Viable cosmology in bimetric theory
  We study cosmological perturbations in bimetric theory with two fluids each of which is coupled to one of the two metrics. Focusing on a healthy branch of background solutions, we clarify the stability of the cosmological perturbations. For this purpose, we extend the condition for the absence of the so-called Higuchi ghost, and show that the condition is guaranteed to be satisfied on the healthy branch. We also calculate the squared propagation speeds of perturbations and derive the conditions for the absence of the gradient instability. To avoid the gradient instability, we find that the model parameters are weakly constrained. 
Antonio De Felice, A.Emir Gumrukcuoglu, Shinji Mukohyama, Norihiro   Tanahashi, Takahiro Tanaka
hep-th astro-ph.CO gr-qc