Cosmology with moving bimetric fluids
  We study cosmological implications of bigravity and massive gravity solutions
with non-simultaneously diagonal metrics by considering the generalized Gordon
and Kerr-Schild ansatzes. The scenario that we obtain is equivalent to that of
General Relativity with additional non-comoving perfect fluids. We show that
the most general ghost-free bimetric theory generates three kinds of effective
fluids whose equations of state are fixed by a function of the ansatz.
Different choices of such function allow to reproduce the behaviour of
different dark fluids. In particular, the Gordon ansatz is suitable for the
description of various kinds of slowly-moving fluids, whereas the Kerr-Schild
one is shown to describe a null dark energy component. The motion of those dark
fluids with respect to the CMB is shown to generate, in turn, a relative motion
of baryonic matter with respect to radition which contributes to the CMB
anisotropies. CMB dipole observations are able to set stringent limits on the
dark sector described by the effective bimetric fluid.
