Testing modified gravity with the eccentric neutron star--black hole merger GW200105

TL;DR

This paper demonstrates that incorporating orbital eccentricity in gravitational wave models from neutron star-black hole mergers significantly improves tests of alternative gravity theories, providing tighter constraints on certain models.

Contribution

It extends eccentric waveform models to include modifications from alternative gravity theories, enabling more accurate tests with eccentric merger signals.

Findings

Eccentric waveform analysis yields tighter constraints on Einstein-dilaton-Gauss-Bonnet gravity.

Including eccentricity prevents false deviations from general relativity.

Dynamical Chern-Simons gravity remains unconstrained due to low spin content.

Abstract

Direct detections of gravitational waves offer a unique opportunity to test gravity in the highly dynamical and strong field regime. Current tests are typically performed assuming signals from quasicircular binaries. However, the complex waveform morphology induced by orbital eccentricity can enhance our ability to probe gravity with greater precision. A recent analysis of the neutron star-black hole event GW200105 identified strong evidence for orbital eccentricity. We extend an eccentric-precessing waveform model to test alternative models with this signal by incorporating eccentric corrections induced by Brans-Dicke, Einstein-dilaton-Gauss-Bonnet, and dynamical Chern-Simons gravity at leading post-Newtonian order. We show that analyzing this event with a quasi-circular model leads to a false deviation from general relativity, while the inclusion of eccentricity improves the bounds on the models. Our analysis of GW200105 places tight constraints on Einstein-dilaton-Gauss-Bonnet gravity, $\alpha^{1/2}_{\rm{EdGB}} \lesssim 2.38\,\text{km}$, and Brans-Dicke gravity, $\omega_{\rm{BD}} \gtrsim 3.5$, while dynamical Chern-Simons gravity remains unconstrained due to the low spin content.