A theory-agnostic hierarchical Bayesian framework for black-hole spectroscopy: a case study on GW250114 in Einstein-dilaton-Gauss-Bonnet gravity

TL;DR

This paper introduces a flexible, theory-agnostic Bayesian framework for analyzing black-hole ringdowns, enabling direct spectral comparisons to test gravity theories with gravitational-wave data, exemplified by GW250114 in Einstein-dilaton-Gauss-Bonnet gravity.

Contribution

It develops a novel hierarchical Bayesian approach that compares observed quasinormal mode spectra directly to theory predictions, avoiding model-dependent waveform assumptions.

Findings

Posterior for the coupling parameter is robust against priors.

Nonzero couplings can be recovered from simulated signals.

Potential systematic bias from Kerr priors absorbing spectral deviations.

Abstract

Black-hole spectroscopy has emerged as a powerful probe of strong-field gravity in the era of gravitational-wave astronomy. In this context, many current tests of modified or extended gravity are implemented by searching for predicted signatures modeled as perturbative corrections to general-relativistic waveforms; however, this approach may introduce model-dependent systematics and limit applicability to broader classes of theories. To complement such methods, we develop a theory-agnostic hierarchical Bayesian framework that connects ringdown observations -- modeled as damped sinusoids -- directly with theoretical quasinormal mode spectra, performing the comparison at the spectral level rather than through theory-specific waveform matching. The framework incorporates a soft-truncation module to account for the finite domain of validity in the theory's parameter space and is equipped with quantitative diagnostics that identify stable analysis time windows. As an illustrative application, we implement the framework within Einstein-dilaton-Gauss-Bonnet gravity and apply it to the gravitational-wave event GW250114, finding that the resulting posterior for the dimensionless coupling $\zeta$ is robust against prior assumptions yet remains only weakly informative over the range considered in this work. We further perform controlled ringdown injection studies across different values of $\zeta$, confirming that nonzero couplings can be recovered while also indicating a potential systematic effect: Kerr-based priors in the $\zeta$ inference may partially absorb spectral deviations arising in alternative theories of gravity. This work establishes a transparent and extensible foundation for future strong-field gravity tests, naturally compatible with the growing precision and modal resolution of next-generation gravitational-wave detectors.