Novel ringdown tests of general relativity with black hole greybody factors

TL;DR

GreyRing is a new black hole ringdown model based on greybody factors, enabling precise, model-independent tests of strong gravity through post-merger signals, demonstrated on GW250114.

Contribution

The paper introduces GreyRing, a novel waveform model based on greybody factors, and a new consistency test of strong gravity that does not rely on overtones or early ringdown data.

Findings

GreyRing achieves mismatches of order 10^{-6} with numerical relativity waveforms.

The new test yields consistent remnant mass and spin measurements for GW250114.

The inferred parameters from GreyRing are comparable or better than traditional black hole spectroscopy.

Abstract

We present GreyRing, a new model for the post-merger signal in black-hole binary coalescences based on the greybody factor of the remnant. The model accurately reproduces the full frequency-domain ringdown signal of a large set of comparable-mass, aligned-spin numerical relativity waveforms, achieving mismatches of order ${\cal O}(10^{-6})$ for the dominant $(\ell,m)=(2,2)$ mode, and typically outperforming state-of-the-art time-domain models. Building on this model, we introduce a novel consistency test of strong gravity based on the greybody factor: the remnant mass and spin inferred from GreyRing can be compared with those obtained through standard black hole spectroscopy. This agnostic test relies exclusively on the post-merger signal and does not require the inclusion of overtones or the choice of very early ringdown starting times, combining the advantages of inspiral-merger-ringdown consistency tests and traditional black hole spectroscopy. We apply the test to GW250114 and find that the remnant mass and spin inferred from GreyRing are consistent with those measured from the full signal. Remarkably, the inferred parameters can be measured with a precision comparable to, or slightly better than, that achieved with standard black-hole spectroscopy. Our greybody-factor waveform model allows for new precision tests of strong gravity using the ringdown signal.