Signatures from metastable oppositely-charged black hole binaries in scalar Gauss-Bonnet gravity

TL;DR

This paper reports on numerical simulations of oppositely-charged black hole binaries in scalar-Gauss-Bonnet gravity, revealing a novel scalar charge flip and associated orbital eccentricity, which could inform tests of gravity beyond general relativity.

Contribution

It introduces the phenomenon of scalar charge sign flip during inspiral in scalar-Gauss-Bonnet gravity, supported by independent simulations, highlighting nonlinear effects in alternative gravity theories.

Findings

Scalar charge can flip sign during inspiral

Transition from dipolar to quadrupolar scalar radiation

Orbit becomes eccentric due to scalar charge dynamics

Abstract

We conduct numerical simulations of inspiraling, oppositely-charged black holes in the class of scalar-Gauss-Bonnet theories that exhibit spontaneous black hole scalarization. For quasi-circular, equal-mass binaries near the existence threshold for scalarized solutions, we find a new phenomenon whereby one of the component black holes can suddenly flip the sign of its scalar charge during the inspiral. We confirm this phenomenon with two independent codes and identify two key signatures thereof: a change in the dominant scalar radiation channel (from dipolar to quadrupolar), and, strikingly, the introduction of eccentricity in the orbit. This scenario offers a concrete example of potential nonlinear departures from general relativity in the inspiral of binary black holes in alternative theories of gravity and is of relevance for the development of new tests of gravity.