Modern cosmology is a huge research area. My own work has touched on only a tiny part of it, mainly concerned with the evolution of the very early universe, and in particular a phase of accelerated expansion known as inflation. This phase is thought to have been driven by one or more scalar fields, which are common within high energy theories of physics. Within this area I’ve had a number of interests and a list of my publications, together with pdf versions, can be found here.
Inflationary Observables. My current research, and the topic of my Royal Society Research Fellowship, focuses on developing the techniques required to test complex models of inflation against observation. The key observable signatures of inflation are the statistics of the primordial density perturbation, produced in the early universe and imprinted on the cosmic microwave background (CMB). Given that the statistics are expected to be close to Gaussian, interest has only relatively recently moved from the two-point function to higher order statistics, starting with the three point function, often parametrised by the fNL parameters. This interest has been driven by increasingly accurate observations. In particular, analysis of data from the WMAP satellite hinted that the primordial fluctuations may be less Gaussian than expected from the simplest models of inflation. Although this hint was not confirmed by the more detailed CMB maps of the Planck satellite, the bounds Planck has put on fNL are extremely tight and can be used to constrain models — but only if we are able to calculate the signatures that a given model predicts.
Recently I have contributed to the task of developing the precision analytical and numerical tools needed to calculate the statistics of the primordial density perturbations predicted by a given inflationary model. In particular, I have focused on complex models with many fields. Multiple fields are generic in realistic models of inflation, such as attempts to embed inflation within fundamental theories like string theory. Moreover, a large non-Gaussianity, large enough that it would have been detected by the Planck satellite, can be generated in a number of ways by such models. A recent highlight has been the development, in a series of papers, of novel techniques for calculating observables: ‘transport methods’. These methods begin by reformulating traditional cosmological perturbation theory directly in terms of the objects of interest, the moments of the probability distribution of the curvature perturbation. These are the observationally relevant quantities. The method is also numerically stable and well suited to numerical simulations. Prior to its development, explicit numerical calculations had been carried out only for two field models. Using the transport method, however, my recent paper presents stable simulations including hundreds of fields. This work has culminated in a open software source package, PyTransport, detailed at transportmethod.com.