Viable cosmological solutions in massive bimetric gravity
  We find the general conditions for viable cosmological solution at the
background level in bigravity models. Furthermore, we constrain the parameters
by comparing to the Union 2.1 supernovae catalog and identify, in some cases
analytically, the best fit parameter or the degeneracy curve among pairs of
parameters. We point out that a bimetric model with a single free parameter
predicts a simple relation between the equation of state and the density
parameter, fits well the supernovae data and is a valid and testable
alternative to $\Lambda$CDM. Additionally, we identify the conditions for a
phantom behavior and show that viable bimetric cosmologies cannot cross the
phantom divide.
