Well, it turns out our cosmic fate will be decided by a fight between two titans.

The two warriors deciding the fate of the universe.

Our universe was born 14 billion years ago in the Big Bang and has been expanding ever since.

For some reason, new empty space is being created out of nothing between galaxies.

Space itself is becoming bigger.

Will it go on forever or will it stop one day?

Today, we think the universe is at the mercy of two cosmic demigods fighting for dominance.

The first one is all the stuff in the universe.

Galaxies, gas, dark matter.

While they're very different, they just want to do one thing, get together.

Matter is attracted by matter, pulled together by gravity.

And gravity also pulls on the universe as a whole, slowing the expansion that started at the Big Bang.

Our second warrior is empty space.

While it seems, well, empty and powerless, empty space has an intrinsic energy.

We don't really have an idea what this energy is or why it's there.

It's really a placeholder that fills a blank in our understanding of reality.

But it's got a cool name, dark energy.

Dark energy pushes things apart, a sort of anti-gravity that accelerates the cosmic expansion.

So, we have all the matter in the universe pulling in and empty space infused with dark energy pushing out.

Whoever wins will kill the universe in fun ways.

But who will win?

It all depends on the mysterious dark energy.

Will its strength stay the same, a common assumption just because it keeps our model simple?

Or will it get weaker?

Or will it get more powerful over time?

1.

Constant dark energy, heat death.

If the strength of dark energy stays constant, it will win the war.

Since space is growing, matter is getting more and more diluted, like sugar in a cup being filled with more and more tea.

But as the universe expands, new empty space is created, which brings more dark energy, which pushes everything apart even more, which creates more empty space, which makes the universe grow even faster.

A feedback loop that will make the universe expand at an exponential rate.

Every 12 billion years or so, it will double in diameter forever.

But while dark energy is winning the war, matter is winning at least one battle.

At short distances, it can keep things together.

Local galaxy bubbles can overcome the push of dark energy.

In a few billion years, our local group of galaxies will merge into a gigantic ball with trillions of stars.

It will soon become our last view of the cosmos.

All other galaxies will be pushed away by the expansion.

For us, it will look like the rest of the universe is rushing away, until in a few hundred billion years, we won't see other galaxies at all.

We'll be alone, surrounded by a seemingly infinite dark void.

In about 100 trillion years, all the stars of our super galaxy will have died out.

All gas that could create new stars has been consumed, and no new gas can come in.

The galaxy will be dark and filled with stellar corpuses.

Over quadrillions of years, white dwarfs and neutron stars will slowly cool until becoming truly dark, turning off the last lights of the universe.

All structures, big and small, will slowly dissolve.

One by one, all dead stars and planets will leave the super galaxy, which slowly dissolves over sextillions of years.

Every object will end up on its own, which means that dark energy takes over again, creating more and more empty space between everything.

Objects will be so far apart, that it will be as if each had a universe for itself.

Not much happens anymore, until in a good old years, all black holes will have evaporated.

In the end, entropy and dark energy won't stop until their job is finished.

Over time spans you might as well call forever, all remaining structures might even dissolve into single particles that will be pushed away from each other by an ever-growing empty space.

Imagine a whole universe with just a single lonely particle traveling through nothingness.

This is the final state, the true end.

The big freeze, or heat death.

A completely featureless, cold and eternally expanding universe.

Nothing will ever be able to happen again. Phew, that was a bummer.

What else could happen?

Two, increasing dark energy.

Big rip.

What if dark energy gets stronger?

In this case, empty space won't just win over matter, it will literally rip it to pieces.

In the big freeze scenario, matter lost the war but won some battles.

But here, matter wins nothing.

Dark energy is growing stronger over time, overcoming the pull of gravity and creating new empty space at smaller and smaller distances.

In this scenario, things will escalate quickly.

It could start as early as 20 billion years from now.

First, dark energy will create empty space between individual galaxies.

Our galaxy will leave its local cluster and begin to drift alone in a rapidly inflating and ever darker cosmos.

Some billion years later, empty space starts to push between individual stars, dissolving the galaxy.

If you live on a planet in a star system, the night sky will start looking sad and gloomy, as other stars are pushed too far away to be seen.

A few million years after the sky turns dark, dark energy starts to create empty space inside star systems.

Your planet is pushed away from its star and all life in the universe freezes to death.

There's not much time left as a few months later, dark energy is creating empty space inside solid objects.

Stars, neutron stars, planets, asteroids, everything solid is being ripped into pieces.

If you're on a spaceship, you only have a short time before you are ripped apart.

Half an hour later, even atoms are destroyed, as new space is being created so furiously that electrons and nuclei are separated.

Now the universe has just a fraction of a second left.

In this final moment, only dying black holes remain, drained and defeated by dark energy.

They're tiny, septillions of times smaller than an atom, and they explode with the power of a trillion supernovae in a trillionth of an octillionth of a second.

Finally, the very fabric of reality is torn to its core, obliterating space-time itself, the Big Rip.

Space and time have lost their meanings, making it impossible to predict what will happen next. 3.

Decreasing dark energy, big crunch.

Poor matter, but there is one scenario where it wins.

If the strength of dark energy decreases with time, and if this reduction is strong enough, the pull of gravity will win, and all the things in existence will move towards each other, unfortunately making the universe collapse into itself.

No one knows when this might begin, but it could be as soon as a few hundred million years.

What will it look like?

As the universe begins to contract, over billions of years, galaxies and galaxy clusters approach each other until they eventually collide.

They're mostly made of empty space, so a collision is like the gentle merger of two clouds.

At any rate, first galaxies and later individual stars get closer and closer.

As the universe goes on collapsing, you might worry about stars and planets eventually crashing against each other.

This will happen, but it's not your worst problem.

If space itself shrinks, this also concentrates all the radiation emitted in the past by all the stars, supernovae, and quasars that ever existed.

Now, empty space is filled with radiation.

The dark nothing between stars is heating up, making life unpleasant and then impossible, as planets just burn.

Slowly at first, then rapidly, space gets as hot as it was after the Big Bang.

Stars are pretty hot, but now the space around them is hotter.

They're literally boiled from the outside.

As the universe collapses into itself, all galaxies and all stars merge into a single ball of hot plasma.

The Big Crunch is complete.

From here on, there are two possibilities.

Either the universe will collapse completely into a singularity, a point of zero size and infinite density, without space and time, the way it might have been before the Big Bang.

Or the universe could bounce back, restarting the cosmic expansion, creating a new baby universe.

Somewhat poetic, really.

Everything died, but everything is reborn.

But to be clear, we have zero evidence that this has happened before or well in the future.

So what will happen?

Most scientists think that dark energy will stay constant, so the likely fate of the universe is heat death, eternal cold, and utter boredom.

Which seems sad, but has a huge upside.

In this scenario, we get to have the universe for the longest.

It gives us trillions of years to expand, jump from star to star, maybe even from galaxy to galaxy.

We might even find a way to keep consciousness around forever.

We don't know.

So we just have to wait and see, and make the most of the amazing universe we have right now.

NASA Jet Propulsion Laboratory, California Institute of Technology