Gravitational radiation in Hořava gravity

Abstract

We study the radiation of gravitational waves by self-gravitating binary systems in the low-energy limit of Ho\ifmmode \check{r}\else \v{r}\fi{}ava 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 Ho\ifmmode \check{r}\else \v{r}\fi{}ava 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 Ho\ifmmode \check{r}\else \v{r}\fi{}ava gravity at low-energies.