Singularity-free gravitational collapse: From regular black holes to horizonless objects

TL;DR

This paper explores how quantum effects might prevent singularities in gravitational collapse, examining regular black holes, horizonless objects, and the importance of dynamical models for realistic scenarios.

Contribution

It discusses the conditions for avoiding singularities in gravitational collapse without relying on specific quantum gravity models, highlighting relationships between regular black holes and horizonless objects.

Findings

Regular black holes and horizonless objects are connected through extremal configurations.

Quantum effects could potentially prevent singularity formation in collapse scenarios.

Dynamical models are necessary for realistic descriptions of gravitational collapse.

Abstract

Penrose's singularity theorem implies that if a trapped region forms in a gravitational collapse, then a singularity must form as well within such region. However, it is widely expected that singularities should be generically avoided by quantum gravitational effects. Here we shall explore both the minimum requirements to avoid singularities in a gravitational collapse as well as discuss, without relying on a specific quantum gravity model, the possible regular spacetimes associated to such regularization of the spacetime fabric. In particular, we shall expose the intimate and quite subtle relationship between regular black holes, black bounces and their corresponding horizonless object limits. In doing so, we shall devote specific attention to those critical (extremal) black hole configurations lying at the boundary between horizonful and horizonless geometries. While these studies are carried out in stationary configurations, the presence of generic instabilities strongly suggest the need for considering more realistic time-dependent dynamical spacetimes. Missing specific dynamical models, much less rigorous statements can be made for evolving geometries. We shall nonetheless summarize here their present understanding and discuss their implications for future phenomenological studies.