 Well, hello everybody. Welcome to the Science Circle. I think shortly after Stephen Hawking died, I told Jess that I would say, oh, I'll give a talk in honor of Stephen Hawking. And since his last paper was related to one of the kinds of topics I like to talk about anyway, I thought, hey, I'll talk about Stephen Hawking's last paper. So disclaimer, actually, there's a lot of disclaimers coming. I'm all about disclaimers. Disclaimer number 72B. Most of the talk actually won't be about Stephen Hawking's last paper. Most of the talk is going to be about the background of understanding our model of the universe that leads into the kind of thing that Stephen Hawking was talking about in his last paper. But that's okay, because if I just talked about Stephen Hawking's last paper, none of us, not even me, would really understand the talk. I was at Caltech for grad school from 1990 to 1996, which used to sound like the present. But nowadays, I teach college, and that's 10 years before all the college students were born. People are going to college now who don't even remember 9-11. It just makes me feel so old. But while I was there, Stephen Hawking would visit Caltech every few years. So I saw him give a couple of public talks, including one where he conceded one of his classic bets to Kip Thorne. And that talk was so overcrowded that I actually wasn't in the same room as him, but I was in another auditorium in Caltech where they were just live streaming. They didn't use those words back then, but they were live streaming the talk there. But he did give a physics colloquium, and I remember the physics colloquium, because I sort of understood the first, I don't know, one or two minutes of it. And then he very quickly got into stuff that was way beyond my understanding of theoretical physics. So again, I got some of the gist of what he was really trying to say a little bit of it, but actually his real arguments on what was going on, it was kind of beyond me. Yeah, so Kip Thorne is, Kip Thorne and Stephen Hawking, you often talk about the two at once. Stephen Hawking is more famous, Kip Thorne now has the Nobel Prize, so you can debate whatever you want. But so today I am going to talk about, there's this idea or this theory that goes along with the Big Bang theory called eternal inflation, which is an extension of inflation, which itself is an extension of the Big Bang. So we will start with the Big Bang. But before that, just a little bit about Stephen Hawking himself. Stephen Hawking may well have been the most famous living physicist. If you walked around on the streets and you asked people to name a physicist, they would by and large just look at you funny. But the few who were actually able to answer the question might have said Einstein, or, you know, they're really antiquarians, Newton. And then you asked to say a living physicist, they probably would have said Stephen Hawking. He was probably the best known living physicist. He was born back in 1942 and pretty early on he was recognized as brilliant, but he also got easily bored. Now there is a problem with people like Stephen Hawking that makes life difficult for people like me. So Stephen Hawking was bored doing stuff that he thought was too easy and too obvious. So once he figured out sort of how you would do a problem, he didn't want to waste any more time on it, which meant that he didn't turn in all his work in college. So now as a professor in college, I am faced with a non-trivial number of students who just don't turn in their work, don't do it all, only do some of it. And not most of them don't claim this, but they can always say, Oh, well, you know, Stephen Hawking didn't turn in most of his work and look how great he did. So clearly it's not important to do your homework. Well, okay, you are not Stephen Hawking. This is an important message to everybody in the world. You are not Stephen Hawking. You have to be as brilliant as Stephen Hawking to be able to not do your homework and still get the stuff. So kids go to school, stay in school. So he did sort of have trouble in college. And I think later on he felt like he was missing some of the background because he didn't learn enough in college. Of course, he was able to pick it up because he was Stephen Hawking and he was really smart. And so he was recognized as quite brilliant. And this is one of the other things that if the students don't do their work, even if they're smart, they may not learn anything and nobody will realize that they're actually smart. So anyway, he goes on to graduate school in theoretical physics and very shortly after he gets there discovers that he has ALS. And I don't remember the expansion of ALS, but it's Luke Gehrig's disease and the best way to describe ALS is that disease that Stephen Hawking had. For most people, they get ALS much later in life than in their early twenties. And for most people, once they get ALS, they have a few years left to live. And Stephen Hawking, in fact, it was predicted that he only had a few years to live when he was first diagnosed. His case was really strange. It progressed slowly and he ended up living with it for 50 years, which is probably surprising. He was famous, most famous in physics. If you ask actual, oh, thank you, Vic, it's anisotropic. That's not really what it says, but anisotropic lateral sclerosis. And it's weird to be both anisotropic and lateral at the same time. So that's why it's not anisotropic, but something else. If you ask physicists, what was Stephen Hawking's biggest contribution, many of them would probably say Hawking radiation, which is a thing that black holes do. Now, and that always surprises people because black holes are not supposed to emit any light or particles at all. But it turns out there's a quantum field theory effect that causes them very slowly to evaporate as random thermalized particles come out of them, according to Hawking radiation. Now, that work he did on that would be Nobel Prize worthy if we were able to confirm it. But we are very far from being able to do any sort of observations to observationally confirm Hawking radiation. So yeah, well, so then that's also what he, I mean, that's what he spent a lot of time working on is the boundaries of physics and that sort of thing. And in fact, that's what this talk is more about the boundaries of physics as we understand it right now. Now, the reason he's famous in amongst the public is probably I mean, there's of course, the great thing about the story of the guy who has this disease that would not just kill but just dealing with it would make most people kind of sort of give up and say, Well, I can't communicate with the world. And he persevered and managed to be very productive throughout his whole life, despite the challenges. So it's a great story. But he also wrote this book back in the 80s called a brief history of time late 80s, that was a bestseller was on bestseller lists for a few years. And this is a book that I think used to be famous for the book that the most people bought and then didn't read. And it was famous for being Oh, that's a very hard book to read. Now, I never thought it was that hard to read. And I reread it, I don't know, six or eight years ago. And of course, by then I had done a postdoc in cosmology. So I'm not the typical audience for this. But it really actually is pretty well written. And yes, it's about heady topics. And you have to kind of pay attention to be willing to think while you're reading it. But it's, it is a pretty good read. So I do recommend it. And yeah, I mean, some say here's the thing is that I think it's reputation for being so hard to read is not well found. And I think it's based on the fact that Oh, he's talking about complicated physics stuff. Therefore, it's hard to read. Now, so it is true that these are difficult topics and difficult to think about. And they blow your mind a little bit. And that's also true for quantum mechanics. But if you can get past the fact that this is weird, and it doesn't seem reasonable. You know, you can, you can learn things about it, even if you don't have a tremendous mathematical background. He also he worked, he spent a lot of his life working in quantum cosmology and other areas relating general relativity to quantum mechanics, which is a big deal. Because general relativity and quantum mechanics are really the extension of quantum mechanics called quantum field theory are the two fundamental theories that physics has of our universe of our world of reality. And very roughly speaking, general relativity is the fundamental physics of the very large and the massive. So it's our theory of gravity. Einstein is the main one who came up with general relativity. Of course, lots of people worked on it. Whereas quantum mechanics or more generally quantum field theory is the theory of all the other interactions, so the nuclear interactions as well as the electromagnetic interaction. And that really roughly speaking deals with the very small so atoms and molecules and that kind of thing. And it turns out that because gravity, when you compare two things of comparable magnitudes, gravity is really tiny compared to all the other forces, you can just completely ignoring it when doing quantum mechanics. But whereas electric charges, this positive and negative so on large scales, they tend to cancel out gravity, there's only positive mass. So on large scales, gravity completely dominates and as a result, almost everything we do in physics, we either don't have to worry about gravity, or don't have to at least don't have to worry about the quantum nature of everything else. Now, yes, certainly happens that you have to mix more than one thing. But for most of physics, we've got these two great theories that are have stood up to all the predictions we've thrown at them. But when you try and use them together in places where it matters, and that would be the very, very early universe, like we're going to talk about today, or the center of a black hole, they don't work well together, they give nonsensical answers. So one of the holy grails of fundamental theoretical physics right now is trying to come up with a working theory of quantum gravity. And that's what string theory may or may not be. So Stephen Hawking worked in that general area, though he often applied it to cosmology. So the study of the universe as a whole, because that's one of the places where it really matters. So this is the this is his last paper, it was submitted, I think not too long before he died, and it was just published. In fact, I think it was just published technically a few days ago, although in physics nowadays, once a paper has been accepted, and often before it was accepted, they upload it to a prefront server so people can see the paper before it was been accepted. So great. And this is in the journal he published in is actually open office, a open access journal. So I approve. Anyway, that's the title. So it's a smooth exit from eternal inflation, question mark. And then you can try and read the abstract very quickly, you'll have no idea what's going on, like what's a deformed Euclidean CFT, right CFT, he doesn't even define the acronym conformal field theory, because he figures anyone reading the paper will already know that. And yeah, great. So you read the abstract, you won't get much out of it. Let's read the title and look at eternal inflation. We have to think about while that sounds, you know, bad from an economic point of view, but that has nothing to do with the inflation we're talking about. What is eternal inflation? And to tell you what that is, I have to tell you about the Big Bang and why the Big Bang has problems. So here's a history of the whole universe. And this isn't the one I usually show. This is a linear history. So each tick on this is a billion years. And here we are 13.8 billion years after the beginning. And very shortly before us on this timescale, a mere 65 million years ago, many of the dinosaurs died. It turns out not all of them because burns are dinosaurs. And, you know, they're still around today. Four and a half billion years ago, the sun formed. And, you know, you look back to the first stars and there's been stars almost all of the universe. And then here, all kinds of interesting things happen right at the beginning. And so a result of this is that a linear scale is not a great way to look at this, because all kinds of interesting things happens in a very short timescale. So this is the one I usually show you. And timescale. So this is the one I usually show. This is a logarithmic scale. And there's a bunch of different things on here like Z and capital T. And don't worry about that. Just look at this little T here for temperature. This is seconds after the classical Big Bang, which almost certainly didn't happen as it was. And here you'll notice I've got this word inflation. And inflation ends somewhere about here. And given that that's what really we think of as the beginning of our universe today, you say, wait a minute, how can the universe be like 10 to the minus 36 seconds after the beginning of our universe? Well, here's the answer. It's 10 to the minus 36 seconds compared to everything else on the diagram that is such a small time interval that if you get it wrong by 10 to the minus 36 seconds, you're not going to care. And then all kinds of fun stuff happens, and I do want to point out things like protons and neutrons form. So at the end of inflation, which is the time we're mostly going to be talking about today, protons and neutrons weren't even a thing yet. The universe back here was a port glue on plasma, very, very exotic, and we've only begun to sort of approximate in our particle accelerators what stuff might have been like then. You get to like 10 minutes after, and then that's when the elements formed, and then the universe went into sort of a boring state for about three or 400,000 years, more like 300,000 years, where it's just plasma. The plasma is free electrons, so usually electrons are on atoms, you have nuclei and electrons, but during the stage of the universe it's too hot. So if a proton, hydrogen nucleus, captures an electron, there's plenty of gamma rays around to hit it very soon and knock the electron back off. So you have this big soup of particles sloshing around, and I think I have a diagram of that. Here it is. Right, you've got the nuclei, mostly hydrogen nuclei, a few other things, vastly overrepresented in this diagram, except for helium, which is represented about right, but everything else is vastly overrepresented. You have free electrons, these little dudes tolling around, and of course nothing really looks like this, because it's all quantum mechanical. And then lots of gamma rays, which I've made little wave things to look like photons, and the universe just was that, interacting constantly. The universe was opaque, which meant any light rays that are light waves or photons really, they weren't able to travel freely very far. The universe was dense enough and there were enough free electrons around that a small time compared to the age of the universe, a photon would scatter or be absorbed or something like that. And yes, the universe became transparent at t equals 300 or 400,000 years. This was two things happened. One, the universe got low enough density that photons were able to travel for times approximating the age of the universe without running into anything, mostly. Also, this is when the electrons got captured by the nuclei unstuck and made the universe atomic. And so ever since then the universe has been transparent. So this is important. The universe started, well the universe started really complicated, but once it was done making elements it was opaque just because light couldn't travel very far and then it transmitted, made a transition to transparent, you can see that transition. So if you think about the universe, it's just big, the universe is big. Ever got that? Okay, good. And if light is flying through it and interacts with something, well that's it. But if light is flying through it and nothing interrupts it, it will keep going until your telescope happens to interrupt it. And your telescope collects a very, very, very tiny fraction of the light that's out there. So if you are looking, mostly, okay, you look out, you're going to see lots of stars and galaxies and stuff like that. But if you look between all of those stars and galaxies and you look at the right wavelength, so it's matched to the wavelengths where most of those photons are nowadays, you will see in all directions the photons that just happen to be reaching you right now from the right distance away in the universe that they bounced off of the plasma at the time the universe made the transition to transparent. And so we call that the surface of last scattering. So here's a picture of this, right? You are here, you're in a galaxy. You'll also notice I have the wavelength getting longer, that's because as the universe expands everything red shifts as well. So even though back here the light that was emitted was largely red light, actually, it turns out red and infrared light, nowadays we observe it as very, very long wavelength radio waves. So farther away is back in time on this diagram and the reason for that is we observe the universe through light, although we've started observing it through gravitational waves as well, but both of them travel at the speed of light. So if you look a billion light years away, you are seeing the universe as it was a billion years ago. So you look the right distance away, you are seeing the universe as it was at the moment of last scattering. If you try to look farther, you won't directly see anything with light because none of the photons from then are around anymore, they all got absorbed. So we see this surface of last scattering or the cosmic microwave background as we call it in all directions and it's often described as the afterglow of the Big Bang and really it's the afterglow of the explosion, well it wasn't an explosion, but the afterglow of the expansion 300,000 years after the Big Bang when the universe became transparent and we do, we see it in all directions left over and in fact this is the discovery of this back in the 60s was really the thing that made astronomers convinced that the Big Bang was a really good model for the history of our universe. So this thing, it's called the cosmic microwave background, turns out to be a really useful thing to look at. First of all just the fact of it is very important because it tells us that the Big Bang is a very good model for what's going on, but then there's all kinds of details that you can look for and the cosmic microwave background that tells you all sorts of different things ultimately that come from fluctuations in the plasma that result the plasma and then depending on how things were going in the plasma at different places, the photons might be a little hotter, a little colder, and then the pattern of that fluctuations tells us all kinds of stuff that's included from stuff that went into the plasma ahead of time. But one of the most important things about the cosmic microwave background is that when you look at it in all directions it's exactly the same temperature, it's a black body which is the spectrum that things emit if they're emitting just because they're hot, and it's exactly 2.73 Kelvin, I think Vic mentioned that in chat, and in every direction of the sky it is exactly the same to one part in a thousand, right? So to within 0.1 percent the cosmic microwave is the same temperature everywhere. So this is a sky map I have here, I'm pulling out a little actual sphere, this is a false color in higher sky if you look at it in microwaves, yes. So, right, and here's that's what the sphere is, and then of course we unwrap it into a sky map to show on slides. Well, okay, so it's extremely uniform, it's not 100% perfectly uniform though, and if you start looking for differences in the temperature from one place to another you discover that in one direction it's a little warmer and the other direction it's a little cooler when you unwrap it into a sky map it looks like a yin yang thing. Now there's another thing here, you'll notice there's like this little extra spot it's part of what makes it look like yin yang. The real reason for that is along this line here is where our galaxy is, and there's all sorts of junk on our galaxy that tends to get in the way and mess up the observations. So along the disc of our galaxy the observations are a little bit messed up, but absolutely everywhere you have this very regular pattern and what that regular pattern is is just unwrapped this, and I have another one here although the green band is a little wider on it. So if I pull this guy out this is what we call the dipole anisotropy, that's a nice word, anisotropy means well so anisotropic means not the same in all directions. So dipole means something that's got two poles like the earth. So this is a dipole because it has a red side and a blue side, one side slightly hotter, one side slightly cooler, but remember we're talking millikelvin, and we're talking one part and a thousand variations. So to get this dipole what you have to do is subtract off the average temperature and just look at the fluctuations around the average temperature, and you get this dipole and this dipole is very easily explained as a Doppler shift. It turns out that relative to the cosmic microwave background photons our galaxy is moving at, I think I have the thing up here, nope I have the temperature range and here's the direction we're moving. I don't remember that it's something like 300 kilometers per second, I don't remember the exact speed, but our galaxy is moving relative to the cosmic microwave background photons. As a result there's a Doppler shift and the ones in front are slightly blue shifted so they look a little hotter, and the ones in back are slightly redshifted so they look a little cooler, and so we see that. So that's great but this is also a really simple pattern, just a, you know, we're moving in a straight line and so we can subtract off the effects of this dipole. So remember we started with smooth to one part and a thousand, we subtract that off. Now this dipole variation when you subtract that off, this is what you're left with, so these are fluctuations that are like one part in 40,000, really small fluctuations. You know if you had a marble that was this smooth it would be perfectly smooth. You couldn't tell that it had any bumps or anything on it, any bumps or divots or pits or anything on it. And of course we're not talking about physical size, we're talking about temperature, little hot spots and cold spots, but it's extremely smooth, but there are variations and these variations you can't explain is just a simple motion of the earth, this all this pockmarking, all the cosmic acne that the universe has is the result of physics of the early universe and there are lots of, oh it's 830 kilometers, so what's the kilometers per second, that's how I usually think. There are lots of little, there's all kinds of lots of physics that's embedded in this in terms of how big are the fluctuations when you look on different scales and how bright are they and those sorts of things and you can tell all kinds of things about the universe by interpreting those fluctuations and that's a big part of what cosmologists do nowadays. All right, so here are some of the things that come out of the cosmic micro background. The first one is, well all right, I have to give you a little bit of background, okay thanks, 230 kilometers per second, so I wasn't too off when I guessed 300. I want to give a background and introduce a jargon term called the horizon and the horizon is just how far away can you see anything? Hey that's just like the horizon on earth, but the horizon on earth comes from the curvature of the earth and so eventually you can't look around the curve of the sphere and so you don't see very far and so that's your horizon. This horizon's a little different, it's due to the fact that the universe has a finite age and you can't see stuff farther away than light has had time to travel. So there is, even though the universe itself is probably infinite or at least freaking huge, there is a region of the universe that we call the observable universe or the visible universe that is the stuff that light has had time to reach us from. There's still more stuff and it's probably very much the same outside this but there is this thing called the observable universe and that boundary is the horizon. So we talk about things inside the horizon. So tagline, the difference between the 43 billion years and the 13.7 billion years age of the universe is that in the 13.7 billion years that light was traveling to us from very far away the universe was still expanding and it took, so during the time it took the photons to get to us the universe expanded by a factor of a little less than four and so I don't think I did that right actually it should have expanded by a factor of a thousand. Whatever, all right distances are hard in general relativity but the universe expanded oh no yeah it did expand by a factor of a thousand, okay. Photons have to swim up hill against the expansion as well so it's all really complicated so the amount of time it took for the light to reach us is 13.7 billion years if the universe was neither expanding or contracting then the thing would be 13.7 billion light years away but because the universe was expanding the combination of the photon making its way toward us and the universe expanding which tended to make the photon have farther to go it's a very zeno sort of thing means that now the things that were the right distance away such that the photon would take 13.7 billion years to get to us are now 43 billion light years away so that's where that difference comes from yeah it'd be much easier if the universe wasn't expanding but then also we wouldn't have redshift to tell us how far away we were looking so I'm just as happy anyway so the horizon is important concept and here's the reason is that all these galaxies out here or the plasma that they form from can't really affect us because the fastest that signals go is the speed of light and so if anything's going to affect you it has to be within the speed of light times your age away from you or the speed of light times the age of the universe if it's going to affect the things that led to you if it's any farther away then it can't there's no way for it to send you a signal so you would expect only to be correlated with things inside your horizon because things farther away are not able to communicate with you but here's the thing look at this cosmic microwave background but just look at the center one here right it's perfectly smooth to one part in 10 to the five but even more than that if you look at the little tiny fluctuations the sort of amount of fluctuation you get you know it the typical size of the spots is about the same everywhere on the sky but if you take the fact that the universe is expanded by a lot factor of a thousand since this light was emitted the size of the horizon the plasma that had had a chance to talk to each other at the time this light was emitted was only this big is this little purple circle here so what that means is there's stuff that looks exactly the same but was not in causal contact with each other they're outside each other's horizon and as such it has no business being the same temperature when things are all the same temperature that's because they've equilibrated with each other and they've had time to send to energy back and forth and reach the same temperature if they're the same temperature but they're not in touch with each other well one way that can happen is if they were in touch before and they equilibrated and got to be the same temperature and then you move them apart from each other right so you can you can put your ice cubes in the freezer and it gets they get down to the temperature of the freezer and they go solid and then you take them out of the freezer and they're still solid why because they were in the freezer so that's fine but the problem here is that given the I mean these things are further apart than anything would have had time to communicate in the early universe and so how the heck did they get to be the same temperature this is a problem there's another problem with the big bang thing and that is the shape of the universe so if you do Einstein's general relativity and you try and figure out what should the universe be like you come up with this thing called the Freben-Robertson-Walker metric technical term but what the Freben-Robertson-Walker metric tells us and it tries to make the minimal assumptions about it is that there's actually three possible general shapes for the universe although it's really a continual because you can have closed universes which are the well the space part of spacetime would be the three-dimensional equivalent of the surface of a sphere so I haven't really drawn that I've drawn the 2d equivalent and it could be lots of different sizes so that's why I see it's a continuum it could be open in which case it's got a geometry that's analogous to that of a saddle or something like that so here again I've drawn a two-dimensional surface or it could actually be flat in which case the space is euclidean like we always expected and here's the thing if you use the equations of general relativity which are working really well to describe everything they tell you that if the universe was not extremely close to flat back in the early days that it would not be anywhere near flat today no today that it's very close to flat you know in fact when when inflation first came up we knew that we were within a factor of two or something more or less of flat which was close enough that the universe had to be very close in the past but in fact today we have we're within less than a percent of the parameters that describe these things away from being flat so the universe's geometry is a thing that it wouldn't be unless it was perfectly tuned just right back in the early days so it would stay that way and whenever you're talking about a parameter that there's no reason why if there's no a priori reason why a parameter has to have some value and then when it's perfectly tuned to be just right that makes physicists suspicious yes it's the Goldilocks cosmos so that was another problem so there was the flatness problem this one in the horizon problem which is how is the cosmic microwave background so smooth even though it represents parts of the universe that should have been in touch with each other and inflation answers both of these and this inflation was originally proposed by Alan Gooth back in 1981 here's the title inflationary universe a possible solution to the horizon and flatness problems so Alan Gooth very famous at the end of his abstract you probably can't read the whole thing here but he says unfortunately the scenario seems to lead to some unacceptable consequences so modifications must be sought and that's what eternal inflation will be want to point out again the unacceptable consequences of inflation have nothing to do with economics because we are not talking about monetary inflation we're talking about the universe inflating so all right what's the deal with inflation well here's a way of thinking about the expansion of the universe like it was for much of the history of the universe so I've got this grid here that represents where the stuff in the universe is so this is the graph paper on which you're putting your galaxies and one way of thinking about the expansion of the universe is that the entire graph paper just scales up and so the galaxies all get farther apart from each other which you can just see is the grid getting farther apart from each other well okay that's great so it's all getting farther apart from each other but at the same time the universe is aging and as the universe ages light has had more time to travel so your horizon gets bigger the older the universe is the farther away you can see stuff because light has had more time to get to you and so this red circle represents your horizon so in this early day the red circle you can only see stuff up to like one grid square away but then a little later hey I can see up to three grid squares away and then later again and then later again so as time goes by you can see more and more stuff farther and farther away and so that's what happened with the cosmic microwave background at the time of the cosmic microwave background only little bits could be in contact with each other but today well we can see lots of those bits because there's been enough time for our horizon to expand and see more of that so that's sort of the normal thing the universe is expanding but the horizon expands at the speed of light because that's how fast light moves and if you want light to get to you there you go all right well inflation is different during inflation your horizon still does expand as normal but the universe expands faster now if you don't believe me you can I don't know take a screenshot or something get the thing as flat as possible take a screenshot and read it into your favorite graphics program and you will see that in fact the red circle is getting bigger from here to here to here so your horizon is getting bigger but the universe expands exponentially and I really mean that exponentially there's a sad trend in society today for people to use exponential as a synonym for a lot and that just makes me sadder than anything in the world well inflation really is an exponential expansion every tiny fraction of a second the universe increases by a factor of a lot and it's the same factor every fraction of a second so it's truly exponential and when that happens the universe expands faster than your horizon so you'll notice here your horizon includes everything up to say two and a half or three grid squares away but now we're only up to two grid squares away and up to only one grid square away so stuff that had had a chance to talk to each other like this had a chance to talk to the center later no longer can talk to each other so things could have come into thermal equilibrium here and all been the same temperature but if the universe after that expanded in an exponential manner if it inflated then okay this and this is the same temperature but they can no longer talk to each other and then if it stopped and slowed down and went back to this right this and this are the same temperature but they're not inside the horizon well a little later they're back in the horizon you say hey look this guy and this guy they have the same temperature even though they were never in contact before well if the universe went through an inflationary phase they would be so the inflation idea solves the horizon problem by saying okay the universe went through a period of exponential expansion now you might start to say well what the heck why would you postulate exponential expansion and it turns out that our modern theories of particle physics it's actually not all that unnatural for the universe to expand exponentially that there are pretty easy ways to generate fundamental fields that would cause the universe to do that and about the flatness problem well imagine that this circle is your one-dimensional universe and this red circle shows you the size of your horizon right so if you can see from here to here you can say hey it's pretty curved but if the universe expands exponentially once again the universe is expanding faster than your horizon does again the red dotted circle is expanding so as time goes by how curved it is inside your your horizon circle gets less and less and less until now here if you just zoom in on this it almost looks like a straight line inside the red dotted circle so an inflationary universe where the universe expands exponentially and as such expands faster than your horizon does can also solve the flatness problem by saying well if it's been through this then every observable universe everywhere will look flat within the horizon because the universe got so big that any radius of curvature got flattened out and Vic said it's like an expanding a bubble or something right imagine or to go blow up a balloon somewhere and look at a little piece of the surface so you get curved and make it really big blow up the balloon but don't let it explode look at the same little piece of the surface and it's almost flat and then blow it all the way up to the size of the earth and it really seems flat right but go outside look at the ground looks flat unless you live in Colorado or something so inflation solves both the horizon and the flatness problems it actually solves a couple of other things as well like how did the universe start expanding in the first place we've known since 1930 or thereabouts that the universe is expanding but for a lot of that time it was just viewed as well that's one of the things the universe can do and that's what it happens to be doing but then you might say well why right how how did that happen why why expanding why not contracting why not something else what got it started well inflation got it started it answers that question there's also a question about where all the magnetic monopoles are I won't really get into that because I'm not convinced it's necessarily a big deal and I like there's a quote that max tengmark gave in a talk I heard 10 or 12 years ago where he said that inflation is what puts the bang and the big bang inflation this exponential expansion which then stops got everything going really fast and then the exponential expansion stopped and it started slowing down and at that point we went into our regular expanding universe that we have today although it turns out that we've started to turn on an exponential factor again and and we're building back to that but that's a different story now is inflation confirmed well it answers a bunch of questions that existed before it was proposed this is not the gold standard for a scientific theory being quote-unquote confirmed really if you want to give your scientific theory a solid test you have to predict something and then see if that prediction comes right and even then you never completely confirm anything you just fail to disprove it so the fact that inflation answered questions is nice it means we should pay attention to it but it doesn't mean that oh hey look here's a scientific theory that's that's past a pop arian test or anything like that okay so it's great it's a nice idea it fits with the big bang in general it fits with the other stuff we know we have ways we can make it go all right but what are the things going to do well it has stood up to some basic tests of looking at small fluctuations in the cmb so there were things that in the you know in the last 20 years when we started really looking at the cmb and and hardcore detail that would have looked different if inflation was wrong so yay so that's good but the best confirmation was actually some specific detailed things about the spectrum of the fluctuations that would have represented the effects of gravitational waves on the cosmic microwave background from inflation was announced back in 2013 something like that kind of a big deal well shortly thereafter we became convinced that we didn't really see what we thought we saw it was actually a systematic error from dust so you know inflation is it confirmed well alan geuscher thinks so right this is from the in fact i think vicks quoted this exact article a little earlier as a review article about inflation alan geus says in my opinion the evidence that our universe is the result of some sort of inflation is very solid of course he's the guy who came up with inflation in the first place so you might think he's biased however i would say that cosmologists as a whole would say yeah you know what inflation probably right i wouldn't say that we say it's confirmed likes a big bang theory as a whole or general activity or anything like that but hey seems like a good idea that's working really well is about where it's at and then if you talk to physicists who aren't cosmologists they'll just kind of snored at you for talking about stuff that is so far from observation that that it's not even worth thinking about if there are physicists who snored at you for thinking about astronomy at all it's true i've met them so but i would say his second statement here is right it is a success of these predictions that just by spending time on the more speculative aspects of inflationary cosmology and when he says speculative aspects that means working towards things that are nowhere near what we could observe he describes elsewhere in this paper that parts of inflationary cosmology were traditionally the realm of theorists who are afraid to venture too close to where experimentalists might be able to actually test their theories so what makes inflation go to explain that here's my one slide for describing how quantum field works in quantum field theory in fact there aren't particles really reality is made up of fields a field what is a field a field is just something that has a value everywhere in space well so here's my example of a field it's actually a two-dimensional field and it is the surface of a lake so you could call the surface of the lake a height field say right because the height of the lake above the ground has a value everywhere along the lake and it's two-dimensional because you just go x and y just where on the surface of the lake you are fields in the universe are three dimensional because every x y and z there's a value so you could imagine like walking around with a temperature gauge thermometer that's what you call temperature gauges and measuring the temperature everywhere in the room and you'll find hot spots you'll find cool spots and you can work out what the temperature field is so that's all a field is just something that has a value everywhere in space so like the height of this lake is a field now in quantum field theory all the things that we call particles are the result of fundamental fields so for example the electromagnetic field is one of the ones that's out there but there was also an electron field and a quark field and a Higgs boson field and all kinds of stuff like this not just the Higgs field so all those are out there and when we see something that we interpret as a particle really it's just an excitation of the field so that's what's going on here right most of the place the field is is flat and boring and it's what we call vacuum state it's not doing anything but right here there's an excitation of the field there's some localized wigglies going on and when there's localized wigglies of the field excitations of the field there's energy associated with that and there can be other properties depending on the nature of the field and the quantum field theory equations that go with it and we interpret that as a particle right so you might say there's a ripple on here or something on the surface of this lake there's a ripple on so so good so those are the localized all right so that's the view of quantum field theory now here's the thing I said this is vacuum so you would normally think of that as nothing right well it turns out that it is possible to have a false vacuum which means even though it's nice and smooth and flat and there's no wiggles to interpret as a particle it's actually not in the lowest energy state so there's no wiggles there's no particle but it's not in the lowest energy state so in principle the field value whatever the field value is and so what is the field value in these quantum field theory fields is this weird abstract thing like a quantum wave function whatever it's a weird thing it's a thing that we put into our theories that we can make predictions with so it's not energy it's not that it's just a thing but it's what I say to say it and so that thing has a vacuum value that when it has that vacuum value and there's no wiggles around there's no particles and then when it wiggles when when when there's a disturbance or an excitation or a different values of the field nearby in space and time we interpret that as a particle well if the overall smooth value of the field is not in its true vacuum state or its ground state and so here's this here's this diagram the idea is that this would be a although it turns out there's two in this cases true vacuum state where it really is at the lowest energy it can be so you see vertical axis here is energy density and then horizontal axis is phi which is the field value whatever that is right so it's just a thing that parameterizes how the physics is going at this point in space and time well and then the idea is that it is possible for some fields to be in this false vacuum where it's it's like balancing a marble on the top of the hill it can stay there but it doesn't really want to stay there and it's not too hard to get it rolling down the hill and so once you get one of these fields rolling and they talk about slow roll cosmology where it's rolling down but not too fast energy is released in that rolling and but the other thing is is because the field is not at its true vacuum value the energy associated with that we would call vacuum energy because of the false vacuum and that vacuum energy can cause a universe to expand exponentially if you put a energy density with this vacuum energy into Einstein's equations you get an exponentially expanding universe so that's what causes inflation would be some quantum field maybe it's the Higgs field maybe it's something else the Inflaton field is what people usually talk about we haven't identified this field specifically except through inflation but if you have a field that's not in its ground state it's in a false vacuum state it's not that there's particles around because there's not excitations there's not wigglies per se but the field as a whole is not in the ground state and if the field starts to transition to the ground state that'll release a lot of energy that'll tend to heat everything up that'll produce huge amounts of particles it will also because I mean the field the universe was already expanding exponentially but if it goes down to the ground state eventually the exponential expansion will stop and in the meantime the universe will have been heated up and all kinds of particles and stuff will have been produced and it'll just go coasting expanding however fast it was and while that sounds a lot like the Big Bang and that in fact is what the Big Bang was really if this inflation idea is right eternal inflation so inflation is this idea Alan Guth had that in the early stages our universe went through this inflationary phase this exponential phase why did that happen you know how did that get started well the early inflation models didn't address that but there is this idea today out there called eternal inflation is that hey wait the everything that's out there which you might call the universe and a lot of people do everything that's out there is inflating all the time because there's fields that are not in their true vacuum state and so there's what we call the bulk that is just everything that's there and the quantum field the inflaton field is not in its vacuum state so everything is moving away from each other and speeding up exponentially all times it's kind of really hard to wrap your brain around that's it you got this bulk everything everywhere speeding up all the time but okay that's great another thing that happens in quantum mechanics is quantum fluctuations so maybe the bulk would look a little bit more like this everything's speeding up everywhere but the field value is not going to be the same everywhere it'll be a little different in different places and if it gets different enough that's going to be where it starts to roll down the hill a little faster and at that spot where it's different enough you might actually get inflation to end as the thing gets down towards its true vacuum and you get a universe so the idea is that the bulk is everywhere and it's always exponentially moving away from all the rest of itself but there might be a region where the field value changes enough that the exponential expansion slows down and stops and what's left behind is extremely heated up still expanding but no longer expanding exponentially just expanding linearly and then it slows down because of mass and then speeds up again because of cosmological constant but again that's another story and that would be a universe a pocket universe is the name that's often given although to preachers who live inside it for weird technical reasons it would actually be infinite so how do you embed an infinite space inside a finite space you wave your hands and you use the word relativity a lot and you're good that same Max Tegmarker talked about at that same talk said that you can embed an infinite space in a finite space and I totally didn't believe him and I pulled him aside later and he drew some pictures on a piece of notebook paper and totally convinced me that it works so and Bob Sherer who was a theorist was standing next to me saying are you buying this but whatever it's it can be done that would be a whole other talk so I'm not going to try and do that right now so the idea is you have this bulk and in most places it just expands exponentially but every so often a piece of it the inflaton decays and you get a little universe embedded another way I've tried to visualize this in the past oh yeah one little thing I'm going to skip this string theory one drive I've tried to visualize this in the past is with a space time diagram where spaces you know left and right and time is up but not left and right but like out of the page and right on the diagram and time is up on the diagram and this is the inflating bulk except that you have to imagine that it's all moving away from each other exponentially and I've divided that out here and every so often you get a little thing here where there's a region where and it kind of expands out from there a region where the inflaton decays and you get a universe and this would be an expanding universe but I've drawn it straight because actually all this stuff is still moving exponentially away from each other and all of these lines should be moving exponentially fast away from each other you know exponentially fast is not the right way to say it fast and speeding up exponentially away from each other so they wouldn't stay all my nice and next to each other but the idea is you have this bulk and then inside here you get a universe and the universe works in such a way that you actually can embed an infinite universe inside here if it goes on for all time and so that would so this is a multiverse here right you've got a whole bunch of these different pocket universes so when people write about this sometimes they describe the universe as the bulk right it's the whole universe and they describe our pocket universe as our region of the universe but you know legitimately our pocket universe is a universe itself because we can't actually get out of it and other pocket universe would be parallel universes because we can't get there from here exponential expansion means that even going the speed of light we won't be able to catch up it's the same problem with that the horizon had early on now all right and so this then okay understanding why does our universe have the value that it does gets into deep debates about the cosmological principle versus the anthropic principle if there's lots and lots and lots of universes it becomes less worrisome that we live in a universe that seems to have all the right constants and parameters such that life can exist right it's like why does the electromagnetic field and the speed of light and the cosmological concept and all these things why do they have the right values such that life that could ask the question could exist well physicists have long hoped that it will turn out that there's only one mathematically consistent theory of everything and that that mathematically consistent theory of everything would be the one we're in and we can predict all the parameters from that and even string theory string theory thought it was going to be able to do that even string theory says that's not the case anymore that there's probably lots and lots and lots of different ways the universe could be well so then what is the the weak anthropic principle just says well so if there's huge numbers of universes we're going to be in one of the ones that can support life right for the same reason that we were born on earth and not in deep interplanetary space even though most of the solar system is deep interplanetary space right we're born in the place where we can live it's just a selection effect so we would be born in the universe that we can live in that's just the anthropic principle now so some cosmologists think well okay that's fine there's lots of universes we're in one we can be in let's just stop worrying about it others are deeply unsatisfied with this and want to be able to use just physics to predict all the parameters are and that's really if you can go back to like Descartes rationalism all that sort of stuff when you're talking about this but I'm not going to right now so you know that's this is still an ongoing debate and there's various different debates about I mean here's here's what even though we can't say there's only one possible universe we would like if we don't want to resort to the weak anthropic principle we would like to say well parameters like the parameters in our universe are actually quite probable you know sort of like you look around our galaxy there's huge numbers of planets there's huge numbers of stars like the sun with planets around them so our solar system doesn't seem all that special because there's lots and lots of other solar systems out there and hey that's good so we live in the kind of place that there tends to be well is the universe the same way well it gets really really difficult to estimate the probabilities of different kinds of universes for a lot of reasons right well first of all because we don't even have a theory that allows us to do it for real and in the kinds of things we do well if there's an infinite number of possibilities and an infinite number of over there and it's an infinite subset that we can be in and when you divide infinity by infinity what do you get oh you know I don't know right and so we have I mean dividing infinity by infinity is actually not a new problem it's been around for a long time Zeno's paradox can be phrased as dividing infinity by infinity and calculus solved that so we have figured out how to either divide zero by zero or divide infinity by infinity in other contexts before relevant to physics the introduction of calculus was very much like that in fact it was exactly that but we don't we don't have the right theory for this whole universe quantum gravity thing to know how to do it right and does it even freaking matter and this is where lots of physicists fall when you start talking about parallel universes their eyes glaze over and they say dude you're not doing physics anymore go go watch Star Trek is it even science well there's two problems here or here mine here's the big problem boring wildly new physics it is strictly impossible for us to communicate with any of the other pocket universes and communicate means includes observe we will never be able to observe these things right so does it even matter if they're out there or not from a scientific point of view this is not something that you could ever test or ever observe so why are we even talking about whether they're there whether or not they exist is not something that we will ever be able to know science or is it just all bullshit well lots of people say it's all bullshit however it is true that other consequences of eternal inflation theories may leave traces within our own pocket universe for example and the best most likely place to look for these would be the cosmic microwave background there may be details on the cosmic microwave background that eternal inflation theories would predict that if we then saw them we'd say hey wait a minute now we have to take this wacky eternal inflation theory seriously and that means that well then we maybe have to take the idea that there's other universes out there seriously even though we can't get there from here until you know somebody invents the cross multiversal wormhole and we go there and Spock has appeared but yeah that's later and we finally get to Stephen Hawking's paper that was the title of this and the question he was addressing is what are the boundaries of the pocket universe is like and the traditional answer involves starting with the starting with the inflating bulk and treating it as classical using just general relativity and then doing quantum mechanics on top of that so it's a semi classical analysis of it and one of the things they conclude is that it's a mess that you get this fractal structure of pocket universes around and the edges of the pocket universes have this fractal structure and it's kind of all over the place and and that's fine you know we could live in that and it could be like that out there and we're just it's on scales big enough that we don't see it even though it's on small scales from the point of view of the bulk but it does make a little less likely that we would actually live in a universe that does seem quite so smooth well what what this paper does is they say well okay but it's it's troublesome to actually go and try and use a mix of classical and quantum physics to estimate this because at the regime you're working about the quantum effects would totally wipe out the classical effect so you're doing it wrong that the title of this paper should have been you're doing it wrong colon and they try to do their own estimations and quite frankly I do not follow the math I don't know quantum field theory well enough in order to actually follow their real argument but what they come up with is this diagram and very roughly speaking the vertical axis is probability and the horizontal axis is our ways the universe could be all screwed up right so one is has something to do with how the mass is all messed up and one has to do with how squished it is and what this says and when zero means the least screwed up and very far from zero would be very screwed up and you'll notice that all of the probability is falling at least for this parameter whatever the heck it is is the least screwed up and even this parameter peaks at the least screwed up and then decays away from that and so that whereas the traditional answer would have lots of screwed up universes Hawking and Herthog are saying well when we actually try and incorporate quantum mechanics correctly we get much less screwed up so we come to the end and here's the here's the takeaway from all of this first of all I didn't really say this but it's background assumption the Big Bang model is rock solid the universe began on a hot and dense state there's huge numbers of observations to back that up so the Big Bang itself not the fact that there was an instant that things got started that you know that's that's more theoretically out there but the idea that the universe started in a very hot and dense state from which elements formed then which dissociated into them or recombined the cosmic microwave background and then stars formed and all that rock solid we've got all kinds of different observations that match predictions that we're really pretty sure about this now but then you get to the very early stage of the Big Bang and our physics starts breaking down and we don't really know what's going on and a flation I would not call inflation rock solid at all I would say that inflation is the leading model the most likely thing for what happened right at the beginning you know what put the bang in the Big Bang what was the Big Bang what got this whole thing started hey look this inflation idea seems to answer a bunch of questions and seems to be working but it's not nearly as well tested as the Big Bang and I think if you take a random cosmologist from day and transport them in time 200 years into the future they would be very surprised if the Big Bang had been tossed out just like Newton has not been tossed out but they wouldn't be all that surprised if inflation had been tossed out because inflation is not on that solid I mean Allen Gooth probably would be but the rest of us it's not that solid but okay it's sort of the best we've got and it actually seems to be working pretty well it's pretty good I mean so I would if I had to bet I would bet on inflation but not my full bet so I wouldn't bet a full nickel which is my maximum bet I'd bet like one or two sets and then eternal inflation is an idea for what was before inflation or what made inflation happen and eternal inflation leads to the suggestion of a multiverse and then finally quantum cosmology is hard Hawking and Herthog argue that the traditional approximations that lead to messy universes are wrong and that in fact in eternal inflation we should get nice universes nice friendly round comfortable smooth universes he's in them I don't think they've actually specifically predicted the cats so anyway that's the end of my talk and I would be happy to take any questions at this point yeah if only you could get a Nobel Prize for giving a talk right great thank you all I um yeah thank you all for your feedback and have a good time there are still a couple more talks coming up in the science circle maybe Shantel or Jess can tell you what they are I don't know off the top of my head but look online and you can see the schedule of what's coming up I think pretty soon we will be going into the summer phase where the talks are not nearly as regular Shantel and Jess are the ones who can really tell you about that yeah sharing science is important is first understanding it that's very true for a whole bunch of reasons but I think maybe the most important reason is why do science at all and you could give the answer oh because we can use science to make stuff that can increase shareholder value and the whole goal of everything is to increase shareholder value and if you give that answer you can tell you're an American because that's how Americans all think nowadays but if I am going to take off my burning cynic hat which is actually the hat I wear almost all the time nowadays and put on my starry eyed idealist hat which is a little dusty I would say that the whole reason we do science or the study of literature the study of history or philosophy any of this is because people with the weird brains that we have are just curious about the world and how things work and we like to think about hard things and scientists who do this stuff that takes lots of training to even be able to think about other ones who are expressing humanity's desire for this and so as such all of humanity I mean some obviously are more interested than other but all of humanity deserve to get some idea of what they figured out because we all just kind of want to know but that I don't know every so often I think thinking that people actually care about stuff maybe I'm just too cynical on all the people want is stuff that'll make them richer but whatever who knows maybe so that's why I think it's very important to share science that if you're not sharing it a lot of it wasn't worth doing in the first place well all right I'm going to head out probably take a little nap I have to enjoy the day because I don't have to do grading today but on Monday I will have a very large number of essay finals to read in handwriting so it'll be tough yeah okay the Anthros Phucine Epoch yeah actually wait if the Anthros Phucine has been going since 2008 isn't it time for a new epic all right I will see you all later