Professor Dave again, let’s talk about the end of the universe.
Logic seems to demand that anything with a beginning must also have an end.
And since we’ve spent a lot of time talking about the birth of the universe as we understand
it, it seems unavoidable that we take a look at some possible scenarios for the death of
the universe as well, as uncomfortable as it may be.
If you think the sun becoming a red giant in a few billion years, swallowing up the
inner planets and then becoming a white dwarf is depressing enough, just think about the
fate of all the planets and stars and galaxies in the entire universe.
Don’t worry too much, things will stay pretty much the way they are now for many, many billions
of years, so it’s not a very pressing issue, but making predictions can help us to understand
the universe, and who knows, maybe long in the future, some conscious entities, perhaps
even our own descendants, could figure out a way to prevent the death of the universe.
First let’s outline a few candidates for the death of the universe.
We know that the universe began as a single point, and has been expanding ever since.
It could be possible that the expansion will slow down, halt, and then reverse, allowing
the universe to contract all the way back to a single point, which we call the Big Crunch.
An interesting component of this possibility is the idea that our universe could be elastic,
with an infinite cycle of big bangs and big crunches.
How long does the universe last each time?
Would it be about a trillion years as calculations suggest?
Would the universe be exactly the same every time or can certain parameters and fundamental
constants change?
If it’s really the same every time, is it exactly the same?
As in, was I saying these words a trillion years ago, and were you listening to them?
It’s a pretty heady concept, with lots of room for philosophical conjecture.
So what is the other option?
If not an ultra-hot, compact death, we could be dealing with an ultra-cold death.
Instead of the expansion turning into contraction, the expansion could continue indefinitely.
Due to the acceleration of the expansion that we measure today, which has been happening
since dark energy began to outweigh gravitational influence a few billion years ago, this does
seem more likely than the alternative, and we currently believe that the universe will
continue to expand until all the galaxies are so far away that they become invisible
to one another.
And then all the stars.
Eventually, matter will be so far apart that it will be impossible for new stars to be
born, and once all the existing ones die, that will be the end of the stelliferous era.
No more stars anywhere, about a trillion years into the lifetime of the universe.
And then, after trillions upon trillions of years, dark energy will be so strong that
ordinary matter on the atomic level will be ripped apart, the electromagnetic and strong
nuclear forces becoming too weak in comparison.
On a long enough time scale, protons themselves will decay.
Even all the black holes will eventually evaporate due to Hawking radiation.
Every point in the universe will reach absolute zero or negligibly close to it, and that’s
the end.
Nothing left but ever-expanding, empty spacetime, which even itself might be pulled apart, which
we can call the Big Rip.
Admittedly, it does sound pretty grim, either way.
But it’s not exactly open and shut.
We don’t even know what dark energy is, so we don’t know whether it is constant
or if it will dissipate over time, allowing gravity to take over again.
There could be some unknown parameters that are throwing off our calculations.
This is big picture stuff, so we don’t have a lot of certainty with it.
But what are we measuring, precisely, besides the rate of expansion?
A slightly earlier era of cosmology, prior to an awareness of dark energy, tried to come
to some conclusion about whether the universe is open, expanding forever, closed, eventually
collapsing in on itself, or flat, delicately balanced in between open and closed, like
rolling a ball up a hill with just enough energy to stay perched at the top without
rolling down either side.
To examine this, we turned to the density of the universe, and the curvature of space itself.
We can get a good estimate of the density of the universe by looking at galaxies and
doing simple calculations with mass and volume.
In terms of curvature, we envisioned a few possibilities.
There is positive curvature, like the surface of the earth, where the angles in a triangle
will add up to more than 180 degrees.
There is negative curvature, which is a little harder to visualize, but looks kind of like
a saddle, and here the angles of a triangle add up to less than 180 degrees.
And then there is the possibility of a flat geometry, where the angles in a triangle add
up to precisely 180 degrees.
This curvature is described by the density parameter, or the ratio of actual density
to critical density, which is represented by the Greek letter omega.
If omega is greater than or less than one, we get positive or negative curvature respectively.
If omega equals one, the universe is flat.
Quite incredibly, current data suggests that the universe is flat overall.
So despite pronounced curvature around massive objects, if we zoom way out, until galaxies
are faint specks of light, we should see a flat universe, where Euclidian geometry more
or less holds true.
Integrating this with what we now know about dark matter and dark energy, we are left with
a spatially flat universe that has an accelerating expansion.
Computer simulations actually confirm this conception of the geometry and behavior of
the universe.
When a cluster of powerful computers simulate what we believe to be the conditions of the
early universe and let things expand to present day, we do get a structure of filaments that
resembles what we see with our telescopes, but only if we consider a flat universe with
about six times as much dark matter as baryonic matter.
However you slice it, with dark energy at work and the expansion of the universe accelerating,
it seems that a Big Crunch is unlikely, and everything is destined to float apart, over
unthinkable time spans, until the universe as we know it is no more.
Don’t be too sad, perhaps from this final void a new universe begins from another quantum
fluctuation, and the whole show starts again, maybe with totally different characteristics.
Even if that’s not the case and our universe was truly one of a kind, if the human race
makes it another couple hundred billion years to see what happens, it will have been a great
run, so let’s keep moving and learn some more astronomy.