How the Universe Works

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.