  We perform cosmological perturbation theory in Hassan-Rosen bimetric gravity
for general homogeneous and isotropic backgrounds. In the de Sitter
approximation, we obtain decoupled sets of massless and massive scalar
gravitational fluctuations. Matter perturbations then evolve like in Einstein
gravity. We perturb the future de Sitter regime by the ratio of matter to dark
energy, producing quasi-de Sitter space. In this more general setting the
massive and massless fluctuations mix. We argue that in the quasi-de Sitter
regime, the growth of structure in bimetric gravity differs from that of
Einstein gravity.
