Bigravitational inflation
  We study the realization of cosmic inflation in bigravity theories. By
analyzing the evolution of scalar, vector, and tensor perturbations in de
Sitter-like spacetimes, we find strong stability constraints on the class of
viable vacua offered by these theories. More specifically, the only stable de
Sitter vacua contain two nondecoupled gravitons (one of which is massive) with
different maximal propagation speeds. We derive an effective theory for the
massless graviton, which is found to propagate at an intermediate speed,
limited by the two maximal values. For inflation, while the spectrum of density
perturbations remains nearly scale invariant, the power spectrum of tensor
modes is found to depart from the usual prediction found in standard slow-roll
inflation. In particular, both the tensor to scalar ratio $r$ and the spectral
index of tensor modes $n_T$ receive sizable contributions from the couplings of
the theory, leading to specific signals that may be tested in future
cosmological probes of CMB polarization.
