --> 00:34:28,526
doesn't dilute away,
as far as we can tell.

604
00:34:28,550 --> 00:34:30,996
So the simplest idea
is that the universe

605
00:34:31,020 --> 00:34:34,750
will simply continue to expand
eternally toward the future.

606
00:34:36,930 --> 00:34:41,706
Just like space,
time will go on forever.

607
00:34:41,730 --> 00:34:46,106
That might sound like a better
fate for life and the universe,

608
00:34:46,130 --> 00:34:47,770
but it's not.

609
00:34:50,040 --> 00:34:53,046
One of the consequences
of this dark energy

610
00:34:53,070 --> 00:34:56,016
that's causing the acceleration
of the universe

611
00:34:56,040 --> 00:35:01,596
is that we eventually are
headed towards the big chill.

612
00:35:01,620 --> 00:35:05,296
I should say, "we're eventually
headed towards the big chill."

613
00:35:05,320 --> 00:35:08,436
So the universe is getting
colder and colder

614
00:35:08,460 --> 00:35:11,436
and things are getting more
and more spread out.

615
00:35:11,460 --> 00:35:13,736
So the accelerated and continual

616
00:35:13,760 --> 00:35:16,006
and forever expansion
of our universe

617
00:35:16,030 --> 00:35:19,706
might make for a frankly
depressing end to time itself.

618
00:35:19,730 --> 00:35:23,246
The ultimate entropy-based
heat death of the universe

619
00:35:23,270 --> 00:35:27,856
where you would walk out
and see no stars in the sky,

620
00:35:27,880 --> 00:35:29,516
see absolutely nothing.

621
00:35:29,540 --> 00:35:32,586
There will come one day when the
very last star in the universe

622
00:35:32,610 --> 00:35:35,450
just fizzles out and that is it.

623
00:35:38,320 --> 00:35:40,296
In the future,

624
00:35:40,320 --> 00:35:44,066
space will be a cold, dark
and infinite void,

625
00:35:44,090 --> 00:35:47,836
where time goes on forever.

626
00:35:47,860 --> 00:35:49,736
There will be nothing to do

627
00:35:49,760 --> 00:35:53,206
but suffer
in the eternal expanse.

628
00:35:53,230 --> 00:35:56,886
It's our inevitable fate
that there's no future edge

629
00:35:56,910 --> 00:36:00,816
of time in the universe.

630
00:36:00,840 --> 00:36:04,126
But even if there isn't an edge
to the universe,

631
00:36:04,150 --> 00:36:09,856
could there be edges
within the universe?

632
00:36:09,880 --> 00:36:12,396
If you wanted to visit the edge
of the universe,

633
00:36:12,420 --> 00:36:15,836
then go find your nearest
black hole and jump on in

634
00:36:15,860 --> 00:36:18,006
because that's a one-way trip.

635
00:36:18,030 --> 00:36:21,830
If you cross this
edge, you will never return.

636
00:36:36,180 --> 00:36:38,286
April 2019,

637
00:36:38,310 --> 00:36:40,496
an international team
of astronomers

638
00:36:40,520 --> 00:36:42,856
makes a special announcement.

639
00:36:42,880 --> 00:36:46,466
And we are delighted to be able
to report to you today

640
00:36:46,490 --> 00:36:51,536
that we have seen and taken
a picture of a black hole.

641
00:36:51,560 --> 00:36:52,830
Here it is.

642
00:36:59,000 --> 00:37:02,246
It's a picture
of a supermassive black hole

643
00:37:02,270 --> 00:37:07,340
at the center of the M87 galaxy
54 million light-years away.

644
00:37:09,180 --> 00:37:14,150
It may also be the first image
of an edge in the universe.

645
00:37:15,420 --> 00:37:17,996
Black holes create
a really interesting scenario

646
00:37:18,020 --> 00:37:22,896
when we think about space
and the universe having edges.

647
00:37:22,920 --> 00:37:27,836
The edge between space
outside and inside a black hole

648
00:37:27,860 --> 00:37:31,106
is called the event horizon.

649
00:37:31,130 --> 00:37:32,576
The event horizon
of a black hole

650
00:37:32,600 --> 00:37:36,146
is a region within which,
once you cross inside,

651
00:37:36,170 --> 00:37:38,746
the gravitational tug
is so strong

652
00:37:38,770 --> 00:37:43,386
that even light cannot escape,
which means nothing can escape

653
00:37:43,410 --> 00:37:46,286
once you cross
inside the event horizon.

654
00:37:46,310 --> 00:37:48,656
So that really
is sort of an edge

655
00:37:48,680 --> 00:37:51,250
because it really does
create a boundary.

656
00:37:56,320 --> 00:37:59,730
The event horizon is
not a physical barrier in space.

657
00:38:01,930 --> 00:38:04,306
Event horizon is an edge
of the part of the universe

658
00:38:04,330 --> 00:38:06,276
we can visit,
but it's not an edge

659
00:38:06,300 --> 00:38:09,176
in the sense
that there's anything there.

660
00:38:09,200 --> 00:38:10,546
You would just pass
right through it

661
00:38:10,570 --> 00:38:12,716
if you actually got right up
to that place.

662
00:38:12,740 --> 00:38:15,156
So it's sort of
a conceptual boundary

663
00:38:15,180 --> 00:38:19,126
between two different parts
of the universe.

664
00:38:19,150 --> 00:38:22,526
If we sent a man to
probe into a black hole,

665
00:38:22,550 --> 00:38:24,320
it would be a one-way trip.

666
00:38:26,750 --> 00:38:31,536
The event horizons of
black holes are a sort of edge

667
00:38:31,560 --> 00:38:34,606
because, once you pass through
an event horizon,

668
00:38:34,630 --> 00:38:38,206
you are cut off
from the rest of the universe.

669
00:38:38,230 --> 00:38:40,776
You can never go back out.

670
00:38:40,800 --> 00:38:45,146
You are outside of our universe.

671
00:38:45,170 --> 00:38:46,986
Once you've crossed
inside that region,

672
00:38:47,010 --> 00:38:50,380
you are never coming back out,
and that's an edge.

673
00:38:52,210 --> 00:38:54,196
Once inside the black hole,

674
00:38:54,220 --> 00:38:58,296
the probe would be in
a separate part of space,

675
00:38:58,320 --> 00:39:02,336
cut off from the rest
of the universe.

676
00:39:02,360 --> 00:39:04,366
Falling through the
event horizon of a black hole

677
00:39:04,390 --> 00:39:07,636
is like jumping over
the edge of a cliff.

678
00:39:07,660 --> 00:39:10,576
You can see the edge
and you can see the edge go by,

679
00:39:10,600 --> 00:39:13,146
and then when you're at
the bottom, you can look up

680
00:39:13,170 --> 00:39:15,616
and see what's happening
at the top of the cliff,

681
00:39:15,640 --> 00:39:17,370
but you can never go back.

682
00:39:19,510 --> 00:39:21,886
At the bottom
of this black-hole cliff

683
00:39:21,910 --> 00:39:24,686
sits a singularity,

684
00:39:24,710 --> 00:39:30,566
a region of space where the laws
of physics go off the rails.

685
00:39:30,590 --> 00:39:33,396
Deep toward that singularity
could be as surprising

686
00:39:33,420 --> 00:39:36,336
as you might imagine
and yet still a possibility.

687
00:39:36,360 --> 00:39:38,806
If you map the space-time
around a black hole

688
00:39:38,830 --> 00:39:40,376
in a very particular way,

689
00:39:40,400 --> 00:39:44,346
there emerges a sort of mirror
universe, a parallel universe,

690
00:39:44,370 --> 00:39:46,006
on the other side
of the black hole,

691
00:39:46,030 --> 00:39:50,800
identical to our own and
traversable by the black hole.

692
00:39:56,080 --> 00:40:01,196
So black holes are not
just edges to our universe,

693
00:40:01,220 --> 00:40:06,566
they may also be gateways
to other universes.

694
00:40:06,590 --> 00:40:08,136
It's highly conjecture,

695
00:40:08,160 --> 00:40:12,366
but if there's ever going
to be a space, or region,

696
00:40:12,390 --> 00:40:15,536
where you're making connections
with, say, some other universe,

697
00:40:15,560 --> 00:40:18,330
a black hole, in principle,
could be a portal to that.

698
00:40:22,370 --> 00:40:23,746
But it's highly unlikely

699
00:40:23,770 --> 00:40:24,946
that anyone will ever want

700
00:40:24,970 --> 00:40:29,586
to venture beyond
an event horizon to find out,

701
00:40:29,610 --> 00:40:33,026
and our pursuits of the other
edges in the cosmos

702
00:40:33,050 --> 00:40:35,956
offer little hope either.

703
00:40:35,980 --> 00:40:41,136
We can never travel beyond
the cosmic event horizon.

704
00:40:41,160 --> 00:40:44,336
We will never be able
to see beyond the edge

705
00:40:44,360 --> 00:40:47,736
of our observable universe.

706
00:40:47,760 --> 00:40:51,906
So can we ever hope to discover
the true edge

707
00:40:51,930 --> 00:40:56,200
of the greater universe
or find out if it even has one?

708
00:40:58,040 --> 00:41:00,416
My feeling is that probably
we should not think

709
00:41:00,440 --> 00:41:03,216
about edges for the universe.

710
00:41:03,240 --> 00:41:05,926
Everything you've ever seen
in your life is finite,

711
00:41:05,950 --> 00:41:10,096
it has an inside
and the outside, it has an edge.

712
00:41:10,120 --> 00:41:11,896
The universe
might not be like that.

713
00:41:11,920 --> 00:41:13,366
It's probably not like that.

714
00:41:13,390 --> 00:41:16,520
There's probably no sense in
which the universe has an edge.

715
00:41:19,560 --> 00:41:22,106
We used to think that
the ultimate limits

716
00:41:22,130 --> 00:41:24,946
on the future life
were set by nature,

717
00:41:24,970 --> 00:41:26,246
we couldn't get off the planet,

718
00:41:26,270 --> 00:41:29,446
or there was nothing beyond
our solar system.

719
00:41:29,470 --> 00:41:31,916
Now we realized
we have this vast,

720
00:41:31,940 --> 00:41:35,256
vast cosmos out there
and that the ultimate limits

721
00:41:35,280 --> 00:41:38,456
are actually simply
our own imagination

722
00:41:38,480 --> 00:41:41,226
and our ability
to do great things with it

723
00:41:41,250 --> 00:41:42,926
rather than self-destruct.

724
00:41:42,950 --> 00:41:46,066
Our future destiny
is in our own hands

725
00:41:46,090 --> 00:41:49,866
and I find that very empowering.

726
00:41:49,890 --> 00:41:52,206
It is beautifully frustrating

727
00:41:52,230 --> 00:41:53,936
to realize how limited we are,

728
00:41:53,960 --> 00:41:56,606
to realize that we're probably
never going to get a true view

729
00:41:56,630 --> 00:41:58,506
of the real extent
of the universe.

730
00:41:58,530 --> 00:42:00,576
We should keep an open mind,
we should be humble,

731
00:42:00,600 --> 00:42:02,446
but I think that we should
give up on the idea

732
00:42:02,470 --> 00:42:03,886
that things should have edges

733
00:42:03,910 --> 00:42:05,346
because that's what
we're familiar with.

734
00:42:05,370 --> 00:42:08,116
The universe
is something special.

735
00:42:08,140 --> 00:42:12,886
What matters to us, and
will only ever matter to us,

736
00:42:12,910 --> 00:42:15,296
is the observable universe

737
00:42:15,320 --> 00:42:17,996
because that's the limit
of what we can see

738
00:42:18,020 --> 00:42:19,896
and that is the limit
of what we can know.

739
00:42:19,920 --> 00:42:22,166
So there is an edge
to the universe,

740
00:42:22,190 --> 00:42:24,190
there's an edge
to what we can know.

741
00:42:24,240 --> 00:42:28,790
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