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One day the universe will die.


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But why? And how? And will the universe
be dead forever?


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And how do we know that?


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First of all, the universe is expanding.


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And not only that, the rate of
its expansion is accelerating.


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The reason: dark energy.


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Dark energy is a strange phenomenon that
scientists believe permeates the universe.


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Until 1998 we thought that the universe
must work a bit like


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a ball that you throw into the sky.


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The ball moves up, but at some point
it has to come down again.


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But the expansion of the universe
is actually speeding up.


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That’s like throwing a ball up
and watching it fly away


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faster and faster and faster.


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Where is this acceleration coming from?


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Well, we don’t know, but
we call it “dark energy”.


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Einstein thought of it first and
then decided it was stupid.


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Now, astrophysicists have
decided it is plausible.


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Trouble is, this is all very theoretical,
and we don’t actually know


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what the properties of dark energy are.


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But there are various theories and they
lead us to three scenarios


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for the end of the universe.


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One: the Big Rip.


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Since its birth, the universe
has been expanding.


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For unknown reasons new spaces
created everywhere equally.


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The space between galaxies expands,
so they move apart.


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The space inside galaxies also expands,
but here, gravity is strong enough


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to keep them together.


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In the Big Rip scenario, the expansion
accelerates up to a point where


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space expands so fast that gravity
can’t compensate for this effect anymore.


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The result is a Big Rip.


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At first, only large structures like
galaxies are torn apart,


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since space between the single objects
expands very fast.


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Next, big bodies like black holes,
stars, and planets die.


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Their gravity isn’t strong enough to keep
them together,


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so they dissolve into their components.


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In the end, space would expand
faster than the speed of light.


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Atoms would now be affected,
and they would just disband.


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Once space is expanding faster than light,
no particle in the universe can interact


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with any other particle anymore.


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The universe would dissolve into countless
lonely particles that won’t be able to


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touch anything else in a strange,
timeless universe.


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Hmm, and you thought you felt lonely!


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Two: Heat death or a Big Freeze.


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In a nutshell, the difference between
the Big Rip and heat death is that


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in a heat death scenario matter stays
intact and is converted


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over an incredibly long but finite period
of time into radiation,


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while the universe expands forever.


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But how does this work?
Let’s talk about entropy.


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Every system tends towards the
state of highest entropy,


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like when we have a latte macchiato.


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Initially, it has different regions, but
over time, they will


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cool down and disintegrate,
until it’s uniform.


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And this also applies to the universe.


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So, while the universe gets
bigger and bigger,


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matters slowly decays and spreads out.


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At some point, after lots of generations
of stars, all the gas clouds necessary


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to form stars will be exhausted,
so the universe will turn dark.


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The remaining suns will die;


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black holes will slowly degenerate
and evaporate over trillions of years


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due to what’s known as Hawking radiation.


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When this process is complete, only a
dilute gas of photons and light particles


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remains, until even this decays.


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All activity in the universe ceases
at this point;


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entropy is at its maximum and
the universe is dead forever.


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Unless… theoretically, it might
be possible


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that after an incredibly
long amount of time,


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there might be a spontaneous entropy
decrease as a result of something called


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“quantum tunneling”, leading to
a new Big Bang.


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Three: Big Crunch and Big Bounce.


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This is the most uplifting scenario.


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If there is less dark energy than we think
or it decreases over time, gravity will be


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the dominating force in the
universe one day.


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In a few trillion years, the rate of
expansion of the universe


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will slow down and stop.


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After that, it reverses.


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Galaxies will race at each other,
merging as the universe becomes


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smaller and smaller.


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Since a smaller universe also means
a hotter universe, temperatures rise


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everywhere all at once.


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One hundred thousand years before the Big
Crunch, background radiation would be


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hotter than the surfaces of the
most stars, which means that


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they would be cooked from the outside.


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Minutes before the Big Crunch happens,
atom cores are ripped apart,


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before supermassive black holes
devour everything.


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Finally, all black holes would emerge into
a supermassive mega-black hole


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that contains the entire
mass of the universe,


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and in the last moment before
the Big Crunch


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it would devour the universe,
including itself.


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The Big Bounce theory states that this
has happened a lot of times


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and that the universe goes through
an infinite cycle of


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expansion and contraction.


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Well, wouldn’t that be nice?


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So what will actually happen to
the universe in the end?


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At the moment, heat death seems the most
likely, but we at Kurzgesagt hope that


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this “dead forever” stuff is wrong and the
universe will start over and over again.


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We don't know for sure either way, so
let’s just assume


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the most uplifting theory is true.


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By the way, we have a Twitter account.


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