70 research outputs found
Classical collapse to black holes and quantum bounces: A review
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.Comment: 31 pages, 8 figures. Published version appearing in the collection
'Open Questions in Black Hole Physics' of the journal Univers
Thermodynamics and gravitational collapse
It is known now that a typical gravitational collapse in general relativity,
evolving from regular initial data and under physically reasonable conditions
would end in either a black hole or a naked singularity final state. An
important question that needs to be answered in this connection is, whether the
analogues of the laws of thermodynamics, as formulated for relativistic
horizons are respected by the dynamical spacetimes for collapse that end in the
formation of a naked singularity. We investigate here the thermodynamical
behaviour of the dynamical horizons that form in spherically symmetric
gravitational collapse and we show that the first and second laws of black hole
thermodynamics, as extended to dynamical spacetimes in a suitable manner, are
not violated whether the collapse ends in a black hole or a naked singularity.
We then make a distinction between the naked singularities that result from
gravitational collapse, and those that exist in solutions of Einstein equations
in vacuum axially symmetric and stationary spacetimes, and discuss their
connection with thermodynamics in view of the cosmic censorship conjecture and
the validity of the third law of black hole mechanics.Comment: 8 pages, 2 figure
Gravitational blueshift from a collapsing object
We discuss a counterintuitive phenomenon of classical general relativity, in
which a significant fraction of the radiation emitted by a collapsing object
and detected by a distant observer may be blueshifted rather than redshifted.
The key-point is that when the radiation propagates inside the collapsing body
it is blueshifted, and this time interval may be sufficiently long for the
effect to be larger than the later redshift due to the propagation in the
vacuum exterior, from the surface of the body to the distant observer.
Unfortunately, the phenomenon can unlikely have direct observational
implications, but it is interesting by itself as a pure relativistic effect.Comment: 6 pages, 3 figures. v2: refereed versio
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 constitute the basis for the
description of formation of compact objects in nature.Comment: 9 pages, published versio
Note on the effect of a massive accretion disk in the measurements of black hole spins
The spin measurement of black holes has important implications in physics and
astrophysics. Regardless of the specific technique to estimate the black hole
spin, all the current approaches assume that the space-time geometry around the
compact object is exactly described by the Kerr solution. This is clearly an
approximation, because the Kerr metric is a stationary solution of the vacuum
Einstein equations. In this paper, we estimate the effect of a massive
accretion disk in the measurement of the black hole spin with a simple
analytical model. For typical accretion disks, the mass of the disk is
completely negligible, even for future more accurate measurements. However, for
systems with very massive disks the effect may not be ignored.Comment: 5 pages, 2 figures. v2: corrected a few typo
Semi-classical dust collapse and regular black holes
Semi-classical corrections at large curvature are employed in toy models of
spherically symmetric gravitational collapse in order to avoid the formation of
singularities. The resulting spacetimes may produce bounces, compact remnants
or regular black holes in place of the usual Schwarzschild black hole. Within
these models, a whole class of collapse scenarios leading to the formation of
regular black holes may be obtained from General Relativity coupled to some
theory of non-linear electrodynamics. In the present chapter we provide a
thorough exposition of semi-classical dust collapse with particular attention
to the conditions for the formation of regular black holes as the endstate of
collapse.Comment: Invited chapter for the edited book 'Regular Black Holes: Towards a
New Paradigm of the Gravitational Collapse' (Ed. C. Bambi, Springer
Singapore, expected to appear in 2023
All black holes in Lemaitre-Tolman-Bondi inhomogeneous dust collapse
Within the Lemaitre-Tolman-Bondi formalism for gravitational collapse of
inhomogeneous dust we analyze the parameter space that leads to the formation
of a globally covered singularity (i.e. a black hole) when some physically
reasonable requirements are imposed (namely positive radially decreasing and
quadratic profile for the energy density and avoidance of shell crossing
singularities). It turns out that a black hole can occur as the endstate of
collapse only if the singularity is simultaneous as in the standard
Oppenheimer-Snyder scenario. Given a fixed density profile then there is one
velocity profile for the infalling particles that will produce a black hole.
All other allowed velocity profiles will terminate the collapse in a locally
naked singularity.Comment: 10 pages, 2 figures, matches published versio
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