Around us, the universe is expanding at an accelerating rate, but that's not going to be the case forever. One day, we'll find the end of the world and everything else — at least, according to scientists. It's the subject of an entire book by astrophysicist Dr. Katie Mack called "The End of Everything (Astrophysically Speaking)." In it, Mack outlines the possible end of our universe as we know it.

There are a few possibilities for the end of the universe, and these mainly depend on whether the rate of expansion of the universe accelerates rapidly, continues as is or decreases enough to actually start reversing universal expansion. Here are four possibilities for the end of the universe, as presented by Dr. Mack.

The Big Freeze

If the universe continues expanding at its current rate, then eventually, entropy will decrease. Everything as we know it functions in the universe because of differentials in heat. However, once the universe expands enough, the differences in temperature may even out. Everything would then be the same temperature, which would be the heat death of the universe, also called the Big Freeze.

This might not seem serious, but if there are no differences in heat, that means the universe can no longer sustain the energetic process that keeps itself functioning. There will be no extra energy available. Stars will die, black holes will evaporate, all matter in the universe will decay and the temperature of the universe will hover right around absolute zero. There will simply be nothing left.

The Big Crunch

The Big Crunch is the counterpoint to the Big Bang. Just as the universe started with one enormous explosion, it could end with a huge collapse. Scientists posit that if the expansion rate of the universe slows and eventually reverses, gravity will become the dominant force. The universe will then collapse in on itself, eventually becoming a small, hot and dense singularity. It's possible that the Big Bang will occur again at that point, creating an entirely new universe.

It's worth noting that while the Big Crunch is still theoretically possible as an end to the universe, it's not a theory that's generally favored by scientists. With the discovery that the rate of the expansion of the universe is actually increasing because of dark energy, it's unlikely the process will reverse itself enough to allow the universe to collapse.

The Big Rip

The universe as we know it isn't just expanding; it's expanding faster and faster. Some scientists think the key to the end of the universe lies in that expansion. Dark energy is the name scientists have given to the force behind the increasing acceleration of the expansion of the universe. It makes up 68% of the known universe, yet scientists know little about it.

If dark energy pushes the expansion of the universe even further, it may lead to the Big Ri p. The forces pushing the universe apart may overwhelm gravity, which is what keeps planets, solar systems and galaxies clumped together. Stars, planets and all other matter in the universe would be flung far apart from one another, bringing about the end of the world. Eventually, everything would be ripped apart at an atomic level.

Vacuum Decay

The Big Freeze, the Big Rip and the Big Crunch are all billions of years off. We don't need to worry about those violent (or quiet) ends to the universe right now. But there is a way the universe could end in an instant, at any moment: It's called vacuum decay.

It takes a lot of science and quantum mechanics to truly understand how a vacuum decay death of the universe would work, but put simply: If a bubble forms within the universe, and the laws of physics as we know them don't apply within this bubble, then it would be the end of our own universe. This bubble would expand, changing the fundamental rules of the universe as we know them, leading to something entirely new.

This might happen because — while the universe is formed by quantum fields that hold it together, and almost all these fields are in a vacuum state, which means they are stable — the Higgs field, which blankets and permeates our universe, is not in a true vacuum state. If it were ever to move toward that, it would lead to vacuum decay.

The end of everything is a long way off, but thinking about the way the universe might end can help us understand our place in the universe now.

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