Hugo Holland is a second year PhD student in Institut d'Astrophysique Spatiale in Orsay (just south of Paris). He works with Julien Grain on stochastic inflation and all things related. Before his PhD, Hugo graduated from Ecole Polytechnique in France and got his masters degree in theoretical physics at King's College London, where he worked on eternal inflation with Eleni Alexandra Kontou.
Title: The separate universe approach in multifield inflation models
Abstract: Primordial black holes could constitute part or all of dark matter but they require large inhomogeneities to form in the early universe. These inhomogeneities can strongly backreact on the large scale dynamics of the universe. Stochastic inflation provides a way of studying this backreaction and primordial black hole production. Because stochastic inflation focuses on large scale dynamics, it rests on the separate universe approach. However, the validity of this approach has only been checked in single field models, but not in multifield models in which we expect strong boosts in the power spectrum, leading to the formation of primordial black holes. I will give an overview of inflation and in particular of stochastic inflation before presenting the validity conditions of a separate universe approach in multifield models by matching it with a complete cosmological perturbation theory approach at large scales.