Instant Genius - The end of the Universe, with Dr Katie Mack

Episode Date: June 13, 2021

Theoretical astrophysicist, author and one of Twitter’s most-followed scientists Dr Katie Mack tells us about the Universe's ultimate fate. Once you’ve mastered the basics with Instant Genius, dive deeper with Instant Genius Extra, where you’ll find longer, richer discussions about the most exciting ideas in the world of science and technology. Only available on Apple Podcasts. Produced by the team behind BBC Science Focus Magazine. Visit our website: https://www.sciencefocus.com/ Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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learn more. And welcome to Instant Genius, the brand new podcast from the team behind BBC Science Focus magazine. In each episode of our new weekly show, you'll hear world-leading scientists

that have a bite-sized masterclasses on the most fascinating ideas in science and technology today. I'm Dan Bennett, the editor of BBC Science Focus magazine. And for this first episode, I'm joined by Katie Mack, a theoretical astrophysicist,

author and one of Twitter's most followed scientists. And she's joining us on the eve of the release of the paperback edition of her book, The End of Everything, which ponderes the ultimate fate of the universe. So here's what you really need to know about eschatology, which is the study of the end of days. To an outsider, it might sound like astrophysicists might be a bit gloomy. why are we interested in how the universe ends?

And I suppose a deeper question is, does it have to end? Is there a reason why we think it's not just going to keep going as it is now? Well, I think the reason that we think it will end is that we see the universe evolving, just as we see how stars have, you know, they are born, they live, if they go through certain stages and then they die, the universe is also changing. It was very different at the beginning. You know, we have this theory, the Big Bang theory that says that the early universe was hot and dense and full of the sort of roiling plasma and the universe expanded and that plasma

cooled and created gas that then condensed and formed stars and so on and so forth. And we can follow through that evolution and see that the universe is still. still expanding. And as the universe continues expanding, galaxies are getting farther apart from each other. Gas is being used up within galaxies and stars are not going to be forming anymore in the far future. And you won't have this constant production of stuff and energy. And you can just kind of follow that through logically. And you find that the universe is sort of a less hospitable place now than it was many billions of years ago in terms of, you know, there's sort of fewer stars forming, you know, potentially fewer planets forming and so on. And it were just evolving more and more

in that direction. So you find that there's not a kind of steady state of habitability of the universe. We are going through a process and the universe is changing. And the far future universe just is not going to be a place where as much can happen. And then that leads, you to inevitably something will occur that will make the universe for all intents and purposes end. And in the book, I define an ending of the universe as the total destruction of all of the stuff within the observable universe, you know, within the sort of region we live. So that doesn't necessarily mean space and time finish, but it does mean that, you know, we don't have stars and galaxies anymore. And we sort of need those to exist. So that there's, it's quite, it's quite

difficult to come up with a scenario where the end of stars and galaxies and planets and life doesn't happen. And is it, is it just a fun thought experiment for physicists, for astrophysicist, or does it help inform the sort of models of how we see the universe playing at? I mean, it is a fun thought experiment in the sense that, you know, we, this is not a practical science. It's not something where you can use this information to build a faster car or something like that. But in thinking about the end of the universe, we are also thinking about our basic models for how the cosmos works, you know, how physics works, what the history and evolution of the universe is, what kinds of laws govern reality. You know, all of those are really relevant questions when you're thinking about the death of the cosmos. And so by considering those questions, we can actually, it can guide our thinking through some of these big fundamental questions that are relevant for a large number of things.

So essentially it's a bit like finding the kinks in different theories and how they might map out. Yeah, there's a bit of a sort of stress testing of your cosmic theories that happens when you're, when you think about the end of the universe. Then let's get into what is, I suppose, the most common or popular idea around the end of the universe that some people might have heard of even in pop culture, which is what's often described as the inevitable heat death of the universe. So what can we expect if we end up there?

Yeah, so the heat death is just the extrapolation of the current evolution of the cosmos, which is that the universe is expanding. And by expanding, I mean that things in the universe are getting farther apart. So, you know, this room is not expanding. Individual galaxies are not expanding, but the spaces between galaxies are getting bigger. And because of that, one thing that happens in that scenario is that things get so far away that you kind of can't see them anymore. And if we extrapolate into the far future, we get the fact that in about 100 billion years,

distant galaxies will be so far away that we won't be able to see them anymore. Everything will be separated from everything else enough that there will be no interactions between, you know, separated galaxies, and the universe will become sort of darker and emptier, you know, with more and more space and comparatively less and less stuff. And so you get to a point where because there are no galaxy interactions anymore, you're not bringing gas together to form new stars. the stars within individual galaxies burn out. You know, you use up all the hydrogen in individual galaxies. And then eventually as time goes on and on, more things become black holes. Those black

holes evaporate and disappear. Matter begins to decay. And ultimately, you're left with the universe in which all that's really left is kind of what you could describe as the waste heat of creation, right? So like everything decays into entropy, into disorder, just because that's a law of the universe that over time more and more disorder happens. You know, nothing is perfectly efficient. Everything kind of wastes away. And so the universe is left with nothing, but that tiny amount of leftover disordered energy radiation. And that's what we call the heat death, where the heat there is, is referring to this. entropy, this disorder, the waste heat of everything.

And am I right in saying, I don't know if this is the right word for it, but is that effectively the most popular or the most common interpretation of how we think the universe might end, or all of the different sort of outcomes that will go through? Are they all equally likely? The heat death is considered sort of the default scenario. It's the working hypothesis, I would say, for cosmologists. merely because it's a very straightforward extrapolation of our current evolution as we observe it. And it relies on fairly simple assumptions for what's causing the current evolution of the universe.

So one of the big pieces that we need to understand to know how the universe is going to evolve in the future is we need to know what's making the universe accelerate in its expansion. So right now we know that the universe's expansion is speeding up. And that's actually quite weird because when we first started measuring the universe's expansion very precisely, we really thought that it was going to be slowing down. We really expected that because the universe is full of matter and matter has gravity and pulls together, something would be, that would be slowing the expansion of the universe. And we were just trying to figure out at that point how quickly the universe was slowing down and whether that meant it would keep expanding forever or collapse back on itself.

And what we found was actually that the universe is speeding up. And so in order to explain that, we had to say, well, there must be something in the universe that's making that expansion speed up, something that acts sort of counter to gravity. Whatever that was, we called it dark energy. We still don't know what dark energy is. One of the possibilities for dark energy is something called a cosmological constant, which is just a sort of property of the universe, property of space that gives it a kind of stretchiness that makes it expand. And if the universe is dominated by this cosmological constant,

if that's the dark energy, then that leads to a heat death and that's all very straightforward. If dark energy is something else, if it's a different kind of something that causes the universe to expand faster, then that could lead to different scenarios for the end of the universe. So really the idea that we're heading toward a heat death is sort of based on this assumption that dark energy is kind of the simplest thing we can think of for what it is. I see. I see. And so that brings me nicely to the kind of next, I suppose, batch of ideas, which is that the universe could end up with a big rip. And so that's a property of the expansion as well, is that right?

Yeah, so a big rip is a scenario that occurs. If you have a kind of dark energy that is sort of more powerful than a cosmological constant, something that's not just a property of space, but something that builds up over time within space. And it's not a scenario that most physicists consider very likely because there are certain sort of theoretical problems with the idea of having that kind of dark energy. but if we did and observationally we can't rule it out yet, then what that would do is instead of just moving galaxies apart from each other in the distant future, it would actually start kind of swelling the space in which galaxies live

and pulling stars away from galaxies and then pulling planets away from stars and building up and building up until you have explosions of planets and dissociation of molecules and atoms. and at some point you can calculate in the future, the entire universe is sort of ripped apart in a spectacular finale. So that's an idea that has been around for a number of years. I don't think it's taken particularly seriously by most physicists because, again, there's sort of issues around how it connects with fundamental principles

we think probably govern how sort of energy moves through the universe. But it's an idea that's kind of fun to think about as, you know, one way that dark energy could go wrong and give us a much more, a much more sort of exciting future scenario. Yeah, it's probably going to be a bit more lively to watch than the heat death. Definitely if you're in the Douglas Adams's famous restaurant at the end of the universe in from his hit-chack's guide, it's probably a little bit more glamorous to watch that while you're dining out. And so then there's, so then there's, There's the ones that I'm kind of, I suppose, I think I first read about it from walking.

I'm not much of a physicist, but they're quite familiar to me, the crunch and the bounce. Can you just talk about those? Yeah, so the big crunch is a scenario that was thought to be how it would go several decades ago when there was a time when we really didn't have a good accounting for how much stuff was in the universe. and how quickly the cosmos was expanding. And so there was this question, as I said before, is the universe going to continue expanding forever, or is the gravity of everything in the universe

going to slow down the expansion enough to pull it all back, right? And in the latter scenario, what happens really is that the expansion slows and slows at some point stops, and then everything starts coming back together, back to the sort of high-density state that we had in the very beginning. And that leads to what's called, a big crunch. At the moment, that seems very unlikely because we know that the expansion is speeding up. It's not slowing down and it doesn't seem to be reversing. But because we don't know what

dark energy is, if it is something that changes over time, if it's some kind of dynamical field or whatever, then in principle, it could be something that is acting to expand the universe now, but could turn around and contract the universe into the future. And so that's, that would lead you to this big crunch, which is one of the more exciting, I suppose, endings of the universe because as astronomers on Earth, we would see it coming. We would see that the universe is contracting, that galaxies are coming toward us. And the ultimate ending point of that is not just that galaxies slam into each other. That itself is dramatic, but not especially dangerous because we see galaxies collide all

the time. And what really happens there is just that the stars get kind of mixed up. You get bursts of new star formation, but it's not like the individual stars collide and explode. There's just a lot of empty space within galaxies. But the thing that gets really dangerous about a big crunch is that not only is all of the matter coming together, but all of the radiation in the universe is also being compressed and condensed into a smaller space. And it's being dialed up in its energy, as it's coming together. And so space becomes hot, right? So as the universe contracts,

you get more and more of the cosmic radiation focused into a smaller space and the radiation hardens. And so you end up sort of cooking the universe from the outside, just from the heat and the radiation of empty space, which is no longer empty because you're now putting so much stuff into it. And so that leads to a very interesting scenario, where space itself can start to sort of cook and explode the stars from the outside in by just having so much radiation concentrated together.

So you're in the hot seat watching everything come towards you while we're talking about the kind of the basic level of radiation in the universe becomes so hot that it's cooking from outside. Yeah, because there's some background radiation in the cosmos now that we pick up that's actually left over from the Big Bang. So the cosmic microwave background radiation is the radiation that's moving through empty space, that's the afterglow of the time when the universe was hot and dense in the very beginning. And we can pick up that radiation, and it's a very, very low level.

It's only a few degrees above absolute zero in terms of what it does to the ambient temperature. But if the universe contracts, then that radiation from the primordial, fire gets concentrated and brings you back to that time of a hot, dense universe. And so you bring that radiation back together to make the universe hot again. But then also you have the radiation from all of the stars that have ever shown in the universe, all of the high energy astrophysics processes like quasars and gamma ray bursts and so on. And so there's all this harder radiation that's also being compressed. And that means that the universe right before a big crunch is actually much hotter and more energetic

than a universe right after the big bang. It's not a symmetric process. So it's in a sense, in some sense, it's almost like the opposite of the heat death. It's everything at times a way. Would it be right to say that that's where a potential bounce could be considered at that point where if things all crunched in? Well, so bouncing cosmologies are interesting because it's not easy to, as a theorist, create a bounce at the end of the universe. A straightforward big crunch will not bounce off of itself. If you just follow the equations to a big crunch, it just crunches and it's done. But there are some ideas that people have brought up over the last few decades that do involve

a cycling universe. And some of them involve a kind of big crunch that has a bounce in it. and some have different ways of, you know, maybe the universe compresses a little bit and then a new kind of energy field takes over and creates a new big bang. There have also been suggestions that you could have a heat death that leads to a new big bang after that. So there are a few ways that you can have a cycling universe or bouncing universe. They're all sort of, I would say, new-ish ideas or, you know, ideas that are still in development. And there are a certain reasons to think that it might be useful to have some kind of cycling universe because

that would give you a way of explaining the initial conditions of our universe as the final conditions of a previous universe and in certain ways that solves some problems of early universe theory that we don't have good answers for. But there are sort of several competing ideas and the cosmology community has not settled on one as being, you know, this is the way it would happen. It brings me to the one that it was completely neat to me from the book. And when my colleague, he's a physicist, really gleeful, she was like, you have to read the chapter about vacuum decay. She was like, that's the really cool one. Yeah. Could you tell us about that? Yeah, yeah. So vacuum decay is my personal favorite as well for how the universe might end. Just because it's a very unexpected

sort of ending. So all of these other endings have to do with, you know, the expansion of the universe doing interesting things and these big sort of cosmic forces, whereas vacuum decay is something where an event on a subatomic scale, a quantum tunneling event. So there's just kind of this random, unlikely, spontaneous transition of an energy field on a subatomic scale could occur that could destroy the entire universe. The way that works is there's an energy field that pervades all the space that's called the Higgs field. You may have heard that the Higgs field because of the Higgs boson, which is this particle that the large Hadron Collider people found that they say has something to do with how particles got mass in the early universe. So what happened there is that there's

this energy field, the Higgs field, and the Higgs field has a certain value to it, a certain kind of energy to it, and that value can change. So it was different in the very early universe, and when it changed, it changed the laws of physics that the universe runs on. So it sets what we call our vacuum state, which is just a fancy word for all the laws of physics are set by what the Higgs field is doing, more or less, or at least the laws of how particles interact with each other in our universe. And so you could have different vacuum states and these different vacuum states set the rules for, you know, what are the particles in the universe? How do they interact with each other? What, you know, do molecules work or not?

You know, kind of these are, this is setting the scene for particle physics and chemistry and all of that. In the very early universe, when the universe was very hot and dense and things were sort of just getting started, the Higgs field had a different value. We did not have protons and neutrons and, you know, electrons. and so on. There were different sorts of particles and different particle interactions. We didn't have the electromagnetic force. We had something called the electro-weak force, which is this whole other way of thinking about particle interactions. So when the Higgs field changed in the early universe, that gave us the particles we have, the masses they are observed to have, and that sort of set up this nice, calm, functional universe where we can have atoms and molecules and planets and stars,

and so on. And so it would be really inconvenient as the Higgs field changed again, because we want to stay in our current vacuum state. We want to have particles coming together to make molecules and atoms and planets and things. But there's a possibility, as far as we know, that the vacuum state we're in is actually a false vacuum, which means that it's not the kind of preferred vacuum state of the cosmos, that there's a transition that could occur again where the cosmos could go into a true vacuum state, a more sort of energetically favorable vacuum state, where the laws of physics would be totally different. And the way that would happen is that somewhere in the universe, the Higgs field would undergo this quantum transition,

this quantum tunneling transition, which is, you know, quantum tunneling is something that happens all the time on this upatomic level. Electrons can sort of show up in different places through random fluctuations on, you know, that's sort of part of the weirdness of quantum mechanics. That can happen to the eggs field and it can happen anywhere in space at any time. One of these transitions can occur and that can kick the universe into this other vacuum state, this true vacuum state. The way it would happen is that there would be this tunneling event would occur at one point in space.

It would create a bubble of the true vacuum at that point. And that bubble would expand outward at about the speed of light and just envelop everything and change the laws of physics everywhere and destroy all structure in the cosmos. And that's known as vacuum decay. One of the fun things, if you want to call it fun, about that is that it is a truly unpredictable thing. There's no way of knowing where or when it would happen. because quantum tunneling is a random process. And we can make calculations about how long we expect it to take. So we can sort of give it the universe a half-life the way you would with radioactive decay.

But you can't say for certain when or where it would occur. And when we do those calculations for how long we think it would take, we find that almost certainly the universe has something like 10 to the power of 100 years left before this is something that could happen, if it can even happen at all, and we're not sure about that. But in principle, it could happen sooner. And so that's, that's one of the things

that if we are vulnerable to vacuum decay, if the university does have this instability built in, then it's a slightly uncomfortable idea. We should probably live a bit more for the present. Yeah, yeah. I mean, we don't know for sure if it could happen. It's sort of a slightly new theory, and there's a lot we're still trying to work out about it.

And if there's some sort of new particle physics process that we haven't accounted for, then it could change the picture entirely. And we have reasons to believe that there will be some kind of new physics that we haven't accounted for. So, you know, I don't think anyone should panic over this. And, you know, furthermore, it would be totally painless. You wouldn't feel it. Nobody, there's no tragic aftermath or anything.

It just, the universe just ends. That's fine. You know, you don't mind. Would we see it coming? No, no, because the bubble expands at about the speed of light. And so by the time you see it, it's already on top of you. But that also means you wouldn't feel it because your nerve impulses don't travel that fast. So, you know, it would just be a very, very calm, you know, very peaceful way to go in a sense because you wouldn't notice.

Yeah, it's pretty good. Pretty good as things go. So do we have any reason to think that the transition would be the same as the one that we saw? or after the Big Bang in terms of... Well, it would be different in the sense that the true vacuum, if we're transitioning into one, would be a different state than the early universe one, but it wouldn't be any more survivable. In fact, it's been calculated that when that transition occurs,

not only does it change the laws of physics and, you know, dissociate all your atoms and so on, it's also gravitationally unstable. So once you're inside the bubble, the space inside that bubble, then also collapses into a black hole. So it's a really very final ending of the universe. So obviously one of the things we've discussed a lot is dark energy

and how that plays into all of this. What are the other factors or possibilities that could change our picture of the end of the universe as we sort of understand more about the way the universe works? Yeah, so dark energy is a big one, but also, you know, there are certain assumptions we make about how the universe works and what physics governs it, that we know are probably incomplete, right? So, for example, we have what we call the standard model of particle physics, which is a

sort of theory that brings together all of the particles that we know about, all their, all their interactions, and we're pretty sure that the standard model of particle physics is incomplete, but we don't exactly know how. So we know that there are certain things that are not included in the standard model that are part of the universe. Like, for example, dark matter is a kind of invisible stuff that sort of holds galaxies together. We think it's probably some new kind of particle. That particle is not part of the standard model. So we'll need to revise the standard model somehow to include dark matter. And when we do that, we might get some insight into how physics works in the universe that might give us a totally different future evolution of the cosmos. And similarly, we have a theory of gravity.

the general theory of relativity. Einstein wrote it down about 100 years ago. It has passed all of the tests we've thrown at it, but we have reason to believe it might not be complete because there are certain inconsistencies between general relativity and quantum mechanics, and we think that somehow we need to revise one or both of those ideas to work better together.

And things like string theory have been proposed as ways of doing that. So we think maybe our theory of gravity needs to be revised. That could also very much change the evolution of the cosmos in the future. And as we learn more about the beginning of the universe, we might find something about the beginning of the universe that tells us about the sort of overall structure of the cosmos that could give us insight into its future evolution,

whether that's because of some kind of bouncing cosmology or if that tells us something about the larger geometry of the universe that gives us a different idea or maybe tells us something about multiverse notion that could change the way we think of the future of the cosmos. So there are a number of things like that where we may have to revise our theories in ways that could have a big impact on what we think will happen in the future. If you had sort of three, what was some of your sort of favorite takeaways that you kind of learned in the process of writing this book because you didn't you know you spoke to a lot

physicists you interrogated a lot of the history what were some of the real facts that you kind of shared with your friends and family after yeah i mean so i i certainly learned a lot about different possibilities for the future of the universe and and you know things that i had not been familiar with before like this whole idea of the radiation cooking the stars in a big crunch like those are there are lots of cool ideas like that that came up in the in the researching of book. But I think the thing that kind of sticks with me most is that, you know, I went around while researching this book and I interviewed a number of, you know, cosmologists and physicists, prominent people in my field who are asking these questions and really trying to dig down

into how is the universe going to end or what is dark energy, what is gravity doing, you know, what can we anticipate for the future? How does vacuum decay work? All of these kinds of questions. And I was asking them questions about, you know, their research and about their expectations for the future evolution of the cosmos and how, you know, how much faith they put in different kinds of theories. But the other thing I did when I went around for these interviews is I made sure to also ask everybody a question along the lines of how does the end of the universe make you feel? And the reason I did that is because when you are researching the ultimate destruction of all things, it changes your mindset, right? It really does affect how you think about, like, what does everything mean? You know, what does it mean to live in a universe that's ending? How do we think about our lives and reality in this context?

And I wanted to know if my colleagues were also struggling with those kinds of thoughts, if they're, constantly thinking about the end of the universe. And it's a really interesting time to get this answer from many different people. And everybody had different ways of thinking about it. You know, some people did find the idea of the heat death really quite depressing and thought that, you know, this is a sad ending for the universe that it just kind of fades away. Some people are researching totally different ideas where we wouldn't have to deal with a heat death where something else would happen and maybe you'd have a new universe at the other end.

You know, I talked to Roger Penrose who has a sort of cycling universe idea that doesn't, we don't have to deal with a heat death. And that's something that I thought sort of brought him hope. And then I spoke with other people who said, actually, I think it's fine. I think it's totally acceptable and right that the universe should end. and we should just be happy with the fact that we've had it as, you know, as long as we have and, and we should just accept our transience and,

and be, you know, very calm about it. And, and, you know, and then other people just found it very troubling. And what does it mean if there's, there's no ultimate future and, and, you know, didn't have any answer at all? So I found that to be, I think, the most, the most interesting part of writing the book.

And so in the book, I include a, a sort of epilogue where I could include some of those discussions and you can see sort of how people are thinking about this as they're working on the science of it. So I thought that was one of the most fun things about it. That was Katie Mac there talking about the physics at the end of the universe. If you want to listen to Katie and I discuss Stephen Hawking, Golden Age of Physics and her love of science fiction. Checking. Checking. out Instant Genius Extra, a subscription podcast available on Apple's podcast app. And of course, do check out Katie's wonderful new book, The End of Everything, which is a concise, fluent guide to some of the most

interesting ideas in cosmology today. Thanks for listening. Instant Genius is brought to you from the team behind BBC Science Focus magazine, which you can find on sale now in all good shops and newsagents. Alternatively, do you come find this online at sciencefocus.com. We'll see you next week. This podcast is sponsored by Name, Audio and Focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist Focal, Name creates high-end audio systems,

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