Classical Collapse to Black Holes and Quantum Bounces: A Review

Abstract

Abstract:In the last four decades, different programs have been carried out aiming at understanding the final fate of gravitational collapse of massive bodies once some prescriptions for the behaviour of gravity in the strong field regime are provided. The general picture arising from most of these scenarios is that the classical singularity at the end of collapse is replaced by a bounce. The most striking consequence of the bounce is that the black hole horizon may live for only a finite time. The possible implications for astrophysics are important since, if these models capture the essence of the collapse of a massive star, an observable signature of quantum gravity may be hiding in astrophysical phenomena. One intriguing idea that is implied by these models is the possible existence of exotic compact objects, of high density and finite size, that may not be covered by an horizon. The present article outlines the main features of these collapse models and some of the most relevant open problems. The aim is to provide a comprehensive (as much as possible) overview of the current status of the field from the point of view of astrophysics. As a little extra, a new toy model for collapse leading to the formation of a quasi static compact object is presented.

Tests for the existence of black holes through gravitational wave echoes

“…In summary, with the exception of boson stars, we do not know how or if UCOs and ClePhOs form in realistic collapse or merger scenarios. [37,116] (but see [117]) [23,24] 2 − 2 holes ClePhOs ∼ [118] (but see [118]) (but see [118]) [23,24] Collapsed ClePhOs ∼ polymers [119,120] (but see [119,121]) [121] Quantum bounces / ECO -ClePhOs ∼ black stars [7,8,[122][123][124][125] (but see [123,126]) [125] Quantum stars * Table 1: Catalogue of some proposed horizonless compact objects. A tick means that the topic was addressed.…”

Born–Infeld inspired modifications of gravity

“…Indeed, from an intuitive point of view, since gravity is a matter of curvature, the blow up of curvature scalars could be seen as an indication of the presence of large tidal forces that would potentially rip apart a physical (extended) observer, which has shaped numerous approaches to get rid of spacetime singularities through bounded curvature scalars [35,21,20,259,254,241,65]. Indeed, the standard lore of the field states that, as the curvature grows to reach Planckian values, an improved theory of gravity properly incorporating quantum effects should avoid the formation of singularities during the last stages of the gravitational collapse [212,41,373,320,49,247]. In this section we will discuss the regular/singular character of the geonic configurations discussed in section 4.4 making use of these concepts.…”

A Model of Black Hole Evaporation and 4D Weyl Anomaly

“…4 We keep using the term "black hole" even though the system is different from the conventional black hole that has a horizon. 5 See also [13][14][15][16][17]. See, e.g., [18,19] for a black hole as a closed trapped region in the vacuum.…”