Chaos in Quadratic Gravity

TL;DR

This paper investigates the chaotic dynamics of test particles around rotating black holes in quadratic gravity theories, highlighting potential observational challenges for future gravitational wave detectors like LISA.

Contribution

It demonstrates that geodesic motion in these theories likely lacks a fourth constant of motion and exhibits chaos near the event horizon, impacting gravitational wave signals.

Findings

Chaotic features are present in orbital phase space.

Chaotic effects are very small and near the horizon.

Detection of chaos with LISA is very challenging.

Abstract

While recent gravitational wave observations by LIGO and Virgo allow for tests of general relativity in the extreme gravity regime, these observations are still blind to a large swath of phenomena outside these instruments' sensitivity curves. Future gravitational-wave detectors, such as LISA, will enable probes of longer-duration and lower-frequency events. In particular, LISA will enable the characterization of the non-linear dynamics of extreme mass-ratio inspirals, when a small compact object falls into a supermassive black hole. In this paper, we study the motion of test particles around spinning black holes in two quadratic gravity theories: scalar Gauss-Bonnet and dynamical Chern-Simons gravity. We show that geodesic trajectories around rotating black holes in these theories are likely to not have a fourth constant of the motion. In particular, we show that Poincar\'e sections of the orbital phase space present chaotic features that will affect the inspiral of small compact objects into supermassive black holes in these theories. Nevertheless, the characteristic size of these chaotic features is tiny and their location in parameter space is very close to the event horizon of the supermassive black hole. Therefore, the detection of such chaotic features with LISA is likely very challenging, at best.