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Credit: Niklas Björling, Stockholm University
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Interview with Hiranya Peiris, Professor of Astrophysics at University College London and Director of the Oscar Klein Centre and Professor of Cosmo-Particle Physics at Stockholm University. Peiris describes her dual affiliation, she discusses diversity in STEM over the past year, and she surveys the current interplay between theory and observation in her field. She recounts her childhood and family heritage in Sri Lanka and the circumstances that led her family to relocate to the United Kingdom. Peiris describes her interests in math and science the opportunities that led to her enrollment at Cambridge as an undergraduate and a formative experience at JPL in California. She explains her decision to pursue a PhD at Princeton, where she worked with David Spergel on WMAP. Peiris discusses her postdoctoral appointment as a Hubble fellow at the University of Chicago to continue to work on WMAP, and her subsequent work as a Halliday fellow at Cambridge. Peiris discusses her work on the Lyman-alpha forest and her faculty appointment at UCL where cosmology was just coming into maturity. She conveys the excitement as WMAP results were becoming available and her contributions to the search for dark matter. Peiris explains why the LSST project is so significant, what it was like to win the Breakthrough Prize, and the gratitude she feels by having eminent physicists as mentors. At the end of the interview, Peiris emphasizes the importance of following inquiry into the most fundamental questions surrounding gravity and space time, and why Stephen Hawking remains an intellectual inspiration to her.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is April 21st, 2021. I am so happy to be here with Professor Hiranya Peiris. Hiranya, it's great to see you. Thank you for joining me today.
Thanks very much for asking me. I'm really honored.
Hiranya, to start, would you please tell me your titles and institutional affiliations? And you'll notice I pluralized them because I know you have more than one.
That's right. So I'm a professor of astrophysics at University College London. And I'm also the director of the Oskar Klein Centre, and also professor of cosmo-particle physics, at Stockholm University.
When did your affiliation begin at Stockholm?
Has it been more or less difficult now that presumably you haven't been traveling back and forth to Sweden, since the beginning of the pandemic?
It's definitely been more difficult, because of course if you work in two places, you like to spend time in both places, but ironically because I have the split appointment and I was spending half my time in each place, I already had a lot of experience managing my group in the other place remotely. So we had been using Slack and Zoom and all of these tools, and really used to working remotely and very effectively for several years. So actually, from that point of view, it didn't disrupt the work that much. Of course, there are other factors that disrupt work, but not that one in particular.
In your capacity as a mentor to graduate students and postdocs, where are you most likely to work with the next generation of astrophysicists?
What do you mean by where?
I mean, either in London or in Sweden, between your affiliations?
Both. I tend to have more PhD students in UCL, just because I'm much more familiar with the British system. So I tend to focus on postdocs in Sweden, and students at UCL. But it's working out really well in both places. This is a topic that is of great importance to me, mentoring students and postdocs is one of the most pleasurable things about the job.
Hiranya, in terms of your overall research agenda, do you look at your dual affiliations synergistically? Do you make sure that they work together, or do you tend to keep your responsibilities and the research that you're doing separate between the two institutions?
I think it's a bit of both. There are some areas of my research which are particular to the research environment and other collaborations within each institution. But increasingly, I do see synergistic developments, you know, where not just me, but other staff in both institutions have started collaborating on common interests as well. So that aspect has been synergistic.
I'd like to ask a question about your work during the pandemic. Not so much on the sociology and traveling side, but just on the science itself. In what ways has remote work, not seeing your colleagues in person, in what ways has that held back the research and in what ways perhaps not doing all of that traveling, not doing all of that commuting, perhaps given you more time or bandwidth to work on some research problems that you might not otherwise have been able to over this past year plus?
I definitely think that it's harder to be creative when you're not in the same room with people, bouncing ideas off each other and doing calculations on the board and trying to get to the heart of a new problem and scope it out. So, I think there's been less creativity and more time for progressing existing projects. Nevertheless, there's been a couple of new projects, new directions, that started during the pandemic, but they have been with existing collaborators. There has been a difficult circumstance with a new student. She is still in New Zealand due to travel restrictions. So, there's been a 12-hour time difference, and it's very hard to have the sort of back and forth discussions that are needed, especially at the start of a PhD, to get someone on board with, you know, what's going on, what to do, introduce them to lots of other people, get them support in the local environment. So, for new people, I think it's been especially hard to integrate into a new environment and a new research team, and so that's where I think most of the downsides have been. Also, I don't know whether it's true for everybody, but when you're trapped in your house and there's no stimulation from going to places and talking to people, or even just being able to like just get beyond ten miles of your house, you know? [both laugh]
Yeah, so I think definitely a lower amount of creativity.
I'd like to ask two broad questions that I think will punctuate much of our discussion. Again, one sociological and the other relating to nomenclature. Just as a historical capsule, we should note yesterday the murderer of George Floyd was found guilty. And this is in many ways a capstone to what has been a very painful year for STEM in the United States. A year of racial reckoning and a year of really serving to commit to making physics and science generally more inclusive. I wonder from your perspective, from the Swedish perspective, from the British perspective, as a woman of color, how did these issues resonate with you in the European and not the American context?
They resonated very much with me, but I've also spent nine years working in the US. So, I know the context in which these events are happening very well. These events did resonate very heavily not just in society, but also within universities, almost in exactly the same way as in the US. Our police doesn't go around shooting people as much, but there are definitely similar kinds of issues that happen between the police force and the black community in London. Extra stop-and-search and so on. I have to say, you know, I have not had the pleasure of teaching a single Black student, despite the fact that there's a large Black community in London.
Sweden is quite different because racial mix is extremely different, and yet especially driven by junior researchers, there was a great awareness of the Black Lives Matter movement, and there we also participated in the community events and days of reflection that had been organized by US Black astronomers. And so, as a woman of color, this of course presents an opportunity to educate and drive change, but also is extremely difficult, because you're confronting personal issues that you've experienced in the past and maybe had to set aside in order to get on, right? And so we've started anti-racism discussions in both places. Sometimes the discussions in those groups reveal a lot of well-meaning but uncomprehending attitudes. These are not people who are actually unhelpful. These are people who want to help, and yet the—
They lack the tools perhaps? The cultural sensitivity.
They lack the tools. Lack the tools, lack the cultural sensitivity, but also there's an aspect of self-education that you have to do before you come to a discussion like this. And many people haven't done that self-education, or don't even realize the need for it. They think that they can come uneducated to such a discussion, when you would never think that way about a physics discussion, right?
One wouldn't come to a physics discussion without any background knowledge, without knowing the facts, knowing the research, and then offer your opinion as if it has the same value as everybody else's. But in these kinds of anti-racism or other equity-related discussions, often physicists come to those discussions thinking they don't have to have learned anything before they talk. So sometimes that aspect of it is distressing. On the other hand, I feel like despite that sort of friction, such discussions are contributing to concrete initiatives to improve the culture and the environment for underrepresented minorities.
One of the exciting developments I think this year is the emphasis that not only should science be a driver for diversity, but we should convey the fact that as a scientific truth, diversity is good for science. In other words, a multiplicity of perspectives actually drives home better scientific accomplishments. And so, I'd like to ask, not so much from even a gender or a racial perspective, but a perspective of national culture.
I'll just share with you one incredible example. So, Andrei Linde explained to me that growing up in Russia, he couldn't just go and get coffee. It wasn't so easy. And that when coffee was available, there was a lot of excitement and you savored it more. And he criticized the American consumerist perspective as it relates to theoretical physics where he said so many of his American colleagues treat theories as throw-away items. Where they'll consider them for a little bit, they won't treasure them, they won't give them all of the attention they deserve, and he said that that actually hurts the science. Which I thought was such a… has nothing to do so much with race or gender, but from his position as somebody who grew up in the Soviet Union.
I wonder if you've ever reflected on that from a more national perspective, not so much from other aspects of your identity?
Yes, so I don't know if you know this, but I was born in Sri Lanka. I moved to the UK when I was 16. So I absorbed the cultural attitudes and you know, the viewpoints of a very different culture. And I can relate to the sort of thing that, you said, Andrei mentioned. Trying to think about problems under constraints actually makes you much more creative. It allows you to see things in a way that somebody who's used to a huge amount of choice or resources might not see. Sometimes people come up with sort of Rube Goldberg machines to explain a set of observations, by constructing a very baroque theory. It's not elegant. And this concept that a theory is elegant is informed by, you know, what constraints do you have to simultaneously satisfy about the known universe, and also make that theory general enough to explain other things as well, right? So, I think that aspect of it really does matter. If you're allowed to write without any constraints, you can come up with complete hodge-podge, whereas if you have to write a sonnet, there's a set of rules. And [laugh] so that somehow sharpens your ability to communicate and visualize and be creative.
So, I think I have also seen that aspect of it definitely in a cultural sense. I also think that there's a separate aspect of… people who've had to translate their thoughts from one culture to another. Not the language, rather the culture, from one place to another, can often see things in very different ways to people who are steeped in the ideas of one culture. You know, most cultures have a religious background to them as well, different sets of ideas. You know, the universe being infinite or being cyclic or whatever. If you come from the Sri Lankan, Buddhist, Hindu cultures, it's completely normal. [laugh] Because those ideas are very familiar. So I think—
It was never a steady state universe, you mean? [laugh]
No. It never was, and so that wouldn't have been surprising, the fact that there was a beginning wouldn't have been surprising, but for different reasons than the Christian worldview. So yeah, I think that mixing ideas and viewpoints from different cultures is another aspect of diversity that really enriches science.
I'd like to ask a very broad nomenclature question. It will also have I think sociological aspects, because these terms mean different things to different generations, and even in different countries. Okay, so we have cosmology, we have astronomy, we have astrophysics, and we have, some people like to say is a different field, particle astrophysics. In your career, where are the boundaries, and where are the overlaps, in these disciplines? And have these boundaries and overlaps changed over time from where you see things?
Okay, so you said astronomy, astrophysics, cosmology, and particle astrophysics.
And you're—feel free to add more if you want.
But those are, that's usually where I'm operating in for your field.
Okay, I see. So, I would say that the core of my work is within cosmology, but also theoretical and observational at the same time. So I try to connect theory with observations. That's my main competence. I have done some work which is very much under the guise of astronomy or astrophysics as well, and increasingly I work on particle astrophysics. And the boundary between particle physics and astronomy, astrophysics as well. I've also worked with theoretical physicists, who are thinking about very fundamental issues. And I also now work with condensed matter physicists. So I'm quite an interdisciplinary sort of person. But I would say all the questions I want to know the answers to are tied in some way to trying to understand the origin and the evolution of the universe.
Given that you work so closely at the interface of theory and observation, is your sense right now generally in the field that theory is leading observation or that observation is leading theory?
Very much the latter. In fact, I would say technological innovation is leading everything. Building better detectors and creating new ways of observing the universe. That's what is leading the field. That is driving observation. And I think theory is very much behind at the moment. And I mean that in several different ways. The fundamental theoretical ideas that could have explained the properties and the empirical parameters that we have measured about the universe, many of those ideas haven't really paid off in the sense that we have not been able to find specific observational evidence for these theories. Like we don't know, what is the nature of dark matter. We can't explain dark energy. We have theories for the origin of structure in the universe, but we haven't found a unique smoking gun.
So those theoretical ideas are still tantalizing and present, but we haven't been able to find specific evidence to narrow down the field for at least a decade. And in the meantime, the amount of data we are gathering is set to explode yet again. Very big surveys across the electromagnetic spectrum are about to start up. Gravitational wave observatories are going to generate huge amounts of data. So we might start to drown in this data, unless we have theoretical concepts to organize our thoughts about what to look for in this data. And I think there is a danger in the field right now.
As you well know, so many theorists in cosmology and astrophysics are lamenting the fact that theory in these fields are somewhat stuck right now.
Yes, I think that's true.
Given the fact that so much data is about to come out, what are you most excited about that will help unstick some of these theories?
So it's an interesting question. I think that in the absence of guidance from theory, the thing to do is to take our model of the universe, which is the Lambda-CDM model, which is an empirical model, right? It's an empirical description of the universe that makes precise predictions. So if you don't know which of a set of hundreds of competing—I wouldn't even call them theories—models is the right way to extend Lambda-CDM or work out its underpinning physics, I think it makes sense to precisely test the predictions of Lambda-CDM and look for deviations from them, without really having too much prejudice about which of the theoretical sectors you're going to focus on. Within that broad landscape, of course you can look for many, many types of deviations in many places. Which gets to the problem we have right now in cosmology, which is about tensions between datasets. I don't know if you've talked to people about tensions?
We have many situations where you're trying to measure the same cosmological parameter with different datasets. And they come up with slightly conflicting answers by, you know, a couple of sigma. Nothing to really send the alarm bells running and definitively point to new physics, and also difficult to distinguish from data systematics. So you know, I think it is actually quite possibly quite a dangerous time for the field. We could drown in these tensions as well. I'm developing methods which are statistically principled, to work out when is a tension signaling something real versus when it's just fluctuation of the noise. That's a much harder problem than one might think, when you're dealing with complicated, high-dimensional datasets. But that's the strategy I will take on this. Then when some very compelling new idea comes along, I will have the tools at hand to robustly test it. Yeah so in general my approach is to try to test the predictions of Lambda-CDM with as much precision and accuracy as I can, with as many independent data sets as I can.
There are a couple of exceptions currently to this approach of testing the predictions of Lambda-CDM where I am investing in testing specific theories. One is that I feel like there hasn't yet been enough work on testing the hypothesis of axion dark matter. So that's something I've invested quite a lot of time into recently, and at Stockholm we fortunately got funding for a research environment which includes Frank Wilczek as well, where we are developing new experimental concepts to detect axion dark matter in the lab, and also trying to connect that with astrophysical tests of axion dark matter. So that's a specific hypothesis we are testing there.
And another thing that I'm very interested in is understanding what's called false vacuum decay, first order phase transitions in the very early universe before the slow-roll phase of inflation, in a theoretical sense. We have some theoretical tools, like the instanton formalism, to make predictions for the rate of bubble nucleation through false vacuum decay. But those theoretical tools are limited in the predictions they can make that are relevant for the very early universe, because of some assumptions they contain.
So, this is why I mentioned collaborations with condensed matter physicists previously. You can make quantum simulators in the lab which are analogues to the false vacuum decay process in the early universe. Here the analogy is in the equations describing the dynamics – we are not actually going to nucleate universes in the lab! If the analogy in the equations is close enough, you can measure something in the lab, and translate those measurements in the lab back into their implications for the analogous process in the early universe. So by doing this, we hope to make concrete and quantitative predictions about the false vacuum decay process in the early universe.
As you indicate, the concern of possibly drowning in the data might suggest that AI machine learning has a critical role to play in data analysis. What role do you see for computers, and where is the human point of connection irreplaceable? In other words, we can have computers do all kinds of things, but we still need people to understand what the computers are telling us.
You've touched on something extremely close to my heart, and this is a new thing I started in the pandemic. It's unclear to me as yet what role, if any, AI will play in solving this problem, but it is a powerful enough tool that we should look into it. So the main problem with the standard deep learning-type frameworks is that their development is being driven by industry for use in very different settings than what we care about in science. In science we care about precision and accuracy and need to be able to understand and validate what the tools are doing and so on. They're black boxes. So we need to develop tools which are what is in the field called "explainable AI" or "interpretable AI" and I think that this could potentially have a big role to play, if we could get those tools to work effectively. Basically, we need to be able to break into the black box and understand why the tool is making a specific prediction.
The second role I see for AI is that currently, our forward-modeling computations—by which I mean, here's a physics theory, here's the data, we need to take the physics theory and usually put it in a very slow computation like a cosmological simulation to accurately predict the final observable we compare with the data. Or else we need to really, really simplify to speed up the calculation, and throw away a lot of information and only use something like two-point correlation functions to compare with the data. Which is like—you know, why did you bother gathering all of this wonderful data to throw most of the information away, right?
So artificial intelligence has a role to play in accelerating this forward-modeling computation. I think there you are on much firmer ground with using these tools because what you care about is whether the tool is accurately reproducing the results of a slow computation, but doing it fast, and you can straightforwardly validate that. It takes hours to weeks, possibly, to run a single cosmological simulation, right? So that means that you need to build a sort of interpolator that is able to make accurate predictions for the parts of parameter space where you haven't run the simulations, right? And so that process is called emulation. So, these emulators can play a role in a variety of settings that are coming up in cosmology where we don't want to throw away information and just calculate two-point correlation functions. AI will also have a role to play in classifying and also looking for anomalies in large datasets. But there the human will have to make the final decision. A human can't look at a hundred million weird looking things, but if you can filter that data with some clever sets of criteria that the human has designed based on what they're interested in, then the human at the end of the day will be presented only with, say, 100 things to then go and follow up and get further observations and so on.
So, I don't think that humans will be replaced by AI anytime soon, but basically what we are doing is we are augmenting our intellectual capacity using tools which can present more digested information for us to look at and to inform our physical thinking. I think that's where the human-AI interface will be and/or should be, in fact. I don't—there are many, many, many papers that come out where it's like, “Oh, here's a big dataset. Here's a black box. I chucked the black box at the dataset. Oh my goodness, there is an unexpected result. New physics!” [laugh] I don't like those kinds of papers. It's just like… you have to be able to poke inside the black box and understand why it's giving that answer and be able to query it in all sorts of ways. So we could also drown in those kinds of papers in the coming years. I hope not.
Do you think the new generation of physicists, particularly undergraduates, as a skill set, are they going to need to understand data analysis in ways that previous generations of physicists never would have been expected to?
I think so. Absolutely. So my generation of students had to learn statistical analysis, because the data revolution was just kind of ramping up when I was doing my PhD. That's the first time I encountered Bayesian statistics, in graduate school. And so then the generations following that, they are all about sampling, Markov chain Monte Carlo, you know, nested sampling, Bayesian evidence, all of that, right? So now the newest generation, on top of knowing all that, has to learn how to use basic machine learning tools. So I do think that is important, and it's a sort of upskilling of the workforce as well, which then has all the benefits for other professions. Because not everybody stays in science. And I think astronomy in particular, and probably particle physics as well, will drive that evolution of the workforce in a very effective way.
Hiranya, our conversation has been very forward-looking up to this point. Let's engage in some history. Let's go back to your first 16 years in Sri Lanka. Tell me about your family.
So, my parents were born in the first post-war generation in the late 1940s, and they were I think the first generation of their families to go to university. They were both civil engineers. They both came from very big families. So, I have a lot of cousins. But our family is smaller. It's me and my sister. My mother was one of the first female civil engineers in Sri Lanka. My dad mostly worked on things related to irrigation, like big dams and so on. And my mom designed bridges. So, they were both extremely supportive of me and my sister and our interest in science and like in many South Asian cultures, you know, education is instilled into you as a huge priority. And that was the case for us as well. And so yeah, I came from a background where I was supported in my interests, I would say, very much.
To what extent was your family or your parents in particular affected by the process of decolonization?
Oh, that's a very good question. I don’t have a very detailed picture of that. I'm not really sure, given their backgrounds, how much they would have been affected directly by colonization. Their parents, my grandparents, were all schoolteachers, and three were school principals. They were very dedicated to improving the local standard of education, both within and outside their workplace. My parents would have been positively impacted by the opening up of higher education following independence to allow people to study in their native language, rather than in English which was the case during the colonial era. As I mentioned, their generation was the first in their families to go to university.
What languages were spoken in your home growing up?
Where did you learn English?
I didn't really speak English—we learned English in school as a second language. I would say I learned to read English because there were so many books lying around. I had these— Do you know Tintin?
Yeah, sure. Of course.
Yes. So, we had the Tintin books in English, and I used to just leaf through them, and at first I followed the story via the graphics, and then one day I remember I could suddenly read it. I think even in kindergarten we were taught some English. I knew how to write and read English. I didn't really speak English until I moved to the UK. I mean of course I knew how to speak it, but I didn't use it as a mode of communication.
What were some of the cultural or religious traditions that really stick out in your memory from childhood?
Well, I'm not religious, but we were raised as Buddhists, right? And this is not Zen Buddhism, it's the original Buddhism. It's very kind of austere, and so we were sent to Sunday school to learn Buddhism, and that involved learning both to memorize the various Buddhist texts which are written in the extinct language Pali, but also meditation and learning Buddhist philosophy. And I think the aspect that stuck with me from that experience is the philosophy of Buddhism, which is not really a religion. It's a system of ethics and a way of thinking about the world. And I think it does give me a different perspective, both on life and on science. So [laugh] but I don't know if I want this to be in an oral history, but you know, I was a very questioning child. And I think I really upset religious teachers by asking for explanations for things. They would just assert something that we had to believe.
And you'd say, “Where's the data?”
No, I would say, “No, that doesn't make sense, because such-and-such-and-such is true of the world, so how do you explain that?” For example, reincarnation is a belief in Buddhism, right? So, I thought, if you have a closed system and there are a certain number of souls in that system, why is the population of the planet increasing? Where are these extra souls coming from? I remember asking this question— So I went to a big girls' school in Colombo in Sri Lanka. My mom had gone to it before me as well. And so, you know, there were like 5,000 students in this big auditorium, and a Buddhist monk had come and was giving his sermon. And we were [laugh] supposed to ask him questions— I think I asked that question and there was this deathly silence. [laugh] I must have been quite young, yeah.
Hiranya, did you grow up with a sense or an appreciation that your mom was a pathbreaker—
—and that you had opportunities as a result of her and her influence on you that you might not otherwise have had?
Absolutely. I think that there was a huge influence—well, I knew she was a pathbreaker, because I would go to work with her sometimes, and she was one of the very few women there. But however, even though you might not think this, just because in a developing country you would expect attitudes towards gender to be worse somehow, but it's not. I don't think I ever felt, while I was living in Sri Lanka, any sense that girls couldn't do stuff that boys could and vice versa. There were cultural restrictions about what girls could do—like what you wear and so on, but not about what you do as a job.
And so, one of the really important memories I have about my mom is both me and my sister would go along with her, either to the office or on site, and she was dressed very elegantly in a sari, and she was one of the only women there. But she was supervising workers who were building a bridge she designed. I felt like, okay, if she can do this, I can do it too, you know? So, I think that having her as a role model was fantastic. I also had several aunts who were medical doctors. So, I saw women all around me that I looked up to in my family who were professional women and doing technical things. Even our prime minister was a woman. And so, it never occurred to me that somehow we should think that women couldn't do any job they wanted. That was something that I learned when I came to the UK.
How backwards of the UK.
I know. [both laugh]
Hiranya, going to an all-girls school, obviously you didn't have anything to compare it to, but is your sense that that was a more productive environment for pursuing your interests in science?
Well, I do have something to compare it to but it's in the UK. So when I moved here, I went to a mixed sixth form college between the ages of 16 and 18. Sorry, you go to university at 18. The last two years before you go to university is called sixth form, and for that I went to a mixed school. So, I can compare with that experience. I don't know how to think about whether it was more productive for science. Girls’ schools can be pretty intense in their own ways with lots of hierarchies and so on. [laugh] I don't know. I think my interests in science were kind of pretty self-contained and singular, and not really related to school. I mostly hated school.
What were the circumstances of your family moving to the UK when you were 16?
Oh well, it was the civil war. It started in the 70s, intensified in the 80s. It was really affecting basically the whole of society by the late 80s.
Including your family's security? Did they feel endangered?
Yeah, like for example my dad's secretary was shot in the street. And you know, every time they went out it's like, you know, are they going to come back? You would see dead bodies in the streets. So yeah, it was pretty bad. And simultaneously, the government was very brutally putting down a Marxist insurrection as well, where you know, young people were being disappeared, like you hear about in South America as well. And because universities were considered hotbeds of insurrection, they just closed all the universities. So, my parents didn't really see any prospects for us, so they moved to the UK.
Did they have family in the UK?
Well, we had an aunt and an uncle who had been living there for quite a while, since the 80s, and they lived in Manchester, which is why my parents moved to Manchester. My mom applied for a job with the city council to design bridges and she got it, so that's how we moved. First I moved with her and then my father and sister came later. And so yeah, I was 16 at the time, and she must have been in her 40s already. So, you know, they were not young when they moved. It was quite a culture shock, I think, for them. And for me as well.
Was it difficult for you, being a 16-year-old, making a big move like that?
Oh absolutely. I mean, we were not, we were quite poor and very... Sorry, this is hard to talk about. [pauses] Yeah, it was quite difficult.
Obviously, you made the adjustments that you needed to. Was science a refuge for you?
Yeah, science has always been a refuge. [pauses] Sorry, this is why I don't like talking about the past.
Yeah, you know, to me science is very... there's truth in science, and truth is also related to justice. And it feels safe. And also, you know, when we moved, I'm very grateful for the opportunities we got. My teachers were very supportive and dedicated even though it was an inner-city school. I wasn't like anybody there, really. Also, I was very, very shy.
Hiranya, tell me about a teacher who was an anchor for you in the UK.
Yeah. So, my applied maths teacher, Dr. Egan, he was very, very supportive in all respects. I mean, as I told you earlier, this is the first time I found myself as a minority in maths and physics class. And let's see, there must have been about twenty-six boys and three girls in my physics class. There were only about seven students in the advanced maths class, and two of them were girls. And you know, he noticed that I was very good. He gave me university level textbooks to work on. The boys complained because I was many, many chapters ahead of them, and he told them that if they wanted, they could keep up with me. [laugh]
And you know, he suggested that I should apply to Cambridge. I think, if he hadn't said that, I wouldn't have dreamed of doing that. It was like a distant, unattainable thing, and so I did apply, and I was the first person from my school to go there. Yeah, I think that, you know, it wasn't just him. All the teachers were very supportive and caring, and it's very strange, because many of the kids there didn't seem to appreciate the opportunities that were there. I feel like, because we had less, you know, when you see an opportunity you take it. You don't take it for granted. So, I think that that was really, you know, a very positive experience, despite the fact that there were kids who didn't really think women should be doing physics, or whatever. The teachers were great.
And they saw something in you.
Yeah. Also, my sister did very well there. She's a very senior medical doctor now. So yeah, so the two of us were the first and second to get into Cambridge.
That is very cool.
[laugh] And you know—
Who's older? You or your sister?
Me. I am the elder. And so, you know—I didn't apply to do physics. I applied to do computer science.
It's important to note for our American audience—
—that you declare the major right away. There isn't this exploration of general studies and then you focus. You have to make this decision at the outset.
Basically that’s true, but in Cambridge in the first year, there's a slightly broader thing than in other UK universities. So I had two computer science modules, one physics, and one maths. And I avoided biology and chemistry like—I don't like memorizing things. [laugh] So anyway, so in the first year, I was still taking the same physics and maths as the natural sciences physics people would have been taking. And so that was partly informed by the fact that, since I was a little kid, I wanted to be an astronaut. But because we were not that well-off, it was like, okay, what's a profession that's going to earn money? That was an important consideration.
My sister wanted to do medicine, which was fantastic because you know, for South Asian families, when their kid wants to do medicine, that's the best possible thing that could happen. But nobody knew what doing physics got you. I'd always really enjoyed physics and maths, but none of my family or relatives knew what a physics degree could get you. Even though my interest was very much supported, I think some in my family might, to this day, not really know what is it that I do? Some of them think I'm still a student.
[laugh] Well you are, in a sense, right?
I am! Yes. But not that sort of student. Either a student or like a schoolteacher. That's their conception of, what is research in physics? It's not a concept that's familiar to people. But I couldn't go to university for two years. I had to work to afford to be able to go. And part of that time, I was in an apprentice scheme called The Year in Industry. And so I worked for Nuclear Electric, which managed the UK's nuclear reactors. So, I was put into this group that was all male. I was the only female there. The young intern woman of color. And they didn't treat me any different. So again I felt like that was very lucky. This was in the early 90s. So what did I have to do? I was given this amazing computer that was just sitting there untouched, actually. It was—do you know Silicon Graphics?
Yeah. I'm not sure they exist anymore, but they used to make these amazing desktops. Very powerful. And it was just sitting there. Nobody was using it. And my task was to model the turbo generator system that is coupled to the reactor to generate electricity, and model its vibration modes and make sure that there were not cracks. And so I had to go to the plants to actually get the data to inform the modeling, and that was also completely all male, and you know, the managers even had pinup calendars on their walls. So that was what the environment was like. Not a woman in sight. And yet that group was, you know, just perfect.
This is a pattern almost from high school where the larger social scene might not be so welcoming, but you were very lucky, it seems, to have been surrounded by the right people at the right place.
Oh I absolutely think that's true. So anyway, I taught myself to code in C and worked out how to do finite element modeling. There was this situation of being kind of being dumped somewhere and told to work out how to do something and not even a sense that, no, you can't do it or it's hard. Just like, you can do it. Here it is. So, I worked it out, and I think that stood me in good stead in a research career. Whereas many students seem to now come into their PhDs having been, somehow having it ingrained in them that everything's really hard and they shouldn't be able to do it, and so they need a lot of guidance along the way, but back then it was like, “Here, here's a thing. Work out how to do it.” So, I don't know what's changed about the education system, but I definitely see this happening now. Sorry, I sound like an old fogey ranting, but. [both laugh]
Was it more a class or a professor that got you more firmly rooted in physics?
So, I've never really liked formal education or exams. So, I think almost as a way of just procrastinating, I ran into—So I was in an ultimate frisbee team in Cambridge, and I ran into David MacKay. I don't know if you know of him?
He must have been a young lecturer. Sadly, he passed away, but you know, he was very influential in my life, because he told me about the Summer Undergraduate Research Fellowships at Caltech. And so, me and my friend Chris Connor, who was doing electrical engineering, spent a lot of time researching projects and applying to this SURF. So, I applied to work on the Galileo space probe to Jupiter, and to my considerable surprise, I actually got it. This was during was my second year at university. So, I had saved up money from a summer job working for a computer company, which was about applying that finite element stuff I'd learned at Nuclear Electric to a different problem. So, I had money to actually get to Caltech, and once you got there, they paid you, right? So, I'm saying this because if you're coming from a lower socioeconomic background, you can't get into physics until there's some kind of startup money. Right? So the fact that that internship was funded is very important. So, I actually got to JPL, and again I encountered a situation where they just said, “Oh here's a time stream of temperature measurements from a spacecraft. Work out how to make a temperature map of Ganymede and the Great Red Spot.” And again, it was—
Yeah, it's like, yeah okay. So, I invented the mapmaking equations [laugh], I worked out the pointing—so you know, you have to see where is the spacecraft pointing, what is the bundle of angles it's looking at in a given time, how do they intersect the planet and then you know, use the mapmaking equation to make a map of temperature. Yeah, I managed to do that, and I remember that this picture, this map came out of my calculations. And I took it to my mentors, and they looked at it and there was a sudden flurry of activity, like now we're going to take down all these maps of the topology of the surface there in that region and we're going to try to correlate the features that we see in the visual image with the temperature, and so on. And you know, this was stuff that no human being has seen before and we are doing it for the first time, right? So that excitement of doing the research was what really made me think: first of all, this is what I want to do, and second, I'm good at it. So then I changed to physics in Cambridge—
Hiranya, did you get a sense at JPL of the complicated relationship between NASA and Caltech and JPL and how all of those things fit together, or were you, you know, young and much more narrowly focused at the task at hand?
So, we lived in Caltech, which was a very odd place. [laugh] There were very few women there as well. And Chris and I, the friend that had applied to the program with me, we were staying in one of the houses at Caltech for students. So I knew Caltech was not the same thing as JPL. Because I was a Sri Lankan national at the time, at JPL I was treated as an alien. I had a badge that had in red letters, "ALIEN." And I had to in principle be accompanied even to the restroom by an American. So that was rather weird. But it was always like an adventure though, because I had never been to America before. And yeah, so I didn't really know anything about the politics or whatever.
What I do know is that after that year, they discovered foreigners were working at JPL on SURF projects and that's no longer possible. So they restricted the foreign intake only to work at Caltech after they discovered that. I think that happened—so I went there in 1996, and in 1997 I went back and I worked more on temperature retrievals from the Great Red Spot. And so after that—I think the following year?—no more foreign citizens were allowed to work at JPL as part of that scheme. So again, I was really lucky. And the mentors I had there were fantastic as well. They are Terry Martin and Glenn Orton and Leslie Tamppari. And they seemed to have the best jobs in the world, from what I could tell, and I really, really wanted to do that. So that's why I changed to physics. I also have another memory that I suddenly barged into—You must think by listening to this that I'm a very outgoing person. I was a very, very shy...
No, but Hiranya, I'll make this observation, and it's quite poignant when you were talking about your transition to the UK. It's that science was an anchor for you. It's where you saw justice. And so, I might just observe that you're only outgoing insofar as you are integrated in science, because it's there where you can express yourself. And that might tell us nothing about how comfortable you would be at a bar, for example. If I may make that distinction, if that makes sense to you?
That's very insightful. I think that's completely right. I'm not at all comfortable in a bar. [both laugh]
But we're here talking about science, and so you are quite comfortable.
Yeah. Yeah, exactly. So I actually, I knew who Kip Thorne was and obviously he wasn't a Nobel Prize winner back then, but he had a certain quality to him that might have been intimidating, but I wasn't at all. I barged into his office to ask him questions about physics. I can't even remember what it was about now – something about what happens to EM fields of objects travelling near the speed of light? I didn't have an appointment and he still talked with me for over an hour, and he gave me his copy of Black Holes and Time Warps. I still have it. It's signed there with the date. And again, it was this feeling of being welcome, right?
And I also think, I have a memory that I rearranged my flight home. By this time I had a kind of inkling I wanted to study in the US for my PhD. I decided I wanted to do a PhD without really knowing what it was. And I rearranged my flight—because I had heard of Harvard—to go and visit Harvard. And again, I emailed Avi Loeb. And it was the same thing, like he had time to talk to me, this random student from the UK. So I felt very welcome. [pause] I feel grateful for the time, for these people that were so kind to a stranger. It meant a lot to me.
And again, a theme here is that just because Kip Thorne and Avi Loeb were kind to you, we should not take that to mean that every luminary in the field is also so kind.
No, but they were.
Again, perhaps you were very lucky at meeting the right people at the right time.
Yeah, yeah. I think that this is the story of my life, I've happened upon these people at the right time. I later worked with John Bahcall too, and I heard all sorts of stories about him, and about how formidable he was, but he was wonderful to me. I'm not going to generalize from that of course. I know people are complicated. But, because of them, I'm here.
Hiranya, after your time in the United States, were you in a hurry to finish your undergraduate and to go on to bigger things?
Not really. So I had two more years and I started enjoying my formal education in my final year of my undergraduate degree, when it was more about the advanced topics. I could take classes from what's called Part III Maths, which you've probably come across. And that's the first time I had a course in cosmology. And again, like similar to what happened in the US, you know, I was treated kindly by people like Stephen Hawking. And I did very well in cosmology. So that was something that was potentially at the back of my mind as a PhD topic, but I was still very broad-minded. And I applied to... I had to work out how to apply to the US. I'd heard about the GRE and so on. And I guess I got these books to practice, and I went and took the test and I scored very well, so when I applied I got into every place I applied.
And did you specifically not want to stay in the UK? Did you focus exclusively on the States?
I did, yeah.
What was your motivation there?
I really liked the non-hierarchical system I encountered when I was in JPL, which was in stark contrast to the very hierarchical, at least at the time, Cambridge situation.
And your sense was that Oxford was not a solution to escape this kind of hierarchical system?
What is this other place you mentioned? I've never heard of it.
[laugh] Very good.
[laugh] Yeah so things have changed completely, but at the time that I was trying to do some research as an undergraduate, I don't think they believed that undergraduates could do research in the UK. That changed I think very quickly after that. But, you know, when I was going around asking for opportunities to do some research, I got blank stares, whereas then I actually got paid to go and do research in an American institution. And I liked the informality, I liked the fact that people called each other by their first names. I felt like it was much better to be in an environment where it's not like seniority means you treat people differently. I know that that's not completely true in the US either, but it was a big contrast to what I encountered in the UK. And so I guess, yeah, I didn't really want to stay. I mean, my family wasn't so happy with that. But you know, you might get the sense I'm quite a stubborn person, and do what I want. And I was like, “I'm going to go.” And I decided to go to Harvard. They accepted me and gave me a scholarship.
Would that be to work with Avi? Was that the motivation?
No. I don't know whether I wanted to work with Avi. I think I wanted to work with Ramesh Narayan, because he was working on black holes. I can't really remember.
And were you focused more on astronomy programs or physics programs for graduate school?
To be honest, I can't remember. [laugh] I must have all the applications somewhere. I think in some places I applied to physics departments and in some cases I applied to the astronomy departments. I think I applied to the CfA. And I think in Berkeley it was physics and Caltech it was physics. In Princeton it was astrophysical sciences. And in MIT it was physics. So it's wherever the astronomers were. That's what I wanted to do. I wanted to do astrophysics. And so, I'd actually gone to visit Harvard to check whether I was sure about going there. And again, on the way back to the UK, I decided to also go to Princeton. This was a complete, like, okay, everybody is telling me to go to Harvard and so I should probably check out one other place. I already knew Caltech. I didn't know Berkeley, I think. But yeah, so I decided to go to Princeton just to check it out. I'd said by this time, like, 95% sure to Harvard. And I'm so glad I did that. Again, it's one of these really lucky breaks. I arrived like in the middle of the night by train, and David Spergel, who was a young professor at the time, picked me up from the train station and took me to meet a very lively and friendly group of graduate students. And —
This is a great image. So David picked you up at the Dinky, was it?
Yeah, it was the Dinky. Or possibly at Princeton Junction. Yeah, yeah.
Do you know how David got assigned to this task? Or did he volunteer?
I have no idea. He was, you know, to me it was like, my goodness a professor has come to pick me up. And I was very touched by that, you know? And so just after one night at that place I was like, this is where I'm going to go. So I told Harvard, “Actually I decided to change my mind.” My parents were concerned because they were not familiar with Princeton. I remember my dad actually came along with me to help install me there, and it kind of felt to me he wanted to make sure it was a real university.
Perhaps you told him that Einstein hung out at Princeton a few decades back?
[laugh] I'm not sure that would have, yeah, I don't know whether that would have made a difference. Everybody has heard of Harvard and Stanford.
Where I come from, right? [laugh]
That's what they've heard of, yeah. But you know, I came to Princeton, and as I mentioned earlier, I was assigned to John Bahcall. Which was terrifying because he was at the Institute. So I had to cycle over there and meet John Bahcall. And again, I'd heard these horror stories about him. And he was really like a nice uncle, you know? He was very, very kind. And I worked on his model of the Milky Way galaxy. So, I don't know if you know how the PhD works at Princeton. So, in the first two years, you do up to four, but usually three, individual research projects with different professors. They take roughly six months each. You were expected to write a paper out of each of those. And then you do your generals exam, and then you do the PhD. And it was a four-year program at the Department of Astrophysical Sciences.
So first I worked with John, then I worked with David, and that was a theoretical forecasting project studying what could you learn about cosmology by combining the not-yet launched WMAP and the Sloan Digital Sky Survey. And after that, I worked with Scott Tremaine on the dynamics of the nucleus of M31. So I worked with very different people with very different styles. And on very different topics. I started a project but never finished it with Michael Strauss on some early SDSS stuff. And so after that, I decided actually— you asked me astronomy or physics. I think through my entire career, I've been a physicist masquerading as an astronomer.
So you know, I'd taken really good courses in the astrophysical sciences department. I passed my astrophysics generals. I could start my PhD. But I'd not learned quantum field theory. And I wanted to learn more GR. So I asked for and was granted the opportunity to essentially by myself learn the syllabus of the physics department and pass their generals exam. That was a horrible and isolating experience. I got the impression that many of the physics grad students looked down their noses at astronomers. The astrophysics courses were fantastic. The physics courses were not uniformly well-delivered. So, I took to essentially working through textbooks by myself and working through past exams and problem sets to kind of teach myself physics. And I think that was very fruitful, even though isolating. And I did pass my physics generals at the end of all that. And then David told me that he would like to have me work on a PhD with WMAP data. The first release. And I was like, "Yeah, that's what I want to do." Because I really liked working with David.
Had you heard about WMAP before David presented this prospect to you?
Of course, yeah. It was known to be the big step coming up in cosmology. It was WMAP in CMB and SDSS for galaxy surveys. Those were both great PhD opportunities available at Princeton at the time. To me, and it's true to this day, CMB is more about physics, and that's what I enjoyed doing, and so that's what I wanted to do. Princeton was a beacon of cosmology at the time, and not just because of senior people like David, although I think he was quite junior faculty probably at the time. There was of course, you know, the huge crowd of amazing postdocs. There was Mattias Zaldarriaga, there was Wayne Hu, there was Max Tegmark. You know, they were all at the Institute. And it really felt like cosmology was where all the action was happening, and that's what I wanted to do, yeah.
Did you spend much time at the Institute for seminars and things like that?
[laugh] Students were not welcome at the Institute. I went there to work with John, but you know, students were not allowed at Tuesday lunch. I do remember going to some seminars, and I found them very... the ones that were more to do with astronomy were fine. John Bahcall sat in the front row and asked very simple questions. Like, you know, so it lowered the threshold for young people like me to feel like, if I didn't understand something, it was okay. John Bahcall is asking a very simple question, so it's clearly fine for me to ask simple questions. But the string theory seminars were horrible. Like, they would just not even... the speakers wouldn't even get two words out before this wolf pack descended and kind of tore them apart.
Yeah, I'm afraid I think the colloquia in the astrophysics department were similar. [laugh] I mean, yeah, even though you know, I had gone there because there's no hierarchy, there clearly was a hierarchy.
There clearly was a very bullying atmosphere in some of these talks, and the opinions of some people clearly mattered way more than the opinions of others.
Did you have opportunity to interact with Jim Peebles at all?
I believe so. I think Jim is one of the kindest people in the field. But I don't believe that we really got an opportunity to interact much with him as astrophysics PhD students. But Dave Wilkinson, before he passed away, was a person that I interacted with a lot.
What was Dave like? What was it like to work with him, to be around him?
Oh he was so amazing. He was such a humble— Jim is as well. But David was also a very humble person that treated everybody the same. He had this idea about doing optical SETI using the telescope we used to host open nights with, along with a similar small telescope at Harvard. I don't remember the technical details of what the setup was now, but I learned to use the telescope from him. And participate in setting up that project, and because I learned how to use the telescope, then I used to help with the open nights for the public. Once we started working on WMAP, we used to have very regular meetings. They alternated between Princeton and Goddard, right? So we would all go on the train to Goddard for these meetings.
So I was the most junior person as a PhD student in the team, but I worked very closely with Licia Verde and Eiichiro Komatsu, who were David Spergel's postdocs. And we were all put into the same office. And we kind of worked in shifts. With David as well. David is a very conscientious parent, so he would go home at, you know, 5:30 or whatever to deal with his kids, and then he would come back to work. We were all like, you know, real night owls, and we worked in shifts. I think Eiichiro was a morning person, so he then took over what we'd done in the evening. [laugh] I mean, we had like six months from the sky closing on the first year’s observing strategy to produce results and come up with a set of seminal papers, so it was very, very intense.
And we all worked as a team, and regarding Dave Wilkinson, I still remember a special moment... So you know, we finally produced a plot of the power spectrum, and the first guess at plotting a best-fit cosmological model on top of the data, and I had to present that at one of those meetings, the regular meetings between Princeton and Goddard. I still remember the smile on Dave’s face when he first saw that. He still had such a childlike quality to him that he was, you know, excited deeply by this.
Hiranya, what was your sense institutionally of Princeton's contribution to WMAP as a broader collaboration?
I think it was for sure a partnership. So of course, Dave Wilkinson was deeply involved in the design, right? David Spergel was the theorist on the team, so he was the interface between the instrument and the data and us, the people doing the analysis. And there was of course Lyman Page and other people in the physics department. And at Goddard, we had Chuck Bennett, who from my lowly position as a PhD student, looking up, seemed to be the best leader that I'd ever seen. He seemed to know that it was important for every single person on the team to feel they were making a contribution, and that contribution had value. And he saw all of it. He was deep in the details, but he also had the big picture.
You know, he would call David's office to crack the whip, I think probably like once a day [both laugh], then we had to give a report of what we are doing. So the project management definitely came from there. And then there was Gary Hinshaw, who was also doing a lot of that work. And the supercomputers we were using were at Goddard, and that was a nice, tight coupling as well between Princeton and Goddard. Then there's the huge effort to measure the beam which was run off the physics department, and that was of course very, very important to getting correct results out and then David's group was responsible for turning the data into cosmological parameters, so that's what I worked on. And so, there wasn't a sense of Goddard versus Princeton. It was a very balanced and very... tightly knit team. Everybody contributed. And so again that was a wonderful experience for me. And another huge stroke of luck to be part of that. And work with all these wonderful people.
Hiranya, as a matter of division of labor—
How did your contributions contribute to what would ultimately be your thesis?
So, the main thing I had to do is something that I actually have repeated in many guises ever since, which was to connect theory to the data through a statistical model. And then implement that in computer code and then use that to measure the cosmological parameters using the data for a variety of cosmological models. So to do that, I worked closely with Licia Verde, and she and I were working closely together to also develop the likelihood function guided by David. And Eiichiro Komatsu contributed to many of these aspects as well. So I worked very closely, I would say, with Licia. Most closely with her. At the time, the currently very standard techniques to use Bayes’ theorem and Markov Chain Monte Carlo to estimate parameters was hidden in obscure language in the statistical literature. It was just starting to be used in astronomy. We didn't really understand this literature well at all.
I remember that Licia and I took the train to New York to talk to Andrew Gelman, who's a brilliant statistician. And tried to decipher this language that we were reading in the stats literature, and then we worked out how to implement that in code. I was also asked to lead one of the papers, which was about testing inflation with the data. I worked closely with Eiichiro Komatsu on that work. And you know some of the choices we made—even the format of that paper and the things we did in it—took hold, right? Subsequent cosmological analyses and subsequent analyses about inflation follow that framework that we set up. We had to come up with it, you know, how do you analyze data, how do you turn that into constraints on theoretical models, what parameters are the best ones to measure? How do you account for theoretical priors? What aspects of the data are interesting? How do you combine likelihoods from different kinds of datasets? All of that. We had to do it very fast on a tight time scale under pressure. So it was very intense, but it was really fun. An intensely creative period.
Who was on your thesis committee besides David?
Oh my goodness. [laugh] There was Paul Steinhardt… goodness, who was my thesis committee? I remember Paul Steinhardt because I was terrified of him at the time.
[laugh] Yeah. Michael Strauss. I think it was also Jim Gunn and Lyman Page. I must have wiped this from my memory. I had like six people on my committee.
Oh, that's a lot.
Yeah. And the format was basically that each of them got 20 minutes to ask me questions while none of the others spoke, and they basically drive you to the edge of your knowledge and beyond. [laugh] And then the next one takes over, and the next one takes over.
Did you ever interact with Paul given his interests in condensed matter? There's very few people who are interested from cosmology to condensed matter. I'm curious if you ever interfaced with him on these topics.
Not on those topics in particular, but I definitely have on theories of the early universe. But it was mostly subsequent to my PhD, rather than at the time. The person I have interacted with, who has that span, is Frank Wilczek. He also has that span of cosmology to condensed matter as well as many other things. I appreciate that, that's really fun. Paul asked me some very difficult—No. Paul asked me some simple question, and he was looking at me over the top of his glasses like that. I was terrified. So I just kind of froze. I remember that. And then he asked me some hard question and I had to work on the board for that, but because I wasn't looking at him, I could solve that one completely fine. So this is like the one memory I'm afraid—No, no, I have another memory. They came up with some off the wall question about what would happen if there were more metals in the early universe? What would the CMB look like? And I could do that one as well, that one was quite fun. But I have blanked the thesis defense from my memory, I'm afraid, other than that. [laugh]
Did David make you feel that your contributions to WMAP were not just for the sake of your thesis and your education, but that you were really contributing to WMAP and what it was doing?
Absolutely. Not just David, but every senior person on that team made the junior people feel like they were making integral contributions to a very important project. And I think that, you know, uniformly was a set of great mentors. But David in particular is an amazing mentor, and yeah, he definitely made me feel like that.
What was most intellectually satisfying to you about your contributions to WMAP, but during the moment where you were at Princeton, and for the broader history of what WMAP accomplished?
It's... [laugh] What's satisfying about it? Do you mean scientifically? What I found most satisfying about that whole experience wasn't necessarily the science. It was the feeling of being part of this incredible team. I can't even describe it. I have not subsequently hit on that again in my career. It was a very, very special team. If you ask anybody on that team, they'll say so as well. It was not too big, and everybody got along, and like one super-brain that was kind of put together from different people's brains.
A neural network, if you will? [laugh]
Yeah. Exactly. Yeah. So, I think that was the most satisfying. In terms of scientific outcomes, I think that the most general point that can be taken away from it was the importance of modeling the data well enough such that you understand in detail all the statistical and systematic contributions, and being very precise about that, rather than waving your hands, right? It was the initiation of the era of precision cosmology. And—
Oh, what do you mean by that? What does precision cosmology mean in this context?
So you can measure a parameter, and you can put an error bar on it. That doesn't necessarily mean that your measurement is correct or that the error bar is correct, and therefore the conclusions that you can make from that measurement are not robust, right? So I think WMAP was a threshold which elevated cosmology from astronomy into the realm of physics. Where quantitative conclusions matter, rather than qualitative conclusions that are often the case in astronomy. I think that was the watershed where, you know, particle physicists could take seriously what we are saying as astronomers and believe that we could say something about fundamental physics from these data. So this was then, you know, incredibly refined by the Planck analysis as well, which is on yet another level again. But that was the start of it. The parameters got refined by subsequent, better data sets. You know, that will always happen. But I think that setup and that way of thinking about analyses very carefully and having multiple checks on things, that very careful way of working, I think has permeated cosmology after that. And that's very important.
From a physics point of view, I hadn't really worked on inflation or thought deeply about the origin of the universe before this. So that paper was very influential in developing connections between high energy physics, theoretical physics, and cosmology, and it drove a lot of theoretical activity in the field and generated a lot of interest from a new community in the field as well. You know, after that paper, I want to know why there's something rather than nothing. I want to know where everything came from, and—
And this research invites the ultimate existential questions, you're saying?
Exactly. Exactly. So you know, even though that's not the only thing I work on, ever since then I'm doing some things, sometimes very, very far out from the rest of the field, to try to get to grips with that question for myself.
And Hiranya, this is a very technical question, but in asking those existential questions, is your perspective more the goal is understanding t=0, or t=epsilon?
I think it always has to be epsilon. [laugh] Zero can always be redefined, right? So I'm mostly interested in trying to understand the initial conditions for inflation. I think there is pretty good evidence that inflation or some process like that, was what led to the origin of structure, right? But I think we've just put the problem further back in time, and then we have to now understand and explain the initial conditions of the process that's generating the initial conditions for the standard Big Bang. And that's what I'm interested in now. It's less about the kind of existential question, but more about how to put this, it—
Okay. Cosmology is an observational science, right? Essentially, we are coming up with a narrative to explain and connect up data sets that we see at different times in the history of the universe. It's a story, right? Always there are assumptions at an earlier part in that story that are needed to then get through to the next part. I want to test the earliest of those assumptions and make sure that we are not on kind of a… sandcastle… I don't know. Like when you're not on a firm foundation, but you build a huge house on it. I want to make sure that the fundamental assumptions that go into these theories are correct. And I want to do that in an empirical way. I'm not interested in doing pure maths. So yeah, that approach is what's eventually now led to this false vacuum decay quantum simulator, for example.
Hiranya, on the basis of approaching inflation and your desire to be on firm empirical grounding, to the extent that there are major unanswered questions for which inflation doesn't have good answers, to what extent either then or now or in the intervening years, have you investigated competing theories to inflation? For example, the kinds of ideas that Robert Brandenberger has advanced.
Yes, so I did actually investigate those ideas when I was working on the Planck project later in my career. Some of those models make different predictions to inflation, for example regarding primordial non-Gaussianity. And they were found to be less compatible with the data. For example, with the ekpyrotic model, the predictions were revised after we found that the original formulation of the model was not compatible with the data. This of course happens repeatedly in model building. So I have investigated those models, but I don't take inflation too seriously either. It's a great idea. It's a toy model though. It's the first version of a story, and we need to flesh it out. But something like inflation, I think, must have happened. I think there's evidence for that.
But if it did happen, I think it opens up lots of very curious consequences, like eternal inflation, and the multiverse and all that. And I think these sorts of ideas need to be tested. So that's my perspective on that. I don't really work on the string gas models or ekpyrotic models or the cyclic universe. I don't work on the theory, I mean. But I have participated in testing predictions made by other people from these theories. I also think that we're probably not going to learn much more by like studying specific models of inflation with toy potentials that kind of come out of nowhere. We should be testing the general features of the theory, broader predictions of the theory, rather than individual specific models. And the chance that any of those specific models is right in every detail is probably measure zero.
For your initial postdoc, was the idea to stay at Princeton just because there was unfinished work to complete? Were you just so happy you didn't want to go anywhere else? Or what were you thinking at that time?
So I spent one year as a postdoc at Princeton, and that was only because it was very secretive, what we were doing in WMAP before the first data release, and I couldn't apply for jobs in the right cycle. So that was an agreement I had with David that I would apply for jobs when data was released. And in the meantime, I would be funded for one year to work on WMAP and kind of finish that up.
This indicates a confidence that the data was going to be released when it was planned to be released.
That's right, yeah. Yeah. There was a small delay. I think due to the shuttle disaster. But we pretty much got it out on time. There's a funny story about that. So because I was working so much on the cosmological parameters paper, the inflation paper was late because the other one was clearly the priority paper to be working on. And Chuck had set a deadline [laugh] by which all the papers had to be on a repository. Like, the latex files had to be there by the deadline. And he'd actually even said something like, “If the paper is not there, it's not going out in the release.” I don't know how serious he was, I should ask him sometime.
So, the inflation paper was the last one to get to the repository, because I had to finish everything else before I finished work on that one. And [laugh] I was about to upload it and there was a power cut on the entire campus. [laugh] And I missed the deadline, and then I uploaded it later, and I actually adjusted the time on the file to show it was submitted before the deadline. Ah, and yeah, I don't think Chuck was very happy about that, but of course the paper worked out. Yeah, so it did all come out in time, and that was due to, I think, the very, very efficient work of the team and the project management by Chuck, as the PI. So yeah, so I think there was confidence that it would come out, because you know, there were only six months we had to do this analysis, and once the data was downlinked and its quality assured, we knew we could do it.
Now, was your subsequent point of contact at Chicago, or was it through NASA?
What do you mean?
Well, you were a fellow, you were a Hubble postdoctoral fellow, but it was held at the University of Chicago.
So I'm curious if the connection was with people at Chicago or it was—
No, no, not really.
It was still through the Princeton group mostly. There was a member of the WMAP team at Chicago, Stephan Meyer. So we used to join the telecons together, but most of the connection still flowed through Princeton for me. And because I was part of WMAP, I was not part of any of the Chicago CMB groups. I was not invited, and I didn't ask. So then I started mostly doing theory during that period. I worked on theoretical connections to early universe physics with Richard Easther. And I worked with Wayne Hu, and that was my first opportunity to co-supervise a student, Cora Dvorkin, who's now doing very well at Harvard, and that was my first experience learning to mentor someone. And a very, very good experience there. And I worked with Dragan Huterer on dark energy. That's like my one paper on dark energy. I realized that, “Well, we're not going to understand dark energy using observations. It is a theoretical problem.”
[laugh] Anyway, I knew that after that paper I don't have anything to say about dark energy. And so I stopped working on that. [laugh] I'm very happy that many dark energy surveys are going ahead, and I'm using that data to do all sorts of science, but I'm not interested in measuring w.
And surely you'll be happy to be proven wrong if they find something really exciting.
Yeah, absolutely. Absolutely, absolutely. I would have bet on the wrong thing, and I'll be extremely happy that everybody else had bet on the right thing. But you know, I think the data is going to be fabulous, but measuring w is not one of my deepest desires. [laugh]
These were a very productive three years for you, though.
Yes, yes. And that's when I started working more with theoretical physicists and again, you know, it's very important to me and my career that I've actually grown as a scientist by being able to work with people in different fields and make connections between them and do projects which require expertise from different angles. I think I'm quite good at that and I got to practice it at Chicago, whereas if I'd gone to Chicago and got into yet another CMB experiment, probably that wouldn't have happened. So it was actually a benefit to me personally that there were these rivalries in the various CMB labs. And I also very much enjoyed being in a department which is building major instrumentation right there. I really enjoyed that, you know, both like deep theory and people building instruments in the same place as people in the middle like me.
So, I had such a wonderful time in Chicago. I think that this is the only place in the US I really still miss. [laugh] I don't know if that makes sense. I mean I've lived in plenty of nice places in the US, but I really miss the vibrancy and atmosphere of living in Chicago. But then I wanted to go back to the UK, because in between my time at Princeton and going to Chicago, I had to go back to the UK to get my US working visa, and I met my partner that summer. And then we had a three-year transatlantic relationship. Which was kind of interesting.
And not ideal, of course.
Not ideal, but I don't necessarily see it as really horrible either. I know some people probably would find that, but I think you know, our relationship is very much strengthened by— this again comes back to that thing that you're saying about constraints. You know, sometimes having constraints really sharpens and brightens things. I think it was fine. So then I went back to the UK, to Cambridge.
Hiranya, I wonder how your perspective of Cambridge may have changed, having been gone and coming back home, so to speak?
Yeah. So as an undergraduate, I very much enjoyed Cambridge. Living there and the atmosphere. But I discovered that being there as an adult wasn't so fun. I'm not talking about the science, I'm just talking about living in Cambridge. And you know, when you're an undergraduate, you're part of a college and you have an amazing social life and a huge range of opportunities to do all sorts of things. And you know, as an adult, it's a very small place. Not as small as Princeton, but a very small place. And when I was in Princeton, I spent a lot of time in New York, and I lived in Chicago. So, I really missed living in a big city, and so from that point of view, I didn't like it so much. I of course really loved being in the same place, finally, as my partner, and—
And what was your title at Cambridge?
So, you know, STFC is the national research council that funds particle physics and astronomy in the UK, and I was an STFC advanced fellow. I was in fact a Halliday fellow because they give you a special name if you are the top of the competition. I was actually [laugh] unbelievably the top of the competition. So I was the Halliday fellow, and this came with like some large amount of extra research money and I used that to set up a small computer cluster. They were very, very opposed to Macs at the Institute, I remember that. They wanted you to use Sun computers. So I caused a ruckus by wanting to bring in Macs, which they refused to support, so I had to do all of that myself.
I had my first PhD student, you know, when I worked with Cora—Wayne was the supervisor and I was the postdoc. I now had my first PhD student, Simeon Bird, who I was responsible for, and I very much enjoyed the opportunity to discuss physics with people like George Efstathiou and Martin Rees. And I started a line of communication between DAMTP, the theoretical physics people, and the astronomers at the Institute of Astronomy. We started going back and forth between the departments for seminars and we started journal clubs where both groups would come. This was not seen positively on all sides. There are historical rivalries between the groups, but I feel I was connected to both, and I connected the groups. And then yeah so—
Hiranya, was it important or feasible to maintain your American collaborations at this point?
I can't remember. I think I was still working quite extensively with Richard Easther, even after I moved—he was at Yale at the time. But yeah, I should look at who I was working with those years. It was a big learning curve, right? I was learning to supervise students and so on. And what I found, actually, different about the Institute was that it was very hard to start collaborations. I don't like working by myself. I find it boring. I find it much easier to sharpen my ideas when I can bounce them off other people. So yeah, I didn’t find the faculty really collaborative. Instead I started working with Andrew Pontzen, who was then a young student, who has basically become one of my closest collaborators ever since. He's now a faculty, he's professor at UCL as well. And that was deeply enjoyable. And I think with my own PhD student, I started to do a thing that I really like to do now. I got into something completely new for me with the student, so that we are both learning together. And so, then you come into a new area with a beginner's mindset. And then you can see things in a way that's different from what everybody else is doing in that area. And so, I started out working—
What was this new project?
Ah, it was the Lyman-alpha forest, which, you know, it's still quite an under-exploited area, although with DESI that will change, but it's very good for measuring the matter power spectrum down to relatively small scales while still mostly in the linear regime, which can tell you a lot about both primordial physics and dark matter. For example, we just used that technique and combined it with the machine learning emulation method that I told you about earlier and used the small-scale power in the Lyman-alpha forest to improve by an order of magnitude the constraints on ultralight axion dark matter. In fact we ruled out the canonical parameter space of that model. At that time I hadn't actually done anything to do with cosmological simulations, hydrodynamical simulations. Simeon and I worked out how to do that together, working also with Licia Verde, and Matteo Viel, who was an expert in modeling the Lyman alpha forest. So that's my trick. I get into an area working with a person who's really good in that field, so I know that I'm learning from the best. And so—
Hiranya, what skill sets did you bring to this brand-new project, and what was really tabula rasa for you?
Never having ever run an N-body simulation was tabula rasa. And what did I bring? I don't know what I bring to projects, David. I don't know. I bring ideas. I bring a vision. I bring a very broad skillset. And I have obscure bits of knowledge because I read a lot. That's what I bring. And I suppose, yeah, I have energy and I'm stubborn and I'm persistent.
This must have been very important for your development as a professional mentor to students as well, this project.
Yeah. I think because the student can see that it's not like the supervisor is like this great expert that knows so much about everything, they feel, I think, that they are an equal. And I think that is a feeling that I was very lucky to experience when I was a student. That people treated me like an equal, right? And I want all my students to experience that.
And probably also to see that even you have to work hard. That these issues, that these problems don't come easily to you, and to some degree that's equal parts comforting and inspiring, perhaps.
I think so. I think it might be also scary. [laugh] Because it's like, “Oh, you know, even the supervisor has to work hard on this.” I'm not afraid of hard work, and I expect my students not to be afraid of hard work. But I think it's more, you know, especially in the UK, people get handed projects which are already defined because the PhD is short. Right?
It's designed to finish rather than to learn, is what you're saying.
That's what I'm saying. So often you can graduate people who are very good with technical skills to solve a known problem. I think what I'm good at, and what I can convey to people, is not that aspect, although obviously I have technical skills. But what I can convey is how to define a research question that is not necessarily mainstream and incremental and so everybody knows how to do it, but like how to define a question, how to scope a project. How to break it down into something that's finite and doable, and then do it. And then do something important with the results. I think that that whole chain is something that I feel I'm good at conveying to a student. And it's something that's actually very hard to convey to a student if you hand them a project where everything's defined, and you just have to do a calculation. That project definition part is easier to illustrate if the supervisor is also learning.
And increasingly because I work in a lot of interdisciplinary collaborations now, I've found that two supervisors with two different backgrounds, learning together with a student, is basically the only way to create lasting interdisciplinary collaborations. Because otherwise, you know, if there’s just one supervisor and one student is trying to get into another area, and there is a second supervisor who is less engaged because they are not learning anything new, there's no skin in the game for that second person. Like why should they spend their time trying to break out of their own silo? Whereas if you're both responsible equally for a student, then you're both learning and the student is learning from both of you. So I think this is a really great model for interdisciplinary work. And it's the only one I've found that works.
Hiranya, perhaps as a window into where cosmology was as you were transitioning from Cambridge to UCL, what was exciting at UCL at that point? Was cosmology really the most exciting thing that was happening there?
Cosmology didn't exist at UCL until 2004, when Ofer Lahav moved there to start a new cosmology group. So it was a very new group and Sarah Bridle, who you might have come across, was also there. So I was very attracted, I suppose, by her presence, because I saw her as a role model. A further huge attraction was because I wanted to live in London, and my partner was working at Imperial College. And I didn't like to live in Cambridge, and I wanted to live in London, and I didn't want my partner to be commuting to London from Cambridge, which is like one and a half hours each way. Possibly more. And so a great attraction was London, and it was very good that we could both get permanent jobs in this wonderful city. It took us seven years to solve the two-body problem.
My partner is Daniel Mortlock, and he's also an astrophysicist. He is also a wonderful collaborator. We've written many papers together. So once I did get a position at UCL I think what I brought there was involvement in Planck. Because I'd joined Planck in 2009. It was very, very hard by then for anybody to get into the Planck collaboration, because there were like 300 people in Planck already and you had to prove that you're bringing something new. Fortunately, they thought I was, so I was brought in.
Which was what? What did you bring in that was new?
Well, I'd worked on the previous big space based CMB analysis with WMAP, and I had a pretty different skillset in terms of model testing for fundamental physics, particularly early universe physics. And I had lots of experience in Markov Chain Monte Carlo analysis and so on. So I brought that involvement to UCL, and I started an early universe cosmology effort there. Okay, what was attractive was this idea that I wasn't under the shadow of great professors who'd already done everything in my field. I really like George Efstathiou, but you know, I don't want to be [laugh] working as a cosmologist in the same department as him. It's like, whoa, what am I adding here, right? He'll always be the cosmologist. Whereas you know, if you go to a new place and you bring something new there, you can develop stuff that is your own. You can—
UCL was not stifled by legacy, is what you're saying.
That's right, yeah. And a young person coming in didn't have to face an enormous inertia from the institution to change the way that things have always been done before and you could do something new. Fast. You could have an initiative, convince people, and then yes, you can do it. And that was something I really appreciated when I moved to UCL. I mean I told you about the Mac issue. I mean that was a small thing, right? Trying to do something new, in a new way, in an old department is not easy. It's not that the UCL department isn't old as well. But cosmology was new, very new, there. And you could set the direction as a junior faculty. So I brought early universe cosmology there and then I got one of these big European research grants, which allow you to essentially establish a new group and do some innovative things. So I worked on Planck in the early part of my junior faculty tenure, and it was a different experience from WMAP. It was a much larger team. Didn't have that very like "you're all a family" kind of feel to it. On the other hand, I learned a lot from that about how to run a big project to a standard that, I mean, the Planck papers are, in my opinion, the epitome of clarity and scope and you know, the text is refined so— I mean, they're so polished.
Hiranya, what exemplifies that observation?
So if you look at the major Planck papers about cosmological parameters, about inflation, about primordial non-Gaussianity, those are the papers I contributed to. They, like each individual section is almost a paper on its own, right? It was a huge amount of work done by a lot of people in different countries. And yet it's been put together in a way that can serve as a definitive document that stands the test of time as the milestone for at least a decade in an area. And that was done by an incredible feat of coordination of how to write papers well.
Planck had an editorial board which is more perhaps a particle physics style thing. They had a style guide. We had style masters for papers. I was the style master for the primordial non-Gaussianity paper. These are very anal people who are like very, very particular about the clarity— like all the plots look wonderful and they look the same. You know, it's like a level of professionalism in writing papers that I've not thought possible before, and after that I was like, “I want all my papers to be like that.” [both laugh] You know, and it made me realize the importance of not just doing science but communicating science. I don't mean like public engagement. Of course, that's important too. I mean communicating science to the community.
Yes. And you know, some of those lessons I've been able to translate into the LSST Dark Energy Science Collaboration, and we are using the same Planck style guide and similar processes for producing papers. All the papers will have this standard. Internal review, you know. These kinds of things were very novel in cosmology at the time. But Planck planted those seeds. And another thing that was different is, WMAP was, you know, you mentioned the two institutions, Princeton and Goddard. Planck is an international collaboration involving several countries. All with different academic environments, different ideas about standards, different communication. And I think that it was very good for me to see that. It was a huge shock to me working in Planck after working in WMAP. In WMAP people were on the same page. In Planck there were so many different perspectives and styles and cultures and levels of contribution, and yet they were to be part of the same project. Because ESA is not about doing cosmology. ESA is about encouraging collaboration between European countries.
Right, it's a different objective, right? I now see this as an incredible strength, but when I was a junior faculty, very naive, you know, it was a shock to the system. [laugh]
We might say, to borrow an American metaphor, that in Little League baseball, everybody has a chance to play in the game regardless of ability.
[laugh] Yeah, yeah. So yeah, so there are many positive lessons from Planck, and I prefer to focus on those. And the results. And I got—
Yeah, so let's talk about the results. What was so incredible about the Planck results?
So the sensitivity and the resolution of the Planck data was a leap forward from WMAP, right? And I would again… So Planck did not discover something that's qualitatively different from WMAP. But it again goes back to this thing I was telling you about, the importance of testing one's assumptions. We could test a set of cosmological models with WMAP while assuming certain other parameters were fixed to certain values, or a constrained set of models. And what Planck allowed you to do is test a much, much broader set of models, which then relaxed many of those assumptions we'd had to make in WMAP in selecting the models, right? Because the data were more informative and more constraining. So, what we found was that the basic Lambda-CDM model established by WMAP survived, we didn’t discover any new physics, but because we had been able to directly test all these extensions to the basic model we could rule out a variety of physical mechanisms.
As a concrete example of this, the theorists, motivated by different types of action you find in string theory, had come up with inflation models which had more complicated kinetic terms in the action, and therefore would generate primordial non-Gaussianity with different characteristics to the standard single field slow roll inflation scenario. This is the three point function, so then you have triangles that can have different shapes, and if the three point function peaks on different types of triangles, like if they're long and skinny or whether they're equilateral, and so on, it tells you something specific about fundamental physics in the early universe. We found that many of those triangles could be ruled out.
And therefore, that means that the constraints on the form of the action were much more stringent. They didn't have to make assumptions about it. More broadly I think that was the key leap forward from Planck. It was more a precision mission rather than a discovery mission. And it firmed up the general direction the WMAP had shown us regarding the basic cosmological model. So some people might think that that's boring, because it's always fun to discover new things, but I think if you're a physicist, you want to be really sure about the direction you are heading in, especially when you make statements that that most of the universe is dark and its expansion is accelerating and so on. You know, you want to be really sure about those things. And I think that that was the key outcome of Planck as a scientific mission.
I think it is going to be relevant that Planck measured things so precisely and also quantified the systematics error bars so well that it is a cornerstone in all these tensions that are now appearing. So when you have a precise and robust measurement, you can easily tell if another measurement is different from it. And then you can then try to work out whether that's signaling new physics or is there something that we haven't understood about the new data that is causing the tension? And so I think that that precision quality of the Planck data really matters. Is that sort of, what you're asking about?
And together, because of course the WMAP results are coming out as well, to the extent that they are complementary, that one reinforces the other, sort of a classical question given the fact that you're right in the middle of really a fundamentally exciting moment of discovery in astrophysics and cosmology, that begs the question as a result of Planck, as a result of WMAP, what new questions can now be asked that weren't able to be asked before? And this can actually bring us close to—right where we are today, circa 2021.
Yeah. I think that this is actually a really important question. Because the outcome was to reinforce the basic WMAP model, right? It didn't actually throw up some kind of intriguing hint that then tells theorists where to go next. So what this did instead was to open up a huge theory landscape that was going like an amoeba in all different directions, rather than in a directed manner.
That is a great metaphor. I love it. An amoeba in all different directions.
Yeah, so I think that ironically while ruling out huge swathes of model spaces, for many different physics questions, it also then opened up an undirected theoretical effort. For example, in modifying gravity. There are like zillions of models coming out that are modifying gravity in slightly different ways. I'm not really sure that is a productive activity. What's the point? What in the data is indicating that we should test this idea versus that idea versus that idea? So I think what it did was to open up a landscape of theory, where there's no evidence that that's the right direction to open up the landscape, and yet people do because they have to be doing something. Do you see what I mean?
So, I think the field is relying right now on this coming revolution of data, the big dark energy surveys, to throw up something new that can guide us. And there's a huge amount of activity on the Hubble tension, because that is seemingly not going away. So people are looking to anomalies to guide the next direction. And I'm not sure that's very healthy. So the—yeah, I unfortunately don't think that some important new question has been asked. It should probably be asked, but I don't think it has. And so I think that maybe that's what the current leaders of the field should think about. The current dark energy survey program was set in motion, you know, already in the late 90s and the early 2000s. That had a scientific agenda that is, you know, by now quite old. A couple of decades old. And we are executing it. We are getting to Stage Four of the program. These amazing new surveys are going to be revealing the universe as never before.
On the other hand, the questions that are being asked in these large collaborations now, it's gone from a few dozen people to hundreds of people in Planck, now thousands of people for the Stage IV surveys. You know? The questions are the same. I think it's sad. I think people are becoming more and more specialized, the new people that are coming into the field. They are specialized on being able to do one aspect of a massive analysis. And they are losing sight of the big picture and they probably don't talk to theorists. Theorists probably don't talk to these people as well because the language they speak is so different, and you know, it risks becoming more like big particle physics experiments which are trying to answer questions that were defined decades ago by other people. And I don't think astronomy should be like that. I think one of the joys of astronomy is that there's so many questions you can ask.
Perhaps, Hiranya, a better way of going about that is one of the things that's so fascinating about dark matter is just the breadth of expertise… All kinds of different scientists who are involved in trying to figure out what it is. So to go back to an earlier comment, a hallmark of your career, that it is so interdisciplinary, that you don't stringently define yourself within theoretical or observational boundaries, right? What is problematic about the breadth of expertise that is being geared towards dark matter? And what perhaps are we being short-sighted about in our frustration that we still are really nowhere with regard to what dark matter is?
I don't think there's anything problematic about the breadth of expertise being directed towards dark matter. Did I say that?
No. I said that.
I'm not asserting it, I'm just saying, in trying to understand why, right?
The amount of brainpower that the scientific community is directing at dark matter, it's remarkable. That's just a historical fact.
It's also a historical fact that dark matter is fundamentally, remains a mystery.
So, I'm raising the possibility, right, that there is potentially some connection to how we are directing our collective brainpower to dark matter, and the fact that we still don't understand what dark matter is.
Yeah, I mean you're completely right, historically, that this has been the case. There's a lot of work going on in it, but I think it has been very narrow. It's basically been about testing the WIMP hypothesis. And you know I think the axion hypothesis is equally compelling, and until the WIMP hypothesis seemed to be not yielding fruit, people didn't really experimentally work much on axion dark matter. I mean that might seem surprising, but even though there's been this huge amount of work on it, it's been quite directed in just one of the ideas that was proposed. So I think theoretical work in dark matter has been very, very broad, but the experimental work has been quite narrow so far. It's been extensive but still narrow, and I think there is an opportunity there. I think that the question of dark energy is almost the opposite.
Like it's such a hard problem, people have even stopped looking at, like it's funny the number of proposals you see, you know, “We want to understand the nature of dark energy, and then…” And then it's like okay, we will measure w. It's not going to answer the question. I think people need to think deeply about the implications of quantum field theory in the cosmological context. And its connection with gravitational degrees of freedom. I think that very few people are doing that. I don't know why. It seems very hard to think deeply about difficult problems. It probably requires a lot of courage, and there's not been any progress really on the theoretical question of dark energy since I started my PhD. I think that's a very under-served area of activity. There's lots and lots and lots of observation activity to measure w. So I think dark matter will eventually crack. And needs more experimental effort on it to crack it. And I think dark energy needs more theoretical work to understand what we observe. And even though there's large communities working on these areas and they're broad, it doesn't necessarily mean the effort has gone necessarily in the optimal way.
This is what I would call a golden historical nugget for the archives. We'll see—
Oh goodness. [laugh]
When we find dark energy or dark matter in 30 or 40 years or something like that, we'll look back and see what you said about it. We'll have to see how well it's aged.
Hiranya, let's bring Stockholm into the picture. Was it a joint appointment from the beginning between the university and the Klein Center? Was that a package deal?
So they wanted to recruit a director for the Oskar Klein Center, and I was asked to apply to it. And I just did. Oskar Klein Center is part of Stockholm University, but includes researchers from KTH, the Royal Institute of Technology. The whole point is that, you know, they built this huge building called the Alba Nova building. And they put everybody doing similar research from these two different universities in three different departments all in the same building. So it's the idea that through co-location, you can stimulate an interdisciplinary research environment. So that's the Oskar Klein Center. But formally, it's founded under Stockholm University. And the suggestion to apply for this came at a point of my career where I'd started to see beyond my own research into issues to do with sub-optimalities in the community and I wanted to take on a leadership role where I could directly play a role in addressing those issues.
Did you find it problematic that you needed to go to Sweden to do this?
There isn't a place like this in the UK.
I think there's very few places like the OKC on the planet. The KICP in Chicago has some of the same range, The Kavli Institute in Stanford maybe? But there isn't a place like this in the UK. And—
Who was or is Oskar Klein?
He was the physicist, the late eminent Swedish theoretical physicist. And so all the things that are named after Klein in physics, except for Klein Bottles, are contributions by Oskar Klein. [laugh] So I think it's basically, there were two things that were attractive, it was the range of research that was being done at the center. It was the opportunity to do the sort of community-related leadership that I wanted to get into. And the third thing was that I wanted to work in Europe. I don't know, I'd never been to Stockholm before I tried for that job, and again, it was like, gut instinct. Like so often in my career. It was like, I went there, and it was like— wow, I really like this. I really like this environment. Loved the city, and so I thought okay, I'd better do a good job, but I wasn't expecting to move. I more or less applied just wanting practice at these kinds of interviews. And then I got the offer. And then I had to seriously think about it. And I didn't want to leave London, in London I feel like I'm at home. I don't feel at home in any particular country, but in this city, I do. So then UCL didn't want me to leave, and Stockholm wanted me even at half time, so that's what we did. We did it like that. So, I agreed to work half time in both places.
Although in reality, though, a half time appointment means essentially 150% time.
Yeah, that's right.
It's never a 50/50.
Yeah, yeah. But I'm one of those people who is easily bored, and so I can get a lot done. If things are stuck in one place, they're not at the other. It works for me. I really like it. With the pandemic it feels like a 200% job, because you can't control your presence by where you are physically.
There's no boundaries anymore.
There's no boundaries, yeah. I refuse to join two Zoom calls at the same time, but you know, it has been quite intense. Yeah so, I did that and that enabled me actually to, again, get into new areas. Like multi-messenger astrophysics. Oskar Klein had hired Stephan Rosswog well before there was any detection of gravitational waves. And he's amazing. And so using him as a core, and the fact that I was working on LSST, and the fact that there was a very active and fantastic transient cosmology and astrophysics group there. We put together a proposal which you know was put in before there was the multi-messenger discovery and was funded immediately after. So from scratch, we kind of booted up a multi-messenger environment. It's now extremely successful. And the axion work was through— it started… I like grassroots stuff. I like stimulating research activities through grassroots efforts rather than top-down planning.
And the axion research environment, which has now attracted a lot of funding, began as a journal club that was started by two postdocs. To which people from different areas started coming. And talking to each other. And through that you know we worked out that it could make a nice environment through connecting a whole bunch of different strands we were all doing independently, and we could bring them together. It was still about getting people in the same place and talking organically, you start collaborations and then organically, those collaborations attract funding, and that allows you to expand the activities and so on. So that worked very nicely. Another thing I am stimulating there is activity around the connection of AI to all these data sets that are coming. I got us into LSST. We attracted a lot of funding to buy into that. And I'm very pleased to tell you, although it's I think confidential until tomorrow, that I got an ERC advanced grant, which I'm hosting at Stockholm and I want to transition out of the directorship role and focus on LSST, just before and just after it starts getting data.
What are you most excited about with LSST?
The discovery space. Not measuring w, although I will help people do that. I'm excited about looking at a huge volume of the universe in a different way than has been done before. I'm excited about the synergy of the time-domain aspect of LSST with the gravitational wave observations that's going to be taking place over the same time scale. So synoptic observations between gravitational wave and the LSST. And even though there are defined science questions that are more or less mainstream that I want to answer with LSST, I'm most interested in the discovery space. And to prepare for that, I'm developing new tools that actually get at the information that's beyond the two-point function in the data. The information in the cosmic web. And that information—
I wonder if you could explain the two-point threshold. What's the significance there?
Ah, so the two-point correlation function is essentially correlating points in space and plotting it as a function of scale. So it's the Gaussian or linear part of the information in the density field. But if you look at the late-time evolution of the universe, there are complex structures arranged into the cosmic web. Filaments of what we think is dark matter that's traced by galaxies. So that intricate structure has information in the higher order correlation functions beyond the two-point. And to get at that information is very difficult. It's a challenging modeling problem, and also, it's a challenging inference problem. And what I want to do is to use artificial intelligence to transition… Let's see if I can try to explain this without completely confusing you.
So, to model this data in principle, you have to run hydrodynamical simulation, right? So that's a completely deterministic process. You give it initial conditions, you give it a model of gravitational dynamics, and at the end, an answer comes out and it's a cosmological simulation, right? However, that simulation is so complex, it's just like the real universe, hopefully, or close to the real universe. Then it's hard to get at the information from it without compressing the information in some way. So, the standard technique is to compress it to the two point correlation function and measure the two point correlation function between different fields, like density and cosmic shear and so on. But I want the extra information, the non-linear information, that's in there.
And to do that, you have build what I think of as mesoscale models. Some intermediate or effective models that are able to accurately link theoretical models, cosmological models, to, for example, the distribution of voids and filaments that we see in the data - effective models for the cosmic web. And it's a very new concept that I'm developing with my collaborators, and it's still not come to fruition, but this recent European grant I got enables me to do high risk, high reward things, and I think that project is a risk worth taking and can pay off. And I'm utilizing explainable artificial intelligence in that program. I want to create compressions of the data that don't throw away so much information like the two-point analysis. But which are still able to be understood by human beings who can use it to inform their physical intuition to build better physical models to do that robustly. But that's a tall order. Let's see where we get to with that.
[laugh] Hiranya, now that we're talking not just about the present but what you hope to accomplish in the near future, for the last part of our talk, I'd like to ask some broadly retrospective questions about your career and then we'll end looking a little further into the future.
So, the first thing I'd like to ask is, and I'm sensitive to as someone who's not comfortable talking about yourself, the last thing that I want to do is burden you and ask you to talk about all of the incredible awards and honors you've been bestowed with over the course of your career. But, I would like to ask specifically about the Breakthrough Prize and your views on how the Breakthrough Prize might offer a more realistic way of recognizing scientific achievement within the fact that we are operating in big science. In other words, there's something anachronistic about the Nobel Prize constraining itself to three people in physics. Because there's no such thing as a three-person endeavor. Right? That doesn't exist. That's very much a 19th century, or even mid-20th century historical phenomenon.
Do you see the Breakthrough Prize not—I mean, not as an alternative or as a competing idea—but do you see the mode of recognizing scientific excellence as being best appreciated through the model or the vision that the founders of the Breakthrough Prize had in mind?
I definitely think that it is an outmoded notion to award a prize to three people, if teams have been involved in the process of that science. Obviously, there might be some branches of physics where that’s still okay, you know, like very theoretical work can still be done by individuals mostly. But certainly, for experimental and observational work, especially that's what's driving the field right now, it's completely true that it's done in teams. I personally feel that as a, you know, a more junior person who has participated effectively in teams, that it was important for my career and the perception of my work that it was recognized as part of a team with that Breakthrough Prize that was awarded to WMAP. And I think probably all the people who were more junior during that time will probably tell you that as well, and so empirically I know that this benefits a broader cross-section of people, than just the people who were at the most senior levels and led the project.
And I don’t know whether the Breakthrough Prize is exactly the right model for it. I think the aspect where previous winners pick the next winners is not great. I think this is something that the Nobel Prize committee gets right. They have a broad consultation about what are the areas, who are the people. They do an extensive investigation about who contributed in what way to the work. And I think it’s important to reward teams, and yet not have an insular community picking who gets prizes.
So maybe with some modification, that could be a very nice model for how to reward and recognize science in the 21st century. I'm personally not someone who is motivated or driven by prizes in any way. I think doing the work is its own reward. I was very happy to be, you know, given a share of that prize, but the real reward was actually getting to do that work, and work with that team at the time. And that's all I need. But I think you know, of course, prizes, awards, are important in terms of recognizing important achievements and milestones and signaling to the broader world that something important was done in physics in a particular area, and they serve a purpose for that. But you know, yeah, it would be better if people were not so motivated by winning prizes.
Hiranya, in your charmed life in science, at being at the right place at the right time and specifically, as you emphasized, feeling so honored that “really important people” were nice to you and took the time and treated you with respect, one thing that I might observe with all of those people that you emphasized, is that they're white men. And that only emphasizes or reinforces the idea perhaps that as a woman of color, one of the things that was so special to you is that the ultimate insiders did not make you feel like an outsider.
Are we at a place yet in science, or do you see that we might get to a place, where a white male student might convey the same story in his interactions with you? In 20 years.
I hope so. [laugh]
To say, “Hiranya spent—" Right? “Hiranya spent an hour with me,” and that you can be viewed where we get to place where these identity things are not, we don't look at them as insider/outsider things. If we can get to a place in science where the excitement or the honor that somebody will feel by spending time with you is because you're simply a great scientist. Do you think we're there now or will we get there in the future?
That's an interesting question. I guess when I talked with those people and they had time for me, I didn't see them as men or white people or even “really important” people or whatever. They were just great physicists, and I admired their work. So that's what mattered to me, that you know, I read stuff by Kip Thorne, Stephen Hawking—you know, it's like it's more about their contribution to physics that mattered to me. I mean this might tell you something, but you know, until much later in my career, I did not educate myself about all the various identity-related aspects that were causing systemic biases in the field. I have very thick skin. I don't react to things. If people tell me some negative “blah blah blah” but it’s not somebody I respect or care about, it doesn't matter to me. That's a selection effect. That's why I'm still here.
But I should hope that a student of whatever background or nationality or ethnicity or other identity would be, you know, inspired by talking to me because they admire my science. Not because I'm a woman of color, but because of my science. And then we would probably have achieved equality. And we should make space for also people who don't have the perspective I just expressed. For them, it does matter that they see representation of their identity. But it's not something that has deeply mattered to me. If that was a necessity for me, I would not be here, because there were hardly any white women, let alone women of color to look up to when I was starting in the field. Nobody. So if I hadn't been able to be inspired by people that didn't look like me, I wouldn't be in the field, right? But I know that this does matter to many people, and so we should work towards representation for that reason as well as the other reason that we talked about earlier, where diverse backgrounds, identities and points of view, lead to better science. It’s also an issue of fairness and justice.
By following your nose and doing what's interesting and fun to you, this has served you very well. But it also would be helpful to hear your perspective on what might be the through-line, given all of the things that you've worked on, given the multidisciplinary approach, given your fluidity between observation and experimentation and theory, what connects it all together for you?
That's a very good question. I think, you know, this was one of the biggest challenges that I faced in my career, especially in terms of getting the first faculty position, right? My interests were so broad that people couldn't put me in a box. And so, they couldn't work out what they were getting when they were hiring me. So, I think, you know, UCL had some vision about that, and I hope I've repaid them well for that. But I think being a broad person in the field right now is not easy. It's not seen as a positive thing. What connects it all for me is kind of, it's personal and hard to describe somehow. To me, it feels like a complete whole. I don't see it as all sorts of different things that I am doing at different times. I see it as a quest to understand the universe through a variety of different perspectives, and it's something I feel a real… like a hunger for. Like I can't stop myself. And the way I would describe it is— do you know what synesthesia is?
It's when your senses are kind of mixed up, in a way. So, I have it. So, I possibly don't perceive the world the same way as you do. And to me, when I am thinking about these questions it feels almost like there's a tactile version of it in my mind. And I know very well when I understand something and when I don't. So if I feel that I can't get a piece of this jigsaw to fit in my mind, that's what prompts me to then shoot off and try to work out how to answer it. Perhaps even by working with somebody in another discipline. Because I think that discipline might have a bearing on the answer to that question. So, to me, it feels like I'm trying to put the jigsaw together, but to other people it might seem like this person is working on a whole bunch of unrelated things. Does that make any sense?
Absolutely. Absolutely. Do you feel like students today, luck or no luck, are operating in a field where things are still fundamentally exciting? And things need to be understood? The way that you benefited so beautifully when you were a student?
I think it's harder, you know, especially in observational cosmology. I think it's become, like I was saying, people being slotted more into cogs in a machine. The machine was set in motion decades ago, and it's going through its program. And so that must be really hard. I try to ensure that my students are able to do things in a way that, you know, induces a sense of discovery and exploration. I think exploration is fundamental to doing good science. But I do think that many students that come into the field right now are assigned projects that are basically about solving some small technical piece of a much larger program. And so, they never see the big picture. And they sometimes even don't know there's a big picture.
There are entire PhD theses written on testing models of something or other where the student doesn't know why that model is important or interesting, they've just gone and done the task of testing it. I don't think that's any good, and it's not good for the field, it's not good for the student. But that happens I think quite frequently. But I wonder whether people would have said that when I was a student as well. I mean there's an element of personality there, right? So I think I'm somebody that thrives in uncertainty, but many people don't like uncertainty. And that's fine too. I mean, it takes all kinds to do good science. [laugh]
Hiranya, we'll end on a very positive note. I always like to end with optimism. So let's establish a speculative roadmap where all of the sociological limitations that are holding the science back: the funding, the stove piping, the personalities, the big science. All of these things. Let's create a roadmap where best case scenario we collectively get over ourselves and we unleash our potential. What can be learned about the universe for as long as you want to remain active in the career? Decades into the future. What don't we understand now that's within the realm of possibility technically and scientifically if we can escape our own worst human collective traits?
Would you like me to be very… Should these be questions I care about or should these be questions that… ?
I think the questions you care about are the big questions that everybody in the field cares about, so I'm asking you as an observer and as a participant and as somebody who cares deeply about these things.
Yes. So, I do want to know where everything comes from and why there is something rather than nothing. And that's a question that can take many, many different dimensions as it cascades down. But fundamentally, that's one question. I also want to know what is gravity? What is space time? I worry that the program of quantum gravity is not going in the right direction, and I feel like we might be missing something there.
Something very big, yes. The approaches for quantum gravity haven't really yet made contact with empirical science and yeah, in the past revolutions in physics have come about through trying to work out what is space and what is time and asking almost childlike questions about what those things are. And I feel like we need to start asking those simple questions again. And see whether, well, see whether we can understand, what is gravity? And similarly, I think we should understand better the implications of quantum field theory. [laugh] So I'm talking about the two old theories rather than string theory. But I think we do not understand the implications of quantum field theory in the cosmological context. So yeah. Think more deeply about gravity and quantum field theory. I want to answer the question, where did everything come from? I do actually think that it is fruitful to think about the end of the universe. It's another extrapolation in the direction we can't actually test, but—
Because it's prophecy? It's not history.
Yeah, it's prophecy, it's not history, but there's… Yeah but still extrapolations can still refine our approaches to deep questions, I feel. And I think that kind of undirected thinking into the future might be helpful. I don't think we understand what goes on at black hole horizons. That is another extreme regime that you know, raises all sorts of fundamental questions about the nature of gravity and how quantum mechanics fits in there. And that's an extreme regime which I think might actually be informed by data in the future, we are starting to be able to study black holes in detail through different empirical methods. I would say despite the, you know, various suboptimalities in the field, I think the experimental directions in the field are progressing very well.
As I said, it's driven by the development of new technologies. And those have led us to very novel views of the universe. I feel like it would be great if the theoretical perspective on these deep questions started developing and exploring and burgeoning again. Like it clearly did in the middle of the last century before my time. It feels like then it got stuck or went in the wrong direction. I'm not a theorist, but sometimes I get so frustrated that I want to try to do theory just so that I'm thinking about something fundamental. But I wish the theorists would think deep thoughts again. [both laugh] Rather than like—it's too much of the shut up and calculate. It should be going back to thinking deep thoughts that might be wrong but are worth thinking. [laugh]
Is there someone in mind? A theorist that—
Is there somebody that you have in mind, a theorist that exemplifies the beauty of thinking deep thoughts?
I was always deeply inspired by Stephen Hawking. And he did his work while, you know, being subject to many limitations. And yet again, it's an example, you know, of just how much can you do under constraints.
And yeah. And if I could have a different set of skills, I would do what he did.
Hiranya, I want to thank you so deeply for spending this time with me, for overcoming your concerns about talking about yourself. My main project always is to show that science is a human endeavor, and I can't think of a better example than you sharing your stories with me. So thank you so much.
Thank you so much for asking me, and I really enjoyed it, actually.