The propagation of gravitational waves can reveal fundamental features of the structure of spacetime. For instance, differences in the propagation of gravitational-wave polarizations would be a smoking gun for parity violations in the gravitational sector, as expected from birefringent theories like Chern-Simons gravity. Here we look for evidence of amplitude birefringence in the third catalog of detections by the Laser Interferometer Gravitational Wave Observatory and Virgo through the use of birefringent templates inspired by dynamical Chern-Simons gravity. From 71 binary-black-hole signals, we obtain the most precise constraints on gravitational-wave amplitude birefringence yet, measuring a birefringent attenuation of $\kappa =-0.019_{-0.029}^{+0.038}{\mathrm{Gpc}}^{-1}$ at 100 Hz with 90% credibility, equivalent to a parity-violation energy scale of $M_{\mathrm{PV}}\gtrsim 6.8\times {10}^{-21}\mathrm{GeV}$.

• Received 15 May 2023
• Accepted 4 October 2023

DOI:https://doi.org/10.1103/PhysRevD.108.084068