174 research outputs found
Constraining gravity models with disappearing cosmological constant
The gravity models proposed by Hu-Sawicki and Starobinsky are generic
for local gravity constraints to be evaded. The large deviations from these
models either result into violation of local gravity constraints or the
modifications are not distinguishable from cosmological constant. The curvature
singularity in these models is generic but can be avoided provided that proper
fine tuning is imposed on the evolution of scalaron in the high curvature
regime. In principle, the problem can be circumvented by incorporating
quadratic curvature correction in the Lagrangian though it might be quite
challenging to probe the relevant region numerically.Comment: 9 pages and 4 figures, minor clarifications and corrections added,
final version to appear in PR
Negative Pressure and Naked Singularities in Spherical Gravitational Collapse
Assuming the weak energy condition, we study the nature of the non-central
shell-focussing singularity which can form in the gravitational collapse of a
spherical compact object in classical general relativity. We show that if the
radial pressure is positive, the singularity is covered by a horizon. For
negative radial pressures, the singularity will be covered if the ratio of
pressure to the density is greater than -1/3 and naked if this ratio is .Comment: 7 pages, LaTeX Fil
A note on the cylindrical collapse of counter-rotating dust
We find analytical solutions describing the collapse of an infinitely long
cylindrical shell of counter-rotating dust. We show that--for the classes of
solutions discussed herein--from regular initial data a curvature singularity
inevitably develops, and no apparent horizons form, thus in accord with the
spirit of the hoop conjecture.Comment: 8 pages, LaTeX, ijmpd macros (included), 1 eps figure; accepted for
publication in Int. J. Mod. Phys.
Spherical Dust Collapse in Higher Dimensions
We consider here the question if it is possible to recover cosmic censorship
when a transition is made to higher dimensional spacetimes, by studying the
spherically symmetric dust collapse in an arbitrary higher spacetime dimension.
It is pointed out that if only black holes are to result as end state of a
continual gravitational collapse, several conditions must be imposed on the
collapsing configuration, some of which may appear to be restrictive, and we
need to study carefully if these can be suitably motivated physically in a
realistic collapse scenario. It would appear that in a generic higher
dimensional dust collapse, both black holes and naked singularities would
develop as end states as indicated by the results here. The mathematical
approach developed here generalizes and unifies the earlier available results
on higher dimensional dust collapse as we point out. Further, the dependence of
black hole or naked singularity end states as collapse outcomes, on the nature
of the initial data from which the collapse develops, is brought out explicitly
and in a transparent manner as we show here. Our method also allows us to
consider here in some detail the genericity and stability aspects related to
the occurrence of naked singularities in gravitational collapse.Comment: Revtex4, Title changed, To appear in Physical Review
Singularities in gravitational collapse with radial pressure
We analyze spherical dust collapse with non-vanishing radial pressure, ,
and vanishing tangential stresses. Considering a barotropic equation of state,
, we obtain an analytical solution in closed form---which is
exact for , and approximate otherwise---near the center of
symmetry (where the curvature singularity forms). We study the formation,
visibility, and curvature strength of singularities in the resulting spacetime.
We find that visible, Tipler strong singularities can develop from generic
initial data. Radial pressure alters the spectrum of possible endstates for
collapse, increasing the parameter space region that contains no visible
singularities, but cannot by itself prevent the formation of visible
singularities for sufficiently low values of the energy density. Known results
from pressureless dust are recovered in the limit.Comment: to appear in GRG; LaTeX, 22 pages, 2 eps figure
The spectrum of endstates of gravitational collapse with tangential stresses
The final state--black hole or naked singularity--of the gravitational
collapse of a marginally bound matter configuration in the presence of
tangential stresses is classified, in full generality, in terms of the initial
data and equation of state. If the tangential pressure is sufficiently strong,
configurations that would otherwise evolve to a spacelike singularity, result
in a locally naked singularity, both in the homogeneous and in the general,
inhomogeneous density case.Comment: 9 pages, revtex4; accepted for publication in Phys. Rev.
Gravitational Collapse and Cosmological Constant
We consider here the effects of a non-vanishing cosmological term on the
final fate of a spherical inhomogeneous collapsing dust cloud. It is shown that
depending on the nature of the initial data from which the collapse evolves,
and for a positive value of the cosmological constant, we can have a globally
regular evolution where a bounce develops within the cloud. We characterize
precisely the initial data causing such a bounce in terms of the initial
density and velocity profiles for the collapsing cloud. In the cases otherwise,
the result of collapse is either formation of a black hole or a naked
singularity resulting as the end state of collapse. We also show here that a
positive cosmological term can cover a part of the singularity spectrum which
is visible in the corresponding dust collapse models for the same initial data.Comment: 18 pages, no figure
Interplay of fission modes in mass distribution of light actinide nuclei 225,227Pa
Fission-fragment mass distributions were measured for 225,227Pa nuclei formed
in fusion reactions of 19F + 206, 208Pb around fusion barrier energies.
Mass-angle correlations do not indicate any quasi-fission like events in this
bombarding energy range. Mass distributions were fitted by Gaussian
distribution and mass variance extracted. At below-barrier energies, the mass
variance was found to increase with decrease in energy for both nuclei. Results
from present work were compared with existing data for induced fission of 224,
226Th and 228U around barrier energies. Enhancement in mass variance of 225,
227Pa nuclei at below-barrier energies shows evidence for presence of
asymmetric fission events mixed with symmetric fission events. This is in
agreement with the results of mass distributions of nearby nuclei 224, 226Th
and 228U where two-mode fission process was observed. Two-mode feature of
fission arises due to the shell effects changing the landscape of the potential
energy surfaces at low excitation energies. The excitation-energy dependence of
the mass variance gives strong evidence for survival of microscopic shell
effects in fission of light actinide nuclei 225, 227Pa with initial excitation
energy ~30 - 50 MeV
Black holes vs. naked singularities formation in collapsing Einstein's clusters
Non-static, spherically symmetric clusters of counter-rotating particles, of
the type first introduced by Einstein, are analysed here. The initial data
space can be parameterized in terms of three arbitrary functions, namely;
initial density, velocity and angular momentum profiles. The final state of
collapse, black hole or naked singularity, turns out to depend on the order of
the first non-vanishing derivatives of such functions at the centre. The work
extends recent results by Harada, Iguchi and Nakao.Comment: 13 pages, LaTeX format. To appear in Physical Review
Photon redshift and the appearance of a naked singularity
In this paper we analyze the redshift as observed by an external observer
receiving photons which terminate in the past at the naked singularity formed
in a Tolman-Bondi dust collapse. Within the context of models considered here
it is shown that photons emitted from a weak curvature naked singularity are
always finitely redshifted to an external observer. Certain cases of strong
curvature naked singularities, including the self-similar one, where the
photons are infinitely redshifted are also pointed out.Comment: Latex file, 14 pages, no figures, one change in the reference.
Accepted for publication in Phys. Rev.
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