Hey, smart people, Joe here.

Now, I don't want to alarm you, but the world is going to end.

All of this, gone.

Poof.

Adios.

The Sun swells into a red giant.

The oceans boil away, planet scorched.

And scientists are certain that all of this will happen.

And I'm sorry if you're just hearing this from me, but I want you to know the truth.

Now, on the bright side, this isn't gonna happen for four to five billion years.

In a universe bigger than we can fathom, across eons of time, stars like the Sun and planets like Earth, they blink out of existence all the time.

But could all of it end?

I mean, everything?

The whole universe?

Yes!

And it probably will.

[energetic music] I've been thinking about the end of the universe a lot lately.

And I know what you're thinking.

"Hey, Joe, you okay?

COVID finally getting to you?"

Don't worry.

That's not why.

It's because I've been reading this awesome book, "The End of Everything (Astrophysically Speaking)" by my friend, astrophysicist Katie Mack.

It is great.

And I have never had more fun learning about how everything will cease to be...forever.

[bell chimes] The big crunch.

This is Andromeda, a trillion stars whizzing around a super-massive black hole, and the Milky Way's closest galactic neighbor.

And it's headed this way right now.

Andromeda is currently around 2 1/2 million light-years from Earth, but it is closing that distance at 110 kilometers per second on a crash course with our galaxy.

In about four billion years, the galaxies will start to merge into one.

Now, across the cosmos, galaxies collide on a regular basis, and we can see these collisions with powerful telescopes, even simulate these collisions on really, really, really big computers, that go beep, boop, boop and do hard math stuff.

But these galactic mergers are becoming more rare, because giant stuff like galaxies are drifting farther apart.

I don't know, maybe they should check in once in a while, just send a text.

It's not that hard.

You set up a coffee date, catch up on old times.

Actually, galaxies are drifting apart because the universe is expanding.

And that expansion all started with one big push, the big bang.

Think of the universe like this slinky.

It started here, and since that push, the space between each rung of the spring has been getting slightly bigger.

That's like neighboring galaxies.

And as a result, the ends have gotten way farther apart, because of the collective distance between all these small expansions happening within its boundaries.

All that started with the big bang.

The question is, will that initial push from the big bang eventually fizzle out?

Think about it.

Thanks to gravity, everything is always attracting everything else.

That's why Andromeda and the Milky Way will one day collide.

Now, the farther apart two objects are, the gravitational attraction between them does get weaker.

But it never reaches zero.

It seems like the gravity of everything in the universe should eventually pull it back in.

So what happens if that expansion stops and goes in reverse?

Doom.

In a collapsing universe, the space between galaxies would get smaller and smaller.

Collisions would become more frequent, stars and planets flung out of orbit.

Black holes ignite and merge, becoming even more massive.

Matter whirling into these giant black holes is superheated by friction so hot that it shoots out jets of radiation scorching anything in their path.

I mean, imagine rubbing two sticks together so hard that the fire that they create emits x-rays.

It's like that.

During these galactic crashes, some new stars will be born, even new planets, and some of them might even have time to develop life, if they're not in the path of a sterilizing x-ray beam.

But eventually, all of these too will be destroyed by fire.

As the universe collapses, all of the energy that's ever been emitted by every star or drawn into any black hole will be squished into a smaller space and squeezed into higher energy wavelengths.

This concentrated radiation will become so intense, the temperature of space will rise until nuclear explosions rip stars and planets apart, leaving space full of hot plasma.

At this point, the universe resembles the early moments after the big bang.

The temperatures and densities are so high, we don't actually have a way to describe them except really, really, really hot.

That's three reallys, so you know it's bad.

[bell dings] But what if that crunch never happens?

Well, today we think the universe isn't likely to snap back on itself.

But turns out that's not such good news either.

The heat death.

When you throw a ball into the air, it slows down and falls back down.

Okay, everybody knows that.

But in the early days of the universe, things were close together, and so gravitational attraction slowed the outward push of the expansion from the big bang.

And expansion was slowing down, just like a falling ball, for a while.

But in 1998, scientists discovered about five billion years ago the expansion of the universe started speeding up, and it's still speeding up.

Like, what would happen if you threw a ball up in the air and it kept going faster in the same direction forever?

And the bigger our universe gets, the faster it's expanding.

That's weird-- really, really, really weird.

And it's three reallys, so you know it's weird.

There's some mysterious property of empty space pushing outwards in all directions.

And we call this dark energy.

The more empty space there is, the more dark energy, and the more it pushes.

Dark energy might be a source of energy that never runs out and that we find everywhere there's empty space, something that astrophysicists and cosmologists call a cosmological constant.

And as the universe expands, it makes more empty space which means more dark energy, and the expansion gets faster and faster.

On the nearby scale, other forces like gravity and electromagnetism are still strong enough to keep molecules together, your feet on the ground, our planet orbiting the Sun, and our galaxy from falling apart.

But the vast spaces between galaxies are dominated by dark energy.

The universe is already expanding faster than the speed of light, meaning that light emitted from galaxies beyond this point... will always be moving away from us and will never reach Earth.

And as that expansion speeds up, closer and closer galaxies will be outrun and they'll disappear from our view.

Eventually every galaxy will be alone in a dark universe.

Nothing will approach anything else.

The stars already shining will burn out over billions of years.

100 billion years from now, the Milky Way will dim and eventually fade to black.

Even black holes will evaporate.

There's no fuel for new stars, just vast, empty space is left, filled only with dark energy, expanding forever.

A slow death by loneliness, lasting hundreds of billions of years.

It could be worse though!

If dark energy actually gets more powerful over time, that--that would be so bad.

The big rip.

Meet phantom dark energy.

It's like dark energy on steroids.

Phantom dark energy would not only expand the space between things, but expand things themselves.

Remember, in our universe today, fundamental forces are enough to hold things together even as dark energy expands the universe as a whole.

But phantom dark energy would be strong enough to overcome all those other forces.

It wouldn't just push galaxies apart, it would push stars away from galaxies, planets away from stars, tear planets apart.

Eventually, atoms, and molecules, and particles would rip apart as phantom dark energy becomes stronger than the electromagnetic forces that hold them together.

And in the final moments, the very fabric of space would be torn apart.

A big rip.

But don't worry, if the big rip is coming, it's not coming for, like, 200 billion years.

So, at this point it might be nice to remind ourselves that both you and I will be dead by then.

So hooray.

Vacuum decay.

But there's one scenario that might be less far off.

It could happen anytime.

It could be happening right now.

But it destroys everything so fast, you wouldn't feel it.

But it all starts with a bubble.

It's like a self-destruct button for the universe, setting off a wave of apocalypse moving at the speed of light.

How could that happen?

Something called vacuum decay.

Now the Higgs field is a sort of energy field that permeates all of space.

It surrounds us and penetrates us.

It binds the galaxy together.

[clears throat] The Higgs field sets the rules for physics on the scale of particles, like what particles exist, how they interact with each other, what their masses are.

Atoms and molecules, and trees, and people, and planets, for that matter, are only able to hold themselves together and exist because the Higgs field has a particular value.

If that value were slightly different, it could scramble all of that up.

In fact, it was like this in the early universe.

The Higgs field had a different value and there were different particles with different rules.

The Higgs field is pretty stable.

It's settled down here at a low energy.

But there is a small chance that an ultra-high energy explosion, the evaporation of a black hole or a quantum event, could kick the Higgs field to some lower value.

This would create a sort of bubble in the Higgs field, a new kind of space with different rules.

And this bubble would zip through the universe at the speed of light, ripping particles apart, swallowing everything in the bubble.

And if it hit you, you'd be gone before your brain knew anything happened.

Once this bubble destroys everything, the space inside it collapses into a black hole.

And they all lived happily never after.

Don't worry, there's no way that we could trigger vacuum decay ourselves, but the universe might.

Don't lose hope, though.

We have one more scenario.

What if the end isn't the end?

There is another option.

Cyclic universes?

Some other more recent theories point to cycling universes, where the end of one is the beginning of another.

These theories involve ideas like antiuniverses or parallel fabrics of space time.

But they would mean that an end really isn't an end at all.

As we learn more, all of these theories may change.

And we may even find some new ones.

But understanding our universe so deeply that we could one day figure out its ultimate fate, as well as ours, well, that's a nice way to add meaning to a universe that we know won't last forever.

And besides, none of this will happen for a very long time-- like, at least tens of billions of years, maybe more than 100 billion.

I guess, unless it's vacuum dec-- Stay curious.