In Brief
Scientists are in agreement that the Big Bang is the mostly likely theory as to how our universe began, but they have several different ideas about how it might end.

A Grand Finale

If there’s one thing scientists love discussing more than the beginning of the universe, it’s the end. There are literally hundreds of known stellar events that could obliterate the life on our planet before we even knew what hit us. The meteor that touched down in Russia kind of cemented that fact. However, the chances of Earth suffering a life-ending global catastrophe are actually rather slim.

The History of the Universe Compressed into One Year
Click to View Full Infographic

But we know that it is coming. At the very least, it will happen when the Sun transitions into a red giant.

The end of everything else, though, is a little bit more difficult to predict, but that hasn’t stopped scientists from speculating and theorizing. With that in mind, here are four popular theories on how the universe might end.

Note: Astrophysicists believe that the ultimate fate of the universe depends on three things: the universe’s overall shape, its density, and the amount of dark energy within it. The first two scenarios below hinge upon the universe existing in a “flat” or “open” system (one that is negatively curved, similar to the surface of a saddle).

The Big Rip

I’m sure many of you are familiar with dark energy and, more specifically, the role it plays in the accelerated expansion of the universe. One theory of how the universe could potentially end relies on the assumption that the expansion of the universe will continue indefinitely until the galaxies, stars, planets, and matter (potentially even the subatomic building blocks that comprise all matter) can no longer hold themselves together, at which point they rip apart.

This theory is called the Big Rip, and it could result in your next door neighbor (or cat) being ripped apart, too.

In this model, if the universe’s density is found to be less than critical density (the boundary value between open models that expand forever and closed models that re-collapse), the expansion of the universe will continue, as well as the accelerating expansion that is driving the galaxies apart at high speeds.

If the density of the universe ever becomes equal to its critical density, it will continue to expand, but the expansion would eventually start to decrease gradually.

4 Ways The Universe Might End

Finally, if the critical density were to become greater than the density of the universe, the expansion would halt and the universe would start to collapse back in on itself, resulting in a gravitational singularity: one that could ultimately trigger the next big bang.

According to Robert Caldwell, a theoretical physicist from Dartmouth College, if the Big Rip won out over all of the apocalyptic scenarios put forth in this piece, the event would occur in some odd 22 billion years, when the Sun has already transitioned from a main-sequence star to a red-giant (potentially incinerating Earth as a result) and then into a white dwarf.

If Earth did manage to survive intact, the planet would explode about 30 minutes before the grand finale.

The Big Freeze

Another popular scenario for the end of the universe that relies on deciphering the true nature of dark energy is the Big Freeze (also referred to as Heat Death or the Big Chill).

In this scenario, the universe continues to expand at an ever-increasing speed. As this happens, the heat is dispersed throughout space while clusters, galaxies, stars, and planets are all pulled apart. It will continue to get colder and colder until the temperature throughout the universe reaches absolute zero (or a point at which the universe can no longer be exploited to perform work).

Similarly, if the expansion of the universe continued, planets, stars, and galaxies would be pulled so far apart that the stars would eventually lose access to raw material needed for star formation, thus the lights would inevitably go out for good.

This is the point at which the universe would reach a maximum state of entropy. Any stars that remained would continue to slowly burn away, until the last star was extinguished. Instead of fiery cradles, galaxies would become coffins filled with remnants of dead stars.

It has been said that intelligent civilizations in the very distant future will look into the sky and think they are alone. Everything will be so far away, the light from distant stars and galaxies could never reach them due to the expansion of the universe.

Many astronomers and physicists alike believe this may be one of the most probable scenarios thought up at the present moment.

The Big Crunch

The Big Crunch is thought to be the direct consequence of the Big Bang. In this model, the expansion of the universe doesn’t continue forever.

After an undetermined amount of time (possibly trillions of years), if the average density of the universe was enough to stop the expansion, the universe would begin the process of collapsing in on itself. Eventually, all of the matter and particles in existence would be pulled together into a super dense state (perhaps even into a black hole-like singularity).

Furthermore, such an event might have already happened before. Some scientists have theorized that the universe we see is the result of a cyclic repetition of the Big Bang, where the first cosmological event came about after the collapse of a previous universe. This is something called conformal cyclic cosmology.

Unlike the first two scenarios, this model relies on the geometry of the universe being “closed” (like the surface of a sphere). Truly, an event like this would be like a single breath. The universe would “breathe out” the Big Bang, and “breathe in” the Big Crunch. This could be due to either a reversal of dark energy’s current expansion effect or as the result of gravity collecting the entirety of spacetime into a single point.

Similar to this theory (and the Big Bang) is that of the Big Bounce. A sort of symmetry is proposed here: the universe is in a continuous cycle of expanding out and then collapsing onto itself. Effectively, we could be one of many iterations of the universe. Perhaps even more eerie to think about is the idea that maybe each time the universe resets, it plays out the same way. Perhaps the you that is currently reading this article right now is just one you out of 10^googleplex versions that existed before.

Ultimately, the universe may be like the mythical phoenix. In death, it is reborn.

The Big Slurp

I saved the best scenario (or worst, depending on your outlook) for last: the Big Slurp. This theory surfaced not too long ago, after revelations were released about the true nature of the Higgs Boson (most of you probably remember it as the particle believed to play a role in granting mass to elementary particles).

The Evolution of Human Understanding of the Universe [INFOGRAPHIC]
Click to View Full Infographic

In this model, if the Higgs Boson particle weighs in at a certain mass, it could indicate that the vacuum of our universe may be inherently unstable, perhaps existing in a perpetual “metastable’ state — something that has been discussed at length many times before.

If this were the case, our universe might experience a catastrophic event when a “bubble” from another alternate universe appears in ours. If said bubble exists in a lower-energy state than our bubble. the universe could be completely annihilated.

I should note that this is disastrous because it could cause of all the protons in all matter found in our universe to decay. By proxy, so would we. If that doesn’t sound unpleasant enough, this sort of a vacuum metastability event could happen at virtually any moment, anywhere in our universe. The bubble could pop over and start expanding at light-speed until it swallowed us entirely.

Truly, none of these scenarios sound very fun.