Probing gravitational parity violation with gravitational waves from stellar-mass black hole binaries

Abstract

The recent discovery of gravitational wave events has offered us unique testbeds of gravity in the strong and dynamical field regime. One possible modification to General Relativity is the gravitational parity violation that gives rise to the amplitude birefringence in gravitational waves, where one of the circularly-polarized mode is amplified while the other one is suppressed during their propagation. In this paper, we study how well one can measure gravitational parity violation via the amplitude birefringence of gravitational waves from stellar-mass black hole binaries. We choose Chern-Simons gravity as an example and work within an effective field theory formalism. We consider gravitational waves from both individual sources and stochastic gravitational wave backgrounds. Regarding bounds from individual sources, we estimate them using a Fisher analysis and carry out Monte Carlo simulations by randomly distributing sources over their sky location and binary orientation. We find that the bounds on the scalar field evolution in Chern-Simons gravity from GW150914 are too weak to satisfy the weak Chern-Simons approximation, while aLIGO with its design sensitivity can place meaningful bounds. Regarding bounds from stochastic gravitational wave backgrounds, we set the threshold signal-to-noise ratio for detection of the parity-violation mode as 5 and estimate projected bounds with future detectors assuming that signals are consistent with no parity violation. We find that a network of two third-generation detectors is able to place bounds that are slightly stronger than current binary pulsar bounds. Since gravitational wave observations probe either the difference in parity violation between the source and the detector or the line-of-sight integration of the scalar field, such bounds are complementary to local measurements from solar system experiments and binary pulsar observations.