0:00:01.595,0:00:03.589
One day the universe will die.

0:00:04.144,0:00:08.570
But why? And how? And will the universe[br]be dead forever?

0:00:08.570,0:00:10.652
And how do we know that?

0:00:10.652,0:00:13.727
First of all, the universe is expanding.

0:00:13.727,0:00:17.362
And not only that, the rate of[br]its expansion is accelerating.

0:00:17.762,0:00:19.668
The reason: dark energy.

0:00:20.293,0:00:25.479
Dark energy is a strange phenomenon that[br]scientists believe permeates the universe.

0:00:25.479,0:00:29.624
Until 1998 we thought that the universe[br]must work a bit like

0:00:29.624,0:00:31.439
a ball that you throw into the sky.

0:00:31.829,0:00:35.005
The ball moves up, but at some point[br]it has to come down again.

0:00:35.515,0:00:39.063
But the expansion of the universe[br]is actually speeding up.

0:00:39.633,0:00:42.781
That’s like throwing a ball up[br]and watching it fly away

0:00:42.781,0:00:44.769
faster and faster and faster.

0:00:45.269,0:00:47.301
Where is this acceleration coming from?

0:00:48.071,0:00:51.693
Well, we don’t know, but[br]we call it “dark energy”.

0:00:52.603,0:00:55.985
Einstein thought of it first and[br]then decided it was stupid.

0:00:56.535,0:01:00.138
Now, astrophysicists have[br]decided it is plausible.

0:01:00.628,0:01:04.716
Trouble is, this is all very theoretical,[br]and we don’t actually know

0:01:04.716,0:01:06.634
what the properties of dark energy are.

0:01:07.064,0:01:10.845
But there are various theories and they[br]lead us to three scenarios

0:01:10.845,0:01:12.676
for the end of the universe.

0:01:19.856,0:01:21.397
One: the Big Rip.

0:01:22.517,0:01:25.316
Since its birth, the universe[br]has been expanding.

0:01:25.646,0:01:29.453
For unknown reasons new spaces[br]created everywhere equally.

0:01:29.453,0:01:33.230
The space between galaxies expands,[br]so they move apart.

0:01:33.980,0:01:38.391
The space inside galaxies also expands,[br]but here, gravity is strong enough

0:01:38.391,0:01:39.722
to keep them together.

0:01:40.392,0:01:44.053
In the Big Rip scenario, the expansion[br]accelerates up to a point where

0:01:44.053,0:01:48.874
space expands so fast that gravity[br]can’t compensate for this effect anymore.

0:01:49.084,0:01:51.185
The result is a Big Rip.

0:01:51.725,0:01:55.199
At first, only large structures like[br]galaxies are torn apart,

0:01:55.199,0:01:58.633
since space between the single objects[br]expands very fast.

0:01:59.233,0:02:03.159
Next, big bodies like black holes,[br]stars, and planets die.

0:02:03.499,0:02:06.130
Their gravity isn’t strong enough to keep[br]them together,

0:02:06.130,0:02:08.091
so they dissolve into their components.

0:02:08.731,0:02:12.272
In the end, space would expand[br]faster than the speed of light.

0:02:12.862,0:02:16.220
Atoms would now be affected,[br]and they would just disband.

0:02:16.790,0:02:21.069
Once space is expanding faster than light,[br]no particle in the universe can interact

0:02:21.069,0:02:22.908
with any other particle anymore.

0:02:23.558,0:02:28.205
The universe would dissolve into countless[br]lonely particles that won’t be able to

0:02:28.205,0:02:31.712
touch anything else in a strange,[br]timeless universe.

0:02:33.002,0:02:35.910
Hmm, and you thought you felt lonely!

0:02:37.213,0:02:40.139
Two: Heat death or a Big Freeze.

0:02:41.029,0:02:44.405
In a nutshell, the difference between[br]the Big Rip and heat death is that

0:02:44.405,0:02:48.089
in a heat death scenario matter stays[br]intact and is converted

0:02:48.089,0:02:51.915
over an incredibly long but finite period[br]of time into radiation,

0:02:51.915,0:02:53.951
while the universe expands forever.

0:02:54.401,0:02:57.479
But how does this work?[br]Let’s talk about entropy.

0:02:58.249,0:03:01.382
Every system tends towards the[br]state of highest entropy,

0:03:01.382,0:03:04.105
like when we have a latte macchiato.

0:03:04.435,0:03:07.625
Initially, it has different regions, but[br]over time, they will

0:03:07.625,0:03:10.421
cool down and disintegrate,[br]until it’s uniform.

0:03:11.091,0:03:13.457
And this also applies to the universe.

0:03:13.777,0:03:16.366
So, while the universe gets[br]bigger and bigger,

0:03:16.366,0:03:19.075
matters slowly decays and spreads out.

0:03:19.655,0:03:25.138
At some point, after lots of generations[br]of stars, all the gas clouds necessary

0:03:25.138,0:03:30.171
to form stars will be exhausted,[br]so the universe will turn dark.

0:03:30.821,0:03:32.451
The remaining suns will die;[br]

0:03:32.891,0:03:37.877
black holes will slowly degenerate[br]and evaporate over trillions of years

0:03:37.877,0:03:40.773
due to what’s known as Hawking radiation.

0:03:41.893,0:03:47.330
When this process is complete, only a[br]dilute gas of photons and light particles

0:03:47.330,0:03:50.327
remains, until even this decays.

0:03:51.067,0:03:54.371
All activity in the universe ceases[br]at this point;

0:03:54.371,0:03:59.275
entropy is at its maximum and[br]the universe is dead forever.

0:04:00.255,0:04:03.290
Unless… theoretically, it might[br]be possible

0:04:03.290,0:04:05.065
that after an incredibly[br]long amount of time,

0:04:05.065,0:04:10.270
there might be a spontaneous entropy[br]decrease as a result of something called

0:04:10.270,0:04:13.405
“quantum tunneling”, leading to[br]a new Big Bang.

0:04:14.935,0:04:17.750
Three: Big Crunch and Big Bounce.

0:04:18.400,0:04:20.810
This is the most uplifting scenario.

0:04:21.030,0:04:25.569
If there is less dark energy than we think[br]or it decreases over time, gravity will be

0:04:25.569,0:04:27.958
the dominating force in the[br]universe one day.

0:04:28.238,0:04:31.226
In a few trillion years, the rate of[br]expansion of the universe

0:04:31.226,0:04:32.534
will slow down and stop.

0:04:32.804,0:04:34.842
After that, it reverses.

0:04:35.282,0:04:38.635
Galaxies will race at each other,[br]merging as the universe becomes

0:04:38.635,0:04:39.868
smaller and smaller.

0:04:39.958,0:04:43.660
Since a smaller universe also means[br]a hotter universe, temperatures rise

0:04:43.660,0:04:45.092
everywhere all at once.

0:04:45.752,0:04:49.302
One hundred thousand years before the Big[br]Crunch, background radiation would be

0:04:49.302,0:04:52.119
hotter than the surfaces of the[br]most stars, which means that

0:04:52.119,0:04:54.036
they would be cooked from the outside.

0:04:54.646,0:04:57.606
Minutes before the Big Crunch happens,[br]atom cores are ripped apart,

0:04:57.606,0:05:00.406
before supermassive black holes[br]devour everything.

0:05:00.936,0:05:05.106
Finally, all black holes would emerge into[br]a supermassive mega-black hole

0:05:05.106,0:05:07.909
that contains the entire[br]mass of the universe,

0:05:07.909,0:05:10.535
and in the last moment before[br]the Big Crunch

0:05:10.535,0:05:13.881
it would devour the universe,[br]including itself.

0:05:14.671,0:05:17.919
The Big Bounce theory states that this[br]has happened a lot of times

0:05:17.919,0:05:20.738
and that the universe goes through[br]an infinite cycle of

0:05:20.738,0:05:22.407
expansion and contraction.

0:05:22.797,0:05:24.566
Well, wouldn’t that be nice?

0:05:25.336,0:05:27.938
So what will actually happen to[br]the universe in the end?

0:05:28.178,0:05:33.021
At the moment, heat death seems the most[br]likely, but we at Kurzgesagt hope that

0:05:33.021,0:05:38.144
this “dead forever” stuff is wrong and the[br]universe will start over and over again.

0:05:39.114,0:05:42.348
We don't know for sure either way, so[br]let’s just assume

0:05:42.348,0:05:44.222
the most uplifting theory is true.

0:05:48.212,0:05:50.688
By the way, we have a Twitter account.