Speaker
Description
Two universal predictions of general relativity (GR) are the propagation of gravitational waves (GW) along null geodesics and the isospectrality of quasinormal modes (QNM) in Schwarzschild and Kerr black holes. However, in extension of GR one generally finds that QNMs are no longer isospectral and that the GW propagation is no longer geodesic and that it exhibits birefringence --- polarization-dependent speed. We study these effects in a general effective-field-theory extension of GR with up to eight-derivative terms and show that there is a unique Lagrangian that gives rise to isospectral QNMs in the eikonal limit. Furthermore, this is also the only Lagrangian that gives rise to a polarization-independent dispersion relation for GWs, and hence is the only non-birefringent theory. We argue that both properties are related through the eikonal perturbations/light ring connection. Finally, we note that this unique theory is the quartic-curvature correction arising from string theory and argue that other stringy corrections may share similar properties.
