Robert Brandenberger

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ORAL HISTORIES
Robert Brandenberger

Courtesy: Robert Brandenberger

Interviewed by
David Zierler
Interview date
Location
Video conference
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In footnotes or endnotes please cite AIP interviews like this:

Interview of Robert Brandenberger by David Zierler on April 22, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/47043

For multiple citations, "AIP" is the preferred abbreviation for the location.

Interview with Robert H. Brandenberger, Canada Research Chair and professor of physics at McGill University. Brandenberger recounts his childhood in Switzerland as the son of organic chemists, and he describes his undergraduate education at the ETH Zurich in physics. He discusses his graduate research at Harvard to work under the direction of Arthur Jaffe, and he describes his first exposure to cosmic inflation. Brandenberger describes his postdoctoral appointment at the ITP in Santa Barbara where he worked with Neil Turok and Andreas Albrecht, and his subsequent postdoctoral work with Stephen Hawking at Cambridge. He explains his initial ideas on cosmic strings as an alternative to inflation and his encounters with Cumrun Vafa and Slava Mukhanov. Brandenberger describes the origins of string gas cosmology, its implications for a multiverse and how it was received among string theorists. He discusses his faculty appointment at Brown and he explains his decision to move to McGill where the opportunity to work with graduate students was stronger. Brandenberger surmises what string theory as a testable proposition would look like, and he reflects on some of the obvious philosophical implications of unknowability in the universe. He explains the difference between a toy model and a proper theory, and he conveys optimism that string gas cosmology will advance research on dark energy. At the end of the interview, Brandenberger reflects on the idea that string theory is "smarter than we are."

Transcript

Zierler:

OK. This is David Zierler, oral historian for the American Institute of Physics. It is April 22, 2021. I’m delighted to be here with Professor Robert H. Brandenberger. Robert, it’s great to see you. Thank you for joining me today.

Brandenberger:

It’s a pleasure.

Zierler:

Robert, to start would you please tell me your titles and institutional affiliations?

Brandenberger:

Well, I’m professor of physics at McGill University. And I’m also Canada research chair. And before 2004, I was professor at Brown University. But then in 2004, I moved northwards.

Zierler:

Now, do you have an affiliation with the Perimeter Institute as well?

Brandenberger:

Yes. An affiliate of the Perimeter Institute, but you see most theorists in Canada are able to get this status. So, this doesn’t mean that much. But I do visit from time to time.

Zierler:

In what ways is the Perimeter Institute useful for your research and collaborations?

Brandenberger:

Well, you see, in general it’s useful for Canadians. Especially those who live far from the main centers. So, if you’re an affiliate you can visit once a year. And the perimeter pays your travel and puts you up. And so, for people who are at a small college, it’s really a chance to get back in touch with cutting edge research. For example, a former student of mine is now applying for a position at University of Winnipeg, which is a small place. And he is definitely planning to apply for affiliate status as soon as possible if he gets the job. Because that will allow him to continue his research productivity. Now, I’m in a fortunate position because we have a big group at McGill. So, I’m sort of less in need of this connection.

Zierler:

Mm-hmm. Robert, this is a question as much about your work style as anything else, but between your collaborations and the people who are most important for your research, how has it been for you in the past year plus in the pandemic not seeing your closest collaborators in person?

Brandenberger:

Well, I think it affects students much more than it does more senior people because students often live alone in the city which they’ve joined to do their PhD. And so, I can tell that from my students. There are a couple of my students who really are not doing very well psychologically. Others are doing OK. And there’s a correlation between whether the students are from the Montreal region or not. So, now, it does slow down research because you can’t just go to the office which the students are in and have impromptu discussions. The way that I would interact normally with my students is we would meet every day for an informal common lunch. Just everyone who was interested will join in. If it’s a nice day we go outside. Bring our stuff with. Otherwise, we meet in the common room. And that’s a way to make students feel like they care part of a group. And so, it’s this group feeling which is gone. So.

Zierler:

Robert, give me a sense of what you’re working on right now. What’s the research that’s most interesting to you in these past few months?

Brandenberger:

Right. So, I always work on a lot of topics in parallel. But maybe the one thing that I’m most excited about is an attempt to come up with a new nonperturbative formulation of early universe cosmology based on nonperturbative super string theory. So, this is actually an improvement on what Vafa and I did back in 1989. And it’s in parallel to a recent paper by Vafa and collaborators which they put out last fall. So, we are working on nonperturbative—we start with matrix theory which is a proposed nonperturbative definition density of super string theory. And we are trying to develop a cosmology of that. So, in particular, we are trying to work out what kind of perturbations and gravitational waves will be produced in this early matrix phase. So that we can then compare our model with observations. And see, the broader context of this research is a search for an alternative to inflation as a theory for the very early universe. I’ve been working on alternatives to inflation for a long time in parallel to also working on things related to inflation.

Zierler:

Robert, I’d like to ask a nomenclature question. Does the term cosmology, does that mean the same now in the field as it did when you first started?

Brandenberger:

No. So, when I started in the field—actually I got into this field completely by accident. But you can ask me about that later. But at that point cosmology was some theoretical ideas that were related to the early universe and that could solve some problems of standard Big Bang cosmology. And there was very little connection with data because we didn’t have much data [on the] microwave background; anisotropies hadn’t been discovered. We had information distance for about 1,000 galaxies. And since the late 1980s, the information about the distribution of galaxies has just increased by orders of magnitude. So, the field is very different. Also, when I entered the field in the West, people who entered the field of particle cosmology, they didn’t have a background in general relativity at all. So, for them it was completely new. And this was different from the Soviet Union where people had a broad training. There, people who entered the field had a very strong background also in general relativity. So, the field has changed, and I was lucky to sort of have been part of the transformation.

Zierler:

Another broadly historical question. When have advances in observation been most relevant for your theoretical research?

Brandenberger:

It was in the late 1990s. So, the key discoveries then were the discoveries of the acoustic oscillations in the spectrum of the microwave background. The angular power spectrum of the microwave background. So, back in, well, COBE had discovered the first anisotropies, but not the key features which are these acoustic oscillations. So, the spectrum has a big peak at one degree and then subsequent peaks at fractions of one degree. And this ruled out topological defect models as a theory for the origin of structure of the universe. So, if you go back to the mid-1980s, at that point the cosmic string theory of structure formation was a competitor to inflation. In fact, some people thought it was better because it was more based on standard particle physics field theory. It was based on Grand Unified Theory. So, it had a solid connection with what particle physicists knew about. So, we were arguing at that point that it was better than inflation. But this model did not predict acoustic oscillations, in contrast with theories like inflation. So, these discoveries by I think key was the BOOMERanG satellite—not satellite. Telescope. It was a balloon-borne telescope. I think that was key. That told us that indeed in the early universe there were fluctuations of density which was set out like those predicted or studied in 1969-1970 by Zeldovich and Sunyaev and Peebles and Yu. So, these discoveries of the late 1990s confirmed the Zeldovich-Sunyaev, Peebles-Yu paradigm of 1969-1970. And recall this is 10 years before inflation. So, when I start out my talks these days on cosmology. I always start with the slide with a copy of the figure from Zeldovich-Sunyaev where there’s a wonderful discussion of how microwave, acoustic oscillations are created. And how you get the baryon acoustic oscillations. So, the physics of that was understood by a limited number of people in the West. But I think by many more people in the former Soviet Union back in 1970.

Zierler:

Robert, what might be some of the cultural explanations for the divide between the approach to this in the Soviet Union and in the West?

Brandenberger:

Well, there was the Iron Curtain which limited our contact. And the Russian, the Soviet physicists had to publish in Soviet journals. And they generally published in Russian. So, that was a logistical impediment. But then I think culturally what happened is that in the West there was a big divide between particle physics and general relativity. General relativists didn’t care about particle physics and vice versa. Like when I was a graduate student, I never had a class on general relativity because particle physicists just in general didn’t take classes on general relativity.

Zierler:

Robert, let’s go all the way back to the beginning. I’d like to hear about your parents first. Tell me about them and where they’re from.

Brandenberger:

OK. So, I grew up in Switzerland. So, both of my parents were chemists. Organic chemists. And so, therefore I knew from the beginning that I didn’t want to do chemistry.

Zierler:

[laugh]

Brandenberger:

On the other hand, I sort of knew I had a little bit of a knowledge what it meant to do science.

Zierler:

Were they academic chemists or industrial chemists?

Brandenberger:

So, my father was sort of in between. So, he was at university, but doing a lot of service work. He was a forensic chemist. And my mother did some part-time translating of chemical abstracts. This was the days when a lot of families were single career families. So, I grew up in a little town not too far from Zürich. And I didn’t know what it meant to be a student actually. I remember one year before I finished gymnasium, I was asking a friend of mine in the same class, “What does it mean to be a student?” And he said, “Well, you sit in a coffee shop all day and read books.”

Zierler:

[laugh] Sounds pretty good!

Brandenberger:

Yeah. Anyway, but then I decided to study a combination of mathematics and physics.

Zierler:

Robert, going back even further. What were your parents and their families experience during World War II?

Brandenberger:

OK. So, my father, so you know Switzerland was surrounded by Nazi Germany. And all Swiss had to serve in the Army. So, my father did his basic training at the beginning of World War II. And when he was a student then he had to spend maybe one third of each year in active service. Basically protecting the borders. And so, he was very much part of… so these experiences were very important for him. And he always talked about them in later life.

Zierler:

Obviously you wouldn’t know this, but did your parents have an idea about the fate of the Jews of Switzerland? Were they safe or not during the war?

Brandenberger:

OK. That’s interesting. So, my father said that anyone who read independently should know what was going to happen. So, my father says that people can’t claim an excuse not to have known. He said if you listen to Hitler’s speeches then you would know what would be going on.

Zierler:

Was there anyone in your family who was in danger during the war?

Brandenberger:

Uh, can you repeat?

Zierler:

Was there anyone in your family who was in danger or suffered during the war?

Brandenberger:

Um. So, my mother was actually, grew up in the United States. So, they did not suffer in nearly the extent that many other physicists suffered. So, in Switzerland there was rationing, and all pieces of land had to be converted to potato farms. So, people who had houses had to rip up their lawns and plant potatoes. But again, these are minor perturbations compared to what many other people had to go through.

Zierler:

Where in America did your mother grow up?

Brandenberger:

She grew up in New Jersey. Just outside of New York.

Zierler:

And where did your parents meet?

Brandenberger:

They met in Boston when my father was a visiting postdoc. After he finished his PhD in chemistry.

Zierler:

And then they decided to go back to Switzerland?

Brandenberger:

Yes.

Zierler:

What language did they communicate in?

Brandenberger:

OK. They communicated in English. So, you see, I grew up trilingual in Switzerland. I’d speak in English at home, speak in German in school, and speak in Swiss with my friends.

Zierler:

No French or Italian though?

Brandenberger:

French is in school. So, French I learned in school. And Italian, I acquired by osmosis.

Zierler:

[laugh]

Brandenberger:

So, I’ve never taken a single class of Italian. But now here in the Italian part of Switzerland I can communicate with people.

Zierler:

And that’s the language of the town where you’re in? Mostly Italian?

Brandenberger:

Yes, yes. On the other hand, you see, I grew up, I had Latin in gymnasium. So, having Latin gives you a good foundation for learning French, Italian, Spanish.

Zierler:

Robert, growing up, how real was the Cold War for you? The divide of Europe? Was that something that was palpable in your everyday life?

Brandenberger:

Actually less so, I would say. See, one of the effects of the World War on the German speaking part of Switzerland is that there was quite a hostility towards Germany. And in fact, many Swiss, like my grandfather, did not want to see Germany unified. So, I would say the Cold War did not have huge impact. What had impact is the uprisings in Hungary and the uprisings in Czechoslovakia. Each time a large number of people fled and came to Switzerland. So, we were aware of what’s going on in the Cold War, but it was not something that was affecting us in our daily lives. It’s completely different from what happened in World War II because that was affecting the daily lives of everyone.

Zierler:

Robert, as I’m sure you know, many American physicists who are around your age were very much impacted by the Russian launch of Sputnik and the American response and the way that that positively benefitted federal funding of basic science. Did anything similar happen in Switzerland?

Brandenberger:

Yeah. And I’m a little bit too young to answer this question because you’re talking about something that happened in the early ‘60s.

Zierler:

1957 was the launch. And then the response.

Brandenberger:

Right. I mean at that point I was still a baby. [laugh]

Zierler:

[laugh]

Brandenberger:

So, I have to pass on that.

Zierler:

Was your sense more generally that Switzerland was generous in supporting basic science though?

Brandenberger:

See, I was totally naïve. So, I never asked that question before graduate school. See, when I started studying physics and math most of the people in my class, their dream job was to be a teacher at a gymnasium. So, gymnasium is on senior high school level. Because you were very well-respected as a teacher. And this is one thing that I would like to see happen in North America. I would like to see high school teachers very well-respected and also well-paid. So, we called senior high school teachers “professor.” So, I knew that I wanted to go into the teaching profession. I didn’t even know really what field I wanted to get into. So, I could have chosen a geography-history combination. Something like that. But I knew I wanted to go into something related to teaching as opposed to a job where you have to dress up. A nine to five job. So, I had very strong feelings about that, but I was never, I never thought about this kind of funding.

Zierler:

Tell me about your early education. What kind of school did you go to as a young boy?

Brandenberger:

So, I went to the normal primary school. And then after sixth grade, then it forks. So, you can go to a six and a half-year gymnasium. And at that point, about 10% went that direction. And those are the people who then will eventually end up in university. And so, that’s the route I took. I enjoyed school, so I went that route.

Zierler:

Besides negatively defining your interests away from chemistry, from your parents, what was it about physics for you?

Brandenberger:

Well, I knew that physics was the most basic of the sciences. And since I had a very poor physics education in gymnasium, I thought it would be good to learn something. And also, I like math. So, I knew that physics was related to math. And so, I don’t regret that I went that direction. I learned quite a bit.

Zierler:

What schools did you consider for your undergraduate education? What was available to you?

Brandenberger:

See, in Switzerland people go to the university which is close to where they live. So, there were two choices for me. The University of Zürich and the ETH, which is also in Zürich. These are all publicly funded, so you paid an inscription fee which was at that point I think 150 francs per term. And at that point a dollar was worth 4 francs. So, again, I knew that the ETH was stronger than University of Zürich for physics. So, there were no big decisions.

Zierler:

Now is the Swiss system more like the British system where you declare the major right away?

Brandenberger:

Yes.

Zierler:

And it was physics for you right away?

Brandenberger:

Right. With a contrast to the British system. See, in the British system you gradually become narrower even at the high school level. In Switzerland, not. You are completely broad and then suddenly when you join university you become narrow. The program was joint math and physics. And after one or two years you then fork either in physics or math.

Zierler:

What laboratory work did you have? Did you ever consider an experimental career as an undergraduate?

Brandenberger:

No. [laugh] Definitely not. Definitely not. I was very much the mathematical type. So, I’m not very good at technical things. I was never very good and I’m still not good.

Zierler:

What were some of the most exciting things happening in theory that you remember during your undergraduate days?

Brandenberger:

OK. So, let me tell you about the transition between undergrad and graduate school. Because, so you see, the ETH Zurich has a very strong tradition in mathematical physics. It goes back to Pauli and Fierz, Jost. And when I was a student theoretical physics was mathematical physics. Everything else was looked down on. Condensed matter physics, particle physics, completely looked down on. Well, I had to do axiomatic and constructive quantum field theory. So, I don’t know if these terms mean anything to you? You are familiar? OK. So, you want to prove that a certain physical theory makes sense mathematically. Can be rigorously defined mathematically. So, for me this was theoretical physics. And I did my senior thesis in that area with Konrad Osterwalder. And then when I arrived at Harvard as a graduate student then, first day I was shown into the office I was going to share with a couple of other students, in particular a very senior student who was already in his second year. And he very friendly asked me what I wanted to do, what I wanted to study. And all I knew was constructive quantum field theory. So, I said, “Constructive quantum field theory.” And then his answer was, “Oh. This crap. You ought to do gauge field theory.”

Zierler:

[laugh]

Brandenberger:

So, that was my welcome to the different style. Different sides of the Atlantic.

Zierler:

[laugh] Might you share who this fellow graduate student was?

Brandenberger:

Um. No. I would need his permission.

Zierler:

OK. [laugh]

Brandenberger:

He is well known as one of…least socially adept.

Zierler:

[laugh]

Brandenberger:

Theoretical physicists.

Zierler:

I can take my guesses. That’s fine. Robert, did you spend any time at CERN as an undergraduate?

Brandenberger:

No. Only when I was a postdoc, I visited for the first time. But I briefly, and then I spent a sabbatical there later on.

Zierler:

But as an undergraduate it simply wasn’t on your radar?

Brandenberger:

No. It wasn’t on the radar because theoretical physics was mathematical physics.

Zierler:

Yeah.

Brandenberger:

Constructive quantum field theory.

Zierler:

What did you do during the summers? Did you have regular summer jobs, or did you do anything that was relevant to your studies?

Brandenberger:

So, first of all, you see the summer break is not that long. And the way that things work in Switzerland is that you don’t have exams after every class or in every class. You have exams after the first year. Exams after the second year. And then final exams. And these exams are at the end of the summer. So, many people would actually do their military service in the summer. And the summer break is not that long. So, I did not do what students in North America typically do. I had one piece of research experience which was very important. And this was after a classical mechanics class by Klaus Hepp. He gave a very inspiring classical mechanics class. And one of a few very inspiring classes which I took as an undergraduate. And then at the end of the class in June, he basically said, he came to class and said, “Oh, I’ve just found this wonderful book on classical mechanics. Who would like to do a summer study on this book?” This book was Arnold’s book Mathematical Methods of Classical Mechanics. And he only had a draft copy, and it was in Russian. So, there were eight of us who joined in and we divided ourselves into groups of two and we prepared a presentation on one of the chapters. Now, recall this was in Russian. So, that was a very important experience for me. Because it sort of gave me for the first time the feeling for what something like independent work is like.

Zierler:

What kind of advice did you get from your professors about graduate programs? How did you know where to apply to, who to work with? Harvard is a long way from Switzerland.

Brandenberger:

See, at that point people did not typically leave the country. So, then out of my class about of hundred, less than ten actually were going to go outside of Switzerland. So, now I wanted to explore the US, and there was an exchange office at the school, and I went there to talk. And then I was told you never apply to more than three places. OK? So, I didn’t apply to more than three. I applied to three places. Now I applied to Princeton because I’d visited once when I was on vacation to New Jersey. And I knew that it was a good place. Now, I applied to Harvard because it was where my senior thesis advisor had just come from. So, then I also applied to RPI. OK. So, now neither Princeton nor RPI accepted the application which went through the office. Or, sorry, I never heard from them again. Harvard refused my application. They said I should directly apply. So, I directly applied. And so, the reason I got into Harvard is that my undergraduate advisor had just left Harvard and he was an associate professor there and his mentor, Arthur Jaffe, was looking for students. And so, there was personal connections.

Zierler:

Now had you been to the United States before? Were you in contact with your mother’s family at all?

Brandenberger:

Yes. Yes. It’s a small family. So, basically my grandparents on my mother’s side were there. So, I knew a particular suburb of New York and New Jersey. That’s what I knew.

Zierler:

[laugh]

Brandenberger:

But I’d never been to Boston.

Zierler:

What year did you arrive at Boston?

Brandenberger:

’78.

Zierler:

1978. And what were your impressions of Harvard when you first arrived?

Brandenberger:

OK. So, it was, I had an image of what Harvard would be which is something that is probably more applicable to NYU. Namely, a couple of tall buildings in the middle of a central city. So, I was very surprised about the beautiful campus. And then, well, I told you like one of my experiences on my first day. Another experience is that my advisor, Arthur Jaffe, told one of his other students to show me around. So, he brought me to Harvard Square. And there was a red light for pedestrians. And then he said, “Let’s cross the street.” And then I pointed to the red light and then he said, “No, we have to jaywalk, otherwise we’ll never be able to cross the street.” And I’d never heard the word jaywalk before.

Zierler:

[laugh]

Brandenberger:

So, anyway, so coming to the United States and to Harvard was a cultural experience. It opened me up to new physics and to also to the variety of people that at that point were on the Harvard campus.

Zierler:

What was Jaffe working on when you first got to Harvard?

Brandenberger:

Constructive quantum field theory. His dream was to prove that lambda-phi-4 is well-defined mathematically. But then, in about 1980 Jeurg Froelich and Michael Aizenman proved that this phi-4 theory is trivial. So, there is no interacting lambda-phi. There is no lambda-phi-4 theory in four space-time dimensions. So, his research program sort of collapsed a bit in 1980 when I was two years into my PhD

Zierler:

What were some of the most important courses you took at Harvard?

Brandenberger:

It was Sidney Coleman’s quantum field theory class and Raoul Bott’s differential geometry class. These were both two semester classes. Now, I’m sure you’ve heard of Sidney Coleman’s quantum field theory class. But Raoul Bott’s class was also very important for me. So, first of all, it was a time when algebraic geometry, differential geometry, and physics were merging. Witten was at Harvard. Mike Peskin was at Harvard. And so, in my first year at Harvard I did take both of these classes. And I remember I was used to mathematics classes in Switzerland which were taught in Bourbaki style. So, I actually had taken an undergraduate differential geometry class with Beno Eckmann. And so, I was used to going into the class and the professor would start off the lecture with a couple of definitions. And then a theorem and then a proof. So, you’d have definition on the board, then theorem, and then there was a proof. And then afterwards there were a couple of corollaries. But Raoul Bott was just talking on the board and he was drawing diagrams on the board. And I felt hopelessly lost. I was sort of asking myself where’s the definition? Where’s the theorem? And where’s the proof? So, that was the difference in doing mathematics. And in retrospect I’d learned a huge amount in that class because when I teach general relativity and when I teach about manifolds, I don’t teach Bourbaki style. I draw diagrams on the board. The same diagrams which I learned, which I saw in Raoul Bott’s class. So, these were the important classes. Now, you don’t have to take many classes as a graduate student. So, I did take some more, but these were the important ones.

Zierler:

Would anybody have said in the late 1970s at Harvard that explicitly they were working in cosmology? Or was that term too far afield at that point?

Brandenberger:

There were people at the center for astrophysics who said that they were doing cosmology. Bill Press had a postdoc who’s now a professor at University of Maryland, Bei-Lok-Hu. So, I think they would have probably said that they were doing cosmology. But I didn’t know what cosmology was during my first two years at Harvard.

Zierler:

Yeah. What was the process for you developing your thesis topic?

Brandenberger:

OK. Yeah, so, I was trying to lead up to this. So, you see my research in the first two years was mathematical. Was constructive quantum field theory. But the research program had sort of collapsed a bit. And then a friend of mine, Jennie Traschen, who was a student in the applied math department working with Doug Eardley at the Center for Astrophysics. At the end of the summer semester she suggested to a couple of her friends that we should do an independent summer study which no one with a PhD knew anything about. So, we were five of us from different departments. In fact, one of the five was from MIT. And Jennie had found a preprint—so, this was the days of preprints—by a physicist who was unknown to all of us except to myself, unknown to probably most people in the Harvard physics department, namely Alan Guth. And this was his preprint on inflation. And Jennie had found that interesting. And so, we decided that we should teach ourselves the material to be able to understand that preprint.

Zierler:

What did you need to teach yourself? What material was most relevant?

Brandenberger:

Well, so this actually developed into my Reviews of Modern Physics article. So, my job was to teach the other methods of quantum field theory, effective potential, finite temperature field theory, and bubble nucleation. Because in old inflation you generate initial conditions. You argue for them using finite temperature, effects, and you have bubble nucleation. So, this was my job.

Zierler:

What were your immediate impressions of inflation at that very early juncture now that you had the tools you could to understand it?

Brandenberger:

Right. First of all, it’s just completely new to me. And it was fun to do work in that area.

Zierler:

Because it’s so existential? It asks the ultimate questions?

Brandenberger:

No. Because this was the first experience of doing something independent. So, you see, it was sort of an accident that I ended up in cosmology. So, then at the end of the summer I went to Arthur Jaffe and I basically asked him whether I could do my PhD thesis on cosmology. And since he had four students in the same year he was quite happy that two of us departed. And so, he said, “Sure. As long as I don’t have to read it.”

Zierler:

[laugh]

Brandenberger:

So, I was Arthur Jaffe’s student. And in fact, he did read it at the end. So.

Zierler:

Who was there? Who would be your informal advisor as it were?

Brandenberger:

And the informal advisor became Bill Press.

Zierler:

A-ha.

Brandenberger:

So, and Bill Press had, so one of the five was Bill Press’s student. And so, Bill wanted the student to look at cosmological perturbations. Because Bill had already worked with Ethan Vishniac on cosmological perturbations. So, I joined in applying the theory of perturbations to inflation.

Zierler:

Were you in touch with Steve Weinberg at all during this time?

Brandenberger:

I sat in on a class which he gave on gauge theories. But I did not have discussions with him.

Zierler:

Do you know if he was interested in cosmology at this point?

Brandenberger:

Well, yes, definitely. So, he’s one of a few people who was interested [in this] for a long time. In fact, he wrote the book on Gravitation and Cosmology. I think it was back in the 1970s. But he was at that point already only spending a half-time at Harvard. And he was—

Zierler:

What were your main motivations in your thesis research? What was most important to you to understand and to convey to others?

Brandenberger:

OK. So, when I started my PhD thesis, there was a lot of ignorance in terms of cosmological perturbations. That was not a tool that was known to really anyone in the West. Well, no. With one exception. That one exception being Jim Bardeen. So, now, Bill Press and Ethan Vishniac had written a paper called “Tenacious Myths About Super Hubble Perturbations.” Because inflation made it clear that there was a possibility that there was a causal theory that would generate perturbations in the early universe. Bill Press realized that back in 1980. He has a paper in a Nobel Symposium which hardly anyone cites where he basically says quantum perturbations in the inflationary phase could give rise to what we see today. But he didn’t work it out quantitatively and so, we were going to work out quantitatively. And I guess this is what we did. But a bit too late.

Zierler:

What was too late?

Brandenberger:

Well, our papers were 1983. So, it was after the papers which are now quoted. So, in particular, it was long after the paper by Chibisov and Mukhanov. And it was also after the paper by Alan Guth and So-Young Pi and after Hawking’s paper and after the paper by Bardeen-Steinhart-Turner.

Zierler:

Robert, what was your reaction to how rapidly inflation was changing? It was improving from one iteration to another. What might that have told you more broadly about the theoretical value of inflation in those early years?

Brandenberger:

No. You see, it was not changing that rapidly. That was not my impression. There was a transition between old inflation, which Alan Guth already said could not work—there was a graceful exit problem—and new inflation. So, we were always working in the context of new inflation. But then it took until Andrei Linde became a professor at Stanford that most people in North American made the transition between new and large field chaotic inflation. So, I don’t have the impression that there was much change at all. There was the initial idea of inflation. Goes back to, well, we thought it went back to Guth. Obviously, we didn’t know about the previous works.

Zierler:

Were you in contact with Paul Steinhardt at all during these years?

Brandenberger:

Yes. So, my first year at Harvard he was a junior fellow. And-- so I heard talks that he gave at Harvard. And I applied for a postdoc with him once I graduated. But it’s only since then that I really, that I’ve been in contact with him. So, I would say since that time that I applied for a postdoc I’ve always been sort of in touch with him.

Zierler:

Who was on your thesis committee? That’s going to be an interesting question for you given the situation with Jaffe.

Brandenberger:

Right. So, Jaffe was the chair. And then—

Zierler:

So, he was the chair who never read the paper?

Brandenberger:

That’s right. That’s right.

Zierler:

[laugh]

Brandenberger:

And Bill Press, obviously because I worked with him. And then Sidney Coleman because I was also a TA for him. And then you see, we knew that these defenses were not, they were like, it was your chance to talk about the work that you’ve done. So, it was that. So, I invited Steve Weinberg to come. And he assumed that he was part of my committee. And so, when he entered the room, Coleman was there. Bill Press was there. But Jaffe was not there. So, I remember Weinberg looking around and say, “Oh, we can start now.” And that’s when he found out that he was not official committee member. [laugh]

Zierler:

[laugh]

Brandenberger:

I think he was not so happy with that.

Zierler:

Any memorable questions from the oral defense?

Brandenberger:

No. No.

Zierler:

It was very pro forma?

Brandenberger:

Pro forma. No. The memorable question was actually during the seminar which Jennie Traschen and myself gave a year after I had graduated. I don’t know if you’ve heard about the gauge seminar?

Zierler:

Mm-hmm.

Brandenberger:

The Harvard gauge seminar. It was held in this little room which had a couch and a couple of chairs. But usually, almost all people were standing. So, Jennie and I had worked on reheating. So, we basically-- so Jennie and I had discovered that perturbative reheating just doesn’t work. It misses the entire point. So, we worked out a better analysis. And so, we gave this gauge seminar and there was Coleman, Weinberg. And so, we started. And each step Coleman was ahead of us. So, we wrote down the equation and Coleman said, “Parametric resonance!” And sure, for cosmology, that was a new thing. Parametric resonance. But it’s in Volume 1 of Landau and Lifshitz. And it’s in Mathematical Methods of Classical Mechanics. It’s actually the topic which I prepared for this summer study with Klaus Hepp. So, we were so depressed after giving this seminar ‘cause we said, “Oh. This is nothing new. Coleman knew it all. There’s nothing surprising for him.” We didn’t publish our results for six years. But then we did, and this became a paper on preheating. Except that we didn’t invent the word preheating. And then the paper lay there. It was a 1990 paper. It lay there without citations for four years. Until others invented the word preheating.

Zierler:

Robert, what postdocs did you consider? Did you think about going back to Europe after Harvard?

Brandenberger:

No. At that point, I’d become Americanized. So, I did what everyone else did. You just apply to all good places that you would consider including some places in Europe. And so then luckily, I got an offer from Santa Barbara from the ITP which is now KITP. And it was again, also connections because while I was at Harvard, my second de facto mentor was Doug Eardley. He was Jennie Traschen’s PhD advisor. And he was just going to move to Santa Barbara. He was going to give up his tenured faculty position at Harvard to move with his wife to Santa Barbara. And so, he sort of-- it’s through him that I got my first postdoc.

Zierler:

Did you look at the—

Brandenberger:

But for my second postdoc I was actually in Europe.

Zierler:

Did you look at the first postdoc as an opportunity to take on new research or to continue and refine what you had done at Harvard?

Brandenberger:

No. It’s during the first postdoc that I began to do the work on cosmic strings. See, going to the ITP was maybe the best thing that could’ve happened because the ITP was running these half-year programs. So, two half-year programs in parallel. So, each half-year I got exposed to different things. And there were lots of postdocs. And so, I was exposed to new tools in condensed matter physics. And I was exposed to cosmology work from the data side. Because in second semester of my first year there was a program on cosmology. Out of which effectively the Lambda-CDM model emerged. So, and it’s at that point that I started working with Neil Turok, and also with Andy Albrecht. And we were trying to push the counter theory which was a cosmic string model. So, it was a little bit of a competition.

Zierler:

Who did you work with at the ITP?

Brandenberger:

I worked mainly with Neil Turok, who was a postdoc in Santa Barbara. And Andy Albrecht who was visiting. He was officially a postdoc in Texas. But he probably spent half of his time in Santa Barbara. And then I had lots of interactions with for example, Nigel Goldenfeld in condensed matter. Faculty now at Illinois. I don’t know if you know about him?

Zierler:

Mm-hmm.

Brandenberger:

You know him? So, he was very instrumental. And my mentor was Doug Eardley. So, obviously I kept him informed of what I was working on.

Zierler:

Was the ITP an exciting place to be at that point?

Brandenberger:

Yeah. The ITP was a really exciting place.

Zierler:

How long were you there for?

Brandenberger:

I was there two years. See, at that point postdocs were two years.

Zierler:

Mm-hmm.

Brandenberger:

It was very rare to be a postdoc for more than two years. That changed quite soon after I graduated.

Zierler:

Were you publishing a lot at this point?

Brandenberger:

Yeah. I was publishing quite a bit. I was publishing enough to get a second postdoc, put it this way.

Zierler:

And where were you considering for your second postdoc?

Brandenberger:

So again, second postdoc I did the same thing that other people did. I applied everywhere. All the good places. But then what happened, I got an ultimatum from Cambridge, England. From Hawking. He asked me to make up my mind about his offer before New Year’s Day. So, I went to Cambridge.

Zierler:

Hawking played hardball.

Brandenberger:

He played hardball.

Zierler:

What was your connection to Stephen Hawking? When did you first meet him or know of him?

Brandenberger:

Well, actually I knew of him already when I was an undergraduate because Klaus Hepp, who was this very instrumental professor, he actually did teach a sort of a special topics class. Not a full-fledged class. A special topics class. Maybe it was one month’s worth of lectures. And he actually took the Hawking and Ellis book as textbook. So, we knew that he was famous. I knew that he was famous already then. And you see-- OK, when I was a graduate student a classmate of mine, David Grossier, he spent a semester of leading a study group on this same book. I was not part of that study group actually. And it was mainly mathematicians. And I remember David, every week he would come to me and say, “Oh, I found a mistake in this proof in the Hawking-Ellis book.” And then a week later he would come and say, “But the theorem is still right.” So, when I was in Cambridge I asked Stephen once what he thought about that book. And then his answer was, “Hopefully I’ve learned how to write better in the meantime.”

Zierler:

[laugh] What was Stephen working on when you got to Cambridge?

Brandenberger:

So, he was working on quantum cosmology. So, see, I met him in Santa Barbara because he was already working with Jim Hartle. And so, he was visiting Santa Barbara. So, he was working on quantum cosmology. I was not working on quantum cosmology. I never worked on quantum cosmology. But, Stephen had a big group. I really got to appreciate him as someone who has deep physical insights and he gave me lots of advice. We had something very unusual for Britain. We had a lunch seminar because Stephen used his prize money. He was starting to get prizes at that point. He used it to fund this Friday lunch which everyone in the group attended. Two of the students had to buy the food and every week someone would give a presentation on their work. So, I gave a presentation on my work on cosmic strings and Stephen gave very good feedback.

Zierler:

What was it like to communicate with Stephen?

Brandenberger:

OK. So, when I arrived in Cambridge in the beginning of October, ’85, he had just lost his voice. He had this tracheostomy. And he was in the hospital actually and he was really intent to see me because he didn’t want me to desert Cambridge like his previous postdoc. His previous postdoc who’d come from California didn’t like Cambridge and deserted Stephen’s group to join a very small startup company in Seattle called Microsoft. So, he didn’t want to me to do that. Obviously, I had no intention to, but so my first meeting with Stephen was a complete disaster because at that point we had to try to communicate with spelling board. But then by the time a couple of months later that he got back to the department, the computer system was installed and then it worked well. So, you learned how to read his eye movements. So, you would often tell him things and then you would sort of say, “Do you think this is right?” And then you would just look at his eye movement. So, it took time to get used to communicating with him, but he was very communicative. He enjoyed interacting with his group. He was very social.

Zierler:

Were you in touch with Michael Green at all at this point?

Brandenberger:

No. See, I met Michael Green in Santa Barbara because when I was in Santa Barbara this was the time of the first superstring revolution.

Zierler:

Right.

Brandenberger:

And he was visiting for some time. So, I knew Michael Green. But I was not in touch with him. Now, this question comes out of the blue. Some of your other questions I could see where they were coming from. But this one I can’t see where it’s come from.

Zierler:

Well, just the timeframe and the fact that there is the string revolution in 1984.

Brandenberger:

Yeah. That took place when I was in Santa Barbara. We had a study group, where we tried to teach ourselves string theory. And when I arrived in Cambridge during my first year I actually gave a series of two seminars at the Institute of Astronomy to the group of Martin Rees and George Efstathiou on superstring field theory. So, I did do some study on that. But I was working at the time mainly on cosmic strings.

Zierler:

Did you see cosmic strings already as an alternative to inflation?

Brandenberger:

Yeah. See, cosmic strings we saw as an alternative back in 1985. That’s why we got into working on cosmic strings. See, now I’m still working on cosmic strings part-time. But now it’s not an alternative to any theory of structural formation. It’s something supplementary. It’s something that can leave imprints and we can use these imprints to test particle physics models. But it won’t teach us about the main source of structure formation. So, cosmic strings were ruled out in ’98 as a theory that explains all of the structure that we observe in the universe. But at that point while I was in Cambridge it was definitely an alternative.

Zierler:

You said that Hawking gave you positive feedback on cosmic strings. What did he say?

Brandenberger:

No. He gave me positive feedback on specific projects I was doing. He was always more pro-inflation. But he likes controversy. So, he was happy that there was someone in his group working on something on a controversial approach. And he even wrote a paper on cosmic strings himself. He was interested in whether the cosmic strings can lead to early black hole formation.

Zierler:

Now, as a matter of intellectual history did you get into cosmic strings because you saw some deficiency with inflation and were specifically looking for alternatives?

Brandenberger:

No. No. I was still much too naïve. Basically inflation was completely new. Cosmology was completely new. And cosmic strings were something that sounded exciting to work on. And you could make a link between particle physics and cosmology.

Zierler:

What was that link?

Brandenberger:

And then very soon we actually saw that it had the potential to be an alternative to inflation.

Zierler:

And what was that link between particle theory and cosmology?

Brandenberger:

Well, that link was present already in the late ‘70s. It was the domain wall problem. Cosmic strings do not have such a problem. They are the type of defects that can be present in the early universe without causing disasters. And they leave interesting imprints which we worked out. Imprints of the cosmic microwave background. These line discontinuities in microwave anisotropy maps.

Zierler:

When was there the most excitement or optimism that cosmological theory could have a real handle on t = 0?

Brandenberger:

What is t = 0? So, I will throw the question back at you. So, we knew that the standard Big Bang is insufficient. And we also knew that inflation is insufficient. Already back then we knew that inflation has a singularity problem. So—

Zierler:

And when you say that it’s deficiency would people like Andrei Linde and Alan Guth, would they agree with that? Or that’s where the controversy is?

Brandenberger:

No. They would completely agree with that. They would completely agree. In fact, one of the papers that I’m quoting is a paper which Alan wrote with Borde and Vilenkin about this initial incompleteness. There was an earlier precursor by Borde and Vilenkin. No, no. There’s no disagreement with either Andrei Linde or Alan on this issue. There has to be something before inflation.

Zierler:

But that before is epsilon or it’s zero?

Brandenberger:

No. So, now comes the controversy, you see. So, what is the theory that holds before inflation if there was even inflation? Is there, does time begin or does time run from minus infinity to plus infinity? Is time emergent? Now these are questions that we were not asking at all back in—by we, it means people who were doing cosmology related to the data. Hawking was. Hawking was doing quantum cosmology. He had an answer to the question of what is the origin, and this was the wave function of the universe approach. So, I think the first time that I touched on the question of what is of the t = 0, or t = 0 not existing, that’s the work I did with Cumrun Vafa.

Zierler:

When did you first meet Cumrun?

Brandenberger:

So, I met him when he had just arrived at Harvard. I think first as a junior fellow. I had just started my job at Brown. And I would go once a week to Boston. I helped Alan Guth and Alex Vilenkin set up the early universe seminar series. And I had a desk courtesy of Bill Press in the physics department. I had a desk in the office that he wasn’t using. And this was actually quite close to the office that Cumrun was using. So, I was giving seminars and at that point Cumrun approached me and he was asking, “We should think about whether we can embed inflation into string theory.” So, we started talking about that. We met maybe every week. I would come up every Tuesday. And then eventually we realized that, no, inflation doesn’t seem to emerge from string theory. But there’s something better that emerges from string theory. And this is string gas cosmology.

Zierler:

What was Cumrun’s original goal? What does that even mean to embed one into the other?

Brandenberger:

Well, OK. So, Cumrun had heard about all the successes of inflation in terms of explaining the data. And at that point inflation was being advertised as predicting things as well. So, it predicted these acoustic oscillations in the angular power spectrum. But actually it does not predict. That thing was known 10 years before inflation. All you need is the spectrum of super-Hubble fluctuations. Approximately scale-invariant super-Hubble fluctuations. For me, this is an important point. So, anyway, there was all this excitement about inflation and then so, well, inflation was being treated as a scalar field toy model. That’s not very satisfactory so let’s try to see if inflation can arise coming from string theory. So, can we go back to the principles of string theory and see whether in the low energy limit we get inflation. That was the goal. The conclusion was that this was, that inflation did not seem to arise from string theory. But we found something better. And we found something that had no conventional Big Bang singularity.

Zierler:

Was anyone else thinking along these lines? This is a truly unique insight at this point?

Brandenberger:

Actually, there was. Neil Turok had a paper on string inflation. He wanted to get inflation driven by a gas of strings. But that never really took off. I think there was a technical problem with that. But then our work spawned interest in Europe. There was pre-Big Bang cosmology by Gasperini and Veneziano. But our work was at the time when interest in string theory was dwindling. So, there was very little follow-up. In particular in the string community.

Zierler:

And what years are we talking about now when the interest in string theory is dwindling?

Brandenberger:

That’s sort of ’89. And then interest picked up again during the second string revolution. And then you see the second string revolution. After that a large number of people were trying to embed inflation to string theory because at that point observations had come in. They showed the acoustic oscillations in the angular power spectrum. And at that point, there was nothing in the market which was competitive at the level of having worked things out. So, inflation was the first model based on causal physics and on well understood field theory techniques where you can do calculations of the early universe and you can carry the calculations forward to the present time. And so, therefore a lot of string theorists said, “Well, obviously we need inflation. So, let’s try to force inflation into string theory.” So, now, so some people would not agree with the words that I’m using here. Force inflation to string theory. So, there was about a five-year period of activity on that. And I was not working on this at all during this period. And with one caveat which I can come back to afterwards. And then Cumrun had heard about all these activities, and he was basically wondering whether we had missed something crucial. So, he devoted I think one year of the string seminars at Harvard to cosmology. So, he invited all these people working on string inflation model building to give seminars at Harvard. He invited me to talk about theory of cosmological perturbations. My thesis was on cosmological perturbations. And this is difficult stuff. And I made many errors in doing this work and I sort of said, “This is the end of it. I want to do something different.” But it stayed with me, you see. I then wrote this review article with Slava Mukhanov, which is another story that you could pick up on. But why did I write this review article on cosmological perturbations? But anyway, I wrote this review article and so I was considered by Cumrun to be an expert on the topic, so he invited me to lecture on that.

Zierler:

Well, let’s go there now with Mukhanov. When did you first meet him and why did you write this review article?

Brandenberger:

OK. Good. So, I met him in 1987 at a meeting in Hungary during the time that the Iron Curtain was still in existence. It was a very famous conference. Even Zeldovich was there. And there was a Russian contingent. A Soviet contingent. Which came with a KGB agent. So, there was one of this Russian contingent who only showed up in the first couple of minutes of each talk and the first couple of last minutes. And he only looked around who was in the audience. That was obviously the KGB agent. So, that’s when I met Mukhanov. And at that point, in his very bad English, he did present his work on fluctuations. I was actually given a preprint, a copy of his preprint, by Bill Press when I was a graduate student. Not that I really understood it. Because it was not all that well written, and my Russian was not that good. So, Bill Press knew that there was this Russian school. And through Bill Press, I knew that there was a Russian school as well. And I already knew back in 1983 that what the Russian school had done was superior to what was done in the West. With the one exception being Jim Bardeen, whose name appears on the Bardeen-Steinhardt-Turner paper. So, I’m again hesitating here because I’m using words carefully. His name appeared on the Steinhardt-Turner paper. [laugh] So, you see, Bill Press was quite well-known and he was getting preprints of various stuff. So, I have something in my drawer back at McGill. A particular pre-print.

Zierler:

[laugh]

Brandenberger:

Anyway, so I met Slava during this meeting and he’s quite an outgoing person. But he was still in the Soviet Union and when the Iron Curtain broke down, then a lot of the most famous Soviets got job offers in the West. But he was not at that point one of the most famous ones. But the economic situation was disastrous for him and he had a baby at that point. So, he invited me to Moscow a couple of times. And he basically said, “We need to write this review article.” Because he knew that I had done some work on cosmological perturbations and his habilitation thesis was a detailed review of cosmological perturbations. And so, then I had got him a visiting position at Brown for a total of one year. And that’s the time when we wrote this article.

Zierler:

How was it received? This review article?

Brandenberger:

Well, in retrospect, it was obviously received very well. So, it’s become the standard review. I was happy when it was finished.

Zierler:

[laugh]

Brandenberger:

Because it was a lot of work.

Zierler:

Robert, you mentioned a caveat of something else that you were working at we should return to. What was that?

Brandenberger:

Yeah. So, jumping back forwards to inflation and string theory. There was this period roughly 2004-2005 when Cumrun Vafa again was interested in cosmology. He sort of wanted to see whether we had missed something. And so, he listened to all of these talks on inflation and model building from string theory. And he asked me to lecture on cosmological perturbations. And at the end of my visit when I lectured on cosmological perturbations he basically said, “Well, none of what I’ve heard from these other talks is really string theory. We need to return to our old work.” And so, that turned into some work which I consider crucial. This is work that Ali Nayeri, who at that point was a postdoc at Harvard, Cumrun, and I did. And also my student, Subodh Patil, joined in. So, we worked out-- we realized that some of the fluctuations of this string gas in the early universe can give rise to almost scale-invariant spectrum of cosmological perturbations. [It] gives rise to scale-invariant spectrum of gravitational waves with a slight blue-tilt of the spectrum of gravitational waves. So, at that point we turned string gas cosmology into an alternative to inflation. So, and then the work that wanted to return to—so, actually, I had gotten interested in this issue of moduli stabilization. In string theory you have many extra dimensions. We don’t see them today, so they are small. And in the string inflation literature there’s all of this technical work on moduli stabilization. And I realized immediately that string gases can stabilize moduli. So, we worked this out as well. This was the work of my student Subodh Patil. OK. So, let me halt for a moment. [End Session 1] [Start Session 2]

Zierler:

And we’re back. Robert, did you recognize right away that this was an alternative to inflation? Or that was a gradual intellectual process?

Brandenberger:

No, we immediately realized that it was an alternative to inflation.

Zierler:

What did it provide answers to that inflation did not?

Brandenberger:

It comes from string theory. So, it’s a better theoretical underpinning. Now it makes different predictions. So, it predicts a slight blue-tilt of the gravitational wave spectrum. Whereas inflation always predicts a slight red-tilt. And you see, if the BICEP result had been right, that would have been a blue-tilt.

Zierler:

What was the response to the founders of inflation?

Brandenberger:

So, I guess until the work in 2005-2006 with Cumrun on fluctuations, we didn’t really advertise string gas cosmology as an alternative to inflation. But yes, so after that, so when these papers came out then that caused quite a stir. So, a former student of mine, Stephon Alexander, at the time when our work came out he was a postdoc at Stanford, SLAC. And he tells me the story that he was in a group meeting of the Stanford theory group and in comes Andrei Linde with a copy of this preprint. And he says, “This is serious. We have to destroy it.”

Zierler:

[laugh]

Brandenberger:

And he’s been trying to destroy it, so he then had a paper claiming that we did the fluctuation calculation wrong.

Zierler:

How was string gas cosmology received within the broader string theory community?

Brandenberger:

Actually, to my surprise, OK. Again, our work was at the tail end of the period where many string theorists were interested in cosmology. So, our work was I think was poorly received by the subset working on string inflation models. Quite poorly received actually.

Zierler:

What was the criticism?

Brandenberger:

See, the criticism was that we did not do a self-consistent calculation. That our stuff was too qualitative, not quantitative enough. That’s especially a criticism for the background. So, I got that criticism especially from people in the West Coast.

Zierler:

What was your response?

Brandenberger:

Well, so I remember once when I gave a colloquium at the KITP, then I got some good criticism from Kachru and Polchinski that basically, this was related to the stabilization of the moduli. The radii of the extra dimension. And they challenged us. They said, “Well, supersymmetry is broken today. And this will destroy all the stabilization mechanisms.” So, that was what they claimed. Now, this was something that at that point I hadn’t considered. And indeed, it was a possibility at that point it was a reasonable challenge. But then we worked this, and in the process we showed that no, that the dilation can be stabilized at the same time as the moduli breaking supersymmetry. So, basically, this was an example of constructive criticism that led us to improve the scenario.

Zierler:

As an intellectual—

Brandenberger:

I was not claiming that string gas cosmology was better than inflation at that point. So, at that point, if you would’ve asked me, I would’ve said inflation has more than 50% chance of being correct. Not inflation driven by scalar field, but inflation per se has more than 50% chance of being an important ingredient in the evolution of the universe. And I would’ve just said that it is useful to have, to also explore some alternatives in the same way that it was useful in the 1980s to work on cosmic strings as an alternative to inflation. Because when you have two different theories that make different predictions, then that will also motivate observers, experimentalists, to look for interesting things. So, like there was an emphasis on looking for these acoustic oscillations because it was a way to tell apart these two different paradigms. In the same way I’m saying that we should try to aim at getting at the tilt of the gravitational wave spectrum because that will basically tell different scenarios apart. It will tell inflation apart from scenarios which are based on string theory.

Zierler:

Now when we say that string gas cosmology is an alternative to inflation, does that suggest that there’s no other game in town? Are there no other alternatives to inflation?

Brandenberger:

No. Not at all. It is one alternative. But not the only alternative.

Zierler:

No, but I’m saying in the way that the field looks at inflation. Is your sense that inflation is the one on the pedestal and competing theories are looking to inflation to knock it off that pedestal?

Brandenberger:

Yes. Yes, because most of the community thinks that inflation has been established. Like you read astronomy textbooks and they have inflation as part of the standard paradigm. They say the universe started with the Big Bang. Then there was a period of inflation.

Zierler:

And in such elegant, simple terms, what is the alternative statement that string gas cosmology says about the beginning of the universe?

Brandenberger:

So, string gas cosmology is an example of an emergent paradigm in which there’s an early phase in which it doesn’t make sense to talk about standard spacetime. No effective field theory description. And then there’s a phase transition into the standard Big Bang phase of expansion.

Zierler:

So, there is a Big Bang in string gas cosmology?

Brandenberger:

No. Well, so, if you mean a hot early universe, yes. But if you mean an initial singularity the answer is no. The evolution’s completely nonsingular.

Zierler:

How might this relate to theories relating to the multiverse?

Brandenberger:

So, I’m conservative on that issue.

Zierler:

In other words, if the Big Bang is not a—

Brandenberger:

I was never keen on the multiverse because I think you give up the idea of making predictions. For example, the multiverse explanation of a cosmological constant. We just say we don’t know. We give up trying to explain why it is vanishing or why it is so small. And there could be so much interesting physics hiding in there. In fact, I think there is. And if you apply multiverse ideas you’re sort of just giving up. OK, but now there’ve been changes, you see. Now I will no longer say that inflation has more than 50% chance of being correct. I will now say that inflation has substantially less than 50% chance of being right. And this is due to this recent work on the Swampland program and the trans-Planckian censorship conjecture. So, this led to a big change in the way that I view the field.

Zierler:

Tell me about the origins of the Swampland work.

Brandenberger:

Well, you see the way that I phrase it is that if you stick with quantum field theory, you stick to scalar fields, you stick to the way that we do cosmology usually with scalar fields. Maybe classical or semi-classical. Then you can do anything you want with classical fields. Any spacetime dimension goes, any number of fields goes, any potential goes, any field range goes. You can get any cosmology you want with an appropriately chosen scalar field potential. So, that’s this huge Swampland of effective field theories. But if you have a consistent quantum theory of gravity then only some small islands stick out of this Swampland. These habitable islands. So, this is the way that I would describe this. So, superstring theory’s very constraining unlike what people often say is that everything goes in superstring theory. No. It’s the other way around. Superstring theory is constraining. And so, it just looks like shallow potentials, like you need for inflation, just don’t work. You can’t embed them naturally into string theory. And worse than that you have this new conjecture which is this censorship conjecture. (Trans-Planckian censorship conjecture. Bedroya and Vafa, 2019.) So, for me this is a natural generalization of Penrose’s censorship conjecture. So, Penrose says that well, in the context of general relativity you can construct solutions which have—you can construct black hole solutions with charge bigger than the mass. But they are pathological. They have naked singularities. You can’t set up a good initial value problem. And so, Penrose says, well, fundamental physics has to be, cannot allow these solutions. The effective field theory will admit them, but the real physics will exclude them. So, that’s how I paraphrase Penrose. So, now what we do is we say wavelengths smaller than a Planck length corresponds to the black hole singularity. And the black hole horizon which shields the observer from this singularity, that turns into the Hubble horizon. And so our conjecture says that nothing-- that the physics of these trans-Planckian modes has to be hidden from the observer. The wavelengths can never grow to be super-Hubble. And that essentially is a deathblow for inflation in my view. Formulated as an effective field theory. Now it may be true that you have a completely nonperturbative approach to inflation for which the Swampland arguments do not apply. And there are some proposals for that. Dvali has a proposal and my colleague, Keshav Dasgupta, has a proposal.

Zierler:

The reason, Robert, I asked about the multiverse was because if the Big Bang is not a singularity perhaps that might just suggest that it originated from another universe.

Brandenberger:

Yeah, I could talk about that too. So, there could be what Slava Mukhanov calls “the Russian doll universe.”

Zierler:

Yeah.

Brandenberger:

And in fact, we have a paper on that, which we call the Matryoshka universe.

Zierler:

Matryoshka being, of course, Russian for the Russian doll. [laugh]

Brandenberger:

See, it’s ironic that I’m using the Russian word and Slava used the English word.

Zierler:

What is your reaction to Linde’s proposition that the multiverse is a testable proposition?

Brandenberger:

I have to admit that I didn’t read about that. My prejudice is to strongly disagree with that.

Zierler:

If the physics community generally accepts inflation as standard, with the caveat that we can’t know for sure, what might we be waiting for observationally or experimentally for verification one way or another?

Brandenberger:

You mean falsification?

Zierler:

Yes.

Brandenberger:

The blue-tilt of the tensor spectrum.

Zierler:

How would we arrive there and what would that look like?

Brandenberger:

OK. So, what we need to do is we need to measure gravitational waves on very different length scales. For example, if you find B-mode polarization in the microwave background and you can verify that it’s from the primordial gravitational waves and at the same time you find gravitational waves using improved pulsar timing arrays. So, then you have the amplitudes of the gravitational wave spectrum at these two different length scales then you can go after the tilt.

Zierler:

Is LIGO geared towards asking these questions?

Brandenberger:

No. LIGO doesn’t have the sensitivity and LISA— will not, either. The most promising right now looks like pulsar timing arrays and B-mode polarization to give you the two data points. It may be the case that an improved LISA will also be able to do it.

Zierler:

Robert, tell me about the intellectual partnership with Cumrun. Who did what? What did each of you bring to this partnership?

Brandenberger:

He brought the knowledge of string theory and I brought the knowledge of—in particular in this later work—cosmological perturbations.

Zierler:

So, the sum is obviously greater than the parts?

Brandenberger:

Definitely. So, this is the way that I like to work. I like to work with people who can provide a very different background. I think that’s the way you make progress. See, even when inflation started, inflation was a product of bringing together ideas from different areas of physics. Condensed matter physics and field theory and particle physics. So, that gave rise to a revolution. You combine these different ideas. It also gave rise to topological defects and cosmology. The same combination of ideas gave both inflation and topological defects. And I think that the time is right to have a different combination of fields to make a qualitative step forward.

Zierler:

Where is bounce cosmology in all of this?

Brandenberger:

OK, so bounce cosmology is also an alternative scenario. Time runs from minus infinity to plus infinity. You need new physics that will provide the bounce. Now, personally, I actually think that the Ekpyrotic scenario is a very promising one. Among bouncing scenarios, it’s the Ekpyrotic bouncing cosmology which is the most promising one because in the Ekpyrotic bouncing cosmology you don’t have to finetune initial conditions. Anisotropies are diluted, curvature is diluted. So, an Ekpyrotic bounce gives you a lot of the same successes as inflation, as large field inflation. So, now I don’t know if you’ve interviewed Paul Steinhardt.

Zierler:

I have.

Brandenberger:

You see, Paul will say that the Ekpyrotic scenario does better than inflation in terms of solving the various problems. So, I’m not sure it does better, but it does well. So, yes, I think—so, when I give talks about early universe cosmology, I mention the inflationary scenario, the bounce scenario, and the emergent scenarios as three scenarios to pursue. In the bounce scenario you also assume that fluctuations start out as vacuum perturbations like in inflation. In the emergent scenario, no. There they are produced more in a thermal way. So, I think these are three interesting scenarios to pursue. And maybe there are others. But the one thing is that among bounce scenarios it’s definitely the Ekpyrotic one which is the one that is worthwhile to consider.

Zierler:

By definition, must they be mutually exclusive?

Brandenberger:

No. But you see, the beauty of a scenario for me is if you can take one scenario to explain everything. So, if you have to merge two scenarios to explain things, to me it’s less interesting. So, see in fact, I think if you talk to Andrei Linde he would say, “Well, I’m happy with string gas cosmology to explain the resolution of the singularity. But there’s inflation to explain the perturbations. There still was inflation because it’s so beautiful.” That may be it’s perfectly possible that you have an emergent scenario and then afterwards you have inflation. So, these scenarios are not exclusive. But I would like to try to get the best out of one scenario before mixing scenarios together.

Zierler:

Now when you say that in the bounce scenario we need new physics, where does trans-Planckian physics come into this?

Brandenberger:

Well, so when I say you need new physics, in order to get a contracting universe to become an expanding universe, you can’t do that if you stick to general relativity and the usual types of matter. So, therefore, you need to change your action. And that’s where the trans-Planckian physics enters. So, I have proposed a new version of the Ekpyrotic scenario where we use an S-brane motivated by string theory to give you the transition from contraction to expansion. So, that’s where the trans-Planckian physics enters. It does not enter into the evolution of the inhomogeneities. It’s safe from that. So, the Ekpyrotic scenario or string gas cosmology automatically satisfy the trans-Planckian censorship conjecture. They are not in trouble with this non-unitarity at the level of fluctuations.

Zierler:

And you mentioned cosmic strings coming to an end in 1998.

Brandenberger:

As the source of 100% of the fluctuations.

Zierler:

Has that been resurrected at all? Or things are where they were since 1998 in that regard?

Brandenberger:

No, in that regard, there’s nothing to resurrect. It’s clear. But it doesn’t mean that you shouldn’t give up on cosmic strings because you know, many particle physics models beyond the Standard Model have strings. Have string solutions. And there will be a network of strings forming in the early universe. They will have imprints. Now we know that we can actually constrain particle physics models by looking for imprints of cosmic strings and not finding them. So, we can set an upper bound on the energy scale of the phase transition and beyond the sand in bottle physics which produces the strings. So, as a mechanism that produces 100% of the structure that we observe today, strings are dead since ‘98. And they will not be resurrected. But they have great interest. And strings forming at a lower energy scale, they could give rise to lots of interesting things. They could explain supermassive black holes. They could explain the origin of primordial magnetic fields. And that’s an active area that I’m working on. So I’m probably spending at least one-third of my research effort in that direction because it’s lots of fun and it combines astronomy with particle physics. And not many people are working on this. So, it’s a great field if you don’t work fast. I don’t work fast. I like to work slowly. And also, it’s a great area for student projects because if you ask a good question then independent of how the result turns out you will have an interesting result. Cause if you find evidence for strings it would be wonderful. It would be something completely new. If you find no evidence for strings in the window that you’re looking at, you will have further constrained particle physics models. So, for students it’s a win-win situation.

Zierler:

Robert, what have been some of the advances in black hole research that have been useful for your work?

Brandenberger:

You know I know too little of that to…I don’t think I should say much. So, you know if you take Einstein’s action then there’s a singularity inside of a black hole which also means that the theory breaks down. So, in the same way that there’s a singularity in the early universe according to this effective field theory, there’s a singularity inside of a black hole. Now my view is that the ultraviolet complete theory has to resolve both. So, I think for people working on fundamental theory these are two areas to apply their ideas to. Now we’ve recently taken an idea from string theory which we used to construct this new version of the Ekpyrotic scenario. And we applied it to black holes. And what we find is the Matryoshka universe. But you see, I feel I’m a bit of a dilettante when it comes to black hole research.

Zierler:

Robert, we’ve been so closely engaged purely in scientific ideas. To get back to the community, the sociology-- first, was Brown a good home department for you to do your work? Was it a good department to be in at the time?

Brandenberger:

OK. Well, so you see, when I got the offer from Brown I asked a friend of mine who’d been an undergraduate there whether it was going to be a good place for me. And he said, “Yes! For someone like you, it’s going to be a good place.”

Zierler:

What does that mean do you think? Someone like you?

Brandenberger:

For me, I always viewed the teaching as my job and the research is like the hobby.

Zierler:

And Brown’s emphasis on pedagogy was going to work for you?

Brandenberger:

Well, no, but Brown is not, as a research university, is not big enough to compete with Harvard. But it has the best students. In terms of the quality of undergraduate students it can compete with Harvard. So, in that sense, it’s a good place. And it’s close to Boston, so.

Zierler:

And that was always important for you? Your role as a teacher?

Brandenberger:

Yeah. I enjoy working with students.

Zierler:

What were some of the key classes you taught at Brown?

Brandenberger:

I taught a mathematical methods class. I taught GR. I taught astronomy and undergraduate cosmology. Mostly.

Zierler:

Did you take on many graduate students at Brown?

Brandenberger:

OK. That was my biggest problem. The biggest problem was to get funding for graduate students. Our group—actually, I may have to move because a couple of rain drops. So, just, these are just a couple of rain drops, so, just…OK. I think this should be OK. It was not supposed to rain, but you see it’s in a mountain region and you never can tell. OK. We were talking about students.

Zierler:

And the funding issues for graduate students.

Brandenberger:

Right, right. Now our group at Brown was five faculty and we typically had money for one postdoc. If we were lucky, two postdocs. And then maybe the group as a whole could take on three graduate students. So, that meant at most one for me. So, that was a problem. And I was using kind of other tools to try to get more funding. At one point I had applied for, under the Education Department, under the GAANN program, Graduate Assistantships in Areas of National Needs. So, I would say that was the biggest struggle I had. Especially compared to McGill. At McGill, right now I have nine students. And a lot of students have their own funding. Universities in Canada are public, so you have no tuition to pay. You just pay the living expenses for a student. Good students have their own funding. So.

Zierler:

Was the graduate issue one of the things that compelled you to look for opportunities beyond Brown?

Brandenberger:

OK. So, it was a two-body problem.

Zierler:

A-ha.

Brandenberger:

I’d gotten married and there was no position for my wife in Rhode Island. She’s also an academic. And then she got a job in Canada.

Zierler:

What year did you arrive at McGill?

Brandenberger:

2004.

Zierler:

Were you aware? Did you have colleagues that you knew of there already? Or was this all new to you?

Brandenberger:

No, I did have-- someone I was a postdoc with at Cambridge is at Université de Montréal. So, I knew him and I’d been to Montreal to give a seminar in his group. And so, I’d met some people, Cliff Burgess and Jim Cline. And then when they found out that my wife had gotten a position in UQAM – University Quebec a Montreal. So, they invited me to be visitor and then the position opened up.

Zierler:

What were your impressions of the physics department? I don’t know much about it at McGill.

Brandenberger:

OK, so the physics department is a bigger physics department than Brown. I would say, OK, and especially for me it’s actually a better department. There are more people for me to interact with.

Zierler:

Among your colleagues you mean?

Brandenberger:

Among my colleagues.

Zierler:

Who else was doing cosmology at Brown? Or was anyone else?

Brandenberger:

That’s the point. When I came there was no one else and while I was there we established an experimental group. So, Rick Gaitskell doing dark matter research and Greg Tucker doing microwave background. And Ian Dell’Antonio who does weak gravitational lensing. So, I scientifically I had lots of connections with them. But on the theory front it was, I mean I did talk to Antal Jevicki and then later on David Lowe. I was happy there.

Zierler:

And so who was at McGill? Who gave you more opportunities for collaboration within the department?

Brandenberger:

Now, you see, I don’t have the history of working with senior people, but more with junior people. So, it’s more the interactions. Like when we have our theory seminar at McGill then all of the faculty are interested if it’s a cosmology talk. They don’t just pretend to be interested, but they do their own calculations. Just the colleagues that I have, they have a fairly broad range of interests. I’m lucky in that respect.

Zierler:

When did you get involved in baryogenesis research?

Brandenberger:

Can you ask the question again?

Zierler:

When did you get involved in baryogenesis? Looking at baryogenesis?

Brandenberger:

I’ve done very little work on that. You’ve looked at my CV.

Zierler:

Yeah.

Brandenberger:

OK. You’ve looked at my webpage. So, I was actually thinking of eliminating that from the list.

Zierler:

It doesn’t really belong.

Brandenberger:

Well, you see, the origin of the matter-antimatter asymmetry is an important question. And in the context of cosmic strings, I did some work on it. And very recently as well, but it’s not one of my main areas of interest. There are a lot of people who are working on it. And I don’t see, I don’t have really new things to contribute. I sort of like to work on areas where I think I can contribute new things.

Zierler:

You’ve spoken publicly about the testability of string theory as it relates to cosmology. And of course, as you well know one of the fundamental criticisms of string theory is that it’s not testable.

Brandenberger:

Yes.

Zierler:

So, how do you counteract that and are the ideas that you present universal to string theory or are they limited to your cosmological area of expertise within string theory?

Brandenberger:

They are limited to my cosmological expertise. But I believe that the cosmos is going to be where we will collect most of the data. So, I am actually quite a vocal opponent of the view that string theory is not testable. So, I’m sometimes more vocal than some string theorists on that. And I will mention this blue-tilt of the tensor spectrum as an example. So, what I will say is the following: that if BICEP had indeed measured a blue-tilt of gravitational waves instead of just dust, then this would have confirmed a prediction which was first made in a context of string theory. So, then I will say I’m not a string theorist, but string theorists should celebrate this. But will I say that will this prove string theory? No, it will not prove string theory. But it will be a success. So, the Standard Model was very successful. It is not completely right. So.

Zierler:

Broadly conceived, what would it look like to prove string theory?

Brandenberger:

See, I think you can only disprove something. So, what I will say is that you should make lots of predictions and you should try to verify as many of the predictions that you can. And each time you verify your prediction you should be happy and celebrate. And I think you should be a little bit modest and this will not prove a theory. Now, there are also theoretical points. So, the theory should be, it should solve some theoretical mysteries. So, it should be the best on the market in terms of solving theoretical mysteries. And here I will again say that in terms of quantization of gravity in the context of unifying all forces of nature, string theory is the best in town.

Zierler:

In your recent work where you talk about the unknowability, of not being able, science or the universe not really allowing us to understand how it came into existence. Have you ever thought about the parallels that many religious traditions have on exactly that same idea? That these are unknowable concepts? They are beyond what humans can understand?

Brandenberger:

I know that there’s a boundary there. There’s an area where these subjects meet. I’m very much aware of that, but you see, if you go back 200 years people studied natural philosophy and philosophy in general. So, any scientist would take a minor as philosophy. Nowadays it’s separated, so.

Zierler:

Right.

Brandenberger:

So, I don’t have the background from the philosophy side to contribute, nor from the theology side. But I think it’s an area for discourse. And in fact, I talk quite a bit to philosophers of science. In particular to philosophers of cosmology.

Zierler:

And to clarify this point, the unknowability is not a matter of us waiting for some new theory or technology or observational advance to shed light on this. What you’re saying is that those limitations transcend any advances that we might be able to make ourselves.

Brandenberger:

That’s what I was saying, but that’s a belief. That’s my belief. Which is, now I’m speaking as a complete amateur.

Zierler:

Yes, but your amateur belief is informed by your scientific expertise, of course.

Brandenberger:

Nah. I don’t think so. I don’t think so. But you see where I think I can say a little bit based on my scientific expertise is that I think we will never know where the boundary is. Things that now appear to be in the domain of philosophy and theology, we might suddenly realize that no, they are the domain of science. Like why is space 3-dimensional? Why is spacetime 4-dimensional? Well, this was in the domain of philosophy and theology. But now not. Now it’s in the domain of science. But I do think there’s always going to be a boundary. See, another area where you could make this explicit is in terms of the question about the origin of the universe. In Standard Big Bang cosmology and inflationary cosmology, there’s a Big Bang singularity. Now there’s a philosophical problem. What does it mean to have space and time emerge?

Zierler:

And by singularity you simply mean there was nothing and then there was something.

Brandenberger:

There was first nothing and then there was something and also things were infinite. Temperature was infinite when this something emerged. So, I mean both aspects. So, now if you look at a bouncing cosmology, specifically the Ekpyrotic scenario, well, you don’t have that question. Because the universe was always there. Time was always there. But you still have to ask why do we come from a contraction phase? And why do we start cold and large? So, there’s still a philosophical problem even in that scenario. Will be a different one. So.

Zierler:

Do you see a specific philosophical problem with the singularity suggesting that the universe must have created itself?

Brandenberger:

I don’t know what that means, the universe created itself.

Zierler:

Well, if there was a singularity what else besides the universe could have created the universe?

Brandenberger:

So, that point I agree with you. But I still can’t answer the question.

Zierler:

But the point is you’re saying that the question is unanswerable.

Brandenberger:

Right. How can the universe create itself? So. But now we are venturing in the direction where I’m not, where I don’t want to go because I simply cannot say anything educated.

Zierler:

What are the future prospects of string gas cosmology do you think? Where is the field headed from here?

Brandenberger:

We need to improve the framework. Right now it’s too qualitative. We have to make it more quantitative.

Zierler:

Is the issue gathering more data or better interpretation of the data that we have?

Brandenberger:

No, no. It’s on the theoretical side. So, the scenario is good enough to do calculations, to make predictions for observations. But it’s turning the model into a theory. So, you see, if you work with scalar fields and inflation, it’s a toy model. We want to see inflation coming from a theory. That was the dream for people working on string inflation. In a similar way I view the scenario Vafa and I proposed as a toy model. And we need to turn it into a theory. So, our toy model is based on some nice ideas. Or my ideas which I find nice. Ideas that come from string theory. It’s not yet something that mathematically minded string theorists will accept as a theory.

Zierler:

What does it look like for a toy model to graduate into a theory? What does that transformation look like?

Brandenberger:

So, in this context it’s something very ambitious because we will have to start with a nonperturbed formulation of string theory. And people don’t agree on what it is. Actually, I can move back to the sun. The rain has stopped. Just a few drops. So, basically it will mean starting with a nonperturbative formulation of string theory and then trying to study where the string gas cosmology emerges. Or something like string gas cosmology. And I’m very optimistic that the answer will be yes. So.

Zierler:

If it is yes, how will you know when you get there?

Brandenberger:

Well, experts in string theory will say that we’ve made progress. So, I will have to revert to higher authority here because I’m not a string theorist myself.

Zierler:

[laugh] As an outsider’s perspective, but somebody who’s very close to string theory, nowadays string theory has diverged. There’s not just one, there’s many kinds of string theory. And there’s some string theorists who are more purely involved in math and those more you know, involved on the physics side of things.

Brandenberger:

Right.

Zierler:

Is your interface with the string faced community broad? Do you concentrate your collaboration, your communication with a particular kind of string theorist?

Brandenberger:

Yeah. It’s the string theorists who are working on string theory as a theory of space, time, and matter. So, the people who are working on string theory as the theory that unifies all forces at the quantum level. So, I wouldn’t interface with people who are working on details of string amplitude calculations or specific mathematical features.

Zierler:

And when you say unifying all forces of course you include gravity.

Brandenberger:

That’s right.

Zierler:

What are the prospects do you think for unifying gravity?

Brandenberger:

Well, string theory is a candidate for that. At the perturbative level we know that it works. The point is that we don’t have a complete theory at the nonperturbative level. But there are proposals.

Zierler:

How has your research, if at all, contributed to other fundamental mysteries in cosmology such as dark matter and dark energy?

Brandenberger:

So, to dark matter not much. And to dark energy the research program that I have is right now not yet related to string theory. Although I think ultimately there will be a relationship.

Zierler:

What might that look like when that relationship is achieved?

Brandenberger:

Well, see, if I knew that I would have publications.

Zierler:

[laugh] Tell me about some other things in the past five years, a little more broadly in the recent past, that you’ve been working on.

Brandenberger:

Well, see, one thing that, let me ask you a question because when you emailed me you sort of mentioned that you had interviewed a lot of people and concerning inflation that you wrote you thought that there was something hiding.

Zierler:

Not hiding, but when you talk to people in inflation the excitement from 40 years ago was much greater than it is now. That if you talk about what was happening in 1981, 1982, 1983, it seemed like we would be a lot further along than we currently are. And that made me think well it would be very interesting to get your perspective as somebody who’s proposed a competing model.

Brandenberger:

Right. So, I think most people, most physicists, and most astronomers have been told that inflation is going to be part of the story. And so, I would say a lot of people are, who worked initially on inflation, are sort of happy their work seems to have done something important.

Zierler:

Meaning that it’s in the canon so to speak?

Brandenberger:

Yeah, it’s in the canon. And so, obviously when you create something new, when new ideas come together, this is an exciting period. And simply what I think, there’s been, people have not made similar progress. So, maybe specifically, the merging of string theory ideas with cosmology have not yet been successful. Completely successful. I think they—

Zierler:

Although it might be hard to define what completely successful exactly looks like.

Brandenberger:

Right. I would say completely successful, it would mean either embedding the current favorite paradigm of cosmology into string theory or else creating a new paradigm which is viewed as at least as successful or maybe better. So, that’s how I—so see, I think we’ve made-- we are making attempts, or we’ve made attempts to propose something new. But it has not made it to the, it hasn’t been able to replace inflation.

Zierler:

Yet.

Brandenberger:

Yet. Right. So, I’m excited. Excited actually in a different way than I was excited back when I started in the field. Cause when I started in the field everything was new to me. And everything was exciting. And it happened to be in a period where important new ideas were being developed. But now I’m excited because I see the challenge. Namely to be specific to combine cosmology and string theory in a nice, consistent way.

Zierler:

Robert, tell me about some of the most important work that some of your graduate students have done as a sign that the field is headed in an exciting and an optimistic place.

Brandenberger:

Right. OK. Well, you see, I will try to avoid that question. But I’ll go back to saying that I view my role, my job, as that as a teacher. And teaching has various levels, but mentoring students is part of it. And so, I’ll just point to the fact that a large number of my students are now faculty. And they are excited about the field as well and trying to contribute. And every student is different, so every student is contributing in a different way. So, there’s Mark Trodden, University of Pennsylvania. There’s Stephon Alexander, extremely creative. Have you ever interviewed him?

Zierler:

No. I should.

Brandenberger:

You should. You should. So, you see, some of the students are still working with me on trying to solve, to unravel this mystery. Try to push cosmology a step ahead. Now as a student you are also supposed to move away from your advisor.

Zierler:

Right.

Brandenberger:

Go a different direction. So.

Zierler:

Robert, we haven’t talked about computers at all. To what extent are computers important for your research or not?

Brandenberger:

Well, that’s a handicap which I have. So, I don’t like computers. So, I had a bad experience when I was a student with computation. So, that’s a limitation for me because given that we have all these observations, high quality observations, it is absolutely important to make quantitative predictions. Not just qualitative. And here I really need collaborators to be able to turn qualitative into quantitative predictions. And this is becoming increasingly important. But you know there’s a division of labor. You have some new ideas which first of all, you first develop the qualitative predictions and then someone else has to come in and help out with the quantitative predictions. So, right now we are finishing a paper on how cosmic strings which are superconducting could explain the EDGES signal in the 21-centimeter absorption. I don’t know if you’ve heard about it?

Zierler:

I have not. No.

Brandenberger:

OK. So, there’s a new window to explore the universe which is in 21-centimeter radiation. It’s different from optical and microwave. And with that you can explore the distribution of matter before star formation, but after recombination. So now, there’s an experiment which has detected an absorption signal at a particular, in a narrow range of redshift. About 17. And so, the qualitative idea is now cosmic strings produce things that give rise to some extra absorption and these things are already present at high redshift. So, you see, qualitatively that you can make a connection with observations without doing computer work. But it is not enough now because we know the feature, the exact redshift dependence of the feature. And with pencil and paper you cannot just work this out well enough to be competitive. So, now I’m teaming up with someone who has a numerical code and then we can produce, predict the signal. Now maybe something else that I can mention is that when I was, started out in the field, theoretical cosmology was one unified area. There were people like Paul Steinhardt who were doing both fundamental theory development and connections with data. And now what’s happened is that the field is divided. There’s hardly anyone who does fundamental theory development and applications to the data.

Zierler:

This is a problem you think?

Brandenberger:

This is a problem.

Zierler:

People are too specialized.

Brandenberger:

People are too specialized. This has slowed down the progress. There’s a couple of notable exceptions. David Spergel and Ue-Li Pen are exceptions. But this is also a reflection of the fact that cosmology has grown so much.

Zierler:

Yes.

Brandenberger:

We went from having essentially no data to having a huge wealth of data and more is coming in every year. So, it’s a fun time to be working in cosmology.

Zierler:

And as you suggest, collaborations are limiting if you’re only relying on other people for a segment of knowledge, whereas somebody like as you said earlier, Paul Steinhardt, crossed those boundaries. Or at least those boundaries didn’t exist at that point.

Brandenberger:

Yeah. No, he really crossed the boundaries. He’s quite remarkable.

Zierler:

Robert, we’ve worked right up to the present, so I’d like to ask for the last part of our talk to ask you about some broadly retrospective questions about your career. So, the first is talking about things that are fundamentally mysterious, the unknowns in the universe. What over the course of your research at the beginning was not well known in the field—not just you, but in the field generally—that is really well-known and agreed upon today?

Brandenberger:

I would say the distribution of matter in the universe on large scales. And the microwave background anisotropies. So, we didn’t know that such anisotropies existed. Now we know they exist. We know that they are approximately scale-invariant. We know many properties of that. And we know now that the distribution of galaxies converges to homogeneity on large scales. We didn’t know that when I entered the field. So, the standard Big Bang, the cosmological principle was sort of a principle that we made for simplicity. And now we know that the universe actually looks approximately like it is based on the cosmological principle. So, yeah, now we also know that the universe, that there’s this mysterious dark energy component. So, we didn’t know that there was this issue. So, in a certain sense there are more interesting questions than there were when I entered the field. There are new ones. So, it’s not that we just start out with a set of questions and then we slowly solve one and there’s one less. You solve another one, two less. Actually, sometimes when we develop a new theory, we solve something, we suddenly see there are very interesting new questions which are emerging. And so, in this aspect, it’s a dark energy mystery which is a key one.

Zierler:

You mentioned earlier than Stephen Hawking liked controversy. Not political controversy, but scientific controversy. Did you stay in touch with him? Did you get a sense of how he was following these developments after you left the postdoc?

Brandenberger:

I stayed in touch with him. So, well, I can give you a little story. So, there was this debate about the Ekpyrotic scenario in the two years after it was first proposed. And Neil Turok at that point was professor in Cambridge. And he organized a little conference. Maybe quite a big conference. And Stephen was also one of the organizers. There were a couple of other people. And so, this was basically a conference to discuss the Ekpyrotic scenario. And then together with Fabio Finelli, I had found that there were some mistakes in the original paper. In fact, a key mistake in what was claimed about the fluctuations. I had a preprint on that. And then a couple of the junior core organizers knew about that. And so, they went to Stephen and said, “Is it OK if we give Robert 15 minutes to present his view?”

Zierler:

[laugh]

Brandenberger:

And then Stephen answered, “No! He needs half an hour!” So, because Stephen was looking forward to the controversy.

Zierler:

On the basis that controversy is good for science, it’s good to have a robust disagreement over ideas.

Brandenberger:

I think so. But I’m not sure. I don’t know Stephen well enough to answer that question.

Zierler:

On that basis, do you think that your work has been useful for inflation? Have you and Cumrun forced inflation to improve?

Brandenberger:

[pause] Unfortunately, no. See, our work with cosmic strings did do it, it forced inflation to improve. It forced people working on inflation to really study the literature on fluctuations and to show that their model gave the same predictions that Zeldovich and Sunyaev had worked on. Now I have, I’m disappointed that the work on string gas cosmology has not led to a similar cross fertilization. So, it’s more controversial.

Zierler:

Why do you think that’s the case? Why has that cross fertilization been lacking?

Brandenberger:

Well, some people feel they deserve a Nobel Prize for inflation. And therefore, they don’t want a criticism. And then also, there’ve been attacks on inflation which I think go overboard. So, for example, when Paul Steinhardt claims that inflation is not a good model, like it doesn’t make any predictions, I think these are just historically wrong because the simple model of inflation predicted a scale-invariant spectrum of cosmological perturbations with acoustic oscillations that explains what is now observed.

Zierler:

And that rises to the threshold of being predictive?

Brandenberger:

Yeah. So, if you were to find that the universe has a large spatial curvature, that would rule out the canonical, the simple inflationary models. At that point, Slava Mukhanov would give up on inflation as he says in public all the time.

Zierler:

As you may know, John Schwarz likes to say that string theory is smarter than we are. Does that resonate with you?

Brandenberger:

Yeah. That resonates.

Zierler:

What does it mean to you?

Brandenberger:

Well, to me it means that we do not understand it. We don’t have a nonperturbative formulation string theory. So, it does not reveal its deeper structure.

Zierler:

Again, it brushes up against philosophical considerations about string theory being a human construct as opposed to being a purely objective illustration of physical reality.

Brandenberger:

Right. But what physical theory is not a human construct? So… I don’t think string theory is any different in that respect. As you see, the reason why I make this second statement is I think string theory does make predictions for observations.

Zierler:

Exactly.

Brandenberger:

It’s not just…so.

Zierler:

Robert, on that point, looking forward to the future for my last question. Best-case scenario, what will it look like in terms of combining testability and string theory with real world observations?

Brandenberger:

OK. The best-case scenario is that we suddenly have a way to see how dark energy emerges from the nonperturbative structure of string theory. And that leads us to make very specific observation predictions. See, it will be something that is neither cosmological constant nor quintessence. It has a chance to be absolutely new. That’s my best-case scenario.

Zierler:

What will it look like when we get there and how will our understanding of the universe’s origins be improved as a result?

Brandenberger:

See, that I don’t know. Basically, it will say in this best-case scenario we will have quite clear evidence that the key aspects of string theory are part of the overall picture. And I think at that point there will be new questions that will have to be answered.

Zierler:

But at that point those are questions that we cannot even conceive of yet.

Brandenberger:

That’s right. That’s right.

Zierler:

Do you see that in historical terms even from your career that there are questions that you can ask now that could not have been asked 30-40 years ago?

Brandenberger:

Just talking about dark energy. Yes. Yeah, I think that’s the best example.

Zierler:

Robert, it’s been great fun spending this time with you. I’m so glad we were able to do this. I’d like to thank you so much.

Brandenberger:

Yeah. It was fun.