Surfing gravitational waves: can bigravity survive growing tensor modes?

TL;DR

This paper examines whether bigravity theories can be consistent with cosmic microwave background observations despite the presence of rapidly growing tensor modes, suggesting that fine-tuning or modifications are necessary.

Contribution

It introduces a linear analysis with a cut-off to control tensor mode growth in bigravity, highlighting the need for fine-tuning or theory modifications for observational viability.

Findings

Growing tensor modes are incompatible with CMB data without adjustments.

A cut-off in tensor perturbations can mitigate growth but requires fine-tuning.

Inflation ending at GeV scale can help suppress tensor mode growth.

Abstract

The theory of bigravity offers one of the simplest possibilities to describe a massive graviton while having self-accelerating cosmological solutions without a cosmological constant. However, it has been shown recently that bigravity is affected by early-time fast growing modes on the tensor sector. Here we argue that we can only trust the linear analysis up to when perturbations are in the linear regime and use a cut-off to stop the growing of the metric perturbations. This analysis, although more consistent, still leads to growing tensor modes that are unacceptably large for the theory to be compatible with measurements of the cosmic microwave background (CMB), both in temperature and polarization spectra. In order to suppress the growing modes and make the model compatible with CMB spectra, we find it necessary to either fine-tune the initial conditions, modify the theory or set the cut-off for the tensor perturbations of the second metric much lower than unity. Initial conditions such that the growing mode is sufficiently suppresed can be achieved in scenarios in which inflation ends at the GeV scale.