Exact self-accelerating cosmologies in the ghost-free bigravity and massive gravity

TL;DR

This paper derives the most general cosmological solutions in ghost-free bigravity where the physical universe is homogeneous and isotropic, and the second metric is inhomogeneous, showing late-time acceleration due to graviton mass.

Contribution

It presents the first comprehensive cosmological solutions with a homogeneous physical metric and an inhomogeneous second metric in ghost-free bigravity, valid for any parameters.

Findings

Universe exhibits late-time acceleration from graviton mass

Solutions include matter sources and are broadly applicable

Perturbed universe remains approximately homogeneous and isotropic

Abstract

Within the theory of the ghost-free bigravity, we present the most general cosmological solution for which the physical metric is homogeneous and isotropic, while the second metric is inhomogeneous. The solution includes a matter source and exists for any values of the theory parameters. The physical metric describes a universe with the late time acceleration due to the effective cosmological term mimicked by the graviton mass. When perturbed, this universe should rest approximately homogeneous and isotropic in space regions small compared to the graviton Compton length. In the limit where the massless graviton decouples, the solution fulfills the equations of the ghost-free massive gravity.