Stable Cosmology in Generalised Massive Gravity
  We present a thorough analysis of the cosmological perturbations in
Generalised Massive Gravity. This is an extension of de Rham-Gabadadze-Tolley
theory where the translation invariance in the St\"uckelberg field space is
broken. This allows the mass parameters to be promoted to functions of the
St\"uckelberg fields. We consider an exact cosmological background in this
theory and study the stability of perturbations. We derive conditions to avoid
ghost, gradient and tachyonic instability. The cosmology is an extension of the
self-accelerating branch of the constant mass parameter theory, but now all
five massive graviton polarisations propagate. For concreteness, we consider a
minimal version of the theory where cosmology undergoes an accelerated
expansion at late times and show that the perturbative stability is preserved
for a range of parameters.
