I suspect that most college students, myself included, spend a sizeable chunk of our time worrying about the future. If we eat our vegetables and exercise, this future may extend another 80 years. We may have kids and grandkids, but beyond that, how far into the future should our concern actually extend? Well, if you’re a cosmologist, your interest may extend 10^100 years into the future. That’s about a trillion, trillion, trillion, trillion, trillion, trillion, trillion times older than the current age of the universe.
Let's zoom out of the usual narrow focus on ourselves and our planet and consider the fate of the universe all the way down the road. You may not find this relevant since it so far into the future, but I think this ultimate fate has deep philosophical implications for our lives today. After all, what’s the point of writing that essay if the whole universe is bound to end at some point?
I’d like to talk about four possibilities for the end of our universe. They all depend upon two parameters: gravity and dark energy. Gravity is somewhat familiar to us. We know that its role in this cosmic play is to hold stuff, like galaxies and clusters of galaxies, together. Dark energy is not as familiar. We haven’t seen or detected it, so why do we think it exists? Because something like it must exist to explain our other observations.
If you look at other galaxies far away from the Earth, they appear to be moving away from us. It was Edwin Hubble, the namesake of the Hubble Space Telescope that takes all those wonderful images, who concluded from this observation that our universe is expanding. More specifically, the space between things is expanding. Further discoveries have led us to believe that not only is the universe expanding, but that the expansion is getting faster. We call this an accelerating expansion.
Because gravity, as far as we know, is only attractive if the universe is expanding, something must be causing it. We call the mysterious space stuff that causes expansion dark energy. You can imagine the interaction of gravity and dark energy as balance of power, gravity pulling stuff together and dark energy pushing stuff apart. The 'winner' of this cosmic battle will determine the ultimate fate of the universe.
Cosmologists believe that the amount of dark energy in the universe is constant. If this is the case, then dark energy must be beating gravity, since the universe is expanding at an accelerating rate. If this pattern doesn’t change (and there is evidence that it won’t) then the universe will continue to expand forever. However, there is only so much energy and matter in the universe, so if space continues to expand it will eventually spread so far apart that the universe will become very, very cold.
In this scenario, matter will collapse into black holes that will evaporate, and the universe will be vast, dark, empty, and freezing. This sad state of affairs is referred to as the “Heat Death of the Universe,” and yes, this is what that card means in Cards Against Humanity. This is currently our most probable prediction, yet we are still far from understanding dark matter entirely.
If dark matter does not stay constant in the future, we would be looking at a very different fate. If the amount of dark energy increases over time, then gravity will continue losing the battle until, eventually, the universe expands so violently that everything—even the tiniest particles—will be ripped apart. This scary possibility is called the “Big Rip.”
On the other hand, if the amount of dark energy decreases then gravity might make a comeback. In this case, expansion would eventually stop, reverse into collapse, and the universe would implode in an event called the “Big Crunch.” There is even a hypothesis that after a big crunch, there would be another big bang, and thus we’d have a cyclical universe, not unlike that suggested in Hindu philosophy.
A big crunch or big rip both require that dark energy behave in ways that have never been observed. But because knowledge surrounding the nature of dark energy is still so limited, we cannot rule them out as possibilities.
In all of our cosmological models, one thing is relatively certain: the universe will eventually evolve into a state that is incompatible with the existence of complex matter, and therefore the existence of life. It appears there is no way around it—our days on this planet are numbered. What that means for your essay, however, is still up for debate.