The universe today is happy and healthy,
with exciting things going on. But at some point the night will turn
dark. Everything that once was will peacefully sleep forever. But what
is the last thing that will ever happen and when will it be? It turns out there
is such a thing and you probably haven’t heard about it. Let's travel to the end of
the universe and look at the last thing. After a messy birth our universe was a sleepy
baby, warm and dark, filled with swirling clouds of hot hydrogen and helium. The story of creation
is a story of this gas and where it will end up. Shortly after, the universe got busy making
the first generation of stars. They were massive and lived violent lives, forging new
elements, only to release most of them when they blew up. Countless stars were born and
refined the gas available in the universe, cycling matter around, each generation giving
most of its gas and fresh elements to the next. But not all gas is returned. Every time a
new generation of stars forms, they also make more and more red dwarfs that burn slowly
and live for trillions of years. When they die, they don’t give their gas back to the universe
but turn into white dwarfs. So red dwarfs lock up more gas forever. Some more gas is locked
forever in other remains of dead stars: neutron stars and black holes. Which is bad, as
it reduces the material available for new stars. Today the universe is a great
home for us and will remain so for billions of years. But most of
the gas has been used up or trapped. Over 90% of the stars that will
ever be born have been born already. To get to the last thing to ever happen,
all other things need to happen first. The next few hundred billion years
will be fun and a great time for galactic exploration – but step by
step, large stars and stars like our sun will die out. Eventually almost all
the stars will be red dwarfs slowly dying. The End of Everything – but not quite In a few trillion years the
cosmic gas will finally have run out. About 88% of the mass of
every galaxy will be white dwarfs, 2% neutron stars and black holes, and about
10% gas giants and sad brown dwarf losers. White dwarfs are the corpses of old stars, not
much bigger than earth but on average as massive as half our sun, some even much more. This makes
them the third densest objects in the universe, after neutron stars and black holes.
About a 1,000,000 times denser than the sun today. Since they used to be active
stars, their surface can be as hot as 150,000 degrees. White dwarfs are dim, hot, dense
spheres that don’t do anything anymore. But eventually even white dwarfs will die because
they are slowly losing their heat – it takes at least 10 trillion years, more than 700 times
longer than the current age of the universe. As they do their cooling down, the universe
around them will irreversibly grow dark, as more and more white dwarfs burn
out, and turn into dead husks: black dwarfs. Spheres of death, as cold as space
itself, invisible against the dark backdrop. Over trillions and trillions of years, every object in every galaxy is
eventually either ejected into the void or its orbit decays and it will fall
into the central black hole and be destroyed. In about a quintillion years, all galaxies
have evaporated and every object is on its own, in the center of its own observable universe,
emptiness as far as can be seen in any direction, traveling through black nothingness.
Still, there are things that will happen. Black holes are dying. Slowly. They’ll fizzle
away by emitting Hawking radiation until they are so small that they die in a final flash
of light. This will take about a googol years, 10 to the power of 100 years, until the last
super massive black hole dies. A number so absurd, there is nothing to compare it to.
Maybe some living beings could have survived around black holes – but even
this science fiction option ends now. After this unsettling amount of time we are not
even close to the end. Now is the time of the black dwarfs. It turns out there is some weird
physics going on inside the dead husk of stars! The Weird Physics of Black Dwarfs A black dwarf is a sphere the size of earth,
as massive as a star but almost as cold as absolute zero. Stars stay alive because
their intense heat in their cores – so why do black dwarfs not collapse into
a black hole? What keeps them together? Deep inside a black dwarf, matter is squeezed
to densities millions of times greater than anything we see on earth. The pressure is so great
that electrons can’t combine with nuclei to form atoms. Instead matter is weird, degenerate: the
nuclei are compressed by the weight of the star, locked into a rigid lattice, while the
electrons form a plasma between them. And these electrons hold the star
together. We are simplifying, but imagine matter as a subway train
and electrons as passengers. If there are empty seats passengers spread out because
they care a lot about their personal space. But as a black dwarf is so incredibly
dense, this is like squishing more and more passengers into our train. Gravity
is pushing in, trying to collapse it. The Passengers are forced to sit and stand
close together, which they hate. And so the passengers, our electrons try to push
out against gravity as hard as they can. This way the electrons that are having a horrible
time in the crowded train that is the black dwarf, hold up the star. Everything else in
the universe may have crumbled already, these tiny particles push against each other
until the end of time. Or they would – if quantum mechanics didn’t ruin everything. Simplifying a lot: when particles get close
enough, sometimes they can jump at each other and fuse together. A process called ‘quantum
tunneling’. This happens constantly in stars because of their intense heat. It is one of the
key reasons stars can fuse elements into new ones. But it also happens at a temperature near
absolute zero. Just, well, mind numbingly slowly. This is the final step to creating the last
interesting thing to ever happen in our universe. Here, in this lone black dwarf something fantastic
occurs. Nothing happens for a trillion years. Nothing at all. Can you imagine that? But
then! A single fusion reaction: two carbon nuclei combine by quantum tunneling to become
magnesium! Another 100 trillion years pass. It happens again! Then nothing for another bazillion
years. Oh! Two oxygen nuclei combine into silicon! As eons pass, the nuclei in the frozen black
dwarf slowly combine. Making new heavier nuclei. And these take even longer to fuse,
but given enough time they eventually will. Remember the breathtaking amount of time
it took for a supermassive black hole to evaporate? That's a brief moment in
comparison to what's going on here. The difference between a second and
trillions of years has lost all meaning. Over a time so absurd that it has no name,
nuclei keep fusing into heavier elements. Until, when silicon nuclei fuse, they
form Nickel-56. Nickel-56 is radioactive, which means it is unstable. And when it
eventually decays and turns into iron, it emits two positrons – antimatter electrons. And these two positrons, find two electrons and annihilate them
and themselves. Which is a problem. Remember how the uncomfortable electrons
produce the pressure to hold the star together? Destroying the electron means
fewer friends to help them hold up the star. Losing an electron does not give
them more space to scratch their butts, it just makes gravity squeeze harder, the
walls closing in on those that remain. In the case of the most massive black
dwarfs this is catastrophic. Bit by bit, the black dwarf turns into a sphere of
iron, and more electrons are annihilated. For at least 10^1000 years – almost,
but not quite forever – there is no visible change in the entire universe.
And then, finally, the last thing to ever happen happens. The black dwarf has lost
one too many electrons. It can no longer support its immense mass and goes into
an uncontrolled collapse - a supernova. It first implodes and then explodes
as bright as a galaxy and fills the empty universe with light again! A beautiful
moment nobody will get to enjoy. And then, as quickly as it began, it's all over. Darkness again, emptiness. That was the last thing that will ever happen.
The universe may now be truly dead. But don’t be bummed out about it. This is so far
away in the future that “forever” hardly describes it. Today the universe is the best place it
could be for us. And you can sleep tight tonight, knowing the last interesting thing that
will ever happen is forever long away. For now, there are still plenty of interesting
things to discover - and the younger you start discovering, the more you will understand
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