Constraining gravitational wave amplitude birefringence and Chern-Simons gravity with GWTC-2

We perform a new test of general relativity (GR) with signals from GWTC-2, the LIGO and Virgo catalog of gravitational wave detections. We search for the presence of amplitude birefringence, in which left versus right circularly polarized modes of gravitational waves are exponentially enhanced and suppressed during propagation. Such an effect is present in various beyond-GR theories but is absent in GR. We constrain the amount of amplitude birefringence consistent with the data through an opacity parameter, which we bound to be 0. 74 Gpc^-1. Our constraint is derived under an assumption that all GWTC-2 events have a common distance. This result for is statistically significant, with a Jensen-Shannon divergence of 710^-2 bits compared to an uninformative distribution on. We then use these theory-agnostic results to constrain Chern-Simons gravity, a beyond-GR theory with motivations in quantum gravity. We bound the canonical Chern-Simons lengthscale to be ₀1. 010^3 km, in agreement with other long-distance measurement results.