Stability of the Self-accelerating Universe in Massive Gravity
  We study linear perturbations around time dependent spherically symmetric
solutions in the Lambda_3 massive gravity theory, which self-accelerate in the
vacuum. We find that the dynamics of the scalar perturbations depend on the
coordinate choice for the background solutions. For particular choices of
coordinates there is a symmetry enhancement, leaving no propagating scalar
degrees of freedom at linear order in perturbations. In contrast, any other
coordinate choice propagates a single scalar mode. We find that the Hamiltonian
of this scalar mode is unbounded from below for all self-accelerating
solutions, signalling an instability.
