Gravitational waves and the polarizations in Hořava gravity after GW170817

Abstract

The gravitational waves of Ho\ifmmode \check{r}\else \v{r}\fi{}ava gravity, their polarization states, and their possible observational signatures are discussed. Using the gauge-invariant variable formalism, we find the three polarization modes in Ho\ifmmode \check{r}\else \v{r}\fi{}ava gravity excited by the three physical degrees of freedom contained in this theory. In particular, the scalar degree of freedom excites a mix of the transverse breathing and the longitudinal polarizations. The constraints from the previous experimental observations are taken into account, especially including the speed bound from the observations of GW170817 and GRB 170817A. We find that the Ho\ifmmode \check{r}\else \v{r}\fi{}ava theory is highly constrained. Within the experimentally allowed parametric space, we study whether the pulsar timing arrays and the Gaia mission can be used to distinguish the different polarizations. After calculating the cross-correlation functions between the redshifts of photons and the astrometric positions of stars, we conclude that it is possible to use pulsar timing arrays and the Gaia mission to tell whether the scalar polarization exists.