Observational constraints on massive gravity
  The ghost free massive gravity modified Friedmann equations at cosmic scale
and provided an explanation of cosmic acceleration without dark energy. We
analyzed the cosmological solutions of the massive gravity in detail and
confronted the cosmological model with current observational data. We found
that the model parameters $\alpha_3$ and $\alpha_4$ which are the coefficients
of the third and fourth order nonlinear interactions cannot be constrained by
current data at the background level. The mass of graviton is found to be the
order of current Hubble constant if $\alpha_3=\alpha_4=0$, and the mass of
graviton can be as small as possible in the most general case.
