We know the universe won't last forever because we can look back in time and see how it's changed. Because it takes time for light to travel from distant galaxies to our telescopes, the light we receive is already old — billions of years old, in some cases. That means that if those telescopes can reach far enough into the universe, we can see the way the universe was billions of years ago. What we see is that galaxies used to be closer together than they are today, and everything has been moving away from everything else faster and faster as time has gone on.

That's one piece of evidence that the universe is expanding at an accelerating rate. Scientists aren't sure what the mysterious force is that's causing this, but they've given it a name: dark energy, which is some sort of substance or field that exerts a sort of reverse gravity. It pushes while gravity pulls. But the universe can't go on expanding forever. Something's gotta give, and when it does, it'll likely happen in one of these ways.

An expanding cloud of smoke eventually dissipates into nothingness, and that's exactly what will happen to our universe if it succumbs to heat death. If there's enough dark energy in the universe, it will cause everything to keep moving farther and farther away from everything else. Galaxies will no longer be visible to each other and new stars will stop forming. Eventually, the atomic particles that make up everything will decay into subatomic particles, and those subatomic particles will move so far away from each other that they can no longer interact.

The universe will be dark, cold, and motionless, which is why this scenario is also called "The Big Freeze." In fact, the "heat" part is solely in reference to the fact that it's the result of entropy, not the temperature that would result. This is the most likely fate for our universe.

We assume that dark energy has pretty much leveled off to become a constant force. But what if it hasn't? If dark energy were to weaken to a point where it couldn't counteract the force of gravity, the opposite would happen. In the Big Crunch, the expansion of the universe eventually reverses. Galaxies would get closer and closer together, eventually colliding with each other and collapsing under the force of their own gravity. All of the matter in the universe would fall in on itself until everything went back to the way it began: as a singularity, an infinitely dense speck. Given what we know about dark energy, the chances of a Big Crunch are fortunately slim.

In the heat death of the universe, dark energy is more or less constant. In the Big Crunch, it weakens. In the Big Rip, dark energy gets stronger over time, making it so galaxies not only move away from each other but start expanding from within. Galaxies, stars, and planets would be ripped apart, and eventually, dark energy would become so powerful that it would tear apart molecules and atoms.

"If the Big Rip is coming, it's not coming for a very long time," said theoretical astrophysicist Katie Mack in a talk at the NECSS conference in July. "We have at least 120 billion years before we have to worry about this. So you can rest easy — it'll be a while."

While the last three scenarios wouldn't occur for billions of years, vacuum decay could happen at any moment. It's based on the nature of something called the Higgs field, which permeates our universe and varies in strength based on its potential. You can think of its potential like water in a waterslide. The higher up the waterslide you go, the more energy the water has. When it reaches the very lowest energy possible — say, into the pool at the bottom — you'd call that the vacuum state. Here's the problem: There are two possibilities for the vacuum state of the universe. It might be a true vacuum, in which case the energy of the Higgs field really is the lowest it can go. We're chilling out in the pool at the bottom of the slide. But it also might be a false vacuum, in which case there's another lower-energy state we don't know about — the pool happens to have a giant bathtub plug, and pulling it can make all of the water rush into an even lower state.

If an event with enough energy occurs in the universe at any time, it could pull that plug. Even more alarming, it could happen without any prodding at all: Quantum particles have the ability to "tunnel" through a barrier, whether from one side of a wall to another or from one vacuum state to another, so random quantum fluctuations could do this too.

"That would be bad," Mack said, "because the true vacuum has different constants of nature than the false vacuum. Constants of nature are things like the charge of the electron or the mass of the particles or even the strength of gravity sometimes. So if you take the molecules that you're made of and put them into a true vacuum state, those molecules don't hold together anymore. Total destruction."

All it would take is one lone particle flitting off to somewhere it shouldn't, and reality would cease to exist. Sweet dreams!

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