Cosmology and perturbations in massive gravity
  We study perturbations around some cosmological backgrounds in the dRGT
theory of massive gravity. We develop a general formalism to calculate the
perturbations around any background. We derive the Lagrangian for fluctuations
in the small scale limit, and for the open FRW solution we repeat the analysis
around the full background. We find that the perturbations display similar
properties: the longitudinal modes of the massive graviton are instantaneous at
quadratic level, but they acquire a time-kinetic term at cubic order.
