Gravitational waves in bigravity cosmology

TL;DR

This paper investigates gravitational wave perturbations in bigravity cosmology, revealing inherent instabilities that require fine tuning, and discusses implications for tensor-to-scalar ratios in low-scale inflation models.

Contribution

It demonstrates the general instability of gravitational wave perturbations in bigravity cosmology and quantifies the necessary fine tuning for viable models.

Findings

Gravitational wave perturbations are generally unstable in bigravity.

Fine tuning of initial conditions is required for stability.

Models can produce large tensor-to-scalar ratios even at low inflation scales.

Abstract

In this paper we study gravitational wave perturbations in a cosmological setting of bigravity which can reproduce the {\Lambda}CDM background and large scale structure. We show that in general gravitational wave perturbations are unstable and only for very fine tuned initial conditions such a cosmology is viable. We quantify this fine tuning. We argue that similar fine tuning is also required in the scalar sector in order to prevent the tensor instability to be induced by second order scalar perturbations. Finally, we show that due to this power law instability, models of bigravity can lead to a large tensor to scalar ratio even for low scale inflation.