Causality constraints on black holes beyond GR

We derive causality constraints on the simplest scalar-tensor theories in which black holes differ from what General Relativity predicts, a scalar coupled to the Gauss-Bonnet or the Chern-Simons terms. Demanding that time advances are unobservable within the regime of validity of these effective field theories, we find their cutoff must be parametrically of the same size as the inverse Schwarzschild radius of the black holes for which the non-standard effects are of order one. For astrophysical black holes within the range of current gravitational wave detectors, this means a cutoff length of the order of kilometers. We further explore the leading additional higher-dimensional operators potentially associated with the scale of UV completion and discuss their phenomenological implications for gravitational wave science.

Automated summary

Causality constraints on scalar-tensor theories involving black holes suggest that within the effective field theory regime, the unobservability of time advancement necessitates a cutoff length approximately the size of the inverse Schwarzschild radius of the black holes. For astrophysical black holes detectable by current gravitational wave detectors, this implies a cutoff length on the order of kilometers. Additionally, implications for gravitational wave science are discussed by exploring potential higher-dimensional operators associated with the scale of UV completion.