On the 6th Mode in Massive Gravity
  Generic massive gravity models in the unitary gauge correspond to a
self-gravitating medium with six degrees of freedom. It is widely believed that
massive gravity models with six degrees of freedom have an unavoidable
ghost-like instability; however, the corresponding medium has stable
phonon-like excitations. The apparent contradiction is solved by the presence
of a non-vanishing background pressure and energy density of the medium that
opens up a stability window. The result is confirmed by looking at linear
stability on an expanding Universe, recovering the flat space stability
conditions in the small wavelength limit. Moreover, one can show that under
rather mild conditions, no ghost-like instability is present for any
wavelength. As a result, exploiting the medium interpretation, a generic
massive gravity model with six degrees of freedom is perfectly viable.
