Description
Linear-perturbation theory has proven to be an extremely powerful tool to compare inflationary models with observational data. Recently, the newcoming high-precision observations call for predictions beyond linear perturbations. Such effects are known to be relevant for example in the production of primordial black holes or scalar-induced gravitational waves. The δN formalism proposes to capture some of these non-linearities by describing the universe as a set of homogeneous patches evolving independently. Originally formulated to describe scalar perturbations only, I will show how it can be generalised to also describe gravitational waves and their correlation functions. As an application, I will discuss a model where a U(1) gauge field sources the gravitational-wave signal.
