Causality Constraints on Black Holes beyond GR

TL;DR

This paper establishes causality constraints on scalar-tensor theories with black holes differing from GR, linking the cutoff scale to black hole size and discussing implications for gravitational wave observations.

Contribution

It derives causality bounds on modified gravity theories involving scalar couplings to curvature terms, connecting the cutoff scale to black hole properties.

Findings

Cutoff must be comparable to inverse Schwarzschild radius for significant effects.

Astrophysical black holes imply a cutoff length of about a kilometer.

Higher-dimensional operators could influence gravitational wave phenomenology.

Abstract

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 at the km. 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.