Before I begin, I'd like to give you a bit of a content warning. I want to let you know that this is a very heavy topic. The matters I'll be discussing here may be a tad distressing to some people. Okay, moving on. One day in the far, far future, the Universe will die. Now, you may be wondering; how? and why? how do we know this? first off, the Universe is expanding, and has been expanding since the Big Bang. Also, the rate of expansion is increasing. The reason of this accelerated expansion? Dark energy. Dark energy is a weird phenomenon that scientists think permeates the entire Universe. Up until the year 1998, scientists thought the Universe worked a bit like a yo-yo, if you will. No matter how far down it goes, it always comes back up. They though the expansion must slow at some point. However, the expansion, like I said before, is actually speeding up. Now, where is this speeding up of the expansion coming from? We don't really know, but this is what we call dark energy. Albert Einstein first thought of this idea, and then decided it was silly. Now, however, astronomers think this idea is very plausible. The problem is, this is all extremely theoretical, and, we aren't really sure what the properties of dark energy are. However, there are a few theories, and they lead us to a few conclusions of how to Universe will end.
Theory One: The Big Rip.
Since the birth of the Universe, it has been expanding. Not only that, new space is created everywhere in the Universe equally. (Weird, right?) This means the space between Galaxies moves apart, making them move farther and farther away from each other. The space inside stellar objects like nebulae and galaxies is also expanding, the gravity in such objects is strong enough to hold them together. However, the Big Rip theory states that one day, the expansion will be so fast and so powerful that the gravity of these objects will not be strong enough to hold them together. First, big objects like galaxies will be torn apart. Over time, stars, planets and black holes would too be torn apart, leaving behind atoms. When the Universe begins expanding faster than the speed of light, atoms too would dissolve. The Universe would then be a strange place, full of particles that would never be able to touch, whizzing around for, well, who knows how long?
Theory Two: Heat Death.
The Heat Death Theory states that over an extremely long yet finite amount of time, matter stays intact but degenerates into radiation, while the Universe expands forever. How does this work? One word: entropy. All systems tend to fall into the highest level of entropy. Think of it like a Latte Macchiato. When served, it generally has three regions, but over time, these regions will cool down, dissolve, and become one region. This also applies to the Universe. While the Universe gets larger and larger, and, along with it, matter slowly decays. After some time, the gas in the gas clouds needed to form stars will run out, and the Universe will turn dark. The remaining long lived stars will die, and black holes will degenerate and evaporate. (I know!) This is caused by a theory called "Hawking Radiation." Black holes are thought to take (drumroll, please), 10 to the 92nd power years. That's an almost incomprehensible number. I mean, the Universe is only 14 billion years old. Think about that for a moment. I could go deeper with this, but that's a whole 'nother blog post. All activity in the Universe ceases at this point. Entropy is at it's fullest capacity. The Universe is hereby dead forever. Unless, theoretically, after a mind-bogglingly long time, part of this dead Universe would be spontaneously jostled, leading to what's known as "quantum tunneling, which would result in a new Big Bang. Weird!
Theory Three: The Big Crunch + The Big Bounce.
If there is actually less dark energy than we think there is today, or if it slowly runs out over time, gravity will be the most pervasive and dominating force in the Universe. If this happens, the Universe will shrink, and the galaxies will get closer and closer together. Since a smaller Universe means a hotter Universe, the temperature will rise dramatically. It would get so hot that background radiation (the microwave radiation remaining from the Big Bang), will become so hot that stars would be cooked, if you will, from the outside. It will then get so hot atoms will tear apart. And it'll keep shrinking. Soon, all that we would be able to see would be the galaxy. Then it would shrink down to the size of the solar system. Then to the size of the earth, and, although this may sound strange, it will keep shrinking. It will shrink so small, the universe will be the size of a subatomic particle. There is one idea that gives me some hope, however. This is where the Big Bounce Theory comes into play. The Big Bounce Theory states that this expansion and contraction has happened an infinite amount of times, and will keep happening. The Universe will just keep expanding and contracting. Wouldn't that be nice?
I haven't thought about this subject deeply enough to decide upon which theory I support. At the moment, though, the Heat Death theory seems the most likely.
When I think about all of this, though, I can't help but remember those first people who looked up to the sky and wondered what was beyond our vision. From the great minds of the Scientific Revolution like Galileo, Copernicus and Kepler, to Greeks like Hipparchus and Aristarchus. It is because of them that we know as much about the heavens as we do today.