Below is a selection of some specific topics on which I do research; a full list of my papers can be found here.
Inflation is a period of exponentially rapid expansion which we believe happened in the early universe, sowing the primordial perturbations which we can observe as temperature anisotropies in the cosmic microwave background and which eventually grew into stars and galaxies. I have studied various models of inflation, as well as how fine-tuned its quantum initial state must be.
Many theories—including string theory—seem to predict many universes, with the possibility of quantum tunnelling transitions betwen them. These events, which generally proceed via bubble nucleation, would offer an observational window on the multiverse, and I have studied the consequences of transitions which change the effective number of spatial dimensions, as well as investigated the possibility of bubbles acting as seeds for further nucleations.
Although Einstein's general theory of relativity is the most experimentally succesful theory of gravity we possess, we know it must be altered at high energies, and there are also reasons why one may want to do so at large, cosmological distance scales. For my D.Phil. thesis I studied theoretical aspects of multi-gravity, a theory in which there are multiple, interacting copies of general relativity.