Causality Constraints on Black Holes

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 scalarcoupled to the gauss-bonnet or the chern-simons terms.Demanding that timeadvances 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-standardeffects are of order one.For astrophysical black holes within the range of current gravitational wave detectors, this means a cutoff length at the km.We further explore the leading additional higher-dimensional operators potentiallyassociated with the scale of ultraviolet completion and discuss their phenomenological implications for gravitational wave science.