Dark energy survivals in massive gravity after GW170817: SO(3) invariant

The recent detection of the gravitational wave signal GW170817 together with an electromagnetic counterpart GRB 170817A from the merger of two neutron stars puts a stringent bound on the tensor propagation speed. This constraint can be automatically satisfied in the framework of massive gravity. In this work, we consider a general SO(3)-invariant massive gravity with five propagating degrees of freedom and derive the conditions for the absence of ghosts and Laplacian instabilities in the presence of a matter perfect fluid on the flat Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological background. The graviton potential containing the dependence of three-dimensional metrics and a fiducial metric coupled to a temporal scalar field gives rise to a scenario of the late-time cosmic acceleration in which the dark energy equation of state w_DE is equivalent to -1 or varies in time. We find that the deviation from the value w_DE=-1 provides important contributions to the quantities associated with the stability conditions of tensor, vector, and scalar perturbations. In concrete models, we study the dynamics of dark energy arising from the graviton potential and show that there exist viable parameter spaces in which neither ghosts nor Laplacian instabilities are present for both w_DE>-1 and w_DE<-1. We also generally obtain the effective gravitational coupling G_eff with non-relativistic matter as well as the gravitational slip parameter η_s associated with the observations of large-scale structures and weak lensing. We show that, apart from a specific case, the two quantities G_eff and η_s are similar to those in general relativity for scalar perturbations deep inside the sound horizon.