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FRW Cosmology in Ghost Free Massive Gravity

We study FRW homogeneous cosmological solutions in the recently found ghost free massive gravity. In previous works it was shown that when the additional extra metric, needed to generate the mass term, is taken as non-dynamical and flat, no homogeneous FRW cosmology exists. We show that, when the additional metric is a dynamical field, a perfectly safe FRW universe exists. FRW solutions fall in two branches. In the first branch the massive deformation is equivalent to an effectively generated cosmological constant whose scale is 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 large, generically the cosmological evolution greatly differs from GR at the early time. In the small xi region, the evolution is similar to GR and the universe flows at late time toward an attractor represented by a dS phase.