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

Within the recently proposed ghost-free bigravity theory, 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 generic values of the theory parameters. The physical metric describes a universe with an effective cosmological term mimicked by the graviton mass, which causes the late time acceleration. 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.