Interview with Cosmologist David Marsh

David Marsh is a Postdoctoral Researcher at the Perimeter Institute for Theoretical Physics and his answers are a must-read for everyone who wants to make a career in cosmology.

Marsh is a passionate skateboarder who was featured, for example, in Nature News and the Time Magazine Online.  We talk about his biggest discovery, his advice for beginner students, his favourite books and much, much more.

Without further ado, here is the interview:

Physics Insider: What are you currently working on?

Marsh: I work on ways to constrain the nature of dark matter using cosmological and astrophysical observations. I’m particularly interested in ultra-light axion dark matter, and constraints coming from the cosmic microwave background (CMB), but I’m also becoming interested in effects on galaxy formation.

Physics Insider: Why is this important?

Marsh:We have no idea what the dark matter is composed of.
A lot of focus in the last few decades has been on so-called “thermal WIMPs” that are relatively heavy, or more generally “cold dark matter” (CDM). But as constraints on those models get more stringent, people start looking further afield. Light axions have a few advantages. Firstly, they affect cosmological structure formation, so we can use what we actually know about dark matter, that it forms large structures in the universe, to limit the axion mass. WIMPs are just too heavy to affect structure formation, so we can’t get any clues about it from that area. Axions may actually play a role in solving some of the problems CDM faces on small scales, for example in the formation of dwarf galaxies.

Generally, by studying dark matter models  that can have effects on cosmology we can learn about what dark matter is or isn’t. We can rule out axions and confirm CDM, or we can discover evidence for axions. It helps to construct models with readily falsifiable effects. They point us to new observations and new ways to think about data. Either way we learn something about dark matter: we either discover it is an axion, or provide more strength to CDM as it knocks down competing theories.

Finally, axions are ubiquitous in string theory models, so we might be able to use constraints on axion dark matter to constrain the string theory landscape!

Physics Insider: What was the biggest advance/discovery in your field in the last 20 years?

Marsh: This is a totally personal opinion, but I think it had to be the Wilkinson Microwave Anisotropy Probe (WMAP), which really inaugurated precision CMB cosmology. Before WMAP there was a collection of disparate pieces of evidence that clung together into our cosmological models, but with WMAP we were able to bring that all together into what is becoming the “standard model of cosmology”. The CMB as measured by WMAP was able to be combined with measurements of supernovae to give us the best precision evidence that the universe is filled with the unknown substances of dark matter and dark energy, and that it had begin with odd “super-horizon, scale invariant” initial conditions (which can be explained by inflation).

Physics Insider: What was your biggest discovery?

Marsh: In October of last year, together with three collaborators, I analysed data from Planck (the successor of WMAP) to put limits on the particle mass and composition of dark matter. We didn’t discover what the dark matter is, but we were able to say a lot about what it isn’t. We used state-of-the-art computational and statistical tools to place robust limits on axion dark matter mass, and on what fraction of the dark matter is allowed to be axions.

Physics Insider: What is your advice to a student who wants to make a career in your field?

Marsh: Cosmology requires an understanding of a wide variety of areas of physics, so pay attention to all of it. Of course things like General Relativity (GR), quantum field theory, and statistical mechanics are necessities. I’ll list what I wish I was better at now, as a good guide for what are important tools for a theoretical cosmologist. On the theory side, differential forms and other formal techniques in GR, as well as supergravity to understand string models. I think Bayesian statistics isn’t taught in enough detail at an undergraduate level. This is *the* key tool in modern cosmology. Good coding practice is very important: learn a language like python and learn how to use high performance computing. Don’t think statistics and computing are something that, as a theorist, you can ignore! You’ll be left behind without them. Finally, don’t neglect presentation style. If your work looks good and reads well, people pay more attention.

Choose an institution and PhD advisor that you can get along with. I was lucky to have brilliant and fun advisors, but some people have horror stories. Don’t work too far outside, or too far inside the mainstream. If you’re outside, you can be brilliant but no one will listen, but if you’re inside a trend you’ll never have original ideas.

Physics Insider: If some fairy would offer to answer you one question about nature; what would it be?

Marsh: Is there a multiverse.

Physics Insider: How far do you think are we away from answering this question?

Marsh: It could be impossible even in principle. Other universes are separated from us by a causal horizon, and the laws of relativity prevent us from ever seeing them. Now, if we are really lucky, they could affect us. For example, early in the history of our universe we may have collided with another universe, and this could have left an imprint on the CMB. But that is completely up to chance, the multiverse could be out there but we might not have had such a collision and so we have no way of knowing about it. Improved measurements of the CMB and large scale structure might improve what we know about whether this happened, or they might not.

Physics Insider: If you could give your 20 year old self one piece of advice, what would it be?

Marsh: Pay more attention in computational physics.

Physics Insider: What math is necessary to be able to work in your field?

Marsh: Lots! All the calculus, in particle physics, especially complex analysis! All the linear algebra. Group theory. Bayesian statistics. Like I said before, I’d like to be better with differential forms, which means Riemannian geometry and related fields.

Physics Insider: Which books do you recommend to someone who wants to do research in your field?


Physics Insider: Which books did influence you the most?

Marsh: I’m not really much of a textbooks person…

I’ve never read a book on statistical mechanics, so I can’t recommend one. But I had brilliant teachers for it and cherish my notes. Stat mech is awesome.

Physics Insider: What was the best physics or math book you’ve ever read?


Other “best books” though not textbooks are:

Physics Insider: What do you wish you would’ve known earlier in your career/ when you started studying physics?

Marsh: Pay attention to computing and statistics. They’re at least as important as QFT and GR!

Physics Insider: Mr. Marsh, we thank you for this interview.