A Metric for Testing the Nature of Black Holes

TL;DR

This paper introduces a new Kerr-like black hole metric that remains regular outside the event horizon for any spin and deviations, enabling more comprehensive tests of black hole nature through electromagnetic observations.

Contribution

The paper presents a novel Kerr-like metric valid for all spins and deviations, overcoming limitations of previous metrics and facilitating observational tests of black hole properties.

Findings

The metric is regular outside the event horizon for all spins.

It allows for testing black hole nature with current and upcoming observations.

Examples demonstrate its application across the electromagnetic spectrum.

Abstract

In general relativity, astrophysical black holes are uniquely described by the Kerr metric. Observational tests of the Kerr nature of these compact objects and, hence, of general relativity, require a metric that encompasses a broader class of black holes as possible alternatives to the usual Kerr black holes. Several such Kerr-like metrics have been constructed to date, which depend on a set of free parameters and which reduce smoothly to the Kerr metric if all deviations vanish. Many of these metrics, however, are valid only for small values of the spin or small perturbations of the Kerr metric or contain regions of space where they are unphysical hampering their ability to properly model the accretions flows of black holes. In this paper, I describe a Kerr-like black hole metric that is regular everywhere outside of the event horizon for black holes with arbitrary spins even for large deviations from the Kerr metric. This metric, therefore, provides an ideal framework for tests of the nature of black holes with observations of the emission from their accretion flows, and I give several examples of such tests across the electromagnetic spectrum with current and near-future instruments.