The Expanding, Accelerating Universe#

First off, we know the universe is getting bigger. It’s expanding. But here’s the really wild part: the speed of this expansion is actually picking up. It’s accelerating!

• We used to think the universe was like throwing a ball up – it goes up, but gravity should eventually slow it down or make it fall back.
• But the universe is acting more like throwing a ball up and watching it just fly away faster and faster.

Enter: Dark Energy#

What’s making it speed up? Scientists call it dark energy.

• They believe dark energy is a strange, mysterious phenomenon that fills the whole universe.
• Back in the day, Einstein thought about something like it, then decided it was a silly idea. Now, astrophysicists think it’s quite likely.
• The tricky part is, this is all pretty theoretical. We don’t actually know what dark energy is like or what its exact properties are.

But, based on different ideas about dark energy, scientists have come up with three main ways the universe might end.

Scenario 1: The Big Rip#

• Since the beginning, the universe has been expanding. Space is being created everywhere, equally, for reasons we don’t know.
• The space between galaxies expands, pushing them apart.
• The space inside galaxies expands too, but usually, gravity is strong enough to hold everything together there.

In the Big Rip idea:

1. The expansion accelerates so much that space expands too fast for gravity to keep things together anymore.
2. First, large structures like galaxies get pulled apart because the space between their parts expands super fast.
3. Next, big objects like black holes, stars, and planets die. Their own gravity isn’t strong enough to hold them together anymore, so they just break apart into their basic stuff.
4. In the very end, space would be expanding faster than the speed of light.
5. Even atoms would start to be affected and would just break up.
6. Once space is expanding faster than light, no particle in the universe can bump into or interact with any other particle anymore.
7. The universe would dissolve into countless solitary particles that can’t touch anything else in a strange, timeless state. Hmm, and you thought you felt lonely!

Scenario 2: Heat Death or a Big Freeze#

Putting it simply, the difference between the Big Rip and Heat Death is that in Heat Death, matter stays whole for a really, really long time (but still a limited time) and slowly turns into radiation, while the universe keeps expanding forever.

How does this work? It’s about entropy.

• Every system naturally moves towards a state of maximum entropy. Think of a latte macchiato: at first, it has distinct layers, but over time, it cools down and mixes until it’s all uniform.
• This applies to the universe too. As the universe gets bigger and bigger, matter slowly breaks down and spreads out.
• Eventually, after many generations of stars are born and die, all the gas clouds needed to make new stars will be used up. The universe will turn dark.
• The stars that are left will eventually die out.
• Black holes will slowly shrink and disappear over trillions of years because of something called Hawking radiation.
• When all this is done, only a very spread-out “soup” of photons (light particles) and other light particles will remain.
• Even this might eventually decay.
• At this point, all activity in the universe stops. Entropy is at its absolute highest.
• The universe is dead forever.

Unless… theoretically, it’s possible that after an unbelievably long time, there could be a random drop in entropy. This could happen because of something called quantum tunneling, and it might even lead to a new Big Bang.

Scenario 3: Big Crunch and Big Bounce#

This one’s the most hopeful story!

• What if there’s less dark energy than we think, or if it fades away over time?
• In this case, gravity would become the strongest force in the universe one day.
• In a few trillion years, the universe’s expansion would slow down, stop, and then reverse!
• Galaxies would start rushing towards each other, merging as the universe gets smaller and smaller.
• Getting smaller also means getting hotter, so temperatures would soar everywhere, all at once.
• About 100,000 years before the final collapse (Big Crunch), the background radiation would be hotter than the surface of most stars, basically cooking them from the outside.
• Just minutes before the Big Crunch, the cores of atoms would be ripped apart.
• Then, supermassive black holes would start eating everything around them.
• Finally, all the black holes would combine into one giant, supermassive “mega-black hole” containing everything in the universe.
• In the very last moment before the Big Crunch, this mega-black hole would swallow the universe, including itself.

The Big Bounce theory adds to this, suggesting that this collapsing and rebounding has happened many times before. The universe might go through an endless cycle of expanding and contracting. Well, wouldn’t that be nice?

What Will Actually Happen?#

So, what’s the final act for the universe? Right now, Heat Death seems like the most probable outcome.

But, we (at Kurzgesagt) really hope that whole “dead forever” thing is wrong and that the universe will just keep starting over and over again.

We don’t know for sure one way or the other, so hey, let’s just choose to believe the most uplifting story is true!

• Video URL mentioned: https://www.youtube.com/watch?v=4_aOIA-vyBo
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• Subtitles for the video were done by the Amara.org community.