Inflationary perturbations in bimetric gravity

TL;DR

This paper investigates how primordial perturbations evolve during inflation in a bigravity model, finding suppressed tensor perturbations compatible with observations but stringent limits from scalar and vector sectors.

Contribution

It provides a detailed analysis of inflationary perturbations in bigravity, demonstrating suppression of tensor modes and exploring constraints from scalar and vector perturbations.

Findings

Tensor perturbations are sufficiently suppressed to avoid instability.

The model is compatible with current observational data.

Stringent limits are derived from scalar and vector perturbations.

Abstract

In this paper we study the generation of primordial perturbations in a cosmological setting of bigravity during inflation. We consider a model of bigravity which can reproduce the $\Lambda$CDM background and large scale structure and a simple model of inflation with a single scalar field and a quadratic potential. Reheating is implemented with a toy-model in which the energy density of the inflaton is entirely dissipated into radiation. We present analytic and numerical results for the evolution of primordial perturbations in this cosmological setting. We find that the amplitude of tensor perturbations generated during inflation is sufficiently suppressed to avoid the effects of the tensor instability discovered in Refs.[1,2] which develops during the cosmological evolution in the physical sector. We argue that from a pure analysis of the tensor perturbations this bigravity model is compatible with present observations. However, we derive rather stringent limits on inflation from the vector and scalar sectors.