Travis Khachatoorian:
You're listening to Sagecast, the podcast of Pomona College where we talk with faculty and alumni who are making a difference in the classroom and in the world. I'm Travis Khachatoorian.

Marilyn Thomsen:
And I'm Marilyn Thomsen. This season we're doing things a little differently. We're handing over the mic to professors who are guest hosting conversations with alumni on the topic of what's next in their fields of expertise.

Travis Khachatoorian:
Today, Associate Professor of Physics and Astronomy Philip Choi is leading the conversation. He studies galaxy evolution, galaxy interactions, and optical and infrared imaging.

Marilyn Thomsen:
His guest is Pomona alumna Erica Nelson, class of 2008. She's Assistant Professor of Astrophysics at the University of Colorado Boulder. Erica's research on how galaxies form and evolve has led to interviews on 60 Minutes and NPR Science Friday and scores of peer-reviewed articles.

Travis Khachatoorian:
Here's a bit of background to help you understand this conversation. JWST stands for James Webb Space Telescope, the biggest and most powerful telescope currently orbiting the earth. It was launched into space in late 2021 and released its first images in mid 2022 that led to a flurry of new discoveries, including some headline-making ones by Erica and her colleagues. Here's their conversation on what's next in our understanding of the universe.

Phil Choi:
Okay. So welcome, everyone. My name is Phil Choi. I'm a professor of astronomy and astrophysics at Pomona College here. My guest today is Erica Nelson, a Pomona Sage and alum from class of 2008. Welcome, Erica.

Erica Nelson:
Hi, thanks for having me.

Phil Choi:
I should also note that Erica was one of my very first students as a faculty member at Pomona College, and we were a bit of crossroads. Scientifically, we were doing very similar things back then, but since we've kind of completely diverged. So for me, this is a real joy to get an update on the field and from all people a former students.
So Erica, you have been kind of so prolific. There's a lot of audio and write-ups about your work in the past year and a half especially. I kind of want to take a little bit of an arc here, and I wanted to hear, or at least have you start with your own evolution, your own journey to help us understand how you got where you are.

Erica Nelson:
This is one of those things where it's like I'm a little bit proud, a little bit embarrassed to admit this, but I decided that the thing I wanted to do with my life is be an astrophysicist when I was in first grade. I did a project, which perhaps not the right word for what it was as a 6-year-old, on black holes, and I was just absolutely sold.
I did another one in fifth grade on the Hubble Space Telescope, which ended up being what I worked on for my PhD thesis. In particular, the thing that drew me in was the fact that we can use really powerful telescopes to effectively look back in time. I found that concept so mind-blowing, I just couldn't imagine spending my days thinking about anything else. So that is what I do now. It is I spend my days using the most powerful telescopes that we can build to look back to close to the beginning of cosmic time.
So I went to Pomona College and had just a fabulous experience working with Phil and the other Pomona College faculty on starting my astrophysics journey. I remember my first astrophysics classes and just being absolutely blown away by, in particular, the scale of the universe, but also the incredibly complex physical processes that rule the universe on large scales. It's really mind-blowing.
And one of my favorite memories from Pomona, actually, that really kind of solidified that I wanted to teach and help the next generation was we had this computer programming assignment for Phil's class and it was awful. I was so bad at coding. I remain bad at it. Honestly, I do it every day, and I remain not very good.
But Phil took the time to sit with me and work through this until midnight, like three days before it was due, and it was just such an act of kindness and faith that this would serve some purpose, and it really launched me on my journey. Phil, you're also, actually, the one that got me to apply to graduate school. Without you, I would not have actually done the application.

Phil Choi:
I had completely forgotten about that story. But now that you say it, it just brings it all back. I think this was a stars or stellar evolution class. Is that the one potentially?

Erica Nelson:
It was. Yeah, we were sitting-

Phil Choi:
Oh, yeah.

Erica Nelson:
... [inaudible 00:04:56] bracket.

Phil Choi:
That's right. That's right. No, that's fantastic. That's a great memory. It's funny that you mentioned coding because I think that your admission that you embrace the fact that you're not an expert coder is one that I actually also embrace. That I feel like when astronomers write code, it works. It does what it needs to do. It's not beautiful. It's not meant to be beautiful. We're not selling it, and a million people won't use it. So I agree with you completely.
Okay. So I know that I'm anxious to get to the sciences, and so I-

Erica Nelson:
Yeah. [inaudible 00:05:27].

Phil Choi:
... really want to know more about what you've been up to recently. I think for the sake of our audience, because not everyone has taken intro astronomy with you at University of Colorado, maybe we can offer some background here. The process, I think, of the things that you do and the way that you do them is really, really important and valuable because the details may change. And so maybe we can start by just thinking about what are the tools that you use as a practicing observational astronomer and how do they allow you to kind of start piecing together the histories.

Erica Nelson:
Astronomy is different from other sciences in that it's not experimental in the way that you can't take a galaxy, which is... There's billions to trillions of stars. They're absolutely immense. They are massive. They are large. You can't take one of these, and you can't put it in a lab and poke it and see what happens.
You also can't... There's no time scales in the context of galaxies that are relevant to, say, a PhD thesis. Nothing is happening over the timescale of five years. And so that makes astronomy really different as a science because it is both remote, you can't actually touch the thing, and it is observational. You can't actually design an experiment and carry it out in the same way.
And so in some ways, that makes it very challenging. It also makes the development of interesting and novel methods really, really important. And so that's been one of the focuses of my career, is to develop these new methods for how we can understand how galaxies form and evolve and how black holes form and evolve.
This is just physics. This is not my insight. But the key here is that because... So light travels very fast, but it doesn't travel at an infinite speed. So if you are looking at something that's very far away, it actually takes time for that light to go from that object to us.
So for instance, if the sun were to instantaneously become a black hole, we wouldn't know about it for eight minutes because it takes eight minutes for the light from the sun to travel to us. Our nearest galaxy is about 2.3 million light years away. It's coming towards us. There will be an epic collision in about 4 billion years, but it still takes that light 2.3 million light years to get to us.
So if we have powerful telescopes like the Hubble Space Telescope and, most recently, the James Webb Space Telescope that allow us to see to really, really great distances, it means that we can see this light that's been traveling for, in the case of the James Webb Space Telescope, almost the entire age of the universe. We're seeing light from close to the beginning of time itself. And so we're able to see how the universe evolved and how the galaxies in it evolved for almost the entire history of the universe, and that allows us to piece together their origin story and our origin story.

Phil Choi:
We've introduced the idea of Redshift and look-back time, the fact that you can effectively... As you look back at more distant things, you're looking back at the early universe. I'm going to get us to the frontier of our knowledge, which is where, Erica, you're working right now. To understand that and to put that into some context, I wonder if you could try and frame what it is that we know so that we can help understand what are the things that we're exploring, what are the areas of unknown?
The reason I want to frame it this way is I think a lot of folks, when they think of science, they think of it as a bundle of facts, and it's sometimes difficult to communicate the process of discovery. And so if you think about what we knew 20 years ago with Hubble, 10 years ago with new telescopes, and then the big extraordinary exciting discoveries that we're making now just in the past months, days, weeks, can we put those things into some context?
So our audience probably has all heard about the Big Bang. When I agreed to do this podcast, I thought, "Oh, one of the fun things is I'm going to ask you the most ludicrous question I can think of," and this is it. Can you take us on a broad stroke tour of the history of the universe? I want the elevator pitch history of the universe from the beginning of time until now. And I'm not thinking about the Barenaked Ladies Big Bang Theory version of that history, so we don't have to get all the way to modern day.

Erica Nelson:
[inaudible 00:10:19].

Phil Choi:
But I do think it's useful for audience to understand what do we mean by the Big Bang because you're really... You're pushing the very beginning of galaxy formation. So I think that it would be helpful if we could just frame what did the universe look like then and why do we think we know anything or why do we have any theories about what galaxies should have been like.

Erica Nelson:
You know, I'm not super into history, but there is an interesting history here, which I think is relevant, and that is the context for the study of galaxy formation itself. And that is about almost exactly a hundred years ago, humanity did not know there was any other galaxies besides the one that we live in, the Milky Way. So we live in the Milky Way. It is hundreds of billions of stars, and there are other galaxies like the Milky Way. There was hundreds of billions of them, trillions in our observable universe.
But before, about a hundred years ago, we had no idea. We thought the entire universe was just our galaxy, and it was with these advances in telescope technology and also an advance in our understanding of stellar astrophysics that allowed us to actually determine that there was galaxies outside of our own. So really, it's these physical insights and these new technologies that have allowed us to completely change our understanding of our place in the universe.
So after that point, we began to realize that not only are there galaxies outside of our own, but that those galaxies that are themselves hundreds of billions of stars are moving away from us. And so we went from thinking that... When you look in the sky, it seems, just based on your own observations at night, that the universe is finite in space and infinite in time. There's no reason when you look at the sky to think, "Oh, there was a starting point." And so we'd always thought of the universe that way.
And then, once we saw that galaxies were expanding away from us, the universe was expanding, that immediately changed how we see the entire universe and our place in it. We went from the universe being infinite in time and finite in space to the universe being infinite in space and finite in time. There was a time in the past when all of the matter, all of the energy, everything that we know as our universe was compressed into an infinitesimal point. And from that point, that was the Big Bang, an inflation, and our universe has been expanding ever since.
So if you play that video backwards and start at the Big Bang when the universe was infinitely hot and infinitely dense and all the matter and energy was compressed into a single point, you start expanding from that and you start forming the first elementary particles, and then as things cool and expand the first atoms gradually. And then eventually, much, much later, you start forming the first stars. And then eventually, after that, you form the first galaxies.
And the whole big picture is that you form little things first, and then gradually those little things coalesce together to form bigger things as a result of the force of gravity. But you really expect, because we think gravity is the only force that operates on large scales in our universe, that it takes a lot of time for very, very large, very massive objects to form very massive galaxies. So we can go from our observations of the early state of the universe to predictions for the galaxies that will reside in that universe as a function of cosmic time.

Phil Choi:
Wow, Erica. That was amazing. Really well done. You did both the history of the field of 20th century astrophysics, cosmology, and you dovetailed that with the history of the universe from early times to now. Impressive. Okay.
Okay. So now I think we've set the stage. So now we can understand we're looking at the early times. And just to be clear, your work is not exclusively on these baby galaxies. You're interested in the evolution. So maybe could you take us and maybe frame your overall big picture questions that you're interested in? What drives you to do the work that you do? What are the kind of large-scale things that you're interested in before we really get to the most recent findings that are driving this controversy?

Erica Nelson:
Yeah. So I think big picture, my goal is to understand how the universe started with roughly constant density. It was fairly uniform. And I want to understand how it went from that relative to the uniform density to all of the structures that we have in the universe now, like tables and chairs and humans and planets and galaxies.
And the thing I am most interested in is how that structure evolved on large scales, on the scales of galaxies, how these immense objects that are kind of the gravitational building blocks of the universe came to exist, how they evolved, how they come to look as they do and have the properties that they have. And in particular, how the galaxies and the supermassive black holes that we believe reside at the centers of all galaxies, how those things co-evolve, how those things grow up together.

Phil Choi:
So maybe now we can really dive in. Part of the reason that you have had a lot of notoriety and press around some recent discoveries... I think many of our listeners are probably somewhat familiar with some of these ideas, but maybe we can introduce the JWST. The description that you've given is, really, this is in the modern stage. JWST launched a couple of years ago and there were some surprises. So maybe you can... Before we introduce the surprises, what did we expect or what did you expect when we turned on the lights?

Erica Nelson:
Yeah. So to give you a bit of context on the reason JWST was launched at all was when the Hubble Space Telescope launched, it was mainly launched to study nearby galaxies in great detail. And the director at the time used his director's discretionary time to take the telescope, the most valuable telescope that we've ever had, so the most valuable resource to astrophysics that we've ever had, and he decided that the thing that would be best to do is to look at a completely empty patch of sky for two weeks. And people were pissed, I hear. I was a child at the time. You know, because it seems like an objectively bad idea, a waste of time.
But what we found in this completely empty patch of sky was tens of thousands of galaxies. This is called the Hubble Ultra-Deep Field. So we looked at this completely empty patch of sky, and we saw these two surprising things, one where we saw these really old galaxies a really long time ago. So that indicated that galaxies had to start forming much earlier than we thought.
And the other thing we saw, in the words of a quite prominent astronomer who is then a postdoc... She is now the director of a large institute. She looked at these objects, these galaxies in this image, and they were messy, they were clumpy. They looked absolutely nothing like modern galaxies do. And she said, "These galaxies are a... mess." And it was with that realization that the field of galaxy evolution was born.
The galaxies that we were looking at when we were actually able to look to greater distances, they looked nothing like the organized spirals and ellipticals of today. And so it indicated that galaxies have to evolve a lot from early cosmic times to now, and they're undergoing a continuous evolution throughout the history of the universe. There wasn't this monolithic collapse that formed these objects. They really evolved over a long span of cosmic time, and so we need to be able to actually watch that process unfold.
And so the thing we needed to be able to do that was we needed a telescope that was very powerful, so could see to great distances, which means a long time ago. And the other thing is that because that light that was emitted such a long time ago has been traveling through an expanding universe for such a long time, its wavelength is red shifted. So light that was originally emitted in wavelengths that we can see is red shifted into the infrared. And so to actually see how these objects are evolving, we need to turn on our infrared eyes and look at these early cosmic times with an infrared telescope. And so we needed a telescope that was capable, that was really big, could see in the infrared, and that yielded a whole ton of requirements for this telescope that were really challenging. The James Webb Space Telescope really push the technical boundaries of what we can do with telescope technology, imagers, spectrographs. And it really launched astrophysics into a new era, but it was a very challenging telescope to build.

Phil Choi:
Since you mentioned the JWST, I thought I would just... And this may or may not persist. I do want to shout out to Spitzer, my former institution. As a precursor, we often talk about JWST and Hubble being its precursor, but I, as a Spitzer alum, think of Spitzer, the first infrared telescope of the great observatories, really being as the JWST's DNA is there.
And I'll also shout out to all of the people who work on these instruments. There's a fantastic IMAX movie that came out about a year or two ago that profiles JWST, the journey, and really talks to a lot of the scientists in the field, our colleagues, the engineers, the scientists, the program managers. If folks have not seen that, I encourage you to go catch it in IMAX somewhere. It's playing locally at the California Science Center. It is mind-blowing. I almost wept. And I may have actually wept and I'm too embarrassed to say that, but it really is... It's amazing.
Okay. So now that we have the backstory, let's dig in. So what did we learn once we actually did launch JWST? I think there were some people who thought, "Oh, we're going to see more of the same," in the same way that we can extrapolate things. What was your expectation? Did you think that you were going to reveal something spectacular, or do you just think we were going to maybe fine-tune some of the things that we already knew?

Erica Nelson:
You always hope that with new telescopes you find something completely new, right? Otherwise, it's like what's the point of even launching it? But you don't know what you're going to see, right? It's like I went in with all of these questions that I wanted to answer, and then I looked at the first images and realized the questions were completely different than I had anticipated. And there was these objects that were just completely unexpected. And so I think I kind of threw out what I was planning to look at because of discovery. It was this moment where it was completely unexpected what we would find.
So we're all sitting around waiting for Biden to release the first images. We were all kind of waiting anxiously to immediately start analyzing the data as soon as it was released by Biden. And so a bunch of my colleagues and I jumped on this data immediately. The most surprising thing we saw was we saw these objects that they were very, very red. They were completely invisible to the Hubble Space Telescope and actually also to Spitzer because they were too faint. And so we saw these objects.
When you observe the sky, the thing you see is you see red fuzzy dots. As people who are looking at the absolute frontier in our understanding of the universe, the thing we see is red fuzzy dots. And so we have to bring all of the physics and astrophysics we know to bear to try to infer what those objects are. And so the big thing we have to do first is we have to figure out how far away they are because that is the key to... That's the linchpin to figuring out all their other properties. We use software to fit how much light there is in different wavelengths and come up with the best stellar population synthesis model for these objects.
And what happened when we did that, my colleague Rachel Bezanson and I, is we found that there was a couple of these objects that we had found that were incredibly far away, very close to the beginning of the universe, like within the first billion years of cosmic time, and the masses that we inferred for them were already as or more massive as our Milky Way galaxy is now. And it was one of those moments when we immediately realized this has to go into Nature, and it has to go into nature Tomorrow. We didn't actually get it in the next day. We got it in two days later. That was Thursday, Thursday at midnight, and we got the paper in Monday morning.
And it was such a big deal because the universe shouldn't have had enough time yet to form things that were that massive. According to all of our models, there just should not have been enough time to make something that big. And so it was incredibly surprising and really challenged our views of how galaxies form in the early universe.

Phil Choi:
So I remember when this came out. It was huge news. Within the community, everyone was talking about it. And the fact that you were able to get this, do this analysis so quickly, put out the result... And everyone understands that it's preliminary, but sometimes I think there's a misunderstanding about how this process works.
So I think that you can imagine that there's some critique about, "Oh, we shouldn't talk about things until we know them." But that really, to me, is misguided because we really want to engage in the process. The moment we see something, we're excited. We're not claiming we understand it, but the excitement of something new, drawing people's attention, to me, was really the point of that paper. Do you feel like it had that effect? And what was the community's reaction to it? How did that influence or impact, do you think, the field for the next year and a half and ongoing?

Erica Nelson:
So we weren't the only ones to see these objects. If you look at JWST images, what these things look like is little red dots, and we see little red dots all over all of these images. So there has been hundreds of papers since then on these types of objects, and we are still figuring out what they are. There's been dozens of different ideas on what these things actually are, and we are just starting to be able to get more detailed information on them.
I'm the principal investigator of a program to get spectroscopy, so that allows us to study their physical properties in much greater detail. And the really strange thing is we expected it, once we got the spectroscopy, it to kind of be an open and shut case for what is physically happening in these objects, and it made the story even weirder. And so we've been seeing these really surprising things.
One of the things that I think is surprising to people is that some of the most luminous objects in the universe are actually black holes, are actually supermassive black holes. And so there is some models that can explain some components of the light that we're seeing from these objects by growing supermassive black holes, growing supermassive black holes of a type that we have never seen before.
And the reason black holes can, surprisingly, be so luminous, which we need to account for in our models, is that when mass falls into a black hole, there's an immense amount of gravitational energy released when that happens, and some fraction of that energy can be converted into light. And we see that light in the form of these accretion disks around the objects. We see some of that in the form of these black holes blowing out enormous jets. And so they show up in different ways, but the reason they're so luminous is because there's so much energy available from matter falling into them.
But there's also some evidence that in a lot of these objects there is mission that is not typically explained by black holes. It's typically explained by stars. And so the community is completely undecided on what these objects are. They're a complete mystery, and we're still working really hard to solve it. We're actively trying to come up with new models for understanding these objects, for explaining the light that we're seeing from them, because we've never seen anything like some of these objects before.

Phil Choi:
So I'm going to dig a little bit deeper on the black hole topic, and pun totally intended there. When these discoveries first emerge, one of the things that I think got people excited is scientists love to break things and tear them down and rebuild them. And these observations that were released, these little red dots, from my read, from my distant read, it really felt like, wow, we might've just broken astrophysics. We might've broken all the models that we've spent generations learning about. Something new is here. Obviously you want to be very careful with that type of statement. But it turns out something maybe even more interesting is going on that maybe it's not broken, but there are pieces or assumptions we've been making that we've really have just been not quite there.
And I think what you're alluding to a little bit is this idea of the AGN contribution, the black hole contribution I should say, is something that maybe we had in our minds, but it sounds like you're seeing things in a whole new light. We're starting to kind of unveil something that people hadn't been talking about prior to these little red dots. Is that what I'm hearing?

Erica Nelson:
Yeah, yeah. There's been a long-standing problem in extragalactic astrophysics, I mentioned earlier, which is that at the center of effectively every massive galaxy is a supermassive black hole, and we do not know how those supermassive black holes got there. It has been a mystery for decades because we don't have a way to explain how those objects got so massive. And so there's been a lot of different theories, theoretical work on that. My colleague and collaborator, Mitch Begelman at University of Colorado is one of the leading theoretical experts in this.
But basically, one of the possibilities for how we can get these objects so massive is we can essentially have what we call a direct collapse black hole. So most black holes are the end product of the evolution of stars, actually. So the most massive stars, once they have used up all of their fuel, there is no longer anything to support them against the immense force of gravity.
So during a star's life, it is powered by nuclear fusion. That is also a fact that surprises people often. But once the nuclear fuel is exhausted, then those stars collapse. And if they're massive enough, then there's nothing that can prevent them from collapsing into black holes. But that only gets you to black holes that are on the scale of the mass of a star. And the thing we need at the center of these galaxies is black holes that are millions to billions of times the mass of stars. And so how you actually grow something that massive has... According to most of our physical laws, that's not possible. And so it's been a subject of a lot of debate how you actually get a black hole that is that massive.
So one of the possibilities for some of this light we're seeing in the very early universe that we've recently found potential evidence for is that there are actually these... It is the formation of a supermassive black hole directly from a cloud of gas. Without forming any stars, you collapse the thing directly. And so they're these kind of... They almost appear as black hole stars. So stars, they look like stars. But effectively, instead of being powered by nuclear fusion, they are powered by growing supermassive black holes inside of them. But they're very strange objects. This is still very much a work in progress.

Phil Choi:
Wow. That is really fascinating. I will add a little anecdote that as a graduate student at any phase, there are always these big questions in the field. And some of them get resolved on time scales of a decade or a career, and some of them may not be resolved for hundreds of years or generations. It's just unclear. But to imagine this kind of new field opening up right now, having emerged from this little discovery and having... This will clearly be one of the big questions in our field for the next... Pick a timescale.
I love the idea that people are going to be gravitating to this topic, but we won't know is this going to be one of these things that we learn about, figure out on the timescale of a career or could this be one of these you just have to wait and see type things. That's really exciting. And I love the fact that some little person out there might be listening to this podcast and thinking, "I'm going to be the one to solve that problem."
So, thank you, Erica. I love it. We have not talked... Just for context, black hole stars are not something in the canon of things we tend to talk about because they're just not there yet. And so to hear it first from you in this space is really... I just feel super privileged, so thank you, Erica.

Erica Nelson:
Yeah. I think it's the most exciting time in my career for our understanding of the universe on large scales, our understanding of the formation of galaxies and black holes. And I am so excited to discover the first galaxies. I think that that places pretty strong constraints on our models of the universe, and I'm really excited to discover those. I'm really also excited to try and discover the first stars. And technically it's challenging, but it's not impossible, so of course we're going to try. And just to really truly understand the formation of the first lights that lit up cosmic night, the first sources of light in our universe is something I'm really excited to uncover.

Phil Choi:
All right. Thank you so much, Erica. It's been such a pleasure.

Erica Nelson:
It's been great to be with you. Thank you so much for having me.