Tests of Gravitational-Wave Birefringence with the Open Gravitational-Wave Catalog*

The routine detection of gravitational-wave events from compact binary coalescences has allowed precise tests of gravity in a strong field, dynamical field, and high energy regimes. In this work, we report the results of testing gravitational-wave birefringence using compact binary coalescence events from the 3rd Open Gravitational-wave Catalog (3-OGC). Birefringence, an effect where the left- and right-handed polarizations of gravitational waves follow different equations of motion, occurs when the parity symmetry of gravity is broken. This arises naturally in the effective field theory extension of general relativity. Using Bayesian inference and state-of-the-art waveform modeling, we use all events in 3-OGC to constrain the lower limit of the energy scale at which parity violation effects emerge. Overall we do not find evidence for a violation of general relativity, and thus we constrain the parity-violating energy scale to 0.11 GeV at 90% confidence level, which is an improvement over previous results by one order of magnitude. Intriguingly, we find an outlier, GW190521, that supports the existence of birefringence over general relativity with a higher match-filtering signal-to-noise ratio and Bayes factor. The inclusion of birefringence for GW190521 gives consistent sky location estimation with a possible electromagnetic counterpart reported Zwicky Transient Facility (ZTF). However, the inferred MPVMPV​ from GW190521 is in tension with the combined constraints from other events, we thus hypothesize that the non-zero deviation may be caused by the limitations of the existing waveform approximants. More gravitational wave detections in the future from compact binary coalescence will further improve the constraints on birefringence and shed light on possible systematics for GW190521-type events.