Original subtitles

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

So, can we ever find

the edge of the universe?

captions paid for by discovery communications

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...

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

But is this the edge of the universe?

The universe all around us is filled with galaxies,

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

If you traveled 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's 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.

Then when you look at galaxies, then 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 going to 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 is 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

centered 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.

One of the wonderful things 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 accurately 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.

And 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.

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

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

that relativity says nothing goes faster

than the speed of light.

Okay, you've been lied to.

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.

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 10 feet away from you.

Expand that by a factor of 2

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 10 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.

Imagine you're a sprinter on a racetrack.

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 sort of teasing us to say, "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 forever.

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.

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 forever.

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 they'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 says that space outside the observable universe

is pretty much the same as our own cosmic neighborhood.

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 universe

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.

This incredibly violent inflation process

is actually monkeyed with the very fabric of space itself,

so that a lot of the things that we were taught

that are laws of physics are different there.

So in an 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 could imagine is a super large-scale structure

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

and 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 wall.

If our fish swimming around in the arctic near an iceberg,

there will be a domain boundary

between the water being in the liquid state

where I am and the solid state inside the ice.

So a domain wall is just a wall between two domains.

If it's water, this could be ice,

this could be liquid.

If you're talking about space,

this could be a kind of space maybe you can live in.

This could be a kind of space where you don't want to 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 somehow 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.

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 age 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 forever?

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 largest scales?

If we are 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.

Okay? 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 nonstarter.

You can't jump on 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 want to 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.

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.

Baryonic 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 percent accuracy

that the universe is flat.

If the universe is flat,

we could set off traveling into the cosmos

and continue traveling forever.

There may be no edge to our universe

because a flat universe can be an infinite universe.

Now we're thinking of the universe as something

that really does go on forever,

that the stars and galaxies never have an end,

and how can something truly infinite really exist?

Infinity is weird because it's a concept of,

because it's endless.

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

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.

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.

But 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.

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 who would have another version of you,

another version of me.

It's 100% guaranteed that there is 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's it expanding into and what did it expand from?

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 want to think of the big bang as an explosion in space,

like it happened some place,

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.

This is a tough thing to think about.

Think about it this way.

In an infinite universe, the galaxies go on forever

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 forever.

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 forever.

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 age.

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,

it expanded and cooled, it's very natural to wonder

whether or not someday the expansion will stop,

reverse, and come back, and that's a big crunch.

In a big crunch,

our expanding universe would begin to contract.

Stars and planets would smash into each other.

Galaxies would collide,

and all of the life left in space would be compressed

with all the other matter into a singularity.

If this theory is true, then the universe

would have both 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 a 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.

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 forever

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.

The universe is not only expanding,

but it's being driven by dark energy

to expand faster and faster and the 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 will go on forever.

That might sound like a better fate for life and 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 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 and infinite void,

where time goes on forever.

There will be nothing to do

but suffer in the eternal expanse.

It's our inevitable fate that 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?

If you wanted to visit the edge of the universe,

then go find your nearest black hole and jump on in

because that's a one-way trip.

If you cross this edge, you will never return.

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.

It's a picture of a supermassive black hole

at the center 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.

The 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 is not a physical barrier in space.

Event horizon is an edge of the part of the universe

we can visit, but it's not an edge

in the sense that there's anything there.

You would just pass right through it

if you actually got right up to that place.

So it's sort of a conceptual boundary

between two different parts of the universe.

If we sent a man to probe into 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, 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 by 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 going to be a space, or 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

that anyone will ever want

to venture beyond an event horizon to find out,

and our pursuits of the other edges in the cosmos

offer little hope either.

We can never travel beyond the cosmic event horizon.

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 the 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 realized we have this vast,

vast cosmos out there and that 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 realize how limited we are,

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

of the real extent of the universe.

We should keep an open mind, we should be humble,

but I think that we should give up on the idea

that things should have edges

because 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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