2,620 research outputs found
Initial data and the end state of spherically symmetric gravitational collapse
Generalizing earlier results on the initial data and the final fate of dust
collapse, we study here the relevance of the initial state of a spherically
symmetric matter cloud towards determining its end state in the course of a
continuing gravitational collapse. It is shown that given an arbitrary regular
distribution of matter at the initial epoch, there always exists an evolution
from this initial data which would result either in a black hole or a naked
singularity depending on the allowed choice of free functions available in the
solution. It follows that given any initial density and pressure profiles for
the cloud, there is a non-zero measure set of configurations leading either to
black holes or naked singularities, subject to the usual energy conditions
ensuring the positivity of energy density. We also characterize here wide new
families of black hole solutions resulting from spherically symmetric collapse
without requiring the cosmic censorship assumption.Comment: Ordinary Tex file, 31 pages no figure
O acervo raro da biblioteca Milton de Albuquerque, da Embrapa Amazônia Oriental.
bitstream/item/63592/1/Oriental-Doc187.PD
Bibliografia de recursos naturais da Amazônia brasileira.
bitstream/item/145920/1/BIBLIOG-RECURSOS-NATURAIS.pd
Toward a Midisuperspace Quantization of LeMaitre-Tolman-Bondi Collapse Models
LeMa\^\i tre-Tolman-Bondi models of spherical dust collapse have been used
and continue to be used extensively to study various stellar collapse
scenarios. It is by now well-known that these models lead to the formation of
black holes and naked singularities from regular initial data. The final
outcome of the collapse, particularly in the event of naked singularity
formation, depends very heavily on quantum effects during the final stages.
These quantum effects cannot generally be treated semi-classically as quantum
fluctuations of the gravitational field are expected to dominate before the
final state is reached. We present a canonical reduction of LeMa\^\i
tre-Tolman-Bondi space-times describing the marginally bound collapse of
inhomogeneous dust, in which the physical radius, , the proper time of the
collapsing dust, , and the mass function, , are the canonical
coordinates, , and on the phase space. Dirac's
constraint quantization leads to a simple functional (Wheeler-DeWitt) equation.
The equation is solved and the solution can be employed to study some of the
effects of quantum gravity during gravitational collapse with different initial
conditions.Comment: 9 pages, 1 figure, Latex file. Minor corrections made. A general
solution of the constraints is presented. Revised version to appear in Phys.
Rev.
Cyclic deformation of bidisperse two-dimensional foams
In-plane deformation of foams was studied experimentally by subjecting bidisperse foams to cycles of traction and compression at a prescribed rate. Each foam contained bubbles of two sizes with given area ratio and one of three initial arrangements: sorted perpendicular to the axis of deformation (iso-strain), sorted parallel to the axis of deformation (iso-stress), or randomly mixed. Image analysis was used to measure the characteristics of the foams, including the number of edges separating small from large bubbles N-sl, the perimeter (surface energy), the distribution of the number of sides of the bubbles, and the topological disorder mu(2)(N).
Foams that were initially mixed were found to remain mixed after the deformation. The response of sorted foams, however, depended on the initial geometry, including the area fraction of small bubbles and the total number of bubbles. For a given experiment we found that (i) the perimeter of a sorted foam varied little; (ii) each foam tended towards a mixed state, measured through the saturation of N-sl; and (iii) the topological disorder mu(2)(N) increased up to an "equilibrium" value. The results of different experiments showed that (i) the change in disorder, Delta mu(2)(N), decreased with the area fraction of small bubbles under iso-strain, but was independent of it under iso-stress; and (ii) Delta mu(2)(N) increased with Delta N-sl under iso-strain, but was again independent of it under iso-stress. We offer explanations for these effects in terms of elementary topological processes induced by the deformations that occur at the bubble scale
Divergence of the Quantum Stress Tensor on the Cauchy Horizon in 2-d Dust Collapse
We prove that the quantum stress tensor for a massless scalar field in two
dimensional non-selfsimilar Tolman Bondi dust collapse and Vaidya radiation
collapse models diverges on the Cauchy horizon, if the latter exists. The two
dimensional model is obtained by suppressing angular co-ordinates in the
corresponding four dimensional spherical model.Comment: 16 pages, no figures, LaTeX fil
Quantum Radiation from Black Holes and Naked Singularities in Spherical Dust Collapse
A sufficiently massive collapsing star will end its life as a spacetime
singularity. The nature of the Hawking radiation emitted during collapse
depends critically on whether the star's boundary conditions are such as would
lead to the eventual formation of a black hole or, alternatively, to the
formation of a naked singularity. This latter possibility is not excluded by
the singularity theorems. We discuss the nature of the Hawking radiation
emitted in each case. We justify the use of Bogoliubov transforms in the
presence of a Cauchy horizon and show that if spacetime is assumed to terminate
at the Cauchy horizon, the resulting spectrum is thermal, but with a
temperature different from the Hawking temperature.Comment: PHYZZX macros, 27 pages, 3 figure
Hawking radiation from the quantum Lemaitre-Tolman-Bondi model
In an earlier paper, we obtained exact solutions of the Wheeler-DeWitt
equation for the Lemaitre-Tolman-Bondi (LTB) model of gravitational collapse,
employing a lattice regularization. In this paper, we derive Hawking radiation
in non-marginally bound models from our exact solutions. We show that a
non-vanishing energy function does not spoil the (approximate) Planck spectrum
near the horizon. We can also reliably compute corrections to the Bogoliubov
coefficient because our solutions are exact. The corrections are obtained by
going beyond the near horizon region and are shown to introduce additional
greybody factors, which modify the black body spectrum of radiation from the
black hole.Comment: 14 page
Combined tools for Surgical Case Packages contents and cost optimization: a preliminary study
This paper presents a solution proposal based on mathematical and statistical tools to optimize Surgical Case Packages of an Operating Room (OR) in a Portuguese public hospital that it is the most complex environment in a hospital. In this particular hospital, more than 27000 surgeries/year are performed, employing, sometimes, misadjusted composition of standard surgical packages and non-optimized grouping of surgical instruments. Problem consequences are, among others, high transport of
various surgical cases packages; high number of open cases and delays in surgical times following surgery. These type of problems are waste that do not add value to the service in the context of Lean Healthcare and must be eliminated using the most suitable tools. After the analysis, different tools were used: combinatorial analysis to optimize surgical cases composition and statistical analysis to identify the instruments usage and surgical basic case patterns. An optimization model was developed which produced a sterilizing initial solution of 135.24€. By identifying the most commonly employed instruments, it was concluded that some instruments have never been used and others rarely and some patterns were identified. The results achieved
were based on minor sample and in a form of data collection that needs some adjustment
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