Cloud Hosting

Sara Rigby: Could you give us a quick description of what your book is about, please?

Katie Mack: Yeah. My book is about the end of the universe. So in the book, I go through several different possibilities for how the universe might end and talk about how we are trying to figure that out in physics and astronomy and what it would look like if you were there to see it.

SR: Why does the universe have to end at all? Why can we not keep on as we are? It seems seems to be doing pretty well to me.

KM: Yeah. Yeah. Well, for a long time, though, there was an idea that maybe the universe could just be in a steady state and, you know, unchanging forever. But once the Big Bang was discovered, once it was found that the universe started out in this sort of hot, dense state and thats been expanding since then, it became clear that that the universe changes and evolves over time. And then the number of possibilities for its remaining sort of reasonably pleasant decreased rapidly now.

Now, its at the point where we can see that the universe is expanding and we can see that, in fact, the universe is expanding faster and faster all the time. And when you get to that point, its just the natural evolution is toward something where things that exist in the universe now will all be destroyed at some point in the future.

And there are a few different possibilities for how that can happen. But the idea that everythings just going to kind of keep going as as is does not does not work in the kind of universe we live in.

SR: We havent any idea of when this is going to happen. Id like to get this out of the way. Yeah. Is this something thats going to happen within a reasonable human timescale?

KM: Theres theres no theres no reasonable expectation that its something that wouldnt be in the very, very, very far distant future. So technically, theres a lot we dont understand about the universe and things could happen unexpectedly.

And in one of the one of the possible end of universe scenarios I talk about in the book, vacuum decay is based on a random process. But that in principle could happen at any time. But based on our understanding of how that physics works, we wouldnt expect it to occur anytime within the next ten to the power of one hundred years.

And even then, were not sure if its possible at all. So, you know, people do get worried about, you know, oh, it could happen in a moment. There are a lot of things that could technically happen in a moment that we dont worry about. And this one we very much should not worry about. For the most part, were talking about things that are so many trillions and trillions of years in the future that its its hard to even come up with words to explain that sort of timescale.

SR: Right. So if its not something that, you know, we even expect necessarily humans will even be around for. Why do you think its important to us to care about whats going to happen at the end of the universe?

KM: I dont know if its important that we care. But I think that we do. I think that its just part of human nature that we are interested in where we came from and were interested in where were going. And we use we in this case to mean that the bigger picture, that much larger universe.

But I think that were were interested in our our environment and in our story and in how we fit into the story of the cosmos and to the whole the whole narrative of of existence. And so its something that I think were just basically curious about. And there are reasons why, as a physicist, its an interesting thing to study because by extrapolating a theory to its ultimate conclusion to take stretching it to the limits you do. It does help you learn something about the theory about how the physics works.

Its a useful exercise to go through in any theory or model of the universe. So its a useful thing to do from a physics perspective to do these sort of thought experiments and to to extrapolate. But I think just as as people, we just we just want to know this stuff.

SR: And if so, there are in your book, you cover five different ways that the universe can end. Could you just give us a very, very brief outline of what those five different ways are?

KM: Sure, sure. So the first one I talk about is the big crunch. This is the idea that the current expansion of the universe might at some point reverse and everything could come crashing back together, creating conditions very much like the sort of hot, primordial soup we came out of initially. And that was unlikely. Now, based on our understanding of how the universe is expanding and speeding up in its expansion.

So that leads us to thats when the heat death, which is the one that we think is probably most likely if you talk to physicists and cosmologists, the heat death is it sounds counterintuitive to call it the heat death. Ill explain what that heat there refers to. But its also sometimes called the big freeze is where the universe continues expanding and expanding faster and faster indefinitely into the future.

And what that does is it kind of just dilutes everything and it makes galaxies move farther and farther apart from each other and everything gets more and more separated and isolated. And you end up with each galaxy sort of in its own sort of sphere of darkness where it cant see other galaxies. And and at some point, stars burn out and black holes evaporate and matter decays. And you just end up in this sort of cold, dark, empty, lonely universe.

And the only thing left in that universe is like a tiny amount of sort of waste heat from creation, sort of. All thats left is this this extremely low level radiation. Thats just the leftover leftover sort of detritus from from everything that ever was. And thats called the heat death.

And thats thats the saddest story. It does seem to be the kind of consensus model based on just extrapolating our current expansion into the future. And then the other the other three are sort of more speculative ideas that for various reasons people talk about in in the cosmology literature.

So one of them is called the big rip, where whatever is making the universe expand faster. Right now, we call that dark energy, depending on what kind of dark energy it is. It could be something that doesnt just separate galaxies apart and make them more isolated, but could actually pull the stars off of galaxies.

It could become more powerful over time and start disrupting structures in the universe sometime in the future. And it would pull galaxies apart, would pull planets away from their stars. And eventually, at the sort of final moments, it would destroy planets and stars and atoms and rip apart space itself.

And thats something that is not the most favourite idea. But its a its something that we cant rule out based on the data yet. But all we can say about it really is that we were fairly sure. Sure. It cant happen within the next two and a billion years or something like that. So, you know, as we get better data, well probably just push that number back and back. But we may not ever be able to say for certain that dark energy wont get weird at the end and destroy the universe that way.

And then the next one is called vacuum decay. And this is the one I mentioned that could technically happen at any moment. But again, dont worry about it. It almost certainly wont. Its where theres a sort of instability built into the universe.

And it means that the universes is vulnerable to a kind of quantum event occurring somewhere in space that would create a bubble of a different kind of space, that would expand through the universe at about the speed of light and destroy everything in its path. And thats a thats a fun one to me because it combines some interesting ideas in particle physics and cosmology.

And its just this very sudden, unexpected thing where at some moment the universe would basically just cease to exist. You know, there would just be this bubble. It would destroy everything that everythings done. So thats an interesting one. And my personal favourite, because its the most dramatic. And then the final the final one I talk about is really a set of different ideas that all have in common some kind of cycling cosmology.

So I call that the scenario bounce, but its really just some any kind of idea where you have an end of the universe. That then transitions to a new beginning and so on. Over and over again. Or even just previous ones. Maybe something that has. There was a previous universe before ours that led to our universe. Or at the end of our universe, there you new once some kind of idea like that.

And there are several possibilities for that. Somewhere you have kind of a big crunch that leads to a big bang. Somewhere you have. He does. That leads to a new big bang. So theres that theres a variety of ways you can get to that. But those ones are interesting because in principle, in certain models, you could have some information passing from the previous stage to the next one. And so it brings up a kind of way that something could live on past the end of the universe, which to many, is it an appealing idea?

SR: Wow. So theres a sort of rebirth of the universe in that sense?

KM:Yeah, yeah. I mean, it would be it would be a different universe, you know, and probably there would be no trace of anything of us. But the idea that there could be is is intriguing, I think, to a lot of people, including a lot of physicists.

SR: Right. So I sort of think if the big crunch, the big rip and the big bounces all being kind of related in a sense, which is right.

KM: In the sense that theyre all sort of based on the dramatic motions of the. Yes, it to me.

SR: So it sounds like they all are a result of the way that the universe is is expanding and moving at the minute. Yeah. So its like, well, you know, given that we know how were expanding in a minute, whats going to happen? Is it going to come back on itself? Is going to rip? Or is it going to say is that the mechanism which determine whether the universe, which sort of turned back on itself and into the big crunch or whether which, you know. Right.

KM: Yeah. So it all for the really for the heat death. And the big rip and the and the big crunch.

The thing thats governing the possible the possibility is there is dark energy. So, you know, we dont know what dark energy is. All we know is that as of about five billion years ago, the expansion of the universe was speeding up. And theres no theres nothing in sort of ordinary matter or energy that could do that. And so dark energy is is some component of the universe that makes space expand faster.

That sort of counteracts the gravity of everything thats kind of trying to pull matter in to pull space back in. And so we know because we dont know what dark energy is. We dont know for sure how its going to act in the future. Our sort of baseline assumption is that dark energy is just a cosmological constant. Its just a property of the cosmos. That space has this sort of stretching is built into it. And that leads you to a heat death where the universe expands faster and faster and just fades away eventually.

But if dark energy is something more dynamical, more interesting, that changes over time and some in some interesting way, then you could end up with something where dark energy gets more powerful and rips the universe part or changes nature, changes direction and pulls the universe back together. Maybe that could lead to some kind of bounce as well, although some of the bounce models have sort of extra components or extra things involved to make those stuff happen.

But yeah, so dark energy is the big kind of question in in trying to figure out whats going on with the future expansion of the universe. And then when it comes to vacuum decay, the big question there is trying to better understand particle physics and how that works in our universe, because thats what would break down and create this this change in in how, you know, the new kind of space would be a kind of space where particle physics acts differently. And thats that would be the the thing that would destroy everything.

SR: Basically, just like to go back to a dark energy for a moment. If thats the sort of mediating factor, the thing that we dont know enough about, how are we going about learning about the dark energy? And like, do we have any good theories about what it could be at the minute?

KM:Well, yeah. I mean, aside from a cosmological constant, theres the other ideas. The dark energy is whats called a scalar field, which is a kind of a kind of field that has some some value all throughout space.

Weve only we dont we will we only have evidence of scalar fields existing in physics in one other context. And thats the Higgs field thats associated with the Higgs boson, which is this particle that that the Large Hadron Collider discovered. It has something to do with how particles get mass. So a kind of stuff called a scalar field. Were pretty sure that those things can exist in in nature.

And if dark energy is something like that, then it could be something thats changing over time that does weird things to the universe. And and we also have reason to believe maybe there was a scalar field that was involved in the very early universe for for a very rapid expansion phase called inflation. So theres theres a theoretical construct for what what dark energy could be if its not just a property of space.

But as for figuring out, you know, the properties of dark energy, theres there arent that many possibilities to do that. Its actually quite hard to study because whether its a cosmological constant or a scalar field, its something that seems to be totally uniform throughout space. Invisible, untouchable. And all it does is make the universe expand faster. And so thats not an easy thing to study.

You cant capture that in the lab.

And so the the tools we have to study it are the expansion rate of the universe, which we study by looking at very, very distant objects, which were seeing as they were in the past, and see how theyre moving through the universe.

And then by looking at the how things like clusters of galaxies built up over time. By looking again deep into the past.

And those kinds of things allow us to to study the the effects the dark energy has had on the cosmos over time. And that gives us some clues as to how it works. There are also some possibilities that if it is some kind of new aspect of physics, like like a scalar field, there are certain versions of that that could interact with things in laboratories.

So there are some laboratory experiments that are looking for specific, specific kinds of dark energy or things associated with dark energy. So there are some laboratory possibilities, but it is a hard thing to study. And right now our best tools are things like galaxy surveys and there are some of those that are coming up that will help us to much better study the the evolution of the cosmos over time.

SR:So what do you look for in a Galaxy survey?

So you just you look at as many galaxies as you can find and you try and measure how theyre moving, how old they are, how far away they are and so on as a way to kind of trace out the expansion history of the universe. So theres a new instrument being built. The bureau, Rubin Observatory.

Its going to carry out a survey called the LSST, and that will be studying something like billions of galaxies through the universe. And its a survey of galaxies in the hopes that will the part of the sky that the telescope can see. And it will be it will be telling us a lot more about how just how matter is distributed through through our cosmos.

Then there are other tools we have, like studying the cosmic microwave background, which is the sort of afterglow of the big bang. And by looking at that, we can learn something about the early universe. We can learn something about the components of the universe. And that can also give us some more clues about dark energy and how its behaved over time as well.

SR:I think in your book, you described the cosmic microwave background as being a way to look directly at the Big Bang. Is that right?

KM: Yeah, yeah, yeah. So its its a its a wild thing. When we when we look out into the cosmos, when we look at very distant objects, were looking back in time because the light from those distant objects took a long time to get to us. So if we look at a galaxy thats billions of light years away, then it took the light billions of years to get to us. And if we look farther and farther away than we see parts of the universe that are so far away that it could take, you know, thirteen point eight billion years for the light to get to us the universe.

Thats how old the universe is. And so if we think that the universe started as this hot, dense sort of space filled with sort of roiling plasma, which is what which is sort of what the Big Bang Theory is built on, that the universe was hot and dense in its early times. But hot and dense everywhere.

It wasnt just a single point. It was the whole universe was hot, intense at some at some early time. Then it sends the reason that if we look anywhere in the universe, if we look far enough away, we will see parts of the universe that are so far in the past that they are still on fire.

From my perspective, theyre still in that hot, dense phase. And so we can actually look out into the cosmos and see that that primordial fire from which all of our cosmos was born and the light that we see in every direction, if we look for one of way, is this this leftover light from the big bang, the light directly coming from that fire to us travelling across billions of light years to come to us. Were seeing the final stages of that primordial fire when we look out into the coals. And I think thats I think thats amazing that we cant see that.

SR: And lets lets get back to the heat death of the universe. So how thats related to thermodynamics, isnt it? Yeah. Yeah.

KM: So, yeah, technically, you dont get to heat death until you get to the maximum entropy state of the cosmos. So entropy is a sort of measure of disorder. So the more disorder something is, the higher the entropy. Sotheres this very strict rule in physics called the second law of thermodynamics.

And what this says is that over time in any closed system and we think in the universe as well, the entropy can only increase. And this is why, you know, you cant have a perfectly efficient machine. You always lose a little bit of energy to to friction or something. You cant have an a perpetual motion machine because entropy increases. Theres always a little more disorder.

You always lose a little bit of energy to waste heat or something like that. And. So if thats the case in the universe, which it seems to be, then over time. All of the processes in the universe will be a little bit inefficient and things will degrade and decay and sort of fall apart. And so in the far, far, far future of the universe, you get closer and closer to the maximum entropy of the cosmos.

So you get to the point where entropy can no longer increase because everything is degraded. Everything is is dissipated into pure waste heat. All of the energy is disordered. And when you get to that point, when you have the maximum entropy state, then that is truly the heat death, because that means that basically nothing can happen anymore.

If if entropy has to increase, thats thats just a totally total solid law of physics that entropy can only increase. Then you cant get to maximum entropy and then do something that would create more entropy. So so at that point, you know, there can be little random fluctuations or something that might, you know, rearrange energy a little bit.

And then it would come back down to this. This magic moment, you say. But you cant you cant do anything productive. You cant build anything anymore. You cant even blink. You can you can do technical calculations that say you cant even think anymore.

Everything will be, you know, totally disordered.

SR: Is the heat death the same as saying that the universe will be the same temperature everywhere?

KM: Yeah, itll be itll be a uniform temperature. There might be, you know, random fluctuations here and there. That would settle out again. But, yeah, everything would be this this uniform temperature. And its and its a calculable temperature of kind of the background of the universe after after it reaches maximum entropy.

Its a very small number.

SR: So why is that the most likely explanation for whats going to happen to our universe?

Well, we think thats the most likely just because if you take the kind of expansion were having now where the universe is expanding and its speeding up in its expansion, then what that does is it kind of separates everything out and. And every sort of galaxy can only go through. So, you know, its own evolution with stars dying and things like that.

And then things will decay. And thats thats just part of its just kind of itll all sort of decay into entropy in its own space. And then once everything in each region decays, then all thats left is you basically you actually get a tiny, tiny bit of radiation from the cosmic horizon, which is sort of the.

A region around each point out to which that information cant pass anymore, but that theres a theres a kind of horizon that occurs in a space that thats expanding faster and faster all the time.

And that that kind of horizon has a little bit of radiation associated with it. And that that ends up being all thats left in the universe is just this tiny little bit of radiation thats basically, you know, just its just waste heat, more or less.

SR: All right. Thats something to look forward to.

KM:Yeah, its a bit of a its a bit of a sad, sad ending.

There are there are some interesting theories about how you could have random fluctuations that could lead to a new big bang or or even weird little entities fluctuating out of this this empty heat death universe.

So there are some interesting theories about about strange things that can happen if you just have a universe thats basically empty. But you leave it alone for an infinite amount of time. All the weird things can occur. And so in the book, I talk about some of the stranger hypotheses in there.

SR: So what can you give us an example?

KM: Yeah. So theres this theres this really weird sort of thought experiment thats been around for a while where if you think that if you want to have a universe that sort of where you you kind of randomly fluctuate out of a heat death universe and create a new big bang, if thats if thats an idea that you want for the origin of the cosmos, which which would make sense if you want a universe where you have an end of universe and the new beginnings here and there and branching out of some larger space, then the problem with that is that you can calculate that, that thats a very unlikely thing to happen, right.

To have that random fluctuation of a whole new universe. Its its its its very improbable. Much more probable is that only just like one galaxy would randomly fluctuate out of the sort of soup. And more more probable even than that. Its just one planet. Would would fluctuate out of it and then more probable than even that.

Just just one person or or even even more probable because it requires getting fewer particles together would be just a single brain, like a single human brain that thinks that its living in an entire universe with a whole past that had a big bang and in the cosmic evolution and everything like that. And this is actually this is actually a problem in physics that that that because that single human brain is more probable to occur than the entire universe.

You cant you cant say for sure that that we you know, we are not just imagining all of cosmic history. This is a fairly bizarre problem. Its called the Boltzmann brain problem. Right. And and its not that its not a problem because, you know, because you we actually think these these things would happen. But its a problem because its hard to figure out how these probabilities make sense.

If if you calculate that thats something more likely to happen than the universe, existing problems like that, and I know it is that we have to be we have to be really careful with how we how we suppose a universe might might come out of this kind of state. And. And you have to if you if you set up a system where where its more likely that were just imagining the cosmic history, then that that cosmic history actually existed, then youve probably set up a bad problem in physics.

And so its one of these things that physicists worry about when when constructing possible models of the universe.

SR: Now, lets talk a bit more about vacuum decay.You mentioned earlier that its the result of an instability in the universe, which brings about what you call in the book a quantum bubble of death. But I think thats what makes it my favourite theory.So what exactly is this instability?

KM: Right. So, OK, so I mentioned before the Higgs field, which is a kind of an energy field that pervades all of space. And the Higgs goes on. Is this particle that was discovered at the Large Hadron Collider that is somehow associated with this Higgs field?

Now, the Higgs boson was was called by some the God particle because because the Higgs field was associated with how particles got mass in the early universe. And so, you know, sort of the creation of of matter in some way has something to do with with the Higgs particle through the Higgs field.

But the Higgs field is really the important thing, not the particle itself. But because weve detected the particle, we can learn something about the Higgs field by measuring the mass of the particle and how it interacts with other particles and so on. And unfortunately, what we seem to be learning about the Higgs field is that it it looks like based on current data, it has a vulnerability to changing its value.

So the Higgs field, its this energy field that pervades all space. It has some value associated with it, some sort of number. And the value the Higgs field has determines how. Physics works how particles work together. The masses of the particles, which particles even exist, how the fields, how the forces of nature work together. And in the very early universe, the Higgs field had a different value and there were different mix of particles, different kinds of forces of nature. And.

And, you know, matter, atoms and molecules and things couldnt exist at that time because the laws of physics just werent set up that way. When the Higgs field changed to the value it has now, that allowed the creation of protons and neutrons, electrons and molecules and all of these things. Right. So if the Higgs field were to change again, that would be very bad for us as as creatures built out of atoms, molecules.

Because we we want our particles all together. We want physics to work the way it does. So unfortunately, the data currently point to the idea that theres that the current value of the Higgs field is not sort of the value that the universe would in some sense prefer that that there theres some other value that if you if you disturb the Higgs field enough, it would it would switch to that at that other value and be more stable.

There means that if you could somehow cause the Higgs field to change value at one point in space, then every point around it would also change value and would create a bubble of this kind of space with different laws of physics, different mix of particles and so on, that would then expand out at the speed of light and destroy everything because it would turn. It would put it into this this different kind of space.

This is called a true vacuum with different laws of physics. Now, fortunately, disturbing the Higgs field seems to be something that we cannot do, that even, you know, astrophysical events cannot do that. That doesnt seem to be plausible, but Im not sure wed want to either.

No, we wouldnt want to. But Im just saying, dont worry about particle colliders. They cant do this. Dont worry about that. About that.

But but one what can do that? What can switch the Higgs field to this other value? Is quantum tunnelling, which is a a process that happens all the time with with subatomic particles. We we we find quantum tunnelling in laboratories where a particle might be on one side of a barrier and then suddenly appear on the other side. And thats thats just something that happens in quantum mechanics.

And we we even use this in all our electronics and things like flash memory. We use it for certain kinds of microscopes. We make use of the fact that quantum tunnelling happens as a way to kind of slowly leak particles into into the machines and so on. Like there are quantum tunnelling is thing that totally happens all the time in in physics. And unfortunately, it could also happen to something like the Higgs field.

And if it did, if the Higgs field quantum tunnelled, its different to a different state, its somewhere in the cosmos, then that would also create this cascade, that would create this bubble, that would expand and destroy everything. And because quantum tunnelling is not something that we can deterministically predict, we cant say exactly when it will happen or where that means that its its just a random event that we we cant we cant say when or if it might occur, but we can put a timescale on it because there are sort of probabilities associated with that.

So we can say that its very, very unlikely to occur within the next ten to the power of one hundred years or maybe five hundred. So thats a long time, much longer than the age of the universe. We probably dont have to worry about it, but its intriguing because we dont know when it would happen if it if it were going to happen. We dont know for sure if it could happen, because the calculations that lead to the idea that the vacuum decay is even possible are based on assuming that we understand particle physics in all its detail.

And were we know that theres theres aspects of particle physics that we dont understand yet. So there might be something that comes into this picture and changes it entirely. But but its an intriguing possibility. And it is something that physicists worry about.

You know, how, how or what kinds of assumptions were making about. About particle physics and about cosmology.

Continued here:

Dr Katie Mack on how the Universe is going to end - BBC Focus Magazine