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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 by vitac...
www.vitac.com
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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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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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There are galaxies
that we can see today
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that in a few million years,
say, we won't be able to see
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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
251
00:14:45,470 --> 00:14:46,976
will be completely invisible
252
00:14:47,000 --> 00:14:50,070
because they'll be beyond
the edge of the universe.
253
00:14:53,140 --> 00:14:55,926
So eventually, every last galaxy
254
00:14:55,950 --> 00:14:57,726
will be so far away from us
255
00:14:57,750 --> 00:15:01,326
that light cannot reach us
through that expanding space.
256
00:15:01,350 --> 00:15:04,096
It's almost as if you're driving
through a dark desert
257
00:15:04,120 --> 00:15:06,836
in your car
and the very, very last town
258
00:15:06,860 --> 00:15:08,696
that ever exists
has gone over the horizon
259
00:15:08,720 --> 00:15:10,990
and they'll never be
any light again.
260
00:15:14,360 --> 00:15:16,806
We can see less
and less of the universe
261
00:15:16,830 --> 00:15:18,976
as we go into the future.
262
00:15:19,000 --> 00:15:21,016
What a strange thought.
263
00:15:21,040 --> 00:15:25,210
So that means we should build
all the telescopes we can now.
264
00:15:27,240 --> 00:15:29,886
There's a limit
to the universe we can see,
265
00:15:29,910 --> 00:15:33,726
even with the most
advanced telescopes.
266
00:15:33,750 --> 00:15:36,226
But what lies beyond
267
00:15:36,250 --> 00:15:39,150
is one of the biggest
mysteries in astronomy.
268
00:15:41,260 --> 00:15:44,506
The greater universe
could be stranger
269
00:15:44,530 --> 00:15:46,630
than our wildest imagination.
270
00:15:49,000 --> 00:15:50,176
When you stand on the beach
271
00:15:50,200 --> 00:15:51,406
and you look at the horizon
272
00:15:51,430 --> 00:15:53,816
and you kind of think,
"oh, what beautiful lands
273
00:15:53,840 --> 00:15:55,276
are there beyond the horizon?
274
00:15:55,300 --> 00:15:58,586
Things I've never
imagined before."
275
00:15:58,610 --> 00:15:59,886
It's so natural.
276
00:15:59,910 --> 00:16:03,286
It's so human to ask,
"what lies beyond that?
277
00:16:03,310 --> 00:16:05,850
What is the true extent
of the universe?"
278
00:16:21,160 --> 00:16:23,146
The observable universe
279
00:16:23,170 --> 00:16:25,370
contains trillions of galaxies.
280
00:16:27,940 --> 00:16:31,140
It's about 92 billion
light-years across...
281
00:16:33,380 --> 00:16:34,886
...but astronomers believe
282
00:16:34,910 --> 00:16:37,780
this isn't the full extent
of the universe.
283
00:16:41,020 --> 00:16:44,266
What we don't know is how much
of the universe
284
00:16:44,290 --> 00:16:46,266
is our observable universe.
285
00:16:46,290 --> 00:16:49,306
It could be a tiny,
microscopic speck of this
286
00:16:49,330 --> 00:16:51,766
much more vast universe.
287
00:16:51,790 --> 00:16:53,836
We just don't know.
288
00:16:53,860 --> 00:16:56,106
We have no idea
how much stuff there is
289
00:16:56,130 --> 00:16:57,576
outside the observable universe,
290
00:16:57,600 --> 00:17:01,416
but because by definition, it's
outside the observable universe,
291
00:17:01,440 --> 00:17:04,216
we really don't know right now.
292
00:17:04,240 --> 00:17:06,286
So what is out there?
293
00:17:06,310 --> 00:17:10,686
One theory says that space
outside the observable universe
294
00:17:10,710 --> 00:17:16,126
is pretty much the same
as our own cosmic neighborhood.
295
00:17:16,150 --> 00:17:18,626
It's just more universe.
It's just like here.
296
00:17:18,650 --> 00:17:21,236
It's just far enough away
that we can't see it.
297
00:17:21,260 --> 00:17:24,706
So it's not like there's bizarre
places where time runs backwards
298
00:17:24,730 --> 00:17:27,090
or aliens have two heads,
well, yeah, maybe.
299
00:17:29,570 --> 00:17:30,946
But further out
300
00:17:30,970 --> 00:17:33,446
in the deepest parts
of the greater universe,
301
00:17:33,470 --> 00:17:35,400
all bets are off.
302
00:17:37,670 --> 00:17:40,556
We expect that as you go sort of
twice or three times
303
00:17:40,580 --> 00:17:42,056
beyond the observable universe,
304
00:17:42,080 --> 00:17:44,926
it's probably very similar
to the universe we inhabit.
305
00:17:44,950 --> 00:17:48,426
But if you go a thousand times
or a million times farther,
306
00:17:48,450 --> 00:17:50,150
who knows what you might see?
307
00:17:52,190 --> 00:17:55,220
It might be very, very different
if we go far enough away.
308
00:17:57,860 --> 00:17:59,776
Strangely, it all comes back
309
00:17:59,800 --> 00:18:02,206
to the expansion of the universe
310
00:18:02,230 --> 00:18:05,400
and one crucial detail
in that process.
311
00:18:08,070 --> 00:18:09,146
There was a brief moment
312
00:18:09,170 --> 00:18:11,286
in the very early history
of the universe
313
00:18:11,310 --> 00:18:15,316
where its expansion
accelerated hugely.
314
00:18:15,340 --> 00:18:17,386
This acceleration
is called inflation,
315
00:18:17,410 --> 00:18:20,826
and in a brief moment,
the universe itself expanded
316
00:18:20,850 --> 00:18:23,550
at multiple times
the speed of light.
317
00:18:25,820 --> 00:18:29,436
Inflation was a formative moment
318
00:18:29,460 --> 00:18:31,006
for our universe.
319
00:18:31,030 --> 00:18:32,476
By the time it stopped,
320
00:18:32,500 --> 00:18:35,530
the universe's basic
characteristics were set.
321
00:18:37,930 --> 00:18:39,846
There are these
fundamental constants
322
00:18:39,870 --> 00:18:42,586
that describe the phenomena
in our universe,
323
00:18:42,610 --> 00:18:46,510
the fundamentals of matter
and light and space-time.
324
00:18:48,140 --> 00:18:49,786
But some scientists think
325
00:18:49,810 --> 00:18:52,386
there could be regions
of the greater universe
326
00:18:52,410 --> 00:18:54,880
where inflation
has never stopped.
327
00:18:57,150 --> 00:19:02,236
The idea is the greater universe
is expanding at an insane speed,
328
00:19:02,260 --> 00:19:05,406
but here and there,
occasionally a little region
329
00:19:05,430 --> 00:19:10,746
will stop inflating and
just expand at the normal rate.
330
00:19:10,770 --> 00:19:12,746
Inflation can end somewhere
331
00:19:12,770 --> 00:19:15,016
and that gives rise
to the universe we live in,
332
00:19:15,040 --> 00:19:18,946
while inflation
continues somewhere else.
333
00:19:18,970 --> 00:19:21,056
Parts of the greater universe
334
00:19:21,080 --> 00:19:22,356
that continued to inflate
335
00:19:22,380 --> 00:19:25,650
would be left
with different laws of physics.
336
00:19:27,280 --> 00:19:29,326
This incredibly violent
inflation process
337
00:19:29,350 --> 00:19:33,366
is actually monkeyed with
the very fabric of space itself,
338
00:19:33,390 --> 00:19:35,266
so that a lot of the things
that we were taught
339
00:19:35,290 --> 00:19:39,976
that are laws of physics
are different there.
340
00:19:40,000 --> 00:19:41,276
So in an essence,
341
00:19:41,300 --> 00:19:43,176
inflation gives us
a very natural way
342
00:19:43,200 --> 00:19:46,076
to make this patchwork quilt of
different parts of the universe
343
00:19:46,100 --> 00:19:48,946
where things seem different.
344
00:19:48,970 --> 00:19:52,086
So what we could imagine
is a super large-scale structure
345
00:19:52,110 --> 00:19:55,656
where there's different regions
of the universe, domains,
346
00:19:55,680 --> 00:19:58,980
and each domain has different
local laws of physics.
347
00:20:01,950 --> 00:20:03,466
These different parts
of the universe
348
00:20:03,490 --> 00:20:07,550
are separated by frontiers
called domain walls.
349
00:20:11,360 --> 00:20:14,176
We have similar boundaries
on earth.
350
00:20:14,200 --> 00:20:17,006
Whenever you have something that
can be in many different states,
351
00:20:17,030 --> 00:20:19,876
you can end up with domain wall.
352
00:20:19,900 --> 00:20:25,516
If our fish swimming around
in the arctic near an iceberg,
353
00:20:25,540 --> 00:20:27,216
there will be a domain boundary
354
00:20:27,240 --> 00:20:29,486
between the water
being in the liquid state
355
00:20:29,510 --> 00:20:33,296
where I am and the solid state
inside the ice.
356
00:20:33,320 --> 00:20:36,866
So a domain wall is just a wall
between two domains.
357
00:20:36,890 --> 00:20:38,826
If it's water,
this could be ice,
358
00:20:38,850 --> 00:20:39,896
this could be liquid.
359
00:20:39,920 --> 00:20:41,336
If you're talking about space,
360
00:20:41,360 --> 00:20:43,906
this could be a kind of space
maybe you can live in.
361
00:20:43,930 --> 00:20:47,706
This could be a kind of space
where you don't want to be.
362
00:20:47,730 --> 00:20:51,606
Crossing a domain wall
would be very bad news
363
00:20:51,630 --> 00:20:54,376
for anyone who dared to try.
364
00:20:54,400 --> 00:20:57,016
Cross that domain wall
and the laws of physics
365
00:20:57,040 --> 00:20:59,316
could change dramatically.
366
00:20:59,340 --> 00:21:01,316
The number of dimensions
could change.
367
00:21:01,340 --> 00:21:03,926
If we were somehow able to
travel to places in the universe
368
00:21:03,950 --> 00:21:05,326
where the laws of physics
are different,
369
00:21:05,350 --> 00:21:07,696
we would die
370
00:21:07,720 --> 00:21:10,496
because all of the chemistry
going on in our bodies
371
00:21:10,520 --> 00:21:15,096
depends very, very sensitively
on the laws of physics.
372
00:21:15,120 --> 00:21:19,060
So you could just dissipate like
Thanos snap and you're gone.
373
00:21:21,060 --> 00:21:22,746
Domain walls
might be the closest
374
00:21:22,770 --> 00:21:25,400
we get to locating
an edge in the universe.
375
00:21:26,970 --> 00:21:29,316
Depends on how
you define the edge.
376
00:21:29,340 --> 00:21:32,686
If it is the realm where the
laws of our universe operate,
377
00:21:32,710 --> 00:21:36,640
then these domain walls are in
essence the age of the universe.
378
00:21:40,780 --> 00:21:42,720
But this is all just theory.
379
00:21:46,590 --> 00:21:48,396
If we ever really are
to work out
380
00:21:48,420 --> 00:21:50,866
what the true size
and shape of the universe is,
381
00:21:50,890 --> 00:21:54,536
we're going to have to look
for clues that are close to us.
382
00:21:54,560 --> 00:21:58,046
Clues that could
answer the ultimate question,
383
00:21:58,070 --> 00:22:01,946
how big is the rest
of the greater universe
384
00:22:01,970 --> 00:22:04,840
and could it go on forever?
385
00:22:20,420 --> 00:22:22,806
For tens of thousands of years,
386
00:22:22,830 --> 00:22:28,136
mankind has gazed in wonder
at the vastness of the cosmos,
387
00:22:28,160 --> 00:22:32,476
but just how extensive is it?
388
00:22:32,500 --> 00:22:34,176
If we could answer
that question,
389
00:22:34,200 --> 00:22:39,356
it might help us to understand
our place in the universe.
390
00:22:39,380 --> 00:22:41,686
One of the fundamental
questions in science
391
00:22:41,710 --> 00:22:44,986
is how big is the universe?
392
00:22:45,010 --> 00:22:47,726
To answer the question,
"how big is the universe?"
393
00:22:47,750 --> 00:22:51,066
We have to answer the question,
"what shape is the universe?"
394
00:22:51,090 --> 00:22:52,996
And by shape, I mean geometry.
395
00:22:53,020 --> 00:22:57,766
I mean, how is the universe
curved on its largest scales?
396
00:22:57,790 --> 00:23:00,176
If we are to discover
that the universe
397
00:23:00,200 --> 00:23:04,206
does have some
sort of geometric curvature,
398
00:23:04,230 --> 00:23:09,746
then this might imply that it
wraps around in on itself
399
00:23:09,770 --> 00:23:12,386
over incredibly large distances.
400
00:23:12,410 --> 00:23:16,126
And that if you could travel
in one direction long enough,
401
00:23:16,150 --> 00:23:18,126
you would end up
at your starting point.
402
00:23:18,150 --> 00:23:19,796
Another version is
that the universe
403
00:23:19,820 --> 00:23:22,626
is more like
an infinite flat plane.
404
00:23:22,650 --> 00:23:25,126
Okay?
No curvature at all.
405
00:23:25,150 --> 00:23:27,666
The further you travel,
well, the further you get
406
00:23:27,690 --> 00:23:30,560
and you never get back
to where you started.
407
00:23:32,800 --> 00:23:34,376
To work out the shape
of something,
408
00:23:34,400 --> 00:23:37,476
we would normally just step back
and take a look.
409
00:23:37,500 --> 00:23:42,746
But clearly moving outside
the universe is a nonstarter.
410
00:23:42,770 --> 00:23:44,146
You can't jump on a rocket
411
00:23:44,170 --> 00:23:47,386
and fly a thousand times larger
than our cosmic horizon
412
00:23:47,410 --> 00:23:49,216
and see what the shape
of the universe is.
413
00:23:49,240 --> 00:23:51,126
We just can't do that.
414
00:23:51,150 --> 00:23:54,156
Our human perspective
on the larger universe
415
00:23:54,180 --> 00:23:55,456
is so limited.
416
00:23:55,480 --> 00:23:57,596
So if we want to figure out
what the larger shape
417
00:23:57,620 --> 00:23:59,596
and scale of the universe is,
418
00:23:59,620 --> 00:24:01,960
we're going to have to be
very clever indeed.
419
00:24:04,790 --> 00:24:06,576
One way to be clever
420
00:24:06,600 --> 00:24:08,976
is to think of the geometry
of the universe
421
00:24:09,000 --> 00:24:11,746
in its simplest terms.
422
00:24:11,770 --> 00:24:14,076
When we talk about
the geometry of the universe,
423
00:24:14,100 --> 00:24:16,616
we really are talking
about geometry.
424
00:24:16,640 --> 00:24:19,786
In order to do geometry,
you have to take measures.
425
00:24:19,810 --> 00:24:21,986
You need a cosmic ruler
to do this,
426
00:24:22,010 --> 00:24:24,386
and it turns out
there's a great cosmic ruler
427
00:24:24,410 --> 00:24:27,250
known as baryon
acoustic oscillations.
428
00:24:29,690 --> 00:24:32,426
Baryonic acoustic
oscillations are ripples
429
00:24:32,450 --> 00:24:36,166
in the cosmic
microwave background,
430
00:24:36,190 --> 00:24:38,660
the oldest light
in the universe.
431
00:24:40,960 --> 00:24:42,676
As the universe expanded,
432
00:24:42,700 --> 00:24:47,300
these ripples were imprinted
in space in a uniform way.
433
00:24:48,770 --> 00:24:50,646
They provide a cosmic ruler
434
00:24:50,670 --> 00:24:53,856
to measure vast
distances over time,
435
00:24:53,880 --> 00:24:56,186
so we can gauge
if the universe expands
436
00:24:56,210 --> 00:24:59,910
in curved space
or over a flat plane.
437
00:25:02,820 --> 00:25:04,826
When we use these cosmic rulers
438
00:25:04,850 --> 00:25:07,096
to try to back out
the shape of the universe,
439
00:25:07,120 --> 00:25:09,336
we're sure
to a few percent accuracy
440
00:25:09,360 --> 00:25:12,936
that the universe is flat.
441
00:25:12,960 --> 00:25:15,076
If the universe is flat,
442
00:25:15,100 --> 00:25:17,846
we could set off
traveling into the cosmos
443
00:25:17,870 --> 00:25:20,570
and continue traveling forever.
444
00:25:22,670 --> 00:25:26,356
There may be no edge
to our universe
445
00:25:26,380 --> 00:25:30,926
because a flat universe
can be an infinite universe.
446
00:25:30,950 --> 00:25:32,956
Now we're thinking of
the universe as something
447
00:25:32,980 --> 00:25:34,826
that really does go on forever,
448
00:25:34,850 --> 00:25:38,866
that the stars and galaxies
never have an end,
449
00:25:38,890 --> 00:25:42,966
and how can something
truly infinite really exist?
450
00:25:42,990 --> 00:25:47,706
Infinity is weird
because it's a concept of,
451
00:25:47,730 --> 00:25:50,046
because it's endless.
452
00:25:50,070 --> 00:25:52,800
What does that mean? Who knows?
I don't know.
453
00:25:55,500 --> 00:25:59,046
Infinity is a concept
more than anything else.
454
00:25:59,070 --> 00:26:01,016
Our brains aren't
evolved for that.
455
00:26:01,040 --> 00:26:03,786
We evolved living in the plains.
456
00:26:03,810 --> 00:26:06,426
We were apes looking for food.
457
00:26:06,450 --> 00:26:09,426
We weren't evolved to think
about the universe
458
00:26:09,450 --> 00:26:12,166
and all of this stuff.
459
00:26:12,190 --> 00:26:14,566
I just can't stop
contemplating this stuff.
460
00:26:14,590 --> 00:26:17,466
The idea of infinity
and these large numbers
461
00:26:17,490 --> 00:26:19,506
and even the tininess
of everything.
462
00:26:19,530 --> 00:26:20,976
It's nuts.
463
00:26:21,000 --> 00:26:24,130
Yeah, thinking about infinity
makes my head hurt a little bit.
464
00:26:28,440 --> 00:26:31,586
An infinite universe
has profound implications
465
00:26:31,610 --> 00:26:34,310
for understanding our place
in the cosmos.
466
00:26:36,380 --> 00:26:39,310
It guarantees we're not alone.
467
00:26:40,920 --> 00:26:42,926
But if the universe is infinite,
468
00:26:42,950 --> 00:26:44,866
then there could be
an infinite number of galaxies
469
00:26:44,890 --> 00:26:49,296
that have planets with life,
an infinite number without life,
470
00:26:49,320 --> 00:26:51,636
then because life
did appear here on earth,
471
00:26:51,660 --> 00:26:53,076
it's physically possible,
472
00:26:53,100 --> 00:26:54,606
therefore,
it will definitely happen
473
00:26:54,630 --> 00:26:57,076
elsewhere in the universe.
474
00:26:57,100 --> 00:26:58,406
In a flat universe,
475
00:26:58,430 --> 00:27:03,386
alien life could come in
an infinite number of forms,
476
00:27:03,410 --> 00:27:07,616
but there is an altogether
stranger guarantee.
477
00:27:07,640 --> 00:27:10,726
If the universe has no edge,
this means that
478
00:27:10,750 --> 00:27:14,680
things that seem like they are
impossible become possible.
479
00:27:17,390 --> 00:27:20,236
Every possible arrangement
of matter,
480
00:27:20,260 --> 00:27:23,236
every possible history.
481
00:27:23,260 --> 00:27:27,706
A galaxy of a solar system
of a planet like earth
482
00:27:27,730 --> 00:27:28,976
is possible
483
00:27:29,000 --> 00:27:32,576
and is happening right now
in parallel to us
484
00:27:32,600 --> 00:27:35,070
somewhere over there.
485
00:27:36,610 --> 00:27:39,486
So that means that there
has to be another place
486
00:27:39,510 --> 00:27:42,956
that has a galaxy just like ours
487
00:27:42,980 --> 00:27:44,856
and it would have an earth
just like ours.
488
00:27:44,880 --> 00:27:49,696
It would have people who would
have another version of you,
489
00:27:49,720 --> 00:27:51,166
another version of me.
490
00:27:51,190 --> 00:27:55,366
It's 100% guaranteed that there
is another Max Tegmark out there
491
00:27:55,390 --> 00:27:58,790
having exactly this conversation
and in fact many of them.
492
00:28:00,860 --> 00:28:03,546
An infinite universe
full of Max Tegmarks
493
00:28:03,570 --> 00:28:05,606
may be a strange concept,
494
00:28:05,630 --> 00:28:08,146
but what's truly mind-bending
is understanding
495
00:28:08,170 --> 00:28:11,216
the physics of a flat universe.
496
00:28:11,240 --> 00:28:14,556
If the universe is infinite
and it's expanding,
497
00:28:14,580 --> 00:28:20,196
what's it expanding into
and what did it expand from?
498
00:28:20,220 --> 00:28:24,626
Was there ever an edge
to the universe?
499
00:28:24,650 --> 00:28:28,796
Fortunately, the answer is that
it doesn't make sense
500
00:28:28,820 --> 00:28:30,766
to ask that question.
501
00:28:30,790 --> 00:28:33,076
Everything is expanding,
502
00:28:33,100 --> 00:28:36,106
including the universe
that we exist within.
503
00:28:36,130 --> 00:28:39,076
So in fact, it's not expanding
into anything
504
00:28:39,100 --> 00:28:40,700
because it is everything.
505
00:28:45,670 --> 00:28:47,756
To help understand
what's going on
506
00:28:47,780 --> 00:28:49,356
in an infinite universe,
507
00:28:49,380 --> 00:28:51,610
we need to go back
to the big bang.
508
00:28:54,280 --> 00:28:57,596
We want to think of the big bang
as an explosion in space,
509
00:28:57,620 --> 00:28:59,326
like it happened some place,
510
00:28:59,350 --> 00:29:02,236
but there wasn't any place
before the big bang.
511
00:29:02,260 --> 00:29:05,506
Space existed inside
of the big bang itself.
512
00:29:05,530 --> 00:29:07,776
So it's not an explosion
in space,
513
00:29:07,800 --> 00:29:10,276
it's an explosion of space.
514
00:29:10,300 --> 00:29:12,346
We're sometimes told that
at the big bang,
515
00:29:12,370 --> 00:29:15,776
the universe started out very,
very small and then got big,
516
00:29:15,800 --> 00:29:19,716
but how can a finite point
become infinite?
517
00:29:19,740 --> 00:29:21,756
Well, if the universe
is infinite,
518
00:29:21,780 --> 00:29:25,526
then it was also infinite
at the big bang.
519
00:29:25,550 --> 00:29:27,826
This is a tough thing
to think about.
520
00:29:27,850 --> 00:29:29,296
Think about it this way.
521
00:29:29,320 --> 00:29:31,896
In an infinite universe,
the galaxies go on forever
522
00:29:31,920 --> 00:29:35,566
and now there's a great
distance between every galaxy.
523
00:29:35,590 --> 00:29:36,606
But once upon a time,
524
00:29:36,630 --> 00:29:38,106
the galaxies
were closer together,
525
00:29:38,130 --> 00:29:40,706
say half their current
distance apart,
526
00:29:40,730 --> 00:29:42,076
but they still went on forever.
527
00:29:42,100 --> 00:29:45,746
The universe was still infinite.
528
00:29:45,770 --> 00:29:47,176
In a flat universe,
529
00:29:47,200 --> 00:29:51,846
space was infinite
from the beginning.
530
00:29:51,870 --> 00:29:54,356
There was never
a single point in space
531
00:29:54,380 --> 00:29:56,986
where the big bang happened.
532
00:29:57,010 --> 00:30:00,596
It happened everywhere.
533
00:30:00,620 --> 00:30:05,596
An infinite universe offers
infinite possibilities
534
00:30:05,620 --> 00:30:08,050
but no edge to space.
535
00:30:11,190 --> 00:30:14,106
But there may be
another kind of edge,
536
00:30:14,130 --> 00:30:18,670
one that will only reveal itself
if the universe dies.
537
00:30:37,420 --> 00:30:42,336
We live in an infinite
and expanding universe.
538
00:30:42,360 --> 00:30:46,030
Space has no edge.
It goes on forever.
539
00:30:51,030 --> 00:30:55,916
But there could be a different
kind of edge to our universe,
540
00:30:55,940 --> 00:30:59,046
an edge of time.
541
00:30:59,070 --> 00:31:01,186
The universe seems to have begun
542
00:31:01,210 --> 00:31:03,256
13.8 billion years ago
in the past,
543
00:31:03,280 --> 00:31:06,356
so there's some inclination,
some impression,
544
00:31:06,380 --> 00:31:08,156
that it's finite in time.
545
00:31:08,180 --> 00:31:12,026
What we call the big bang is,
as far as we understand it,
546
00:31:12,050 --> 00:31:14,636
a beginning,
a start of the universe.
547
00:31:14,660 --> 00:31:16,906
The universe has a finite age.
548
00:31:16,930 --> 00:31:20,636
Now, does it have an edge
in the future?
549
00:31:20,660 --> 00:31:22,536
We used to think
that time would someday
550
00:31:22,560 --> 00:31:25,146
come to a catastrophic end,
551
00:31:25,170 --> 00:31:28,516
along with the planets,
galaxies,
552
00:31:28,540 --> 00:31:30,770
and all life in the universe.
553
00:31:33,440 --> 00:31:35,686
If we know there's a big bang,
if we know the universe started,
554
00:31:35,710 --> 00:31:38,526
it expanded and cooled,
it's very natural to wonder
555
00:31:38,550 --> 00:31:40,756
whether or not someday
the expansion will stop,
556
00:31:40,780 --> 00:31:44,020
reverse, and come back,
and that's a big crunch.
557
00:31:49,120 --> 00:31:50,336
In a big crunch,
558
00:31:50,360 --> 00:31:53,776
our expanding universe
would begin to contract.
559
00:31:53,800 --> 00:31:56,700
Stars and planets would smash
into each other.
560
00:31:58,870 --> 00:32:02,116
Galaxies would collide,
561
00:32:02,140 --> 00:32:05,386
and all of the life left
in space would be compressed
562
00:32:05,410 --> 00:32:08,940
with all the other matter
into a singularity.
563
00:32:17,590 --> 00:32:20,266
If this theory is true,
then the universe
564
00:32:20,290 --> 00:32:25,736
would have both a beginning
and an end of time.
565
00:32:25,760 --> 00:32:27,936
If we live in a universe
that will expand,
566
00:32:27,960 --> 00:32:30,876
stop expanding,
and then go back into a crunch,
567
00:32:30,900 --> 00:32:34,276
then it has,
in effect, two edges.
568
00:32:34,300 --> 00:32:38,646
But there's a much
stranger possibility.
569
00:32:38,670 --> 00:32:41,686
Perhaps the end is
but a beginning,
570
00:32:41,710 --> 00:32:43,786
where the universe
is a oscillating universe.
571
00:32:43,810 --> 00:32:45,686
It has a big bang
like beginning,
572
00:32:45,710 --> 00:32:47,426
it expands to a maximum size,
573
00:32:47,450 --> 00:32:49,426
and then goes back
into a big crunch
574
00:32:49,450 --> 00:32:51,780
and does that over and over.
575
00:32:53,520 --> 00:32:55,536
We could be residents
of a universe
576
00:32:55,560 --> 00:32:58,706
created from the ashes
of another,
577
00:32:58,730 --> 00:33:03,430
a single universe in a stream
of bouncing universes...
578
00:33:06,570 --> 00:33:11,446
...each full of galaxies,
planets, and life.
579
00:33:11,470 --> 00:33:14,986
But our most recent observations
of the universe suggest
580
00:33:15,010 --> 00:33:17,940
a big crunch isn't in the cards.
581
00:33:20,120 --> 00:33:23,580
Once again, dark energy is key.
582
00:33:26,250 --> 00:33:30,336
For a while we didn't know
if the expansion of the universe
583
00:33:30,360 --> 00:33:32,666
was going to slow, stop,
584
00:33:32,690 --> 00:33:35,306
and reverse itself
because of gravity.
585
00:33:35,330 --> 00:33:36,706
There are all these galaxies
in the universe
586
00:33:36,730 --> 00:33:38,846
and they're pulling on
each other by their gravity.
587
00:33:38,870 --> 00:33:40,516
And if the expansion
isn't fast enough,
588
00:33:40,540 --> 00:33:43,146
that gravity might be strong
enough to stop the expansion
589
00:33:43,170 --> 00:33:44,916
and re-collapse the universe.
590
00:33:44,940 --> 00:33:46,216
Now with dark energy,
591
00:33:46,240 --> 00:33:48,656
we know that there's no way
that can happen.
592
00:33:48,680 --> 00:33:51,186
The universe is going
to expand forever
593
00:33:51,210 --> 00:33:55,026
because dark energy is
pumping it full of acceleration.
594
00:33:55,050 --> 00:33:57,526
In order for there
to be a big crunch,
595
00:33:57,550 --> 00:34:01,136
our understanding of dark energy
would have to change a lot.
596
00:34:01,160 --> 00:34:03,736
That is, dark energy would have
to be extremely weird
597
00:34:03,760 --> 00:34:07,906
and turn off in some very funny
way for the universe
598
00:34:07,930 --> 00:34:10,930
to suddenly stop expanding
and re-collapse.
599
00:34:13,270 --> 00:34:14,876
Without a big crunch,
600
00:34:14,900 --> 00:34:19,246
there is no future edge to time.
601
00:34:19,270 --> 00:34:20,956
The universe is not
only expanding,
602
00:34:20,980 --> 00:34:22,686
but it's being driven
by dark energy
603
00:34:22,710 --> 00:34:26,056
to expand faster and faster
and the dark energy
604
00:34:26,080 --> 00:34:28,526
doesn't dilute away,
as far as we can tell.
605
00:34:28,550 --> 00:34:30,996
So the simplest idea
is that the universe
606
00:34:31,020 --> 00:34:34,750
will simply continue to expand
eternally toward the future.
607
00:34:36,930 --> 00:34:41,706
Just like space,
time will go on forever.
608
00:34:41,730 --> 00:34:46,106
That might sound like a better
fate for life and the universe,
609
00:34:46,130 --> 00:34:47,770
but it's not.
610
00:34:50,040 --> 00:34:53,046
One of the consequences
of this dark energy
611
00:34:53,070 --> 00:34:56,016
that's causing the acceleration
of the universe
612
00:34:56,040 --> 00:35:01,596
is that we eventually are
headed towards the big chill.
613
00:35:01,620 --> 00:35:05,296
I should say, "we're eventually
headed towards the big chill."
614
00:35:05,320 --> 00:35:08,436
So the universe is getting
colder and colder
615
00:35:08,460 --> 00:35:11,436
and things are getting more
and more spread out.
616
00:35:11,460 --> 00:35:13,736
So the accelerated and continual
617
00:35:13,760 --> 00:35:16,006
and forever expansion
of our universe
618
00:35:16,030 --> 00:35:19,706
might make for a frankly
depressing end to time itself.
619
00:35:19,730 --> 00:35:23,246
The ultimate entropy-based
heat death of the universe
620
00:35:23,270 --> 00:35:27,856
where you would walk out
and see no stars in the sky,
621
00:35:27,880 --> 00:35:29,516
see absolutely nothing.
622
00:35:29,540 --> 00:35:32,586
There will come one day when the
very last star in the universe
623
00:35:32,610 --> 00:35:35,450
just fizzles out and that is it.
624
00:35:38,320 --> 00:35:40,296
In the future,
625
00:35:40,320 --> 00:35:44,066
space will be a cold, dark
and infinite void,
626
00:35:44,090 --> 00:35:47,836
where time goes on forever.
627
00:35:47,860 --> 00:35:49,736
There will be nothing to do
628
00:35:49,760 --> 00:35:53,206
but suffer
in the eternal expanse.
629
00:35:53,230 --> 00:35:56,886
It's our inevitable fate
that there's no future edge
630
00:35:56,910 --> 00:36:00,816
of time in the universe.
631
00:36:00,840 --> 00:36:04,126
But even if there isn't an edge
to the universe,
632
00:36:04,150 --> 00:36:09,856
could there be edges
within the universe?
633
00:36:09,880 --> 00:36:12,396
If you wanted to visit the edge
of the universe,
634
00:36:12,420 --> 00:36:15,836
then go find your nearest
black hole and jump on in
635
00:36:15,860 --> 00:36:18,006
because that's a one-way trip.
636
00:36:18,030 --> 00:36:21,830
If you cross this
edge, you will never return.
637
00:36:36,180 --> 00:36:38,286
April 2019,
638
00:36:38,310 --> 00:36:40,496
an international team
of astronomers
639
00:36:40,520 --> 00:36:42,856
makes a special announcement.
640
00:36:42,880 --> 00:36:46,466
And we are delighted to be able
to report to you today
641
00:36:46,490 --> 00:36:51,536
that we have seen and taken
a picture of a black hole.
642
00:36:51,560 --> 00:36:52,830
Here it is.
643
00:36:59,000 --> 00:37:02,246
It's a picture
of a supermassive black hole
644
00:37:02,270 --> 00:37:07,340
at the center of the M87 galaxy
54 million light-years away.
645
00:37:09,180 --> 00:37:14,150
It may also be the first image
of an edge in the universe.
646
00:37:15,420 --> 00:37:17,996
Black holes create
a really interesting scenario
647
00:37:18,020 --> 00:37:22,896
when we think about space
and the universe having edges.
648
00:37:22,920 --> 00:37:27,836
The edge between space
outside and inside a black hole
649
00:37:27,860 --> 00:37:31,106
is called the event horizon.
650
00:37:31,130 --> 00:37:32,576
The event horizon
of a black hole
651
00:37:32,600 --> 00:37:36,146
is a region within which,
once you cross inside,
652
00:37:36,170 --> 00:37:38,746
the gravitational tug
is so strong
653
00:37:38,770 --> 00:37:43,386
that even light cannot escape,
which means nothing can escape
654
00:37:43,410 --> 00:37:46,286
once you cross
inside the event horizon.
655
00:37:46,310 --> 00:37:48,656
So that really
is sort of an edge
656
00:37:48,680 --> 00:37:51,250
because it really does
create a boundary.
657
00:37:56,320 --> 00:37:59,730
The event horizon is
not a physical barrier in space.
658
00:38:01,930 --> 00:38:04,306
Event horizon is an edge
of the part of the universe
659
00:38:04,330 --> 00:38:06,276
we can visit,
but it's not an edge
660
00:38:06,300 --> 00:38:09,176
in the sense
that there's anything there.
661
00:38:09,200 --> 00:38:10,546
You would just pass
right through it
662
00:38:10,570 --> 00:38:12,716
if you actually got right up
to that place.
663
00:38:12,740 --> 00:38:15,156
So it's sort of
a conceptual boundary
664
00:38:15,180 --> 00:38:19,126
between two different parts
of the universe.
665
00:38:19,150 --> 00:38:22,526
If we sent a man to
probe into a black hole,
666
00:38:22,550 --> 00:38:24,320
it would be a one-way trip.
667
00:38:26,750 --> 00:38:31,536
The event horizons of
black holes are a sort of edge
668
00:38:31,560 --> 00:38:34,606
because, once you pass through
an event horizon,
669
00:38:34,630 --> 00:38:38,206
you are cut off
from the rest of the universe.
670
00:38:38,230 --> 00:38:40,776
You can never go back out.
671
00:38:40,800 --> 00:38:45,146
You are outside of our universe.
672
00:38:45,170 --> 00:38:46,986
Once you've crossed
inside that region,
673
00:38:47,010 --> 00:38:50,380
you are never coming back out,
and that's an edge.
674
00:38:52,210 --> 00:38:54,196
Once inside the black hole,
675
00:38:54,220 --> 00:38:58,296
the probe would be in
a separate part of space,
676
00:38:58,320 --> 00:39:02,336
cut off from the rest
of the universe.
677
00:39:02,360 --> 00:39:04,366
Falling through the
event horizon of a black hole
678
00:39:04,390 --> 00:39:07,636
is like jumping over
the edge of a cliff.
679
00:39:07,660 --> 00:39:10,576
You can see the edge
and you can see the edge go by,
680
00:39:10,600 --> 00:39:13,146
and then when you're at
the bottom, you can look up
681
00:39:13,170 --> 00:39:15,616
and see what's happening
at the top of the cliff,
682
00:39:15,640 --> 00:39:17,370
but you can never go back.
683
00:39:19,510 --> 00:39:21,886
At the bottom
of this black-hole cliff
684
00:39:21,910 --> 00:39:24,686
sits a singularity,
685
00:39:24,710 --> 00:39:30,566
a region of space where the laws
of physics go off the rails.
686
00:39:30,590 --> 00:39:33,396
Deep toward that singularity
could be as surprising
687
00:39:33,420 --> 00:39:36,336
as you might imagine
and yet still a possibility.
688
00:39:36,360 --> 00:39:38,806
If you map the space-time
around a black hole
689
00:39:38,830 --> 00:39:40,376
in a very particular way,
690
00:39:40,400 --> 00:39:44,346
there emerges a sort of mirror
universe, a parallel universe,
691
00:39:44,370 --> 00:39:46,006
on the other side
of the black hole,
692
00:39:46,030 --> 00:39:50,800
identical to our own and
traversable by the black hole.
693
00:39:56,080 --> 00:40:01,196
So black holes are not
just edges to our universe,
694
00:40:01,220 --> 00:40:06,566
they may also be gateways
to other universes.
695
00:40:06,590 --> 00:40:08,136
It's highly conjecture,
696
00:40:08,160 --> 00:40:12,366
but if there's ever going
to be a space, or region,
697
00:40:12,390 --> 00:40:15,536
where you're making connections
with, say, some other universe,
698
00:40:15,560 --> 00:40:18,330
a black hole, in principle,
could be a portal to that.
699
00:40:22,370 --> 00:40:23,746
But it's highly unlikely
700
00:40:23,770 --> 00:40:24,946
that anyone will ever want
701
00:40:24,970 --> 00:40:29,586
to venture beyond
an event horizon to find out,
702
00:40:29,610 --> 00:40:33,026
and our pursuits of the other
edges in the cosmos
703
00:40:33,050 --> 00:40:35,956
offer little hope either.
704
00:40:35,980 --> 00:40:41,136
We can never travel beyond
the cosmic event horizon.
705
00:40:41,160 --> 00:40:44,336
We will never be able
to see beyond the edge
706
00:40:44,360 --> 00:40:47,736
of our observable universe.
707
00:40:47,760 --> 00:40:51,906
So can we ever hope to discover
the true edge
708
00:40:51,930 --> 00:40:56,200
of the greater universe
or find out if it even has one?
709
00:40:58,040 --> 00:41:00,416
My feeling is that probably
we should not think
710
00:41:00,440 --> 00:41:03,216
about edges for the universe.
711
00:41:03,240 --> 00:41:05,926
Everything you've ever seen
in your life is finite,
712
00:41:05,950 --> 00:41:10,096
it has an inside
and the outside, it has an edge.
713
00:41:10,120 --> 00:41:11,896
The universe
might not be like that.
714
00:41:11,920 --> 00:41:13,366
It's probably not like that.
715
00:41:13,390 --> 00:41:16,520
There's probably no sense in
which the universe has an edge.
716
00:41:19,560 --> 00:41:22,106
We used to think that
the ultimate limits
717
00:41:22,130 --> 00:41:24,946
on the future life
were set by nature,
718
00:41:24,970 --> 00:41:26,246
we couldn't get off the planet,
719
00:41:26,270 --> 00:41:29,446
or there was nothing beyond
our solar system.
720
00:41:29,470 --> 00:41:31,916
Now we realized
we have this vast,
721
00:41:31,940 --> 00:41:35,256
vast cosmos out there
and that the ultimate limits
722
00:41:35,280 --> 00:41:38,456
are actually simply
our own imagination
723
00:41:38,480 --> 00:41:41,226
and our ability
to do great things with it
724
00:41:41,250 --> 00:41:42,926
rather than self-destruct.
725
00:41:42,950 --> 00:41:46,066
Our future destiny
is in our own hands
726
00:41:46,090 --> 00:41:49,866
and I find that very empowering.
727
00:41:49,890 --> 00:41:52,206
It is beautifully frustrating
728
00:41:52,230 --> 00:41:53,936
to realize how limited we are,
729
00:41:53,960 --> 00:41:56,606
to realize that we're probably
never going to get a true view
730
00:41:56,630 --> 00:41:58,506
of the real extent
of the universe.
731
00:41:58,530 --> 00:42:00,576
We should keep an open mind,
we should be humble,
732
00:42:00,600 --> 00:42:02,446
but I think that we should
give up on the idea
733
00:42:02,470 --> 00:42:03,886
that things should have edges
734
00:42:03,910 --> 00:42:05,346
because that's what
we're familiar with.
735
00:42:05,370 --> 00:42:08,116
The universe
is something special.
736
00:42:08,140 --> 00:42:12,886
What matters to us, and
will only ever matter to us,
737
00:42:12,910 --> 00:42:15,296
is the observable universe
738
00:42:15,320 --> 00:42:17,996
because that's the limit
of what we can see
739
00:42:18,020 --> 00:42:19,896
and that is the limit
of what we can know.
740
00:42:19,920 --> 00:42:22,166
So there is an edge
to the universe,
741
00:42:22,190 --> 00:42:24,190
there's an edge
to what we can know.