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.
