Causality and quasi-normal modes in the GREFT

The General Relativity Effective Field Theory (GREFT) introduceshigher-derivative interactions to parameterise the gravitational effects ofmassive degrees of freedom which are too heavy to be probed directly. Thecoefficients of these interactions have recently been constrained usingcausality: both from the analytic structure of 4-point graviton scattering andthe time delay of gravitational waves on a black hole background. In this work,causality is used to constrain the quasi-normal mode spectrum of GREFT blackholes. Demanding that quasi-normal mode perturbations decay faster in the GREFTthan in General Relativity -- a new kind of causality condition which stemsfrom the analytic structure of 2-point functions on a black hole background --leads to further constraints on the GREFT coefficients. The causalityconstraints and compact expressions for the GREFT quasi-normal mode frequenciespresented here will inform future parameterised gravitational waveforms, andthe observational prospects for gravitational wave observatories are brieflydiscussed.