Axion gravitodynamics, Lense-Thirring effect, and gravitational waves

Abstract

We investigate physical implications of a gravitational analog of axion electrodynamics with a parity violating gravitoelectromagnetic theta term. This is related to the Nieh-Yan topological invariant in gravity with torsion, in contrast to the well-studied gravitational Chern-Simons term quadratic in curvature, coupled via a dynamical axion-like scalar field. Axion gravitodynamics is the corresponding linearized theory. We find that potentially observable effects are of over 80 orders of magnitude stronger than for its Chern-Simons counterpart and could thus be in reach for detection by experiments in the near future. For a near-earth scenario, we derive corrections to the Lense-Thirring geodetic effect and compare them to data from satellite-based experiments (Gravity Probe B). For gravitational waves we find modified dispersion relations, derive the corresponding polarizationdependent modified group and phase velocities and compare them to data from neutron star mergers (GW170817) to derive even stronger bounds.