Mild bounds on bigravity from primordial gravitational waves
  If the amplitude of primordial gravitational waves is measured in the
near-future, what could it tell us about bigravity? To address this question,
we study massive bigravity theories by focusing on a region in parameter space
which is safe from known instabilities. Similarly to investigations on late
time constraints, we implicitly assume there is a successful implementation of
the Vainshtein mechanism which guarantees that standard cosmological evolution
is largely unaffected. We find that viable bigravity models are subject to far
less stringent constraints than massive gravity, where there is only one set of
(massive) tensor modes. In principle sensitive to the effective graviton mass
at the time of recombination, we find that in our setup the primordial tensor
spectrum is more responsive to the dynamics of the massless tensor sector
rather than its massive counterpart. We further show there are intriguing
windows in the parameter space of the theory which could potentially induce
distinctive signatures in the B-modes spectrum.
