One day, the universe will die.

This goes far beyond the eventual death of life on Earth as a result of global warming or a freak asteroid collision. This is bigger —much bigger.

The way that the universe is constructed—or more accurately, is being constructed—leaves three incredibly conceivable possibilities for it's demise. Let's check them out!

1. The Big Rip

Despite the enigmatic nature of the creation of the universe, scientist have discovered that the universe was not created in its entierty all at once; rather, the universe is expanding and continues to expand at an accelerating rate due to the existence of a mysterious force Einstein termed "dark energy."

For unknown reasons, new space evenly manifests between all bodies. Galaxies move farther apart from each other. New space is also created between planets and stars within galaxies; however, gravity is strong enough to keep them together. But for how long?

The Big Rip hypothesis suggests that there will be a time when space between bodies is created so rapidly that it will override gravitational pull, causing an explosive dissolution of every object in the universe.

First, large bodies such as galaxies are destroyed. Then, slightly smaller structures such as black holes, stars, and planets are broken apart, all the while the universe continues to expand at an accelerating rate. When the universe expands faster than the speed of light, all components in the universe rip apart. Molecules are broken apart into the particles that form them and those particles move so rapidly in the universe (now expanding at the speed of light) that no particle can ever touch one another.

The universe is reduced to a never-ending space where only molecular particles fly past one another without ever making contact.

2. Heat Death / The Big Freeze

The difference between The Big Rip hypothesis and Heat Death is that in a Heat Death scenario, matter, instead of being ripped apart, is instead converted (over a very long time) into radiation. Why?

The universe is like a tall vanilla espresso with extra foam. When the barista hands you your drink, there are different regions of temperature in the cup (near the foam the temperature will be cooler than near the bottom). If you wait a bit, the contents will ultimately reach the same temperature. Leave the coffee on your desk, and within hours, the entire coffee is as cold as your cubical.

The universe works in the same way. While the universe gets bigger and bigger, matter (like planets) dies and spreads out. Eventually, all the gas clouds necessary to form stars have dissipated, and after the remaining stars and suns die out the universe goes go dark. Black holes also die due to hawking radiation.

After the black holes evaporate, only a gaseous mixture of photons and light particles remains. This gas also eventually decays. All universal activity ends here forever...

...that is unless, after an extremely long time, there is a spontaneous fluctuation of entropy caused by quantum tunneling, generating a big bang.

3. The Big Crunch & The Big Bounce

This has to be the most hopeful scenario of the three.

We do not know much about dark energy, other than that it seems to be causing the universe to expand at an accelerating rate, but if there is less dark energy than we think in the atmosphere, one day it might run out and gravity will once again be the dominating force in the universe.

If this happens, the rate of the expansion of the universe will slow down and ultimately stop. Then it will start to reverse, and the universe will start to get smaller. Galaxies will race towards each other and the temperature in the universe will rise all at once.

Background radiation will be hotter than the surfaces of most stars, and minutes before The Big Crunch, atoms will be ripped apart and massive black holes will consume everything.

All the black holes will merge to form a super-gigantic-mega blackhole, containing all of the mass of the universe, and moments before The Big Crunch, it will consume the rest of the universe and itself.

Crunch.

The Big Bounce hypothesis states that the universe has gone through The Big Crunch before, many times actually. In fact, the universe goes through a never ending cycle of expansions and contractions in which the universe is devoured and then reborn.

Conclusion:

At the moment, Heat Death seems to be the most likely end, but since we don't know for sure, we can always assume that the most uplifting theory is true!