All language subtitles for How.The.Universe.Works.S08E03.Edge.Of.The.Universe.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

NARRATOR: Islands have edges.

Planets have edges.

Even galaxies have edges.

But what about the universe?

As explorers,

as curious humans that we are,

we're obsessed with boundaries
and limits,

and we want to know -
does the whole thing,

the universe have a limit?

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

It depends on
what you mean by "edge".

The edge of what we can see?

The edge of where we can go?

Or the edge of reality itself?

Looking out to the edge
of the universe is
tremendously important

to understand our place
in the universe itself.

We're talking about our universe,
we're talking about the thing

that we exist within,
the most fundamental thing there is.

We're driven to understand it.

There's always a desire to push
the knowledge to the edge.

So, can we ever find
the edge of the universe?

2016.
The Hubble Space Telescope turned

toward a dark patch of sky
in the constellation Ursa Major.

It captured an image
of an indistinct blob of light.

The glow is from a distant galaxy
called GN-z11.

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

..but is it the edge of the universe?

The universe all around us
is filled with galaxies,

so it's natural to say,
"Would there be a final galaxy?

If you travelled far enough away,
would you finally be

at the very last galaxy
in the universe,

looking out into empty space?"

That's a difficult question
to answer...

..because there is a limit
to how far we can see.

It all comes down
to the speed of light...

..and the age of the universe.

The key to understanding
the edge of the universe

is that light travels
very, very fast,

but not infinitely fast.

It takes time for it to get
from one place in the universe
to the other.

You open the curtains,
light fills the room,

it doesn't seem to travel at all.

But over the vast distances
of the universe

you actually notice
this travel time.

Even the sun,
93 million miles away,

the light takes eight minutes
to get to us.

When you look out at the stars
we start to think of distance
in terms of light-years,

because it takes years for the light
to get from those stars to us.

And then when you look at galaxies
you're talking about

millions or billions
of light-years.

When we look at the light
from galaxy GN-z11...

..we're seeing light emitted
13.4 billion years ago.

You can't really even find a galaxy
too much farther away than that,

because the universe
is only 13.8 billion years old

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

So we're not gonna find
too many more galaxies
farther away than this.

If things are far enough away,
there is no way

that light can get to us
in the age of the universe.

What this means is
there's a hard limit

to the edge of the universe
that we can see,

and this is set
by the age of the universe.

GN-z11 sparked into life early
in the history of the universe...

..just 400 million years
after the Big Bang.

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

If you look in any direction at all,

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

no galaxies,
nothing but very, very hot gas,

and that sort of forms
a shell around us.

That outer shell
is the cosmic microwave background.

It's the oldest light
in the universe.

The echo of the birth
of the universe,

the Big Bang.

The edge of our universe,
the very furthest thing
that we can see,

is one of the earliest relics of
the formation of
the universe itself.

That is
the cosmic microwave background.

We call this the edge
of our observable universe.

So we have an observable universe,
but beyond that, even if
there are things out there,

there's no way we can see them

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

As the name states,
the observable universe

is simply the part of the universe
we can see.

We can think of
the observable universe
sort of like a spotlight

centred on wherever
you're standing right now,

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

But if you move a little bit to
the left, a little bit to the right,

your observable universe
actually moves with you.

For someone living in galaxy GN-z11,

a totally different part
of the universe would be observable.

So that distant galaxy

is at the edge
of our observable universe...

..and we are at the edge
of their observable universe.

We have different spotlights.

A wonderful thing to think about
is that there are other spheres
around other galaxies.

There are other aliens
looking up into the sky tonight

wondering what the true extent
of the universe is.

The true extent of our universe
doesn't end with galaxy GN-z11.

But when astronomers use
the Hubble Space Telescope

to measure the distance to GN-z11...

..they find something shocking.

It's 32 billion light-years away...

..three times further
than thought possible.

So, if nothing can travel
faster than light,

and we measure the distance
to this galaxy...

how can it be
32 billion light-years away?

There hasn't been enough time
in the history of the universe

for light from GN-z11 to reach us.

There must be
some mistake here, right?

At this point, your brain
is probably thinking

of leaping out of your skull
and running around screaming.

Trust me, I know.

I'm an astronomer,
I've been doing this my whole life,

and this stuff twists
my imagination up.

It's really hard to grasp this.

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

and only 13.4 billion years old?

GN-z11 is further away
than it should be...

..because something strange
is going on with our universe.

It's expanding!

If the universe is expanding,

then where does its edge lie?

And can we ever reach it?

13.8 billion years ago,
a speck of energy burst into life.

We call it the Big Bang.

Space and time pushed out
in all directions.

Ever since,
our universe has expanded.

But the way it's expanding

makes finding an edge
a major challenge.

The universe is expanding,

and expands according to
a very simple law

that the farther away
a galaxy is from us

the faster it appears
to be receding away from us.

The furthest galaxies are moving
at very high speeds.

The most distant galaxy
we've ever spotted, GN-z11,

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

..in just 13.4 billion years.

That's faster
than the speed of light.

We can measure the speeds

with which galaxies
are moving away from us.

And many, many galaxies
are moving away from us

at speeds faster than
the speed of light.

Sounds like it's breaking
the law, right?

There's this idea you've been told
that relativity says

nothing goes faster
than the speed of light.

OK. You've been lied to.

PHIL: Space itself
can do what it wants.

It makes the rules,
it can break the rules.

That rule applies to matter,
not to space itself.

Space can expand
at whatever rate it wants.

A simple way to think
of this expansion law

is imagine standing
on an infinite rubber sheet

that stretches all the way
out into the distance

and you're standing
on the same place -

you can mark it with a little X.

Now, all the sheet expands
in every direction.

So if it expands by a factor of two,

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

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

But another galaxy
was ten feet away from you.

Expand that by a factor of two
and now it's 20 feet away from you.

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

where another galaxy moved ten feet.

So the more stuff there is,
the more elastic

between you and another galaxy,
the more it seems to expand
away from you.

Expansion means
our observable universe

stretches for a colossal 46 billion
light-years in all directions,

92 billion light-years across...

..and getting bigger by the second.

This number
is so incomprehensibly large

that it's difficult
to wrap your brain around.

There are trillions of galaxies
within this volume.

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

If we were to travel to the edge
of the observable universe...

..we would enter
even more unfamiliar territory.

Imagine we're
in an ultra-fast spaceship.

We leave the solar system,
then the Milky Way.

As we travel deeper
into intergalactic space,

things start to get really weird.

For every million light-years we go
from the Milky Way...

..the galaxies move away from us

at around 13 miles per second faster.

We have to accelerate
just to keep up.

But the galaxies keep on moving,

always beyond our reach.

PHIL: Imagine you're a sprinter
on a race track.

If you're running towards
the finish line, it may take you
a few seconds to cross it.

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

If it's moving away from you
at the same speed you're running,
you'll never reach it.

And if it's moving faster
than the runner,

then even faster runners
won't reach it.

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

Beyond a certain distance,
galaxies are racing away from us

faster than the speed of light.

It's a line called
the cosmic event horizon.

And 97% of galaxies
we see in the observable universe

are beyond this line and unreachable,

including GN-z11.

They're teasing us, "Look at me.
What a nice piece of real estate,"

but we know even if
we started going there now,

we could never reach them.

Anything that has crossed
the cosmic event horizon

is out of our reach for ever.

But that's not the full picture,

because the expansion rate
of the universe is changing.

A little over 20 years ago,
astronomers discovered

that the current rate
of the universe's expansion

is accelerating, it's speeding up.

Everyone thought that
the expansion of the universe

should be slowing down
because of gravity.

Galaxies have mass,
they have visible matter,

dark matter,
light has energy as well.

All known forms of energy slow down
the expansion of the universe

because they act
in an attractive gravitational way.

Imagine our surprise when we found

the universe wasn't slowing down.

It wasn't even going
at a constant speed.

It's speeding up with time.

That was a real jaw-dropper.

Astronomers discovered
that distant galaxies

were speeding away from us faster
than the laws of physics predicted.

That is one of the most
bizarre things you can imagine.

I know it doesn't sound like much,

but imagine holding a rock
in front of you and letting it go,

and instead of falling down,
it falls up.

That is as big of a shock
as what the astronomers got.

We did not expect the universe
to be accelerating its expansion.

Astronomers suspect
a mysterious force is at work...

..dark energy.

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

causing this accelerating expansion.

And the origin
and true physical nature

of dark energy is a big mystery.

Thanks to dark energy,
more and more galaxies

are crossing the cosmic event horizon

and leaving the observable universe.

These galaxies
are lost to us for ever.

There are galaxies
that we can see today

that in a few million years, say,
we won't be able to see

because the edge
of the observable universe

has basically moved in closer
than that galaxy.

That's going to happen all the time,

and in a trillion years
or something like that

all these galaxies
that we see in our sky

will be completely invisible,
because they'll be beyond the edge
of the universe.

So eventually every last galaxy
will be so far away from us

that light cannot reach us
through that expanding space.

It's almost as if
you're driving through a dark desert

in your car and the very,
very last town that ever exists

has gone over the horizon, and
there'll never be any light again.

We can see less and less
of the universe

as we go into the future.

What a strange thought.

So that means we should build
all the telescopes we can now!

There's a limit to the universe
we can see

even with the most
advanced telescopes.

But what lies beyond is one of
the biggest mysteries in astronomy.

The greater universe
could be stranger

than our wildest imagination.

When you stand on the beach
and you look at the horizon

and you kind of think,
"Oh, what beautiful lands are there
beyond the horizon,

things I've never imagined before?"

It's so natural,
it's so human to ask,

"What lies beyond that? What is
the true extent of the universe?"

The observable universe contains
trillions of galaxies.

It's about
92 billion light-years across.

But astronomers believe this isn't
the full extent of the universe.

What we don't know is
how much of the universe

is our observable universe.

It could be
a tiny, microscopic speck

of this much more vast universe.
We just don't know.

We have no idea
how much stuff there is
outside the observable universe,

but because, by definition,
it's outside the observable universe

we really don't know right now.

So what is out there?

One theory is that space outside
the observable universe

is pretty much the same
as our own cosmic neighbourhood.

It's just more universe,
it's just like here.

It's just far enough away
that we can't see it.

So it's not like
there's bizarre places

where time runs backwards
or aliens have two heads...

Well, yeah, maybe.

But further out, in the deepest parts
of the greater universe...

all bets are off.

We expect that as you go,
sort of, twice or three times
beyond the observable universe,

it's probably very similar
to the universe we inhabit.

But if you go a thousand times,

or a million times farther,
who knows what you might see?

It might be very, very different
if we go far enough away.

Strangely, it all comes back
to the expansion of the universe,

and one crucial detail
in that process.

There was a brief moment
in the very early history
of the universe

where its expansion
accelerated hugely.

This acceleration
is called inflation,

and in a brief moment,
the universe itself expanded

at multiple times
the speed of light.

Inflation was a formative moment
for our universe.

By the time it stopped,

the universe's basic characteristics
were set.

There are
these fundamental constants

that describe
the phenomena in our universe.

The fundamentals of matter,
and light, and space-time.

But some scientists think

there could be regions
of the greater galaxy

where inflation has never stopped.

The idea is the greater universe
is expanding

at an insane speed,
but here and there, occasionally,

a little region will stop inflating

and just expand at the normal rate.

Inflation can end somewhere

and that gives rise
to the universe we live in,

while inflation continues
somewhere else.

Parts of the greater universe
that continued to inflate

would be left
with different laws of physics.

These incredibly violent
inflation processes

actually monkeyed with
the very fabric of space itself,

so that a lot of the things that
we were taught are laws of physics

are different there.

So in essence, inflation gives us
a very natural way

to make this patchwork quilt
of different parts of the universe

where things seem different.

So what we can imagine
is a super-large-scale structure

where there's different regions
of the universe, domains,

then each domain has different
local laws of physics.

These different parts of the universe

are separated by frontiers
called "domain walls".

We have similar boundaries on Earth.

Whenever you have something that
can be in many different states,

you can end up with domain walls.

If I were a fish swimming around
in the Arctic

near an iceberg,

there would be a domain boundary

between the water being
in the liquid state where I am

and the solid state inside the ice.

A domain wall is the wall
between two domains.

If it's water, this could be ice,
this could be liquid.

If we're talking about space,
this could be a kind of space,
maybe, you can live in,

and this could be the kind of space
where you don't wanna be.

Crossing a domain wall
would be very bad news

for anyone who dared to try.

Cross that domain wall
and the laws of physics

could change dramatically.

The number of dimensions
could change.

If we were able to travel
to places in the universe

where the laws of physics
are different, we would die...

..because all of the chemistry
going on in our bodies

depends very, very sensitively
on the laws of physics.

So you could just dissipate
like Thanos' Snap and you're gone.

Domain walls might be
the closest we get

to locating an edge in the universe.

It depends on how
you define the edge.

If it is the realm where
the laws of our universe operate,

then these domain walls are,
in essence, the edge
of the universe.

But this is all just theory.

If we ever really are to work out

what the true size and shape
of the universe is,

we're going to have to look
for clues that are close to us.

Clues that could answer
the ultimate question.

How big is the rest
of the greater universe...

..and could it go on for ever?

For tens of thousands of years,

mankind has gazed in wonder
at the vastness of the cosmos.

But just how extensive is it?

If we could answer that question,

it might help us to understand
our place in the universe.

One of the fundamental questions
in science is,

"How big is the universe?"

To answer the question,
"How big is the universe?"

we have to answer the question,
"What shape is the universe?"

And by "shape" I mean geometry,

I mean, "How is the universe curved
on its larger scales?"

If we were to discover
that the universe does have

some sort of geometric curvature,

then this might imply
that it wraps around

in on itself
over incredibly large distances,

and that if you could travel
in one direction long enough,

you would end up
at your starting point.

Another version is that
the universe is more like

an infinite flat plane, OK,
no curvature at all.

The further you travel, well,
the further you get,

and you never get back
to where you started.

To work out the shape of something,

we would normally just
step back and take a look,

but clearly, moving outside
the universe is a non-starter.

We can't jump in a rocket and fly

a thousand times larger
than our cosmic horizon

and see what the shape
of the universe is.
We just can't do that!

Our human perspective on
the larger universe is so limited,

so if we wanna figure out
what the larger shape and scale
of the universe is,

we're going to have to be
very clever indeed.

One way to be clever

is to think of the geometry of
the universe in its simplest terms.

JAMES: When we talk about
the geometry of the universe,

we really are talking about
geometry.

In order to do geometry,
you have to take measures.

You need a cosmic ruler to do this.

And it turns out
there's a great cosmic ruler

known as
baryon acoustic oscillations.

Baryon acoustic oscillations
are ripples

in the cosmic microwave background...

..the oldest light in the universe.

As the universe expanded
these ripples were imprinted in space

in a uniform way.

They provide a cosmic ruler
to measure vast distances over time

so we can gauge
if the universe expands

in curved space or over a flat plane.

When we use these cosmic rulers

to try to back out
the shape of the universe,

we're sure to a few per cent
accuracy that the universe is flat.

If the universe is flat,
we could set off into the cosmos

and continue travelling for ever.

There may be no edge
to our universe...

..because a flat universe
can be infinite.

Now we're thinking of the universe
as something that does go on
for ever,

that the stars and galaxies
never have an end.

And how can something
truly infinite really exist?

Infinity is weird because we...

It's a concept of...
Cos it's endless.

What does that mean? Who knows?
I don't know. (LAUGHS)

PHIL: Infinity is a concept
more than anything else.

Our brains aren't evolved for that.
We evolved living in the plains,

we were apes looking for food.

We weren't evolved
to think about the universe

and all of this stuff.

HAKEEM: I just can't stop
contemplating this stuff.

The idea of infinity
and these large numbers

and even the tininess
of everything, it's nuts.

Yeah, thinking about infinity makes
my head hurt a little bit.

An infinite universe
has profound implications

for understanding
our place in the cosmos.

It guarantees we're not alone.

If the universe is infinite,

then there could be
an infinite number of galaxies

that have planets with life,

an infinite number without life,

then because life
did appear here on Earth,

it's physically possible,
therefore it will definitely happen

elsewhere in the universe.

In a flat universe, alien life could
come in an infinite number of forms.

But there is an altogether
stranger guarantee.

If the universe has no edge,
this means

that things that seem like
they are impossible become possible.

Every possible arrangement
of matter,

every possible history of a galaxy,

of a solar system,
of a planet like Earth

is possible and is happening
right now

in parallel to us
somewhere over there.

So that means that
there has to be another place

that has a galaxy just like ours...

..and it would have
an Earth just like ours.

It would have people,

it would have another version
of you, another version of me.

It's 100% guaranteed that there's
another Max Tegmark out there...

..having exactly this conversation,
and, in fact, many of them.

An infinite universe
full of Max Tegmarks

may be a strange concept,

but what's truly mind-bending
is understanding the physics

of a flat universe.

If the universe is infinite
and it's expanding,

what is it expanding into,

and what did it expand from?

And was there ever an edge
to the universe?

Fortunately the answer is that

it doesn't make sense
to ask that question.

Everything is expanding, including
the universe that we exist within.

So, in fact, it's not
expanding into anything

because it is everything.

To help understand what's going on
in an infinite universe,

we need to go back to the Big Bang.

We wanna think of the Big Bang
as an explosion in space,

like it happened someplace,

but there wasn't any place
before the Big Bang.

Space existed inside
of the Big Bang itself,

so it's not an explosion in space,
it's an explosion OF space.

We're sometimes told that
at the Big Bang

the universe started out
very, very small and then got big,

but how can a finite point
become infinite?

Well, if the universe is infinite,

then it was also infinite
at the Big Bang.

And this is a tough thing
to think about.

Think about it this way.
In an infinite universe
the galaxies go on for ever,

and now there's a great distance
between every galaxy.

But once upon a time, the galaxies
were closer together, say,

half their current distance apart.

But they still went on for ever -
the universe was still infinite.

In a flat universe,

space was infinite
from the beginning.

There was never
a single point in space

where the Big Bang happened.

It happened everywhere.

An infinite universe
offers infinite possibilities,

but no edge to space.

But there may be
another kind of edge,

one that will only reveal itself
if the universe dies.

We live in an infinite
and expanding universe.

Space has no edge -
it goes on for ever.

But there could be a different kind
of edge to our universe -

an edge of time.

The universe seems to have begun
13.8 billion years ago,

in the past,
so there's some inclination,

some impression
that it's finite in time.

What we call the Big Bang is,
as far as we understand it,

a beginning,
a start of the universe.

The universe has a finite edge.

Now, does it have an edge
in the future?

We used to think
that time would someday

come to a catastrophic end...

..along with the planets, galaxies,
and all life in the universe.

If we know there's a Big Bang,
if we know the universe started,

expanded and cooled,
it's very natural

to wonder whether someday
the expansion will stop,

reverse and come back.
And that's a Big Crunch.

In a Big Crunch,
our expanding universe

will begin to contract.

Stars and planets will
smash into each other.

Galaxies will collide.

And all of the life left in space
will be compressed

with all other matter
into a singularity.

If this theory is true,

then the universe will have
a beginning...and an end of time.

If we live in a universe
that will expand,

stop expanding,
and then go back into a crunch,

then it has, in effect, two edges.

But there's
a much stranger possibility.

Perhaps the end is but a beginning

where the universe
is an oscillating universe.

It has a Big Bang-like beginning,
it expands to a maximum size

and then goes back
into a Big Crunch,

and does that over and over.

We could be residents of a universe

created from the ashes of another.

A single universe in a stream
of bouncing universes...

..each full of galaxies,
planets and life.

But our most recent observations
of the universe

suggest a Big Crunch
isn't in the cards.

Once again, dark energy is key.

It's pushing our universe apart.

The universe is being
dominated right now by dark energy,

and because the dark energy
doesn't seem to be fading away,

it's easy to imagine
eternal expansion toward the future.

Dark energy seems to be overpowering
the large-scale effects of gravity.

But some theories suggest

that this strange force
could switch sides.

It's possible that
the currently repulsive dark energy

will someday change sign and
become gravitationally attractive.

If that's true, and if there's
enough of this energy

or if it grows in magnitude,

then the universe would slow down,

eventually come to a stop,

and then reverse its motion
and collapse in on a Big Crunch.

So in that case,
despite the current acceleration,

the universe could ultimately
reverse its motion

and collapse in on itself.

Dark energy really is little more
than a word for our ignorance,

and until we can figure out more
about what this stuff is,

we will not know
our ultimate destiny

and whether things are gonna
keep flying apart

or come crashing together or what.

But most scientists think
a Big Crunch is unlikely.

For a while we didn't know
if the expansion of the universe

was going to slow, stop,
and reverse itself

because of gravity.
There are all these galaxies

in the universe and they're pulling
on each other by their gravity.

And if the expansion
isn't fast enough,

that gravity might be strong enough
to stop the expansion
and re-collapse the universe.

Now with dark energy, we know
that there's no way that can happen.

The universe is
going to expand for ever,

because dark energy is pumping it
full of acceleration.

In order for there to be
a Big Crunch,

our understanding of dark energy
would have to change a lot.

That is, dark energy would have to
be extremely weird

and turn off in some very funny way

for the universe to suddenly
stop expanding and re-collapse.

Without a Big Crunch, there is
no future edge to time.

SEAN:
The universe is not only expanding,
but it's being driven by dark energy

to expand faster and faster.

And dark energy doesn't dilute away
as far as we can tell,

so the simplest idea
is that the universe will simply

continue to expand eternally
toward the future.

Just like space,
time would go on for ever.

It may sound like a better fate
for life in the universe,

but it's not.

One of the consequences
of this dark energy

that's causing the acceleration
of the universe

is that we eventually are headed
towards the Big Chill.

I should say we're eventually headed
towards (DEEP VOICE) the Big Chill.

So the universe
is getting colder and colder

and things are getting
more and more spread out.

So the accelerated, and continual,
and forever expansion
of our universe

might make for a frankly
depressing end to time itself,

the ultimate entropy-based
heat death of the universe

where you would walk out
and see no stars in the sky...

..see absolutely nothing.
There will come one day

when the very last star
in the universe

just fizzles out, and that is it.

In the future, space will be
a cold, dark, infinite void

where time goes on for ever.

There will be nothing to do
but suffer in the eternal expanse.

It's our inevitable fate

if there's no future edge of time
in the universe.

But even if there isn't
an edge to the universe,

could there be edges
within the universe?

(APPLAUSE)

April 2019.

An international team of astronomers
makes a special announcement.

And we are delighted to be able
to report to you today

that we have seen
and taken a picture of a black hole.

Here it is.

(APPLAUSE)

It's a picture
of a supermassive black hole

at the centre of the M87 galaxy,

54 million light-years away.

It may also be the first image

of an edge in the universe.

Black holes create
a really interesting scenario

when we think about space,
and the universe having edges.

This edge between space
outside and inside a black hole

is called the "event horizon".

The event horizon of a black hole
is a region

within which once you cross inside,

the gravitational tug is so strong

that even light cannot escape,

which means nothing can escape

once you cross
inside the event horizon.

So that really is sort of an edge,

because it really does create
a boundary.

The event horizon isn't
a physical barrier in space.

The event horizon is an edge of
the part of the universe
we can visit,

but it's not an edge in the sense
there's anything there.

You would just pass right through it
if you got right up to that place.

So it's sort of
a conceptual boundary

between two different parts
of the universe.

If we did send a manned probe
to a black hole,

it would be a one-way trip.

The event horizons of black holes
are a sort of edge

because once you pass through
an event horizon,

you are cut off
from the rest of the universe.

You can never go back out.

You are outside of our universe.

Once you've crossed
inside that region,

you are never coming back out,
you know, and that's an edge.

Once inside the black hole,

the probe would be
in a separate part of space...

..cut off from the rest
of the universe.

Falling through the event horizon
of a black hole

is like jumping over
the edge of a cliff.

You can see the edge,
and you can see the edge go by,

and then when you're at the bottom,
you can look up

and see what's happening
at the top of the cliff,

but you can never go back.

At the bottom
of this black hole cliff

sits a singularity,

a region of space where
the laws of physics go off the rails.

Deep toward that singularity
could be as surprising

as you might imagine,
and yet still a possibility.

If you map the space-time
around a black hole

in a very particular way,

there emerges
a sort of mirror universe,

a parallel universe
on the other side of the black hole

identical to our own and traversable
via the black hole.

So black holes are not just edges
to our universe...

..they may also be gateways
to other universes.

It's highly conjecture,
but if there's ever gonna be

a space or a region
where you're making connections

with, say, some other universe,

a black hole, in principle,
could be a portal to that.

But it's highly unlikely
anyone will ever want to venture

beyond an event horizon.

And our pursuit of the other edges
in the cosmos

offers little hope.

We can never travel beyond
the cosmic event horizon...

..and we will never be able
to see beyond the edge

of our observable universe.

So can we ever hope
to discover the true edge

of the greater universe,
or find out if it even has one?

My feeling is that probably
we should not think

about edges for the universe.

Everything you've ever seen
in your life is finite.

It has an inside and an outside,
it has an edge.

The universe might not be like that.
It's probably not like that.

There's probably no sense
in which the universe has an edge.

We used to think
that the ultimate limits

on the future life
were set by nature.

We couldn't get off the planet,
or there was nothing beyond
our solar system.

Now we realise we have
this vast, vast cosmos out there

and the ultimate limits are actually
simply our own imagination,

and our ability to do
great things with it

rather than self-destruct.

Our future destiny
is in our own hands,

and I find that very empowering.

It is beautifully frustrating
to realise how limited we are,

to realise that we're probably
never going to get a true view

of the real extent of the universe.

We should keep an open mind,
be humble,

but I think we should
give up on the idea

that things should have edges
cos that's what we're familiar with.

The universe is something special.

What matters to us,

and will only ever matter to us,

is the observable universe

because that's the limit
of what we can see,

and that is the limit
of what we can know.

So there is
an edge to the universe -

there's an edge to what we can know.

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