TAMEST Member Profile: Katherine Freese, Ph.D. (NAS), The University of Texas at Austin

Katherine Freese

TAMEST Member Katherine Freese, Ph.D. (NAS), The University of Texas at Austin, is a world-renowned theoretical astrophysicist who recently discovered a possible new type of star, powered by dark matter itself.

Her team utilized data and images provided by the James Webb Space Telescope and found several objects that appear to be almost as old as the universe, a million times as massive, and a billion times as bright as the sun. If confirmed, “dark stars” as they are called, could reveal the nature of dark matter, one of the deepest unsolved problems in all of physics.

Dr. Freese received her B.A. from Princeton University, where she was one of the first women to major in physics and received her M.A. from Columbia University and her Ph.D. at the University of Chicago under her advisor, famed astrophysicist and one of the world’s foremost experts on the Big Bang Theory, Dr. David Schramm.

Dr. Freese is known for her work in theoretical cosmology at the interface of particle physics and astrophysics and has long been working to identify the dark matter and dark energy that make up an estimated 95 percent of our universe.

TAMEST spoke with Dr. Freese about what this recent discovery could mean for our understanding of the universe and why early mentorships are so vital in the trajectory of rising researchers.

How did you find your career path in cosmology and astroparticle physics.

When I was working toward my Ph.D. as a Columbia University student, I started out in experimental high energy physics and from there I went off to the Fermilab, which is an accelerator an hour west of Chicago.

I wanted to get to know Chicago, so I looked at what I could do that would be interesting, and I found a course in cosmology taught by Dr. David Schramm at the University of Chicago. I decided to drive an hour in to audit the class and it really changed my life.

I also started reading a 1972 textbook, Gravitation and Cosmology: Principles and Applications of General Theory of Relativity, that is still really the best, by the famous Nobel Laureate and TAMEST Member Steven Weinberg, Ph.D. (NAS), The University of Texas at Austin. Many years later he was also instrumental in bringing me to UT Austin, and unfortunately, he passed away in 2021. He was the greatest physicist of my time and one of the founders of the standard model of particle physics.

I was reading the book and taking the course from Dr. David Schramm, and he ended up asking me if I wanted to do a project with him because I was at the top of his class even though I was auditing it. I came back to him and asked him to be his student. Since I really was at the top of the class, he said yes, and I transferred to Chicago for my Ph.D. He was very inspiring and so I’m talking about the importance of a mentor. He really changed my life, and I could just go on and on about him.

David Schramm was one of the founding members of the field of astroparticle physics, which is the intersection between the field of particle physics – the smallest things in the world – and the field of astrophysics – the biggest things in the world. It was a brand-new field, and he was one of the leaders. I got to participate from the beginning, which was a very exciting endeavor.

You’ve quite literally written the book on dark matter (The Cosmic Cocktail: Three Parts Dark Matter). What fascinates you the most about dark matter and dark energy?

I’m going to state this surprising fact about the universe. Everything in our daily existence – our bodies, the air, the walls, the stars, the planets, etc. – all are made of atoms, which are made of quarks, and all add up to only five percent of the universe. We believe the other 25 percent is dark matter, and the remaining 70 percent is dark energy.

How can you not want to know what 95 percent of our universe is made of? I’m living in an era where we have some ideas for what could make up that dark matter. For example, it could be some kind of fundamental particle that hasn’t been discovered yet.

I’ve worked on some of those ideas, proposing what you would do to solve this problem. Because of my work, we’ve done calculations that have actually pioneered the experimental efforts and had experiments built off of our papers, which is really exciting.

Most of our galaxy is made of dark matter, and we are very sure about that, and so we’ve got to figure out what exactly it is.

You are the Director of the Weinberg Institute for Theoretical Physics at The University of Texas at Austin. Tell us more about the mission of the Institute and the vital research coming out of it.

I mentioned Dr. Steven Weinberg, one of the greatest minds of our time, both in terms of the science he did and what a wonderful human being he was. He was also really good at writing about science to the public – he really was a master at that. Because he passed away two years ago, we now have the Weinberg Institute of Theoretical Physics that I am directing. The Institute has three centers within it: The Particle Physics Center, The Center for Gravitational Physics, and The Texas Center for Cosmology and Astroparticle Physics.

The mission is that we work together to do great science. We are hiring young people, students and post-docs, to work on different projects in these three different areas. The other thing we do is the Steven Weinberg Memorial Lecture. The inaugural one was given last year by Nobel Laureate Dr. Frank Wilczek.

I’m excited to report that Nobel Laureate Dr. Jim Peebles will give our next Weinberg Memorial Lecture in spring 2024. Dr. Peebles is a founding father of the early days of cosmology and a wonderful speaker. We plan to hold such lectures every other year on the UT Campus, and will also broadcast via Zoom, so that everybody in the world can participate. Other efforts we do at the Weinberg Institute includes hosting visiting professors and collaborating across disciplines. I’m actually hosting a visitor right now, who will give a seminar to our community.

You’ve been lucky to have wonderful mentors in your career, what does it mean to you to be able to return the favor and mentor the next generation of scientists in your field?

It means a lot to me. At this point in my career, I always tell people the most important thing I do is help young people in their careers. That means thinking about what kinds of future positions they can have and helping them do great research.

In 2014 I was the Director of NORDITA, the Nordic Institute for Theoretical Physics, which is one of the premiere theoretical physics institutes in the world. That got followed by a 10-year grant at Stockholm University, from the Swedish Research Council, and they gave me 15 million dollars over 10 years to do theoretical astroparticle physics, which is a huge grant.

At my peak I hired seven post-docs, which is more than Fermilab or Stanford had. These people all did really well, and I am very proud of that. One student, for example, became the Kavli Newton Fellow at Cambridge after he graduated, and then he got a permanent position in Italy three years post-Ph.D., which is unheard of. Another of my postdoctoral fellows also obtained a permanent position elsewhere in Italy just four years after her Ph.D. Many of my Ph.D. students have gone on to great post-doc positions, including at Stanford, Princeton and the Center for Computational Astrophysics in New York.

It’s very important to me that these people do well, and I am proud of that record. I also want the same to be true when I graduate students from UT Austin.

Recently your research team found three bright objects that might be “dark stars.” Tell us more about how this discovery came to be.

This all started in 2007 when I had a Visiting Miller Professorship at the University of California Berkeley. There was a student, Douglas Spolyar, whom I had known from my days as a professor at the University of Michigan. He was a graduate student at UC Santa Cruz and wanted to work with me on a dark matter project.

We ended up looking at the very first stars that formed and we asked a question that the astronomy community hadn’t really taken seriously. These stars form inside protogalactic objects called mini-halos and there is a huge amount of dark matter that is at the center of these mini-halos. The question is: what does the dark matter do to these stars that are attempting to form?

These are collapsing clouds of hydrogen, that normally you’d think would collapse until they start burning fusion. Instead, we realized that if dark matter is of the type that self-annihilates, then the annihilation products would turn into other particles. Then, those particles would get stuck inside this hydrogen cloud by interacting with the hydrogen and getting trapped in there. This means you are dumping all the heat and energy that used to be in the form of the mass of the dark matter particles and dumped as a heat source into the cloud.

Well guess what? It took a while to figure this out, but it is a star! And we called it a dark star, but it is made almost entirely of hydrogen and helium. The dark matter is .1 percent of the mass, but the dark matter part is an incredible heat source. These things are very weird though. They are 10 AU in size, which means the radius is 10 times the size between the earth and the sun, yet they are very cool. There is nothing hot there, there is no core with fusion or even any core at all. There is only dark matter annihilation happening throughout, and because they are cool, there is nothing to prevent more mass from creating onto them.

I’m describing work that took course over several years, in a collaboration with Dr. Doug Spolyar and Dr. Paolo Gondolo. We realized they could become super massive and could grow as much as a million times of the sun and a billion times as bright.

In order to find this your team analyzed images from the James Webb Space Telescope. Tell us more about the discovery.

We predicted that supermassive dark stars would be early really bright objects. Back in 2010, Nobel Laureate Dr. John C. Mather, the Senior Project Scientist of the James Webb Space Telescope (JWST) who got the funding going for the program, asked us what they should look for in the telescope to identify dark stars. So, we made predictions. It was the Ph.D. thesis project of my student Cosmin Ilie.

And then, once the JWST started to release data, these interesting objects came out in the last year, and we went and compared the data to our predictions. One thing that’s very important is that the spectra match, which means the amount of light at different wavelengths that we predicted matches against our observations.

Basically, one single dark star is as bright as an entire galaxy of small stars. Unfortunately, at the moment we can’t tell the difference between a dark star and an entire galaxy. However, the JWST has found more than 700 objects that could be from the early universe, and if those are all galaxies then that’s a problem for the standard cosmological model that I talked about at the beginning, which states five percent of this and 95 percent of the rest. That model doesn’t predict that many galaxies. So, if some of those are dark stars that’s actually good for the standard model of cosmology.

What comes next to confirm the existence of “dark stars”?

Well, we need better spectra. We have observed all we can from these objects and they are not going to tell us anymore. However, in the future there will be tons more.

The James Webb Space Telescope is just getting going and if there are some objects that are lensed that means there’s a bunch of mass in front of them that amplifies the brightness. So, whatever you’re looking at will get a magnified image and you’ll get more information about the spectra. And by these spectra, it only has to have hydrogen and helium in it to be a dark star because back in the early days that’s the only thing you had from the Big Bang.

All the other elements – carbon, nitrogen, etc. – were all created in much later generations of stars. So, if you see evidence for that, it means it’s an entire galaxy containing stars. Comparing the spectra in later upcoming magnified objects will allow us to look for the ones that only have hydrogen and helium. There’s also a particular absorption line that would be kind of a smoking gun for dark stars, a helium line.

What was your reaction when you made this discovery?

We are not sure yet because we don’t know if the objects are dark stars or early galaxies, but the situation is super exciting! I’m chomping at the bit to get more data and to find one that we really can identify as a dark star.

That’s to come and once we do know what it is, then we learn about the dark matter, too, which is really a 90-year-old puzzle at this point. So, these are exciting times.

What makes you the most passionate about your work?

I enjoy the creativity. We come up with new ideas and we do it by talking to each other. So, it’s collaborative. It’s really fun, idea oriented and I enjoy the process immensely. Then, if as a result you can actually explain something about nature no one knew before … well, isn’t that cool!

You were inducted into the National Academy of Sciences in 2020 and became a TAMEST member. What does being a member of NAS and TAMEST mean to you?

It’s an enormous recognition and I’m really grateful for it. It’s huge.

One thing I really admire about TAMEST is the ability to nominate a young researcher as a TAMEST Protégé. Recently, I nominated a young assistant professor who was just hired a few years ago at UT Austin, Dr. Kim Boddy. I nominated her as my TAMEST protégé to come to the conference and to try to help her also get recognition and meet and network with academy members. Being a TAMEST member allows me to have another mentorship opportunity to help young researchers gain access and awareness in their field.

I am still very active in my research, so unfortunately, I cannot yet participate in all of the opportunities that being an NAS and TAMEST member presents in terms of doing public activities that could help society and help the public. However, I look forward to being more involved once my research slows down.

What made you come to Texas to live and work?

I would say two things. First, I love Austin. It is a really fun town and the people are incredibly friendly. It’s easy to meet people here and to make friends, it’s wonderful. Second, I was on Sabbatical here in 2012, and I can tell you that the theory group as well as the physics department as a whole is an amazing place to work. I’ve been really happy from day one.

The tone set within the theory group really encourages interaction between different groups – cosmology, particle phenomenology, string theory, etc. All groups really talk to each other, which produces a collegial atmosphere conducive to doing new and exciting science. It makes everyday fun to go to work.

A big part of that was Dr. Weinberg, who every Tuesday would host a free lunch at the AT&T Center where people could come and network and hear a discussion about something interesting. Now that he is no longer with us, we continue this tradition, and it really means we get to interact and find out what everybody else is doing. We’ve all ended up being friends and that kind of environment can be very rare in theoretical physics communities. The department as a whole is filled with really interesting people, and I really appreciate the civility and interactions I get to have with them.

Is there anything else you’d like to add?

I do some kind of sport every day whether it is swimming or biking or going to the gym. I used to play water polo and went to the collegiate National Championships twice after starting to play at the age of 32. I was born to play that game because I grew up swimming and playing baseball.

Unfortunately, I don’t play anymore but I do swim in the Gregory Gym Aquatic Complex when it isn’t so hot. It’s a wonderful facility and I call it one of our best recruitment tools!


Katherine Freese, Ph.D. (NAS) is the Director of the Weinberg Institute for Theoretical Physics, Director of the Texas Center for Cosmology and Astroparticle Physics (TCCAP) and the Jeff and Gail Kodosky Endowed Chair in Physics at The University of Texas at Austin.


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