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.
Inflationary Observables. My current research 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 parameter. 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 by classical ‘super-horizon’ evolution in such models. A recent highlight has been the development, in a series of papers (Mulryne, Seery and Wesley 2009 and 2010, Mulryne, Seery and Anderson 2012 and Mulryne 2013), 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.
String Cosmology. During my postdoc in Cambridge, I began to be interested in a class of complicated inflationary models motivated by string theory. In particular, I studied the infinite order differential equations which occur in non-local scenarios motivated by string field theory. I developed a novel numerical approach, solving the full non-linear and non-local equations as an initial value problem (Mulryne and Nunes 2008) for the first time. My interest in stringy models and their observational consequences led to an invitation to contribute an invited review, ‘Towards an observational appraisal of string cosmology’ (Mulryne and Ward 2011), to the Classical and Quantum Gravity journal’s focus issue.
Loop Quantum Cosmology. My early research, conducted mainly during my PhD, was focused on loop quantum cosmology (LQC). Among other studies, I investigated how LQC effects lead to novel dynamics which might have played an important role in how the very early universe evolved. For example, modifications to the Friedman equations allow a scalar field to evolve up its potential, perhaps setting the initial conditions for a subsequent phase of inflation. The same modifications also allow the universe to undergo successive bounces, and the possible consequences of this was investigated in my paper `An emergent universe from a loop’ (Mulryne et al. 2005), which was later reviewed in Nature. My later work on understanding inflationary observables in the context of LQC (Copeland et al. 2007) was reviewed in Nature Physics.