Research

My research goal is to obtain a better understanding of how Nature works, on various levels and in a number of different contexts. This is a very brief overview of how I think about what I'm doing; for more details see my annotated list of publications, or just skip to my CV.

For most of my career, my focus has been on the intersection of cosmology, gravitation, and field theory. The idea is to explain the actual behavior of the universe we observe by creating and testing new models of fields and interactions. Some of my favorite topics have included dark matter and dark energy, the arrow of time, inflation, extra dimensions, modified gravity, and possible violations of fundamental symmetries. I was at least partly responsible for some of the first models of Lorentz violation in field theory, interactions between dark energy and ordinary/dark matter, and accounting for cosmic acceleration by changing Einstein's theory of general relativity.

These days my physics research is a bit different, focusing on two big themes. The first theme is the foundations of quantum mechanics, especially connections to cosmology and emergent spacetime. I'm a proponent of the Everett ("many-worlds") approach to quantum mechanics. It's a great theory, but raises a number of questions to which the answers are not yet clear, including the origin of probability and how the furniture of our semi-classical world (space, time, fields) emerges from an underlying wave function in Hilbert space. In addition to studying the basic features of Everettian quantum theory, I've been applying it to questions in cosmology and quantum gravity, such as Boltzmann Brains, the Big Bang singularity, and connections between spacetime, entanglement, and quantum information.

The second theme is the evolution of entropy and complexity. The arrow of time (the difference between the past and future) in our observable universe can be traced to low-entropy conditions near the Big Bang, and I've proposed models to help explain that puzzling cosmological feature. As entropy grows, complexity first appears and then eventually fades away, and I'm interested in understanding the processes by which that happens. On small scales, including within cells inside our bodies, occasional fluctuations downward in entropy can be crucial in the dynamics of complex systems, and it's important to develop new conceptual tools linking information to physical processes. Finally, on intermediate scales, I am interested in connecting the entropic arrow of time to our everyday notions of causality, memory, and free will.

I've also become increasingly interested in the foundations of physics/philosophy of science, as well as philosophy more generally. Some of my "physics" work above, e.g. on quantum mechanics or the arrow of time, relates directly to current work by philosophers of physics. I'm hoping to expand on that connection in the near future, to work more seriously on issues of emergence, causation, and metaphysics. In my book The Big Picture I coined the term "poetic naturalism" to describe a view where there is only one world, the natural world, but there are many ways of talking about it, each of which can be thought of as potentially capturing something true about reality. In ambitious moments I hope to elaborate on that vision in more rigorous ways.