What's at the edge of the universe

and what happens if we try to get there?

You might be thinking, wait, how is there

an edge to the universe if it's infinite?

This gets talked about a lot.

And people usually say one of the following.

The universe defines all of space and time that exists.

So that's one definition of universe.

But is it even part of our universe if we can never

interact with it?

And what if our universe is very, very different

beyond the edge?

The universe is infinite because general relativity tells us

that the universe is demonstrably flat

and therefore the galaxies go on forever.

But how flat?

Are you sure you measured the universe's curvature

with infinite precision?

And my favorite, we can never know and never test it.

So it's not even a scientific question.

Oh, yeah.

I'll science any [BLEEP] question I please.

This is "SpaceTime."

OK.

Sorry.

Before we get carried away, let's talk about the edge

or edges of the universe and what

it might take to get there.

In a previous episode, we talked about the size of what we

call the observable universe.

We even gave you a number, 93 billion light years

in diameter, 46 billion in radius.

Go ahead and watch it again.

It'll be useful.

While you're at it, we're also going

to be talking about the CMB.

So it wouldn't kill you to watch that episode also.

OK.

Let's start with that 46 billion light year number.

We defined that as the current radius of the known universe.

It's the distance to that blob of the CMB, the most

distant thing we can see in that direction.

Now, it's not 46 billion light years to that actual blob.

It is currently 46 billion light years to whatever galaxy

or galaxy clusters that blob evolved into,

racing away from us with the expanding universe, as it did.

We call this the particle horizon of the universe.

It's the current instantaneous distance

to the most distant part of the universe that could possibly

have a causal connection to us.

Anything inside the particle horizon

is referred to as the known universe.

Now when I say the "current instantaneous distance,"

I mean it's the distance that you

would have to travel only if the universe froze in its expansion

and you were traveling through static space.

In cosmology, this sort of instantaneous distance

is basically what we call the proper distance between two

points.

But nothing actually ever travels the proper distance.

That's not how spacetime works.

The shortest path in spacetime is

defined by the geodesic, the path

of light between two points.

Even light takes time to make any journey.

So we have to factor in the time interval, especially when space

is changing.

To travel to the particle horizon,

we need to move through expanding space.

And the closer we get to our destination,

the more space will have expanded over the remaining

distance.

How far would you have to travel?

Bad news, you'd have to travel infinitely far,

even if you were in a spaceship that could

travel at the speed of light.

Just as black holes have event horizons, so too do universes.

The event horizon of a black hole is that point beyond which

we can never receive information because light from that point

is redshifted into oblivion.

It's a boundary to the observable universe.

There's a region of this universe from which we can

never receive any new signal.

That is, any signal that's emitted today.

This is because the distance that signal

has to travel to get to us will be expanding faster

than the speed of light before the signal reaches us.

The same thing applies to our light speed spaceship.

We can ever get to anything beyond the cosmic event horizon

because that space will be moving away from us faster

than light before we reach it.

Now, here's where it gets weird.

The event horizon of the universe

is actually closer to us than the particle horizon.

Given our best measurements of cosmological parameters,

we think that the cosmic event horizon is around 16

billion light years away.

This means that there are galaxies

that we can see now that we could never reach or even

communicate with.

We're sort of seeing ghost images

from outside the part of the universe

that we could ever interact with.

As our universe expands, more and more of it

will cross the event horizon and eventually almost

all of that will be lost to it forever.

Kind of sad, really.

And, of course, all bets are off if we can

break the cosmic speed limit.

So let's do just that.

There's no doubt that Einstein was

right in setting that limit for objects moving through space.

But two regions of space can have

superluminal relative speeds.

That's actually the motivation behind the warp drive, which

we might get to in a later episode

if you're up for the challenge.

But for now, let's just assume we have a nice Alcubierre-class

warp-ship and we burn the mass energy of entire stars

to chase the particle horizon.

What do we find?

Almost certainly, just more universe.

Bummer.

Remember, the particle horizon is just

defined by the limit of our current view.

Move to my left, and my observable universe

moves with me.

Wait a minute, and my particle horizon expands.

Travel to the particle horizon instantaneously and you'll

see the Milky Way as a cute baby CMB blob on your new particle

horizon.

And presumably, a pretty similar distribution of galaxies

and clusters all around you.

But what if we keep going?

What's far beyond that edge?

Well, that all depends on the geometry of the universe.

On the largest scales, the geometry of spacetime

is very flat.

It's lumpy on small scales due to stars and galaxies,

but smooth on large, sort of like ripples on the ocean.

Measurements of the distribution of galaxies and the CMB

confirm this flatness with very high, but not infinite,

precision.

If spacetime really is perfectly flat, then,

with the most simplistic application

of Einstein's equations, we get that the universe is infinite.

Now, people claim this a lot.

So if it's true, what happens if you cross the particle horizon?

The universe just goes on, and on, and on, and on, and on,

and on, and on, and on, and on, and on.

And infinity is its own amazing beast.

And there are many types of infinity,

including some that involve infinitely repeating versions

of this bit of the universe.

But is our universe really perfectly flat?

Think about it this way.

The surface of the Earth looks pretty flat to us

because we really can't see the curvature locally.

But get some perspective by taking

a ride on the International Space Station

and it's clearly curved.

What if the curvature of the universe

is so small that we're just not seeing far enough

or measuring precisely enough to detect it?

It's very possible that the universe has curvature just

inside the uncertainty range of the best measurements to date.

If that curvature is positive, then it

may be that the universe is really

the surface of a hypersphere, the 3D surface of a 4D sphere.

In that case, our warp-ship would eventually

travel all the way around this curved hypersphere

and get back to where it started.

OK, so how far would it have to travel?

Based on a recent estimate of the minimum radius

of the curvature of the universe,

you'd need to travel an absolute minimum

of 18 times the distance to the particle horizon

to get back to where you started, assuming expansion

froze for the whole journey.

We also have to keep in mind that these geometries assume

that we can just extrapolate Einstein's equations

in the most simplistic way.

In addition, although general relativity is pretty cool,

it's not a theory of everything.

Non-crazy ideas for the origin of cosmic inflation

suggest that our universe may just be a slowly expanding

bubble in an exponentially infinitely-inflating

multiverse.

Now, bubble universes may be finite in size regardless

of internal geometry.

And so they may have a true edge.

But what's on the other side?

Are the laws of physics, or even the number of dimensions, the

same?

Tell us what you think in the comments.

We'll cross that edge into the multiverse in another episode

of "SpaceTime."

Squishina and others ask whether it's contradictory or circular

to use Einstein's theory of general relativity

to prove itself?

Well, the only way to test a theory is to use it.

However, you're right in thinking that just one

prediction is not enough.

A model has to make multiple independent and accurate

predictions to be accepted as a theory.

And there are few theories with as

many independent and accurate predictions

as general relativity.

For example, the predictions GR makes for planetary orbits

can give us a mass for the Sun.

And that mass predicts the deflection angle

for light passing the Sun perfectly,

that is its gravitational lensing effect.

The same with galaxy clusters.

The galaxy orbits give us a mass for the dark matter

in the clusters and the lensing gives us a mass consistent

with this.

Shadowmax889 asks why stars and planets

aren't filled with dark matter?

Now, this is a great point.

Dark matter is cold and clumpy, which

means it can bunch together to form galaxies.

But it doesn't interact with itself

in any other way besides gravitationally.

This means there's a limit to its clumpiness.

To collapse completely into a star-sized object,

it would have to lose a lot more energy.

This is really hard unless you can

interact electromagnetically.

By comparison, the cold hydrogen gas that fills our galaxy

clumps together in giant clouds.

But then these clouds radiate light in different ways,

allowing the gas to cool even more and collapse into stars.

Dark matter doesn't do that.

So it stays sort of puffed up.

MrLewooz asks if we can please stop throwing monkeys

into black holes?

Don't worry.

No monkeys were harmed in the making of "SpaceTime"

and any events that can be consistently assigned

to our clocks at PBS.

Izvarzone informs us that upon winning a Nobel

Prize for discovering dark matter particles,

he or she would spend all of the prize money on Phoenix cases.

And, yeah, I guess that's cool.

But I only play 1.6.

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