Inflation and the cosmological (not-so) constant in unimodular gravity

Abstract

We propose a mechanism for generating an inflationary phase in the early universe without resorting to any type of scalar field(s). Instead, this accelerated expansion is driven by a dynamical 'cosmological constant (CC)' in the framework of unimodular gravity (UG). The time dependent CC can be related to an energy diffusion term that arises naturally in UG due to its restrictive diffeomorphism invariance. We derive the generic conditions required for any type of diffusion to generate a realistic inflationary epoch. Furthermore, for a given parameterization of inflation (in terms of the Hubble flow functions), we show how to construct the corresponding diffusion term in such a way that a smooth transition occurs between inflation and the subsequent radiation dominated era, hence reheating proceeds naturally. The primordial spectrum is obtained during the inflationary phase by considering inhomogeneous perturbations associated to standard hydrodynamical matter (modeled as a single ultra-relativistic fluid). We demonstrate that the resulting spectrum is equivalent to that obtained in traditional inflationary models, and is also independent of the particular form of the diffusion term. In addition, we analyze the feasibility of identifying the variable CC, responsible for the inflationary expansion, with the current observed value.