Direct imaging rapidly-rotating non-Kerr black holes

TL;DR

This paper explores the possibility of observing non-Kerr black holes with non-trivial horizon topologies through direct imaging, which could reveal deviations from classical General Relativity and quantum gravity effects.

Contribution

It proposes that rapidly rotating non-Kerr black holes may have observable horizon topologies different from Kerr black holes, potentially detectable with future imaging technology.

Findings

Horizon topology changes at high spin parameters.

Accretion can induce topology transitions, violating Cosmic Censorship.

Potential observability of quantum gravity effects near the black hole center.

Abstract

Recently, two of us have argued that non-Kerr black holes in gravity theories different from General Relativity may have a topologically non-trivial event horizon. More precisely, the spatial topology of the horizon of non-rotating and slow-rotating objects would be a 2-sphere, like in Kerr space-time, while it would change above a critical value of the spin parameter. When the topology of the horizon changes, the black hole central singularity shows up. The accretion process from a thin disk can potentially overspin these black holes and induce the topology transition, violating the Weak Cosmic Censorship Conjecture. If the astrophysical black hole candidates are not the black holes predicted by General Relativity, we might have the quite unique opportunity to see their central region, where classical physics breaks down and quantum gravity effects should appear. Even if the quantum gravity region turned out to be extremely small, at the level of the Planck scale, the size of its apparent image would be finite and potentially observable with future facilities.