Non-Linear Bigravity and Cosmic Acceleration

We explore the cosmological solutions of classes of non-linear bigravity theories. These theories are defined by effective four-dimensional Lagrangians describing the coupled dynamics of two metric tensors, and containing, in the linearized limit, both a massless graviton and an ultralight one. We focus on one paradigmatic case where the coupling between the two metrics is given by a Pauli-Fierz-type mass potential. We find that the generic feature of bigravity cosmologies is to exhibit a period of cosmic acceleration, which in certain cases has specific anisotropic features. This leads us to propose bigravity as a source of a new type of dark energy ("tensor quintessence").