PhD candidate, theoretical cosmology
Canadian Institute for Theoretical Astrophysics
Department of Physics, University of Toronto
njcarlson@physics.utoronto.ca
Find me on:
arXiv,
ADS,
ORCiD,
Google Scholar
and LinkedIn.
My name is Nate Carlson, I am a PhD candidate in the Department of Physics at the University of Toronto and the Canadian Institute for Theoretical Astrophysics (CITA). I study theoretical astrophyiscs and cosmology, my work involves interfacing our theories of the early universe and the formation and evolution of structure across cosmic time using analytic methods and simulations. I'm particularly interested in searching for non-Gaussian signatures left over from primordial physics in the large-scale structure of the universe as we see it today.
My work connects many different aspects of research that is ongoing at CITA. I work with the Early-Universe cosmology group to study the distribution of energy in the universe following inflation and re-heating and use these as the initial conditions for cosmological simulations. These simulations allow us to see how early-universe physics is manifest not just in the large scale structure of the universe, but precisely how it would appear in direct observations of various different cosmological obsrevables. This in turn relates to other efforts at CITA such as the evolution of star formation across cosmic time and the formation of the first cosmological structures in the epoch of reionisation.
To probe the physics of the very early universe, I search for signs of primordial non-Gaussianities. The distribution of energy in the universe after inflation is inferred to be very nearly Gaussian from our observations of the Cosmic Microwave Background. This is expected from our standard model of cosmology in which the density fluctuations in the universe are seeded during inflation from Gaussian quantum fluctuations. However, our theories of inflation predict that there should be some deviation from purely Gaussian statistics just after inflation. Typically, the deviation from Gaussian statistics (or non-Gaussianity) is treated perturbatively with leading order coefficient \( f_\mathrm{NL} \), however recent work by my collaborators at CITA (Morrison et al. in prep.) shows that any multi-field inflation model could form a form of non-Gaussianity that is not well modelled by this approach. We've dubbed these Primordial Intermittent Non-Gaussianities or PINGs.
One of the main topics of my PhD thesis work has been developing novel ways of searching for this form of non-Gaussianity. I have been characterising the effects that PINGs in the large scale structure of the universe using cosmological simulations. These are the largest structures in the universe, the interconnected network of galaxies, clusters, and superclusters known as the Cosmic Web.
In this research programme, I've worked extensively with the Peak Patch and WebSky cosmological simulations. Respectively, these generate realisations of the distribution of dark matter (DM) halos in the universe and produce mock observables from catalogues of DM halos. In addition to running the simulations with new non-Gaussian initial conditions, I have made extensive updates to these codes to improve accuracy and speed, to optimise for the architecture of UofT's Niagara Supercomputer, and to add new observables to our mock maps.
These updates are set to culminate in the upcoming public release of the WebSky2.0 mocks. Stay tuned for these as well as a python repository that I have written for manipulating and running post-processing on these maps. For more on this exciting new work, check out the CITA Large Scale Structure Mocks webpage!
I will add these soon, in the meantime, see my ORCiD ID or ADS above.
For a more complete history of my research and academic history, see my CV here.
I have given a number of talks on my research for both expert and public audiences, I've listed some that have been recorded below:
Annoyingly, some talks I'm quite proud of weren't recorded. For some of those, I've linked PDFs of my notes here:
As a more senior graduate student at CITA, I have done some service to CITA, including being organiser of the weekly CITA cosmology lunch meetings, hosting visiting students, post-doctoral fellows and professors, and serving on the local operating committee of the Pan-Canadian Reionisation Workshop held at CITA in 2023.
For current and prospective CITA graduate students, I was the primary author of the CITA Graduate Student's Handbook, drop me a note if you'd like to get access to it.
I have a lot of experience as a TA, I've taught General Relativity I as well as a number of first-year physics laboratory classes at UofT. I've also been involved in a number of public outreach activities. For the full list of courses I have TAed and outreach activities, see my CV above.
In December 2023, I gave a public talk titled "What the biggest things in the universe can tell us about the oldest and smallest" there is a pretty low-quality recording of it on YouTube, my slides can be found here.
Some other miscellaneous advice for for students: if you pursue graduate studies in physics at a public university, you will almost certainly get emails from cranks. Don't reply to them. Obviously. It's not worth your time. If, however, responding becomes unavoidable (whether because they show up at your institution or are someone you know) I think an educational response is useful. Here's an example from educator Neil deGrasse Tyson of a tactful response.
\[ G_{\mu\nu} = T_{\mu\nu} \]