Parity-violating gravity and GW170817

TL;DR

This paper investigates how gravitational wave observations, especially GW170817, constrain parity-violating theories of gravity, revealing that such constraints can limit the parity-violating sector to Chern-Simons gravity due to GW speed deviations.

Contribution

It demonstrates that GW170817 constraints can be used to restrict parity-violating gravity theories beyond Chern-Simons gravity by analyzing GW propagation speeds.

Findings

GW propagation speed deviates from c in generic parity-violating gravity.

GW170817 constrains the parity-violating sector to Chern-Simons gravity.

GW speed measurements can limit parity violation in gravity theories.

Abstract

We consider gravitational waves (GWs) in generic parity-violating gravity including recently proposed ghost-free theories with parity violation as well as Chern-Simons (CS) modified gravity, and study the implications of observational constraints from GW170817/GRB 170817A. Whereas GWs propagate at the speed of light, c, in CS gravity, we point out that this is specific to CS gravity and the GW propagation speed deviates from c, in general, in parity-violating gravity. Therefore, contrary to the previous literature in which only CS gravity is studied as a concrete example, we show that GW170817/GRB 170817A can, in fact, be used to limit gravitational parity violation. Our argument implies that the constraint on the propagation speed of GWs can pin down the parity-violating sector, if any, to CS gravity.