Black hole binaries in shift-symmetric Einstein-scalar-Gauss-Bonnet gravity experience a slower merger phase

TL;DR

This paper investigates how shift-symmetric Einstein-scalar-Gauss-Bonnet gravity alters black hole binary mergers, revealing a transition from faster inspiral due to scalar radiation to a slower late-stage merger caused by non-linear effects.

Contribution

It provides the first non-linear numerical simulations showing the late-stage deceleration of black hole mergers in this modified gravity theory.

Findings

Scalar dipole radiation accelerates early inspiral.

Late-stage merger is decelerated compared to GR.

Non-linear effects significantly alter the merger dynamics.

Abstract

In shift-symmetric Einstein-scalar-Gauss-Bonnet gravity, stationary black holes have a non-vanishing scalar charge. During the inspiral, the phase evolution is modified by several effects, primarily an additional scalar dipole radiation, which enters at -1PN order. This effect accelerates the inspiral when compared to general relativity, when including corrections up to 2PN. Using fully non-linear numerical simulations of quasi-circular, comparable mass binaries, we find that in the late stages the orbital dynamics are altered so that the overall effect is instead a decelerated merger phase for the modified gravity case. We attribute this to a change in the conservative dynamics, and show that at the late inspiral stage more energy must be emitted in scalar-Gauss-Bonnet gravity to induce a given change in frequency. In longer signals, this should lead to a distinctive switch between a faster and slower frequency evolution relative to general relativity as the binary approaches merger. This work suggests we should revisit existing constraints on the theory that are obtained assuming PN approximations apply up to merger, or based on order by order approximations that neglect backreaction effects on the metric, and shows the importance of including non-linear effects that modify the gravitational sector in the strong field regime.