Curvature perturbations of quasidilaton nonlinear massive gravity

We study the cosmological perturbations of the recently proposed extension of nonlinear massive gravity with a scalar field. The added scalar field ensures a new symmetry on the field space of the theory. The theory has the property of having a flat de Sitter solution, in contrast to the standard de Rham, Gabadadze and Tolley massive gravity. The tensor part is the same as that of the standard de Rham, Gabadadze and Tolley and shows gravitational waves with a time -dependent mass that modifies the dispersion relation. We obtain the curvature perturbation of the model on superhorizon scales for a specific choice of omega = 0 and find that the theory does not allow a constant curvature perturbation on the superhorizon scales and we have always a growing mode. The consistency of equations restricts the parameter space of the theory.