Semiclassical gravitational collapse and avoidance of singularities

General Relativity (GR) predicts that, under standard conditions for normal matter, when gravity prevails over other forces, a space-time singularity must form [1]. On the other hand singularities are not expected to occur in the real universe as they may lead to pathological problems for the causal structure of the space-time. Several recent studies suggest that there may be mechanisms that prevent the formation of singularities and that these are likely to be related to quantum correction to GR that appear in the strong field regime. We considered some mathematical toy models describing complete gravitational collapse of a massive cloud that classically lead to the formation of a singularity. In order to fully characterize the final stages of collapse we would then need to use a theory of quantum-gravity which we do not possess yet. On the other hand semi-classical approaches are possible and they provide valuable hints at how gravity behaves in such extreme conditions [2]

Semiclassical gravitational collapse and avoidance of singularities

General Relativity (GR) predicts that, under standard conditions for normal matter, when gravity prevails over other forces, a space-time singularity must form [1]. On the other hand singularities are not expected to occur in the real universe as they may lead to pathological problems for the causal structure of the space-time. Several recent studies suggest that there may be mechanisms that prevent the formation of singularities and that these are likely to be related to quantum correction to GR that appear in the strong field regime. We considered some mathematical toy models describing complete gravitational collapse of a massive cloud that classically lead to the formation of a singularity. In order to fully characterize the final stages of collapse we would then need to use a theory of quantum-gravity which we do not possess yet. On the other hand semi-classical approaches are possible and they provide valuable hints at how gravity behaves in such extreme conditions [2]

Thermodynamics and the naked singularity in the Gamma-metric

We investigate a possible way of establishing a parallel between the third law of black hole mechanics, and the strong version of the third law of thermodynamics. We calculate the surface gravity and area for a naked singular null surface in the Gamma-metric and explain in what sense this behaviour violates thermodynamics

The instability of black hole formation in gravitational collapse

We consider here the classic scenario given by Oppenheimer, Snyder, and Datt, for the gravitational collapse of a massive matter cloud, and examine its stability under the introduction of small tangential stresses. We show, by offering an explicit class of physically valid tangential stress perturbations, that an introduction of tangential pressure, however small, can qualitatively change the final fate of collapse from a black hole final state to anaked singularity. This shows instability of black hole formation in collapse and throws important light on the nature of cosmic censorship hypothesis and its possible formulations. The key effect of these perturbations is to alter the trapped surface formation pattern within the collapsing cloud and the apparent horizon structure. This allows the singularity to be visible, and implications are discussed

Compact objects from gravitational collapse: an analytical toy model

We develop here a procedure to obtain regular static configurations as resulting from dynamical gravitational collapse of a massive matter cloud in general relativity. Under certain general physical assumptions for the collapsing cloud, we find the class of dynamical models that lead to an equilib-rium configuration. To illustrate this, we provide a class of perfect fluid collapse models that lead to a static constant density object as limit. We suggest that similar models might possibly constitutethe basis for the description of formation of compact objects in nature

Gravitational collapse with tangential pressure

Using the general formalism for spherical gravitational collapse developed in [1], we investigate here the final fate of a spherical distribution of a matter cloud, where radial pressures vanish but tangential pressures are non-zero. Within this framework, firstly we examine the effect of introducing a generic small pressure in a wellknown black hole formation process, which is that of an otherwise pressure-free dust cloud. The intriguing result we find is that a dust collapse that was going to a black hole final state could now go to a naked singularity final configuration, when arbitrarily small tangential pressures are introduced. The implications of such a scenario are discussed in some detail. Secondly, the approach here allows us to generalize the earlier results obtained on gravitational collapse with non-zero tangential pressure, in the presence of a non-zero cosmological constant. Finally, we discuss the genericity of black hole and naked singularity formation in collapse with non-zero tangential pressure. The treatment here gives a unified and complete picture on collapse final states, in terms of black hole and naked singularity formation, generalizing the earlier results obtained for this class of collapse models. Thus the role of tangential stresses towards determining collapse endstates emerges in a straightforward and transparent manner in our treatment

The instability of black hole formation in gravitational collapse

We consider here the classic scenario given by Oppenheimer, Snyder, and Datt, for the gravitational collapse of a massive matter cloud, and examine its stability under the introduction of small tangential stresses. We show, by offering an explicit class of physically valid tangential stress perturbations, that an introduction of tangential pressure, however small, can qualitatively change the final fate of collapse from a black hole final state to anaked singularity. This shows instability of black hole formation in collapse and throws important light on the nature of cosmic censorship hypothesis and its possible formulations. The key effect of these perturbations is to alter the trapped surface formation pattern within the collapsing cloud and the apparent horizon structure. This allows the singularity to be visible, and implications are discussed

Recent developments in gravitational collapse and spacetime singularities

It is now known that when a massive star collapses under the force of its own gravity, the final fate of such a continual gravitational collapse will be either a black hole or a naked singularity under a wide variety of physically reasonable circumstances within the framework of general theory of relativity. The research of recent years has provided con- siderable clarity and insight on stellar collapse, black holes and the nature and structure of spacetime singularities. We discuss several of these developments here. There are also important fundamental questions that remain unanswered on the final fate of collapse of a massive matter cloud in gravitation theory, especially on naked singularities which are hypothetical astrophysical objects and on the nature of cosmic censorship hypothesis. These issues have key implications for our understanding on black hole physics today, its astrophysical applications, and for certain basic questions in cosmology and possible quantum theories of gravity. We consider these issues here and summarize recent results and current progress in these directions. The emerging astrophysical and observational perspectives and implications are dicussed, with particular reference to the properties of accretion discs around black holes and naked singularities, which may provide charac-teristic signatures and could help distinguish these object

Recent developments in gravitational collapse and spacetime singularities

It is now known that when a massive star collapses under the force of its own gravity, the final fate of such a continual gravitational collapse will be either a black hole or a naked singularity under a wide variety of physically reasonable circumstances within the framework of general theory of relativity. The research of recent years has provided con- siderable clarity and insight on stellar collapse, black holes and the nature and structure of spacetime singularities. We discuss several of these developments here. There are also important fundamental questions that remain unanswered on the final fate of collapse of a massive matter cloud in gravitation theory, especially on naked singularities which are hypothetical astrophysical objects and on the nature of cosmic censorship hypothesis. These issues have key implications for our understanding on black hole physics today, its astrophysical applications, and for certain basic questions in cosmology and possible quantum theories of gravity. We consider these issues here and summarize recent results and current progress in these directions. The emerging astrophysical and observational perspectives and implications are dicussed, with particular reference to the properties of accretion discs around black holes and naked singularities, which may provide charac-teristic signatures and could help distinguish these object

Thermodynamics and the naked singularity in the Gamma-metric

We investigate a possible way of establishing a parallel between the third law of black hole mechanics, and the strong version of the third law of thermodynamics. We calculate the surface gravity and area for a naked singular null surface in the Gamma-metric and explain in what sense this behaviour violates thermodynamics