This Cosmologist Knows How It’s All Going to End

Why is it important to study the universe, even at a time when there are more urgent problems that require the attention of scientists?

Well, it’s not like every person on earth can suddenly become an epidemiologist or a doctor or a respiratory expert and put all of their energy into that. We can’t all just drop everything and become different people. I know that there are a lot of physicists trying to do some kind of disease modeling, trying to put their skills toward that effort. And I don’t know if anything will really come of that or not — but I think that there’s a limited scope for that sort of respecialization.

But in terms of “why think about the universe when the world is difficult and things are happening?” — Well, why have the arts, why have music? Why literature? It’s part of the human condition to be curious and to want to understand the universe in every possible way. Part of what makes us human is those kinds of questions, that kind of curiosity, that kind of joy. And I think that we can’t — we can’t just shear all that away because things are bad.

Your book looks at how astrophysical objects — including the entire universe — come to an end. What made you want to investigate this?

I think it’s just — it’s the biggest, most dramatic thing that you can think of. The destruction of the whole universe: There’s nothing bigger and more dramatic than that. I like those big questions. I like things that are kind of hard to imagine, but that could have consequences that are just impossibly huge. Those are a lot of fun.

In your book, you talk about your influences, and you mention Stephen Hawking in particular. How did his work affect you?

I read A Brief History of Time, and I saw some documentaries about Hawking. And I was just so fascinated by all these mind-bending things — like black holes and the Big Bang and time travel — you know, really mind-boggling topics. And just the idea that these are things you can actually study and learn something about, and understand in a quantitative, concrete way — I thought that was amazing.

I did meet him, actually; I guess I was probably 15. Hawking was giving a talk at Caltech. And my mom took me and a friend of mine to go see the lecture. As we were leaving the lecture theater, somehow we ended up going the same direction that Hawking was. So we sort of ran into him on the walk to the car. And I was too shy to say anything. But my friend went up to him and said: “My friend would like to say something to you.” And so I went and I said something like, “I really admire your work.” And he said, “Thank you very much.” So that was my first encounter with him.

Many of us now interact with scientists on social media. It’s probably safe to say that more people read your tweets than will ever take a college-level physics course. What is it like to be a scientist in the public eye?

I am keenly aware of the responsibility that comes with having a platform, and having a voice, and then having an influence on people. And the more visibility you have, the more responsibility you have to ensure that what you’re saying is responsible and not harmful in whatever way. I’m very aware of that, and I worry about that all the time.

I’ve also had a few people tell me that — that I inspired them one way or another. And that’s been very affecting, certainly. And a little scary. And I’ve been told by some people that seeing a youngish woman in this field has given them the confidence, as women, to approach it themselves. I’ve heard that from a few young women in high school or college.

Let’s talk about the long-term fate of the universe. For many years, if you picked up a cosmology textbook, it would list three possible fates — that it might expand forever; that it might collapse in a “big crunch”; or, in a sort of borderline case, that it might expand forever but just barely. How have those options changed?

Those are the three possibilities that make sense in a universe without a cosmological constant, or without dark energy. But what we learned in the late ’90s was that there is something that is causing the expansion to accelerate. So it’s very hard to see how a big crunch would happen.

Is this where the idea of the “heat death” of the universe comes in?

Yes. The heat death of the universe is the end state of a universe that’s ruled by accelerated expansion forever. Every gravitationally bound system — galaxies, clusters of galaxies — gets more and more isolated from one another. And then each one ends up alone, and everything else gets carried farther and farther away such that they lose contact. So in the ultimate future, if we’re heading toward a heat death, our little group of galaxies, the Local Group, will be isolated. We won’t be able to see other galaxies at some point. We won’t even see evidence of the Big Bang, because we won’t see anything else that’s out there. And as that carries on, eventually star formation halts, because there’s no new material being brought in. The stars you have burn out. A lot of things fall into black holes, then the black holes evaporate. Particles decay. And if you leave that alone long enough, eventually you get a universe where the only thing that’s left is a few strange particles and some radiation.

And in some sense this is the end of time?

From a physics perspective, if you define the arrow of time to be the direction of increasing entropy, once you reach the heat death, the arrow of time ceases to exist. And if there’s no arrow of time, I don’t know what the point of time is anymore.

In your book you mention other possible fates, like the “big rip.”

If you have a kind of dark energy where the energy density is not constant, but is increasing over time, then — then you get this, this thing called phantom dark energy. And that leads to this horrific destruction of the universe in a finite time. Dark energy will start to overwhelm the gravitational binding of every galaxy. And the dark energy in this room is going to start overwhelming the binding of the stuff in this room. It starts to pull apart things that should not otherwise be affected by the expansion of space. So space itself is destroyed, basically.

How seriously do physicists take this scenario?

I think that people don’t take it seriously just because it’s hard to envisage a fundamental theory that would make that happen and be consistent with other things we assume to be true about the universe. But in terms of the data, we can’t rule it out, and we may never be able to rule it out.

As terrifying as the big rip is, vacuum decay is perhaps even more terrifying.

Vacuum decay is my favorite, for sure, for a few reasons. One, because it is a very dramatic kind of idea. But also because it seems to come right out of left field.

Vacuum decay is the idea that the universe that we are living in is not fully stable. We know that when the universe started, it was in this very hot, dense state. And we know that the laws of physics change with the ambient temperature, the ambient energy. We see that in particle colliders. We see that if you have a collision of high enough energies, then the laws of physics are a little bit different. And so, in the very early universe — the first tiny few microseconds or whatever it was — it went through a series of transitions. And after one of those transitions, we ended up in a universe that has — that has electromagnetism and the weak nuclear force and the strong nuclear force and gravity. It created the laws of physics that we see today.

And if that’s true, then some disturbance of our current universe could end up sort of kicking the laws of nature into a different state in which the laws of physics are different, with different fundamental particles and fields. If that happens, then at the point in space where that occurs, you get a bubble of this “true” vacuum, this other vacuum state of the universe, forming. And that bubble would expand at about the speed of light throughout the universe and destroy everything it encounters.

Do physicists take the idea of vacuum decay more seriously or less seriously than the big rip?

Much more seriously, because it doesn’t violate any fundamental principles we know of. However, I would say that most physicists, although we take it very seriously, we mostly don’t believe it’s going to happen. And the reason for that is that the way you get to vacuum decay as a possibility is to say that the Standard Model of particle physics — our current understanding of how particle physics works — is the whole story. [Editor’s note: Most physicists believe that the Standard Model is not the whole story.] So a lot of people would say, you know, this is an interesting problem and we do take it very seriously — but we think it’s not going to happen.

And from our point of view, the decay would be instantaneous?

It’s painless; you don’t see it coming. You don’t notice when it happens, because you can’t feel it. You can’t see it. And then you don’t exist anymore. It’s technically inconsequential, right? It’s hard to think of it as being too tragic, in that sense.

If the universe is fleeting, how is all of the stuff that we do worth doing?

That’s a huge thing that I’ve wrestled with in the course of writing this book, and I don’t think I came to a solid conclusion. It’s different from a personal death, because people think about their own death and they think, well, I’ll live on in some way through my children or my great works, or just the impact I had on the people around me. There will be some legacy to my existence in some way. But if it’s the whole cosmos that’s ending, that is no longer true. I think there’s a point at which you did not matter. And I don’t think we have the emotional or philosophical tools to wrestle with that.

Correction: June 22, 2020

One of the interview questions asking about vacuum decay was incorrectly presented as being about the big rip scenario. The Q&A has been updated to correct the error.