Massive gravity: nonlinear instability of the homogeneous and isotropic
  universe
  We argue that all homogeneous and isotropic solutions in nonlinear massive
gravity are unstable. For this purpose, we study the propagating modes on a
Bianchi type--I manifold. We analyze their kinetic terms and dispersion
relations as the background manifold approaches the homogeneous and isotropic
limit. We show that in this limit, at least one ghost always exists and that
its frequency tends to vanish for large scales, meaning that it cannot be
integrated out from the low energy effective theory. This ghost mode is
interpreted as a leading nonlinear perturbation around a homogeneous and
isotropic background.
