A Professor Returns to Columbia, Where She First Explored Dark Matter as an Undergraduate

Kerstin Perez (CC’05) first came to Columbia from Philadelphia, to study physics as an undergraduate. After finishing her doctorate at the California Institute of Technology, Perez was awarded a National Science Foundation Astronomy and Astrophysics Fellowship at Columbia. And now, after stints at Haverford College and MIT, Perez returned to Columbia this summer as the Lavine Family Associate Professor of the Natural Sciences in the department of physics. Perez is an astroparticle physicist. She builds instruments that look out to space and try to measure what the universe is made up of at its tiniest, subatomic scales. She focuses in particular on identifying the particle nature of dark matter.

Columbia News caught up with Perez to discuss dark matter, her next Antarctic research project, and how science needs to change in the wake of the pandemic.

How would you explain your field to a non-physicist?

If you zoom out and think about the universe at its biggest scales—galaxies and whole clusters of galaxies—these are objects that we know are held together by gravity, just like gravity is holding me down in the chair right now, gravity is holding those objects together. Dark matter is the material that is out in the universe, creating gravity. It's making the universe clump up into the structures we observe. We don't know what it is, but we know it exists.

How do we know it exists?

Essentially, if you look at a galaxy, and you count up all the visible matter, and you see how fast it's spinning around, the observable matter is not enough matter to hold it together, so there must be something else, and that something else is what we call dark matter. 

Imagine you have a bucket of bricks on a string and you're spinning it around and around and around. And if you see that bucket going, you can infer something about how strong that string must be. And gravity is that string holding it all together. But if you add up all the visible matter that's making gravity, it's not making enough gravity to hold it together for the speed it’s moving. Galaxies should just be spinning off into space, spinning apart, but they're not.

We have this whole array of particles on earth that everything around us is made up of: We know that in an atom, electrons orbit a nucleus that’s made up of protons and neutrons. Those protons and neutrons are made up of smaller things called quarks; the electron has some heavier cousins, the muon and the tau; those are related to some very light particles called neutrinos.

But in the case of astrophysics, it can’t be just those particles that are out there: We know how to detect those particles. We know how to detect the structures that those particles would make. And it's just not that that’s holding matter together, so we know we need something new. 

You’re currently working on the GAPS experiment, an Antarctic balloon mission that is set to launch next year. What is that experiment looking for?

That experiment is looking for low-energy anti-nuclei. It’s measuring anti-protons and looking for anti-helium and anti-deuterons, which have never been seen coming from outer space.

Why would we look for that? I always say, you've probably never thought about an anti-deuteron in your life. And there is a very good reason for that: Astrophysics can't really make anti-deuterons. Outer space creates all kinds of particles. Stars are being born and dying, and spitting out protons, and creating carbon, hydrogen, and helium. But we have never seen a complex anti-nucleus like an anti-deuteron or anti-helium coming from outer space.

But there are a lot of well-motivated theories for what dark matter could be that predict that you would see some anti-deuterons coming from space. So this is the basic idea of that mission: We’re building an instrument to look for anti-deuterons, and if we see a few, it would confirm that dark matter or some other new physics that we haven’t yet discovered is causing them to appear.

What happens if the experiment does not observe any anti-deuterons?

It’s very possible.

We’ve built a lot of dark matter experiments, and we have not conclusively found dark matter. So what I usually tell my students is, look, we all want to build the experiment that finds dark matter, or evidence of dark matter interactions, go to Stockholm, meet the king, win a Nobel Prize, the whole thing.

Probably that's not going to happen. And so fundamentally, we have to be happy with our jobs being to tell theorists that they're wrong, and to say go back to the drawing board, come up with some more ideas of what dark matter could be, because it's not this. That's where we are right now. We're all planning to find it. We're all hoping to find it. But fundamentally, a lot of our measurements are checking things off the list, like, “no, no, no, no, no, not this.”

In addition to the GAPS experiment, I use data from NuSTAR, an X-ray telescope in outer space, to find evidence of dark matter. And I’m also working on the International Axion Observatory, which is a new experiment that’s looking at particles emitted by the sun with the same goal, observing dark matter interactions.

Early in the pandemic, you wrote an op-ed for Inside Higher Ed, where you said that a feeling of “cultural dissonance” rather than a lack of ability is the reason that some students from minority backgrounds leave undergraduate programs in STEM and that professors need to change how they teach in the classroom. What in particular do you think needs to change?

I think the biggest thing that needs to change is the attitude that we as professors are gatekeepers of science, and that it is our job to impose obstacles that we call standards between students and what they want to study. We need to change to a mindset of: We have these students who are there at our institution; clearly they are intelligent, you don't get here by accident. Clearly, they know how to approach schoolwork. And so the question is why some students who show up on day one to study physics aren’t studying it at the end of four years. I think what a lot of the research shows us is that it makes a difference whether your attitude is “hey, you're here, prove yourself,” versus “you’re here, you're interested in physics, and I am here to help you reach the high standards that I'm setting.” Those are two very different messages.

If you’re a student hearing, “prove yourself, I'm here to decide if you're good enough,” there's going to be a subset of students that are like “great, I love a challenge.”

But there's a lot of students who are going to say, “OK, maybe I'm not good enough.” And what the research will show is that if you are a woman, or a student of color or a woman of color, you're more likely to react that way.

And that's a loss to physics. Physics isn't a done deal. We have a lot of open questions that we don't understand. And so having people walk away is a loss.

Have you seen anything change in the two years since you wrote that?

Well, the entire world has changed. I remember writing that and I wrote it from a somewhat hopeful point, because I had already seen how the pandemic was making us able to talk about this in STEM departments in general. All of a sudden, we could no longer be like, “physics is objective, you can either do it or you can't.” Every professor I know in the early days of the pandemic spent time on the phone with an individual student who didn’t have somewhere to go home to when campus closed, for example. It really ripped back this lie that the problems are the problems and everyone has an equal opportunity to do well on them. Everyone is actually bringing a lot of their own life into the classroom with them. 

One other thing I’ve seen change is that we’re having to reckon with the fact that the students who are coming to us out of the pandemic had very different backgrounds in what their education was like over the last few years. You could have students who were at a private school who were in-person, in class from fall 2020 onwards, no problem. Then you had students whose whole high school day was three hours online, and physics was 20 minutes of that. Should we take those two groups of students and say, well, the ones who were in class all day, those are the ones who get to be physicists? That’s a whole conversation we’re only starting to have.