Improved Constraints on Non-Kerr Deviations from Binary Black Hole Inspirals Using GWTC-4 Data

TL;DR

This paper uses GWTC-4 gravitational wave data to place tighter, theory-agnostic constraints on deviations from the Kerr black hole metric, reinforcing General Relativity's predictions.

Contribution

It updates previous bounds on non-Kerr deviations using a Bayesian framework and GWTC-4 data, employing parametrized corrections within the post-Newtonian approximation.

Findings

Deformation parameters are consistent with zero, indicating no evidence for non-Kerr deviations.

Tighter bounds on the Johannsen metric parameters compared to previous GWTC-3 analysis.

Results support the Kerr hypothesis and the validity of General Relativity.

Abstract

Gravitational wave (GW) observations of binary black hole (BBH) mergers provide a unique opportunity to probe the nature of spacetime in the strong-field and dynamical regime. We present updated constraints on deviations from the Kerr metric using BBH inspirals from the fourth Gravitational-Wave Transient Catalog (GWTC-4). Building on our previous GWTC-3 analysis, we employ a theory-agnostic framework in which non-Kerr effects of the Johannsen metric are incorporated as parametrized corrections to the GW phase within the post-Newtonian framework. We perform Bayesian parameter estimation on a selected subset of GWTC-4 events to constrain the deformation parameters $\alpha_{13}$ and $\epsilon_3$, yielding significantly tighter bounds compared to earlier results. When varied individually, the deformation parameters are found to be consistent with zero, providing no evidence for departures from the Kerr geometry. Our results reinforce the validity of General Relativity, particularly the Kerr hypothesis, and highlight the progress enabled by GWTC-4.