- My name is Katie Mack.
I'm a theoretical cosmologist,
which means I study the universe
from the smallest to largest scales, from beginning to end.
So we know the universe had a beginning
and we also know that it will have an end.
We don't know for sure
which way the universe will come to an end.
There are several possibilities.
Most of them involve something called dark energy.
Now, we don't know what dark energy is.
It's some mysterious force
that's making the universe expand faster.
What we do know is it will probably kill us all in the end.
The most likely end of the universe
is something called the heat death.
Now, we know the universe is currently expanding,
spaces between galaxies are getting bigger,
the universe is becoming emptier and darker and colder.
And the heat death is the ultimate end of that process,
where the universe has become so big and so diffuse
there's essentially nothing in it but the waste heat
of every process of nature.
The heat death is driven by dark energy,
and dark energy is a mysterious thing,
but our best guess is that dark energy
is something called a cosmological constant,
just a property of space
that makes it expand faster in every direction.
But dark energy could be something weirder.
It could be something that gets more powerful over time.
It could be something that changes nature entirely.
Now, if it is something that gets more powerful,
then it would lead to something
more extreme than a heat death,
something called a big rip.
A big rip is where the universe expands
and expands and expands and gets faster and more powerful.
And instead of just moving galaxies away from each other,
the expansion will start building up within objects,
ripping galaxies apart, ripping planets apart,
and eventually destroying the entire universe.
We don't know why
it started making the universe expand faster.
We don't know what it'll do in the future.
And there are other possibilities.
It could get more powerful in its expansion.
It could even change entirely
and change from expansion to collapse.
And that would lead to what's called a big crunch.
Instead of galaxies getting farther apart,
they'd start coming closer together.
The universe would get more dense and hotter
and we would all end in a fiery inferno.
So those are three possibilities
where dark energy would be the doom of the cosmos,
but there are other things
that could also happen to the universe
that would destroy all structure
or change its nature entirely.
So one question I get a lot is,
after the end of the universe, could it start again?
Could we have a new universe after ours is over?
And there are several theories
that involve a cycling universe.
One that goes from a big bang to an ending,
to a new big bang.
Some of those involve a collapse and a re-expansion.
Some involve a universe that expands out to a heat death
and then triggers a new cycle.
We don't know for sure which of these might happen,
but as we study the beginning of the universe,
we are hoping to find signs of the possibility
of a previous cycle or just insights
into the origin of our own cosmos.
In 2012, physicists at the Large Hadron Collider
discovered the Higgs boson.
This is a particle that has to do
with how matter got its mass in the early universe.
Really, it's connected to something called the Higgs field,
which is a sort of energy field that pervades all of space.
The Higgs field sort of tells physics
how to act in our universe.
It determines how particles interact, which particles exist,
what their properties are.
In the very early universe, the Higgs field was different.
It had a different value
and led to different kinds of physics,
different kinds of particles,
and different kinds of interactions.
In the very early universe, the Higgs field changed
and it set up the laws of physics that we experience today.
Some physicists now believe
that the Higgs field could change again,
and that would be really bad.
If the Higgs field does change again,
it would create a bubble of a new kind of space
wherever the transition occurs.
And that bubble would expand outward
at about the speed of light
and destroy everything in its path.
Anything that's caught in the bubble
would experience a change in the laws of physics
and would disintegrate.
And some theorists say that after that,
they would collapse into a black hole.
This process is called vacuum decay.
The scary thing about it
is that technically it could happen at any moment.
However, you should not worry about vacuum decay.
For one thing, you wouldn't see it coming.
It wouldn't be scary.
It wouldn't be painful.
There's nothing you could do about it.
There'd be no tragic aftermath.
The universe would just be over.
But also, our estimates are that
if vacuum decay is going to happen,
it would be very, very, very long from now,
maybe 10 to the power of a hundred years at the least.
Also, our ideas about vacuum decay are wrapped up
in our ideas about particle physics,
and we know those are incomplete.
There are things that our current theories can't explain,
like dark matter and dark energy.
There are places where we know
that the theory has to be changed.
And when we change the theory,
we may find a totally different answer
to how the universe will end.
So we have to keep studying the beginning of the universe,
we have to keep studying what the universe is made of,
and we have to keep doing experiments
that tell us about particle physics,
because all of those things
can give us insight into our future.
If you'd like to hear more about any of these possibilities
or how we're studying the end of the universe,
check out my hour-long lecture
on the Royal Institution website,
or check out my book,
"The End of Everything Astrophysically Speaking".