And so to lecture on cosmology's mad idea, inflation, and the creation of a universe. Please welcome Professor Chris Linton.

Thank you. Thank you. Thank you for coming out. Thank you for joining us. Happy Equinox for those who celebrate. We need to mum up these astronomical seasons. I'm not sure about this one, if I'm honest. I'm an observer. I've told you from the stage several times that I'm an astronomer because I grew up looking at the stars. And the science that excites me is that which is testable by observing our universe. And in many ways, the remarkable thing about my subject is that we've got quite so far as we have by looking up at the universe in a myriad of different ways, as many ways as we can possibly think of. We've managed to determine how stars are born and live and die. We've established that there are planets scattered throughout the cosmos, we're making progress on looking for life. And in the grand questions of cosmology, we have a coherent story, though we don't understand all the details, that takes us back, as we'll hear in a bit, to the first few minutes of the universe's existence. But there are scientists out there, many of them my colleagues and friends, who aren't encumbered by the need to observe things. They're people who like chalk a lot mostly and spend their time around blackboards. And theorists, of course, can follow their scientific curiosity beyond what's observable. And we can ask questions like, what happened before the Big Bang? Or what is the nature of time, or do wormholes exist, and so on. And these are sensible scientific questions, but they're not testable. And so I've considered them for the purposes of a professor of astronomy as outside my remit. And today's lecture lies exactly on the fault line between those two things. It's a theory, it's about a theory that has, I think, been the biggest change in our understanding of the universe in my lifetime. It's gone from something that was way beyond the idea of testable hypothesis to something that's well established, and you can argue, and I might argue by the end, uh, that uh we have tested this fundamental idea. This is the idea that tends to go by the name of inflation. It is a very simple idea, which I shall explain, which is that the universe, which we know has been expanding since that phase that we call the Big Bang 13.8 billion years ago, but at the very beginning it had a period of rapid expansion. That's it. That's the whole theory, more or less. And yet that has profound consequences, and we have reasons for invoking it. Now, as well as this being slightly outside my wheelhouse, it also breaks one of the rules of Gresham Astronomy lectures, which is that as one of my colleagues told me when I first arrived, astronomy is easy to lecture on, because we just put up beautiful pictures. And as long as I sound like I'm saying something, you have a great time. Unfortunately, there's no picture of inflation. So just to tick that off, this is a completely gratuitous picture of Mars sent back by the Curiosity Rover which celebrated its 5,000th Sol, a Martian day, on uh on the surface of the red planet just at the end of August. This is a combination of a picture taken on August the 30th and one on September the 2nd. And it has no bearing whatsoever uh on this evening's lecture. What is more relevant is I think to remind you of the grand picture that we're playing with with cosmology. And really, here we're back on firm observational territory. So when we look up at the night sky, and this is a new picture from the Vera Rubin Observatory, which those of you who saw my last lecture will know all about. Uh, this is a gorgeous picture of the sky. Um, there are a few bright stars here, but most of the things here are galaxies, systems of billions of stars, rather like our own Milky Way. Um, and this is a region of space called the Cosmos field, which was selected so that you're looking straight out into deep space. And you see that the universe is populated with galaxies, but as we look deeper, we see that the most fundamental of observation of all is that the number of galaxies changes. So there's something that's different about the early universe or the distant universe than the nearby one. And as we look even deeper, say using Hubble Space Telescope or the new JWST, we see that those galaxies are different, they're younger, and they're also redder. And that redness is because of the expansion of the universe that's happened in the time that the light has been traveling towards us. It's what we call redshift. So when we do this game of looking back in time, we get a history of the universe unfolding for us that goes back, as I said, to this time 13.8 billion years ago that we call the Big Bang. Now we know that's 13.8 because we've measured the properties of the universe. And we've measured the properties of the universe by looking at how things are swept along in the cosmic expansion. As I've talked about from the stage before, we do this by looking carefully at different distant objects. Uh this technique, for example, was pioneered by Edwin Hubble, astronomy's premier pipe smoker, uh, who used the telescopes that were coming into existence at Mount Wilson and Mount Palomar in California in the start of the 20th century to be able to resolve individual stars in galaxies, in this case in the nearby Andromeda galaxy, and to pick out those stars in those galaxies that are variable, that are changing in brightness over time. In doing that, he found some that had periodic changes of brightness in a particular sawtooth pattern, which were recognized as being the equivalent of Milky Way stars called Cepheids. And Cepheids, and there's a gratuitous picture of one in the top right there, that's uh R. S. puppis taken recently with Hubble, the stars in the middle, and you can see that it's surrounded by clouds of dust and some gas that is being shocked by material released from the central star. But all Cepheids share a property in that they pulse. As they pulse, their brightness changes. So R. S. puppis between brightest and faintest is about five times difference, a factor of five variation in its brightness over the course of just 40 days. And that pulsing, the speed of it, is related for reasons of stellar structure to the brightness of the star, the intrinsic brightness. We'd call it the luminosity of the star. And that means that if I can count the pulses, I know how bright the star really is. If I know how bright the star really is and how bright it is in the sky, then I can look at the difference and that gives us a distance in the same way that if I was holding a 60-watt light bulb, you'd know whether I was a mile away or a meter away. Uh, it's a very simple observation. If you can find enough of these CFITs, Hubble did this. He compared these results with the redshift, that expansion of space, and found what we now call the Hubble-Lemaitre law. Um, each dot here is a galaxy. Um, the further right it is, the further away it is as measured by CFIDs. Um, the higher up it is, the faster it's being receding because of this expansion of the universe. And you can see there's roughly a straight line. This is Hubble's data. Here's the Hubble Space Telescope version going out over a significant chunk. That's uh on these galaxies range up to something like one and a half million uh light years away. That can't be right. Um a half billion, that's better, light years away. Um million here, billion there. Um you can see there's this relation. The further away a galaxy is, the faster it's receding from us. Well, that's the Hubble on the matral law. The further away a galaxy is, the faster it's receding. And just from that idea, you get to this picture that space is expanding and we have a big bang. So I always show this visualization. This is uh uh an Escher uh drawing called cubic space division. But cubic space division sounds like they should be an 80s band, but they're not. Um I think it's 80s. Um but in this simple picture, imagine you that you live on one of the cubes, and in this universe, we have the rods between the cubes expanding. And each rod expands at the same rate. Let's say, does it really matter, 100 miles an hour. If you imagine yourself in that world, first of all, you'll assume that you're static on your cube, everyone else is rushing away from you, just as we see that every other galaxy is rushing away from us, and that's true regardless of which cube you find yourself or imagine yourself standing on. And then you could see that you also have this property that the further away a cube is from you, the faster it will be receding. Because let's say you imagine yourself down here. This cube in the middle will be going in that direction to 100 miles an hour, because there's one rod between us and it, but the one in the top right corner will be expanding at twice that rate. And so we recover this Hubble, Hubble law. The further away a cube is, the faster it's receding from us. Further away it is, the faster it is. Replace the cubes with galaxies, the rods with space itself expanding, and you naturally explain the observations that Hubble made of these galaxies. And having done that, of course, you can run the movie backwards in your head, and you discover that there was a period where all the cubes were very close together. And you could work out when that was just by running this picture backwards. And that's what we do that leads us to this idea that there was a big bang, or rather that the universe was began in this place where everything was squashed close together. We usually say the universe was in a hot, dense state. Whether it actually had a time equal zero, well, we'll we'll come back to that at the end. But basically, just at the beginning, you have a hot, dense universe. And that's a testable prediction. I can observe things that are determined by that hot, dense state. And the most famous of them is the cosmic microwave background, which was observed by the Kobe satellite, by a satellite called WMAP, and more recently by the European Space Agency's Planck satellite, which mapped the cosmic microwave background and provided uh this picture of the whole sky. And what you're seeing here is the blobs are regions of slightly different temperature, which we'll see many times during the course of this evening. So, why because the universe was in a hot dense state, why does that mean we get this sea of radiation coming from all directions, which we call the cosmic microwave background? Well, I like to use this as my example. This is uh an art piece called Blind Light by Anthony Gormley. So we're outside it, and this is this is essentially a box of fog with some lights in it. And if you imagine a light bulb in the middle of this box, and you imagine it emitting a photon, a particle of light, you can uh think of a beam of light if you prefer, but our photon will emerge and it bounces off a water molecule, and then it hits another water molecule, and it will bounce around within the box until it happens to get, by sheer chance and a random walk, to the edge. And then if it scatters in the right direction, it will then go through the glass and shoot out and travel through the rest of the galaxy, in this case, hitting a camera shortly thereafter. So this is rather like the situation in the early universe. We're in a hot, dense state, so hot that particles fly freely. Electrons aren't constrained by atoms. And so any light in this early universe doesn't travel more than a centimeter or so before it hits an electron and it scatters. And they bounce around from electron to electron to electron to electron for the first 400,000 years of the universe's existence. But the universe is expanding, it's cooling, because that's what happens when things expand, as we'll come back to. And so eventually the electrons are all captured, and suddenly, for the first time, there is space in space. And at that point, the light can travel across the whole universe. So that's what we see as the cosmic microwave background. It's like the outside of this box which appears to be glowing. We see this point in time, about 400,000 years after the beginning, where light could first travel across the universe. So the existence of this cosmic microwave background, discovered in the 1960s and discussed in an excellent lecture just a year or two ago, told us that this picture of a hot, dense Big Bang was really the right one. And that's the fundamental observational evidence that got that into my talk and allowed me to talk about that cosmology as something that we've tested. But there are other really remarkably accurate predictions of this model of the universe as a hot, dense state. So for example, as well as light knocking around the early universe, the particles that are there, particularly protons and neutrons, are also colliding with each other. They're moving fast enough. And there's enough of them, remember it's hot, particles move fast, it's dense, there are lots of them in a small space, that they collide. And in the first few minutes of the universe's existence, we essentially have a universe that's acting as a nuclear fusion reactor. So just as the sun produces energy by merging hydrogen into helium in the extreme conditions in its core, so the whole universe was able to support nuclear reactions. And this complicated plot, this is a simul uh a calculation, it's not a compute simulation, this is blackboard equation stuff, basically, that tells you about the production of various different ingredients in the universe. And in particular, you start off with neutrons and protons, so that's the brown and the blue line right at the top. You start off with a universe almost entirely composed of those things. And then they combine, look particularly at the purple line and the lighter purple line, the lilac line, that produce helium. And so you form out of these protons, you form helium. Until such a point that the universe has expanded, it's cooled down, and such reactions are no longer possible. And that happens about here, which is about three minutes after the Big Bang. So we produce helium and a little bit of beryllium and a little bit of lithium in these first three minutes, which were documented in this still amazing book by Stephen Weinberg that first came out in the 1970s, which is describes the extreme physics of this early universe. But he chose the first three minutes because we can now look at the hydrogen and helium that we see around us today and compare it to these predictions and check that the big what we think happened just after the Big Bang really did happen. Just last week there was a particularly sensitive test of this. A bunch of astronomers led by a guy from Minnesota decided to test this prediction. What you have to do is you have to look at various cosmological objects, various objects in the universe, and compare the amount of hydrogen to the amount of helium. And the only reason this is complicated is that stars turn hydrogen into helium. That's what a star does for most of its life. And so you want to try and look at places where there's been relatively little star formation so that you can get as close as you can to the primordial mix of hydrogen and helium. So what you do is you look at really boring galaxies. And these are the really boring galaxies in cresctic, the dots at the middle. These are the closest things we've got in the local universe to um to pristine conditions. So there has been star formation in these galaxies, but not a huge amount. And the team stared at them with a marvelous thing called the large binocular telescope. LBT is, I think it might be my favorite if the other telescopes aren't listening, just for the absurdity of we're gonna have one eight-meter telescope, but what if we put two mirrors on it? Um and it they're on one mount and the whole thing moves around the sky. And what you're supposed to be able to do is combine light from both mirrors into the same instrument. Um, it's proved tricky. Um, getting the alignment correct for lots of measurements is quite hard. Um I also love the fact that when I was there, which is a while ago now, somebody had dropped a spanner during its construction. And inside this giant telescope, which has a frame large enough to hold and move two eight-meter mirrors around, you can hear the spanner rattling as the thing moves around the sky. And they've got one hole at the bottom, and they're hoping that one day the spanner will fall out. But it's been going for 20 years and it hasn't yet. Anyway, Evan Skillman and team looked at their boring galaxies with the LBT, and they measured the amount of hydrogen to helium. And they get numbers that are incredibly consistent with that prediction from the first three minutes. They could have found that those galaxies have less helium than expected, and that would have falsified the Big Bang. So this is a direct prediction about our universe which we can test and which holds up even when we have spanner-infested telescopes of high technology uh pointing at distant galaxies. So I really want to emphasize that the Big Bang itself is well established and well tested. We've got the microwave background, we've got uh these measurements. And there was also a paper out this week by Michaela Weller, who's here from Ohio State. She looked at nearby stars using the Kepler Space Telescope and again looked at the amount of helium in the star and looked at what we know about how stars convert hydrogen to helium, but the results she and her team get are consistent with this Big Bang prediction. So this is something we can test now. I've shown you a graph. Here's a gratuitous picture of a star-forming region. Uh, this is from JWST. Let's all enjoy that for a minute. Right, the cosmic microwave background. So um I told you already that we had this light coming from our box of fog, from our early universe. So with the Big Bang nuclear synthesis, the predictions of hydrogen helium, we can go back to the first few minutes. Cosmic microwave background takes us back to the first few hundred thousand years. And when it was first observed, this is a map of the sky. This is real data. I haven't just used a shape tool and filled it in. I've plotted the data from the Kobe satellite. What you can see is that, oh, and the colour here is temperature. So what you can see is that wherever you look in the sky, this radiation is the same temperature. Essentially, it's emitting at the same wavelength from wherever you look. It's incredibly uniform. If you look a bit more closely, if I change the the axes so that we look for tiny differences in temperature, we do see this. Uh we see that one half of the sky is bluer than the other half. Um, that's an illusion. That's just because we're moving. So the earth goes around the sun, sun goes round the center of the Milky Way, the Milky Way is moving, you add all those effects together, and you find that we're moving in a particular direction in the sky towards a point called the solar apex. Um, and because we're moving, we get a red shift or a blue shift. And so you get this dipole pattern. So you you can remove that. So we can remove that, and we look for tiny differences in temperature now, and we see this. So now we're seeing some blobs, but we've got this big stripe across the center. Well, that's actually our galaxy. So dust in our galaxy also emits at the wavelengths that we detect the cosmic microwave background. And so we have to be careful when we think we're looking at the early universe, we have to be careful to remove the effects of our galaxy. But we can do that mathematically, and we're left with this picture, which was first seen in 1992 by the Kobe satellite, that showed small ripples in space. These are places where the difference in temperature from the hottest to the coldest bit is about one part in 10,000, and that corresponds to a change in density as well. So some of these places, the cold ones, have a little bit more stuff than the rest. So the early universe was nearly but not completely smooth. And this was a really important moment. I think if there was a place where you could say that it ceased to be possible, scientists can believe anything, um, but it became very hard not to believe in the Big Bang model was here. Because in these tiny seeds imprinted in light 400,000 years after the Big Bang, we see the beginnings of the process that led to the structure that we see around us. So gravity acts on these tiny differences. We can see this in computer simulations, and we start with these small differences, and they're exaggerated over time by gravity. The places that already have more stuff attract more, places less stuff lose out, and you end up with the lumpy universe that we see in computer simulations, and which we can compare to the universe around us today. And a lot of modern cosmology takes the CMB, using computers, predicts forwards to what we think we might see in the universe today, and then this is real data from the Sloan Digital Sky Survey that I've shown many times, showing the universe of galaxies that trace the large scale structure that's actually embedded in the structure of the cosmos 400,000 years ago. So we have a big bang. It was hot, it was dense. It was nearly but not quite smooth, the universe it produced, and then we go on to form the universe that we have today. But we want a theory of the universe, theory of cosmology, a theory that describes things on the grandest scales. Not just to explain the details of how you form the lumpy universe that we see, but we want to explain the bulk properties of the universe as well. We'd like to explain its fundamentals. And I still get people mentioning to me that the thing they most understood in the 18 and a half Gresham lectures that I've given so far is when I was talking about the Sun and I pointed out that the Sun is massive and hot. That was more than a year ago now, so I can confirm that's still true. But bulk property is the universe. So the universe, I suppose it is massive. It is big as well, which we'll come back to. But actually the two most fundamental properties that we need our theory to explain is that the universe is isotropic and that it's flat. Anyone need a picture of Mars at this point? Are we okay? Are we good? Okay. I've got a gratuitous picture coming up, I think. So let's take this by one. So let's let's talk about isotropic. This is also linked to the idea of it being homogenous. So this basically means things are the same everywhere. So if something is isotropic, it's the same in all directions. So in thinking about the universe, that means if I look in this direction, I should see the same as if I look in this direction, and see the same as if I look in this direction, and see the same as I look in this direction. If it's homogeneous, that means it's the same everywhere, so I should be able to take this part of the universe and put it over here and put this part over here, and you shouldn't be able to notice the difference. We live on a planet. This is different from that. Our experience of the universe isn't that it's isotropic and homogeneous. Up is different from down. And indeed, even if you go beyond the planet, we live on a galaxy which has a shape. We live, I've just shown you in this large scale structure. So what we really mean here is that on large enough scales, if you take a big enough chunk of the universe, the universe is isotropic and homogeneous. In other words, if I take a large chunk of the universe in that direction, I could swap it with the bit over here. And although the individual galaxies would be different, on any bulk property, if I looked at the dense overall density, if I looked at how clustered the galaxies are, if I looked at how many of them there are in any direction, I shouldn't see any difference. It's almost an assumption, but we would like a theory of the universe that explained it. The second, and we could see that actually in the cosmic microwave background. This map from the European Space Agency's Planck satellite shows those tiny fluctuations, but you can see that I could reverse this image, and I don't think you'd notice the difference. Or I could take the bottom left and top right quadrants and swap them over, or I could take uh an octant, an eighth of the thing on the left and swap it to the right, and the bulk properties of this distribution wouldn't change. And this is surprising. And the reason it's surprising is this. Remember, this is our observable universe, but in the past it was much smaller. In the past, it was small enough that we could think about how light travels across it. And actually, our observable universe is big enough that at no point has light had time to go from that side of it to this side of it, or from that side of it to this side of it, or from that side of it to this side of it. The volume of the universe on these scales that's causally connected, that's had light to travel across it, is about the size of double the full moon, and this is the whole sky. So that means that there's been no time at any point in the history of the universe, going back to the Big Bang, for the bottom left to communicate with top right, or for any information to travel from bottom right to top left. What this means is this apparent homogeneity, this isotropic nature, the fact that all the bits look the same, becomes a cosmic conspiracy. How have they agreed what they're going to look like? It's as if you had two people you you know hadn't had time to communicate with each other, but yet they both say exactly the same phrase to you. There has to be some underlying conspiracy. One answer is that the Big Bang just does this. Right? Whatever physics we don't remember, we don't understand the Big Bang, so maybe it just produces isotropic universes. It's not a great end to the lecture. Uh another option is that we got lucky. Maybe the Big Bang could have produced all sorts of other universes, but we happen to live in one where this property holds. But you can work out the odds with some guess as to what the underlying physics is, and then you have to get stupefyingly lucky. Um, or maybe there was some physics before the Big Bang that ensured this would happen. But that's like the first explanation again. That's still cheating, right? I might as well say that the great galactic god Zorb likes isotropic universes, and to please Zorb, uh, which we should do, uh, we are uh it'll come up later in the year. Um genuinely. Um we we need we need to arrange this. So this is a problem. It was known as the horizon problem. Now I said the universe is flat, uh, and this is a strange word to use as well, but um what we really mean here is about the geometry of the universe. There's a famous dictum by the great general relativist John Wheeler, who said that the essence of Einstein's theory was that matter moulds space, he said space-time, but thinks space. And then he said space tells matter where to move. And that's general relativity in a sentence. It's not even the hard bit of the lecture. Um, but um what that means is that when you put stuff in the universe, the geometry of space changes. And so if you have an empty universe, um you may have what's called a flat universe. One where the geometry you learn in school holds. If you draw a large triangle, then the angles add up to 180 degrees. If you put lots of stuff in, space will bend, and you can end up in a closed universe, one where the geometry is more like that on the surface of a sphere. And if you have some other configuration, you can have geometry rather like that on a saddle. And the reason we care is that because we're talking about the amount of matter, this determines the future of the universe. So there's our Big Bang. This is how big the universe is over time. If you have an open universe, a saddle one, it will expand forever. If you have a closed universe with lots of stuff in it, then it will collapse to a big crunch or a nab nig, which is the opposite of Big Bang. Um, or, not my joke, still funny, uh, I think. Um, or you can live right in the middle, where we have just enough stuff to slow the expansion of the universe, but never to stop it. And this picture is now complicated because, as you know, if you've heard my lectures, we have this mysterious thing called dark energy that's causing an acceleration of the expansion. But that doesn't matter for these purposes. And the weird thing is we live in this flat universe to a remarkable degree of precision. And it's rather like the horizon problem. You could just say that the Big Bang produces flat universes. But if you pick randomly how much stuff there is in the universe, you're never going to land on the lucky number in the middle. And yet that's where we seem to sit. So either the Big Bang only does this, either something before the Big Bang does this, or we got stupefyingly lucky. It's a very similar problem to the horizon problem. So we've got these two properties of the universe that we would like our theory to explain, which the theory of the Big Bang, as I've explained to you so far, can't touch. And actually there was a third problem. When physicists were worrying about this in the late 70s and early 80s, um, there were three things you need to know about the universe. It's isotropic, it's flat, and it's apparently devoid of magnetic monopoles. Now I know lots of you were thinking that, uh, so I did think I'd put it in. Um the universe was hot and dense. It behaved a bit like a nuclear reactor. Well, before that, it was even hotter and even denser, and all sorts of weird particle physics happens. And one of the things that should have happened, according to the theorists, when temperatures and conditions were extreme in the first fraction of a second after the Big Bang, is that all sorts of exotic particles should have been produced, and some of them should have hung around. And in particular, particles called magnetic monopoles. So when we have a magnet today, a magnet always has a positive and a negative end. You think bar magnet or something like that. Magnetic monopoles are sort of relics where you'd only have one or the other. And we have never seen one, despite years of looking. And yet the theory said that if we lived in this hot, dense state, the universe should have been full of them. This thing really bothered a physicist called Alan Guth, who was working on this in the 70s. Um, and before we get to his idea, here's a gratuitous set of videos of all sky cameras showing the northern lights above Canada. Magnetic is the link, you see. Anyway, Guth was worrying about why there weren't magnetic monopoles. And one evening, he realized that a solution that he'd been considering also solved the horizon problem and the flatness problem. And this is an actual quote from his notebook. He wrote, capital letters, spectacular realization. Colon. This kind of supercooling, so he'd been considering, an idea that I'll tell you about in a second, can explain why the universe today is so incredibly flat and therefore resolved the fine-tuning paradox pointed out by Bob Dickey in his Einstein Day lecture. So he'd been to a talk, he came back, he sat down, and he realized he could solve this problem. And then he drew a nice box with two lines around it, and then went home for the evening, which I love. I think this is great. This notebook's now in the collection of the Adler Planetarium in Chicago. If you go there, it is on display. Um there aren't many times in a scientific career where you get to write spectacular realization, and there are even fewer where you're right. So Goth's idea was really simple. It's this idea of inflation that I told you about at the start, and it can be presented modestly. Take um the head of a pin, I don't know, that size, and all you have to let me do to solve these problems, all you have to let me do is instant very nearly instantaneously go from something the size of the head of a pin to the head of a to the size of a grapefruit. That's it. From there to there. So the universe forms. There's a big bang, and normally the red line here, we think the expansion just proceeds throughout the first few million years of existence. In Guth's model, you start with a really small universe, it then inflates really quickly, and then it goes back to normal. And just that one idea solves the three problems. Firstly, it means that the universe you end up producing, and we now think that what's called the E-folding time, that the doubling happens about 80 times at least. So you go from pin to grapefruit, your magnetic monopoles, which were floating around, get diluted by this massive expansion. So there should be, you can do the calculation, there should be fewer than one of these in the entire observable universe. They were there at the beginning, but you've spread them out. You can explain the flatness problem because you can now have any shape of universe you want, any geometry, but when you blow it up via inflation, we're only looking at a tiny part of it. So one of the consequences of this idea is that the bit of the universe that we can see is a tiny fraction of the whole. And so that means that whatever the underlying geometry, we're going to think it's flat. If you stand on a football, it becomes instantly obvious that it's round. If you stand on the earth, you have to do some thinking. But if the earth was billions of times larger than it was, no measurement we could make would convince us that it was anything other than flat. So we no longer need a coincidence. We're just looking at a tiny part of a much grander geometry. And we can also explain this problem of the conspiracy, the fact that we have a homogeneous and isotropic universe, because way back at the beginning, before this inflation, our bit of the universe was much, much smaller. There was enough time for light to bounce all around it, for temperatures to equip uh to equilibriate, to get a nice homogeneous universe before you blow it up. And so it's a really brilliant, lovely idea, and it solves these three problems. But is it testable? Like, how do we have to think to be able to establish whether this actually happened or not, as opposed to being just a nice idea in a in a notebook? Well, I think I don't want to get too lost in the physics of it, but I think it's important to know that this sort of thinking where magic, apparently magic things like pin to grapefruit in a few seconds, um, can actually happen, it is a fairly natural way of thinking for a particle physicist. Because we've learned that as well as the sort of particles and forces that we're used to thinking about, the universe is perfaded by what quantum physicists call fields. And the most famous of these is the Higgs field, uh, which you'll have heard of because the large hadron collider uh seen here. This is the Atlas experiment underground in Geneva with particles whizzing around. Um, the Higgs field can produce particles called the Higgs boson, and there's Peter Higgs with the machine that discovered the Higgs boson, which you sometimes call the God particle, which I don't like, uh, but it was the holy grail of particle physics for many years. And it's a particle that, well, I was gonna say the particle gives particles mass. But actually that's not true. What's happening is this Higgs field, which pervades all of space, interacts with particles, and gives them their mass. So we have a Higgs field which is acting on things. And so to explain inflation, we invoke the presence of another of these fields, these properties of space, uh, and we call it the inflaton, which sounds like a transformer, but if you say it enough, it starts to sound like a sensible thing. So we invent this thing called the inflaton, and we say that what it did was it caused the universe to expand. And if that's hurting your head, here's a picture of Jupiter. Um, we have the inflaton, it transitions from one state to another, causing an expansion of the universe, and it does other things as well, and we can look for evidence of them. So one of the things we can do is we can look for gravitational waves caused by these things. Now I talked about gravitational waves in a previous Gresham lecture, but they're caused when massive things move in the universe. So this is a simulation in a supercomputer of two black holes merging together, and in green, you have a visualization of these gravitational waves. These are ripples in space-time spreading out through the cosmos caused by the merging of these two black holes. And we think this is an inevitable result of whenever masses move in the universe. Now, you need big masses moving a lot to cause gravitational waves that we could detect, but we are able to detect them with facilities like LIGO. And it turns out that if you're going to inflate the entire universe, if you're going to have this rapid expansion, then you should have a period where these gravitational waves are produced in the early universe, they propagate through the universe, and they should affect the microwave background. Imprinted on this light from 380, 400,000 years ago after the Big Bang, we should see the signature of the inflationary period. We look for it in polarized light for technical reasons. So there's a measurement you can make that we hope will show this. And the people who've been trying to do this best run this telescope. It's at the South Pole, because it turns out the South Pole's an excellent place to do astronomy. Um weather's pretty good, it's a desert, it doesn't actually snow very often. Um the skies are clear, and you get six months of night. So if you want to make a stable measurement for a long time, you could do that, and then you get the other six months of the year to repair your telescope and get ready. So this is a telescope called Bicep. And in 2015, astronomers working on Bicep put out this paper that said they detected B-mode polarization. So that's the signature. What they're claiming here is that they found the imprint of these gravitational waves on the cosmic microwave background. So this was proof of inflation. It's one of the most memorable days of my career because I was asked by NewsNight to make a two-second explain, a two-minute explainer of what had been found. I'm on 40 minutes so far, I think, this evening. I think I'm doing a better job than I did on Newslight. But we tried to convey the excitement, and they had an interview with Clem Pright from Minnesota, who's one of the leading scientists who happens to be married to a colleague of mine. Um, and it was really exciting for about 24 hours. Because remember, I said when I was talking about the microwave background, that you have to remove the effect of the Milky Way of our galaxy. And what it became really apparent really quickly was that the bicep team, who thought they were in a race to win a Nobel Prize, they thought their rivals, the polar bear consortium, made of physicists, not polar bears, uh, but with another up to a telescope, they thought they were right behind them. And so they rushed to do their analysis and they published it. It turns out they were miles ahead. They could have taken time to do it properly. And instead of carefully modeling the dust and the effect of the Milky Way, they used a cell phone picture of a slide that somebody had presented in another conference to map the dust. And they'd misunderstood the model. And they basically had screwed up. And in front of the world, they'd announced that they'd made the scientific discovery of the 21st century, and a day later, almost everyone who knew about the subject knew they were wrong. It took them a couple of years to admit it. There's a brilliant book written by one of the leaders called Losing the Nobel, which I quite recommend as a study of scientific hubris. Uh, it's quite something. So we haven't detected this yet. There are people building new experiments now. In fact, BICEP are still going, they're still funded, they've got a more sensitive telescope. But one of the problems is that there are lots of models, ideas about how inflation that works, which wouldn't produce a detectable gravitational wave signature. You perhaps the gravitational waves were not powerful enough to leave something that we could even plausibly detect. So not finding this thing doesn't disprove the idea, but it doesn't provide any proof. What we do have, though, is a clue in the pattern of dots on the screen here. So inflation provides a natural explanation for why we have a universe that has these primordial ripples. Imagine a universe filled with, I don't know, anything you like, magnetic monopoles, giraffes, um, glasses of water, it doesn't it doesn't matter before inflation. Once inflation kicks in, you produce such a large universe that your giraffes are scattered. Uh, so are your glasses of water, so are your electricals, so is anything else that was there. Inflation's really good at producing nothing, it empties out the universe. And so you might think that what we should expect is an empty universe. In fact, that's what Guth realized when he first did his work. And actually, that's not very helpful because we don't see an empty universe. We see a universe that's got galaxies and things in it. We see these ripples in the microwave background, but it was realized at a remarkable conference. Actually, this is sort of into the early 80s. There was a conference convened by Stephen Hawking in Cambridge with many of the great physicists of the time there. In fact, I've got the group photo. There we go. Studying the very early universe. Um I think the black and white is to save us from 80s fashion, if I'm honest. Uh I particularly I will happily take bets on the colours on this tie. Um, anyway, um, this group of people, over the course of a few weeks in Cambridge, worked out that there's a solution to this problem. The solution lies at the heart of quantum physics, because quantum physics tells us that empty space is not empty. Empty space is actually a broiling sea of what are called virtual particles, particles that appear and disappear extremely rapidly. It's just part of what quantum tells us space is actually like. And it sounds like an insane idea, but you can measure the effect of these particles. There's a thing called the Casimir effect, whereas if you take two plates in a vacuum, you can actually measure the knocking of these particles against each plate as they appear, bounce off, and then disappear again. It's a remarkable thing that this quanta obscure quantum effect can be measured. And so that must have been happening in the early universe just as inflation was happening. And so what starts off as tiny fluctuations on the quantum scale, the scale much smaller than atomic nuclei, boom, when inflation happens, they get blown up to the size of the dots in the microwave background. And so the ripples in this early light that go on to produce the galaxies and the structure that we see today are actually the microscopic quantum fluctuations from before inflation. And so one of the things this idea does is it connects our theory of the universe with our theory of the very small in a very beautiful and profound way. And by the mid-80s, this was well understood, and we could they could predict that you'd get things this size, depending on what your assumptions about inflation are. And furthermore, it predicts something about the nature of these fluctuations. It says you should have slightly fewer large ones than small ones. If anyone wants to ask why in the questions, I'll have a go. But it's a fundamental property of whatever you assume about inflation that you don't have as many large fluctuations as small ones. And we can now measure that. And this map from Planck shows that we have slightly fewer large fluctuations than small ones. It's not, in the technical jargon, it's not scale invariant. This then is evidence that this happened. It was a prediction made by inflation. So, as well as explaining the horizon problem, the flatness problem, and the lack of monopoles, it predicted something about a measurement that was made 30 years later. And that's what moved this theory across my line from theory to observation. I think we've got evidence, if not proof, that this mad idea that the universe expanded rapidly right at its beginning because of something that we call the inflaton. I think that happened, because I think we have evidence in the universe around us today that's true, that we're talking about direct evidence about something that happened in something like a billion, billion, billion, billion, billion, billion, billionth of a second after the Big Bang produces a testable prediction that we can look at now. Now I'm pretty sure there's one question left in your minds uh at this point, which is why was there a block of cheese on the first slide? And I find cheese useful to think about, particularly when I'm thinking about inflation. Not as useful as I want to recommend, by the way, there's a brilliant book that came out a couple of years ago by Will Kinney who talks about this stuff. And it's technical, but it's detailed and it's understandable. So if you want to know more about this, do read Will's books. He uses a pint of beer instead and thinks about the bubbles in a pint of beer, but we're going to talk about cheese. And the point is you can imagine that this inflation happens on a grand scale. So we have a universe that consists of this inflaton. This is expanding in all directions at incredibly rapid speed. And then the thing that distinguishes our universe is not that inflation happened, but that it stopped, that we're in a bubble, a hole in the in the Emmental in this case, where we transitioned from this existence where all that was happening was inflation, this great emptying out and expansion of space. And then here, slowly, and slowly is important by physics standards, we ended up with a bubble where that's not happening anymore. But that's the unusual bit, that that we're in a bubble, not that we were once in this other state. And so you can imagine a much grander picture, a bigger block of cheese picture, uh where scattered throughout the universe, some grander universe, there are more than one bubble. There are places where inflation stops. And this is called eternal inflation. It's determined by a race between how fast the accelerated expansion happens and how often you produce bubbles, how often they stop. Those two quantities are linked in a deep and fundamental way. But actually, you end up with these isolated universes scattered throughout an infinite space. And so this is a multiverse model. It's a well-founded way of getting to the idea that there are many universes out there, perhaps an infinite number. In fact, they have to be an infinite number in this picture. And that means that in each one of them, in each one of them, something different will have happened. There'll be a universe with a different set of physics, perhaps, or a universe where history went a bit differently, or a universe where this lecture made sense, or, you know, almost anything is possible in this grand picture. And I really like this because we've got to something I would have put firmly in the crazy theory bucket, right? The idea that there are multiple instances of the universe. But we got there via it's one step from the things that we can test. Whether we'd ever be able to test this directly is a different matter. I think we have to resolve almost certainly to thinking that we live in one bubble and our bubble won't interact. But I did talk to a uh a colleague in Cambridge, Hiranya Perez, a few weeks ago, who's working with uh part of her team, Alex Jenkins, to create a simulation of this. So this is a calculation that shows red is a slow part of the universe in a blue expanding one. And you can see in this model there are multiple bubbles coming together and they interact. If we live in this universe, we might see the signatures of some of these interactions on the cosmic microwave background. And in fact, Harania believes that there may be experiments we can do in the lab in quantum world, on the realm of the very small, that would help us understand whether or not we live in this giant block of cheese. I think that's a suitable ending for a lecture. So we'll go back to my picture of inflation. I'll give you some pretty galaxies. I'm happy to answer any questions. Thank you very much.