Marcelle Soares-Santos

Zierler:

OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is March 12, 2021. I am so happy to be here with Professor Marcelle Soares-Santos. Marcelle, it is great to see you. Thank you so much for joining me.

Soares-Santos:

It's a pleasure to be here. Thank you for having me here.

Zierler:

I should say thank you so much for joining me six months later because amazingly, on the day we were supposed to do this, you went into labor and gave birth to your baby boy, Francis. So, a late congratulations from me, and I'm so happy that we're finally able to get together.

Soares-Santos:

Thank you. I'm delighted to be here. It is exciting, and I think I'll have a new perspective to add this time around when we talk, being a physicist and a mom now.

Zierler:

Marcelle, to start, please tell me your current title and institutional affiliation.

Soares-Santos:

I am an Assistant Professor of Physics at the University of Michigan in Ann Arbor.

Zierler:

When did you come to Michigan?

Soares-Santos:

I moved here in July of 2020. Before that, I was an Assistant Professor at Brandeis University in the Boston area.

Zierler:

So, you made this move right smack in the middle of the pandemic.

Soares-Santos:

Yes. And 20-some weeks pregnant.

Zierler:

It must’ve been difficult specifically trying to get to know your new colleagues virtually.

Soares-Santos:

That was one of the many challenges. Although I have to say that I had a little bit of a head start there because I have had collaborators here at UM for many years. So, I'm originally from Brazil. And when I came to the US the first time in 2008, one of the first groups of people that I interacted with and collaborated with was actually a physics and cosmology group here at U of M. So, I knew already a few people, but of course, it's just a subset of a large department. And many of my colleagues, I unfortunately, have not yet had a chance to interact much with them in person.

Zierler:

The question everybody is dealing with, particularly in the world of experimentation, where there's a necessity to have somewhat of a physical presence, either in the laboratory or with your colleagues: how have you been faring during the pandemic and the mandates of remote work?

Soares-Santos:

This has been a challenge in many ways. I think that in my particular situation, I am somewhat lucky that many of the datasets that I use for my research are possible to be obtained via remote observations, using telescope facilities remotely. Also, there's a large body of data that we collected before the pandemic that are being used, so throughout this past year, we were able to produce papers, and publications, and so on, and so forth. But it is true that a big component of our work at the physical location of the experiment is being either delayed or done under more challenging circumstances. And I think that it is putting an added pressure on everybody.

Zierler:

In what ways have not just remote data analysis of actual observations or experiments, but computer simulations or the rise in computational power made you think it's possible that more and more of your work might actually be detached from physical observation?

Soares-Santos:

We use simulations a lot in my field. But I think it's not that one thing replaces the other but complements it. So we're able to, for example, validate methods for data analysis and investigate whether some approaches will be useful or not or test what are the most relevant sources of systematic uncertainties. And we are able to do that in-depth thanks to the ability of those new innovation resources. And in order to do this level of analysis without those simulations, it would be very difficult, if not impossible in some cases.

Zierler:

Marcelle, let's take it all the way back to the beginning. Let's go to Brazil. I'd like to ask first about your parents. Tell me a little bit about them and where they're from.

Soares-Santos:

So, my parents are from Brazil. My mom is from Vitória, which is a beautiful city on the coast of Brazil, where my family lives today. And my father is from a very small village inland in Brazil, in a state called Minas Gerais. And so, my father moved to Vitória maybe 40-some years ago, and that's where he met my mom. And I was born there. And my father is a technician in electronics. So, I need to give some context for that. So in Brazil, it is something that occurred maybe in the 70s that in each state of Brazil was created a federal technical high school. So, that was part of a big effort of industrialization in the country at the time. So, many people who did not have access to, say, go to a university and become an engineer would instead go to those technical schools and become a technician in some area or another. So, for my father, that is what happened.

He grew up very poor, and through this technical education, was able to get what at the time was a good job at a mining company. This mining company had a post on a port on the coast of Brazil, so my father moved there. And a few years later, when I was 4 years old, there was an opportunity to move to a place called Serra dos Carajás, which is a village in the middle of the Amazon Forest. At the time, this mining company found a huge reservoir of iron ore. I think it's still the largest mining operation in Brazil. And certainly, it was in the 80s. So, off we go, me, my parents, and my little sister. I had also a brother. He was born there in Carajás. And so, that's where I grew up. So, from 4 to 14, that's where I lived. So, that is part of the story. So, my mom was working from home and raising us. And both of them have always been very supportive of me and the idea of studying or becoming a physicist or going in that direction. But they don't have higher education degrees.

Zierler:

Did your father involve you at all in his work? Did you understand the technical aspects of what he did? And did that rub off on you in any way?

Soares-Santos:

I think that he had a big influence on me. So, one of my first memories as a kid was from learning to read. I was probably 2 years old or so. He would get a physical newspaper back then and cut out the big letters from the headlines. And he would play that to learn how to read. And obviously, I still couldn't really form words or anything like that, but I got to know the alphabet that way. And I was proudly showing off my skills and getting treats and chocolate from people by saying, "Oh, I can read this." So, learning was something that was part of my experience from the very beginning, and he was a big component of that.

Zierler:

Growing up, what were some of the most important religious or cultural traditions that were formative in shaping your identity and your family's identity?

Soares-Santos:

So, my parents are Catholic. So, that is their main background. And in particular, they are very active in the community. And this was something that was important. I would say for my mom, I think that's been particularly important because when we moved to Carajás, it was a very small place, thousands of miles away from my grandparents and cousins. I have a huge, huge extended family. But we were really far away from everybody. And I think that the church, for example, working in the community and so on, that provided her with a sense of community that was very helpful. And still is today because they're both very active in the community right now in Vitória.

Zierler:

What kinds of schools did you go to growing up?

Soares-Santos:

It was private school. So, basically, in the village, in Carajás, as part of the incentive package for people to move there, the company provided a very good school. That's where I studied. At least until maybe 7th grade or so because that's when my father retired, and we moved back to my hometown. And that's when I went to high school, which was a public school.

Zierler:

Did you get interested in science early on?

Soares-Santos:

Definitely. I didn't have a name for it, but very early. I didn't know what it was called yet. Some kids are great at art, or sports, or that stuff. I'm terrible at all of those. The only things I thought I was good at were puzzles, figuring things out, math, things like that. So, I thought that was my thing. And I didn't know what it was called or even that there was a profession where you just happen to be studying how the universe works. But once I figured that out, it was clear that was the path I wanted to do. Over the years, I changed my mind a lot about what I was going to do in the world of physics because it's a very vast world. But I think that physics was pretty much in the cards from the beginning.

Zierler:

To the extent that Brazil might be a conservative society in some ways, were you ever made to feel growing up that science was not appropriate for a girl to be interested in? Were you ever discouraged along those lines?

Soares-Santos:

Not that it was not appropriate. So, I think the way sexism manifested itself, I believe, is not in a sense that anybody told me that I couldn't do this or that, at least in my experience. It's something that you notice that most of the people referred to in the textbooks that you read, the great discoverers of laws of physics and so on, most of them don't look like me. And those things indirectly will shape your notion of belonging or not in that world. So, it's more subtle than that. I don't think I ever had somebody tell me that, "No, it's not for you. Don't do that." I think some of it happens, but you just look around to see what people look like, and you realize that, "Oh, maybe I'm in a place that may not be a place for me."

Zierler:

Did you have a good curriculum in math and science in high school?

Soares-Santos:

Yes. So, when I went to high school, I also went to a technical school. One of the ones I was referring to earlier. And so, these days, those schools no longer only focus on the technical aspect. In fact, they have a very strong curriculum in math and science. They are public schools, which in Brazil, means you don't have to pay tuition or anything, and they're very competitive schools. You have to pass an admission exam. So I went to a technical school in my hometown in Vitória. And there, I did get a lot of experience in physics, and math, and all that.

Zierler:

Between your grades, your family’s financial situation, and geographic limitations, what kinds of schools did you apply to for your undergraduate education?

Soares-Santos:

For undergraduate education, I only applied to the university in my hometown. I didn’t apply anywhere else. So, I have to say that this is a little bit different between Brazil and the US because the selection process for the public university–also, it has changed since I was a student. But each university has their own admission exam. And it's something where you have to go there three days and do exams in math and geography one day, and then physics and chemistry the other day, etc. And so, it is not like in the US, where you send your application, and then you're going to be accepted or not.

You have to physically go there and take an exam, and it's one different exam for each school. And they're far from each other. So, I only applied for the university in my hometown. And it was a good university, they had a physics program, so I went there. And my thought was that after a four-year degree, I would seek an opportunity to go to a PhD program elsewhere. They didn't have a PhD program at the time. Nowadays, the physics department there has a PhD program. So, that was my path. I only applied to the university in my hometown.

Zierler:

In the British system, you know what your major is going to be from the beginning. And in the American system, there's a more open course of study. Is the Brazilian system more like the United States or more like Great Britain?

Soares-Santos:

More like in Britain. When you apply, you apply specifically for that major. So, everybody does the same exam. But when the grades for the admission exam are ranked–so say there are 50 slots for physics. People are ranked, and the 50 people who chose physics as their first choice get in. And it gets a little bit complicated because you have first choice, second choice, third choice. But basically, when you enter, if it's physics that you're doing, it's physics that you're doing. And now that I'm a professor, I can see that from a different perspective. That means from day one, you get more physics and math in your curriculum as an undergrad than you might get in the US, depending on how soon you decided you want to become a physicist.

Zierler:

And this is always something that amazes me. In the United States, there's no expectation you should have any idea what you want to major in as a high schooler. So as a high schooler, what was it about physics? How did you know you wanted to go into that field?

Soares-Santos:

By the time I started high school, I already knew I wanted to do physics because it was not clear to me, for example, that I was interested in figuring out how nature works and going deeper into the type of questions we ask as kids. "Why this? Why that?" So, it was clear to me that I wanted to do physics already. So, by the time I started college, it wasn't a question anymore, it was the thing I wanted to do.

Zierler:

To the extent that you had an appreciation that there was a binary in physics between experimentation and theory, what did you gravitate toward more as an undergraduate?

Soares-Santos:

I thought I was going to be a theorist. Early on, I saw myself as somebody who'd be coming up with new concepts and ideas and figuring out the mathematical constructs that would represent nature. And part of that could've been probably because of the department I was in was really strong on the theoretical side, and the opportunities for experimental research in fundamental physics at least, not on the applied side, were more limited. So that's what I thought I was going to do. And so, I worked on calculations of predictions for the spectrum of primordial gravitational waves in a certain scenario with the non-Lambda CDM Cosmological Model. And there's even a paper out there I'm a co-author on with the results of that. And I really thought I was on the path of being a theorist back then. If you asked me at that point, that would've been my answer. It also may have been a traumatic experience because of the Millikan Experiment, which never works.

I discovered that the charge was not quantized because I could not reproduce that experiment at all. But overall, I thought I was going to be a theorist until about halfway through my master's studies. That is also something that's a little bit different from the US, that in Brazil, it is very common to do master's and PhD as two separate units. So, you have a two-year master's with coursework and then master's thesis. And then, you do a four-year PhD, where there is a little bit more coursework, maybe not as much as in the master's, in a longer period. So, when I got to the part of really working on my master's thesis, that's when I decided that I really wanted to become an experimentalist.

Zierler:

Did you ever give any consideration to doing graduate school outside of Brazil?

Soares-Santos:

I did consider a little bit of that when I was about to finish my degree and transitioning to graduate school. But at the time when I really started looking at the mechanics of it, it was a little bit too late for that particular academic cycle. So, instead, what I focused on was seeking an opportunity to do a period of research abroad with a fellowship. And this was what I pursued instead. So, I did my master's and PhD at the University of São Paulo in Brazil and about halfway through the PhD, came to the US with a fellowship. That approach worked very well for me. But doing the entire PhD in the US wouldn't have worked for me.

Zierler:

And this is when you were at Fermilab during your PhD.

Soares-Santos:

Yes, yes. So, the story there was, when I was considering where to go with the fellowship, the way it works is that the student applies to go to a certain institution abroad and needs a letter from a professor of that institution saying, "If awarded, I will be willing to host this student here." And at the time, Scott Dodelson was a professor at the University of Chicago and a researcher at Fermilab. He came to Brazil for maybe a summer school or something like that, which I attended. And I really liked his lectures. And I asked my supervisor in Brazil at the time if he would be willing to make an introduction so that he would consider the possibility of allowing me to come to the US and work with Scott on that fellowship. So, I didn't participate in that conversation, but I'd imagine that my supervisor talked with him and said, "I have this student. She's not bad. She's applying for this, so what do you think?" And probably what he said was, "OK, if she comes for free, what harm is there?" Or at least that's how I imagine it. But afterwards, I wrote an application and got the scholarship. And I showed up in Chicago's O'Hare Airport in March 2008.

Zierler:

How was your English at that point?

Soares-Santos:

English was not a problem for me.

Zierler:

You learned it throughout school?

Soares-Santos:

I learned during the school year, starting maybe from 1st grade. But as a parallel thing outside of formal school. So, in Brazil, it's common that in school, you have English as a second language, but oftentimes, it's focusing more on just reading. And that's one of the reasons why sometimes it's really hard for us to pronounce words correctly because we don't get many opportunities to practice that. But there are schools where you can enroll to just learn English as a second language. And so, that's what I did. At the time, this was, again, one of the things that my father was influential in because he had a hard time learning English for his job, to understand the manuals of the machines he was going to fix. So, he thought it would be important for me and my siblings to learn that from the beginning. So, he made a point of making sure that we would enroll in that school and attend it. I hated it, I have to say, in the beginning at least. But afterwards, you get used to it. And so, by the time I came to the US, English was not really a problem for me.

Zierler:

We'll talk more about the science, but when you first came to the United States, I'd like to ask how you felt others perceived you in the United States may have been different than in Brazil with regard to your ethnic identity or racial identity.

Soares-Santos:

I have a hard time with this question because I think my understanding of this question has also changed as I've been living here longer, than I did then.

Zierler:

The question is first impressions in 2008, when you might have never thought about these things before as a Brazilian coming to the middle of Illinois, of all places.

Soares-Santos:

So, my first impression was that in the US, the view of South America and Brazil is they think we're all kind of in the same package. And so, things are very different and very nuanced to me between the difference between being Brazilian, being Chilean, or being Colombian. It was clear that that was on most of the people I would talk to. People get put in the same bucket, and it doesn't work. Another thing that I thought was interesting was, when you look at me, there's no doubt that I'm a Black person, right? But I'm not someone who's experienced being Black in the US, where there's a completely different history and experience. So, I don't think that I was feeling that I fit in with the Black community in the US. So, I think those were some of the impressions I had. Nowadays, I see it differently because I see more clearly that if you're seen from 5,000 miles away, another country, another continent, that sometimes–the same way that for me, the US was all one thing, where now I see clearly that it's very different, such as the East Coast and West Coast. So, clearly, that perspective has changed in me as well. Also, as I live here, I'm learning more about the differences in the ways that African Americans and people of African descent in Brazil have similar problems, but manifest sometimes differently.

Zierler:

Did you find Fermilab to be a welcoming place? Did you feel comfortable from the beginning there?

Soares-Santos:

Yes. It was very cool, I have to say. But it is interesting because Fermilab is a place that breathes physics, and it’s very different in that regard from my university, where you have an English department, next to literature, next to physics. But at the same time, I feel like I needed that. So, it was really amazing to be somewhere that just breathes physics. And a lot of it was very different from the physics that I do. Cosmology is just a subset of the program that exists there. But that meant, also, that I was learning something new every day. So I started attending seminars and talks, being hungry to learn more. And it was a little bit intimidating at times. And it was also out of this world to experience things like going to a seminar, where I'm hearing about a result for the first time, before it's announced outside. So you really feel like you're in the heart of where physics is happening, while oftentimes, back in Brazil, you're reading a paper that has already gone through a review process, etc. It seemed far more remote from where the discussion is really happening. So that can be a little bit intimidating, but it's also extremely exciting, and I thought that was very good.

Zierler:

Did your experience at Fermilab clarify what you wanted to do for your own thesis research back in Brazil?

Soares-Santos:

So, at the time, I was studying galaxy clusters as a way to map out the history of expansion of the universe. So, there were a few things that clearly changed the direction of my PhD because of my experience at Fermilab, and one of them was that while before, I was planning to use a much smaller dataset, and I think the dataset I had access to was more limited, suddenly, I was in a group that had access to a huge amount of data, and computing resources, and so on. And that opened up a number of possibilities. Another thing that I learned was working in collaboration with a larger group than just my PI and another student. For example, one thing that I learned that wasn't in the picture were the techniques for measuring the masses of clusters of galaxies using gravitational lensing analysis. And this was something where by working with colleagues, post-docs, and other people at the lab, I learned that I could incorporate that into my thesis, which was great.

Zierler:

Did you leave that fellowship really wanting to come back to Fermilab? Did you know that that's what you wanted to do after you defended?

Soares-Santos:

I left with an offer to come back, so yeah. When I got towards the end of that time, I had learned about how academic job cycles work in the US. So, I had applied for post-doc positions in the US. At this point, I had narrowed it down to institutions that would be members of the Dark Energy Survey because I wanted to work on that dataset at the time. DES was not yet operational, but it was up and coming. I knew that it would be an important component in the years to come. So, I applied to institutions that were members of DES. I couldn’t say I wouldn’t have gotten the job otherwise, but it clearly made a big difference in terms of learning how to present your work and what makes you competitive when applying for a job. So then, I got some interviews and so on, and a couple of offers, including from Fermilab, to become a post-doc there. So, when I went back to Brazil to defend my thesis, I already knew that I'd be coming back the following year. And that was great. It was a really exciting time to join Fermilab and go to the next stage of my career.

Zierler:

The PhD thesis is a great opportunity to demonstrate your interest in specialization in a particular area of astrophysics that could be applied to any number of collaborations. So, what did you see your identity as an astrophysicist at that point in terms of using your thesis research as a launchpad for the next collaboration, the larger research that you wanted to be a part of?

Soares-Santos:

I used to call my thesis the franken-thesis. But I thought it was, really, a toolbox. So, the big thing for me was galaxy clusters. The big question was cosmology, but galaxy clusters are the main observables that allowed me to answer the cosmology questions.

Zierler:

And what are those questions? What are the big questions in this period?

Soares-Santos:

So, the big thing that we want to know is, what is causing the expansion rates of the university to accelerate? We have no idea what that is. We have an estimate that approximately 70% of the energy content of the universe today is this thing we call dark energy, which causes the expansion of the universe to accelerate. I find this very amazing and humbling at the same time because all of this stuff that we’ve spent years learning in physics accounts for such a small fraction of the total of the universe. It’s also great because it means if you’re in the job of learning what that means, you have a job for life. There’s a lot to learn. And galaxy clusters are awesome because they are the largest gravitationally bound structures in the universe, so they are the most sensitive to whatever is happening in the cosmos in terms of the acceleration rate. So, for example, if this dark energy is changing with time, it will cause the rate of growth of the clusters to change.

So, if I can count number of clusters as a function of cosmic time as a function of mass, I have an observable that allows me to map it back to what is going on in the cosmos in dark energy. And so, I really like that idea, something so simple as a number count helping us answer such a complex question. Of course, the devil's in the details, so getting to identify the clusters reliably, and getting the masses, etc., each and every step is very complicated. And then, more than a decade later, we're still trying to get that data. But it is a powerful observable, so it is worthwhile investing in it. So, by the time I finished my PhD, the way I saw myself is, "OK, I'm this person whose specialty is using these clusters for cosmology. And in my toolbox, I have algorithms that allow me to identify the clusters out of millions or hundreds of millions of objects in the sky. I have tools to identify which galaxies are members of clusters or with a certain probability and whatnot. And then, associate the mass to them, and do the modeling, and then get to the cosmology. And this was something that I was eager to apply to the DES data that was upcoming, and it became during the post-doc years that this was a big focus of what I wanted to do.

Zierler:

What research did you join when you got back to Fermilab?

Soares-Santos:

When I got back to Fermilab, I joined the Dark Energy Survey. And at the time, the camera for the survey was in construction.

Zierler:

The construction was at Fermilab?

Soares-Santos:

Yes. With contributions from collaborating institutions and so on. But the lab was the heart of the experiment at the time. And this was interesting because I realized at that point, although it was not required by my job description, that it was an opportunity to get involved in the instrumentation, the actual construction of the experiment, which is something that, in some areas of physics, that's the standard, that people work on all phases of their experimental setup. But in my field, it's not very common. Oftentimes, either because the timing doesn't work well, you join the experiment, and it's already built, or for whatever other reasons, it's not so common. But I had this opportunity, so I went to my supervisor at the time, and I said, "Hey, I would like to get that experience, getting involved with the instrument." And she was like, "Not a problem. We can arrange that." So a year later, I find myself being an expert in the instrument. And that was about the time that we were shipping all the parts to install and put on the telescope. And I went there, and it was wonderful. It was really a different type of learning experience that I thought was amazing.

Zierler:

Maybe it's going to sound like a dumb question, but with the Dark Energy Camera, how can we build a camera to take photos of something we don't understand?

Soares-Santos:

What we are going to do there is to take images of the sky, identify objects such as galaxies and clusters of galaxies, and we'll use those objects to infer what's going on with the dark energy. So, the name may be a little bit misleading, but the idea is that we study dark energy by studying the bright objects we can observe.

Zierler:

Of course, seven years later, we still don't know what dark energy is. So, the two ways of looking at that are, what do we know we don't know as a result of the dark energy camera, and in what ways is it possible it hasn't gotten us any closer to understanding what dark energy is?

Soares-Santos:

So, we did make significant progress since then. And we're still making progress in this area, so we're not at the final answer yet. But the main ways in which we make progress are, for example, if you take galaxy clusters specifically, ten years ago, when we started with DES, in clusters, you had the idea that the number count would be an observable that is sensitive to dark energy. But an actual measurement that would take advantage of that fact was not really quite there. The uncertainties and so on were just too large. Now, we have made significant progress with measurements of the number counts of clusters, also measurements of power spectrum of galaxies, and all correlations, and rate of growth of structures in the universe, and so on. We have made significant progress where we have measurements, where we can quantify the parameters of the cosmological model.

More than that, another thing that's important to notice is that DES is, in comparison to other surveys in the same category, particularly cool because we were able to do analysis of clusters, and weak lensing, and supernova, and combine them all together from the same dataset. And this allows us to have fine control of the systematic uncertainties. Because as I said earlier, the devil's in the details, so you really want to get those uncertainties under control. So, it's not that we have an answer of what that dark energy is today, but we have sharpened our toolset for achieving that. And we're getting measurements of those cosmological parameters. The main ones are the rate of expansion today, the equation of state of dark energy, and a parameter that has to do with the rate of clustering, how quickly the structures form in the universe.

So, we're making progress in those areas, measuring those parameters, with levels of uncertainty that are good enough to begin distinguishing between one model and another. Because the idea would be that the cosmological constant of Einstein is a model for dark energy that fits the data. But until now, the uncertainties on those measurements were so large that there were families of models that could live under that tent. And now, what we're doing is shrinking that parameter space so that we can restrict the models, and we are making progress in that regard. Which also means that we are finding some surprises. You probably have heard about the whole tension of the Hubble constant parameter measurements.

Zierler:

What is so surprising about that?

Soares-Santos:

So, the story there is, one of the main parameters that are relevant for the problem of dark energy is the rate of expansion of the universe today. So, when we talk about the accelerated expansion of the universe, we're saying, "This rate of expansion is growing." But pinning down what is the rate of expansion today is an important parameter. So, there are different methods of measuring this quantity. And one, the gold standard, is currently to use supernova observations. And these measurements are hard to make. And they're beautiful because we measure those numbers with percent level, a couple-percent uncertainty. It's also possible to infer what the rate of expansion of the universe today is by observing the cosmic microwave background.

That signal is coming from 300,000 years after the Big Bang, so 13 billion years ago. And it has to do with the fact that back then, the universe was so compact and so hot that photons and electrons were really coupled together, interacting very closely. So what happens is, when we look at the radiation from that time, we're really seeing an imprint of that scale of interactions. So we make an all-sky map of the cosmic microwave background radiation, we look at the angular size of those blobs in the sky, literally, where is the typical scale that the photons could travel in that plasma. And then, we can say, "OK, it looks in the sky like it's this big. So, if I translate that for 13 billion years afterwards, that means that the rate of expansion in the universe today is this number." And I compare this number with the number that the supernova people give me, and they're close, but not quite the same. They disagree with each other at a level of, right now, greater than five sigma.

So, it is not a fluke. And you can play this game in many ways. You can dice and slice your data and combine it in different ways, etc. The discrepancy is there. It is not going away. It is not a fluke. And more precise measurements just make it worse instead of better. So, usually, when this type of discrepancy comes along, there are two options. Either there's some systematic uncertainty that nobody thought of, or there's new physics. So, the possibility that it's systematic uncertainty, where all of these multiple probes, multiple observations, different surveys help because we try to address the problem different ways. Although, it could be that you're dealing with a nasty conspiracy of systematic effects all going the same direction. On the other side, if it is physics, that is a place where the parameters of dark energy, for example, if dark energy is changing throughout the cosmic history, you could account for that difference. And so, measuring that in detail can be a way of indirectly figuring out what is going on and what dark energy is or how it evolves. So, that is the current situation with this discrepancy.

And then, what happens is that because DES is a survey that has multiple observables, for example, we've collected thousands of supernovas, millions of clusters, we can make internal casts of consistency for those parameters, and compare as well with other external data sets, and so on. And what we find is that we see this type of tension between different measurements in our data, and it's not only the rate of expansion, it's also the rate of growth of structure, and other observables are showing the same type of tension. So unless you're really in an unlucky scenario where there's one source of systematic uncertainty that nobody thought of, which can explain all of this, it is likely that we're getting close to figuring out what that dark energy might be. Because we're getting to experiments that are sensitive enough that they're giving us different answers when we make different assumptions. So, it gets more and more exciting.

Zierler:

Beyond Fermilab, to what extent did being a member of the Dark Energy Survey allow you to really broaden your contacts, the kinds of people you were working with, the kinds of institutions that were supporting the Dark Energy Survey? To what extent was it useful in broadening your professional research world?

Soares-Santos:

Those collaborations really give you an opportunity to work and learn from different people, different institutions, and different backgrounds, and also to show your work to people as well. So I think this was a big component for me in terms of helping me learn. And then, later, after I was already in my fourth year as a post-doc and applying for faculty-level positions in 2014 or so, I was thinking already about going to the next stage of my career, a new direction for my research program as an independent PI. And that was a time when the LIGO collaboration came to the community, saying, "We're going to start our first season of observations with the advanced LIGO detectors next year." So, that would be 2015. "And we're inviting members of the astronomy community who are interested in pursuing follow-up observations attempting to find the electromagnetic counterparts.” So, when I first saw this, I was just like, "Wow, OK. That's not relevant to me."

But then, thinking a little bit more about it, and thinking about the possibility of using these gravitation wave events as a new cosmological tool–imagine the possibility of making the measurements that I was talking about that were discrepant, now with an independent observable. That would be the perfect way to try to figure out where we are in this parameter space. And more than that, the localization areas in the sky are huge because the gravitational wave detectors are not very good at localizing the objects. So, if anyone has a chance at finding the counterparts, it would be us because we're using a wide field of view camera. Oftentimes, people talk about how they have to raise millions of dollars to build an instrument to do it. In this case, it looked like we had the instrument at hand. So, the more I thought about it, the more I thought that would be a perfect next project for me.

Not that I don’t love clusters anymore, but it would be my next passion in thinking about the next stage of my life as a physicist. And in order to do that, there were a number of new things that I needed to learn. I had never worked with transient object analysis before, and I didn’t have the expertise. But I was a member of this collaboration with amazing experts in supernova analysis that are very close to what we needed to do and with people who had expertise in spectroscopic follow-up observations, and simulations. And pretty much all the pieces that we needed in order to put together a successful program were there, were people that I already knew, or were just one or two degrees away.

So, it's not to say, "If I would not have been a member of the collaboration, I would never have been able to do this," but it did help. And it was something where I feel that being a member of the collaboration was a huge, huge leg up in getting this program off the ground and to be the success that it was. We were successful in finding the electromagnetic counterpart of the first neutron star merger ever detected, and I am certain that I would not have been able to ramp up my level of expertise alone in such a short period of time.

Zierler:

One of the things that's so interesting about dark energy collaborations is it takes so many different scientists from so many different intellectual perspectives and technical expertise. What did you feel were your talents, the things that you were good at that you contributed overall to the collaboration?

Soares-Santos:

I think that has changed over time. So, early on, I developed the expertise, as I said, in the instrument, the camera, and really became one of the few people that could answer the questions that were relevant in the very early days when we were putting the instrument together and figuring out problems in the day-to-day operations. And other than that, on the science side, I think that being able to do this complex analysis, where we're really trying to put together different datasets that are very large sometimes and that are extremely complex and distill the physics out of it in the analysis is also something that was part of my contribution. And then, there are other things that are not as technical. I think that in the process of putting together this group to do the gravitational wave follow-up program, I also learned a lot about how to run a group and a team of people in a way that would be fulfilling and inclusive to a lot of people. And I think that's one of the areas where I've also contributed.

Zierler:

On the personal side, at what point did you start to realize you were making a life for yourself in the United States, that you would not be going back to Brazil? A post-doc is always a temporary kind of thing. Maybe you'll go back, maybe you won't. But at what point did you start to think, "This is where I want to be?"

Soares-Santos:

I'm hesitating because I'm still thinking, "Is this where I want to be?" So, the way I see this, and I think other people will probably see it very differently, but I go where the physics takes me. And so far, at least for me, it's all happening here, so it's natural that I want to continue on this path. I don't know, maybe in the future, who knows what will happen? But so far, I feel like my vector of motivation, the resources I need, and the networks of people I work with are all here. And so, each year, I'm becoming more and more bound to this place. But it was not thought that way when I came here at first. And of course, at every stage, when you start applying for jobs for the next stage, then you have to make some decisions. And then, another thing that comes into play is you start building a life that includes physics, but is not only physics.

Zierler:

Like a 6-month-old, for example.

Soares-Santos:

That also becomes a component. So, my husband is also a physicist, and for three years, we were trying to converge on the same side of the Atlantic. At one time, he was a lecturer in the UK. So if the right opportunity for me had come there, I would probably be in the UK right now. But at the time, instead, a better opportunity came for us to reunite here. And so, here we are.

Zierler:

Kind of a broad question about Fermilab. Of course, the history precedes you, but by the time you got to Fermilab, its reputation in particle astrophysics was already solid. But there was a long history where astrophysics was not very central to Fermilab, it was much more high energy physics. Did you feel like you were part of that transition historically? Were these things still in flux by the time you got to Fermilab? Did you notice those kinds of things?

Soares-Santos:

By the time I got there, that transition had already mostly happened. So, what I saw was a number of clear traces of that unique history. So, for example, a number of the more senior colleagues, in particular, in the group had been physicists working, for example, at the Tevatron, and they brought their expertise, for example, on silicon detector development to work on the CCDs of the camera, which are also silicon-based. Many of them also brought in experience from managing these large collaborations to DES. Today, DES has over 500 people. But it didn't start that way. And many people who joined, yes, were not coming from a background of experiments that large. So some of our colleagues from those experiments brought that in. And what we were talking about early on, the simulation side, our colleagues in the particle physics world used their Monte Carlo simulations very heavily. So, that tradition was also an area in which they clearly helped us make fast progress. So, a number of those things, it was clear that I could trace back to that history. But when I arrived, most of that was already in place.

Zierler:

Were you considering ever making a career at Fermilab? Is that something that was a possibility available to you?

Soares-Santos:

So, after my post-doc, I was an Associate Scientist at Fermilab, which is the tenure track equivalent position at the lab. So, yes, I was considering that as a possibility. At the same time, I have to say that I see myself a lot in this role, as a faculty member at a university, and I felt that the experience of running my own research group as a PI and so on was also something that I wanted to have. But for a while, I was considering continuing my career at the lab, yes.

Zierler:

Tell me about the discovery in 2017 for which you were an invited speaker for the National Science Foundation.

Soares-Santos:

Oh, that was awesome. So I mentioned a little bit earlier about the project. So, it started, really, as an idea where I thought we were a perfect match to the problem at hand. And I had no idea that we were going to actually make such a discovery so soon. Less than two years from the beginning of the observations by the advanced LIGO, we were able to make this detection. So, it was really spectacular to be part of that discovery. Also, this was something where there was a much larger community involved than just DES and LIGO. And this was quite interesting as well for me, not only in terms of career, but as a scientist and member of this community. So, this was really exciting. We were, I think, thrilled to be part of the discovery, and I wasn't really, early on, expecting that they would invite me to be part of the team there giving the news to the world. But that was particularly amazing.

Zierler:

When did you go on the academic job market?

Soares-Santos:

So, my first round in the academic job market was in 2014. And then, after that, I was still trying to figure out a solution to my two-body problem. And in 2017, we converged at Brandeis University. So, that period was the search.

Zierler:

Did you take on graduate students at Brandeis?

Soares-Santos:

Yes. I have four graduate students working with me right now.

Zierler:

Who are still at Brandeis?

Soares-Santos:

They moved here with me. So one of them, Alyssa Garcia, is in her fourth year, so she's the student that I have that's most advanced. So, when I moved here, part of the story is that it really wouldn’t make sense to leave them behind, and I really wanted to make sure that the group could come here and be as productive as it could be. So, it was part of the negotiation there to make sure that we'd be able to move together. And I'm glad that worked out.

Zierler:

Tell me about some of your more recent work in gravitational waves.

Soares-Santos:

Since 2017, we had one more observing campaign, where we did follow-up observations of gravitational wave events that LIGO and Virgo had seen. We have not yet found any other spectacular event with an optical counterpart like 2017. But we did make progress in many different ways along those lines. One of them is, with the simulations that we do, we are able to make a complete study of the probability of detection and so on. So, even if we don't find anything, we can put constraints on the physical parameters. Knowing that there was a gravitational wave event in that region, then we looked there, and we found nothing, we can put limits on the physical properties. For example, what's the maximum amount of mass that could be ejected from that system, maximum velocity, and opacity? This is important because since we have only one observation so far, we cannot tell if that one particular event is typical or an outlier in the distribution.

So, these type of studies allow us to fine tune the searches we're going to do in the future and hope to make more detections and more discoveries in the next round. But that's one way that we make progress. The other way, which is one of my favorite topics lately, is we've made progress in using the event for which we don't have optical counterparts for cosmology, the idea being that–I'm realizing I didn't explain the connection between the gravitational wave event and cosmology, which is the bottom line that we want to get at. So, the idea there is that, in the same way that we use, for example, supernova as cosmological probes, where we get the distance information from the fact that the supernova are standard candles, and we measure the redshift and do the distance versus redshift plot, for the gravitational wave events, we can do a similar analysis. But the distance information and the Y axis of the plot is not coming from the fact that those are standard candles, but from the fact that they are what we call a standard sirens. That means that you can infer the distance from the gravitational wave signal alone.

And this is a beautiful result from General Relativity. It's really amazing. And it's a very clean result as well, meaning that if you compare it with clusters, for example, where there are tons of sources of systematic uncertainties, the number of systematic uncertainties there is much reduced. Now, in order to do that, to put the dot in that imaginary plot, you need to detect the optical counterpart. But what if you don't have an optical counterpart? Can you still use this event for cosmology?

Zierler:

Why would you raise the question, "What if you don't have an optical counterpart?" Why would that come up?

Soares-Santos:

That would come up either because the counterpart's too faint, and we were not able to detect, no matter the fact that we have a great telescope, maybe it's just too faint for it, or it could be that instead of two neutron stars, you could have, for example, mergers of black holes. And those mergers, at least as far as we know, don't really emit any electromagnetic signature. The reason for that is, the electromagnetic signatures that I'm talking about are coming from the result of the neutron star material being disrupted in the final moments before the merger, and all that neutron material going through the nuclear reactions heats up like a ball of fire in the sky that becomes very bright for a short period of time that we can see with our camera. Now, if these components of the merger are black holes instead of neutron stars, then there's nothing there to be disrupted like this. So, this mechanism doesn't work. So, the reason why you would want to use these dark events for cosmology is that it would add to your sample. Instead of having only the subset of the events for which you found counterparts, now you can use all of them. The ultimate dream analysis that I have here in my mind would be something where we actually combine both things. We would use all of the events, regardless of the existence of a counterpart or not, and then you would get the subset of those that do have a counterpart and enhance your result of that. But in order to do this, we need to figure out a way to use this dark sirens cosmology.

And so, in the more recent papers that our group put out, we did look specifically into that, into, “How can we make a measurement of this rate of expansion of the universe without knowing which galaxy in that region of the sky was the one galaxy associated with the event?” So, basically, you do a giant likelihood analysis with a probabilistic assignment where each galaxy has a little probability to be associated with the event, and you come up with an answer. This technique is, of course, not as precise as it would be with individual, one-to-one association with a counterpart. But we would win the game with the fact that there are many, many more events than there are events with counterparts. So the papers we put out recently show that we would need maybe a factor of 10 or 20 more advanced to achieve the same level of sensitivity that you would achieve for events with counterparts, and that is great news, so I'm excited about that.

Zierler:

This is a great place to ask your understanding of terms that mean different things to different people at different times. So there's cosmology, there's astrophysics, particle astrophysics, and there's astronomy. Where do you see these fitting in together, where do you see the separation, and in what field is your area of expertise most relevant?

Soares-Santos:

That's a very interesting question. Sometimes, I have the impression that this is a superficial distinction between astro and physics. I have the impression that this probably comes from the fact that early on, when we were studying the cosmos in ancient days, it wasn't clear that the same physics that happen in the lab also happens up there. And as a way to have reconciled those two views, I often think that the astronomy or astrophysics component is more referring to the same physics, just happening in a specific context. Obviously, if you ask different people, they will have very strong opinions about this. But I tend to see myself as a physicist trying to understand a question about the fundamental nature of the universe, and that is what dark energy is. That question about dark energy has to do with cosmology in the sense that it will tell us about the fate of the universe, the evolution of the universe and its components, and it uses astronomy data. And so, that's the way I connect the dots there. But I have to say most of the time, those distinctions are a little bit arbitrary.

Zierler:

Tell me about your decision to move from Brandeis to Michigan. Was Boston not cold enough for you?

Soares-Santos:

Oh, my. This was hard because I really liked it at Brandeis in the department. My colleagues there were wonderful. But really, Michigan made both Bjoern and I an excellent offer. And so, at that point, we had to look and try to think of future a decade or so from now, and we thought this was going to be the right place for us. So, that was more or less the process there. And then, once we came to that decision, it became clear, "If we're doing this, we have to do this now. Because delaying it longer wouldn't help."

Zierler:

And as you said earlier, it was eased to some extent because you had already worked with some of your future colleagues at Michigan. They were already collaborators.

Soares-Santos:

Yes. So, that made some of that a bit easier.

Zierler:

In what ways is it more advantageous just to be at a bigger school?

Soares-Santos:

I think there's a big difference in terms of the resources that are available, especially for young faculty members. I think that makes more of a difference. When you're establishing yourself, establishing your group.

Zierler:

Obviously, as we've said before, you got to Michigan in the middle of the pandemic, where most of your work is through the computer. But once we get through this thing, hopefully September, to what extent will that give you an opportunity to build a lab in Ann Arbor? Is that something that you're planning on doing?

Soares-Santos:

Yeah. Actually, lab renovations are ongoing.

Zierler:

Was that part of the package, that you would get support for building a lab from scratch?

Soares-Santos:

Yes. So, this was part of the whole package deal. And so, this is happening now, the renovations. And hopefully, we'll be able to move in right when we're actually able to go in. So, that's part of the plan. And my goal with that in particular is for my group to play a role in developing the next generation of experiments. We already now have experiments that are planned and being built now. So that's DESI, and that is the LSST camera. And I am involved in those experiments, and my group's helping with those. But I think that we're not stopping there. The game is just in the beginning. While we're getting better and better at doing this, and we're really going to be going towards the next-next generation of those experiments. In particular, as you probably know, one of the things that's happening right now in our community is the whole snowmass process of planning for a timeframe of ten years and so on. So, the way I see my group going forward in that area is that we're going to have the opportunity to play an important role in those future experiments.

Zierler:

Of course, the opportunity to build a lab from scratch forces you to really think, what do you want to do with this lab? What are the questions that you want this lab to answer? With that in mind, what are the most compelling questions for you as this lab comes to fruition?

Soares-Santos:

One of the important things on the technical aspect is that we're going to need the ability of observing tens of thousands of spectra, for example, of orders of magnitude more objects than we're able to do today. So, being able to assemble a focal plane that has enough density of the fiber position so that you can take the spectrum of those objects will be a big challenge. We don’t know yet how to do that. So, talking about specifically what we can do at the university lab to address those questions, diving into that is one of the things that we want to do. And so, the idea would be that we really think of this next generation of experiments as making a leap in quantity and quality of data so that we can get the amount of data we need in detail. On the imaging front, we are already making a giant leap with the LSST camera with respect to, say, DES. But on the spectroscopic side of things, the corresponding leap is not yet there, and I think that that's going to be the next big one.

Zierler:

Since we've already worked our way up to the present, what are the things in the short term, say five years, where there's really opportunity for fundamental discovery? What are the things that you would be looking at?

Soares-Santos:

So, in this very short term, what I am really excited about is to see this gravitational wave program, specifically the cosmology with gravitational waves program, go from a measurement that is done with ones and twos individual rare events to a large sample, where you actually see those error bars shrinking to achieve percent level precision, and it will get harder as we go, and systematic uncertainties will creep in. But it means, also, that we're getting sensitive enough to answer the questions we are interested in. What is dark energy? Is this discrepancy that we see within CMB and supernovae really new physics? And if it's new physics, what is it? So, I don't think that in five years, we will get to the bottom of the question, but we will certainly see big improvements in the sensitivity and the uncertainty levels with more events.

Zierler:

Dark energy is a singular. It implies that it is a thing as opposed to many different things. Is there anything to read into that? Are we sure that it's only one thing? Or could it be many things?

Soares-Santos:

We are not sure at all. I think it's just our lack of knowledge makes it simpler to talk about it as a singular. But it doesn't mean that's the case.

Zierler:

Do you think that the discovery of dark energy, or dark energies, will be more like a eureka moment, like gravitational wave detection, where we don't see it, and then we detect it? Is it going to be dramatic? Or do you think it's going to be more like an incremental series of independent discoveries, where people start to connect the dots and say, "This is what it is?" How do you think that will play out?

Soares-Santos:

It's hard to tell. But from what we've seen so far, I think that the tensions that we're seeing in measurements done by different experiments will be the key. And I'm not sure if somebody will have a brilliant eureka moment and figure out a way to put everything into place or not. But clearly, whatever solution comes about, it will be in trying to disentangle what's going on with those tensions between different measurements.

Zierler:

Which tells us what? Again, more likely that it will be gradual or it will be sudden?

Soares-Santos:

Hard to tell. If I were to bet, I would imagine it will probably be a more gradual process.

Zierler:

But you sound optimistic. You sound like these things are really within reach, that you and your collaborators are on the right track.

Soares-Santos:

Yeah, I think so. Otherwise, we're in trouble. Again, it is a marathon, not a sprint. But we will get there.

Zierler:

Who are the greater funders for dark energy? Is it more DOE or more NSF?

Soares-Santos:

DOE plays a big role. We, of course, use NSF facilities a lot. So there's also that component there. But under the umbrella of the cosmic frontier in DOE, HEP (high energy physics). That is one of the big funders for this side.

Zierler:

What are the other things, broadly speaking, in physics that you’re excited about beyond your immediate field? Other things that may actually be relevant for your research at some point.

Soares-Santos:

So, one thing that I'm not sure will be particularly relevant for my research but I'm really excited about has been the g-2 result that is coming up on I think next month or so. That was the announcement I saw. And I am really excited about that one because, first, I have good friends from Fermilab who are working on that experiment. And when I was there, I also saw the big enterprise of bringing the big magnet from Brookhaven to Fermilab to do this experiment, and I feel like it will be great to see the results, to know what they'll tell us. But these types of precision measurements are oftentimes the places where you see interesting discrepancies that later lead to new physics. So, this is one that I'm excited about.

Zierler:

On the social and cultural side of things, last year, 2020, was a difficult year as a period of racial reckoning in this country. You alluded to it before about when you first got to Illinois, you had certain different feelings about your experiences were not the experiences of Black Americans. To the extent that all of the racial injustices and horrors from last year sparked a broader conversation within STEM about diversity and inclusivity, what was your perspective on that as a woman of color in physics, but not somebody who shared the history of so many of your Black physicist colleagues, for which these were much closer to home in some regards?

Soares-Santos:

So, I have two perspectives on that. On one side, I feel that it is amazing to see this question become front and center in the world of physics. And I wish that something similar would actually happen in Brazil, where we don't necessarily talk about these issues as openly. Or at least in my experience, we didn't. So, this is one aspect. I feel excited about the prospect of seeing change happen. Not that talking about it would automatically fix the problem, but it is something where I have the impression that there's more than talk going on, and maybe the changes are not happening as quick as one would hope, but they are coming. And I think this is good. And so, on one side, I feel that it's very positive to see what's going on. On the other side, it also brings to the surface a number of things that I would rather not think about. And that comes for me as something that can be a little bit uncomfortable.

Because one of the things that is amazing about physics is that I don't necessarily have to deal with these problems every day. I would much rather think about dark energy and about instrumentation for problems than some of this stuff. And then, suddenly, this space, which for me is my beautiful physics space, being entangled with everything else that is going on. So, that part makes it difficult for me. And I understand that it's necessary because you don't make progress if you don't think about it and don't face the problems. But at the same time, it feels likely that this space of physics is now a place where I have to deal with this stuff as well. But at the same time, I hope that means we're making progress. I choose to interpret it that way.

Zierler:

Have you chosen to be active in some of the community-wide movements in STEM like Shut Down STEM, Particles For Justice? Have you been involved in those groups on a more organizational level? Or do you prefer to deal with these things more as an individual?

Soares-Santos:

I think dealing with it more as an individual so far. And sometimes, getting involved if there's an opportunity in particular to, I don't know, contribute to a panel discussion or something like this, but not at a level of being one of the real organizing forces behind it. I'm not involved at that level.

Zierler:

One of the most important things I've heard over and over again in terms of enhancing diversity and inclusivity among underrepresented groups is simply to be seen, to be out there, to be teaching, to be presenting at conferences. Does that resonate with you? Does that feel to you like, without specifically being engaged in these topics, just by being a woman of color who's doing excellent work and is out there in the world, who's doing PBS documentaries, that that's part of the solution? Encouraging more people who look like you to say, "I can become a part of this as well?"

Soares-Santos:

Yes, I hope it does. The way I see it is that the one place I can contribute most effectively is trying to be the best physicist that I can. Because if I can be that person and be successful, then maybe I can inspire other people. But more than that, it just means that whatever barrier that exists, it shows that it can be broken. So, I think that's one aspect of it. But also, I have to say that another way I think about this is, oftentimes, I think back when I was making a decision. As I said, in Brazil, when you're a teenager, and you're going to college, you have to make a decision on what course to apply to. And I remember that one of the concerns that my parents had at the time about my choice of doing physics was about the ability to pay the bills at the end of the month. Because it is not a career that is traditionally known for being well-paid in Brazil.

So, I remember having this conversation with them and saying that one of the things that I perceive as a big difference between the poor kids and the rich kids when they reach the point of making that decision when they're 17 years old is that, oftentimes, the poor kids have no choice. You go to where you're going to be able to pay the bills next month, you live paycheck to paycheck, and you don't even have time to sit and ponder, "What do I really want? What are the big questions?" These types of questions are just for rich people. And although I'm not rich, we're not rich, I know that my parents made many sacrifices in their lives in order to give me a good education, so that I could go there and ask those questions and make my decision. So, I'm not going to waste that now and choose something else. So, I often go back to that conversation because I feel like as a way to honor the sacrifices they made, and it may sound grandiose, but there are generations of people who worked as slaves in my family, not going very far, is to be the best physicist that I can and rise as high as I can. Maybe that is not the right approach, I don't know. But that's the way I kind of choose to go about it.

Zierler:

To go back to the safe space, as you call it, which is, I think, another way of saying that in outer space, there's no racism or sexism, right? Those concepts don't exist out there. Those are all human constructs.

Soares-Santos:

Yeah. Exactly.

Zierler:

So, to go back to that safe space, or maybe that happy space, give me a sense of what you’re working on right now, the things that are sort of on your agenda this week, last week, next week.

Soares-Santos:

So, right now, there are a few things that are present. One of them is, on the galaxy clusters that I was talking about from my thesis, we are at a point where we have a really good method for estimating the masses of the clusters, and this goes by an estimate of the total mass of the stars in the galaxy. So, it’s, like, a step removed. But this is working quite well. As a result of a long study with a post-doc in my group, we got to a point where we’re getting good precision measurements there. So, one of the things that I've been discussing with Maria Pereira, who's a post-doc, and a student in my group, Johnny Esteves, is, how do we apply this in an optimal way in the next round of analysis of DES data?

So, what we want to do is to try to beat down the systematic uncertainties by a factor of two if we can so that we can really see what is currently a hint of a discrepancy between the measurements of galaxy clusters and measurements of galaxy lensing the other method. If that discrepancy is real or if it is a source of the systematic uncertainty. So, this has been quite exciting. So, we are now looking into validation tests with simulations and stuff like that, and hopefully, that will be coming about very soon. Another thing that I'm working on as well as a member of the faculty here at UM, I'm now also a member of DESI, which is a spectroscopic instrument, and this is new to me. The heart of my experience is on imaging instruments, not spectroscopic instruments. So, there, I'm diving into the technical challenges related to the focal plane, and how to optimally point to the targets in the sky, and so on. But that's another thing that is at the forefront of my mind now.

Zierler:

Now that we’ve worked our way up to the present, for the last part of our talk, I’d like to ask a broadly retrospective question about your career, and then we’ll look to the future, even though we’ve spent so much of our time in the future already. One thing we haven’t really talked about, because you’re so focused in the world of observation, and instruments, and experimentation, is the interplay between theory and experimentation over the course of your career. Now, of course, in the history of physics, it’s a duality or a dialectic. The theory advances the observation and experimentation, the experimentation and observation advance the theory. So, in your career, in the things that you've worked on from a graduate student all the way to the present, what's your sense of that overall dynamic in terms of experimentation driving theory and theory driving experimentation?

Soares-Santos:

In my field, I think that the experimenters are ahead of the theorists a little bit. We really have very little guidance at the moment for what dark energy might be. So, these experiments that we’re doing are really trying to poke the problem from different sides, and to see what works, and to shake loose the parts so that we can have a foundation to build the theory on. So, that's the way I see things going. I mentioned that earlier on, when I was doing my master's, I was on a theory track. And part of the reason why I thought that was not the right track for me was the fact that I had the feeling that the foundations, the observational pieces were not there to allow us to build the theoretical construct. And I think that largely, right now, this is still the case in some ways. But we are getting better because now we're having enough sensitivity to probe different families of models and so on, and that means we can make advances on the theory side as well.

Zierler:

Do you think that makes your job all the more difficult, given that theory is not providing much guidance? If we look at the discovery of the Higgs, you don't know it's there until you actually see it, until the experiment actually provides the evidence that it's where it is. But with the Higgs, there was a tremendous amount of theoretical confidence that it would be more or less where it was found. Are you operating in a totally different environment where what you're looking for is that much harder to find than what the particle physicists at the LHC were looking for?

Soares-Santos:

I think there's a big difference there. For the Higgs, for example, you had a very specific prediction of a particle with this mass or mass range. Here, we don't have that. We don't even know if it can be a particle or what type of scalar field–we don't know. So I think that, with the dark energy problem, maybe it's a little bit similar to back in the late 1800s and so on, when people had the problem of the black-body radiation, for example, where classical physics couldn't explain the observation, and Planck comes along and suggests a brilliant idea that fixes the problem. I think it might be something like that, where we are at the point where we have experimental data that's not fitting well with the theory, we have some discrepancies with different experiments that I've done in different regimes, and making sense out of this will require probably a big change in our theoretical understanding. But we don't have a theory prediction for a thing to look for. So, that’s why the experiments we’re doing are not searches for this particle or that particle. And we’re doing this overall scanning of the parameter space, and trying to figure out inconsistencies, and bring out mapping of the parameter space.

Zierler:

Of course, the other dark that we haven't mentioned is dark matter. Do you think it's possible that advances in understanding dark matter will be relevant for understanding dark energy and advances in dark energy might be relevant in understanding dark matter? And I only ask because with both dark energy and dark matter, nobody has any idea. So, why wouldn’t it be the case that they might help each other?

Soares-Santos:

Exactly, we don’t know. We don’t know what the relationship is between them, if any. The key to dark matter, I would say there’s at least a series of models and a theoretical framework to believe that it may be a particle. So, there are experiments looking for that. For dark energy, we don't have this. Actually, on the dark matter front, Bjoern is working on the LZ experiment.

Zierler:

It's all astrophysical mystery at your household.

Soares-Santos:

Absolutely. Between Bjoern and I, we're trying to figure out only what is 95% of the universe. So, there, it's a little bit different because at least in the current theoretical framework of particle physics, there is a possibility that there are dark matter particles, and you can look for them. In the dark energy world, we don't have the possibility of even trying to do that. Now, I do think that there might be a connection, but it is unclear at the moment what the connection is. We'll know more once we get more data. But it's unclear at the moment.

Zierler:

Last question, looking forward. You have a long career ahead of you. Do you think there will be fundamental discoveries that will allow you to ask questions that you don’t even know to ask at this point?

Soares-Santos:

I hope so. I think that’s the fun part, that it’s like this Russian doll model. There’s always another layer and another layer. So, definitely. There are two things that I find really fascinating. We're making progress really quickly in this field. So, what I hope will happen is, not only are we going to explore further the tensions, the models, etc., that we have today, but we will have new insights that will allow us to build other experiments and dig deeper. I think that's something that's probably going to be ahead in our future.

Zierler:

Because you have interest in being a science communicator to the broader public, what are the things that everybody wants to know? The broader audience out there. How does the universe work? What is space time? Where does it all come from? From your vantage point, your area of expertise, what are the most inspiring and informative things that you can tell the broader audience that is interested in your work?

Soares-Santos:

One thing I like to tell people is that it may sound puzzling or discouraging to know that 95% of the universe are things that we don't know, but what I tell people is, "Look around you. Look at how much we have transformed the world around us by knowing only what 5% is. So imagine what we can accomplish if we get the other 95%." So that's enough motivation for me, and I hope that motivates others to try to attack problems instead of being discouraged.

Zierler:

It's been so fun speaking with you. I'm so glad that we stuck with each other over these past six months and were able to do this.

Soares-Santos:

Me, too.

Zierler:

I'm so appreciative of your time, and so many people will gain so much from this discussion. So, I really appreciate it.

Soares-Santos:

Thank you. Thank you for having me.