Photon rings in a holographic toy model

TL;DR

This paper investigates the theoretical relationship between photon rings around black holes and their holographic dual descriptions using a simplified Warped AdS3 black hole model, providing analytic insights into quasinormal modes and resonances.

Contribution

It introduces a holographic toy model that analytically explores the connection between photon rings and quantum resonances in black hole physics.

Findings

Analytic expressions for quasinormal mode frequencies in the toy model

Demonstration of emergent conformal symmetry in the photon ring structure

Mapping of classical QNMs to Ruelle resonances in the dual theory

Abstract

Light circling around an astrophysical black hole can spend a long time skirting its unstably bound photon orbits before escaping to infinity. To a distant observer, this orbiting light would appear as a bright ring encircling the image of the black hole. Though not yet resolved by radio-interferometric observations from the ground, this ``photon ring'' will be the target of future space-based black hole observations. Motivated by this experimental prospect, studies have sought to elucidate the theoretical connections between the photon ring -- an observable, classical effect -- and the putative holographic description of black holes in quantum gravity. General relativity predicts that the detailed structure of the photon ring encodes the high-frequency (eikonal) spectrum of quasinormal modes (QNMs) emitted by a perturbed black hole as it rings down, and also that the photon ring displays an emergent conformal symmetry that acts upon this spectrum. In holography, the classical QNM frequencies are expected to map to Ruelle resonances of the dual quantum theory. In this paper, we explore these connections in a lower-dimensional toy model based on Warped AdS$_3$ black holes that shares many features with the (3+1)-dimensional Kerr background -- including a photon ring at finite radius -- while still providing analytic control of the QNM frequencies.