Follow these links to see slides from a some recent talks I've given about my work. The papers themselves can be found in the research section.

• Oxidised dark energy

  Here I describe some significant improvements to no-go theorems for creating accelerating four-dimensional universes from dimensional reduction. The improvements are of three main types: the energy conditions can be weakened (to the null energy condition), time-dependent compactifications are treated, and non-de Sitter expansion is included.

• Primordial black holes in the dark ages

  Primordial black holes (PBHs) are very poorly constrained for almost any choice of PBH mass. In the past few years 21cm radiation has attracted great interest as a potential probe of physics during the cosmic dark ages before luminous sources turned on. 21 cm radiation could place very stringent constraints on energy injection during the dark ages - and thus potentially the PBH population. Here I describe work with Katie Mack (Princeton) where we forecast the signal from PBHs that one might observe in 21cm radiation. Our novel contribution is developing a computer code which tracks the photon population over time accounting for the stringly wavelength-dependent interactios of photons with the intergalactic medium.

• Primordial magnetic fields and cosmic string networks

  Large-scale magnetic fields are observed in galaxies but the mechanism by which they originate remains both mysterious and controversial. I make the case that they must have started out as very week seed fields that were amplified by a galactic dynamo. A cosmic string network satisfied the requirements to create the seed fields, and has the advantage that this model makes a number of other predictions. I report on calculations carried out with Diana Battefeld (Helsinki), Thorsten Battefeld (Princeton) and Mark Wyman (PI) on this magnetogenesis mechanism. We correct previous calculations and find that the loops, not the long strings, produce the strongest magnetic fields.

• Scale invariance without de Sitter

  This is a talk about some work that I've done with Andrew Tolley (PI), generalizing the one-field scaling solutions to two fields. This changes the picture for perturbation theory in collapsing universes (such as the cyclic, ekpyrotic, or pre-big bang scenarios). In particular, it's quite simple to obtain a scale-invariant spectrum of adiabatic density perturbations, which has proven difficult to achieve in past models.

• Observing cosmic strings with gravitational lensing surveys

  In this talk I describe some results worked out with Lindsay King (IoA, Cambridge) and Katie Mack (Princeton). We investigated whether searching for cosmic string loops -- rather than long strings -- might provide better constraints on the existence of cosmic strings themselves. We found that it did, suggested searching for compact radio source (CRS) lensing events, and estimated the number of events one would see in CLASS, extended LOFAR, and SKA for various choices of string model parameters.

• Cosmology, chaos, and Kac-Moody algebras

  Near a big crunch, the dynamics of Einstein gravity and supergravity reveals an underlying structure connected with infinite-dimensional groups. The underlying group structure can be changed upon compactification on a manifold M, if one chooses M to have non-trivial topology. Strikingly, the group structure is only sensitive to simple topological invariants of M, such as its de Rham cohomology. I discuss some applications of these results to cosmological models.