Research

Group Members Listing

My main research interest is in the intersection of cosmology (the history and composition of the universe) with high energy physics (particle physics and string theory). In my research, I answer questions about dark energy, dark matter, inflation, and the Big Bang. In this work, I use tools from particle physics, string theory, and general relativity; as I am a theoretical physicist, that involves analytical (paper and pencil) as well as numerical calculations. I describe some of my current active areas of research below, and I'll add more descriptions over time.

My appearances in the media are at the bottom of the page.

Inflation, Dark Energy, and Extra Dimensions in String Theory

During much of the universe's history, its expansion has slowed down due to the gravitational attraction of matter. However, in two significant eras, the growth of the universe has actually accelerated! In fact, one of those eras is the present, and nature of this acceleration will determine the future of the universe. But the other era of acceleration, known as inflation, was in the earliest moments of history, and it is responsible for seeding all the structures we can see today. As it turns out, though, the theory of inflation has one problem: it is ultimately sensitive to physics at the highest energies, including quantum mechanical effects in gravity. So, while it's possible to build working models of inflation by insisting on certain assumptions, we don't yet have a solid understanding of how those models might actually arise in a theory of fundamental physics.

Enter string theory. String theory is at present the only theory of physics that can not only describe quantum gravity but also unify gravity with all of particle physics. As it turns out, string theory requires 6 extra dimensions of space, and it is these extra dimensions that provide the rich structure needed to include known particle physics. And, over the past decade, string theorists have developed many models of inflation and dark energy within extra dimensions. I've made several important contributions to this field.

Many questions remain, however. Most string inflation models are built within our four dimensions, using information about the extra dimensions. What does inflation look like in the complete extra-dimensional theory? We know that the extra dimensions can change shape and size during inflation; does that motion correct our understanding of inflation in some way, and might we be able to measure it? This is essentially a question in extra-dimensional general relativity, and I am actively pursuing an answer. I am also very interested in understanding the role of quantum mechanics in extra-dimensional geometry, since we know that quantum effects are important in driving inflation. As a bonus, answering these questions about inflation may also help us to understand the current acceleration of the universe!

Image credit: Jbourjai. Calabi-Yau. Wikipedia Public Domain.

Gravitational Collapse in AdS/CFT

One of the more astounding discoveries of string theory was the realization by Maldacena that certain gravitational backgrounds are equivalent to specific theories of particle physics living in a smaller number of dimensions. There are by now many examples of this type of duality, but the most elementary and best-studied involves anti-de Sitter spacetimes and particle physics theories known as conformal field theories, so the subject usually falls under the moniker of "the AdS/CFT correspondence." Questions that are very difficult to answer on one side of the correspondence sometimes become very simple on the other side. In addition to a host of theoretical developments, string theorists using AdS/CFT provided the first qualitative understanding of some puzzling results in nuclear physics experiments.

I am interested in dynamics — time dependence — in AdS/CFT, specifically setting up an initial configuration of matter and watching it collapse under gravity. The key question is whether a black hole forms, since that corresponds to thermalization (distribution of energy) in the dual particle physics theory. In other words, imagine you have a bunch of marbles. If you thump one marble, does that energy get distributed to all the other marbles?

If the marbles are all held in a finite container, you'd probably guess that it does. This is a lot like the basic AdS/CFT situation, and, a few years ago, Bizoń and Rostworowski conjectured that an arbitrarily small amount of energy in a generic configuration eventually forms a black hole. I am currently investigating the behavior of gravitational collapse in a variety of systems relevant to AdS/CFT using numerical methods. The figure shows the initial black hole radius (inset: black hole formation time) as a function of initial energy when we account for some types of quantum corrections to gravity.

Image credit: N. Deppe, A. Kolly, A. Frey, & G. Kunstatter, arXiv:1410.1869

Quantum Information in Gravity

The basic unit of information, a bit, can take the value 0 or 1. But in quantum mechanics, a quantum bit (qubit) can take a superposition of those values. And multiple qubits can be superposed in such a way that correlate with each other (quantum entanglement). Superposition and entanglement are two reasons quantum computers will be able to solve different types of problems than normal computers, including some very hard ones.

It turns out that quantum information is also important for gravity in addition to computing. Think about a black hole. If some information falls into the black hole, say in the form of a book or accreting gas, it must eventually come out as Hawking radiation from the black hole. How this happens is still a major puzzle!

This century has seen a great amount of progress developing precise definitions for information theory in gravity through the AdS/CFT correspondence (discussed above). I am particularly interested in extending these definitions to less symmetric situations in AdS/CFT and to cosmology.

Dark Matter

I'm not working actively in this area but maintain a professional interest and may do some projects if opportunity arises.

There is very strong evidence, not only from nearby galaxies but also from the farthest distances we can see, that normal matter makes up only a small fraction of the energy budget of the universe. About five times as much is a new type of particle (or several types, perhaps) that we call dark matter — dark because it does not shine. The image to the left is a simulation (not mine) of how dark matter might be distributed through a galaxy like ours.

That image actually suggests a couple of things. One is that dark matter is spread well throughout our galaxy, so there is some of it near the earth. We can try to detect it, and, in fact, many experiments are trying to do just that! Unfortunately, these experiments don't all seem to agree — some see nothing, others possibly see dark matter but disagree on its mass and interactions. I'm part of the community of scientists uncovering particle physics models that explain how these different experiments can actually be consistent with each other. And I'm looking for other consequences of these particle physics models, like how they might show up in particle colliders, for example.

But that's not all! Since dark matter is spread throughout our universe, it could have many effects on astrophysics, and I have to account for those in my particle physics models. On the flip side, I also look for unexplained observations in astrophysics and ask if dark matter can provide an explanation. For example, we've known about a strong gamma ray signal from the center of our galaxy for about 40 years with no consensus astrophysical explanation. What if dark matter causes it? My collaborators and I have shown that at least some models of dark matter can.

Image credit: CosmoO. Dark Matter Halo. Wikipedia Public Domain.

Note on Open Access

My research publications (and preprints) are available here through the INSPIRE database. In accordance with NSERC Open Access rules, all my papers published starting Aug 2014 are either available freely through the SCOAP3 (published version) or arXiv (equivalent content to published version but distinct formatting) links. ArXiv versions of earlier papers may be slightly different than the published versions.

Media Appearances