A novel test of gravity via black hole eikonal correspondence

TL;DR

This paper introduces a new method to test the eikonal correspondence in black hole spacetimes by comparing quasinormal mode spectra with photon ring observations, providing a way to verify general relativity or detect deviations.

Contribution

It proposes a novel observational test of the eikonal correspondence using black hole images and QNM spectra, applicable beyond general relativity.

Findings

The method links QNM frequencies to photon ring features.

It is robust against different emission models.

Applicable to a broad range of scenarios.

Abstract

When adopted in black hole spacetimes, geometric-optics approximations imply a mapping between the quasinormal mode (QNM) spectrum of black holes in the eikonal limit and black hole images. In particular, the real part and the imaginary part of eikonal QNM frequencies are associated with the apparent size and the detailed structure of the ring images, respectively. This correspondence could be violated when going beyond general relativity. We propose a novel method to test the eikonal correspondence via the comparison of two sets of observables from a nonrotating black hole, one extracted from QNM spectra and the other from the lensed photon rings on the image plane. Specifically, the photon ring observables robustly capture the information of the black hole spacetime itself regardless of the surrounding emission models. Therefore, the proposed test of eikonal correspondence can be validated in quite broad scenarios.