The Origins Podcast with Lawrence Krauss - Alan Guth: Inflation of The Universe & More

Episode Date: May 6, 2022

In 1979 Alan Guth, then a postdoc at Cornell, made what is perhaps the most important contribution to our theoretical understanding of the evolution of the Universe in the past half century. His realization that the early universe could have undergone a brief period of what he dubbed as “Inflation” provided the first and to date the only explanation of the large scale properties of the Universe compatible with observations, and based on well-defined, calculable, microphysical physics principles. Since that time, Inflation has become the paradigm of modern cosmology, and it made fundamental predictions about other observables in cosmology that have since been validated by observations of the Cosmo Microwave Background Radiation. I was particularly happy to have Alan on the podcast for a variety of reasons. First and foremost he is a remarkably clear and precise expositor of science. Second, his own history in the field provides, I think, a good object lesson for young scientists who might be struggling. Third, it was important that he provide a counterpoint to the discussion I previously had with Roger Penrose, who has presented his own alternative to Inflation that is much less well-defined at this time. Finally, Alan is a lovely human being, and both a friend, and in some sense a mentor to me (having served on my thesis examination committee when he first came to MIT, and having been a colleague and co-author with me on scientific papers). I hope you enjoy what I found to be a very enlightening discussion about science, and a revealing window into the thoughts of one of the most important cosmologists currently alive today. The audio version is free to all on this Critical Mass site. An ad-free video is available on Critical mass for paid subscribers only, a video version with advertisements is available separately on the Origins Project Foundation YouTube Channel. Get full access to Critical Mass at lawrencekrauss.substack.com/subscribe

Hi, I'm Lawrence Krauss and welcome to the Origins podcast. This episode is with the theoretical physicist and cosmologist Alan Goof, the father of inflation. In 1979, Alan then a postdoc had a sudden realization that changed the world of physics and changed our understanding of the universe. He produced what is now the standard picture of what we think happened in the early universe in order to explain the properties of the universe we see today. day. It produced a sea change in cosmology and gave us a model that allowed predictions to be made, which are compared with observations very favorably, as we discuss. But the interesting thing is,

Alan actually had that realization when he was on what would call a terminal postdoc. Like many young postdocs, he was bouncing around the country and was on potentially his last postdoc when he had that realization that changed his life and, as I say, changed things. Change. Change. physics for the better. Alan is a remarkably coherent, deep thinking, clear speaker. And I've always learned a lot from speaking to him. We talked about his own experiences in physics, what got him interested in physics and his experience of discovery inflation. But we went beyond that to talk about the physics of the universe itself from the beginning of time to the end, whether there is a beginning of time, whether there is an end of the universe.

multiverses, and of course, contrasting inflation with other ideas, including some that have been proposed by Roger Penrose, which Roger and I talked about in an early podcast. I hope you'll find enlightening. As I say, I always find my discussions with Alan in Lightning. He's been a colleague, a collaborator, and before that, a professor of mine. And it was a real pleasure to bring him on the podcast. I hope you enjoy it regardless of the platform in which you watch it, whether you watch the YouTube version or whether you watch the ad-free version on our Critical Mass Substack site. And no matter how you watch it, I hope you'll subscribe to the appropriate platform, our YouTube channel or Critical Mass. The ad-free version, the video ad-free version,

requires a paid subscription to the Substack site, and that money, goes to support the Origins Project Foundation, which makes the podcast possible. So I hope you'll consider supporting it as well. In any case, no matter how you watch it or listen to it, I hope you find it enjoyable and enlightening. Take care. Well, Alan, it is a true delight to see you again. It's been a while, but thanks so much for agreeing to be on the podcast. Great to see you. Yes, we've been, gone back a long way together, in fact, right back to when I was in graduate school. Oh, yeah. I remember. where you played a role for better or worse in allowing me to have a PhD.

So I guess that was for better or a role for works. Well, we'll see again. Yeah, that's right. There's still time. There's still time. Exactly. The jury's still out. There's still time. And one of the things I like about doing this is that I can learn things about people I know well that I didn't know about them before.

So because I try and do a lot of research. And I was intrigued actually because there, some things about your background that reminded me of my own in a way, but I don't know if it's true. So you grew up in New Jersey, I was born in New York, but you grew up in New Jersey, and your parents had a grocery store and dry cleaning business? Is that right? That's right. So did either of them go to university? I think my father went to junior college for a couple years.

My mother did not go to the university at all. Okay, she was a homemaker primarily, or did she work in that business? Okay, she was homemaker. Well, she's primarily a homemaker, but she did help out as a star. Yeah, sure. Now, you know, because my parents owned a little store and neither of them went to university, as it turned out. But I've always intrigued and to find out why, what motivates people to become educated and also go become a physicist. Now, you were a nice young Jewish boy in New Jersey. Your mother wanted to be a doctor, maybe, or no? I think they wanted me to be an engineer. Oh, engineer.

Okay, well, that's good. That's a, which, and, but did they influence you or were they influenced by it? What, what got you interested in science or math at the beginning? What was it? Yeah, a little hard to know for sure. It did not come directly from my parents. My parents certainly expected all my, me and my two siblings, my two sisters had to go to college. that was kind of a foregone conclusion in our community,

even though the parents had not. Yeah, that was more opportunities. Yeah, yeah. It was definitely what was expected. As far as science versus other possibilities, I certainly know that at a very young age, I was interested in science, and I remember what I thought was a marvelous TV program

called Watch Mr. Wizard. Oh, I love Mr. Wizard. You can remember that. Oh, yeah, I love Mr. Wizard. But I think he did continue for quite a while. So you probably got to see some of them as well. Don Herbert. Yeah.

Person who created it. And I thought that was marvelous. And maybe that's what helped me on science. I don't know. Then I started reading some books. I didn't have a large number of books. I remember particularly a book called The Universe on Dr. Einstein, I think by Lincoln Barnett.

And I know that book got me very excited about science. Did you ever read any of those books by Asimov or Gamma or any of those things when you were younger? No. I don't think I read Asimov. I probably did read one, two, three, infinity by Gamov. I was also very impressed by that. Yeah, no, those books and others. Books primarily by physicists were, I think, part of what got me interested in physics.

there was a long road. But you knew, but you had, you said, you had an early aptitude for mathematics. How did you know that? Just be doing well in school or whether there are other things? Um, well, I did well in school, but I remember I was also learning some things on my own. I remember I learned how to calculate square roots on before we learned it in school and felt very proud of myself. I had to calculate a square root. I think I learned it from some book. I forget. how I acquired that book or what book it was. Well, did you, okay, so it was, did you have good teachers in school that encouraged you to do science or one way or another or were they influential at all? Are you mostly self-motivated? Right. I guess I somehow don't remember very well my elementary and seventh and eighth grade teachers. I don't think they had a strong influence on me.

I did have a physics teacher in high school, although I thought was great. By that time, I think I already had decided I wanted to go into physics. So I'm not sure that he turned the course of the tide. But he certainly got me very enthusiastic about it. It was a kind of strange situation. He didn't actually know that much physics, but he had the personality that was incredibly exuberate,

and he was incredibly excited about different things about physics, and it was contagious. Well, I think that, yeah. No, I think that's important. I think that enthusiasm is more important in some sense and knowledge in teachers, especially if teachers are willing to say, I don't know when you ask the questions and say, hey, maybe we can figure this out together. I think that's the best thing a teacher can do. There's no need for them to be encyclopedic at the high school level. It just, hey, this is interesting and we can work out together. And I find it fascinating and I'd like to learn more and that sort of thing. I think, I often say teaching is selling in some sense, but certainly at the high school level, I think it is. It should, it could be. Yeah, I agree. And that is the person this teacher was. Now, I was going to ask you where you, when you, I knew you decided, you went into physics, MIT, but you say you already decide to do physics in high school. Did you ever think of doing mathematics or no, or was it always? Somehow, I think it was always physics. I was, of course, interested in mathematics as,

any theoretical physicist is and enjoyed it. But I guess I always thought it was much more meaningful if it was directed at understanding nature. Yeah, sure. Pursuing abstract axioms. Well, something I can't disagree with, but since you're both theoretical physicists, it's not surprising.

I did a degree in math as well as in physics, but it was so I wasn't certain. But you knew you wanted to go into physics, And you actually, something is interesting me. You attended several different high schools. Is that true? And why? That's what a biography of you says somewhere, and I want to know if it's true.

Yeah, no, it's not true. Oh. So I don't have to explain why. Okay, good. I know where you looked, I thought, I thought there might be a neat story there, but, you know, the story is, it's a, it's the author of the idea. Okay, well, that's good. I grew up in a town called Highland Park.

I went to Highland Park public schools throughout. And they were good. I left high school after three years. Yeah, well, I wouldn't get that. That I knew was true. And you left. Now, you left, interestingly, you left, well, you went into a program at IT that allowed you do a bachelor's and master's degree in five years. But you also left because of concerns about the Vietnam War.

Is that true? That's not relevant for high school, but it is relevant to the five-year program at MIT. Okay, so why did you leave high school after three years? Mainly just the science teachers wanted me to leave. You weren't poorly behaved, right? I was pretty well-behaved. You've always been very well-behaved since the minute I met you, so I can't imagine that you were.

But actually, it was the science teacher, the chemistry teacher, who I would have had the following year, who suggested that I, leave high school and go on to college. And I guess maybe he first talked about with people in the guidance department. And everybody seemed to think it was a good idea. And then they talked to me and I thought it was a good idea. And this was in the spring of my junior year. It was after all of the applications for colleges we do. But they asked me where I wanted to go. And I said MIT. I didn't know a lot about colleges at that point, of course. But my sister had gone to, my sister was three years older than me,

was a student at Leslie College in Boston. And she was dating MIT guys. And I got to meet some of them and came to be impressed that MIT was the school if you wanted to do science. So I told my guidance counselor in high school that, I'd like to go to MIT and he called MIT and they arranged it. Wow, it's a different world back then. I was going to ask about it. It wasn't it wasn't normal. It wasn't a standard procedure to leave before your last year. It wasn't as if the MIT had an official program for anything like that.

No, no, I just came into MIT as a normal freshman as far as MIT as far as the way he did make. And you've found yourself as prepared as the other freshman. I guess, you didn't have to do any remedial work. Yeah, I did. I was surprised. I expected to be outclassed by everybody. I was, I also joined the track team. I was tremendously outclassed there.

Okay. I was the best broad jumper in my high school. But at MIT, there was a freshman who could jump about two feet further than me. You see, well, you know, I figured if you want to do sports and you want to excel, MIT would be the place to go, I would have thought. It would have thought, right. It depends what level you're at. Yeah.

But they're pretty spectacular people in all fields at MIT. So I also at MIT as the best chess player at my high school. So I went to the MIT chess club one day and got wiped off the board by somebody who was playing four of us blindfolded or something. Again, it was just a totally different class. It is. It is. But it still turned out in physics and in math, I was doing. Doing well. And you didn't have to take any math. You know, you'd done enough of the calculus and everything in high school to do that. Yeah. No, it's, uh, um, okay. And physics. So I was going to ask, so MIT, like, you know, the decisions most people make, my experience for going to university, usually are poorly informed ones. Yeah. You know, you get to you know, you visit for a day and it's a sunny day at that day at university or something like that. But it was in this case, it was Boston and MIT students.

So you didn't think of the other. So you really never thought of any other places, any of the Harvard or Princeton or any of that stuff. The Princeton was closer to home, certainly. Yeah, right. Now, I never thought of Princeton. Never thought of Harvard really either. I do, however, recall that I knew a student who was a freshman at Harvard. And when I went to visit MIT, I stayed with him at Harvard,

and visited MIT during the day. And the visit reinforced my belief that MIT was the place to go. It's interesting. And I remember when I got it, of course, I went to MIT. Well, you went to MIT as a PhD student as well, but I moved from a small place in Canada to go do my PhD at MIT. And I remember visiting and staying with a friend of mine, a physicist who was at Harvard.

and I went down to MIT and I just kind of immediately felt like it was yeah it was there was I liked the atmosphere there you know as you know I later went to Harvard but I've always liked the sort of no-nonsense attitude of MIT and the fact that they didn't have a football team which wasn't that influential in my decision but there's no longer the case I'm so you so you wanted to physics and and you weren't turned and during your undergraduate career that was only reinforced you never never thought of doing anything else. That's right. That is right.

And you did something which again is not, so skipping last year of high school is unusual. The other thing in your career that's somewhat unusual is staying on to do a PhD in your institution. Most, a lot of institutions encourage, and I used to encourage my students to go somewhere else. MIT generally does too. And but you couldn't be discouraged. Is that, you're just, you were comfortable there? What was there a reason? You were just comfortable there?

Here in the Vietnam War did play a role. By the year that I would have graduated, the way that I would have gotten my bachelor's if I got my bachelor's in four years, was precisely the year when graduate students were no longer deferred from the draft. So MIT actually did quite a bit to try to help that students do as well as possible.

to stay out of the draft. And one of the things that they made possible was actually this the program, this five-year program was created at that point. For that reason? For that reason. Well, not officially. I wonder. Yes, for that reason. So I was able to officially remain an undergraduate for another year.

Oh, okay. And that would only have been possible by staying at MIT. I see. But then you could have, you could have, you could have after a master's gone to, gone to do a PhD elsewhere, though. Perhaps, perhaps. Actually, though, then there was another draft-related reason to stay at MIT, which is that while graduate students were no longer deferred from the draft,

MIT created something called a full-time teaching assistantship, which involved more teaching than a standard teaching assistantship, and they convinced the Massachusetts draft board that this was a a critical occupation that was worthy of a deferment. So that's why I did the next year. I love him. Another reason I like MIT from my, I mean, recently I've had issues, but, but yeah, no,

that's great. I wonder if, you know, my friend, Omschomsky, who was at MIT, and very strong, the anti-war had any role in encouraging them to do anything. That was, but it's great that they did that. I remember when I was there was a different thing. They really, what they did when I was there was accept an incredible number of Iranian students, Iranian students because of the revolution that was happening in Iran. And they,

you know, they took a lot of students in who wanted to continue their education. So let's applaud them. That's great. You, you, you're the interesting thing, you know, you're not that. You're about six or seven years older than me, seven years maybe. Yeah, seven years older than me. And the interesting thing to me about with when I began to think about the early stages of your was that you were interested in particle physics, as was I. I'm both trained as particle physicist. But that was a kind of a weird time in particle physics. It was kind of a, I wouldn't say a lull, but it was a time.

I mean, there was an explosion in the in the 1970s in many ways, and you were a part of it in a key way later on. But, but you know, the 60s were this time of uncertainty and a lot of confusion in the field. And even though a lot of the key ideas were developed in the 1960s, that later became part of the standard model of particle physics, it wasn't that clear in the early 1970s. And certainly when you began your PhD,

and even when you ended your PhD, that the standard model was a real, was a really, you know, the things were settled with a beautiful theory. The things were still very confused. Why did you focus on particle physics and where were your interest at that time? Right. Well, I agree with your description of the field. There was the standard model in the late 60s and early 70s, it was sort of one of the models that was on the table, the model that later came to be called the standard model.

But there was a lot of confusion, even whether or not field theory was the right language to describe elementary particle interactions. But I think what fascinated me about science from the beginning was the goal of understanding nature at its most fundamental level. And that is what particle physics was trying to do. By the way, I described my interest as if they were kind of constant over time. I think they did sharpen over time. When I was in grade school, I think I knew I wanted to be a scientist. but I didn't really know how to tell one scientist from another. Yeah, yeah, I know.

And probably it was high school. I decided I wanted to go into physics, and probably as well as an undergrad that I decided I wanted to go into theoretical physics and theoretical particle physics, I guess. Any professors at MIT that were in time? Steve Weinberg was there then, wasn't he at, was he at MIT then? He was, yeah. In my later years as an undergraduate, in law, I was a graduate student at MIT. a yes and and I

took general relativity from him while he was writing his famous book and I was incredibly impressed by Steve Weinberg and he said that for one time my thinking. Yeah no I and me too and an amazing amazing physicist, a great loss for humanity when he died recently.

Yes. But did you know and I know and and interestingly a number of the areas of research that you pursued related in one way or another to things that Steve had been thinking about. And I don't want to go too, I don't want to jump ahead too far, but of course, in some sense, the movement of particle physicists into cosmology, which wasn't really kosher early on, something that he kind of also heralded. And I'm wondering that, did that, well, let me ask,

it's going ahead of things, but did, with general relativity, of course, like, everything that Steve did when he wrote a book on it. When he taught a course, he would write a book. When he did either, he'd develop a comprehensive knowledge of things. And of course, Jim's General Relativity's testing ground was astronomy and astrophysics. I suspect that's where he began his interest in cosmology and astrophysics. Did that come across to you at the time? Partly at the time and much more later, Steve really played a crucial role in my getting involved and went back to inflation. And maybe we'll continue the story later and get there explicitly.

But while I was a graduate student, I did take, along with Steve's general relativity class, I did take a cosmology class, which was co-taught by Steve and Phil Morrison. Oh, Phil Morrison. I was wondering if it was a great question. Yeah, maybe two legends, Phil Morrison. Right.

It was amazing. And a wonderful man, I used to as a graduate student. I never took a course from him, but I used to just go into his office and talk and it was always open. And I, you know, I remember trying to emulate that later on. I mean, I just was a lowly graduate student. He said, come on in. And, uh, he was a wonderful, wonderful person. And also committed to issues that were relevant at the time and not just anti-war, but later on,

issues of, uh, uh, missile, you know, of protecting us from, from false claims about, about defense systems in the country and missile defense and other things. He wrote a variety of important books on that and played a role. Always spoke out. I admired that tremendously. Yeah, me too. Yeah, later on, in fact, my interest in the Bolton of Atomic Scientists in some sense came from him as well. Anyway, who was your supervisor at MIT?

Who did you do your Ph.D. with? Francis Lowe. Oh, okay. Great. Another great. He was a direct version too. I did very well. Yeah, and he did a number of key bits of work that one way or another laid the basis of the standard model.

The whole notion of what we'd call the fact that the evolution of theories depends upon scale came out of work that he'd done earlier and led to what's called asymptotic freedom, which we may get to because I'm intrigued. So what was your PhD on? What was your, what was your thesis? Yeah, well, to tell you that you, even though I was working with Francis Lowe, who has previously done this very prescient work that played a important role in developing the standard model, the thesis that I was doing was very anti-standard model and became totally obsolete by the time I finished it. I was working on the quark model, which was, of course, aimed at fundamental physics at its purest form. Yeah. But the version of the quark model I was working on was a pre-gauge theory model where quarks were viewed. I was just doing mesons. So I was looking at cork anti-cork pairs.

And the model was designed as if these were just incredibly heavy particles. So you didn't see them because they couldn't get out very strongly bound to give an overall energy to the bound state. That was the energy of mesons. Well, did you, was that influence, was it the bag model at MIT active at the time? Ken, what's it, Ken Johnson and others, which was kind of a pre, again, a pre, not quite a pre-gauge theory, but a pre-standard, standard model way of thinking about quarks in Meson's Barron. Was you, were influenced by that at the time or no? I don't think that existed yet. I think that came somewhat later. If it existed, it existed, I didn't know about it this time.

I'm intrigued when you say your thesis became irrelevant almost because my thesis was kind of an anti-inflationary proposal for a way to solve some problems in physics, which by the time I even completed it and was and defended it before eminent scientists like yourself, I realized was already irrelevant because inflation was clearly correct at that point, or at least clearly a beautiful idea. I wouldn't say necessarily clearly correct. But, but anyway, It's interesting. At least I view a PhD thesis as I often tell my students this. It's an excuse to get out and do things more than something to labor over. And I'm always amazed. You know, I always like the fact that in physics, more so at Harvard than MIT when I was there, that at Harvard you could sort of staple three pages, papers together and call it a PhD. And you wouldn't waste half a year writing a beautiful dissertation. They just wanted to do you. get out and do things. And I've tried to encourage my students to that, but they always want to

write a nice, you know, they always want to write it all up and everything. I say, yeah, the trend in MIT now is the staple three papers together, Nathan. I remember, I remember the idea was that I had to have sort of more or less three papers worth of work, but I, but I still had to write a comprehensive thesis then. Now you, so you're working on quarks and you say, as you say, anti-stander model gauge theories, which Steve helped pioneer in many ways. And, and, and, and was a sort of one of the great scientists pushing gauge theories and as a as a fundamental understanding and as they now are for all of at least all of the known interactions in nature but you went you went what intrigued me then okay so there was this confusion because at that time

as you say when you did your court model no one knew of this what the theory that would then become the theory the strong interaction was right that people were saying still speculating that the whole notion of field theory was not right and it might would have to go out the window and in Berkeley there were all these people looking at alternatives that were seemed you know inventing things called string theory to try and to try and circumvent field theory and and later on showing you know turned out to be a an incorrect effort although it was later be born in a different form but you went to Princeton after graduating and I was intrigued you were

Princeton is 71 to 74 or something like that or probably 76. That's right. And of course, that was intriguingly the time that gauge theory was used to develop a theory of a strong interaction. And you were there at that time, but if I'm not incorrect, you were not necessarily aware of it or working on different things, right? I was definitely working on different things, and I think I was not even really aware of the development of asymptotic freedom, even though it was an unbelievably important development in particle physics that was happening right under my nose. But I was working on other things and did not notice it.

I was working on things that basically were a continuation of my PhD thesis at that time still. But you, okay, okay, that's, and that's not unusual for postdocs, let's face it. I mean, you're working, you've developed a series of tools. You want to go and, you know, work on them and push them further to demonstrate your abilities. And that's not, so it's not that surprising.

But you, you, the career of yours that I certainly became aware of and that I knew later in, in one of your other postdocs, you really moved into gauge theories, you know, as sort of became a studied the details of quantum field theory of gauge theories in your later work. When did that transition happen? It happened pretty much when I went from Princeton to Columbia. at Columbia while I was at Princeton at least the people Gave series were big at Princeton but among people I wasn't talking to very much I was working mainly with Dave Soper

who was also I guess he was maybe assistant professor at that time or maybe also we started those postdocs together I don't remember when he got promoted to assistant professor and also Merf Goldberger Oh okay Marvin Gildberger for people who look up his name in the cyclopedias.

But everybody called him birth. M-U-R-P-H. Another important figure at Princeton and influenced a lot of people. It's a very important figure, but also towards the end of his career, so he wasn't into gauge theories. So he was happy to work with May on this,

what we now view, has a very old-fashioned approach to the way corks are bound inside of hadrons. And you moved, look, I mean, one of things we'll get into, and no secret is that you, your experience was something that coming a few years afterwards is something I sort of expected in a way. I mean, it was really hard to get a job. And you had a career that looked ultimately like you might, you know, not get a job. And a number of postdocs.

And at each stage, however, it turned up to be very useful because each stage, you got something out of it that eventually led you to the work of inflation, which we'll get to. So Columbia, there was much more emphasis on gauge theories. Who was the people there? Was it? Was it? I was working mainly with Norman Christ. Yeah, okay.

Who later on did lattice gauge theories, right? Later on did lattice gauge theories. That's, by the way, just so we can tell people that, you know, the mathematics of these theories, one of which is a strong interaction is not handleable when the interaction becomes strong. And one way to try and resolve that is to take space and make it a discrete series of lattice points and do numerical computations about things that you couldn't analytically compute. And it's become a very important area, which has really led to wonderful predictions demonstrating once again that the standard model is right.

But this was before his work on lattice gauge theories, I guess. That's right. That's right. But he was very interested in gauge theories. And the interest in sort of phase transitions in gauge theories, which I think you, did that come from there or later on or was added to Cornell? That didn't start until Cornell. Okay. So gauge series, you got to. At this point, we're concerned with things like how exactly should one quantize gauge theories,

which was still somewhat up in the air. And I think we develop methods that are correct, but are not the simplest methods and are not really used. by anybody today. But again, the technology of dealing with field theory was handling difficult fundamental questions with how to handle gauge theories, which on the surface have problems because it looks like many different ways

of describing them. And that's become a useful thing. But it also presents ambiguities and things like terms of making a quantum theory. You can make different quantum versions of gauge that on the surface look very different and sometimes are in general are the same, but it's kind of a fascinating feature of gauge theories. Yeah, I agree.

And okay, so that's, but I mean, there's more technology and detailed. Then you moved after Columbia to Cornell. Is that right? Right. And that's where you began to make the transition. Although you were still working, when you were Cornell, grand you the theory so let me let's put this in perspective you were 71 to 74 to 77 I think at Columbia and then 77 on to almost 80 at Cornell and the field well was that a year at slack yeah I was going to say 79 to 80 at slack and and the field that was an

amazing decade it was a because at the end of the decade it was totally different at the beginning it Everything was confused in 1970, but 1980, there was a standard model of particle physics that was not only accepted but tested and understood and engaged theories, as we point out, had become clearly the right way to understand at least the known forces in nature. And physicists, sort of the chutzpah that came from the very heady success. of particle physics to understand what was known, began to think about what was unknown. And what became clear by the mid-1970s is that there was a, that, that the, this standard model which described, you know, three of the non-gravitational forces in nature, of there only four forces as gravity and the three, four, the, the, the weak, the electromagnetic,

and the strong force, that they could be potentially unified at a scale which was far far away from anything we could ever measure in the laboratory directly at the time. That was an amazing leap that changed sort of ultimately sociology as well as the psychology of particle physics, this notion that one could try and extrapolate by 16 orders of magnitude to try and understand what was going on. And the fact, I guess that it was acceptable, kind of made it more acceptable to think about the kind of physics that you began to think about, right? I mean, eventually. Absolutely, yes.

Yes. And I agree with you, around 1980 was kind of the peak of physicist Kutzpah in believing that if we can write down a beautiful theory, it has a good chance of being right. Yeah. Yeah, no, I remember going to the, it was a little bit. I forget when it was, but it was around then. I went to the first conference on Grand Unification, and it was like a, it was like a, you know, a prize party. patting themselves on the back because they were going to make the next great discovery that was going to change the world. And because the power of what had been done was so strong that, that,

that, I mean, physicists were obnoxious enough at the time, but I think that was probably, well, no, maybe they got more obnoxious when they developed string theory and claimed to have a theory of everything. Maybe that was the peak of obnoxiousness, but anyway. But it's certainly, but your initial interest in guts and granification was the specific mechanics of, so let's step back again, since this is for people other than you and me. So the three non-gravitational forces, nature, could become unified, and that meant that at some very small scale, they would essentially be part of a larger theory. And then they would, as the scale increased, or as happens in physics, as the temperature, the energy, at which interactions are taking place decreases, eventually those theories would begin to look

very different. In physics we call that, well, in physics we call that a phase transition. So one had to try and understand how it was that that one single theory would break apart into three theories. Now that an example of how to have been done was the Electra Week theory that Steve Weinberg and Shelley Glashore and Abdu Salom won the Nobel Prize for. But it was an example of something that was then adopted at a much higher scale. And was it at this stage when Steve Weinberg sort of began influence you more? Was it with guts or what? Why don't you talk us through that?

Yeah, with guts. But actually, there was a particular interaction which I'll get to in my story. Okay. So the sort of the immediate story of how I got into inflation begins when I was at Cornell, as opposed to Ock. And it began really with two events, which were maybe slightly separate from each other, but happened within a few weeks of each other. One was a visit Cornell by Bob Dickey from Princeton. Great.

It was one of the... So he won a Nobel Prize or two. Yeah. Right. Should have, right. And one of the things, and he gave a series of lectures. And one of the lectures, talked about what, I forget if you use the word or not, but what we now call the flatness problem.

And what he explained was that if you trace the universe backwards in its evolution and look at it at a time when it was one second old, at that time, the, if you fix your notion of what the mass density was, the expansion rate had to be just right to an accuracy of about 14 decimal place. for the universe to look anything like what it looks like now. The idea was if it was expanding just a little bit slower, the universe would have soon just be collapsed under its own weight, and it was expanding just a little bit faster, and a little bit means one part in the 14th decimal place, just one point in the 14th decimal place,

it was just a little bit faster. The universe would have flown apart so fast that no structures, like galaxies or stars would ever form. So the question was, how did this incredible fine-tuning take place, what caused it? And he didn't really have an answer to that, but he raised the question. And I didn't really understand these calculations or anything. I'm not sure in what detail he tried to present them. But I was very much struck by that fact, which stuck in my brain for a while.

Okay. Whoever, I guess. I learned it from you, I think. But I learned it from Dicky. No doubt about where I learned it. And then the other event that happened that was crucial is a fellow postdoc at Grinnell, Henry Tye, we probably know, came to me one day. And he, by the way, was also a recent MIT PhD. So we had a common background.

He came to me one day, and he had gotten very interested in these grand unified theories. which were still at this point very foreign to me. They sounded crazy. But having gotten interested in them, he came to me and asked me whether or not grand unified theories would predict that magnetic monopoles should exist. And that was the kind of thing that I had worked on in the past when I was doing gauge theories at Columbia. One of the things we worked a lot on where the nature of magnetic monopoles

and engage theories. Which, in fact, I'd like to go back to that because when people talk about inflation now, one of the things they don't realize, I think for me, when I first heard about inflation, you know, the flatness problem seemed like something astronomers worried about, but they were probably all wrong. But, you know, as a particle physicist, that was kind of my attitude, maybe. But there was a real problem. And at the time, solving what we came along as the monopole problem seemed to me to be,

one of the strongest features of inflation. And now one often doesn't even hear it talked about. And I'd like to go back and give people a little bit of an education in that, especially since it was relevant to your work. When you were Columbia, you were thinking about monopoles. And really, so what, to put in a framework, of course, the difference between electricity and magnetism

is that there are electric charges, whereas magnetism, they're only dipoles. There's little magnets with the north and south pole. And there was a big mystery. Dirac made an amazing proposal, which should have been right. Maybe in the end, it may have some theory. It may still be right. It may still be right.

That a way to understand the remarkable quantization of charge. In fact, all that all charges come in multiples of, say, the charge of free particles, charge on the electron, say, could be understood if there was a single monopole in nature, a magnetic monopole. And that was a profound and amazing. very Diracian argument. But it required being put in by hand. But then the real change, and maybe you can walk me through it,

was when Gerard Atouft showed in one of his many ground-baking papers in the 1970s, and he really did lead the field in so many ways, that if you had these grand unified theories, what were called non-a-billion gauge theories, unlike electricity and magnetism, which is called Nabiling gauges, these complicated gauge theories that could encompass all the forces, known forces of nature than gravity, that magnetic monopoles could actually naturally result as real solutions of the theory. When did you first hear about that and what was your work on that?

Okay, yeah, that's one of the things I first learned when I was at Columbia. And our work was somewhat related to that. You, by the way, didn't quite state it accurately. The original Magnetic Monopoles were discovered, I guess, independently by Tuft and Polyakov, which is one addition to what we said. But also it was not in, it was not in Grand Unified Theory. It was a much more simplified gauge theory. A non-a-billion gauge theory. That's what I really mean. Yeah. Okay. Of which ultimately, which in standard, which occurs in even the standard model in even the standard theory is a non-agrified gauge theory. Right, right. Although the standard model does not derive to magnetic monopoles. Yeah. Yeah. Yeah. Yeah.

But models that are not any more complicated can give rise to monopoles. And that's what Tuft and Polyakov discovered. And it's exactly what you said, that while Dirac had a theory of monopoles that one could put into the theory by hand, one might say, what Tufton Polyacloth discovered is that once you write down this gauge theory, which ostensibly does not have any magnetic monopoles, you can nonetheless discover that there are solutions to the equations, that correspond to kind of twisted configurations of fields that form topologically stable knots,

and that those knots behave as magnetic monopoles. And once you formulate the theory, it's just no way to stop them from happening, it becomes a possible particle or a possible state within the theory. And as particle physicists are want to do, when there's a particle that comes out of the theory that you can't see, make it heavy. Right. And so the idea was that, you know, because we know, we know when had ever seen at that time still,

until Valentine's Day a little bit later, no one had ever seen a magnetic monopole and still haven't. The idea was that they were somehow, if they exist in nature, they were heavy. And you're absolutely right. The original theory of Tufton Polykov was a much simpler theory, not a billion theory, very similar to the electroweak theory in some ways, but a different grade group, as we call it.

But the great thing was that when you had gran unification, the scale of gran unification was pushed up so that any sort of kind of new particles associated with this unified theory would be at an energy scale, 16 orders of magnitude but heavier than the proton or 15 at the time, 15 orders of magnitude, every proton. And therefore, if monopoles existed,

they were so heavy that you would never produce him in accelerators and therefore they weren't a problem ostensibly, ostensibly. but they became a problem. So what did it? So Henry was the one, did he, so I want to, I mean, your focus wasn't on cosmology and astrophysics. I don't know if you were thinking about it at all.

No. So did, sort of when you were talking to Henry, did he tutor, he was beginning to think earlier on about cosmology, I think, right? Yes, that's right. What are you in cosmology at all? I was not thinking about cosmology at all. Did Henry tutor you at all at the time when you're working? Or where did you learn your cosmology then?

I probably learned my cosmology mainly, well, let's say. I certainly learned a lot of the background by reading Steve Weinberg's the first three minutes. Me too, yeah, popular book, which shows that you made a lot of popular books, yeah. But before that, I was certainly dealing with the fundamental equations of cosmology. and probably I learned them from Steve Weinberg's general relativity. Yeah, yeah. I still knew from graduate school. Okay.

And then, but Henry sort of convinced you that this problem was worth thinking about. Well, let me tell them more detail. Okay, good. Well, that's why I'm asking. I gave you a leading question. Right, okay. So I'll follow your question. So Henry came to me one day and asked me whether or not,

grand unified theories would produce magnetic monopoles, which is not generally known at the time. There were probably other people who knew like this. It's a fairly straightforward deduction from things which were known. But I didn't know what a grand unified theory was. He did have to tutor me about that. And truth, I remember thinking at the time that I never really understood the standard model in full either. Now I can understand the standard model as a subset kind of unified theories. Okay. Okay, great.

But in any case, Henry did a good job of educating me on those things. And then it was obvious that, yes, Grand Unified theories do predict that magnetic monopoles should be part of the particle spectrum. It should be a possible kind of particle for the same reason as Tufton Polyakov discovered in the simpler theory. And also, as you already said, because the scale of Grand Unified theories is,

high, these monopoles would necessarily be at that scale. In fact, even a little bit above the scale of other particles introduced by the Grand Unified Theory. So I told Henry, yes, it predicts they should exist, but these theories are crazy anyway, and these monopoles are even crazier. Nobody has any possibility of ever seen one. So why should we care? But did you write that up? Actually, I didn't, wasn't aware. So did you write, were you the, you presumably the two of you the first to point out that monopoles were a consequence of grandification did you write that up no we didn't write it up you see i know alan i knowing you well enough having written papers with you that you don't like to write things up that's right that's right

i i don't know if we ever looked carefully enough to discover if we were the first people to realize this certainly other people did shortly after so yeah there's no harm to the field by our failing to write it up But when I told Henry that we're obviously never going to see these particles anyway, without missing a beat, he came back and said, well, why don't we try to figure out how many of them would have been created in the Big Bang? Okay. And that was the start of everything.

That was. Now, and that was the start of everything because it led to a, because so did, now, did you guys write that up? How many would be created the Big Bang? Well, okay, the story has several twists. Okay. Because I know, I mean, yeah, okay, go on. Okay.

Well, let's see. First of all, both of us were working on other things, and I thought it was kind of a crazy thing anyway. I guess obviously these grand unified theories are a stab in the dark. So I was not too quick to start to work on it. So maybe six months or so dragged by. Between, I guess, the fall of 1978 and the spring of 79. And in the spring of 79, here's where Steve Weinberg comes in.

Steve Weinberg came to Cornell and gave a series of lectures about his work on grand unified theories and bariogenesis. You might recall that Steve was one of the first people who worked on that question. Sure. How was the net excess of barions over anti-barion matter over antimatter created? Let me just interrupt for a second to say that. In my mind, that was a crucial moment because that's when thinking about cosmology became kosher for particle physicists. Because that was a particle physics problem in some sense with a, I mean, that was a problem in cosmology that could only be solved by particle physicists in some sense.

And so his work on that and his example of being a well-known scientist made it, made it kosher. I know it dramatically affected me. I was doing my PhD at the time to be able to think about those questions. apply fundamental particle physics to the universe. Before then, it kind of seemed like science fiction. You know, you could go back to a second, but that wasn't really particle physics. But anything before that was kind of just, you know, science fiction.

But here was a real problem and a crucial problem. Why is the universe made of matter, not antimatter, that had no solution, and it had to have a fundamental explanation in terms of particle physics. And he was kind of, he was one of the people who spearheaded that. So, sorry, go on. I thought I just put that in perspective. No, I completely agree with you.

Thanks for saying it. Okay. And the effect that you described is exactly the effect they had on me. Okay. So while I had been dawdling about thinking about these things, and maybe we should emphasize to the audience that we're talking now about time scales in the early universe like 10 to the minus 35 seconds, decimal point 34 zeros, very tired writing.

Yeah. It's just unimaginable that we're going to talk about such times. And it had seemed totally crazy than me because of these unbelievable. early times that we would necessarily be trying to make statements about. But after Weinberg's visit, I decided, well, if someone of Steve Weinberg's stature can work on this, why should I? And that's really when Henry and I got to work on trying to figure out how many magnetic monopoles would have been created in the Big Bang if Grand Unified theories were a correct description of particle physics. And you wrote that up? Eventually, yes, but well, not exactly,

Again, this is a story of many twists. We fairly quickly, and I don't remember exactly how long, maybe a month or something like that, came to the apparent conclusion that far too many magnetic monopoles would be created, that the universe should just be swimming with magnetic monopoles, with a number of magnetic monopoles comparable to the number of protons. And there obviously aren't,

if you don't see them, But even beyond that, there was a fun calculation that we did at the time of the effect that this would have on the age of the universe. The magnetic monopoles weigh about 10 to the 16 times as much as a proton. And if there are comparable numbers, that would mean the mass density of the universe and these hidden monobiles would be vastly larger than we imagine the mass density of the universe to be. and that would mean the universe would be slowing much faster in this expansion from the Big Bang. And that would make the universe, given other observations, fix vastly younger than you would have thought, if you do not include these models. And it ends up being about a week old.

Which was, and there was reasonable evidence that it was older, just from your postdocs, actually. That's right. That's right, just for my postdocs. We thought it was cute that it came out to be such a biblical number. Yeah, exactly. In fact, actually, I'm surprised you weren't adopted by the, anyway, by the biblical creationists. But yeah, a week old, that would be perfect.

So we were convinced that there had to be some mechanism that prevented this magnetic monopole glut. But we were scooped on publishing that. So in fact, we didn't publish exactly that. Well, the problem is John Prescol, you know, was that's where I learned. I mean, he was a graduate student around some time as me a little bit earlier. And he, and he produced the paper at least showed the problem of the fact. Well, you know, I wasn't clear to me who showed how many there would be. But what he worked out in detail was that, you know, the only solution to that is to try and get rid of them, you know, if they could somehow get rid of each other annihilate or something.

and in a pretty lovely paper demonstrated that any kind of mechanism you could think about would not get rid of the blood of them early on. Had you worked on that part too or you just worked on the production of them or did you think about getting rid of them as well? My guess is that we didn't think about getting rid of them until we read John Prescott's paper.

I'm not entirely sure, but I don't remember thinking trying to get a handle on that. Okay, but that created this. That was one of the fuzzy issues, which is why we're going to publish immediately. It required a lot of different thinking. I remember being impressed by a paper a lot of thinking from different parts of physics to try and decide whether monopoles you could get rid of it. And there was very powerful arguments and convincing arguments in general. And I've written papers since then on ways to try and get rid of them.

But you have to sort of stand in your head. But that created what was then a, and increasingly seen, at least to try and, to me as a graduate student and around that time, yeah, it's still a graduate student, an emerging problem that was severe, you know, that particle physics predicted if these grand unified theories were right, I mean, it was another reason for me to perhaps not believe in gran unification is to say, if these theories are right, they produced something that clearly wasn't there with such abundance that, and it wasn't there, and that suggested the theories were wrong. It was a really clear and pressing problem, at least for some people who were beginning

to think about cosmology as I was at that time and you were. But that's what really motivated is that, well, anyway, so let me ask you, was it, the paper you eventually wrote on inflation was written as a solution to the horizon and flatness problems, I think. I don't think monopoles were in the title at all, right? They were not in the title, but they were in the article. In the paper, yeah, yeah, yeah. What, so did that, were you thinking about?

So what led to the realization that I, were you, let me, how can I put this? You weren't really motivated in developing inflation, the realization that inflation happened, that the universe could expand exponentially, naturally as a result of a phase transition in early times that might be associated with grandification. It was kind of a natural expectation that could easily happen. And it would solve all these problems. But it was more, was it more of the flatness problem that you were thinking about? the time than monopoles or why do you want to walk us back and i'm sure this is a story now

this particular story you've told a gazillion times but why don't you walk us back through that i like to i like in these things to try and focus on things you haven't spoken out a million times but okay uh yeah well we um after john prescott's paper came out uh henry and i felt scooped But we wanted to try to salvage what we had done. We didn't think it would be appropriate to just publish another paper saying we found it, depending on what Don Peskel found, especially since we think his derivation, at least I thought his derivation was better than anything we had at the time. But so we decided to concentrate on the question of whether or not there's any way around this problem.

Is there anything we can introduce to change either grand unified theories or one's model of the early universe? to make grand unified theories consistent with cosmology. And now there's sort of two parts, the part that Henry was part of and the part that happened as he was leaving on a trip to China. But while we're working together, and this did lead to a paper that we wrote together, we came up with the idea that extreme supercooling at this phase transition could prevent this magnetic monocall glut. The idea is that the idea is that the The idea is simply that the excess of magnetic monopoles happened because the magnetic monopoles were described in these theories as twisted knots of fields.

And if the phase transition happened quickly, people had been assuming and we assumed initially that the phase transition happened instantaneously when the temperature fell to the right value, then there's not time for these knots to smooth themselves out and you end up with a very knotty field, which means a large number of magnetic monopoles. But if there was a huge amount of supercooling, so the universe cooled to temperatures well below the natural temperature for the phase transition without the phase transition happening yet, and that's always a possibility. Then there'd be time for these knots to smooth themselves out before they get ironed in. and we argued that that could be the explanation of why there's so few monopholes. While we were doing this work, and in the paper that we eventually wrote,

we were really blindly assuming that as the super cooling was taking place, the universe would go on expanding exactly as it would have otherwise. Which of course is wrong. Which of course is very wrong. Yeah. I'm pretty sure it was Henry who told me that I should look at that. And what happened is one night in December of 1979, I went back to my rented house.

At this point, I moved to Slack for the year. So I went home to my rented house in Menlo Park, California, and wrote down the equations on a piece of paper. And as you know, but the audience may have trouble believing, once you write down the equations, it's just so obvious. The real question is just why wasn't obvious to everybody before this? You know, by the way, you've probably heard this,

but I know many of my colleagues, well-known physicists, a number of whom won the Nobel Prize, who all said the same thing to me. Why didn't I realize that? Yeah, you know, because it's amazing. When you look at it, it's one of those things where the minute you see it, It's just so clear. Yeah.

Right. It's very simple physics. It's a differential equation you could show to a freshman. A freshman would know how to solve it. It's, you know, it's weird that, you know, and you don't take things. It's often a fact, I've often said this about, and this example where maybe it was cosmology had forced it, but I found in my career, and I guess I've done a lot of things that are more related to at various times to particle accelerators than you have. but it wasn't until there was a real experimental problem that I began to think seriously about the theory. There are papers I could have written well in advance of an experimental development

that I never thought seriously enough about until the data showed it to me. And I guess maybe that was an example. People didn't play with the idea until there was a real reason to think about that problem. And that's why I wanted to bring up monopoles in some sense because while we don't, while history now doesn't use them as doesn't think of them as a key, but it was that real problem in some sense that caused someone like you to think seriously

about the equations. Yeah, yeah, no, definitely was definitely magnetic monopoles that got me into this. It was only after, on that same night, but after I realized that the system has kind of started exponentially expanding, that I realized that it would solve the fightness problem too. And that got me extraordinarily excited.

Yeah, I've seen your, I've seen everyone senior notebook with the explanation marks. Right. And the double box. Yeah. Exactly. That's. And then the horizon problem was later and we'll have to have to frame that for people. But, but it's a more subtle, well, in some sense, it's a more subtle problem. But it's an important one. And of course, inflation naturally solves that too. And do you want to, do you want to explain that? Sure. The horizon problem is the problem, is the problem, of understanding how the universe got to be so uniform. And this uniformity is seen most clearly observationally

through the cosmic microwave background radiation, which of course we view as the heat, as the afterglow of the heat of the Big Bang. So it fills the universe, and we see it coming from all directions. And when we measure its intensity in different directions, we find that after we correct for the motion, of the Earth through it, which changes things a little bit.

It's just unbelievably uniform, same in all directions, to an accuracy of about a part in 100,000, which is incredibly uniform. Incredible, unbelievably. Yet, it was pointed out long before my work in cosmology, that if you trace back these photons of the CM of the cosmic microwave background that are arriving at Earth today from two opposite directions, trace them back to where they came from.

When they originated, those two points were separated from each other by roughly 50 or 100 times the distance that light could have traveled up until that time. So there's just no way that the point over here could know anything about the point over there, but somehow they sent out photons that agreed in temperature to an accuracy of one part in 100,000. And that's an incredible mystery when you think about it. Was it Jim Peoples who, I mean, I learned that, I think, from People's First, but I don't know who first pointed out the horizon. I mean, there were people who thought about it. But, and again, I think it was one of those things we don't take seriously.

What was that? I think Rindler is sometimes credited. Rindler? Okay. Okay. Again, it's one of those things you don't, you know, to try and, again, frame for people the transition that occurred in the way people thought about things. It was probably an understood problem, but no one thought seriously about

applying fundamental physics to the early universe. It just seemed so crazy that I think people said, well, yeah, that's a problem, but there's so many other problems. Why worry about that one? Because it's not a big deal. Yeah. Yeah. Yeah. That's right. People do not take it terribly seriously. At least particle physics, they're not there too much at all. Some people were thinking about cosmology, were puzzling about it, but didn't really have a handle about what to do about it. So you realized it solved the horizon problem after you realize it solved the flatness and monopole problems. That's right.

In fact, I only learned that the horizon problem existed and did not invent it myself, so I learned it from others. And that only happened until a few weeks after. It was all rather coincidental that it all happened within a few weeks. But a few weeks later, I was having lunch. at the cafeteria at Slack. And the group at the lunch table was having a conversation about the horizon problem, motivated by a paper that had just come out by Tony Z.

And I don't remember what kind of a possible solution he was talking about, but he did bring up the problem and close our lunch table to talk about it. So I asked the people at the lunch table, what is this horizon problem? and they explained it to me. And I don't remember how long it's up, but certainly didn't take very long before I realized that, yes, inflation gets around that problem too. Well, okay, now.

My list. Yeah, no, then it was, then you were very excited, but I'm still, we'll get to something that's, again, uniquely Alan Gooth-like, it seems to me, but we'll get there. I mean, knowing as I do, but before we do, let's explain to people, and that means let's you explain to people,

why inflation solves all these problems. So let's give a little brief primer. Okay, refrimer of what inflation does and how it solves these problems. Okay, looking back at it, rather than giving the historical narrative that we just talked about, inflation, I think, is the best way to start to think about it is as an answer to the question of what drove the Big Bang? What was the repulsive force that drove the universe? into the gigantic expansion

that we're still seeing the aftermath of. Curiously, the theory called the Big Bang Theory says literally, absolutely nothing about what caused the expansion. The Big Bang theory as it existed before inflation was purely a theory of the aftermath

of a bang. In its description of the universe, it starts with the universe already uniformly expanding and how the universe, that happened, there was no previous explanation. Inflation explains this through a repulsive form of gravity. Now, repulsive gravity is certainly not widely known among the public and certainly never taught to me when I was in school. Newtonian gravity is entirely attractive. Two masses attract each other, always, never the other sign.

But in general relativity, it does turn out, and this was known really from very early on in Einstein's work, general relativity can produce repulsive gravity as well as attractive gravity. And the key is that in general relativity, gravitational fields are produced not only by mass densities, which are equivalent to energy densities with E equals MC squared, but not only is it these mass densities that can create gravitational fields, but pressure can also contribute to the gravitational field. And that's built into general relativity at the heart. Yeah, and it's very hard because it's because of it,

Lauren, because of the structure of space time. Yeah, yeah, maybe since you started, we'll elaborate a little bit on that. If general relativity was designed to be consistent with special relativity, And in special relativity, if you know the energy density in one frame, that's not enough information to know the energy density in another frame. And transforming between frames is crucial. So one always has to be explaining things in terms of quantities that can allow you to know what happens in one frame in terms of what happens in the second frame. So to make the energy density complete, you actually have to combine it with nine other quantities. but if everything is spherical symmetric, it is just the energy density and the pressure that survives.

So we'll work in that simplification. If I know if everything is spherical symmetric and I know the energy density and the pressure here, I can talk about a moving frame and I know what the energy density and the pressure is there. So pressure is necessarily part of the equation. And it's this whole what's called the energy momentum. some tensor that is the source of gravity in general relativity. So it includes both energy densities and pressures. And energy densities, as far as we know, are always positive. One can build theories of negative energy densities, and they may happen as quantum fluctuations,

but nonetheless, they don't happen big time, and they're not relevant here. But pressures can actually have either sign. They can be positive or negative. Now, we're accustomed to thinking the pressures in an ideal gas model where it are caused by particles bouncing off the wall, and that seems to only allow a positive pressure. And it's true that that model only allows a positive pressure. But you really don't have to go very far to see the possibility of a negative pressure. The simplest example that I know of is to just imagine a piece of rubber and to keep things isotropic same in all directions. Imagine pulling out in all directions on this piece of rubber, it will pull back. That's negative pressure. It's not large enough to see the gravitational

effects of it, but it is negative pressure. Well, that's a good example. Okay, that's good. I like to say, I've always said that my short version of what you just said is that gravity doesn't always blow. Sometimes it sucks. That's right. That's right. Exactly. Exactly. Your short version is very accurate. And but but I think the function isn't my way to look at this negative pressure. And but to put it the framework, this sounds very mathematical to people are listening and it is, but I think I want to put it back in the framework. What you realized was that the specific conditions for energy and momentum, when you're in the middle of this super cooling phase,

when a phase transition hasn't completed, when there's energy density that's stuck in empty space, if you want to say it because that hasn't yet been released because the field has not yet cool to its or not yet relaxed to its preferred states. So it's storing energy that really doesn't want to be there, that that particular type of energy momentum was exactly the kind of energy meant you were talking about, which had negative pressure. Exactly.

Yes. What you just said is absolutely right. I would have said it next. I'm sorry. Okay, well, I wanted to let us believe us back in. And now let's see your turn. Right. Yeah, I go off on tangents.

No, it's sorry. No, no, it's great. Right. Tengents are often more interesting then. No, it's good. Okay. Right. So the super cooling leads exactly to that kind of state with a large negative pressure. In fact, the pressure is almost exactly equal to the negative of the energy density. And that turns out to be more than enough negative pressure to drive the region through the gravitational repulsion into a phase of exponential expansion. And in case any of the listeners don't know, exponential expansion means that in a certain very small amount,

of time, the region doubles in size, and then in the same amount of time, it doubles again, and then doubles again, and then doubles again. And these exponentials build up very quickly into very large amounts of total expansion. So the universe could have, just to give people a sense of this, in 10 of the minus 35 seconds, the universe could have expanded by how much? Yeah, kind of the minimal numbers to make inflation work at grand unified theory scales is about a hundred doublings and the time period might be about 10 to the minus 35 seconds. And for people understand, 100 doublings is about 10 to the 30th, right? Or something like that.

10 to the 25. So that, I mean, that's amazing to think about. It's one of these things that you, it can happen, but it's hard to imagine. Actually did happen. The universe could have expanded by a factor one with 25 zeros in a time frame of 10 to the minus 35 seconds. Yeah, that's right. now it's mind-boggling, which is why it took me so long to take it seriously myself.

And now, so why does that solve the problems? Yes, okay. Right, no, it says they're fairly simple explanations for how it solves all three of these problems. Let's start with monopoles. It solves the monopole problem in a very simple way, really. The monopoles are produced dominantly when you reach the critical temperature, but then we're going to have this huge amount of exponential expansion after. So it doesn't actually lower the number of monopoles that are produced.

But what it does is to stretch space out by this factor 10 to 25 or so, diluting the monopoles fantastically. So you start out with a glut of magnetic monopoles. But by the end of the exponential expansion, the density is entirely negligible. You probably have one at most. And it might have been discovered at Stanford in 1982 by my friend Las Cabrera. But generically, I think it would be one, right? I mean, the universe would be less than one horizon reason.

So it would be one, one monopole at best in the whole universe. Yeah, I think they're even less. It's much less because the rise much bigger, but, you know, but one is, one at most. One being upper limit, right? Yeah, yeah, yeah. Yeah, one would certainly be an upper limit. And you write it. Might be that one that was found.

1982 or Valentine's Day, 1982. Yeah, but most likely it wasn't. Anyway, so the monopole problem is immediately solved by just, diluting them away. Flatness and horizon. Okay. Horizon is similar really. I'll do that next. The horizon problem, the way I described it, you'll recall, is that when if we follow two photons backward, two photons of the cosmic microwave background arriving from opposite direction, we follow them backwards to their source. Those two points were separated from each other by far greater than there could have been any communication between those two points in the conventional cosmology. What inflation does is it

adds this period of exponential expansion. And that means that if we follow the observed universe backwards in time, when we come to the inflationary period, it looks like exponential contraction gone backwards in time. And it means that before inflation started in the inflationary model, the region that is destined to become our observed universe is, is vastly smaller than anybody had previously thought. And it's while the universe was so vastly small that there was plenty of time for everything to become uniform, plenty of time for light to cross that region

zillions of times over and smooth everything out. And then once the smoothness is established, inflation takes over and stretches this tiny region to become large enough to include everything that we see. So the uniformity is just preserved. by that stretching. And we'll come back to that in a number of contexts. But, okay, but that's great.

And then the flatness problem? Flatness problem is, I think, most easily understood by imagining, for example, that the universe might be closed rather than flat. So there's our closed universe. All inflation does there, it's really very simple again, is it stretches it fantastically. And if you look at a tiny patch of the surface of a sphere, it looks flat. just like the earth looks flat does. Yeah, Kansas, yeah.

Okay, and it's that simple. It naturally solves these three fundamental problems. And the key thing that I want to stress here is that it's natural. It's not, it came out of thinking about a theory that had not been designed to explain the universe in any way. If there was a, if anything like grand unification, or if there were phase transitions in the universe, which are again generic because we know there was one, that separated the weak in electromagnetic interactions.

We know that such things happen in the early universe, that this is kind of the rule rather than the exception, that something like this is generic, and it's almost impossible to imagine that it couldn't happen if you had phase transitions. And you would sort of, I'm not, I want to, you would agree with me here because I know I've sort of, people have been beaten up on me being,

by being a little bit hard on Roger Penrose when I was talking about this subject. But I really want to stress it because I think it's true that this is kind of a generic behavior that is not at all invented to somehow solve some problems, but it's a behavior that happens and it turns out to solve the problems. Yeah, no, I think that's entirely right. And I maybe should add to make Andre Linde happy, that's even a little bit more general than what you describe.

Everything you said is true. But even without phase transitions, if you have a potential energy surface in field space, describing the potential energy as a function of the values of fields, Whenever you have a hill, if you start on top of the hill, which if you have random fields in the early universe, you'd expect somewhere we'll start on top of the hill, you can roll down the hill in exactly the same way. And you don't even need hill top. If you have a hill. A valley. It was Andre Linda who pointed this out.

If you start up high enough on the hill, even if you don't have a plateau, you can still have inflation as it rolls down and produces all of these same solutions. in these problems. Yeah, in fact, in fact, a very important kind of inflation that would, yeah, well, we'll get to Linday and that. And one of the things I should say is this statement about there's enough time to solve the horizon problem is something we'll come back to because, you know, I do want to talk about, sort of the discussions you've had, but Roger Penrose's concerns about inflation. And in some sense, they come back to that question of that early time, whether there was enough

time to get things uniform. But we'll get there. So that's inflation in a nutshell, which I think is essential to talk about because I want to spend time now talking about the more subtle questions that are still open questions. The beauty of inflation was there immediately. If nothing else, it got you a ton of job offers, finally in a field where it was hard to get a job. And I remember hard, it was a job back then. But I've got to ask you this. And this is what I mean by the prototypical Alan Guthian thing. You first talked about inflation in January 1980, this amazing thing.

And the paper appeared nine months later. So when it was it, were you just too busy talking about? I mean, everyone knew. I knew you'd been doing it. And I'm pretty sure I knew you'd been doing before I ever saw the paper. I don't know. But you obviously started talking about. about it and must have gotten a lot of requests to talk about it. But what was the reason for the delay?

Well, two reasons, I guess. One is just the goothian aspect of it. I do write slowly and try to make sure everything word I say is accurate. Yeah, you do. That is a characteristic of you, which I've admired tremendously. You try to make sure everything you say is precisely accurate and often catch me when I'm just sort of, you know, approximating things. So I, I, I really appreciate that, but go on. Okay, but there was another significant reason, which is that when I started giving talks about it in January 1980, I could point to these wonderful successes,

which I thought was real, and a lot of other people got very excited about it too. So it really did catch on very quickly on the particle theory community. But there still was an unsolved glitch. Yeah, a real problem. Which is how does inflation end? The assumption that I wanted to make at the end of January, but I knew I couldn't show, was that when the phase transition finally happened, it would happen by bubbles forming in different places, randomly as supposed to happen in the first order phase transition, and that these bubbles would then grow and collide and merge. And the hope was that they would merge smoothly enough so that it would be compatible with the smoothness of our universe.

inflation clearly created fantastic smoothness, which was one of its wonderful features, and that's more or less the horizon problem. But when inflation ends, it clearly creates a certain amount of chaos. And I didn't know if that would work out right or not. So most of the spring, while I was traveling around giving talks, I was also worrying about this question and talking to a number of people at the places I went to give these talks. And in fact, I acquired the crucial piece of knowledge to understand this problem at Cornell talking to, what's his name? Not Hans Bay. I'm going to take a pass. Okay, okay. It doesn't matter. Don't worry about that. But the thing is, I mean, it's a natural segue. This was a real problem. And in fact, there was no,

I mean, within the context of the original idea, there was no solution to that problem. And I suspect you were one of the people who showed that, right? Yeah, well, that's right. With the help of this mathematician at Cornell. Whose name? His name I can't remember. Unheralded, but a mathematician somewhere. Right.

You can write me later. He declined our offer to be on the original inflationary paper. Oh. Okay. But it was his insight that allowed me to show. I just had to extend it a little bit, really. He showed more or less on a checkerboard model how this system like this would work.

And I filled in the cross the teeth and dotted the eyes to show that it would work in a three-dimensional model as well. So the bottom line is that in the original model with a strongly first border phase transition, it really just did not work. So that's amazing. I mean it's a graceful exit problem. Yeah, that's what word to have down,

the graceful exit problem. which you showed there was no graceful exit. And it's amazing. I mean, that must have been a huge disappointment. I mean, I know it's an experience. I think a lot of us theoretical physicists have had. I've had it numerous times during the care where you come up with an idea and it looks to you to be great.

And you think, oh, it's all. And then and then you get craps out. And, and you know, this is just going to do everything. Oh, no. And then and the good thing about science is that, is a scientist. when they realize that, they generally are honest about it and don't try and sell it. I know some people who haven't been, but, but, but, and you just have to say it doesn't, you know, it's a neat idea, but it doesn't work. But it's actually useful to publish

neat ideas that don't work because it motivates other people to think about ways that might work, or at least, you know, and, and so I've learned that. There are a lot of times I never published something and I realized I wished afterwards because it became significant at the time I thought it's just not going to work. But you publish it and, and it was a big, disappointment to you at the time or did you just at that time had you been such a convert that you knew there had to be something it was it smelled too right i was still i was still pretty confident that that would hold up in the end um that it just seemed to have too much going for it and certainly the failure at the end of the inflationary period uh separated in time from the success at the

beginning and during the inflationary period. So I did have significant hope that somebody would find a better ending and that the whole thing could be made to work. But of course, I was disappointed. I certainly would be much happier with everything. Yeah, yeah, yeah. But it's interesting with such a amazing idea that already before anyone knew how to get out of inflation and before anyone found a graceful exit, you had already, I mean, that your career was now, I mean, you already had numerous job offers. And I was around. I was in Boston.

And every place I was at, whether it was MIT or Harvard, was talking about who would try and get Alan Goose there at the time. And so, yeah, it was that significant that there was a face transition in the physics community that suddenly, I think everyone thought that this is just too good an idea not to be true. And by the way, it was interesting as a physicist versus astronomers. because most of my after that, my career,

I've always been in both departments. And for a long time, it was kind of interesting to see that the physicists had already decided the universe had to be flat because that was a generic prediction of inflation. And we'll point out that inflation can do many things. And when it didn't look like the universe was flat, a lot of people came up with inflationary models that didn't produce a flight universe.

But those, I would think, are fair to say are the exception rather than the rule. But the astronomers, they, that's garbage. they didn't believe the universe is flat, at least for a long time. And it was an interesting bifurcation that the theoretical physicists were so enamored with this idea

that they were convinced, even in the absence of any evidence, the universe had to be flat. It's really kind of amazing. So the physics community was convinced that it was great, you got a job, and then what happened? Well, I'll say,

in terms of the theory development, the next very important step was one that would not involve me, except I read about it, which was the discovery of what was initially called the new inflation in our universe, which was the first solution to this graceful exit problem. It was discovered independently by Andre Linday in the Soviet Union and Albrecht and Steinhart at the University of Pennsylvania. And the idea was that instead of having the phase transition happened sharply by the formation of many little bubbles,

which is what we call first order phase transition, it would be a type of second order phase transition. The way it was often described at the time, which I think is still good, is it's a phase transition that's more like the congealing of jello than the boiling of water, something that happens gradually over a region of space, more or less uniformly.

And that could produce regions of uniformity more than large enough to describe the universe that we observe. So the simple way to frame that for people is to say that instead of being many bubbles of new phase forming, our universe existed inside of one bubble. One big bubble, right. And that, by the way, is one of the reasons one could argue that there's not more than one monopole in our universe because the monopoles tend to form at the intersection of bubbles. And so that was a complete, and that's generally, you know, that's the picture. Then that works, at least. It works in principle.

We'll get to the fact that, well, I want to ask about your biggest disappointments and your biggest excitements regarding inflation before we move on. But, well, look, let me say, let me ask you, I'm going to lead you rather than, well, maybe I should, but I'm going to lead you. Go ahead. Some people could say, well, inflation is a postdiction. Look, you know, these problems and inflation solves them, but what the hell does it predict? And the real surprise to me, and I want to ask if it's to you too, and you certainly could say no,

was the surprise that came a couple of years later. And I'm surprised I'm very sad about because I was at Harvard at the time and I was invited to this meeting in Cambridge. And Harvard wouldn't pay for me to go. And it seemed like. At the meeting, everyone who was at the meeting basically wrote a paper on this subject. But, and you certainly did. But the real surprise, which really to me smacks, it's when a theory makes a prediction that you didn't expect.

It solves a problem, but when it makes a prediction and then it's shown to be correct. And that was this prediction that the universe wasn't exactly smooth. The biggest mystery in some sense in fundamental physics, if you think about it, was not, you know, why are we here? which is not why is the universe so smooth, but why isn't it exactly smooth? What caused these small lumps that eventually became galaxies? And I think it's fair to say no one had ever expected other than having a theory of everything

or a religious experience saying God did it, to have a fundamental physics understanding in our century of why this might happen. And what you showed and others is that it's just quantum mechanics. If you have inflation and you have quantum mechanics, Boom. And you get something which is later on been seen in the microwave background, a small, beautiful what's called isotropic spectrum, but a scale and variance spectrum, as it's called.

And we don't have to go into those details too much, which has been the subject for observers of Nobel Prizes. But it's a generic prediction of inflation. So do you want to, was that for me, that was the biggest surprise or the biggest thrill. I don't know if for you, was it a bigger thrill to discover inflation or to discover that inflation predicted those? That's what I want to ask you. A little hard to say, I guess. The story of the density perturbations is one that I think drags out over a much longer period of time. Yeah. The calculations were done actually in an incredibly exciting way.

Yeah. This Nuffield workshop that you alluded to. It was unbelievably exciting meeting of us there were. four different groups that are working on the same subject. Yeah. We would talk to each other late at night and compare notes. And initially, people got all kinds of different answers. But by the end of the meeting, we all agreed,

and we all went out and wrote four different papers, giving the same conclusions. And that way was extraordinarily exciting. However, at the time, at least in my own assessment of the problem, it was only a game. I never believed that anybody would ever measure these crazy things, these tiny fluctuations and the cosmic microwave background radiation

fluctuations that are only one part and 100,000 of the radiation itself. And the radiation itself had only been discovered a decade before because it was so faint. It was very hard to see. We figured that you'd never be visible because even if it was there, there'd mean just too much garbage out there to get in the way.

I mean, an astrophics full of dust and all sorts of things. I for one never, and I ran meetings on the subject, but I never felt that the observers were ever be able to disentangle all the noise and get to the signal. Yeah, exactly, me too. So I was shocked when Kobe came along and gave the first measurements of these non-uniformities

and then more shocked with balloon experiments like Maximo that showed the evidence of this first peak in the spectrum, which was also, a key prediction of inflation where that beach should be, and it seemed to be right where it was predicted, and then WMAP and then Plunk and a number of other ground-based measurements, incredibly exciting. I completely agree. And I also agree that certainly when inflation was first put together, when I wrote my first paper, none of this was anywhere near anything I was thinking

about. So it really was a surprise. It was a surprise, and it Really, I guess to me, when I think about theories, as I say, the fact that when a theory, the event for one reason comes out to have an explanation of something else that's right, it really makes you, if you didn't have some kind of faith, if you want to use that word, and I hate to use that word, but that the theory is right, when it starts making other predictions of things and they turn to be right, it really gives you great confidence. So I guess for me, it's hard to know.

It was a great excitement about inflation. And I think it was really more physical physicists, again, patting themselves in the back saying, we've got a great theory of the early universe. But whether you really believed it, just, hey, we can do this was with the excitement about it. But then it was, I think it's really the fact that it predicts what's been seen that really is more convincing that, hey, it's really probably right. Yeah, no, there is an incredible hand of detail. detail that's seen in the cosmic microwave background. And it's even more than the cosmic microwave background.

The fluctuations continue to shorter scales with the barrioan acoustic oscillations and Lyman Alpha forests. And the predictions of inflation seem to work wonderfully. They work wonderfully. Now, let's, you know, I want to put the devil's advocate. I want to put a perspective on this because you might say if it's so great, you know, it, why, why, you know, why, you know, why isn't known the Nobel Prize? and all this other, you know, arbitrary prizes.

You've won a lot of prizes, so that's okay. But, but, but, but, uh, it's fair to say, first, it's interesting to me, again, it's sociological that while inflation predicts it, other people had basically said the only kind of spectrum you could have that's consistent with observations without knowing why would be the spectrum that's seen. So, so it's kind of, you know, in some sense that the, the cosmological theorists that already said, But this has to be what's there because anything else would have been long inconsistent with either too many black holes or too much stuff that you would have seen already. So the fact that it kind of had been guessed in advance might have been one reason people to say, well, you know, obviously any mechanism has to produce that.

And sure, inflation produces it, but maybe there's some other mechanism that produces it. I think it's fair to say from the sort of devil's advocate point of view saying, well, yes, this is one example, but how do we know it's the right example? Okay. The other one is, I think my biggest disappointment of inflation, and I want to see if it's yours. And people be mad at me for just not asking you and throwing this out, why. But is the fact that there isn't, you know, it wasn't like the standard model that there's some model that works, you know, is that inflation, there's a whole lot of models. And some of them work, some of them don't. But none of them, it's not as if everyone's point to say, that's the particle physical. model that clearly produces inflation. It's obvious. That's the fundamental physics. There hasn't been

such a thing. And I think that's for me been the greatest disappointment. I don't know how you feel about it. Well, there certainly was a stage in the development of the standard model that was the saying. Yeah. There are a number of different field theories that were being considered. Quantum field theory itself is very flexible. And the physics of inflation is really just quantum field theory plus general relativity. Yeah. So it has all the flexibility of quantum field theory built into it. So it is true that one can produce inflationary models, which have rather different

predictions from others. Surprisingly, I think what we're seeing in the universe is really the predictions of the simplest possible inflationary models, which to me is kind of a miracle. It didn't have to be that way. Certainly didn't. Inflation could be entirely right, but it could be a much more esoteric kind of model that is producing the inflation. So I think it's going to be a very slow process to narrow down the physics of what drove

inflation to a specific model. It may take a century to reach a specific model. I don't know. It's hard to see how we can reach a single model anytime soon. But did you think, now it's a class of models. Yeah, but that's easy. I mean, that's a kind of, I agree. that's a sage analysis.

But did you think maybe that early on that there might be a single model that would jump out, that it would, you know, that would, that would seem so obviously right? Or was that, you know, just ask, you know, didn't. Yeah. Well, I guess, I think probably the, that's, you're asking me to think back about my own belief in psychology as time involved. Yeah, it's hard to remember what you thought back in. I think the answer is that I probably did believe that through the calculation of these density perturbations.

And what I believed is that the SU5 grand unified theory had to be it. Yeah. I guess it was just so simple and so elegant and seemed so much the natural extension of the standard model of particle physics. Yeah. But then, of course, that ran into trouble in several ways. The prediction that the theory made for the lifetime of the proton got shown to be wrong. And also from the point of view of density perturbations, the initial calculations we did of density perturbations show that they were of the right form, as the predictions had the right form. But when applied to something like the UNified, the SU5 grand unified theory, it produced density perturbations that were far too large.

So it had to be some other theory. And I think at that point is probably where I realized that even though we were. we're pretty lucky with the standard model and guessing the right, the Grangian fairly early and the history of it. It might take longer this time. Yeah, no, that's right. I mean, it's, again, it's worth talking about this. In the psychology of the field, I think, in 1980 and between 1980 and 82,

this one Grand Unified theory looked just so clearly right and so simple that everyone, I think particle physicists assumed it would just be a matter of time because there was actually a way to look for it, the K of the proton, that it would be seen and everything would fall into place. And then it wasn't. And then things started getting more complicated. And not just in inflation, but in particle physics itself.

And particle physics has also moved in the direction of not knowing what, if any kind of gran unification happens and lots of additional complications that have caused physicists to look in lots of directions, including string theory. So that's that same kind of. initial optimism has sort of diverged into realizing that it's got a lot more. I was going to say a final solution.

I don't like that word, but that if there is a final solution, it's a lot more complicated and a lot longer road to get there. And many people, myself included, I'm not sure there is, you know, that that's the right way of even thinking about things. But anyway. So what we're talking about that, though, let me mention one other big shift in my faith and guesswork in theoretical physics is, as you might guess, the cosmological constant. The fact that the vacuum energy appears to very distinctly not be zero.

While until 1998, I was very much convinced that there was only one simple solution to that question. It had to be zero. I know. I mean, that was a different plan that was discovered to be non-zero. And that further blew apart any faith I had that we could just guess the laws of physics. Yeah, no, no. I mean, and obviously that hits home for me because, as you know, I, as you may or may not remember, I proposed that the data had told us the cosmological conflict wasn't zero in 1995. But the only reason I did it was, I mean, besides the fact that the data, in my opinion, pointed that way, was because I was convinced some of the data was wrong. It was really a paper to say, look, this is so stupid and so crazy that some of the data must be wrong.

And no one was more surprised than me than when it worked out to be exactly what we said it was there in 1995. I mean, it was really because it just is so crazy. You're right. And that was the other big surprise that's changed things. But in a sense, in a sense, however, it validates inflation as an idea as well, right? Because we now know we live in a universe that's inflating, albeit at a much smaller rate than the one that you argue. would solve the problems of cosmology. But we are living in an inflationary universe right now, right? That's right. That's right. So it's a clear demonstration that's possible, although I think

most theoretical physicists never doubted that it was possible. So that's not too big a thing among theoretical physicists. But as far as selling the idea of the public, it probably helps a lot to be able to say, I know that the universe is inflating now. It does. But of course, that leads to another question. And I want to start talking about sort of these open-ended issues, some of which will lead us to Penrose. In the last, if you don't mind, maybe 15 or 20 minutes of this or so, we'll get to do two hours or so. The, it does, the many you say that, of course, we remember the public or anyone, you think, well, okay, the original inflation ended. What's going to happen in our, in our universe, is that can end? And it really depends on the amazing thing is that most theoretical physicists somehow think,

or I'd say the majority, I think that what we're seeing now is this fundamental energy of empty space and not something that's stuck in a field that's going to have a phase transition. But it could. And I don't know if you have any thoughts or if you have any since we've now established that we're all crappy at guessing what the actual answer is. It's fair to say. What's your bias? What do you think?

Do you think it's fundamental or it's something that may one day dissipate? I'm not sure if I would describe it as fundamental, but I guess I think it's more likely not to be something that will dissipate. I have in mind a complicated landscape of states that have different vacuum energies, metastable states that can undergo transitions from one to the other, and that the vacuum we see is one of those states. I think it's less likely that it would be something like a scalar field

that was in the process of rolling down the hill and its potential energy diagram, just because that involves more parameters and it seems to me that that involves more special choices and is less likely. Well, let's talk about likely unless... I will go back to reiterating that. I do think we're not.

very good at guessing these things. So that could be wrong. And that, and you know, it's great. Let me point out again for people. The fact that we're not that good at guessing things is a strength of science, not a weakness, because it means we have to keep looking. Nature will tell us the answer.

And that's why we have to keep doing experiments and observations, because it's the only where we're going to find the answer. It's not going to be a bunch of people sitting in a room without windows arguing at a blackboard. It's going to be some other way. But having said that, let's talk about your preferred, I mean, let's talk about internal inflation, your preferred kind of models, where you think inflation is now, and where you think sort of the issues are and what's the most attractive possibilities. Okay, very good. Yeah.

Now, we should definitely talk about eternal inflation because I have a strong interest in it and a strong belief that that's probably the way the universe works. And that seems very attractive as well. And of course, it'll get us back to the beginning too, but go on. Right. Yes, it will. So to set the stage, eternal inflation refers to the fact that for most, if not maybe even all, successful versions of inflation, when inflation ends, it ends because of random things that happen. So it doesn't end all at once throughout space, uniformly. It ends in places and continues in other places. and in the places where it ends,

it begins a Big Bang evolution, which leads to a local universe. I like to call them poppy universes. And when inflation ends over here, a pocket universe forms here, and then the pocket universe forms there and then there and there. And meanwhile, the whole background is exponentially expanding. So the number of pocket universes that is produced is ultimately infinite,

and even the rate of it could be. being produced grows exponentially. So we get not just one universe, but for free, we get an infinite number of universes. This is often coupled with the idea that the underlying laws of physics very likely allow different kinds of vacuum to exist. And this is certainly a feature of string theory,

but if string theory is wrong, it could very well be a feature of some other fundamental theory. Right now, string theory is, I think, by far the best guess we have for a single theory that might describe the world. So that would mean that when these different pocket universes form, they can form in any one of the different possible vacuah that the theory allow. And that can mean that the pockets could look very different from each other. In these different kinds of vacua,

because there'd be different kinds of ways the vacuum can bend and place, different ways it can be excited. the low energy laws of physics would look completely different from one pocket to another, even though I am talking about a system where I'm imagining that the ultimate underlying laws of physics are the same everywhere. I don't really know how to talk about anything that would go beyond that. It's hard enough to understand one set of laws of physics to speculate about other sets that might exist elsewhere as far as I'm concerned, not a useful exercise. So I'm only talking about a region of space that is government.

by a single set of laws of physics, but nonetheless, different pockets can behave very differently. And that gives underpinning to these anthropic arguments that previously, for me, seem somewhat crazy. But applying it in particular to the cosmological constant, which is one of the big problems and theoretical physics and cosmology. And this is the problem of why is the vacuum energy density so incredibly small by particle physics standards, by what seems like a reasonable standard to compare with the vacuum energy that we observe is about 10 to the minus 120th of what we might expect. Well, I'll use the word, what we might expect is what we call the plunk scale. Yeah, yeah.

And that doesn't have to be what everybody expects, but it's a reasonable number. even if it's 10 of the 60 times smaller it's still it's still 10 of the 60 times smaller than that that's right exactly even if even if you don't want to go to the blind scale even if you just want to talk about the energy scales of the LHC for example you're still 60 orders of magnitude away from that is the observed value of the vacuum energy is still about 60 orders of magnitude smaller than what you would estimate so it's a big mystery but one of the possibilities of the possible resolutions, which I think is a very real possibility, is that with this huge infinity of pocket universes, each of which would have its own vacuum energy, some tiny fraction of them would have vacuum energies as small as what we observe. And then one wants to argue that those are the only places where life would form.

It's certainly the case that if we had a pocket universe where the vacuum energy was at this plunk scale, the expected scale by some standards. If that vacuum energy were positive, the pocket universe would implode in 10 to the minus 43 seconds or something like that. And if it were negative of that magnitude, it would fly apart in 10 to the minus 43 seconds. So it's very easy to believe that life would never form

in pocket universes that are typical, which have vacuum energies of the order of the Planck scale. So the argument goes that perhaps the reason why we observe such a tiny vacuum energy is just that life only forms in universes that have vacuum energy is that small. And that explains the problem. Well, let me let me let me step back and parse this a little bit. One of the things I do, and it'll be the rare case where I'm trying to make what you say more precise rather than the other round. Okay, go ahead. Because we've maybe, well, we've debated this in different contexts, but I think one would say, it's fair to say that where life like ours would form.

I mean, so the argument works, even if you don't, I mean, there could be life in other, there could be living systems in universes that are very different than ours. So, I mean, we can't imagine. And by the way, one should point out that the person who came up with this argument was Steve Weinberg again, having you with the cosmological constant in 1987 before it was observed. as well. And the point is that she deserves credit for coming up with the idea, but he certainly is the first major physicist to support the idea. It's to support the idea. Well, yeah.

I mean, in 1987, it was modern physics. It's a fair place. I saw it written down anyway, but I, and the idea is really even simpler almost than said, is that you need galaxy, as far as we know, you need galaxies to form us, because you need stars,

and they needed planets for you and I to be here to talk. And in order to have galaxies form, you have to not be expanding too fast because if there are pulsive force is bigger than this sort of detractive force that causes matter to collapse, you wouldn't have any matter collapsing. And so to have a universe with galaxies and stars, it turns out the cosmological it can't be much bigger than what it's seen to be. And lo and behold, it has that value. And that certainly lends credibility.

But I think it's fair to say that it's important to point out that we don't know the locus of possible life. I mean, it could be that life could exist in universes with vastly different laws of physics. It just wouldn't be life like ours. And those life forms might be asking the same, exactly the same question. Why does their universe seem so nicely tuned to their existence?

And I think it's worth pointing out that what the multiverse that you're talking about does naturally is explain potentially why the energy of empty space is what it is in a universe that looks like the universe that allows us to live in. It doesn't say that there couldn't be other universes with life where people wouldn't be asking that question. I think that's it. The reason I'm saying that's important is because some people think this is an incredible fine-tuning problem or the multiverse is invented by people like you and me to just either get rid of God or to try and finesse this problem. And the point is, once again, it wasn't developed as something anyone wanted. I don't think you or I probably would have liked the idea of a multiverse when we're studying physics. We wanted there be one universe that has to look like the way it does and that there aren't other possibilities.

We're sort of drag, kicking and screaming to realize that this is a likely possibility, first of all. And then secondly, that if it's a likely possibility, it happens to potentially be a natural explanation of something that's very weird, namely why the energy of empty space is what it is. So I think I want to just sort of once again stress that we're trying to explain why it is natural in a universe that looks like the universe we live in to have a cosmological, to have an energy of empty space that's absurdly small. The multiverse solves that problem. Okay, I guess actually I do have a slightly different take. Yeah, I know, I know. We've made it this in the past, but go on. I want to hear it.

Right, right, right. I guess I have a stronger criterion for believing that the explanation is plausible. It's probably the right way to phrase it. I would say that if the life like us was really an important restriction and that there was lots of other lives that was not like us, that they outnumber us by a huge amount, that that would make it not a satisfactory explanation. The way I would look at it is I would say,

if we are very unusual, you know, say we had a spot on our left ears, identified us as being unusual types. And if you convince me that people who have spots on their left ears should live in the universe like us, but for every such person there's a million who don't have spots on their left years, it would see a completely different world.

I would say that that's not a good explanation for the world that we see. Why do we happen to be the people who have spots on our left ears? So I would like, in order to accept this explanation, that it ends up predicting that essentially most, doesn't really have to be all, but most life in the very general sense would live in regions that have very low cosmological constants. Okay.

And I think that's possible. also. Yeah, absolutely. It is. It is plausible. And I think, and this is where we get to, I think it's important. This is where we get to the subtle questions that really are the kind of things that people debate now and hope and may one day be resolved by observation or experiment. And we'll get to that. Is this, these, these sort of subtle questions of whether, of how, of the kind, The problem with all of this, to be fair, is that when one is trying to calculate how likely or unlikely something is, you have to have, you have to know about probabilities, and you have to know about the face space of possibilities. And the reason that we can vastly differ in what we say is that we don't know the probabilities. We don't know the phase space. So this is all, and until we do, it's fair to say, I think this has to remain sort of speculative.

Yes. I completely agree. Yeah. And it's perfectly okay for us to disagree. And I even claim that we have good evidence that we're right. Yeah. And, you know, we don't. And so it's an open question.

But it is a fascinating fact that I think, once again, a corollary of inflation. I think I said some people use a different way of pronouncing that word, but I say corollary. Anyway, is an unexpected result. is the fact that a multiverse is a generic prediction of inflation rather than the other way around that. It really comes back to your original problem. Inflation doesn't end gracefully. Yes.

And the point is in the multiverse, it doesn't end gracefully. There's universes being created all the time, and some of them may collapse or other, you know, I mean, there's big bangs all the time. And it's going on eternally. And it never stops for the reason that you kind of realized at the beginning that you couldn't solve what you wanted. And so the fact that inflation goes on internally is a generic property, but it's not a negative. It turns out to be a positive because one of the side benefits besides predicting fluctuations is that it gives a possible explanation of something that's really difficult to understand otherwise, namely why the energy of empty space is what it is. Now, let me point out for people who are going to say that, okay, well, this is all speculation. you guys aren't you know this is all science fiction and this has been important to me because

I've written about this a lot and thought about this a lot and and predicted some things in advance but this can be tested and people say how could you ever test the existence of the universes and and as you know I've been thinking a lot from the early stages of Kobe and maybe you don't I think you know about observing what are called gravitational waves from inflation which one could say, as we said earlier, inflation is an idea, and it comes from just the merging of quantum field theory and general relativity. And therefore, it's an idea more than a model. And therefore, there's lots of possibilities. And some people would say the negative feature of inflation is that can explain almost everything, no matter what it is an inflationary model could probably explain it.

And that's been pointed out as a negative by some people who like to criticize inflation. But one of the things that inflation does produce almost uniquely is a background of gravitational waves of a certain type, which could be detectable. And for a while, we thought had been detected for a brief moment of excitement in your life and mine. we thought had been discovered, these certain modes in the cosmic microwave background that would look for gravitational waves, which are pretty well unique prediction of inflation. And so I would say that these ideas are testable, and tell me if you think I'm overstating it, that these ideas are testable because if we could measure gravitational waves from inflation, we could hone in on the kind of inflationary model that might exist.

And that model would tell you if there was internal inflation. And if that model told you that, then it would be clear indirect evidence that multiverses existed, even though we'd never be able to take them correctly. And I would have said that that's not much difference than the indirect evidence that Adams existed well before you could ever see Adams. Am I overstating the case? No, and I think that's right. And I do sometimes make exactly that point,

that we are narrowing down the possible physics of the field that drives inflation, the inflaton field, and we can hope to eventually narrow it down so much that we'll be able to tell whether the inflationary model that best fits the observed universe also predicts a multiverse. I think it's a very important principle, which I think is real, that you don't have to confirm every prediction of a theory to decide that the theory is right. If you confirm enough predictions and don't find any discrepancies, you can become convinced that the theory is right. That certainly is ultimately what's true about everything.

Even Maxwell's equations predicts lots of things, huge scales that we've never measured and never will measure. But we believe that if we ever observed those scales, they would have been Maxwell's equations. There's no reason to believe that it wouldn't be anything else. And the simplest argument is that it. And I think that's for me, what it's fascinating is that turns what is metaphysics and definitive. because a lot of people criticize multiverses, and the point is, we may eventually, it's plausible, although not guaranteed, that we will be able to have measurements that tell us that there are other universes out there without ever knowing, without ever clearly ever being able to measure them directly. But empirically, because the theory, if it explains 50 things that are measured and the 51st you can't measure, as you just say, well, it's highly likely to be there. And so we'd accept it. Just the way we accepted atoms well before we could ever see them.

And that, so that argument against inflation, I think, is, or against multiverses is really misplaced, and I wanted to stress it. First of all, because they came out of the theory, they weren't put in. And secondly, we were able to measure them. And the other thing, so measuring gravitational waves, which I've been pushing for since Kobe, I think, is important for that. But also recently, I've been excited because, as you probably know, I argued with my colleague Frank Wilczek, that if you can measure gravitational waves from inflation, you'd also prove that gravity is a quantum theory. because those they wouldn't because if if gravity wasn't quantum mechanical, then those inflation, inflation wouldn't produce gravitational waves. And I think that's really remarkable.

I don't know if you're familiar with it, but I'm very excited. Now that you mentioned, I did hear about it. And yes, I agree. I agree. Because I guess I assume gravity has to be one mechanical, so I don't worry about it much. But you're right, it's one of these fundamental questions. And there are some people who think that quantum, and there are some people who think the solution to this problem of the,

of merging quantum mechanics and gravity is that quantum mechanics stops being true at some fundamental scale, including people maybe even are at a tuft and others. So there are some reasonably good physicists who would argue that maybe you don't have to have both of them. And that's why I find the possibility of empirically measuring it to be so exciting. Because if you did, then it would just remove that whole possibility

of philosophical and maybe physics speculation. Okay, in the last few minutes, let's go back to the beginning. because, you know, inflation generically predicts, as we say, that the universe is eternal, which means it continues going on once it starts, but it has to start. And I'm happy to say that you and I agree about, well, many things, but you were the first person that I've ever heard say the ultimate free lunch, that inflation gives us a universe, which is the ultimate free lunch, but it may even be. be more simple than that, that quantum mechanics can actually produce universes for free,

because the total energy of our universe could say be zero. And as you know, that was the arguments I gave in my book, the universe from nothing. And inflation gives the motivation of that. But I think we are agreed that that seems to be both of us the most likely possibility, that universes can literally space and times can pop into existence from nothing and the ones that can survive are the ones that probably have zero total energy in one way or another and end up looking like ours, right? Yeah, no, I certainly think it is important that our universe is consistent with having zero total energy

with the negative energy of gravity canceling the positive energy of everything else. And I think we know that that is a highly accurate statement, if not exactly true. And I think that does mean that our universe can arise from nothing. in terms of what the history of our universe actually is if we followed it backwards, I now have actually two favorite alternatives, one of which is the one you just mentioned, creation from nothing through a quantum transition. I'm also now rather keen about an idea, which I think is really originally Sean Carroll's, the idea of a two-headed arrow of time.

I've heard you mention that, and I'm suspicious about it, but okay. Okay, so I'll describe it for you and their listeners. The idea, and we claim that this is a generic behavior for many kinds of mechanical systems, a crucial feature of the underlying mechanical system, and I'm now thinking of our universe, as being one mechanical system, a crucial feature for this to be possible is that there has to be no upper limit to the entropy of the system. The system has to be capable of getting bigger and bigger with more and more entropy. And an eternally inflating universe certainly seems to meet that belt. It seems to be able to increase its entropy forever without any clitches.

So we haven't mined eternal inflation here, but we think of it really in terms of a simpler model. The idea is that if you start such a system in a more or less arbitrary state and follow it into the far future, one would expect the entropy to grow, if you start at most any state, for a long time the entropy grows. So here's our space time diagram. We start here. Starting here makes it a kind of a logical beginning, but it won't really be a chronological beginning, because I'm going to talk about what happened before. Logically, we started some arbitrary state here and followed to the future, entropy grows. But if we follow it to the past, because the laws of physics are completely time symmetric, and we just chose a random state, everything that we said about the future, about the future applies also to the past,

entropy grows to the past. So if you define the arrow of time as the direction that entropy is growing, it becomes a two-headed arrow of time. I don't know if I'm entirely on the screen here. Yeah, you are. With the arrow of time in the future pointing to the future and the hour of time in the past,

plain to the past, and a finite region in the middle where the arrow of time might not be defined. That means that in almost all of space time, really space time be infinite and only a finite region where the arrow of time is ill-defined. So the arrow of time would be well-defined everywhere, almost everywhere, I guess is the way the mathematicians would describe it, everywhere except for a set of measure zero. And it gives a very natural explanation for the existence of an arrow of time.

It seems to be almost unavoidable. Yeah, I mean, people have tried to tie the arrow of time to a, growth of entropy in good and bad ways. Stephen Hawking, as you know, tried to do it in a very bad way once because he'd argue that, that, you know, somehow time would flip if the universe was contracting. And I think we all agree that's not true. Yeah, so I guess I'm, you know,

I knew I wanted to give you a chance to give an explication, if not an explanation of that idea. But what it does do, and again, I think it's interesting, because it does suggest in a way that's not that different than maybe, at least in spirit, than Hartle Hawking is that there doesn't have to be beginning of time, that while, you know, that there doesn't have to, time doesn't have to have a beginning, even if it, even if the universe isn't eternal in that sense, in the traditional sense, when you go back to what you think is the beginning, it may not be a beginning.

Right. The notion of time might change dramatically. And that's one example of it. But I mean, all of those are, I find, I mean, those are just, yeah, there's just speculations because, of course, we don't have a theory of the beginning. Yes, it's, and I think it's worth, but it's important because it obvious, it's one of those things, which, as you know, I've had the misfortune of having to debate at least one, um, apologists who, who used a result of yours and our good friend Valenkin, and, um, which seemed to suggest that the universe had to have a beginning. But that was only if you, that's true. If you assume there's, there's no, there's nothing new, no new physics, basically. And then, and we all expect there to be new physics

at some way. So it's fair to say that. Okay. But if we go back to this beginning, the last thing I want to do talk about is to touch on, on, on, on, because I did just, as you know, do a podcast with Roger Penrose. And he's been a long time critic of inflation. And I've, I've always thought for the wrong reasons myself. And I'm sure, to be fair to him, you and I are, you know, of the same view here. so we may be giving him short shrift. Some people, as I say, thought I gave him a kind of a hard time when I talked to him. But I think I was being fair in the sense that what we now know suggests that inflation is kind of like cancer in a way in the sense that you can't, it just, you can't get away

from it in one way or another. And the hard part is getting it away from it, not having it happen. And therefore, to argue it somehow the world is fine. tuned and he would go back to this question of entropy at the very early times and i and i and i think you've listened to the argument i gave him but i want i want to give it to you here and then i'd like you to pick it apart especially if i miss if i said it incorrectly is that there will inevitable as he might argue that it's incredibly improbable to have a region as you say you solve the horizon problem because the original universe is very very small and therefore this time for it to

homogenize there's time for and he would say well but you know entropy arguments would suggest it'd be very in homogeneous and that it you know you wouldn't expect to find a region which would be uniform enough for inflation to happen but my argument is that that probability argument doesn't work because the minute there is such a region you're guaranteed you're guaranteed that almost all of space in the long run has to be part of that region because inflation is eternal. Is that argument, is my argument wrong or right? I think it's essentially right. There is a catch, which is that we don't really know how to talk in any rigorous way about fractions of space, time. Yeah. This measure problem, which we haven't mentioned yet, but maybe we should

tell all our listeners about the measure problem. Okay, why don't you? The problem is, this is a feature of eternal inflation that we don't completely understand. Some people argue that because we don't completely understand it, it must mean the whole theory is wrong. That's not the way I look at it. I look at it at something that we don't yet understand. Yeah, yeah. It's like I used to say that evidence of not understanding something is not events for God. It's just evidence that you don't understand stuff. Yeah, that's right. It's important. That's right. It's fairly human to not understand something. Yeah. So I think we just need to accept that. So the problem is that because eternal inflation produces an infinite number of these pocket universes,

trying to describe the statistics of them, what fraction of them are blue or property that you might invent. The number that are blue is infinite, the total number is infinite. The fraction that are blue is infinity divided by infinity, which is not mathematically well defined. It can be defined, but you can find in many different ways. I think that's the way to say it.

Not uniquely defined, I'll be qualified that. Yeah. But yeah, not uniquely defined probably is actually a much better way of saying it. Yeah. Because our goal is to find a way to define it by some kind of a regularization procedure. And we do have ways of defining probabilities in eternally inflating universes, ways that we've just made up. But which gives sensible answers. It also turns out, by the way, that sensible sounding recipes can sometimes give very nonsensical answers when you think about it more carefully.

That was one of the surprises in this business. We won't go into details there. Yeah. So it's a non-trivial issue to decide if a recipe gives sensible answers or not. And we do have recipes that give sensible answers, but what we still don't really have a clue about, as far as I know, is anything that really determines what the right answer should be. And that's important.

So there definitely is an important missing piece in this puzzle. And for that reason, I don't like to talk about, fraction of spaces are inflated and what fraction happens. It does depend on having an answer to this question. I guess so, but I guess my point was that once it happens, it's hard to stop and then. Yeah, no, that's certainly true. And it's hard to beat an exponential. Yeah, yeah. No, all that is true. So I think at the intuitive level, what you said is definitely right. And at a more precise level, we certainly do have ways of defining probabilities, which we can think of us being part of our assumptions of the theory for now, although in the end, I think we definitely want to have a way of

deriving the procedure for not getting probabilities. We do have ways that make everything fit together, everything's plausible. Well, I want to give you a chance to just sort of respond here clearly to this question of entropy, which I still, I think it's a non-issue. I kind of get a feeling like it's a non-issue, but it's a very big issue to Roger. And so why don't you, if you want, to take a few minutes to describe what you don't think, well, what your take on the problem is? Yeah, sure, sure. Okay, my take is that if whatever we assume happened before, if we could imagine starting with a nascent universe, and in my two-headed hour of time picture, this would be a region near the neck where the hour of time is not that well defined yet. As long as one can find in that region, I'm thinking this is a big region, which has lots of little regions, as long as one can find in that region, a little stack that has the right conditions to start inflation.

And those are pretty generic. It just means that you have to have in this model of scalar fields, we need to have, for example, and there are other possibilities. but for example, we need a local maximum and a potential energy function in a region of space where the field happens to be sitting on the top of that maximum. As long as something like that happens somewhere in the region, and it has to have a certain minimum size, but that minimum size is very small. Then the physics of inflation takes over and builds an entire universe from that. And the initial entropy is the entropy of a tiny spec. So I don't think there's any problem explaining why it's small compared to the energy of the interpy of the universe today. I think it's exactly what the theory predicts.

And what the theory predicts in terms of entropy is I think exactly what everybody thinks happened. The reason starts to exponentially expand. When it does that, the entropy density goes down to essentially zilch because of the huge expansion. One second. Come back here. Sorry, I'm telling my dog, go on. Yes. Okay.

Come back. So during the exponential expansion, the total entropy, the entropy density in this region disappears to zilch because it just gets so incredibly diluted. And meanwhile, the space time just naturally develops into the cedar space. That's really a theorem that if the energy momentum tensor is dominated by this vacuum-like energy, the energy associated with a field on top. of the hill, for example, that leads to evolution of space, which is called the sitter space, which is exactly what inflation uses. It becomes smooth because of the exponential expansion.

And then at the end, locally, something happens like the scalar field starting to roll down the hill. That releases a huge amount of energy into the fields without exciting the metric, because the interactions with the metric are incredibly weak. Gravity is incredibly weak. And you're left with exactly the state that Penrose describes it as incredibly implausible. A state of high intensity of the manner and a smooth spatial background. Yeah, no, I mean, his argument, a priori is that, yes, why does matter have so much entropy? Is there so much stuff?

But gravity doesn't, namely, why is it so smooth? And that, if you ask the question that way, abstract. it sounds unusual. But then if you have a physical mechanism that makes it happen, it's not so unusual, I think. And this is a generic physical. And in fact, the first thing I guess I learned from you, maybe, I don't remember,

but when I heard first trade about inflation, is that it produces an incredible amount of entropy. One of the big problems of understanding is why we have a hot universe. Inflation produces, what it's great at is producing entropy. And it does. And so it's not too surprised. I mean, that's what it's built to do.

almost. That's really the reason it solves all the other problems in a way. Yeah, yeah. In some ways we should maybe say that the flatness problem can be rephrased as the problem of where did all the entropy go on? How do we develop so much entropy? And the way to make the connection is that the flatness of the universe really is described by how many particles do you have over a region of a specified amount of curvature. Exactly, exactly. And when, and it's fair to say, just to make a nice ring, things together, then that very problem that, as I said, caused particle physicists to think about cosmology, the why do we live in the universe of matter and not antimatter is also a question of why we

live in a hot universe, which is really why there's, why there are between a billion and 10 billion photons for every particle of matter, which is another way of saying, why is there so much entropy in the universe? So it all comes back down to that question in a sense. So I think it's, that's where we, I guess, where you and I would differ from Roger is that phrasing the question abstractly makes it seem like there's a problem, but there's a natural mechanism to solve the problem. The other thing I want to argue about, because again, I think people love this idea of an eternal universe that gets expanding, you know, that some of the future is the past and it's a cycle because it's a beautiful picture and people have thought about it a lot

over the years, many different versions of cyclic cosmologies because it's so nice not, you know, to have that. But I think it's fair to say that as I tried to argue with Roger, and I tried to be fair about it in a way, is that inflation requires no physics. You don't have to know any fancy quantum gravity. You don't have to, it just have to know basic physics, and it's a mechanism that automatically happens. To do this other stuff, you have to assume all sorts of things we don't know and try and explain, explain away things that are, that are, that we explain away things instead of explaining things, I guess is what I'm thinking about. And to some extent, also not necessarily yet make predictions. And so those are my, the reason I were going to fall in one camp versus another,

I would fall in the inflation account. Now, we've been, he has been here to, but I, you know, he did have his time with me. But, but how do you, view those same issues? Let's say. The fact that inflation is just known physics and CCC is requires physics. Well, it seems to me require physics we don't even know about. I don't know. Nobody besides Roger even thinks about.

Yes. Yeah. No, no, I think that's an important point and I completely agree. The underlying physics of inflation is just quantum field theory and general relativity, both of which are well-established parts of a toolkit of theoretical physicists and knowledge of physicists of all types. And we don't really have to assume anything new about principles of physics. While Roger's cyclic cosmology, I think you made both points very clearly, requires this mass fading, which is just a new feature of physics that Roger would like to assert happens with no real basis.

And then the disappearance of entropy at the end of the cycle, which I find somewhat flabbergasting, really. He makes this huge point of saying that we need to explain why the entropy is so low. And then he provides a theory where the explanation is, well, maybe it all disappears. Yeah, I know. It's just basically solving the problem. I'm saying it's like that, I think I said it's him like that famous Sydney Harris cartoon where there's this little gap and says then a miracle happens. I think you should be a little more specific there.

And look, it's fair to say that when we have these debates, we're not being unfair to each other because, first of all, extraordinary claims require extraordinary evidence. And, and, you know, so it's important. It's a sign of respect, I think, not disrespect, that one would argue with someone like Roger, who's clearly made, you know, profoundly important developments in physics to say, well, you know, you have, but this one we have a problem with. and it's necessary to ask what the problems are.

And I think, you know, and Roger came back to me afterwards and saying, you know, these things, I didn't really thought about that aspect of inflation, and we're going to work on that problem. And so I think it's part of the, I think right at the forefront, that's where we can have these arguments. And you and I've had debates about many things

at the forefront, even though we fundamentally agree about the fundamental physics. And that's a great part about physics. And the greatest part is that it's not going to be decided by who's a better arguer. It's going to be decided by nature if it ever gets decided at all. And that's what makes science worth continuing.

And I'll just end, I want to ask you, what's the future? What do you think? What's the, where do you think inflation, dark energy, cosmology, where do you see the greatest opportunities? And it's something, by the way, just to advertise,

that we'll be talking about a greater length when you and I get to be together at a public event that was delayed, that was supposed to be two days ago, and it'll be delayed until November because of my mother's unfortunate demise. But anyway, so what's the future? Okay. Well, I agree with, I think,

Yogi Barra, who says the hardest thing to predict is the future. Yeah. It's, I don't know. I mean, certainly we will continue to make, I think, various significant strides in pinning down the details of the behavior of the universe. searches for possibly finding the gravitational waves that you mentioned, which would be so crucial and helping us learn about the early universe. And coupled with more precise measurements from the cosmic microwave background, maybe we'll discover non-gassianities, which should be there at some small level in inflationary models. And exactly what they look like will tell us a lot about the details of the early universe that we don't know now.

So I can anticipate a very significant amount of progress along those lines, especially with things like 21 centimeter tomography, becoming a bigger thing. The idea is that with only looking at the cosmic microwave background, we're seeing essentially a sphere around us, but with other methods that are beginning to come online, we can see the whole value. And that gives a lot more information, a lot more statistics.

better ways of pinning things down. On the more hyper-theoretical side of thinking about things like eternal inflation, there I feel that we've kind of gone as far as we can go in doing the kind of thinking that we've been thinking about, which is basically exploring recipes for things like how to define probabilities. I think we've already explored a large swath of recipes,

and we have a good idea of what might be consistent with what we observe and what isn't, but there's still a large class that's consistent with what we observe, and we're still, as I said, pretty clueless about what the underlying principle might be that determines the right answer. And that's for, I think, the best hope for an answer materializing in the next decades. is through a better theory of quantum gravity, through really understanding the quantum mechanics of space time, which right now we really don't.

Even people who have faith in strength theory don't really know how to even address the relevant questions. Yeah. So there's certainly a lot that we don't know there, and hopefully it's a lot that we will actually learn. I think it's going to be slow, but over the next decades. Well, I think, thanks.

And I think, you know, I also like to say probably what we'll learn is the things we don't even know we're asking the questions about. And that's, you know, with the new, you know, science progresses in this field of cosmology has progressed with new tools. And you're saying topography, 21 centimeter lines one, gravitational waves is another whole new area window on the universe. And generally, each new window on the universe surprises us. So I'm, I'm, and so as a theorist, I continue to sort of look to experiment. more than theory, I think. But nevertheless, it's nice to be guided by wonderful theorists and have them as colleagues.

And it was a wonderful gift to the world that December night, not just because you discovered something that I think it may be an amazing feature of the universe. And the fact that we can even talk with any seriousness, as I've written in the past, about a universe when it's 10 to the minus 35 seconds after the Big Bang, or when the size of the universe is smaller than the same, size of a single atom. The fact we can talk about that with any kind of seriousness is an amazing

thing. And the fact we can do it and come up with interesting predictions we can thank you for. But it's also, I'm very happy that besides what it did for physics, what it did for you is very important because knowing you as a colleague, it's been a gift to all of us that it got you a job and you could be there to teach the rest of us. And since the time, I've known you for many, many years as an honest and wonderful human being. So thank you very much. Thank you for those kind words. Okay. Thanks again for your patience. And I hope it was fun for you. I think it'll be a nice way for the audience. So thanks again. I hope you enjoyed today's conversation. This podcast is produced by the Origins Project Foundation, a non-profit organization whose goal is to enrich your

perspective of your place in the cosmos by providing access to the people, who are driving the future of society in the 21st century, and to the ideas that are changing our understanding of ourselves and our world. To learn more, please visit Originsproject Foundation.org.