FRW Cosmological Perturbations in Massive Bigravity
  Cosmological perturbations of FRW solutions in ghost free massive bigravity, including also a second matter sector, are studied in detail. At early time, we find that sub horizon exponential instabilities are unavoidable and they lead to a premature departure from the perturbative regime of cosmological perturbations. 
D. Comelli, M. Crisostomi, L. Pilo
hep-th astro-ph.CO gr-qc