IRA FLATOW: One day the universe is going to end, not just the Earth, not even just our galaxy, all of it, every star, every nebula. Nothing we have ever done will remain. My next guest is someone who has spent a lot of her time thinking about the end, more specifically how will that ending happen. There are, it turns out, a lot of possibilities for the end of something as infinitely large and massively energetic as our universe.

Try the big crunch, or the big rip. How about a heat death? Or my favorite, vacuum decay.

But which will it be? Will it be fast or slow? How soon could it happen?

Would we get a new big bang at the end of it all and a new universe of stars and planets? The jury is still out. But telescopes and particle colliders are giving us clues to what may come.

And here to help us wrap our head around the finite nature of our infinity is Doctor Katie Mack, who tackles our universal demise in her new book, The End of Everything. Dr. Mack is a cosmologist, an assistant professor of physics at North Carolina State University in Raleigh. Welcome, Dr. Mack.

KATIE MACK: Hello, thanks. Thanks for having me.

IRA FLATOW: So tell me– I’ve got to say this. This is an awfully cheerful book about the end of the universe. Is that how you feel?

KATIE MACK: I think I’m just excited about big things happening in the universe. And I guess I have some professional remove from the idea of everything really ending. Although, there are some points of the book where I do sort of wrestle with that idea that we will have no legacy in the cosmos ultimately. And that is a scary idea. But it’s fun to think about these big powerful, destructive forces.

IRA FLATOW: Does that bother you that we’re not going to have any legacy? I mean, do you think about that as a scientist?

KATIE MACK: I do think about it sometimes. And sometimes I think that’s fine. We’re just doing our thing. And someday that’ll be over. And that’s how it should be.

And other days, it’s a little bit unsettling. I mean, I’m not fully comfortable with the idea that I’m going to die. And I’m certainly not comfortable with the idea that the whole universe is going to die. But it seems like that’s the way of things.

IRA FLATOW: Give us a short tour of what could possibly go wrong to end our universe.

KATIE MACK: Well, there are several possibilities based on different ways that we look at the data and how we extrapolate what’s happening now in the universe into the future. So in the book, I talk about the big crunch was just this idea that the expansion of the universe that’s currently happening now could reverse and everything could come crashing back together. That’s probably not how it’s going to happen. But that’s an idea that’s been kicked around for decades as one of the possibilities.

IRA FLATOW: Let me stop you there. And tell us why that’s probably, you as a scientist, know or feel it’s probably not going to happen that way.

KATIE MACK: Well, when we look at the expansion of the universe, one of the things that we can observe about it now is that the expansion of the universe is speeding up. That means that the distant galaxies that are moving away from us are moving away from us faster and faster all the time. And that suggests that there is something in the cosmos that’s accelerating the expansion of the universe.

We don’t know what that is. We call it dark energy. But in the presence of dark energy, it’s hard to imagine that accelerated expansion stopping and turning around and everything coming back.

Now we don’t know for sure because we don’t understand dark energy. But based on what we do know about it, it looks like it’s probably something that’s just going to keep going the way it’s going. And the universe will continue to expand forever.

IRA FLATOW: OK, let’s go to option door number two.

KATIE MACK: Well, that’s the heat death. This is what we think is probably the most likely based on how we see the universe evolving now. Again, this is just the universe keeps expanding forever. And the end result of that is that everything gets more and more isolated.

So galaxies get farther apart from each other. Everything gets more and more contained in its own little space. It’s harder for things to interact with each other over these increasing distances.

And so over time, the universe just gets more and more diffuse, darker, colder. There are no new stars forming after a while. And particles decay and everything kind of fades and decays away. And ultimately, you’re left with basically just the waste heat of the cosmos.

IRA FLATOW: OK, let’s go to option number three.

KATIE MACK: Well, this is a dramatic one. This is called the big rip. And this is based on the idea that maybe dark energy is not what we currently think it is. So our best guess about dark energy right now is that it’s something called a cosmological constant. Basically, just a property of the cosmos that spacetime has this expansion built into it.

And so if you have some space, it’ll have a tendency to expand. We think that that’s probably what the dark energy is. We’re not entirely certain.

If it’s something else, if it’s something that isn’t just a property of space but some kind of field in the universe that changes over time, it could be something that gets more powerful over time. And if that’s true, then it could actually, instead of just moving galaxies away from each other, it could start pulling galaxies apart and pulling stars away from the planets, then pulling apart planets and stars themselves, and eventually ripping apart atoms and tearing space itself asunder.

IRA FLATOW: Very interesting. I love talking about dark energy, dark matter. I mean, we talk about we don’t know what 96% of the universe is made out of, right?

KATIE MACK: Yeah.

IRA FLATOW: Dark energy and dark matter. So then how can we make predictions about anything if we don’t know what most of the universe is made out of?

KATIE MACK: Well, we don’t know what it is. But we know a lot about how it acts. For dark energy, it’s– whatever it is, it’s something that’s making the universe expand faster. And we can see the expansion of the universe. We can see how galaxies are moving apart from each other.

And because we can see into the past in the universe by looking at very distant things, we’re looking at things as they were billions of years ago. We can see how that expansion has changed over time. And so we can deduce what dark energy is doing to our universe from those observations.

We can survey galaxies all over the universe and see how they’re moving and how they’re relating to each other. With dark matter, dark matter is a very different phenomenon. Dark matter is some invisible matter.

So it’s something that has gravity. It has mass. But we can’t see it.

And it seems to be the stuff that’s holding galaxies together. It’s this extra gravity where there are clumps of this invisible matter. And galaxies and clusters of galaxies and so on are embedded in these clumps of this invisible dark matter.

Now we don’t know what dark matter is made of. But we can map it out. We can figure out what it’s doing and where it is by how it affects the regular matter of the stars and the galaxies that we do see in the universe.

So we might see stars at the edges of galaxies moving around faster than we would expect. And we can say, oh, that must mean there’s extra gravity holding them in. We know a lot about where it is and how it acts. We just don’t know what it’s made of.

IRA FLATOW: Now we have an option number four that we– is a relatively new comer, is it not?

KATIE MACK: Yeah, yeah, so my personal favorite, cosmic apocalypse, is called vacuum decay. It’s been around since maybe the 70s or 80s. But it’s gained a lot of prominence recently because as we’ve learned more about particle physics, and as we’ve done things like discover the Higgs boson, which is this particle associated with the Higgs field, a kind of energy field that pervades all of space, it’s allowed us to understand our model of particle physics a little better. And what we found is that it looks like maybe particle physics or maybe the physics that governs the universe isn’t quite as stable as we thought.

What we’re predicting now is that the way that physics works in our universe is not really the only option. And the Higgs field, which is this energy field, it can have different properties. And it can change.

And so there is a possibility that somewhere in the universe, there will be a quantum event that changes the Higgs field in that point, in some single spot in the universe. And that would create a bubble of a different kind of space where physics works differently inside that space. And that bubble would expand outward at about the speed of light and just destroy everything in the universe.

And it would be an unpredictable event. We wouldn’t know when or where it would happen. But it would be a totally inescapable bubble of doom.

Now there are reasons that we’re not convinced that that’s definitely going to happen because first of all, it depends on the idea that we really understand all the particle physics. And we’re not quite that arrogant. We know that there are pieces missing to our puzzle. And so it may be that something will come up that will prove that vacuum decay can’t happen. But the current equations point that way, which is a very interesting consequence of the calculations people have been doing recently.

IRA FLATOW: If there’s a bubble that bubbles up inside our universe, could there be a bubble that bubbles up outside our universe, too, and have another universe?

KATIE MACK: Well, outside of our observable universe, the region of space that we can actually see with telescopes and so on, we have no idea what’s going on out there. There could be vacuum decay happening there. There could be new universes popping up out of the larger space we’re embedded in.

There could have been many universes being created around the same time as ours, or popping out of some larger space. There’s a lot that could happen beyond the realm of our observable universe, which is this volume about 46 billion light years across that we can actually get any information from.

IRA FLATOW: So what we visibly can see is not what might be out there. There might be a lot of other stuff out there that is beyond our horizon.

KATIE MACK: Yeah, and our horizon is defined by how far away something can be where the light from that thing can have reached us by now in the age of the universe. So if there was a galaxy 46 billion light years away, and the light left that galaxy at the moment of the creation of the universe, it would only just be reaching us now. So that defines the distance we can see. Because anything farther away from that, there just hasn’t been enough time for the light to reach us.

Turns out that because the universe is expanding faster and faster, we’re actually never going to be able to see things that are beyond that point because they’re being carried away from us faster than light can travel by the expansion of the universe. So there’s this real hard edge to what we can see. There’s a lot of interesting mysteries around what might be beyond our horizon.

IRA FLATOW: Now I know as a theorist, you’re not just inventing these ideas out of thin air. You’ve mentioned that there’s data guiding everything in those theories. And my question is, where does the data come from that’s most useful and important to you?

KATIE MACK: Well, there are a lot of different things we can study. We can look at how galaxies are moving through the cosmos. And during the expansion of the universe, we can see galaxies moving apart.

And we can look at very distant galaxies and see how they’re different from galaxies that are nearby. And that tells us something about how the cosmos has been changing over time. And we can look at actually the background light from the Big Bang itself.

One of the most important pieces of data we have about the early universe, about the evolution of the universe comes from the fact that we can see the afterglow of the Big Bang. So if we look at a galaxy that’s 5 billion light years away, we’re looking back billions and billions of years. If we look farther away than that, if we look as far out as we can see, we can see the universe as it was at the very beginning because it took 13.8 billion years for the light to get to us from that point.

We can see it as it was in the very, very early stages of the cosmos. And what we see when we look as far as we can see is we see this glowing light. This light from a universe that is so young.

It’s just finishing up the Big Bang. And it’s still in that primordial fire stage. So the early universe was hot and dense and sort of roiling with plasma. And we can see that because we can look so far away that we’re looking so far back in time that we see that hot, early young universe.

And that light has in it patterns of little blips of higher density here and lower density over there. And it tells us what the seeds of the structure of the universe looked like and how the universe went from being this fiery, roiling plasma state to this big, cool cosmos with galaxies and clusters of galaxies and so on in it.

IRA FLATOW: We’re going to take a break. And when we come back, more cosmology and deep thoughts with Dr. Katie Mack, Cosmologist and Author of The End of everything. This is Science Friday.

I’m Ira Flatow. If you’ve just joined us, we’re talking this hour about the end of the universe with my guest Dr. Katie Mack, author of the new book The End of Everything. You know, I like the way you write in your book.

KATIE MACK: Thank you.

IRA FLATOW: Because you write just like a regular person, sort of speaking in language that we can understand. And you write that there’s a point– you’re describing the heat death of the universe. And you put it in all caps. But it gets even stranger, right? You go on to say the universe is frickin’ weird.

KATIE MACK: Yes.

IRA FLATOW: You’re a cosmologist. How do you think the universe is weird if you understand it so well?

KATIE MACK: I mean, I’m a cosmologist. But I’m also a person who lives a day-to-day life. And I get used to things making sense in my normal existence. And when you start to study these concepts in cosmology and these really deep time things that particle, physics, and quantum mechanics, it really is very different. And it really is very weird.

And sometimes you have to step back and say, this is strange. This is not our usual experience. And you have to take notice of that and just give yourself a chance to experience the oddness and the wonder of how weird physics really gets.

IRA FLATOW: Well, how do people react to you when you say that you don’t know something? I mean, does the public is– you speak to the public a lot. Can they accept scientists saying I just don’t know? It’s freaking weird.

KATIE MACK: So what I try and do is I try to emphasize what we do know or what we have good evidence for versus what we’re still trying to figure out. Science is always a process of trying to get better and better approximations to what’s really happening out there in the universe. We come up with theories.

We come up with models of how things fit together, how everything works. And then we try and test those theories. We test those models. And we try and get a better and better approximation.

We are developing tools to understand and describe our universe. But we don’t know for sure every time we develop a new tool, every time we come up with a new theory if that’s the ultimate answer or not. And we don’t fool ourselves by saying we’re definitely describing true reality. We’re just trying to get an idea of how we can think about this, how we can model this, how we can find useful ways to describe it.

And so I try and say like, sure, there are things we don’t know. There are things we’re figuring out. But we do have very good useful theories that predict a lot of the data that match what we see. And we can use those. And we can understand our universe better by further developing those and further testing and verifying what we know.

IRA FLATOW: When do you give up on a theory that you can’t collect enough data for? I’m thinking of string theory. It’s entering middle age now. Or any other theory that you want to universe, when do you just say well, we can’t test it out, we have to go to something else?

KATIE MACK: I think it’s very hard to fully set something aside and say, I’m not going to think about that. There are areas of physics where it’s very, very hard to get data because you would need to take a particle collider and do collisions in energies we can’t possibly reach. Or we’d have to– you’d have to take data like right at the edge of a black hole. And we can’t get there.

There are all sorts of things like that where we’re trying to understand something about the universe that really only shows up in places we can’t really test easily. But what we can do is we can come up with models that are consistent, that mathematically fit together well in all the places we can test. And we can see what those consequences are for other areas where we can’t test it.

We’re always trying to find new creative ways to look at the data where maybe we wouldn’t need to build that solar system sized collider. Maybe we could look very carefully at the data we have and test the theory that way. So I don’t think we’re at the point where we give up trying to describe things.

But it is important, I think, to always keep in mind that if you come up with a new model, you need to be able to compare that to other models and do some testing that tells you at least if it fits the data a little bit better than another model or not. So it’s always about taking what we have and saying, is this a better fit than this other idea we have?

If so, we’ll maybe move toward that one. If not, then we’ll stick with what we have and try and come up with something new. So I haven’t given up on understanding these things. But I do think that ultimately what we want is a picture of the universe that is useful and that tells us something new.

IRA FLATOW: Albert Einstein was asked many times whether the human mind has the ability to actually comprehend the universe. And he once said the most incomprehensible thing about the universe is that it’s comprehensible.

KATIE MACK: I think that I’m constantly fascinated by how much we can know. We can see the Big Bang. We can see the background light from the time when the universe was still on fire.

We can see the parts of the universe that, from our perspective, still are on fire. We can study that. And we can see the little variations in the temperature of that primordial fire and use that information and put that into simulations and come out with a whole universe full of galaxies in our numerical simulations.

We can watch the expansion of the universe. We can calculate to very high precision how much matter and energy and dark matter and dark energy and all that stuff is in the cosmos. It’s astonishing how much we know and how well we know it. Whether or not we’ll ever get to the point of ultimate truth of exactly how the whole cosmos is put together, we’re working on that. But it is fascinating how much we do know.

IRA FLATOW: I’d like to get one more listener in, Harriet in Hawaii, who asks, what if the end of the cosmos is just the end of the story we’ve been telling ourselves about the cosmos? How do we know the universe isn’t capable of many big bangs?

KATIE MACK: Well, it’s very possible that there could be cycles in the universe where the universe goes through an ending and then a new beginning of a different kind of phase of the universe. You can even have regions of the universe that are beyond our observable universe or beyond our horizon, where the universe is beginning again or a new universe is popping out over there somewhere.

You can have regions of space where the universe continues even after ours is over. So it’s very possible that there are larger spaces and that there are more things before and after the timeline of our observable universe. When I talk in my book about the end of the universe, what I’m really talking about is the process by which everything, all structure inside our observable universe is ultimately destroyed. And that’s pretty– we’re pretty sure that’s going to happen. But what happens beyond that, or before that, or after that, we don’t know.

IRA FLATOW: Well, it’s not like quite ending on a sad note because we know how it’s going to end. But it’s all part of great stuff in your book, Dr. Mack. Thank you very much for taking time to be with us today.

KATIE MACK: Thank you. It’s always fun to talk about ultimate destruction.

IRA FLATOW: Dr. Katie Mack, Assistant Professor of Physics at North Carolina State University in Raleigh, and author of the new book The End of Everything. I highly recommend it. It’s a great read. And you can see an excerpt of it on our website sciencefriday.com/theend.

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