Space: The Next Generation

How Will the Universe End?

Not with a bang but a bubble.

Illustration by Juliana Jiménez Jaramillo

Illustration by Juliana Jiménez Jaramillo

Cosmologist Alex Vilenkin gave a lecture at the Hayden Planetarium in Manhattan earlier this year called “The Universe and Beyond.” I attended the talk, along with a few of my friends, and we all had the same reaction afterwards: “That was awesome! I didn’t understand a thing.” I understood even less of the lecture than the rest of my friends, which pretty much makes me the poster child for regression toward the mean, because Alex Vilenkin is my father.

There was one section of his talk that particularly intrigued me and that I regretted not understanding even more than the rest. It also happens to be the subject that has the biggest potential to harsh all of our eternal mellows: How will the universe end? So I asked my dad if he wouldn’t mind doing a Q&A with me regarding the end to end all ends, you know, kind of an End of the Universe for Dummies. Mostly because I am his daughter, he decided to oblige.

Me: I need an easy metaphor to even begin to grasp the idea of the end of the universe. So I’m kind of picturing the universe as this nice, merino wool blanket and then suddenly this giant moth appears and eats the whole thing.

Dad: (confused) A mop?

No, moth.

Well, it’s going to happen a bit faster than that. All of a sudden a tiny little bubble will appear. It can appear anywhere—under your chair, or somewhere in Andromeda, very far away—and this little tiny thing starts growing at a speed that’s pretty close to the speed of light. And as it expands, all things that it engulfs turn into an alien form of matter. It may be approaching us right now. Say it nucleated at Andromeda some millions of years ago, it may be expanding toward us at the speed of light. But we don’t get much of a warning. So the good thing about it is you don’t really have to worry about it.

So what would happen to the Earth? It would just go, “FLOOP!” and not exist anymore?

Yeah. Inside of this bubble, ordinary matter as we know it does not exist. It’s made up of different kinds of particles. So everything will be turned into some other stuff that we just don’t know about. But aside from the fact that the end will come very quickly, the other piece of good news is that the probability of the universe ending at any given moment is extremely low.

Like how low?

We can’t really tell. It depends on particle physics at very high energy, so we can’t reliably calculate it. But back-of-the-envelope estimates give you extremely low numbers, like trillions and trillions of years from now. The probably of it occurring while our sun is still active and burning is almost nil. So most likely it will happen when the sun is already gone and, you know, we might not be around.

This is really helping me prioritize my cosmology-based freakouts: No. 1, sun goes cold; No. 2, universe dies. But you’re saying it could happen now?

It could happen now, yes.

Is it also possible that it won’t happen at all?

It is what the current theories of particle physics and cosmology tell us. Do we have 100 percent certainty that they are correct? Not really. Because actually the predictions for the end of the universe have changed a couple of times in the past few decades.

Wow. So what were the failed theories?

Well, for most of the 20th century, cosmologists thought there were actually only two options. We know we had a Big Bang some 14 billion years ago, and we know that the universe is expanding and that its expansion is slowed down by gravity. So there are two possibilities. One is that if the universe has enough matter, and its gravitational pull is strong, expansion will stop at some point and this will be followed by contraction. All the universe will recollapse to a big crunch. Initially the universe was very hot, then it expanded and cooled down. In this scenario, it will happen in reverse. The universe will contract; it will heat up and we’ll end up in fire.

Another possibility is that if there is not enough matter, the universe will keep expanding until it cools down to absolute zero. Stars will die and the universe will become this sort of unfriendly, extremely cold space where galaxies are separated by huge distances and will continue flying ever farther apart. Astronomy will become a very boring subject.

The end in fire was ruled out by the discovery of dark energy in the late 1990s.  Dark energy is the energy of empty space. Each cubic meter of space carries a tiny amount of energy, but there is a lot of empty space in the universe. All this energy produces a large gravitational force, and a remarkable thing about the vacuum is that its gravity is repulsive. This repulsive gravitational force causes the universe to expand faster and faster. And this is precisely what the astronomers found—that the universe is now expanding faster than it did before. If it were not for the bubble, the accelerating expansion would continue forever, leading to the big chill.

I definitely like the current scenario—of the bubble swallowing us with lightning speed and turning us into alien matter—the best. It sounds peaceful, actually.

I agree. Though none of these scenarios is very pleasant …

Are the questions of how the universe ended and how it began actually the most difficult questions for physicists? Or just the most existentially interesting to the rest of us?

They are very difficult. The beginning of the universe always attracted more attention, and for good reason: In cosmology you can see the past. Cosmology is the history of the universe. The same way that paleontologists unearth bones that reveal the history of evolution, cosmologists look at distant objects to actually see how galaxies were billions of years ago (since light from those objects travels to us over billions of years). So we have a lot information about the beginning, but as you start looking farther and farther into space, things become difficult because space becomes opaque. If you’re looking at objects so far away, you’re getting closer and closer to the light emitted by the Big Bang, at that time the universe was very dense and it was kind of filled with hot plasma, and light doesn’t really propagate freely in that stuff. And there is a threshold beyond which we cannot really see, at least in light, in electromagnetic waves. (It’s possible we’ll be able to see deeper into the past using gravitational waves.) What I’m trying to say is that we know a lot about the past, but we can’t see into the future. We can only extrapolate given the theories we have now.

So do we basically have the Big Bang figured out now?

Cosmology for the last century or so was the study of the aftermath of the Big Bang. There was a great explosion, but what actually caused the explosion was a mystery. Now we have the theory of inflation, which my friend Alan Guth thought up in the early ’80s, that explained to a large degree what could have happened at the Big Bang. And now some of the predictions of this theory have been confirmed by the data. So it seems this theory is on the right track. We now understand what happened prior to the Big Bang, but you kind of move the boundary further, because you can always ask: And what happened before that?

But why can’t you ask the same question about the end of the universe: What happens after that? Will there ever be another Big Bang?

That’s true. St. Augustine wrote a wonderful book called Confessions in the fourth century. He was trying to answer the question of what God was doing before he created Heaven and Earth. Augustine was wondering: If God was just hanging out doing nothing, why did he suddenly burst into action after an eternity of idleness? And the answer he came up with was that when God created the universe, he created time as well. So there was no concept of “time” before the creation of the universe. Some cosmological theories have this property as well, that space, time, and matter all came into being at the same time. As for the end … we don’t really know what happens after that. Or if there is any “after.” But if the “bubble” theory of how the universe ends is correct, then although we and our surroundings will sooner or later be engulfed, beyond that, the vacuum will continue expanding and there will be other bubbles forming. So this kind of “boiling” of the vacuum never ends, it continues forever.

I have one more question, Dad. One of the friends I brought along to your talk, Dan Engber (who is something of a contrarian), mentioned that he often found the programming at planetariums somewhat depressing because it seems designed to make us feel small and insignificant. Do you ever feel this way while pondering the universe? Or is it just the opposite—that you feel like some kind of supermensch because you can actually understand all this stuff?

I don’t feel sad that I am small and insignificant. I think it is amazing that we understand so much about the universe. I’ve felt depressed for a different reason, because according to these modern theories the universe keeps going forever. And even though our local region will kind of succumb to an evil bubble, in different places there will be different Earths and in that scenario, things will repeat themselves. So there will be other Earths that are pretty much exact copies of ours. So of course most of the different civilizations will be nothing like ours, but there will also be ones just like ours. So what I’m sad about is that we’re not unique in the universe. But small and insignificant … ? We are.