Black holes: on the universality of the Kerr hypothesis

TL;DR

This paper critically examines the universality of the Kerr hypothesis for astrophysical black holes, proposing criteria for alternative models and demonstrating that non-Kerr objects can mimic black holes while passing key tests.

Contribution

It introduces a minimal set of criteria for alternative compact object models and provides concrete examples that challenge the universality of the Kerr hypothesis.

Findings

Non-Kerr black holes can pass dynamical tests

Some models match astrophysical observables

The Kerr hypothesis can be challenged at macroscopic scales

Abstract

To what extent are all astrophysical, dark, compact objects both black holes (BHs) and described by the Kerr geometry? We embark on the exercise of defying the universality of this remarkable idea, often called the "Kerr hypothesis". After establishing its rationale and timeliness, we define a minimal set of reasonability criteria for alternative models of dark compact objects. Then, as proof of principle, we discuss concrete, dynamically robust non-Kerr BHs and horizonless imitators, that 1) pass the basic theoretical, and in particular dynamical, tests, 2) match (some of the) state of the art astrophysical observables and 3) only emerge at some (macroscopic) scales. These examples illustrate how the universality (at all macroscopic scales) of the Kerr hypothesis can be challenged.