13 research outputs found
Gravitational Collapse of Circularly Symmetric Stiff Fluid with Self-Similarity in 2+1 Gravity
Linear perturbations of homothetic self-similar stiff fluid solutions,
, with circular symmetry in 2+1 gravity are studied. It is found that,
except for those with and , none of them is stable and all have
more than one unstable mode. Hence, {\em none of these solutions can be
critical}.Comment: latex file, 1 figure; last version to appear in Prog. Theor. Phy
Fractals and the Distribution of Galaxies
This paper presents a review of the fractal approach for describing the large scale distribution of galaxies. We start by presenting a brief, but general, introduction to fractals, which emphasizes their empirical side and applications rather than their mathematical side. Then we discuss the standard correlation function analysis of galaxy catalogues and many observational facts that brought increasing doubts about the reliability of this method, paying special attention to the standard analysis implicit assumption of an eventual homogeneity of the distribution of galaxies. Some new statistical concepts for analysing this distribution is presented, and without the implicit assumption of homogeneity they bring support to the hypothesis that the distribution of galaxies does form a fractal system. The Pietronero-Wertz's single fractal (hierarchical) model is presented and discussed, together with the implications of this new approach for understanding galaxy clustering
Probing the stability of gravastars by dropping dust shells onto them
As a preparation for the dynamical investigations, this paper begins with a
short review of the three-layer gravastar model with distinguished attention to
the structure of the pertinent parameter space of gravastars in equilibrium.
Then the radial stability of these types of gravastars is studied by
determining their response for the totally inelastic collision of their surface
layer with a dust shell. It is assumed that the dominant energy condition holds
and the speed of sound does not exceed that of the light in the matter of the
surface layer. While in the analytic setup the equation of state is kept to be
generic, in the numerical investigations three functionally distinct classes of
equations of states are applied. In the corresponding particular cases the
maximal mass of the dust shell that may fall onto a gravastar without
converting it into a black hole is determined. For those configurations which
remain stable the excursion of their radius is assigned. It is found that even
the most compact gravastars cannot get beyond the lower limit of the size of
conventional stars, provided that the dominant energy condition holds in both
cases. It is also shown---independent of any assumption concerning the matter
interbridging the internal de Sitter and the external Schwarzschild
regions---that the better is a gravastar in mimicking a black hole the easier
is to get the system formed by a dust shell and the gravastar beyond the event
horizon of the composite system. In addition, a generic description of the
totally inelastic collision of spherical shells in spherically symmetric
spacetimes is also provided in the appendix.Comment: 29 pages, 10 figure
Levi-Civita Solutions Coupled with Electromagnetic Fields
The local and global properties of the Levi-Civita (LC) solutions coupled
with an electromagnetic field are studied and some limits to the vacuum LC
solutions are given. By doing such limits, the physical and geometrical
interpretations of the free parameters involved in the solutions are made
clear. Sources for both the LC vacuum solutions and the LC solutions coupled
with an electromagnetic field are studied, and in particular it is found that
all the LC vacuum solutions with can be produced by
cylindrically symmetric thin shells that satisfy all the energy conditions,
weak, dominant, and strong. When the electromagnetic field is present, the
situation changes dramatically. In the case of a purely magnetic field, all the
solutions with or can be
produced by physically acceptable cylindrical thin shells, while in the case of
a purely electric field, no such shells are found for any value of .Comment: Typed in Revtex, including two figure
Bounded excursion stable gravastars and black holes
Dynamical models of prototype gravastars were constructed in order to study
their stability. The models are the Visser-Wiltshire three-layer gravastars, in
which an infinitely thin spherical shell of stiff fluid divides the whole
spacetime into two regions, where the internal region is de Sitter, and the
external is Schwarzschild. It is found that in some cases the models represent
the "bounded excursion" stable gravastars, in which the thin shell is
oscillating between two finite radii, while in other cases they collapse until
the formation of black holes. In the phase space, the region for the ``bounded
excursion" gravastars is very small in comparison to that of black holes, but
not empty. Therefore, although the existence of gravastars cannot be excluded
from such dynamical models, our results do indicate that, even if gravastars
indeed exist, they do not exclude the existence of black holes.Comment: Revtex4, 12 figures. Version published in JCAP, 06 (2008) 02
Bounded excursion stable gravastars and black holes
International audienceDynamical models of prototype gravastars were constructed in order to study their stability. The models are the Visser-Wiltshire three-layer gravastars, in which an infinitely thin spherical shell of stiff fluid divides the whole spacetime into two regions, where the internal region is de Sitter, and the external one is Schwarzschild. It is found that in some cases the models represent the 'bounded excursion' stable gravastars, where the thin shell is oscillating between two finite radii, while in other cases they collapse until the formation of black holes occurs. In the phase space, the region for the 'bounded excursion' gravastars is very small in comparison to that of black holes, but not empty. Therefore, although the possibility of the existence of gravastars cannot be excluded from such dynamical models, our results indicate that, even if gravastars do indeed exist, that does not exclude the possibility of the existence of black holes