The End of the Universe

How is it all going to end?
25 October 2020
Presented by Ben McAllister, Adam Murphy
Production by Ben McAllister, Adam Murphy.


A nighttime view of the Milky Way from Earth.


In this episode of Naked Astronomy we're taking a look at all the ways that the universe could come to a close. From the dramatic to the slow and ponderous, we're chatting about how space will evolve. Ben McAllister and Adam Murphy are joined by theoretical cosmologist Katie Mack, from North Carolina State University, and author of The End of Everything (Astrophysically Speaking) to chat about the death of everything...

Ben -  For as long as there have been human beings, we’ve wondered where we come from, and where we’re going. Long have we looked to the skies, searching for answers - for an understanding of the origin of the Universe. For millennia, debate has raged - does the Universe even have a beginning, or is it infinite, and eternal? Many scientists and philosophers have believed different things at different times. And then, about a hundred years ago, things changed.

Adam -  It was the Golden Age of astronomy, an amazing era where fantastic new instruments like radio telescopes, combined with new, detailed mathematical theories of cosmology to enable us to develop a deeper understanding of the Universe. In the 1920s Edwin Hubble made the critical, historic observation that the Universe was expanding - in every direction, at all times. This observation, combined with some theory, led to our most popular model of the beginning of the Universe - the Big Bang.

Ben - According to the Big Bang theory, about 13.8 Billion years ago, all of the matter in the Universe was condensed into an extremely small, hot, dense state - which began to expand. Things were extremely chaotic and complex in the early, tiniest fractions of a second of the Universe - we don’t really understand what was happening, since the fundamental forces of Nature which we now know about were still developing.

Adam -  A short time into history, a period of extremely rapid expansion known as “inflation” occured, which lasted about 10-32 seconds. After inflation, the Universe’s growth slowed considerably for about the next 10 billion years - but lots of interesting things were still happening in that time! Over the first second of the Universe’s life, as it cooled, the fundamental particles which we are familiar with today all “froze out” of the primordial energy bath. Tiny particles called quarks started fusing together to make protons and neutrons, and there were a bunch of electrons kicking around as well.

Ben - Over the first few minutes some of the protons and neutrons started fusing together as well, to make the earliest atomic nuclei - leaving us with things like hydrogen and helium, and a very small fraction of heavier things. Now, just a few minutes after the Big Bang it's still too hot, and there is too much ambient radiation around for ATOMS to form, we have atomic nuclei - protons and neutrons smooshed together - but the electrons are still floating around largely freely, and there are tonnes of photons around, which are particles of light.

Adam - This continues for a long time, like, 400,000 years. The Universe continues to expand, and cool - and eventually it reaches a point where it is cool enough for atoms to form. That is to say, electrons can be captured by the nuclei which were formed, and we end up with full, proper, honest to goodness atoms - albeit only of simple, very light elements. Nothing fancy like metal just yet. Prior to this time photons of light couldn’t travel around freely, because there were too many electrons kicking around in free space, and they would always bump into each other, and interact. But now that the electrons have been captured by the atoms, the photons can travel far and wide, and the Universe becomes transparent.

Ben - At this point gravity starts to play a huge role in the evolution of the cosmos. Due to the force of gravity pulling together any two objects with mass, the early matter starts to be drawn together into clumps. As the matter continues to get closer and denser, it heats up. This matter is almost entirely hydrogen, and helium - and at a certain point the pressure and temperature is high enough that something amazing occurs. It ignites in a thermonuclear explosion. The first stars are born.

Adam - These first stars are like massive nuclear furnaces, smashing hydrogen and helium together into heavier, more exotic elements. We aren’t sure exactly when this happened, but we think it might have been about 100 million, or a few hundred million years after the big bang, so - yeah, that’s a long time between the formation of atoms and the formation of stars. It sure is, and due to the lack of light-emitting objects in that time, astronomer's call it the Cosmic Dark Ages.

Ben - Anyway, eventually these first stars ran out of fuel, and died in enormous, fiery explosions called supernovae - which scattered the heavier elements that they had so patiently created all across the Universe. And it’s a good thing they did! Because all the complex, heavy matter that makes up things like planets and human beings was forged in the core of a star - if not one of these first stars, one of their descendants.

Adam - Around this time, over the 100 million to billion year timescale, so much matter has coalesced in some regions that the first galaxies start to form. Cosmologists think that our own Milky Way was formed something like a few hundred million years after the Big Bang. And from the formation of the first stars and galaxies, the cycles continue - stars are formed, they smash light elements into heavy elements, then explode and scatter their material across the Universe, then new stars form from the rubble.

Ben - More and more galaxies continue to form and on and on it goes. Then, about 9-10 billion years after the dawn of everything...a few curious things start to occur. Firstly, a little rocky planet which we will come to call “Earth” forms in one of the outer spiral arms of the Milky Way - made of material which has been vomited out of exploding stars. This little planet cools, and some weird things start bubbling away in a chemical soup.

Adam - And around the same time - in a cosmological sense, something even weirder happens. According to our observations, the expansion of the Universe….starts to accelerate again. So the Universal expansion, which was so rapid initially, and then gradually slowed down over the 9 or so billion years that stars, galaxies and planets were forming, has been getting faster and faster over the last 4 billion years or so.

Ben - It’s a vaguely terrifying thought - kind of like we’re in a runaway train of a cosmos, growing and growing at an ever increasing rate. But what does it mean for the fate of the Universe?

Adam - Well, thankfully, we’ve brought in a proper expert: Dr Katie Mack. In addition to being a theoretical cosmologist, Katie has just written a book called The End of Everything (Astrophysically Speaking), and she was good enough to sit down with us and talk about it.

Katie - So there are several different ways the universe might come to an end, and some of them are so far in the future that it's very hard to even come up with terms to express those numbers. Others could in principle happen much sooner. So, it just depends on where you think things are going. For the most part, there's really no credible possibility for it to end before the Earth is destroyed by the expansion of the Sun and so on. So this is not an immediate threat, but it's something that's interesting to think about in terms of just kind of understanding our place in the universe and the narrative of everything.

Ben - Absolutely. But I'm glad to know that we humans will have met a, probably a much fierier end by the time any of these things occur.

Adam - Thank heaven for small miracles.

Katie - We've only got about a billion years on this planet before it's entirely uninhabitable. So, you know, we should plan ahead at some level. Yeah.

Ben - Oh, if we make it that long.

Adam - Yeah. Unless we do something bad.

Ben - Yeah. And something else happens. Okay, great. So, it sort of depends, I guess then on how the universe is going to end. Well, I'm given to understand there are a few different major theories, about how the universe is potentially going to end. So could you talk us through what some of them are.

Katie - In my book that I've just written, The End of Everything, I go through five different options, different ways that people talk about where things could go in the future, and the most common, or most accepted version is called the Heat Death. And this is where the expansion of the universe that's currently going on now continues forever. So, you know, right now we see the distant galaxies are moving apart from one another. And the whole universe seems to be getting bigger. There's more and more empty space. So if that just continues and keeps speeding up, which it appears to be doing now, then over time, everything will get farther apart. There'll be fewer interactions between galaxies, fewer stars forming, and everything kind of just fades away. The stars burnout, eventually matter decays, black holes evaporate, and you end up in a cold, dark, lonely, empty universe, and that's called the Heat Death.

Ben - Interesting that it's called the Heat Death. When it seems more like a cold death.

Katie - Yeah. So in this case, heat is used in the technical term to mean sort of, disordered energy. And what's happening is that all that's left in the universe at the very end state is kind of, the waste heat of everything in the universe, sort of decaying and falling apart. So it's the descent into pure entropy. If you want to think about it like that.

Ben - Sounds pretty boring, really, like this just slow, fizzle out.

Katie - It's definitely not the most exciting possibility. It's maybe the one that is least violent in some sense, but it's just a slow fade into black. And you know, it is the most accepted, and it's the most likely based on what we currently know about the expansion of the universe. But there are other possibilities. When we look at the expansion of the universe, for a long time, there was a question about whether the universe would continue expanding forever or stop at some point, and everything would sort of turn around and come crashing back together. We think that's probably not going to happen, because the expansion seems to be speeding up, and it's not clear how it would turn around and stop, but whatever is causing the expansion to speed up, it's mysterious something we call dark energy, and we don't really understand dark energy well enough to know that it's just going to keep doing what it's doing. And so if it were something that could change over time in a certain way, then it could reverse the expansion potentially, and create the conditions for everything to come crashing back together. And that's a process called the Big Crunch. That would be a very violent and fiery end for everything. So that's another possibility, and the Big Crunch is something that was thought to be the most likely end of the universe, for quite a while, like in the sixties and seventies, people thought that that was most likely how we would go. So there's been a lot of research in terms of sort of, what that would do to everything. If it all comes crashing back together, which is fun to read about

Ben - Like a reverse Big Bang, I suppose, like some kind of awful cacophony.

Katie - Yeah. Except kind of worse, because in a Big Bang, you just start in this sort of fiery hot state. The universe is full of plasma and then everything expands and it cools. But in the case of the Big Crunch, you're not just going back to that same sort of hot plasma. You're not just compressing all the background radiation from the Big Bang. You're also compressing all the background radiation from all the stars that have ever shone. So it gets like actually much hotter as you're approaching a Big Crunch, than it was the Big Bang. And so the coolest thing about the Big Crunch is that as the compression is happening, because you're compressing all this hard radiation, and hardening it in the process, you actually get to a point where the surfaces of stars ignite, because the radiation from just the background of space is so hot and you get these thermonuclear explosions all along the surfaces of stars. It's really awesome.

Adam - So the space explosions themselves start exploding.

Katie - I mean, everything, like the stars are at some point hotter on the outside than on the inside. And they just like, everything is, and just empty space is hotter than stars at some point,

Ben - That sounds like it'd be something to see if there were any human beings around to do so, which of course we know there won't be. Okay. So am I correct then in understanding that, the fact that we think it's more likely to be the boring Heat Death, as opposed to the Big Crunch is only something we've come to understand since we've had observational evidence for the fact that the expansion of the universe is accelerating?

Katie - Well. So I think there was a while before the acceleration was discovered, where it was looking more like the universe would continue expanding forever, but not speeding up. The speeding up is the weird part. Okay. So the idea that a universe could start with a Big Bang and then expand forever is not that weird. As long as the Big Bang was powerful enough, and there's not enough gravity to slow it down and stop it, then, you know, an expanding forever universe is fine.

Ben - Fine until the Heat Death.

Katie - Yeah. Yeah. But I mean, it's not offensive to physical sensibilities, but a universe that starts speeding up for no apparent reason, about 9 billion years into the story, that's weird. And that's why we need this extra ingredient we call dark energy. We need something to step in and cause the accelerated expansion so long after the Big Bang, when the universe had been slowing down, suddenly it picked up around 5 billion years ago and it's been speeding up ever since. And that's weird. And if it's just a cosmological constant, just a sort of property of space, a term in the equations that Einstein wrote in there, then we know what's going to happen. It's just, the universe will keep expanding forever. It'll keep it speeding up. You'll get to a Heat Death. That'll be it. But, there are possibilities that dark energy could be something else. And I mentioned one of them could cause the expansion to turn around. There's also a possibility that the expansion could get way more extreme. So if you have dark energy, that's in the form of something we call phantom dark energy, where it gets more powerful over time, then that can lead to what we call a Big Rip.

Ben - Ooh, that sounds scary.

Katie - Yeah. Yeah. So this is where, not only does the dark energy, sort of move galaxies away from each other. It also starts ripping them apart from the inside. And so over time you'd get more and more dark energy and the expansion of space would get more and more violent, and it would start to tear apart galaxies, and then solar systems, and then stars and planets, and eventually even, you know, molecules and atoms and particles themselves. And you would end in this fantastic Big Rip at some time in the future.

Ben - Where what, you've just got individual particles just like, scattered around, rapidly accelerating away from each other.

Katie - What's technically happening in a Big Rip is that the dark energy is building up in every point in space. And so there's more and more of this expansion happening everywhere. And that's what's able to do things like unbind galaxies and solar systems, but then at the very end, what's happening is that the scale factor of the universe goes to infinity. Now the scale factor is a measurement of the distances between things, right? So the distances between galaxies or whatever, and we use the scale factor to measure sort of, the size of the universe, how much it's expanded over time. And if the scale factor goes to infinity, that means that no matter how close together two points were, at the time the scale factor goes to infinity they're infinitely far apart. So, you know, that's really a kind of, tearing apart of everything in space, and space itself, in some sense,

Ben - It's like an infinitely large expansion of a finitely small distance, right? It doesn't matter how small you make it, anything times infinity is infinity. That's a high school, high school maths class, one we all remember.

Katie - Yeah. And it's not clear physically, exactly what that would do to everything. If you, if it would be such an influx of energy that you would have a Big Crunch, or some kind of other horrific result, but in any case, it would be bad for the stuff in the universe.

Adam - Where would something like that start? Because rips generally need a weak point before they start. Is it just an everywhere kind of thing?

Katie - It's everywhere. Yeah. So the expansion is uniform, right? And dark energy is everywhere. And so it would be, in the same way that currently the expansion of the universe is happening everywhere. There's no centre from which everything is expanding, the Big Rip would also happen everywhere. Now there's sort of, theoretical reasons not to like the Big Rip, all the theories that do a Big Rip, kind of violate certain energy conditions that physicists think we probably need. But in terms of the data, you can't currently rule it out as being something that could happen hundreds of billions of years in the future. And the theoretical side is a little bit shaky.

Ben - It's such a sinister name, isn't it? Dark energy. It sounds like some real sci-fi stuff, but I guess we're starting to see why it deserves that sinister name. Because it's responsible for so many of these awful deaths. Okay. So Heat Death, Big Crunch, Big Rip. What else we got?

Katie - So my personal favorite is called Vacuum Decay. So Vacuum Decay is, I sometimes think of it as sort of a manufacturer's defect in the universe. It's due to what might be an instability in sort of, the structure of physics in our universe. So the laws of physics, the way that particle physics works, it's kind of all tied into the Higgs field, which is this kind of energy field that pervades all of space. And the Higgs boson is this particle that's connected to the Higgs field. And the Higgs boson was discovered at the Large Hadron Collider in 2012, it was a big triumph of theoretical physics to come up with this particle, and then experimentally, it was detected. And so it was a big deal, right? And the Higgs field, basically it kind of sets the rules for particle physics. So it's sort of this energy field everywhere that has a certain value associated with it. In the very early universe, it had a different value and there were different particles. There were different forces of nature, things kind of fit together in a different way in particle physics. Then the Higgs field changed, set up the rules for particle physics as we have them now. Now we're able to have molecules, and atoms, and matter holds together, and everything like that. And it's great, right? We like it this way. We want it to stay this way. We don't want the Higgs field to change again, and rewrite the laws of physics and make atoms impossible, for example. But there's recent evidence based on measurements of the Higgs boson, and other particles in the Standard Model of Particles Physics, they suggest that maybe the value the Higgs field is at now, is not the stable, most secure value that could have. Maybe there's another value that it would rather be at. And if it were disturbed in some sense, or if there was some kind of quantum transition somewhere in the universe, it would settle to that other value, in much the same way that if you have a ball, in a valley or a boulder, and it's at the top of the valley, it would want to roll down to the bottom.

And maybe what's happened is that the universe is this boulder rolling around in this valley. And it's got stuck in a little divot near the edge, but it's not at the bottom of the valley. And it would rather be at the bottom of the valley. And so in some sense, the Higgs field is in this similar situation, it's in a little divot, but it would kind of rather be at the bottom. And if that's the case, then it's kind of only a matter of time until it gets there, because something could disturb it. That's a little complicated, how to do that in our universe, it's difficult to disturb the Higgs field, but our universe is fundamentally quantum mechanical. The Higgs field is subject to quantum uncertainty. Quantum tunneling is a thing that can happen, where a particle can sort of suddenly appear on the other side of a barrier without having to go through. And that kind of uncertainty, that random motion can happen with the Higgs field in such a way, where the Higgs field at one point in space could just transition to this other vacuum state, this other value. And if that happened...

Ben - And then all of a sudden, bye-bye atoms.

Katie - Yeah, that happened then at that point, yeah. You'd have a little bit of space where the laws of physics are different. You're in what's called a true vacuum state, that would create a little bubble of this true vacuum state with different laws of physics. And that bubble would expand through the universe at about the speed of light, and just destroy everything. Yeah. And that's called Vacuum Decay.

Ben - And there's no way to know when it's going to happen?

Katie - Exactly. Yeah. Because it's a random quantum event. You wouldn't know when or where it would happen. You could only put probabilities on how long it would probably take. And based on our calculations, it probably won't happen until 10 to the power a hundred years from now. So a really long time from now.

Ben - Okay. So the universal probably be gone anyway by then. Right?

Katie - I mean, yeah. Basically, like by that time you're deep into the Heat Death. Like there's nothing left anyhow, whatever, but you know, I mean it's intriguing because in principle it could happen sooner.

Adam - It sounds kind of like "universe.exe has crashed and experienced a fatal error."

Katie - Yeah, yeah. Something like that.

Ben - So is it like, basically, like a number if you like, that sort of sets the strength of the interactions between say, the electrons and the other stuff inside my body. And if that number just decided like, Oh, I'm the wrong number. I want to be a different number. I'm going to change all of a sudden, I just cease to exist.

Katie - It's actually a little worse than that, because it's been calculated. It turns out the true vacuum space, the space that is what would happen inside the bubble. That's also gravitationally unstable. So once you're inside the bubble, once the bubble passes over you, and destroys all your atoms and everything, then you collapse into a black hole. So good times.

Ben - Wonderful. Glad. Glad to know that that could happen.

Adam - I'd rather be a black hole than nothing.

Ben - Yeah, exactly. Exactly. A black hole, rather than just a bunch of scattered subatomic particles hurtling through the void. Okay. So that's terrifying. And I guess that leaves one more of these possibilities that are presented in your book.

Katie - So the last one is, is really a set of different possibilities, that all have a characteristic in common, which is that they are, bouncing cosmologies, or cyclic cosmologies. So theories where the universe could cycle from a Big Bang to some kind of ending, and then back to a Big Bang, over and over again. I talk about a few different possibilities there. Some, where you have a kind of compression, and then a new expansion. Some, where you have Heat Death and then a new Big Bang after that. And there are several different ways you can have a new universe appear after this one. And they're all a little bit more speculative, because these theories are kind of newer and less developed. But, it's an interesting possibility, that maybe even if we're all gonna die, like something will happen afterwards. That could be interesting.

Ben - Yeah. That's kind of encouraging to know that maybe there will be something after, whichever one of these horrible things occurs to us, occurs. So I've heard this concept of like, if we're going to have a Big Crunch, the idea that there might then be an expansion back out, like we're in some kind of massive pair of breathing lungs.

Katie - Yeah. So it turns out that it's actually technically quite difficult to work out that transition, from a crunch to a bang, but a few cosmologists have worked on that. There's an idea called the ekpyrotic scenario. That it doesn't have a crunch exactly. It does have a compression that then transitions to a new Big Bang and expansion, but it doesn't go to a singularity, because it's just very hard to work out how to bounce back from that free collapse, kind of Big Crunch scenario. But there've been examples of that, that have been brought up as well.

Ben - So I suppose the implication of that then, is if we do have observational evidence for the Big Bang, believing we've come from a single point, then we would be pretty confident we're not in the aftermath of a prior Big Crunch.

Katie - We don't know if there ever was a singularity in our past. And when we talk about the Big Bang, we usually talk about, just the idea that the universe was hot and dense in the past, and has been expanding ever since. The idea of a single point is kind of uncertain, and probably not true, because we know that the Big Bang happened everywhere. It didn't happen at one point. And so there may have been a point of infinite density, but still extended space. So that part we just don't know.

Ben - Okay. So we could be in some kind of cyclical universe now where we're not special. We're just one of many, many cycles?

Katie - We could be. Yeah. Yeah. And there are groups of people working on trying to find evidence or not for that scenario.

Ben - Fascinating. So that's something we can potentially have evidence for one way or another?

Katie - Yeah. I mean, that's kind of the big effort in that area right now, is trying to distinguish among all these various scenarios.

Adam - How do people tend to react when you tell them the universe is going to end in these various horrific scenarios?

Katie - I think that there's kind of a mix. You know, some people will find it quite scary to think about the end of the universe. Some people find it sort of thrilling, and amusing, and some people find it sort of comforting to know that whatever happens, it's not our fault. We fit into this larger picture. And, it's kind of nice to think about things that are so separate from our daily lives. And the end of the universe is one of those things. That's just very interesting and disconnected from us and still big and dramatic anyway,


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