Bigravitational inflation

TL;DR

This paper explores how bigravity theories can realize cosmic inflation, analyzing perturbations and stability, and predicts distinctive tensor mode signatures that could be tested through future CMB observations.

Contribution

It provides a detailed stability analysis of de Sitter vacua in bigravity and derives an effective theory for the massless graviton with unique propagation properties.

Findings

Stable de Sitter vacua contain two nondecoupled gravitons with different speeds.

Tensor power spectrum deviates from standard slow-roll predictions.

Tensor-to-scalar ratio and tensor spectral index are significantly affected by the theory's couplings.

Abstract

We study the realization of cosmic inflation in bigravity theories. By analyzing the evolution of scalar, vector, and tensor perturbations in de Sitter-like spacetimes, we find strong stability constraints on the class of viable vacua offered by these theories. More specifically, the only stable de Sitter vacua contain two nondecoupled gravitons (one of which is massive) with different maximal propagation speeds. We derive an effective theory for the massless graviton, which is found to propagate at an intermediate speed, limited by the two maximal values. For inflation, while the spectrum of density perturbations remains nearly scale invariant, the power spectrum of tensor modes is found to depart from the usual prediction found in standard slow-roll inflation. In particular, both the tensor to scalar ratio $r$ and the spectral index of tensor modes $n_T$ receive sizable contributions from the couplings of the theory, leading to specific signals that may be tested in future cosmological probes of CMB polarization.