What came before the Big Bang?

00:00:07
Speaker 1: Everybody wants to know where we came from, how we got to be here. That means understanding the events that set in motion your life, all of its ups and downs. Historians can tell you about the journey of your ancestors, how it shapes your culture and way of life. Anthropologist can tell us the story of humanity itself, why we walk on two feet, and how we got to be so darn smart. But physicists can take us deepest into the past, even beyond the chaotic formation story of our solar system or of the galaxy itself. Incredibly, we can look billions of years deeper into history, to a time before there were galaxies or stars, when the universe was filled with hot, dense plasma. But then we hit a wall. Beyond thirteen point eight billion years ago, the universe was opay, and all photons emitted before then have been lost, reabsorbed, and beyond another three hundred and eighty thousand years, the universe was so hot and dense that our theories of physics no longer apply, like trying to explain steam using fluid dynamics. Our curiosity, of course, is limitless, and so we desperately want to know what came before, What is the original fundamental context of the universe, and what does it say about those of us scratching out a living on a tiny dot of rock. Today we'll take a tour of the current ideas about the earliest moments in the universe, if there even were any, and think about what we might learn in the future. Welcome to Daniel and Kelly's Extraordinary Ancient Universe.

00:02:00
Speaker 2: Hello, I'm Kelly Wiener Smith. I study parasites and space, and today we're talking about where it all began.

00:02:05
Speaker 1: Hi, I'm Daniel. I'm a particle physicist, and I love physics because it subsumes every other question in science.

00:02:12
Speaker 2: So today we're talking about, like, what happened before the Big Bang? Do you think will we ever get solid answers where we're like, Bam, we don't have to debate this anymore. We've got to figure it out. Or is this the kind of thing that we will always be tinkering with. I think either answer is okay and exciting.

00:02:27
Speaker 1: I should say yeah. I don't think there's going to be a moment where we're like, yeah, we figured it out, and here's the rest of the grant money back. By the way, we have nothing else we're curious about, and we don't want to think about it anymore.

00:02:37
Speaker 3: Yeah.

00:02:37
Speaker 1: No, I think we're always going to wonder what about this? What about that? Where did that come from? Why not this? Why the other thing? I think it is a lot of philosophical questions, just thinking about the origins of the universe, but also wondering which answers are acceptable, which ones we go, yeah, that makes sense, let's move on, and which ones were like, wait a second, why that? I think that reveals a lot of about our humanity and our curiosity, which is one of the reasons why I love this topic so much.

00:03:04
Speaker 2: Yeah, and we've talked on past shows about why there are some explanations we get where we're like, oh, that makes sense, we don't have to dig any deeper, and how that can like sort of send us down the wrong path actually by just you know, oh, that makes sense, no more questions needed?

00:03:18
Speaker 1: Yeah, exactly, And whether aliens would find those answer satisfactory or not?

00:03:23
Speaker 2: I know, Dan, the constant question that is ringing in Daniel's mind, what would the alien say.

00:03:30
Speaker 1: Exactly? Well, you know, they're not here to speak for themselves, so I feel like some pressure to speak up for the aliens.

00:03:35
Speaker 2: I'm so glad that they have an advocate in Daniel Whitson.

00:03:39
Speaker 1: Well, look, if they're aliens all over the galaxy, then they are literally the silent majority, right because everybody's speaking for them. So yes, I stand here to speak for the aliens. But today we didn't have an alien on the podcast. We instead had a human being. We thought a lot about cosmology and how to communicate it to the public because we wanted to dig deep into this question of what is the Big Bang and what came before it? And so before we get to that interview, let's hear what folks on the street, by which I mean podcast listeners who volunteered think came before the Big Bang. Here's what folks had to say.

00:04:16
Speaker 3: Our understanding of physics breaks down with a big bang, but one idea so a turn of inflation. It could be that there was something that was just so different from our current experience or understanding that we don't even have the language to describe it.

00:04:29
Speaker 1: According to Pastafarian scripture, the great flying spaghetti Monster made us all hail the cosmic Cannloni. It's just all theories. I guess at the singularity, there was no space time, thus no time, no causation, no before. But how can there be an after without a before?

00:04:57
Speaker 3: That's like asking what the NOI at the Pope. I don't think that's an answer, but prove me wrong, please.

00:05:05
Speaker 1: Nothing came before the Big Ban, because as I understand it, it's when time started. All right. What a wonderful spectrum of answers as usual? Aren't our listeners wonderful? Kelly?

00:05:17
Speaker 2: Yeah, obviously, obviously, clearly they have a good sense of taste for podcasts. But exactly, but we've got a real professional on today to tell us about the mini theories actually for what came before the Big Bang. I think we don't even manage to get to all of the ones we had thought about, because there's a lot of them out there. So instead we dug deep into about two of them. And let's go ahead and jump in.

00:05:41
Speaker 1: It's my pleasure to welcome to the podcast Phil Helper. He's a Fellow of the Royal Astronomical Society and a science popularizer. He's a creator of the popular YouTube series Before the Big Bang. His astronomy images have been featured in The Washington Post, BBC, and The Guardian. He's one of the rare breed of pop physics authors that also who has deep credentials in physics, and together with cosmologist naish Ash forty, he's the co author of Battle for the Big Bang The New Tales of Our Cosmic Creation. Phil, welcome to the podcast.

00:06:12
Speaker 3: Thank you great to be on.

00:06:14
Speaker 2: We're excited to have you. And if I can get the ball rolling. So, like my for my PhD, I studied fish, and I really like things that you can have in the lab and you can see and manipulate, and studying something like the Big Bang, which happened so long ago, sounds really difficult. What got you interested in this question in particular?

00:06:33
Speaker 4: Well, who cannot be absolutely fascinated by the origin of the universe. I mean, I think that's deeply ingrained in right in human nature. I mean, it's no surprise that every culture, well maybe not every culture, where many many cultures have an origin story, something where they can point to and say, this is how the universe came to be. So I think we all have that deep curiosity in us. And the beauty is that science has actually found a way to probe the very origin of the universe and that's cosmology. So why not follow follow us down that rabbit hole and see how far it goes?

00:07:13
Speaker 2: Amazing?

00:07:14
Speaker 1: Well, I like how you appeal to our innate curiosity, but I think in the end there is something subjective, right about what you find exciting? Would you find interesting? What questions you think are worth exploring? And so you know, some of us are excited about fish guts and some of us are excited about the whole universe. So it's all about it, for sure.

00:07:35
Speaker 2: I can see the fish guts in front of me. I like that.

00:07:39
Speaker 3: Oh I have actually published a paper on fish.

00:07:42
Speaker 4: Oh yeah, scientific und yeah, yeah, so I'll have.

00:07:45
Speaker 3: Been interested in fish as well. Okay, good, Literally it's about fish pains.

00:07:50
Speaker 2: Oh that's an important topic, I think.

00:07:52
Speaker 1: So.

00:07:52
Speaker 4: Yeah, we tried to show that the case of fish pain is stronger than some people think.

00:07:58
Speaker 3: Yeah.

00:07:58
Speaker 1: Well. The thing I love about the BANG is that it explains everything. Like literally, we're going back to the beginning and trying to find the cause of you know, planets and life and fish guts and Kelly and everything. So it's all inclusive, but something we have to clear out before we dig deeper into theories about what happened before the Big Bang is what we mean when we say the big bang. And in your book, I really appreciated that you attack this head on. You're like, look, there's a lot of confusion about what this means, and cosmologists aren't even always consistent about what they mean by the words. So can you give us a clear and crisp definition of what we mean when we save the big Bang?

00:08:37
Speaker 4: Well, the answer is now, I can't, because there's all than one definition of the big Bang, and that's kind of the whole problem. People mean different things when they say a big bang. But what we can do, which I hope will be really helpful, is to explain the different meanings and then people might try and infer which meaning is being used in which context. So we sol highlight two definitions of the big Bang. So one is that the big Bang is a theory that says the universe evolved from a very hot, dense state. So it was different in the past, very different in the past than it is today. So it was very hot, very dense, highly curved, and that's very very different to the sort of cold, sparse universe we see today. So that's one definition we call without the hot Big.

00:09:22
Speaker 1: Bang, and that's what scientists mean when they refer to the Big Bang, because we know the universe is expanding and getting less dense as time goes forwards. But we can also run the clock backwards and figure out that it was more dense in the past. And if we run it back thirteen point eight billion years, we get to a very hot and very dense state, and that's it. The hot Big Bang doesn't go any further than that because it can't. That state is so hot and so dense you'd need quantum gravity to predict what happens before that, So it doesn't go all the way to a singularity. It can't without quantum gravity, so it doesn't tell us what happened before or how time began. It assumes that initial state and explains what happens afterwards as time goes forward, and it does it very well, and there's a lot of experimental evidence for it.

00:10:12
Speaker 4: Another definition we call the Big Bang singularity. So that's an idea that says the universe start a denist state that's infinitely dense, infinitely hot, infinitely curved, and that marks the beginning of time.

00:10:27
Speaker 3: So there is nothing before the Big Bang, and this idea.

00:10:30
Speaker 1: The Big Bang singularity is what most people think of when they hear the phrase big Bang. This takes that hot, dense state from thirteen point eight billion years ago and extrapolates it even further backwards in time, but ignoring quantum mechanics. It says, if we just account for gravity, what would happen? So you get a singularity. It's a collapse. That's like how general relativity predicts the singularity inside a black hole, which also ignores quantum mechanics. So we don't really believe it. So the Big Bang singularity is an extrapolation past what we know using an approximation of quantum gravity that we know is wrong, which ignores all of the quantum effects. And there's no experimental data for this extrapolation. It's all very speculative.

00:11:12
Speaker 4: Now here's the interesting thing. And we did a survey of physicists. We went to a conference in Copenhagen, very large conference. It was interdisciplinary. We had theorists, we had observers, we had like numerical relativists, we had people working in quantum gravity or kind and lead some of the leading figures in the field. And we asked various questions about controversies within physics, such as what's the right interpretation of quantum mechanics, what's the best candidate for quantum gravity, what do you think dark matter is? And we got consensus on almost nothing. I can't think of any topic where we got fifty percent for any one thing except one one exception, and that was how should we define the term big And the consensus was that it should only be defined as this evolution from a hot, dense stay. So the idea that the universe began at the Big Bang or there was a singularity is not perceived as the way we should understand the Big Bank. That is a theoretical extrapolation that most people don't trust, I would say.

00:12:21
Speaker 1: And let's dig into that a little bit more. Like, we have this theoretical extrapolation, the Big Bang singularity, as you say, which might predate the hot early universe that you described as the hot Big Bang. And that's very speculative. But we have a lot of evidence for the hot Big Bang. Right when we say the universe was dense and hot thirteen point eight billion years ago, that's not speculation. That's something we have unfairly solid grounds, right.

00:12:46
Speaker 3: That's right.

00:12:47
Speaker 4: I don't think there's any serious doubt about the hot big bank. So that's very well established. We've got lots of independent lines of evidence for that. People will probably be familiar with, you know, the expansion given by the red shifted galaxies. We've got the cosmic microwave background, the abundance of elements, and there's various properties of the cosmic microve background that we should see if the Big Bank had happened, and we see them. So I don't think there's any doubt that there was a hot Big Bang. But the observational evidence stops there. We can't have observational evidence of a singularity. They are sort of hidden from us if they were to exist, but I think most physics would say they don't really exist. They're just part of our physics that breaks down, and we need better physics and that will sort of get rid of the singularities. But how to do that is an open question, and what would replace a singularity is where all the debate is. That's why we call our book the Battle of the Big Bang, because once you concede, Okay, we don't really know the universe began with a big bank singularity. It might have had a prior stay, Okay, so what was that prior's day? And that's the battle because there's all kinds of ideas out there and no one knows which one is right.

00:13:57
Speaker 2: So for the biologist, if I were to be floating in and I was observing the big bangs of the two different varieties happening, what would I be seeing in a hot big bang and what would I be seeing in a big bank's singularity?

00:14:10
Speaker 4: So in a hot big bang, well, I think the name kind of gives you a clue. It's very, very hot and dense basically, but soon you know, as you go back in time. It's a bit like looking at the sun. You know, if you look at the surface of the sun, you can see that it's about five thousand, seven hundred colvin, but you can't it's.

00:14:28
Speaker 3: Opaque, right.

00:14:29
Speaker 4: If you to go through it, all the photons would be bouncing off the materials, you couldn't really see very far.

00:14:36
Speaker 3: It'd be like a fog. So that's this sort of hot, dense state.

00:14:40
Speaker 1: And to clarify, you're saying that the whole universe was hot and dense, like the entire universe was thirteen point eight billion years ago, something like the inside of the sun is now and photons made inside the sun get reabsorbed almost immediately, which is why the sun is opaque. You can't see through it, so it's not a great view.

00:15:00
Speaker 4: Now the singularity again, most people would say it's just a part with a mass breaks down. But but the description involves you know, there was what was called a curve. It's your singularity. So here the temperature would literally be infinite.

00:15:13
Speaker 2: So it's hot too.

00:15:14
Speaker 4: It's also hot, yes, yes, okay, yeah, it would.

00:15:18
Speaker 3: Be hotter, hotter a big bang, yeah, the holter big bang.

00:15:21
Speaker 2: Oh wait, so the hot Big Bang is not the hardest of the Big Bass.

00:15:24
Speaker 4: Not the hot good naming, h guys, it is all right yeah, but no, but we'll come. We'll come to what might have come before the Big Bang, and that could have been something very cold.

00:15:35
Speaker 3: Okay.

00:15:35
Speaker 4: So yeah, that's to do with something called inflationary cosmology. So if we believe that story of the Big Bang or then actually the universe is very cold before the Big Bang.

00:15:45
Speaker 1: All right, So we have this scientific conception that the universe is cold and sparse now and we run the clock backwards, we get to a hot, dense state thirteen billion years ago, and the big question is what came before that? Where did that hot dance state come from? We know a lot about that state and its evolution from that point to now, but not a lot from before that. That's the scientific conception of the Big Bang. But I think if you talk to people in the general public and you ask them about the Big Bang, they mostly think about the Big Bang singularity. They think the universe started from a tiny dot that exploded into empty space. Why do you think there's such a persistent gap between the scientific view and the popular view. I mean, you work on both sides of the aisle. Why do you think there's been a disconnect there.

00:16:30
Speaker 3: Well, I think there's a couple of reasons.

00:16:33
Speaker 4: One, when you're doing animation, and you know, I've got YouTube videos that we try and animate the big fact, you kind of it's hard not to draw bit a light exploding outwards, right, because that's how you're going to animate it. So it's hard for people to not have that visualization, which is, you know, a rough approximation, but it's not precise in their head.

00:16:56
Speaker 1: And I want to be clear here that it's not precise because applies that the universe was empty, that a dot of matter exploded into empty space. This is misleading because the universe was never empty. Matter was always everywhere. If the universe was infinite, then matter was infinite and then expanded everywhere simultaneously. If the universe was finite, then it was small but still always filled with matter. There was never a tiny dot exploding out into empty space.

00:17:25
Speaker 4: But the other thing is, I think two things happened at the same time. Roughly in the mid nineteen sixties, we got the evidence now clear that there was a cosmic microwave background, that's the afterglow of the Big Bang. It was a very hot surface and then it cooled down into microwaves as the universe expanded and stretched, so it stretched that light into into the microwaves, and that was very clear evidence of the hot Big Bank. At the same time, Roger Penrose and Stephen Hawking, roughly at the same time, we're establishing these singularity theorems that say the Big Bang must have been a singularity. So that looked like you've got the theory and the evidence all dancing in harmony. So it would appear then that the universe had this definite beginning of the singularity, and of course one of the authors of these Penrose Hawking. Theorems of Stephen Hawking, and he was an incredible figure popularizing science. And whilst Roger Penrose was a bit reluctant to even user RaSE singularity, wasn't sure that we should trust it this distrapolation, I think Stephen Hawking was much more brash, you know, so he was sort of proclaimed, you know, this is the beginning of the universe. But what hers happened is that some of the assumptions of those singularity theorems have started to unravel. And this happened, you know, in the decades after the singularity theorems were proven. So physical theory is only good as the assumptions that it has, and those assumptions in the peneris hooking theory. We can go over them one by one if you wish, But bottom line is people started to question those assumptions. So then it was like, well, maybe there isn't really a singularity or it maybe it's just an artifact of pushing the equations beyond their domain of validity, and we need new physics to explore what really happened to fourteen billion years ago.

00:19:27
Speaker 2: So The problem is that the popular science conception of the Big Bang hasn't caught up with where we are now in our understanding of the Big Bang. Would that be fair to say.

00:19:35
Speaker 1: Yeah, But I think there's more than that. Two ideas were developed at the same time, the hot big Bang and the big Bang singularity, and they were both just called the bing bang, and so they get confused for each other. The hot big Bang is still a very solid idea scientifically, and the big Bang singularity kind of never was and we've moved on from it. But most people still think of the big Bang singularity when you say the big Bang.

00:19:58
Speaker 3: I think that's right.

00:20:00
Speaker 4: Yes, although there were people even at the time in the sixties saying these singularity theorems don't prove the universe at a beginning, but I don't think they were as loud a voice as those He said it did.

00:20:11
Speaker 1: Sounds like we should blame Stephen Hawking. Yeah, exactly, But it sounds also like a familiar story. You have one story that somebody tells in a compelling way and it sticks in people's mind. It's like, even if it's hard for people to wrap their mind around this idea of the universe, beginning in a point. It is a compelling story, it's easy to tell, and it's stuck in the popular imagination long after people are like, well, we actually only know a certain part of the story. The rest of it's speculative. That's the nuance, that's the subtlety that's hard to convey. I feel like that's sort of what's happening with popular science everywhere, that the details are being lost and the compelling clickbait headlines are getting propagated and embedded in people's minds. As a science popularizer yourself, how do you feel about the sort of state of science journalism right now?

00:21:00
Speaker 3: Yeah, it's depressing.

00:21:02
Speaker 4: I Well, one thing I do want to say, just before I come on to that, another correction that we want to make to the popular I'm saying is that the Big Bang wasn't a point. Rather, it could have happened everywhere. Well if it would have happened everywhere, and in fact, the universe could be infinite today, and it could have been infinitely big even at the Big Bang, So it wasn't a point. But anyway, no, siche journal isn't very very depressing at the moment. I think you see people just trying to get headlines clickbaits. They're not, i think, being rigorous and of course probably underfunded. So you just get people, you know, copying press releases or they're going for like the big angry headline. You know, Oh everyone's got everything wrong, and that's usually also dubious too. So yeah, I'm not optimistic about the state of journalism.

00:21:56
Speaker 1: All right, Well, let's do our best to push back, and so to reiterate, we know a lot about the last thirteen point eight billion years of the universe. We have evidence, it all comes together, we have multiple lines of evidence. It all tells a very compelling story, and it tells us that there was a hot density a long time ago. But then the question, of course is where does that come from? And so let's explore some of those theories. One of the most popular ones is the theory of inflation. Can you give us a quick primer on what inflation is and why it's a compelling explanation for how we got that hot density?

00:22:29
Speaker 4: Right, So before we say what inflation is, it's probably worth saying why people take it so seriously? What the problems that are supposed to solve, And then maybe we can some one will explain what it is. So the problems that inflation are supposed to solve, I, well, there are kind of a few. One is called the monopole problem. So this is that if you think of a magnet, it has a north and a south pole. If you cut it in two, you don't get one with a north pole, one with a south pole. You just get another two magnets with the northern sealf pole. However, theorists in the nineteen seventies thought that in the whole early conditions of the universe you would get something called monopoles, which would literally have only one pole, and these monopoles would be very, very abundant and so should dominate the universe. But yet we don't see any There are no monopoles ever observed, and also they probably be too heavy to allow the universe to spand anyway, So this was called the monopole problem. And actually this was a motivation to come up with inflation.

00:23:30
Speaker 1: That's super fascinating and not something I think is widely enough appreciated. Inflation wasn't conceived of to explain the origins of the universe, but to solve a little technical problem. The theory at the time predicted that when the universe was hot and dense and cooling as it cooled, it shouldn't just make protons and electrons, but it should also make magnetic monopoles, weird particles that have just a magnetic north or south, the way electrons just have a negative and protons just have a positive. The use does this when it has a phase transition as it cools, like when water goes from steam to liquid, it forms droplets. Here, the phase transition was predicted to make all these magnetic monopoles, and they should stick around. They don't go anywhere, So if they were supposed to have been made in the early universe, they should still be here for us to see. But of course we don't see them anywhere we've looked. We can't find any. So that's a problem, and you need to find some way to tweak the theory so it doesn't predict a bunch of monopoles that we know don't exist in the universe. I love how thinking about weird magnets helps us, in the end accidentally understand the origins of the universe. All right, so then tell us what happens next.

00:24:40
Speaker 4: So what happened was to scientists Henry Tyane, and we're trying to solve this monopole problem. And I imagine there could be like phase transitions in the early universe.

00:24:51
Speaker 3: Phase resistion might be familiar with.

00:24:52
Speaker 4: If you have I'm setting out my going and right now is very hot, So if I had some ice similar drink, it's going to melt, It's going to go from solid to liquid. That's a phase transition. So these are the sorts of phag transitions we're familiar with. However, there can be phase transitions that are a bit weird that we're not so familiar with. One of them is called supercooling. So supercooling is if you put some water very very pure. You can do this experiment at home. Who would think you could do an experiment about the very early universe and the origin of the Big Bang in you your breezer at home. So what you have to do is get some very very pure water, keep it nice and still, and put it in the freezer. Leave it in there, and after a while you take it out. It should have frozen. But actually, if it's pure enough and you are careful enough, it will not freeze. It's what's in it. What's called a super cooled state, it will stay liquid. Then if you shake it, it will very quickly freeze. So this sort of delayed transition is called supercooling.

00:25:51
Speaker 2: Why.

00:25:52
Speaker 4: It's just a way that the molecules can be in a certain state that sort of resists being frozen.

00:25:57
Speaker 1: Chemistry. Yeah, chemistry, this chemistry, but it's also super interesting. I know that's unusual for me to say, but this one I love. For freezing. The start, the liquid needs a tiny seed of a solid like an ice crystal or a dust spec or a surface bump to organize molecules into the solid lattice. If no seed is present and the liquid is very pure and undisturbed, the molecules stay disordered, even though they are cold enough to be a solid. It's like a crowd waiting for someone to start clapping. Nobody does anything until the first person starts to clap. And this chemistry trivia is actually important because it helps us understand the universe cooling. It lets the universe cool without making those monopoles. The universe passes that point, but it stays super cooled and doesn't make monopoles the way super cooled liquid doesn't make ice crystals.

00:26:51
Speaker 4: Basically, they had time, because imagine that the universe would be in a super cooled state, and they calculate this would actually stop monopole production.

00:27:00
Speaker 3: We've solved the monopare problem.

00:27:02
Speaker 4: And then they said, well, let's just see if this would affect the expansion rate of the early universe. And what they found was that it would expand exponentially if it's in this super called state. It would expand an incredibly rapid rate. The source of race we're talking about is doubling in size every ten to the minus thirty seven seconds, and so that in words, that is and I think ten trillionth of a trillionth of a trillionth of a second. So that is a very very fast rate of expansion.

00:27:34
Speaker 1: So they did this trick to avoid predicting monopoles would be made, and then discovered that it also predicted that the universe expanded super fast.

00:27:43
Speaker 4: That's awesome, But then what happened was Gooth had heard a lecture about another problem in cosmology called the flatness problem. And the flatness problem basically is that the universe can have a sort of different geometries at large scale. It could be curved, positively curved like a bore or negatively like a saddle, or it could be flat like a piece of paper, and observation showed it was reasonably close to flat in the nineteen seventies. But what had been pointed out by another scientist called Pop Dickie was that those flat solution is unstable. So if it doesn't start out really, really flat, it will be driven away from flatness.

00:28:18
Speaker 1: So if the universe has positive curvature, that means it has enough mass density to curve in on itself and gravity winds and it curves more and eventually collapses and becomes super duper dense. Even if it's very slightly curved in the early universe, it will get more curved and then more curved, and it's a runaway effect. And the same thing happens in the opposite direction. If it has negative curvature, it will get slightly more negatively curved, and then more and then more. It's a runaway effect towards negative curvature. But the universe as we see it today is still very close to flat, if not perfectly flat. That's weird because in order to be flat now, the universe had to have been borne exactly on the knife's edge of flat any tiny deviation from flatness would after almost fourteen billion years, make the universe either collapse or explode. That's the flatness problem.

00:29:07
Speaker 4: So it's very surprising then how the universe could be so flat because it's this unstable solution. When Gooth thought about inflation, he realized that if you're undergo inflation, then actually it'll be driven towards flatness, and the rays you can think about this is it. Imagine have you ever seen like a circle show where people walk on like pilates balls and you're saying that, m okay, quite difficult. I imagine you need some skills for that.

00:29:35
Speaker 3: I imagine. So imagine you were standing aren't that ball? Or you tried and he fell off.

00:29:41
Speaker 4: Let's say, but then you expanded the ball rapidly to the size of the Earth. Be pretty easy to stand on that ball. You wouldn't even notice any curvature.

00:29:49
Speaker 1: I rarely fall off the Earth, that's true.

00:29:50
Speaker 3: Yeah, yeah, I don't fall off the Earth often either.

00:29:53
Speaker 4: So then anything will appear will automatically appear flat if it goes undergoes this exponential experience so real, it would fix this flatness problem. So now it solves two problems at once, and then it turns out it solves many other problems it once.

00:30:06
Speaker 3: I won't go all of them, but I'll do one more.

00:30:08
Speaker 1: Wit. Hold on, I understood how the rapid expansion solves the flatness problem, Like it seems curved, you expand it, all of a sudden, it doesn't seem very curved. How does it solve the monopole problem? Exactly? Like why because you're not putting the universe in a freezer. It's not. No ten year old is doing the experiment on us.

00:30:26
Speaker 4: The monopoles would be thrown out of the horizon because there would only be like a handful of monopoles in any patch. So as the universe expands, they'd be thrown out of the horizon and you wouldn't see any.

00:30:38
Speaker 2: I like that as a solution. Anything that you don't like gets exploded out of the picture.

00:30:43
Speaker 3: Throw it out. Yeah, yeah, exactly.

00:30:46
Speaker 1: I think that's a fascinating sort of bit of science where you're like, let me calculate what I think should have happened in the early universe, and then I'm going to compare that to what I'm seeing, Like, yeah, I don't see a lot of monopoles, so Obviously something must be wrong. I'm missing a piece. What can I add to the universe is like now it's more likely to describe the universe I see today. I think that's super fascinating. Let's digest that and take a break, and when we come back, Phil is going to tell us more about how inflation solved these problems and whether or not we should believe in it. Okay, we're back and we're talking to Phil about the state of the early universe. What we know about the universe before it was hot and dense, and we've been talking about inflation, this theory that the universe expanded super rapidly in a very short amount of time. So tell us, Phil, inflation sounds a lot like the expansion that we have now we talk about dark energy in the universe is accelerating in its expansion. Is inflation a different mechanism for expansion? Is it the same thing, just at a different moment in the universe? How do those two things connect?

00:32:05
Speaker 4: They are similar because the universe is currently accelerating in its expansion. So you could say the dark energy we see today is a very dilute form of inflation, and so that does raise the question are they really the same physics? But I think that's still to be determined, Like there's a lot unknown about inflation, if it even happened. So whether inflation and the dark energy are are the different manifestations the same phenomenon, or whether they're different things that just result in something broadly similar.

00:32:39
Speaker 3: I think it's still an open question.

00:32:41
Speaker 4: But one thing I do want to just highlight is that one of the things that inflation solves, and there are a number of problems, but there's one in particular that I think is the most relevant, and that is how you get galaxy structures. Because if you had a very uniform start at the universe, you've got to ask the question, how on Earth to all these networks of clusters of galaxies form what laid the seeds? And in inflation, the idea is there will be quantum fluctuations and these would be stretched by inflation to create the seeds of galaxies. So amazingly, these enormous, enormous structures, you know, hundreds of thousands of light years across for a particular galaxy, and for clusters of galaxies even millions of light years across.

00:33:26
Speaker 3: You know, these are succeeded.

00:33:28
Speaker 4: By tiny subatomic quantum fluctuations that were stretched by inflation. And I think, out of all the problems that inflation is supposed to address, I think that's the one that's the most impressive, and where cosmologists so really thinking, this is the leading candidate to explain the properties of our universe.

00:33:46
Speaker 2: So I think I get bogged down by this idea that the Big Bang started at a tiny point. And so what I'm imagining is we're looking at before the tiny point where it's inflating, and there's areas where you're going to get galaxies eventually, but then it all kind of such down to a little point and then blows back out again. But that is not the right way to be thinking about it, right because I don't see how stuff that happened before the Big Bang is going to result in galaxies after the Big Bang?

00:34:12
Speaker 3: What am I missing?

00:34:13
Speaker 4: I mean, there are ideas that the universe could undergo cycles like that, I could expand and contract and expand and contract, But that's independent of inflation. So that depends on I guess, on the properties of the dark energy we see today, if it's a cosmological constant, I if the energy of empty space is sort of a constant I should say better, what's driving the accelerated expansion is that empty space has a constant energy, then the universe will not recollapse. However, there have been observations recently suggesting that the dark energy is not a cosmological constant, that it could be some dynamical field that changes over time, and in that case it could recollapse. So that I hopefully address as this issue of you know, getting sack back down. But to answer your question, how could it be that this tiny fluctuating sea of space could have anything to do with what we see today where you need basically density fluctuations. So if you imagine the universe being perfectly smooth, then you wouldn't get the universe we see today because some regions are more dense than others. So those regions that have galaxies in them are more dense, and there are voids in space that are under dense. Also, when you look at the cosmic microwave background, this oldest light that we can see is very very very very uniform, like there's almost no differences across the sky until you get to a point of about one part in one hundred thousand then you start to see these tiny fluctuations. So that's telling you there were definitely density fluctuations in the early universe.

00:35:46
Speaker 3: So they have to.

00:35:47
Speaker 4: Come from somewhere, and inflation is a very nice but not the only possible mechanism to give you those density fluctuations. Now, I said before that the universe could have been infinitely big at the Big Bang. What wouldn't have been infinitely big is our patch of the universe that would have been very very small. So if you're struggling to think, oh, I want to think of this very very small universe, you can think of the observable universe that we see around us. Think of that being very very small. So now in this quantum state, there would be fluctuations. That's just part of quantum theory. And then they get stretched as the universe expands very very rapidly, and they gives you some regions that are more dense than others, and then gravity sculpts them, putting more matter into the denser regions less matter in the under dense regions, and that gives you the seeds for galaxies to form.

00:36:36
Speaker 1: Thanks well, and I think there's a possibility there of getting confused about the two ideas of the Big Bang. Right, what Phils talking about is the hot Big Bang, So not that the universe started from a point, just that the universe started from a hot dense soup. And he's saying that soup is pretty uniform. So how does that soup then grow up to give you galaxies? And the answer is that soup plus quantum fluctuations. We're not zooming all the way back to the singularity with that's singularly aside from.

00:37:01
Speaker 4: That, right, But what's interesting is that during inflation, the universe is as cold as it can possibly be, and it's only when inflation ends that it heats up. So then inflation, we can say is a pre Big Bang model, because a lot of textbooks tell you that inflation happened after the Big Bang. But actually, if we use the definition of the Big Bang that everyone, well not everyone, but most scientists adopt, then we're going to say it's the hot dense state. So inflation happened before that hot dense state. It was cold during inflation, as cold as it can possibly be, and then when inflation ends, it heats up. The universe lots all the energy and the inflating space gets converted into matter and radiation, and then you have our Big Bang. So our Big Bang is like a bubble that appeared from this soup of inflating space.

00:37:50
Speaker 2: Where does the heat come from? How do you go from cold to hot?

00:37:53
Speaker 1: And let me clarify because cold means something very specific here. It doesn't mean low energy like you might imagine. Is Remember that energy can take many forms. It can be in the form of motion or mass or heat, but there can also be potential energy, like when you put a book high on a shelf or a spring that squeezed down, and that doesn't make the universe hot. And sometimes fields like the Higgs field can get stuck in a state with a lot of potential energy, and so that energy can fill the universe and the universe still be cold. Inflation theory says that the inflation field had a lot of this potential energy, which is exactly also what you need to make the universe expand. In general, relativity, matter and energy density make the universe contract. That's gravity, but potential energy like dark energy or the Higgs field or the inflant On field makes the universe expand. So there's a lot of energy in the universe. But it's not heat. It's in the potential energy the inflation field. So the universe is nearly empty, nothing is moving very fast, but it is expanding. Now when inflation ends, that potential energy is converted into normal matter fields and into mass and motion. So then the universe is hot. That's reheating. It went from cold and full of potential energy to hot and full of mass in motion.

00:39:11
Speaker 4: But there's another way you can think of it in that is, when particles pop in and out of existence from the vacuum, they have to go back into existence very very quickly. This is the function of the uncertainty principle. And there's an equation which will tell you how long they can exist for and someone and so forth. Now they pop into resistance in pairs, but in inflation they can't get back together again.

00:39:33
Speaker 3: Because the universe is all ripped them apart.

00:39:35
Speaker 4: So you kind of get you can actually expanding space can create literally create massuin radiation because it rips these virtual particles which would otherwise just appear and disappear out of the vacuum and they form into real particles. And this is similar to what you see in black holes, because in black holes Hawking famously showed that virtual particles can get also get ripped apart. Someone go into the black hole, some would come out and this would emit Radiation's called Hawking radiation. So something similar to that basically, So.

00:40:06
Speaker 1: We have the hot Danse universe a long time ago, which were pretty confident about. There's questions and problems and theoretical issues. People come up with this idea of inflation. The universe before that hot Dnse state expanded super duper rapidly, and that solves a lot of those problems. But doesn't it also just kick the questions down the road. Like you say, if you have this incredible inflation, then it solves those problems. But where does that inflation come from? What causes that?

00:40:33
Speaker 4: So that's an excellent question and the short answer as you don't know. I mean, we're not sure that inflation happened. I mean, some people are pretty confident it happened. And we did a survey physicist about that and we didn't get more than fifty percent saying inflation happened, but they I had to be fair. It was a black hole conference that we were at. If we did a cosmology conference, I guess it would be high. But I think I can't remember exactly what we've got about forty percent, but it wasn't most popular candidate.

00:40:58
Speaker 3: It didn't get past fifty.

00:41:01
Speaker 4: So now, one idea is that inflation is eternal into the past and the future. So let me try and unpack what that means. So you might have heard of the idea of a multiverse. Surely anyone that's witches on the TV these days, we must see some sci fi with a multiverse, whether it's Doctor Strange or the Man in the High Castle or whatever. You know, it's very pervasive now in popular culture, but its origins in the cosmological setting is with inflationary cosmology. So remember I said, you can think of our universe as like this bubble that appears out of inflating sea. But the idea is that it wouldn't produce one bubble, it would produce an infinite number of bubbles. And the way this works is pretty straightforward. Just think of the inflating space as something that decays, like a radioactive particle decays, So it decays with a half life. So when it decays, that's that.

00:41:59
Speaker 3: Moment what we talked about.

00:42:00
Speaker 4: You know where all the energy gets converted into Madgine radiation. Boom, a big bang, the universe is born. But what has happened to the bit that hasn't decayed yet? Well, it's undergoing exponential expansion. So as long as that exponential expansion is faster than the exponential deka, then the amount of inflating space can never go down.

00:42:20
Speaker 3: It can only go up.

00:42:21
Speaker 4: So the analogy I always give is my mum makes this fantastic chocolate cake is really yummy, and I go around there for Sunday afternoon tea. She prints the chocolate cake down on the table and we eat half the cake. Then at some point we go back for seconds, and now the cake is gone. You can't have your cake and eat it right. However, what would happen if when we went back for that second piece the cake had expanded exponentially? Now we could have more cake. We have the same volume of cake that we had in the first bite, you know, but the cakes expanded. Then we go back for thirds. Well, the same thing's happened. The carown faster than we could have eaten it, in which case you rut, the cake can never ever go down, so you get infinite number of pieces of cake and you can have your cake and eat it.

00:43:09
Speaker 1: I think you also get infinite fill in that scenario, don't you.

00:43:12
Speaker 4: Yeah, yeah, you have to have a very big belly.

00:43:16
Speaker 1: All right. So I'm imagining space filled with this inflationary material we don't understand or don't know it's original, and random little bits of it are decaying into our normal universe kind of stuff. So we get these bubbles of normal universes that appear to train in places, but they're separated by this rapidly inflating space.

00:43:35
Speaker 4: Yes, that's exactly right, yep, yep. So then you get this multiverse and it's eternal. Now, when I say it's eternal, there's a bit of a subtlety there. It's eternal into the future, but is it eternal into the past? Now there is a theorem called the Bordais Gooth and Velenkad theorem of that is, it can only be eternal into the future, but it is not eternal into the past. Still have to ask what came before inflation or what might cause inflation. However, there have been people that have disputed this theorem and said, no, it could be eternal into the past. So if you want to know what came before inflation. In that scenario, it's just more inflation, and it just goes back forever and ever for eternity. However, if we take the body Goose for Lincoln theorem, then we have to ask, okay, what came before inflation? And there are a number of suggestions. And I should also add that there are people that don't like inflation. So one idea as well, inflation didn't happen at all, and there's some other thing that happened, but let's stick with the inflation. Since you asked, so, I would say there's a few ideas out there. The first one that probably came on the scene was nineteen eighty two, and this was proposed by Alex for Lincoln. It's called the tunneling from Nothing idea. So the idea is that what if we treat space quantum mechanically. So normally, when they do this cosmological modeling, you're using Einstein's theory of gravity, so you're assuming space time is classical. But if you'd seen in space time as quantum mechanical as the linking did, then the suggestion is that space itself could fluctuate into existence from a state where there was no space, so that's really hard to get your head round that. This is why the tunneling from nothing idea. The idea is what controls the expansion of the universe, so it's partly it's radius. So if you have a small universe, so we'd actually just clapse back in on itself. So how would you go from a small universe to a universe big enough to inflate? At least and classically it seems to be impossible. However, quantum mechanically it can what's called tunnel. It can tunnel from one set to another, so the small one could tunnel to the larger one. And then what he asked was, well, what's the smallest size that the universe could tunnel from? And he concluded it was zero. So that's pretty hard to get your head round. So that's the tunneling idea. It tunneling from nothing idea. Then you have to sink with a hard tour hawking no boundary state.

00:46:05
Speaker 1: Hold on, let's dig into the linking idea a little bit more. Yeah, because this is still pre inflation. This is not an alternative inflation. This thing where does the inflation areyas come from? And you're saying that we're adding quantum mechanics to it, so it's not full quantum gravity. It's certain sort of like semi classical.

00:46:20
Speaker 4: No, no, this is yeah, we'd call it some classic. It's not full quantum gravity. And we should come to full quantum gravity and in a moment.

00:46:27
Speaker 1: And so this is speculating that the inflationary conditions before the hot Big Bang fluctuated into existence. From and I'm just going to kick the can down the road further here from and you said something that has no space time. So it's a universe but without space time, or it's nothing meaning not a universe.

00:46:45
Speaker 3: What's the difference?

00:46:46
Speaker 1: Yeah, well that was gonna be my next question. That was a trap you avoided it.

00:46:52
Speaker 4: There's no space there at all. I mean, there's nothing so physical there. That's at least one conception of it.

00:46:58
Speaker 1: But how can you have the laws of the universe and follow those laws if there is no universe? So there is a universe but without space time, Well.

00:47:06
Speaker 3: That depends on your view of laws. What are laws of physics?

00:47:11
Speaker 4: Now some people would say they're just descriptions, so they can't cause the universe. Another view might view, we don't need a cause because there's no time and time you need time for causality.

00:47:22
Speaker 3: If there's no time, don't worry about what the cause was. You're asking the question what caused?

00:47:27
Speaker 1: But if there's no time, it is.

00:47:29
Speaker 4: A question that is asked in a condition. The condition is time exists. But if the time doesn't exist, then don't worry about causality.

00:47:38
Speaker 1: Yeah, but if you have that, but how can you speculate about a transition and evolution of the universe from this to that? If there's no time in the universe? Right, how does that? How do I wrap my mind around this?

00:47:49
Speaker 4: Well, it is hard to head round. You know, we're answering in analogies. But you know, I saw a mathematical description. So the question is, you know, you compute what would happen if the universe of zero size? And then what is there a tunneling aptitude for one? Yeah, people have challenged this. I mean, I don't want to claim it's always worked out and everyone understands it and it's all agreedable. This is one of like twenty something ideas that we talk about in the book, and this is a problem with it. So I don't you know, some people have said, well, you're always tunneling from one quantum state to another, so maybe we shouldn't consider this this model. However, one idea that Valncan suggests is that actually laws are more than just descriptions that they actually are. They exist in a sort of platonic sense in a sense. And this has some advantages to it because you know, you can ask why is it that objects obey the laws of physics? You know, if you have an electron, let's say it gets excited, it shoots out a photon. How does it know to shoot out a photon? You know, it's all that information written inside the electronic That seems hard to believe. So the idea is that laws are actually sort of photonic objects in some sos it's pretty weird.

00:49:03
Speaker 1: And when you say platonic objects, for those of us who are not reading philosophy papers all the time, you mean that they exist outside of our ability to observe them.

00:49:11
Speaker 4: Yeah, I think that's a fair fair description. Yeah, they don't sit on something else. You know, if you think of a table, all right, it's made of particles, and you might even say they're made out of quantum fields or you know, so the laws themselves have their own sort of physical existence.

00:49:29
Speaker 3: Well, I don't know.

00:49:30
Speaker 4: Physical isn't the right word independent existence, So that may be required by a the Lincoln's model, And if you don't like that idea, then maybe you don't want to entertain the Lincoln's model. But I do think that it's uncertain what the status of laws is. So those that say that cannot be right because laws are just descriptions. I challenged that and say, are you sure that they're just descriptions? You know, how do you solve problem of induction? You know, we ask how do we know that there are these regularities? Why do why don't we think the sun might stop rising tomorrow? You know, if you just think their descriptions, it's hard to solve this problem of induction. But one possible solution is, no, the laws are more than that. They're sort of ironclad objects that the other particles have to sort of obey, And then you can see why induction makes sense. So there's a lot of debate in the philosophical literature about the nature of laws, and I think as long as that is not settled, and I think it's right to say it isn't settled, then you can entertain this view, but you know, let's be honest, it's pretty wacky and out there. So people have come up with other views and maybe, you know, we can get onto those if you want.

00:50:43
Speaker 1: I do want to get onto those, and I'm meant to ask you where the philosophical implications later, But I can't not follow up on that fascinating conversation, you know, because it makes me wonder about the future of this line of inquiry, Like if we discover the heart big being, and then we find proof of inflation, then we find proof of what became before that? Is this an infinite line of study where we're always going to be asking what came before that? Or do you think there's some point at which we find something where like, ah, this caused itself, or this is obviously something which is fundamental. And isn't that just some philosophical sleight of hand where we're like, this requires a cause and now we found something which we define to not require a cause and therefore we don't have to ask questions anymore. I mean, why isn't this going to be an infinite line of inquiry or do you think it will be what.

00:51:27
Speaker 3: It could be?

00:51:28
Speaker 4: But there are sort of ideas out there that might stop that infinite regress, shall we say, I mean the Lincoln thinks that he stops the infinite regress because you know, we've got this solution, if you like. The universe doesn't need a cause because it's quantum mechanical and causes our emergent properties. They exist for the microscopic world, but not for the quantum world. So we don't need to ask what the cause for the universe is. It's a meaningless question maybe, or maybe not meaningless, but it's just you know, you don't need to demand. You can't demand there must be a cause when you get into the quantum realm. So that's one approach. Another approaches that maybe the universalist existed eternally into the past, so you can always just go back one step further and further and there's no end. It just goes on forever into the past. That's certainly an idea.

00:52:15
Speaker 1: Infinite grants for cosmology.

00:52:17
Speaker 4: Yeah maybe, yeah, so it may just go on forever. But that's certainly a logical possibility, and it's certainly something that I think many physicists entertain. Certainly that's one of the battles. Actually the Battle of the Big Bang is was there a beginning even if it wasn't the Big Bang, maybe there was some other beginning, and some physicists think there was, and some physicis think there wasn't. Another idea is that there could be what's called a closed time like curve. So if you've seen the movie Groundhog Day, think of that. So you wake up, you know, and you're just experiencing yesterday all over again. So now there are solutions in ice Sei's Theoria relativity, which describes you know, gravity as a curvature of space time, and there are solutions to say it gets so curve, it curves back on itself and it forms a loop. So there are a couple of models that sort of take advantage of this and say the universe would cause itself because it curves back in a loop. And so there is no origin point of a circle. Even though the circle is not infinitely long, it doesn't have a starting point, so you can't ask what happened at the start.

00:53:31
Speaker 3: There is no start. It just goes around in a loop.

00:53:34
Speaker 4: So in the book Battle the Big Bang, we talk about two different models that exploit this. So there are some of the different ideas. One, maybe there was a beginning and you describe the universe maybe without the language of causality. Or maybe there was no beginning and it's infinitely far into the past. Or maybe there's a sort of loop in time, so there's no starting point, even though maybe it's not sort of eternal in some sense.

00:54:01
Speaker 1: All right, so let's take another break, and when we come back, we're explore some of these other ideas, some proposing the universe has no first moment. Okay, we're talking to Phil about the origins of the universe, what we know, what we might know, and what we can only speculate while smoking banana peels. You referenced earlier, this fascinating idea, the no boundary proposal, this alternative to like a singularity or a first moment. Tell us about this idea. How does it come about and how does it help us avoid a first moment in time?

00:54:51
Speaker 4: Well, it's not clear whether it does avoid the first If I was going to suggest models that avoid the first moment and so on, I might probably go to a another model, like a bouncing model, which was or you know, some other maybe some cyclic models. Now I think I'm more unambiguously don't have a beginning that the no boundary proposal.

00:55:11
Speaker 3: I work with.

00:55:13
Speaker 4: Stephen Hawking and Jim Hartl and Thomas hog to make a film to explain it to the public, and I did get the impression they didn't quite agree on how to interpret it. I think, I think Corking sawy it is like something closer to what for Lincoln proposed, like a universe for nothing. Whereas Jim hart Or I asked him he could just realize the idea of the universe fy nothing, and he's like, what do you mean by nothing? Quantum mechanics is always something.

00:55:41
Speaker 1: I know that hacking special sauce is getting gravity and quantum mechanics to play together, even without a full theory of quantum gravity. He did for black holes, and that's how we came up with his prediction for Hockeing radiation.

00:55:52
Speaker 4: So he started to think, well, what if we do the same for the Big Bang? Because the singularity theorem that he approved with Penrose was just classical. It didn't include quantum mechanics. So he said, loris, let's try and include quantum mechanics. And when it did that, he found that it's cut a long story short, the description that we have with space having three dimensions and time having one dimension changes such that the time dimension turns into a space dimension.

00:56:22
Speaker 3: So now you have four space dimensions and no time dimension.

00:56:26
Speaker 1: Okay, that's really weird. Let's unpack it. So Hawking is saying that the universe used to have four spatial dimensions and one of them became a time dimension. So we had four spatial dimensions, and then we had three spatial dimensions and one time dimension, which we're familiar with. Now that's really confusing, So let's try to think of an analogy. And hawking student's analogy is like, if you're standing at the north pole. It's not only that there's no more northiness, there's also no sense of east or west. East and west have no meaning of the north pole. But as you move away from the north pole, you don't change the number of dimensions on the surface of a sphere. But now the concept of east and west makes sense. It's a direction that emerges smoothly. As you move away from the north pole. One of the directions on the sphere becomes east west. Like the idea here is that the universe has one end where all the directions are spacelike, and as you move away from that end, one dimension changes to have new time like properties. It's pretty hard to wrap your mind around, honestly.

00:57:29
Speaker 3: What this does.

00:57:30
Speaker 4: It actually smooths out the singularity. You don't get this infinite, dense state. You get a smooth state. And think of it unlike the point of a cone. Think of it more like a shuttlecock, like a smooth surface. Now this is bound it. It's just not infinite, but it doesn't have a starting. There's no point on the surface. This is many better than any other point. So you could say it doesn't have a beginning, but you might say, what, it's not infinite to the past either, And then there's some more complications to it, which we go into in the book. But bottom line is whether it has a beginning or not is sort of slightly ambiguous, and it depends on how you interpret the model. But there are other models that I think are They have less ambiguity and clearly don't have a beginning in them. So the most obvious one, I would say, is one with where you replace the singularity with a bounce. So here the idea is that there's so remember we said with the singularity, the universe goes to infinite density. Now, the idea in some theories of quantum gravity, the suggestions being that you will actually get a maximum density. So think of a sponge. You pour water on a sponge and it absorbs the water, but there comes a point where it won't absorb the water anymore. It will switch its properties from being water absorbent to water repellent. So space might be like that, and in certain theories of quantum gravity that's what happen. Spaces are like a limit. So when you try and squeeze more and more energy into space, it gets to a point where it gets fill up, and then if you try and squeeze more in, it will bounce back out.

00:59:10
Speaker 3: It becomes repellent.

00:59:11
Speaker 4: So gravity switches from becoming attractive force to a repulsive force, and that triggers a bounce. So then if we could go back in time, what we would see is, well, if we started today fourteen billionaires, it is a big bang. Roughly you go back in time is getting denser and dancer and dancer and denser. When we get to its maximum density, it just bounces back out again. So you'd get another expanding universe, or if you wanted to think of it as going from the past today, it would be a contracting universe and then an expanding universe, and that core attracting universe could have been contracting for infinitely long ago. You might just have it mirror like. It doesn't have to be cyclic, or though it could be. It could just be one contracting universe mirrored by it one expanding it.

00:59:56
Speaker 3: Something like that, like an hour class.

00:59:58
Speaker 4: So that's that's the idea of a. And I think that is less ambiguous, that it does not have a beginning in this model.

01:00:06
Speaker 3: So that's one idea.

01:00:07
Speaker 4: And of course there are cyclic universes as well, and oh, guys, of other ideas we explaw.

01:00:13
Speaker 2: So this idea that at some point gravity stops pulling things in and starts to repel energy, do we have evidence for that in other domains or like, where did this idea come from?

01:00:24
Speaker 3: Arguably we do.

01:00:25
Speaker 4: The fact that the universe is accelerating in its expansion is an effect of the vacuum having a pressure.

01:00:33
Speaker 3: But think of it this way.

01:00:34
Speaker 4: I mean, this is different to what we're talking about in the quantum gravity case. But I just want to ask your questions, is there some evidence of a repulsive gravity. So in Newton's theory, the only thing that contributes to gravity is mass, so you can't have a negative mass as far as I know, so you can't have negative gravity. Therefore gravity is always attractive. However, in Einstein's theory there's also a pressure term, so it's not just mass, and the pressure can be negative. It's like a suction, and in fact, vacuum has negative pressure, so it is a repulsive gravity force. And the fact that we see the universe accelerating an expansion is direct emperical evidence of repulsive gravity.

01:01:15
Speaker 3: So that is clear that there is repulsive gravity.

01:01:18
Speaker 4: Now that's not necessarily what's going on here at the big bounce, because there you're assuming it's coming from quantum gravity. So there you have to use a different type of geometry. So the geometry that's used in relativity is classical geometry, whereas in these quantum gravity theories it's a quantum geometry, so it's sort of fluctuating. And the idea is that then because it's discrete, it can't have infinite compression. It will bounce back at some point and that and then the gravity switch assigned and you get repulsive gravity. So that's the theoretical trapulation from the quantum gravity theory. So in particular, Luke quantum gravity is that it's a candidate for for quantum gravity. They've done a lot of work on this bouncing cosmology, but there have been ideas even in string theory that you might get a bounce in cosmology from this fact that if you discretize space then it will have a maximum limit of what you can put into it, and then if you try to go beyond it, it pushes back.

01:02:17
Speaker 1: I like these bouncing ideas because they sort of avoid the question of our first moment. They're like, lit's just bounce forever, and that's cool. But I wonder if there's any evidence for that, Like, you know, if the universe compresses to a singularity or a near singularity, some quantum version of it, does it do so in so intensely that all evidence of the previous you know, pre bounce universe is lost? Or can we find some hints, some clues in the universe today that showed that it had to have a bounce.

01:02:46
Speaker 3: Yes, So this is an active research program.

01:02:50
Speaker 4: So, as I said, I think it's in the lib quantum gravity case where there's the most consensus that you get a bounce. As I said, they have been ideas in string theory for a bounce as well, and even in some other proposals of quantum gravity. But in the loop case, loop quantum gravity, which is one of the candidates for a theory to mix ie thereogenerativity and quantum mechanics, as we've got this quantum theory gravity. Because we should point out to the listener if they're not familiar, these two theories are the cornerstones and modern physics. But yet they contradict each other. So we know we need a deeper theory. So we know or we need to modify one of the existing theories, or maybe both of them, but we know we need new physics in some way. So one of the contenders this is theory called loop quantum gravity. And what they've got this program called loop quantum cosmology, where what they want to do is calculate what we should see in the cosmic microwave background if there was a bounce, and they claim that they've already done this and they it matches their observations. Now obviously not everyone agrees, so this is correct so they have to make certain assumptions, and then the question is are they valid assumptions? And then you know, in particular, I'll give you one I think is important or especially important, and I should say, and that is you have to assume whether there was inflation or not. And as we said, not everyone agrees there was inflation, so they have to put in inflation. And then there's different models of inflation. So what they do is they try and say, well, which one is favored by the data.

01:04:21
Speaker 3: Put that in.

01:04:21
Speaker 4: Then we calculate corrections to what we see in the couse of micro background, and then they claim that there's actually much is better than the standard model, but the differences are not big enough to be decisive. And of course, as I said, you could challenge the assumptions. So it's still an ongoing program and people are still working on the theory to see if they can make the predictions more robust. So maybe it doesn't depend on some of these assumptions. The other thing you could try and do is look at black holes and how black holes might behave because there also we think we need quantum gravity, so people have made models of black holes using quantum gravity. And then maybe you could look for signatures of what they might do. Maybe there are signs of quantum gravity in black holes. So those are the two areas that I think are the most exciting about looking for signs of quantum gravity. And if we can find those, then we might know what the right theory of quantum gravity is. And then we might know whether this bounce took place or if it didn't.

01:05:28
Speaker 2: That would be exciting.

01:05:29
Speaker 1: Yes, oh yeah, Well, now we've been looking into the past. Tell us about the future of looking into the past. What do you think we're going to learn in the next ten years or fifty years that could help us understand what came before the hot Big Bang.

01:05:40
Speaker 4: Well, there's a number of avenues we can explore. So one is to look more at this cosmic microwave background. So this is the oldest light that we can see. You can't see beyond that with light. It's admitted three hundred and eighty thousand years after the Big Bang. And the reason that you can't see anything earlier is that if you went earlier, as I said, the university more like the sun, and if you look at the Sun, you can't look into the interior because it's opaque.

01:06:05
Speaker 3: So same with the universe.

01:06:07
Speaker 4: Before the emission of a cosmic microwave background, it was opaque, so you cannot look earlier. But there are these patterns of hot and cold spots, and they give you information that you could try and guess as to what came before. And some theories make different predictions for those patterns of hot and cold spots. Also, there's patterns of polarization, so this is a way that light sort of twirls around. That's a new frontier, So looking for polarization of the cosmic microwave background, and some different models make different predictions for what we might see there.

01:06:39
Speaker 3: Another avenue you.

01:06:40
Speaker 4: Could look at is the distribution of galaxies because that was set in the very earliest moments of the Big Bang, So different models might make different predictions for the distribution of galaxies. But I think the most exciting idea for how we can probe much earlier into the into the Big Bang is something called primordial gravitational waves. So this you might people might have heard in twenty let me think was it twenty sixteen Ligo, which was these giant lasers that they have in Louisiana and Washington State, and they detected sort of a ripple in the fabric of space and time. They're called gravitational waves, and they came not from the Big Bank. They came from colliding black holes. But they send out this ripple in the fabric of space and time, and we can detect them amazingly because they're very, very feeble. When they get to the Earth, they change the distance of like these lasers that they basically have. These lasers, they're aout four kilometers apart and they bounce between mirrors and if there are no gravitational waves, they should stay in phase with each other. When a gravitational wave passes through, they go out phase, and so you can detect their existence. And they have two of them, one in Louisiana and one in Washington State. Initially, I mean they've been more built since then. There's one now in Italy and India and Japanese that they're building one or I don't know what the status of that one is. But so so the idea is have a station stations of them around the world, so these ripples can go through that plasma that you can't see through. So even though you can't see light from the Big Bang, you could in some sense see these gravitational ways.

01:08:25
Speaker 3: You don't see.

01:08:25
Speaker 4: Them, maybe you hear them, you know, they're kind of sound ways actually in the early universe. That were they cause dounways in the early universe.

01:08:32
Speaker 3: They should say.

01:08:33
Speaker 4: So, these ripples in the fabric of space are potentially detectable. Either they might change a polarization of the light of the cosmic microwave background, or we could detect them directly via building something like these giant laser observatories. But the ones that we have today are optimized to see them from black hole collisions. They can't even see them from super massive black holes, which are the really big black holes that.

01:08:58
Speaker 3: Sit in the center of galaxies.

01:09:00
Speaker 4: However, we are building a space based observatory called LISA, the Laser Interformance of Space Antenna, and that will be able to detect super massive black hole mergers. So maybe it could see signatures of quantum gravity in these extreme conditions, So that might help us understand the Big Bang. But beyond LISA, I mean lisa's not going to get launched on the twenty thirties. But then beyond that there are projects, one called the Psycho, another one called Big Bang Observer. These are kind of pie in the sky ideas, right, so we don't think that they're going to be built anytime soon. But often what you do is just dream big, because just say, what would you do if you had like endless money and resources, and the idea is just get something on the table and then you know, maybe decades into the future, maybe people find cheaper ways to do it, and then you can actually launch these things. So the Big Bang Observer, for example, is like Lisa, it's a similar idea, but it has twelve spacecraft. Lisa has three that is happening as being built as we speak. But Big Bang Obsurvey is much much more ambitious. But that could potentially see these ripples from the Big Bang. And now why that is that important because some models predict that there should be these ripples and some don't. So a lot of cyclic models say there are no ripples. And some of these cyclic models are alternatives to the inflationary screen, which says.

01:10:25
Speaker 3: There are these ripples.

01:10:27
Speaker 4: Not only that, but they actually have different properties, so they have different strengths at different wavelengths, and so we call that the spectrum of the gravitational waves. So by probing the gravitational wave universe. And this is an area of astronomy that is brand new. I mean, we only detected the first gravitational wave in twenty fifteen. It's like Galileo first looking at in this telescope in sixteen ten. Think of what a small telescope we had and how radically it changed our view of the universe. Because before Galileo looked through our telescope, most people will sure that we lived in a geocentric universe with everything going around the Earth. And I think mccalilet took through that telescope and he saw that there were moons going around Jupiter. It was clear as clear as day that not everything goes around the Earth. And then he saw the phases of Venus, which was a direct prediction of the helocentric model or particular properties of the phases of Venus. So it was incredibly revolutionary. And now we're at the birth of this new revolution gravitational wave of astronomy. And this could probe which models of the Big Bang or even pre Big Bang universe are right. And that is the incredibly exciting frontier for the universe cosmology in the decades or maybe centrics to come. Depending on whether we decide to find this area of science.

01:11:49
Speaker 1: That's right, it'll finally tell us whether we're in the Marvel Multiverse or the DC Multiverse or another version. Right, yes, exactly right, all right, well, thank you Phil very much for coming on the podcast and talk to us about all these crazy ideas. It's amazing to me we can know anything about the universe so long ago, and we can even have debates about various ideas. I really enjoyed the book. The book is called Battle for the Big Bang, The New Tales of our Cosmic Origins.

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