Self-accelerating Massive Gravity: Covariant Perturbation Theory
  We undertake a complete and covariant treatment for the quadratic Lagrangian
of all of the degrees of freedom of massive gravity with a fixed flat fiducial
metric for arbitrary massive gravity parameters around any isotropic
self-accelerating background solution. Generically, 3 out of 4 Stuckelberg
degrees of freedom propagate in addition to the usual 2 tensor degrees of
freedom of general relativity. The complete kinetic structure typically is only
revealed at an order in the graviton mass that is equivalently to retaining
curvature terms in a locally flat expansion. These results resolve several
apparent discrepancies in the literature where zero degrees of freedom
propagate in either special cases or approximate treatments as well as
decoupling limit analyses which attempt to count longitudinal degrees of
freedom.
