Cosmologist Katie Mack, an assistant professor of physics at North Carolina State University, works at the intersection of astrophysics and particle physics. She’s particularly interested in dark matter, what it may be made of, and how it could be measured.
Mack’s first book, The End of Everything, is available on 4 August. After a quick introduction to the Big Bang and how the universe reached its present state, Mack clearly and lightheartedly describes various universe-ending hypotheses and explains why we should care about them now. The possibilities include the exceedingly drawn-out heat-death scenario, in which continued expansion leaves the cosmos cold and empty, and quicker ends due to either the inherent instability of our universe or a collision with another one.
Mack recently talked to Physics Today about her new book, her research on black holes and dark matter, and her experience as a theoretical physicist living in coronavirus lockdown.
PT: What motivated you to write The End of Everything?
MACK: I’ve always been fascinated by the really big questions in cosmology: where we come from and where we’re going. Some of my recent research connecting particle physics to cosmology has been on vacuum decay. It’s a process where you can get this bubble of different space that expands across the universe and destroys everything. It’s not very likely to happen, certainly not anytime soon, but it’s an interesting possibility. When I was reading up on that, it made me think more about other ways the universe may end and how we figure this all out. I thought it would be a really fun topic for a book. I had given some public talks on vacuum decay, the heat death of the universe, and things like that, and people were always fascinated by those kinds of big, universe-ending, violent events. There are a lot of books about the beginning of the universe, and there are very few about the end. It was certainly a space where there was room to do something different.
PT: Do most physicists agree on the heat-death hypothesis? Or is there a lot of debate about how the universe will end?
MACK: I think heat death is the most favored among cosmologists right now because it seems to be in line with how we understand the evolution of the universe, how it’s expanding right now, and what we think it’s made of. If you go just based on what we understand about particle physics right now, then you would get to the point of saying that vacuum decay is inevitable eventually. The time scale on that is still really long, and most particle physicists would say that something will change our picture enough so that vacuum decay won’t be inevitable. But it’s still very much an open question.
PT: You told Cosmos magazine in 2015 that vacuum decay is your favorite way for how the universe may end. Why is that?
MACK: It’s just so dramatic. It’s out of left field, and it would be very sudden. You wouldn’t see it coming; you wouldn’t notice it happening because it’s so fast. It’s a neat idea that you can measure something unexpected in the data, tweak the equations a little, and suddenly there’s this bubble of death that destroys the universe. We in cosmology get used to thinking of change only on time scales of billions of years, and the idea of something dramatic happening and being so destructive is kind of exciting.
PT: You mentioned earlier that there’s been a lot more written about the origins of the universe. In the book you talk a little bit about how research into the universe’s end may not be quite as respected. Why do you think that’s the case?
MACK: It involves some speculation and some stuff that can’t be tested because we won’t know the way things are headed until billions and billions of years from now. For the early universe, and by “early” I mean within the first minute, you can come up with theories and models of what happened, and then you can look for signatures or some smoking-gun sign that that’s definitely what happened. For the late universe, we’re not going to have those kinds of proof. We can come up with a model and say it fits the data and is a natural extrapolation of what we know, but that’s not going to be testable in quite the same way as early-universe physics is testable.
Part of it is people saying, “Well, it’s not really relevant to us because it’s what’s going to happen in the future.” But I think the end is something that piques our basic curiosity about the cosmos and our place in it. Sometimes just asking those kinds of questions can lead to a better understanding of our theories, and that’s applicable to all areas of physics.
PT: Do you think your science writing has influenced how you do your research?
MACK: I think they’re both influenced by the same thing, which is to get to those big questions and to think about things that are dramatic and interesting. I do find that I write about a lot of broad topics. It gives me a bigger picture of what’s happening in my field in a way that helps me get new ideas for my research. I’ve had situations where I’ve had a good idea for something to calculate because I’ve been talking about the idea in some public setting. Because of the area I work in, between cosmology and particle physics, and because cosmology is such a big field anyway, it’s really useful to have that broader perspective.
PT: You have a huge following on Twitter. What’s been your outreach experience on social media?
MACK: I think social media—and maybe Twitter in particular—can be fantastic for having conversations with nonscientists about science. You learn to share information simply and concisely, and you learn very directly what kinds of questions people have. There are always issues with trolls on social media, but for the most part I’ve found it to be a really fantastic way to share what I do, what I’m excited about, and some insight into how the process of science works. And I’ve made a lot of connections, new colleagues, and friends in the process.
PT: What has it been like to do your work during the coronavirus shutdown?
MACK: In some ways I have it easier than a lot of my colleagues because all my work is theoretical. I don’t need to go to a telescope, and I’m not working in any large collaborations that need to meet in person. So I’m doing much the same kind of work I always do, though I’m doing it at home in my living room instead of in an office. It’s harder to have spontaneous conversations with my colleagues or to talk to someone over lunch. Because I’m trying to synthesize a lot of different ideas, that is a significant disadvantage. My productivity is somewhat impeded, but I’m still making progress on projects. I’m still having meetings virtually with my colleagues.
PT: What are you working on now?
MACK: I have an ongoing project looking into how dark matter may have affected the first stars and galaxies in the universe, assuming that dark matter has some sort of interesting particle physics going on. If dark matter annihilates with itself, that creates energy that affects how stars and galaxies form and evolve. I’m also working with some colleagues on aspects of black holes in the early universe, and I have some stuff I’ve been thinking about for a while that has to do with vacuum decay.
PT: What have you been reading lately?
MACK: I just finished Network Effect by Martha Wells, which was awesome. It’s part of the Murderbot Diaries series, which is also great. I’m reading The Last Emperox, which is John Scalzi’s latest entry in the Interdependency series. I’m reading Hank Green’s latest, which is called A Beautifully Foolish Endeavor. I enjoy science fiction, and I generally figure if there’s a spaceship involved, then it’s escapist enough to satisfy that necessity. I also read Mary Robinette Kowal’s The Relentless Moon, which was excellent as well. The whole Lady Astronaut series is amazing.