Nonperturbative suppression of beyond-General-Relativity effects in quadratic gravity

TL;DR

This paper demonstrates that nonperturbative effects in quadratic gravity lead to exponential suppression of deviations from general relativity in gravitational-wave signals, confirming the effective-field-theory equivalence.

Contribution

It provides a nonperturbative analysis showing how quadratic gravity effects are suppressed in black hole mergers, aligning with effective-field-theory predictions.

Findings

Deviations from GR in gravitational waves are exponentially suppressed in the GR limit.

Black holes in quadratic gravity exhibit additional quasinormal modes.

Observable effects of quadratic gravity are negligible in the nonperturbative regime.

Abstract

Quadratic gravity is a well-motivated extension of general relativity~(GR) wherein the Einstein-Hilbert action is augmented by quadratic curvature terms. This theory is equivalent to GR in an effective-field-theory framework, while the two theories are different at the non-perturbative level. As we have recently shown, black holes in quadratic gravity have a rich linear response, including extra scalar, vector, and tensor quasinormal modes that can be excited in physical processes, even when the stationary solution is the same as in GR. Here, by studying the gravitational-wave emission from point particles plunging into a Schwarzschild black hole in quadratic gravity, we show that observable deviations from GR are exponentially suppressed in the GR limit. This provides a nonperturbative realization of the equivalence between quadratic gravity and GR predicted in the effective-field-theory framework.