Gravitational radiation in Hořava gravity

We study the radiation of gravitational waves by self-gravitating binary systems in the low-energy limit of Hořava gravity. We find that the predictions for the energy-loss formula of general relativity are modified already for Newtonian sources: the quadrupole contribution is altered, in part due to the different speed of propagation of the tensor modes; furthermore, there is a monopole contribution stemming from an extra scalar degree of freedom. A dipole contribution only appears at higher post-Newtonian order. We use these findings to constrain the low-energy action of Hořava gravity by comparing them with the radiation damping observed for binary pulsars. Even if this comparison is not completely appropriate—since compact objects cannot be described within the post-Newtonian approximation—it represents an order of magnitude estimate. In the limit where the post-Newtonian metric coincides with that of general relativity, our energy-loss formula provides the strongest constraints for Hořava gravity at low-energies.