Bimetric gravity is cosmologically viable

TL;DR

This paper demonstrates that bimetric gravity can be made cosmologically viable by tuning parameters, resulting in a model closely resembling DM with a natural cosmological constant and stable perturbations.

Contribution

It shows that by taking the second metric's Planck mass to be small, bimetric gravity avoids instabilities and mimics DM, providing a natural cosmological constant and stable perturbations.

Findings

Instabilities can be pushed to early times with small second metric Planck mass.

The theory approaches DM with a natural cosmological constant.

Helicity-0 mode is not strongly-coupled at low energies in this limit.

Abstract

Bimetric theory describes gravitational interactions in the presence of an extra spin-2 field. Previous work has suggested that its cosmological solutions are generically plagued by instabilities. We show that by taking the Planck mass for the second metric, $M_f$, to be small, these instabilities can be pushed back to unobservably early times. In this limit, the theory approaches general relativity with an effective cosmological constant which is, remarkably, determined by the spin-2 interaction scale. This provides a late-time expansion history which is extremely close to $\Lambda$CDM, but with a technically-natural value for the cosmological constant. We find $M_f$ should be no larger than the electroweak scale in order for cosmological perturbations to be stable by big-bang nucleosynthesis. We further show that in this limit the helicity-0 mode is no longer strongly-coupled at low energy scales.