FRW cosmology in ghost free massive gravity from bigravity

We study FRW homogeneous cosmological solutions in the bigravity extension of the recently found ghost-free massive gravity. When the additional extra metric, needed to generate the mass term, is taken as nondynamical and flat, no homogeneous flat FRW cosmology exists. We show that, when the additional metric is a dynamical field a perfectly accetable FRW solution exists. Solutions fall in two branches. In the first branch the massive deformation is equivalent to an effective cosmological constant determined by the graviton mass. The second branch is quite rich: we have FRW cosmology in the presence of a gravitational fluid. The control parameter xi is the ratio of the two conformal factors. When xi is small, the evolution is similar to GR and interestingly the universe flows at late time towards an attractor represented by a dS phase.