Into the Impossible With Brian Keating - Jo Dunkley: This Is Our Universe (#168)

Starting point is 00:00:00 We've really found out so much and so quickly about our magnificent universe that's happened really in the last decades. Our knowledge has advanced so much. But there are still tons of fascinating and interesting questions. And what is true is that today's astronomers and I won't get to answer them all. But happily someone else will. Maybe some people in this room. Thanks. This book is about our quest as humans to understand the bigger world that we're part of,

Starting point is 00:00:30 universe and it's it's about finding out about our bigger home we want to know what's out there that our earth is just a little bit of a part of any sufficiently advanced technology is indistinguishable from magic a little upbeat music to start this episode of the into the impossible podcast with a good friend and i'm so glad to know that she is my collaborator on the simons observatory a fellow executive member of our executive governance board. And that is Professor Joe Dunkley, joining us all the way from Princeton University. How are you, Joe?

Starting point is 00:01:11 Very well, thanks. Thanks for having me. Joe, you have been a phenomenal inspiration for many reasons, for many millions of people around the world. You are a professor of physics and astrophysical sciences at Princeton University. You've won awards from the Royal Astronomical Society, the Institute of Physics, and the Royal Society for her work on the origins and evolutions of the universe, and we collaborate on the Simon's Observatory,

Starting point is 00:01:34 I couldn't be more proud. And Joe, what we normally do is do what you're never supposed to do, which is to judge a book by its cover. But, you know, what other Bayesian priors do we have, right, Joe? So we'll start off by reading the title, Our Universe and Astronomer's Guide by Joe Dunkley. So, Joe, take us through how you came up

Starting point is 00:01:56 with the concept of the book and what the cover and subtitle really are supposed to convey. Well, so this book actually came up originally when I was over in the UK. I was working there before I moved over to the States. And actually, so it popped up as part of the Pelican Guide of Books, Pelican series of books, which is supposed to be introductions to topics of interest to kind of the lay public, people who would have some, you know, an education, but wouldn't have had a particular educational training in these areas of, you know,

Starting point is 00:02:33 that go into more details about things that often people think are maybe overwhelming or too difficult to comprehend. And so the idea was to, you know, write something that anyone could read and learn more about, you know, this beautiful universe around us. And to really keep the level, you know, at a pitch that you don't need, you know, a degree in physics, or really any equations either to pick up some kind of difficult concepts.

Starting point is 00:03:04 And it's very successfully done as such because it's not only informative, even for a professional astronomer, but it's also extremely well written and delightfully researched. And my favorite are the illustrations, because they're very whimsical, but they're very informative. Let me get one up here that has one of the classic illustrations of space, time bending on a trampoline and i love the fact that you um don't shy away from from going very deep and actually talking in great depth about some of these concepts including things like the acceleration of the universe the composition of stars and how they're made and how they're nuclear fusion reactors the size scale of the universe you have unique analogies um that i guess you worked on

Starting point is 00:03:50 with with somebody credit at the acknowledgements to somebody at princeton i guess can you talk more about your collaborator on the concepts of some of the illustrations? Yes. So, yes, absolutely. I think there's two. So one thing about the illustrations I sort of had in my mind the sort of when I, you know, as a professor, when I'm explaining something to a student or some who I'm trying to teach something to, we typically use a chalkboard.

Starting point is 00:04:14 You know, we try and simplify diagrams. You get a piece of chalk and you to draw the simplest thing you can to get across a concept. And so these diagrams that I put in the book were kind of inspired by that concept of like, this is what I would draw on a chalkboard if I was to be trying to explain to someone the basic idea. So they're not fancy. They're not, you know, you can go to the web or other books. So the beautiful images we have about cosmos, this was to try and get the concepts in mind. Yeah.

Starting point is 00:04:48 But then I worked, a lot of the content. and yeah, the kind of ideas for how to explain these things, I picked up from working with two, you know, fantastic women. One was a school teacher, Eileen Levine, who I taught, co-taught a course with, gosh, about 10 to 15 years ago now in Princeton, where we brought in teachers from New Jersey to do a week-long course on astronomy and cosmology. And the idea was that we were teaching teachers, new content,

Starting point is 00:05:29 they could then take back to their classroom and teach to their classes. And we got a lot of the content of that from a NASA educator, Lindsay Bartolone, who showed me so many of the wonderful materials that NASA had created for educators to use in the classroom as well. So we kind of put together this course based on this, you know, this, this wonderful body of work. And then I learned so much from Eileen as a teacher who was like, that's too complicated. We've got to make this easier. You can't say that.

Starting point is 00:06:04 That's too difficult. And we, I think, got down to a level that was, that worked. Yeah. I think it's something that, that C.S. Lewis coined the curse of knowledge, which is that, that the more that someone becomes erudite and figuring things out, you basically stop being able to relate to your former self when he or she didn't know these advanced concepts, gravitational lensing and origin of the universe. But I think the skill, and I think that's exemplified in your book, is really indicative of the fact that you can relate to readers who are experts, you know, like me

Starting point is 00:06:45 and then, but also novices of this field. I was curious about the hour in the front. In what sense is it our universe? Because it's an astronomer's guide to our universe. Are you saying that the general public can understand and appreciate the universe as much as an astronomer? Or sometimes I feel it's the reverse.

Starting point is 00:07:03 We don't really appreciate what we do and how privileged we are to be astronomers. So what is the meaning of hour in the title? I wanted to convey this feeling, which I believe to be true, which is that this is our home, right? That we are, it's our universe, it's the place where we live.

Starting point is 00:07:21 So as an aside, you know, we don't know if there are maybe more universes out there. There's this mystery, I think, where I shouldn't assume that what we can see is all there is. So there is one kind of, there's the kind of possessive of like, I'm just going to try to describe the bit that we know of. You know, that's in one sense our universe. I'm not going to try and, you know,

Starting point is 00:07:39 speculate too much on what else that could be. But I do think this concept of, like this is our home. It's our, we live on earth, but I want people to feel that this bigger place is also something that people can feel, you know, not ownership of, but belonging to. You know, we belong to this bigger place. And in the same way that, you know, we like to learn, we can live in a particular, you know, I might live in Princeton, New Jersey, but I want to know what else is out there in the world. I want to know, you know, what other countries there are, what other people there are. This idea of it, we're just taking.

Starting point is 00:08:13 that those boundaries a little further about learning what other things they're out there as well. But it's still kind of part of our whole home and our whole story. It's often said that you learn a lot when you teach. I find that. And you're a renowned teacher as well. Do you feel that you learned even more by writing the book? Are there anything that surprised you in writing the book? Or are you so expert as one of the preeminent cosmologists of our time? Did you like, oh, this is, I just want to explain it to people. But do you, did you actually learn anything new in the writing of the book? The thing I learned the most was actually the stories of some of the scientists.

Starting point is 00:08:53 And what sort of happened a little bit by accident is that I got really drawn in by by the personal stories. And in particular, to me, I was drawn by other women scientists who I certainly knew of, but didn't know so many of the kind of the facets of their lives. And so I think probably I knew that the science that I describe in the book, I knew most of that science already. But the way, the stories of the people and the way the things happened were things that I then learned more about in writing this. And that was really fun for me. Yeah, that's wonderful.

Starting point is 00:09:30 And of course, you received the Rosalind Franklin Award. And the book has a wonderful encomium from Dame Jocelyn Bell Burnell, who says, this book is simply superb, beautifully written and very clear, and incorporates all the major recent results and indicates what might come from telescopes now being built. And I want to thank you, by the way, for including her story or speaking about her so accurately and so decisively her contributions were to astronomy. And it inspired me last week, just as I finished reading the book for a second time, your book, to reach out to her. And she's going to be a guest, actually, coming on the show. And she's coming on my show on December 10th. Now, I don't know if you know what

Starting point is 00:10:15 December 10th means, if you recall from my book or from other things you might know. Do you know whose date of death that was? I don't. You'll have to tell me. If you look behind me, there's some hints. So the Nobel Prizes are given out every year on December 10th, which is the day I'm going to interview Dame Jocelyn Bell Burnell. And one of the things I'm going to ask her is about the fact that she was uniformly regarded as having been snubbed for the Nobel Prize won by her PhD advisor and another British scientist. Of course, you won the Rosalind Franklin Award. Talk about the status of women and where you see things. Is astronomy, you know, sort of exceptionally, you know, better, worse? How are we doing? A lot of our colleagues on the Simon's Observatory are women.

Starting point is 00:11:01 Our spokesperson is Professor Suzanne Staggs, and she is, you know, she's a phenomenal leader of many of projects, not only the Simon's Observatory. Talk about how women have, you know, come forth since the time of Dame Bell Burnell's work, Rosal and Franklin, if you want to go back that far, CSW if you want to go back even further. Talk about, you know, how you see things in astronomy. Is there improvement, or do we have a long road to go? I think it's a bit of both. I think there's been significant improvement in the numbers of women who are in astronomy. and I see them now, you know, emerging at the senior level as well. Although I will say that at the scene,

Starting point is 00:11:41 and it's wonderful to see people take on leadership of institutions. I see Risa Wexler in Stanford. Now just Julian Dalcanton has been announced as a new director of Center for Competition, Astrophysics. I keep seeing, you know, wonderful, exciting announcements of senior women astronomers taking, you know, leadership roles. and at the same time we are seeing more junior women come through as well. So I'm overall positive, and if I look even around the physics department where I work,

Starting point is 00:12:11 overwhelmingly the largest number of women are in the area, the subfields of astronomy compared to other areas of physics. Now, that's not to say we shouldn't be working harder across physics, but I think astronomy is doing pretty well, but I don't think we're, you know, we're not, we're not, we shouldn't feel too complacent because I think there are still challenges and we're still, you know, not represented fully. But I think to me, actually, what I'm what I'm also realizing is, you know, as a, as a white woman that, like, there's a, there's an issue with lack of representation of women, but there's a much actually

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Starting point is 00:13:51 Stop cutting me up. Price Line! So, you don't probably know this, but I'm calling you or speaking. in the office formerly occupied by Jeffrey Burbage, who, along with his far more accolated wife, Margaret Burbage, did some of the foundational work of the 20th century in astronomical observations. And in fact, I am blessed to have some of Margaret's old plates that she took long before you were born and even a couple of months before I was born in the early 1970s.

Starting point is 00:14:26 And these are, I treasure these, and it's in her own handwriting, and I just love these. But it's impossible not to kind of, you know, recognize that the kind of challenges that she was facing and how things might have changed. And she was famous for turning down certain awards because they were only offered to women. But you're right. And I think, you know, the challenge I feel, and I was talking about this earlier this week, but, you know, there's a twofold challenge in increasing representation. We all want to do it. But we're also, you know, there's a sense. that the past was really bad, and we do want to make things much better, but it's almost like

Starting point is 00:15:01 we feel like we want to make sure that we do it the right way, that we don't burden our colleagues. I have a colleague who's an African-American neurobiologist here, and, you know, he's always asked to do stuff. He's always asked to be on committees and serve and, like, assuage how different people are feeling and make them feel better. Anyway, as a woman, does that happen as well? In other words, are you put on all these committees for women and status and award? And is that also sort of like a tax that women have to pay, perhaps that male astronomers like myself don't? I think so, yes. I think that I will say I think that, you know, senior members certainly of my colleagues have been careful to try and protect me from that.

Starting point is 00:15:39 But it happens nonetheless because you may not be asked. I think I'm not probably asked to do that much formal work in that respect because people are aware of that tax. but it's the informal or or it's it's the informal work of for example talking to which I love but talking to junior women and and sometimes addressing issues that have to be brought up you know informally people might come and talk to me because they will see me as someone as a woman who you know who they might be able to bring things to um so I and so yeah I do I do definitely feel there is an extra tax and some of it is invisible and some of it's visible. And so being aware of both those parts, I think just people need to keep aware of those things. Yeah. Yeah, I had, you know, Katie Mack on a couple of years ago,

Starting point is 00:16:30 along before her book, and she was saying, you know, it's often very trivialized, like, oh, just give a girl, you know, a pink circuit board, and that'll be enough. But also that she doesn't really want to talk about it all the time. And so I actually don't want to talk about it that much more either, except to bring up that Margaret left this plate. And of course, she was part of the duo that discovered how the quartet, who discovered how nuclear fusion worked inside of stars to produce the elements that you and I are made of and meteorites that are the villains of certain books.

Starting point is 00:17:01 But I want to ask you about some of her more controversial other associations, maybe guilt by association. And that has to do with the origin story itself of the universe. And they believed, along with Fred Hoyle, renowned cosmologist, that the universe didn't have a singular origin. And yet in this book, you make a very convincing argument that there was a single kind of Big Bang,

Starting point is 00:17:21 and there's a lot of information associated with it. Is there room for doubt? I spoke to fellow countrymen and actually a student of Fred Hoyle, giant Narla Kar. And he still has his doubts about the Big Bang. Is there room for doubting the Big Bang paradigm, shall we say? Is there still room for legitimate,

Starting point is 00:17:41 not cranks, not crackpot, but legitimate criticism of the Big Bang. So I would say that, you know, when I talk about the Big Bang and the being a beginning, I'm talking about the, you know, the beginning of the growth of the universe we see around us today. So I think there's this huge open questions about what that point at which we call the Big Bang started from where there was time before there, where there was something before then. I think, you know, that you think about this a lot too. this question of could it have come from something prior to what we call the Big Bang, was there something around before?

Starting point is 00:18:20 So I would not, I don't think our current data tell us about whether what happened or whether there was something around before our expansion began. But I think that the model we have that like, yes, some growth has been happening from a hot, big, that things were in a hot, dense state, you know, 14 billion years ago and have grown since then following this model of expansion and following laws of gravity, there's so much data now for that. I just don't, I don't buy a lot of, you know, concerns about that because we have so much evidence. We have, you know, we can look at the proportions of different elements in the universe.

Starting point is 00:19:02 We see this, you know, this radiation from the Big Bang, just after the Big Bang itself, this cosmic microwave background that we both study. and so much data now with the beautiful telescopes we have, that I think that story is, while there are big questions in it, I think that story has become robust. Yes, and I think, you know, you mentioned it earlier on in the beginning of the conversation

Starting point is 00:19:27 about the concept that it might not be the only universe. It might just be our universe. And you talk a lot in the book about, about, you know, how the universe is, has the appearances as if inflation took place, this early energetic phase of cosmic, I wouldn't say genesis. I think a lot of people conflate the two, that inflation doesn't necessarily have to be the Big Bang, right? Joe? Yeah. So what is most promising about inflation as an expert? And then what are some of the most

Starting point is 00:20:02 significant challenges and things you would like to see shored up? Because you actually give it a very fair treatment. You mentioned our mutual friend Paul Steinhart, two-time guests. on The Into the Impossible podcast. And unlike many other guests that I've had on, you don't assume it's a fate accompli, as our French friends would say. So talk to me about Steelman, the arguments against inflation,

Starting point is 00:20:22 and then let's dive into what the good parts of inflation are. Well, so one of the things that is, one of this fascinating questions that inflation manages to address at least, I'm not sure convincingly solved, but it's one of these open questions. It wasn't really what first inspired inflation But, you know, we see cosmic structure in the universe.

Starting point is 00:20:43 We see galaxies in the sky, like, distributed in this, in a beautiful cosmic web. And the universe is full of features. But if we step back to, you know, the very early times, it was almost featureless. It was kind of this soup of particles, almost completely, you know, uniform density throughout space. but, you know, we had to find that almost featureless universe wasn't completely featureless. There were these tiny lumps, these tiny over-dense regions in the early universe, and they were the seeds of these cosmic structures. They were the things that then grew by gravity of millions and billions of years

Starting point is 00:21:26 to form this beautiful universe around us now. And one of the huge questions we have is how, why were these irregularities in the early universe, in this soup of particles, why were there little regions that were ever so slightly denser than others that there would be the seeds of everything to come later? And inflation, which is this, you know, the idea that the universe expanded extremely fast, fast in the speed of light, just for very short time. It kind of solves that in a beautiful way, I think. And I get to kind of this, this, like, amazing mixture of the quantum mechanics of the very

Starting point is 00:22:09 tiny with the, you know, the general relativity of the very large, that the tiny quantum fluctuations in the very early universe where particles are kind of allowed to appear and disappear because of our uncertainties of that quantum mechanics tells us we should be uncertain of. Those combined with a universe that's growing exponentially fast kind of freezes in these these little ripples, these quantum ripples, into huge macroscopic, enormous scales that then become the seeds of everything else. And that is a love, you know, that's, that's beautiful. The idea that you could, that this works and that this, this kind of matching up of like tiny quantum fluctuations expanded enormously and then they grow then the galaxies that we see. That's very beautiful.

Starting point is 00:23:02 trouble is that that even though sort of part of the inflationary models predicts that indeed you should get these little ripples in the way that we see them, there are real problems with the theory as well. Like it says that there's also high chances of getting something looking completely different. And so this is that, you know, the people who are saying, oh, this actually isn't, this model doesn't really work so well, say that,

Starting point is 00:23:36 that actually there are more, there's more, you're more likely to get a universe that looks nothing like the one we've got than this particular one, which has these, you know, elegant little fluctuations that, that match up what we see. So there, there are, that to me, like this, whatever, whatever, whatever was going on has to explain how we got these, this initial, you know, the initial conditions of the universe. It's got to explain that. But there are other ideas out there. And when you think about those ideas, sometimes I have friendly arguments with my friends, Paul,

Starting point is 00:24:11 and Anna Aegis and others who promote this because eventually their model in bouncing cosmology has to incorporate something like a scalar field. I always tell them, I say, Anna, you know, Paul, can't you do it without a scalar field? In other words, don't replace the kind of unknown, unknown within inflation, which is a scalar field, we have no evidence of, you know, currently, I mean, there is only one known scalar field,

Starting point is 00:24:36 which is the Higgs, still, how, you know, how nice would it be to not have to have this feature? So I kind of feel like it even will retain all these cosmological alternatives, and even Sir Roger Penrose's aeons and cyclic cosmology, they still retain some kind of scalar field for which we have no evidence. So I guess the concept, the question I have is, how should we evaluate these things? I mean, inflation is stunningly successful, but it cannot account. for its own origin. In order to account for its origin, it must invoke the multiverse. And so does that trouble you, or is that just really a signpost that we need better ideas? I think we certainly need better ideas. I think that, you know, it is remarkable that there

Starting point is 00:25:22 are a set of predictions from inflation that we have sort of checked off most of them already. We're now together, right? We're looking for one of the key predictions. of the inflation model of the early universe, which is that it should imprint space time ripples, gravitational waves permeating the early universe. And so we should certainly look for them. But I think even if we find them, I wouldn't be ready to say, oh, inflation happened.

Starting point is 00:25:52 Like this is what it is, you know, problem solved. We actually did do that in 2014. You might remember. Well, we did. But even if that result had been right, I think, you know, that was a, it was not even, that was a mistaken result. Yeah. But even if we had found them, and even when we do, I hope we do find them with

Starting point is 00:26:10 Simon's observatory, or maybe I should hope we don't because that might tell us something interesting too. I'm not sure what I hope. Even then, we're still going to have this gap where we will say, still, you know, do I know what inflation is and why it happened? And so I think that there are still, I think there are issues. And I think having more clever minds thinking about these theoretical models for the early universe, I think is really important.

Starting point is 00:26:45 I feel like we don't quite have enough. Right now I feel like we've got this dominant paradigm, which is inflation. We have these, these, these, you know, interesting alternatives, bouncing models. But we don't have whole communities of other, you know, theoretical cosmologists thinking up other alternatives or subtleties of this, and I sort of wish we did. Yeah. So we're going to nerd out a little bit now because my audience, the into the impossible audience, is nothing, if not the smartest, most brilliant minds in the multiverse. Your thesis titled Modern Methods for Cosmological Parameter Estimation Beyond the Adiabatic Paradigm. First of all, we've had a lot

Starting point is 00:27:22 of people on Julian Barboor, who you probably know from Oxford. He was on the show talking about this entropic arguments that he has for the origin of the universe, an Englishman, but he's not English. His name is England. Jeremy England, we talked about life and the connection of entropy to the origin of life and evolution. But I want to ask you, first of all, what does adiabatic mean in the context of cosmological perturbations? And how does that fit into the inflationary paradigm? And then we'll go deep. Yeah, good. So it's a funny word. It's a word that's used in other ways in other parts of physics, but it's a word that we use in cosmology for saying, you know, I said, I mentioned earlier, we have these sort of,

Starting point is 00:28:00 lumps and bumps in the early universe. And the sort of theory, inflation says that before we had any particles, we had a scalar field. I mean, I know what that is, but we're saying we had the scalar field. And that it was the thing that these quantum fluctuations left the irregularities in. And so then we think that that decayed or changed into the particles we know. So the sort of barion, the normal barionic particles that we are made of, rays of light, photons, neutrinos, and we think also dark matter particles that also brought them mysterious. If that sort of what happened, then all of those particles, all of those different ingredients, the particles, the light, the neutrinos, they should all inherit the same

Starting point is 00:28:52 lumps and bumps as went into that initial scalar field. So anywhere in the universe that was kind of an over-density of energy in the scalar field, that will then turn into an over-density of both the particles and the rays of light and the neutrinos and the dark matter all in one place. And so the adiabatic sort of assumption is indeed that that any over-dense region in the early universe is going to be similarly overdense in all of the ingredients of the universe. It's not like you'll have one lump of matter over here, and over here you'll have a lump of photons. They need to trace each other.

Starting point is 00:29:30 Like if you imagine drawing, my blackboard is not right behind me. If I imagine drawing a line tracing the density of the universe with space, the density of the light and the matter and the dark matter, everything in it should follow the same pattern. And that's, again, what simple inflation would predict, because it all came from this scalar field. Right.

Starting point is 00:29:55 And when we think about adiabatic or engine, One of the main kind of attacks on inflation is that it doesn't, it makes the problem of the arrow of time, you know, particularly pernicious because it's really saying the universe had this extremely low entropy state in the beginning, but it almost has to be added by hand. How does the entropy behave? Let's say something happens at the plank time. We don't understand what. Inflation happens, you know, 10 to the eighth plank times later, 10 to minus 36 of a second. Then it lasts for 10 to the minus 35 of, I don't know, 10 to some small amount of time. And then it emerges. But it has to emerge in this low entropy state. Wrap my mind, my audience's mind around that. How does that get accomplished so that it is actually adiabatic? Not just thermal equilibrium, which, you know, you could kind of make sense,

Starting point is 00:30:47 although it's hard to really think about thermalization of what would become the entire universe effectively. But nevertheless, how does the low entropy get a certain? if not by Fiat, which most cosmologists find anathema. Yeah, well, so I think we're certainly missing big pieces of the story. Still, we usually actually, you know, usually, when we're tracing through how these like perturbations evolve, we just give them an initial condition. And we say, I think what I'm saying is that, you know, if we assume that these fluctuations came from inflation, then we say, okay, we had some kind of, we had, at the end, inflation ended,

Starting point is 00:31:27 we decay and produce particles. But there's still, you know, big uncertainties about that, even that process. But so I think, but then from sort of, when we then do cosmology onwards, right, we say, okay, I'm just going to start with some initial conditions of these ingredients. And I'm not going to exactly worry you right now about how I got from the scale of field into this initial conditions of those particles. I mean, obviously, plenty of people are working on that. But if we're trying to say, what does that?

Starting point is 00:31:57 what are the observables in the universe? We tend to just say, right, what are the initial conditions of these different ingredients? We don't, we don't then have to model that earlier bit. I agree, it's very interesting to understand, try and figure out exactly. I always say, you know, if you want to study, you know, embryology of a chicken or something, you don't actually have to come up with how the DNA got in the chickens, you know, egg in the first place to understand which came first, the chicken or the egg. But nevertheless, it's interesting. And it's always kind of adjacent. It's like I have a, I'll have the famous astronomer. Actually, I had Sarah Seeger over the other night for dinner in San Diego, and she's a world-renowned astronomer,

Starting point is 00:32:35 and a guest past guest on the end of the Impossible podcast. And we were just lamenting the fact that how few astronomers can identify most constellations. I'm not going to put you to the test, because I know you can. But Sarah and I were joking, but it's, it's not too surprising that we get asked about these big picture questions as cosmologists, right? Because we're we're studying, you know, the origin of everything. And so people assume we're going to be philosophically inclined to want to know what came first. But maybe more practical question to answer is I've never fully been comfortable with the notion that, you know, inflation is super smoother. It makes the universe exquisitely flat during the early radiation dominated phase.

Starting point is 00:33:16 But then after 10 to the 60th, or E to the 60th foldings or whatever it is, the best fit model that you and your colleagues have determined, that it emerges, but then it has ripples, and then it has these tiny curvature fluctuations. And those are adiabatic, as you discussed in your thesis and elsewhere. How does that, is that another fine-tuning problem? I guess that's my question for you in a concise form. Does it have to be, because if the perturbations were present

Starting point is 00:33:43 before the enormous expansion, then they too would get diffused and flattened out because they would just amplify outside the horizon? So the perturbations are appearing during the expansion. So every time, well, we've got little quantum fluctuations that are present at every scale of the universe's space as it is at the time. And so as this inflation says I'm exponentially expanding space. So every time I expand double, double space time, I'm imprinting space time, I'm imprinting the, fluctuations, which is why, again, we see these particular scale invariants. We see fluctuations

Starting point is 00:34:28 that look the same, whether we look at like really, really big scales or really small scales. So those, because they're being imprinted during the inflation itself, if they've been imprinted, if they were there beforehand, you could imagine they might have been, you know, washed out. But like, they're actually coming in during inflation itself. due to from the you know from the physics of the of the universe growing coupled with these quantum fluctuations so you end up so that then they get you know so essentially frozen in on big you know on visible macroscopic scales doing it um and but it is this lovely thing that like that makes quite a specific prediction that because inflation had to end and it slowed down you actually even

Starting point is 00:35:14 then you get almost as the kind of same size features at very large angular scales as small, you actually get like slightly smaller features at the smaller scales. And that's a departure just for the list. That's the departure of NS from Unity. Yeah, that's exactly. And what, I mean, it's been, it's been really exciting because it's only been since the Planck satellite that, you know, we saw this for the first time high significance, that actually that's indeed what we see. is this departure from scale invariance.

Starting point is 00:35:47 And so, yeah, I guess we are actually seeing the kind of, if indeed that comes from inflation, again, it might not. We're seeing that kind of the evolution of like how the expansion went during that, during those eFolds. And what else you put in a little caveat out there with appropriate candor, but what other models would predict a departure that could be 7 sigma or 5, whatever you guys have measured it to, significant, departure from NS from one. What, you know, I mean, in other words, what other alternative models, or is it just other

Starting point is 00:36:22 models of inflation that could predict it and evade that test? Yeah, my impression is that even though it's very nice, so it is pretty compelling to me, that, that, that, I think was one of the most important results of the Planck satellite was this, was this measurement. and it has made non-inflation builders' life, I think, more challenging because of this, because it is rather specific. And so I feel sure that there can be other ways of doing it. I would say either there are two things.

Starting point is 00:36:53 Either there's a different way of doing it or we, there's still a possibility that we've got it, that it's not less than one. Because if you want to throw in some other strange physics, you know, we get to. these numbers from the universe by looking at our observations very carefully, but we make assumptions. And so in detecting this NS less than one, this slight scale invariance, we've assumed everything else about the model of the universe. We've assumed that it's what we call Lambda CDM. It's this flat universe. It's got only three ingredients, et cetera. We might have got some of that a bit wrong still, even though we don't think so, even though we've done our best so far to say,

Starting point is 00:37:32 you know, given all we know, this is our best model, there might still be something new physics, in there, some new model behavior that actually could give some slightly different, some infer some different scale dependence, even from the data we have in hand. And sometimes people say things like, you know, inflation's prediction of a flat universe is not really a prediction. It shouldn't count as a prediction or even, you know, or it's not even a problem in some sense. What do you make of that? The classical pillars on which inflation's credulity rests are the observed flatness, absence of spatial curvature, the observed, you know, isotropy of the microrate background, but not perfectly isotropic, and then

Starting point is 00:38:20 the absence of magnetic monopoles, which is a non-observation. So I agree we really, and that was Goose's original motivation, as I'm led to understand. What do you make of the flatness? Should we stop using it as a justification that inflation is well motivated? No, I think the fact that our universe does appear to be almost perfectly or exactly geometrically flat, which means light travels in parallel lines that it's not, that it probably stretches out, it stretches out, it's not like the surface of an orange, but like the surface of a piece of paper. That is a surprising measurement if we made no assumptions about the early universe, because, you know, this is one of the things I remember doing as an undergraduate,

Starting point is 00:39:09 is you can derive that, you know, you can show that if you just have some departure from perfect flatness of the universe and you just let it evolve, it will get much more, much less flat. That was this big problem. It's like this incredible fine tuning you need at the very beginning of the universe, that the universe starts off incredibly close to flat if it's going to look pretty flat today. And so that is no, any viable model has got to address that. And so that was one of the things that inspired inflation. So yeah, anything that's not that, better address that issue.

Starting point is 00:39:49 Right. And then the final thing that I hear a lot about, and I'd love to get your professional, professorial opinion. Remind, we're talking with Professor Joe Dunkley, Princeton University, renowned cosmologist, friend, and collaborator on the Simon's Obser. It's such a pleasure to talk to our friends. And this interview has been two years in the making since our universe came out, and I've been dying to get you on the show.

Starting point is 00:40:11 And something happened in the world of health-related matters that we're not going to talk about, that delayed many times our interviews with our kids and our schedules. But we're talking about right now some of the nitty-gritty nerding out on inflation. So one thing I'm often led to believe from inflation is that it predicts something about quantum gravity. In other words, an observation of primordial B-mode perturbations would in some sense validate the notion that gravity was quantized during the inflationary epoch. What do you make of those kinds of claims? And in what sense is a B-mode, a classical B-mode gravitational wave? And GR is a classical theory, as you know better than anybody.

Starting point is 00:40:54 And what sense is the observation of a classical gravitational wave any bearing on whether or not gravity is quantized? Well, I think it depends on your model. So when what we're all looking, you know, we're busy looking for this, this imprint of gravitational waves on the causing microwave backgrounds. And the physics of that doesn't, you know, is not necessarily assuming that gravity is quantized. You know, we've seen the gravity, those beautiful gravitational waves already from spiraling black holes from LIGO. And it's, again, it's just a prediction of relativity. And so if we. If we. find that imprint from the early universe, I don't think we're instantly saying anything more than we have found that signal. You know, we've seen, we can first of all say, you know, yes, we've seen gravitational waves produced from some mechanism in the early universe. I don't think we can then say with certainty, it therefore must have come through this, you know, this, this mechanism where I've come, you know, quantized gravity,

Starting point is 00:42:03 I've come from quantum fluctuations, et cetera. But it's certainly, but it's one of the parts. I think if we, of course, if we see them, if we detect them, then, you know, people will be busy exploring these options and trying to determine how, with what certainty we could say, that is what happened. But I think that we won't be able to say that with,

Starting point is 00:42:23 I expect, with, you know, there will still be options, I suspect for, you know, other possible mechanisms. And when we look at the inflationary paradigm, one complaint I hear about it is that it's kind of like string theory. It takes up all the best and brightest young minds, and there are two few people working on alternatives to inflation. Do you think that's a problem?

Starting point is 00:42:48 Do you think there should be, you know, I had Mitchie Okaku on who is the father of string field theory along with other people. And I asked him, well, you know, how much should we spend on alternatives to string theory? And he said, well, 50-50, you know. I thought that's, you know, if I made you, director of the NSF, which maybe someday you will be,

Starting point is 00:43:09 but if I made you, you know, in charge of funding and so forth, would you, how would you allocate balance the portfolio of research that's done by theorists or theoretical colleagues? Well, and one thing I would say is I think there were so many, you know, the origin of the universe is a huge question. There are other huge questions too, even in cosmology, you know, that may or may not be connected about the, you know, the invisible universe, the dark universe. But within the early, within the early universe modeling, I think it's really important to support the alternative models, modeling or thinking, thinking of alternatives. And, you know, but in that respect, I'm biased, I always want to have a strong focus on.

Starting point is 00:43:54 Now let's figure out what to measure and connect those theories to what should we best go and measure to really give as many possibilities to the theorists. And one example that we're doing with the Simon's Observatory is we're not only looking for gravitational waves that are kind of one of the things that people are looking for for inflation, but we're also going to measure these tiny flukes, the tiny ripples in space time that produce galaxies. at really fine scales to try and get a broader lever arm on what they look like. Because as I said before, the Planck satellite has said, yeah, these fluctuations look almost scale invariant and they follow this pattern. But we must go and look for anything else

Starting point is 00:44:42 that could be telling us something new. So I think perhaps what I'm saying is that observations, if I was funding projects, you want to fund, I think, things that give you the most, information possible, not perhaps about just one facet, but like about, you know, many different views of a problem. And then, yeah, ideally supporting a variety of theory, theorists. So at the end of the book, you concluded with a beautiful passage, you say, the most exciting discoveries are the ones we least expect, ones that can radically change what we thought was

Starting point is 00:45:15 true and ultimately lead us to a better understanding of our wider world. We look forward to them with eager anticipation. I always think it's kind of funny when I say, like, oh, we have to look for serendipity, because by definition, you know, I'm going to do it. But what do you say to people that, you know, say, look, you know, the Simon's Observatory or some other experiment, you know, we're basically not going to be able to say that inflation definitely took place, as you just said, if we detect B modes, because something else could purport it. Or we're not going to be able to rule out inflation either, right? Because inflation could have occurred, but at too low an energy regime to create observable primordial perturbations.

Starting point is 00:45:51 other goals involve looking for evolution of dark energy. Well, it doesn't seem to really have any hints that it's really quintessence or evolving dark energy. And then looking for neutrino masses. And, well, we have a good lower bound and we have a good upper bound. We just don't know the exact results. Why is it important to do these measurements with precision? Why is that important? And is serendipity, as much as we love it, is that a good justification for doing scientific research?

Starting point is 00:46:19 I think ideally you want to be able to have an experiment that can do both. I think, of course, you need to build something where there is something that you're after. And neutrinos are an interesting example, actually, because we know that neutrinos exist. We know that they have mass and we don't know what they are, what that mass is. We don't actually know if they behave cosmologically like we think they do. So there actually is a huge scope for discovery there, I think. There's whole sector of particles that we really don't fully understand. understand. So to me, that's an interesting area where, you know, you say, yes, I'm going to

Starting point is 00:46:53 build an experiment that can go and measure the neutrino mass, assuming they behave in the universe like they seem to do here on Earth or in the local, local universe. But then I think you want to try and build your experiments in a way that say, that give you the most opportunity to find, you know, stranger behavior that you didn't, that you weren't sure of. So I do think that the ideal experiments, and we're seeing that with large cosmological, surveys actually that you build them to target a few key questions and then you collect a huge amount of data and realize that you could actually look for things in different ways and hopefully find something new. So I think my favorite, my favorite experiments have a bit of both.

Starting point is 00:47:36 And obviously they're probably not going to get supported unless they have a bit of the first one, which is something that you actually think is worth going to measure. Okay. So for the last nerding adjacent question, I want to talk about, ineffective. This is probably the least known, but maybe the most interesting parameter that Simon's Observatory and other projects can go after. What is ineffective? Why do we expect there to be a departure from the standard model? And maybe you can talk about what the standard model predicts for ineffective. And what do you think our prospects are? Yeah. So what this number in effective, it characterizes the, mostly characterizes the number, to begin with, just the number of types of

Starting point is 00:48:17 neutrino particles in the universe, of which we are pretty sure there are three. That's what we've measured, you know, lab-based and local measurements. But that number in a broader sense cosmologically measures how many actually just in general relativistic particles there are, or species of particles, where relativistic particle is something that certainly in the early universe was traveling, you know, almost at the speed of light. And again, we think that it's just three neutrinos, but there may be more particles out there.

Starting point is 00:48:53 And many models for the early universe would say, well, actually, yeah, there should have been additional kind of particles created in the early universe that would also be left behind streaming through a universe now. And the subtlety is if they were produced

Starting point is 00:49:08 or if they sort of stopped interacting with other particles much earlier on than the neutrinos did, then they would, then they would sort of look in our data as if they were some fraction of a number. So we talk about this number, like the number of particles, the effective number of particles, but an extra kind of species of like a novel new relic particle might only contribute like a tenth of a number of particles.

Starting point is 00:49:37 So it might be that we go out and we measure not three, but three point one as this number. And three point one would be really exciting because it would say that there was actually some additional, you know, density stuff in the universe made of rapidly traveling particles that are not in our current standard model of physics. So sadly, we've, well, not a purpose on sadly, but maybe a bit sadly, we've already ruled out the fact that there's a whole other neutrino species, you know, we went down through a few years ago with our new data, we said, okay, four is ruled out with our data. So anything that is in there has got to be just some fraction of a number, which again corresponds to particles that stopped interacting

Starting point is 00:50:17 at a much higher temperature in the universe than neutrinos. But they might be there. And so we're looking for that slight departure from three. Again, you do expect some official departure from three just because of tracking the heat of the early universe. But nonetheless, three neutrinos should give a very specific number and any addition to that will be new physics, new particles. And this will be different from, say,

Starting point is 00:50:42 the potential dark matter candidate called the axione, correct? So do you want to explain what is an axiom, why do cosmologists get so excited about it, and how can we maybe measure it with experiments like the Simon's Observatory? Well, axioms are very interesting. So they're one of the candidates for dark matter. So we have a huge amount of mass in the universe that's invisible to us, about five times as much mass as the matter that we're made of, is dark matter. And until quite recently, I think many people assume that this dark matter was made of what we call WIMPS, weakly interactive, massive particles, something heavier than the electrons.

Starting point is 00:51:23 I think things that we're made of. But now there's a renewed interest, I think, or there always was, but I think it's probably an increase lately for axions, very, very light particles to be the source of dark matter instead. and they'd be incredibly, incredibly light, but, you know, but the total energy of energy density of them would be enough to account for this invisible matter. They're actually sort of different flavors of them. Some of them could be the dark matter, sort of the cold, this cold dark matter. They could also be, contribute a smaller amount of the energy in the universe

Starting point is 00:52:00 and just be very, and contribute to this number that we're looking for. and effective. It does depend on the kind of the kind of axiom, but it's, it's, yeah, we'll, we'll just have to see. Yeah, it'll be exciting. Okay, Joe, we've come to the end, but I want to ask you one of my big picture questions and actually relates to the late, great former countryman of yours, Sir Arthur C. Clarke, who said many things, including for every expert, there's an equal and opposite expert. He said, any sufficiently advanced technology is indistinguishable from magic. And he also said, the only way to discover the limits of the possible is to venture beyond them into the impossible. And that's how I got the name of this podcast. Because I am the co-director of the Arthur C. Clark Center for Human Imagination

Starting point is 00:52:46 at UC San Diego. I want to ask you, Joe, what mysterious aspect of life seemed impossible to a 20-year-old Joe Dunkley, but now makes sense and give you the courage, give yourself the courage and gave you the courage, perhaps, to go into the impossible? Well, I guess I'll answer that in a slightly, perhaps not the same way as envisaged of a knowledge thing, but I, to me, it's the thing of doing something that you didn't think you were capable of, right? So my 20, 30-year-old self wouldn't have thought that I could, for example, write a book, right? This seemed impossible. something that seems something that only, you know, someone else does, right, not me.

Starting point is 00:53:31 And so I think that the thing that I've probably learnt in the past, in the last 10 years or so, is that you can do those things. You can do things that actually seem impossible to do. I had this sort of dream. Like, I'd love to write, like, I'd love to write a book. I'd love to be able to do this. And it seemed impossible. And so then it was some advice and support.

Starting point is 00:53:51 I was like, oh, maybe I can do this. And now, you know, it seems like that's something. achievable. So to me, it's not a scientific knowledge thing that's learning, but it's a kind of what yourself is capable of. Yeah, that's beautiful. In fact, it reminds me of what Audrey Hepburn said. The word impossible tells you that nothing's impossible because the word says, I'm possible. So a little advice from Audrey Hepburn, great scientist, as I'm sure she was. But Joe Dunkley, I'm so pleased to be able to talk with you today and to benefit from the wisdom and knowledge in your wonderful book, Our Universe, available everywhere books are consumed. And I just want to add

Starting point is 00:54:31 further gratitude for being a member of our collaboration and a leader of it. And I look forward to seeing you next week is our face-to-face or screen-to-screen meeting. And I can't wait to hear from you next week, along with all the great stuff you do at Princeton University. Joe Dunkley, thank you so much for going into The Impossible. Thanks, Brian. Any sufficiently advanced technology is indistinguishable from magic. listening to End of the Impossible with Professor Brian Keating. Please support the show by rating, commenting, sharing and leaving reviews. We appreciate hearing from you and it really helps keep our universe expanding. Watch our YouTube channel at Dr. Brian Keating. That's DR. Brian Keating and join our premieres Tuesdays

Starting point is 00:55:17 at 8 a.m. Pacific Time. Follow Brian on Twitter and Medium and support us on Patreon at Dr. Brian Keating. For exclusive content, visit Brian Keating's website and sign up for his informative newsletter at at Brian Keating.com. Into the Impossible is produced with the Arthur C. Clark Center for Human Imagination in the Division of Physical Sciences at the University of California, San Diego. Produced by Stuart Volko and Brian Keating. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building.

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Into the Impossible With Brian Keating - Jo Dunkley: This Is Our Universe (#168)

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