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.
