380 research outputs found
Naked and Thunderbolt Singularities in Black Hole Evaporation
If an evaporating black hole does not settle down to a non radiating remnant,
a description by a semi classical Lorentz metric must contain either a naked
singularity or what we call a thunderbolt, a singularity that spreads out to
infinity on a spacelike or null path. We investigate this question in the
context of various two dimensional models that have been proposed. We find that
if the semi classical equations have an extra symmetry that make them solvable
in closed form, they seem to predict naked singularities but numerical
calculations indicate that more general semi classical equations, such as the
original CGHS ones give rise to thunderbolts. We therefore expect that the semi
classical approximation in four dimensions will lead to thunderbolts. We
interpret the prediction of thunderbolts as indicating that the semi classical
approximation breaks down at the end point of black hole evaporation, and we
would expect that a full quantum treatment would replace the thunderbolt with a
burst of high energy particles. The energy in such a burst would be too small
to account for the observed gamma ray bursts.Comment: 21 pages (10 diagrams available on request
Associations of inflammatory and hemostatic variables with the risk of recurrent stroke
<p><b>Background and Purpose:</b> Several prospective studies have shown significant associations between plasma fibrinogen, viscosity, C-reactive protein (CRP), fibrin D-dimer, or tissue plasminogen activator (tPA) antigen and the risk of primary cardiovascular events. Little has been published on the associations of these variables with recurrent stroke. We studied such associations in a nested case-control study derived from the Perindopril Protection Against Recurrent Stroke Study (PROGRESS).</p>
<p><b>Methods:</b> Nested case-control study of ischemic (n=472) and hemorrhagic (n=83) strokes occurring during a randomized, placebo-controlled multicenter trial of perindopril-based therapy in 6105 patients with a history of stroke or transient ischemic attack. Controls were matched for age, treatment group, sex, region, and most recent qualifying event at entry to the parent trial.</p>
<p><b>Results:</b> Fibrinogen and CRP were associated with an increased risk of recurrent ischemic stroke after accounting for the matching variables and adjusting for systolic blood pressure, smoking, peripheral vascular disease, and statin and antiplatelet therapy. The odds ratio for the last compared with the first third of fibrinogen was 1.34 (95% CI, 1.01 to 1.78) and for CRP was 1.39 (95% CI, 1.05 to 1.85). After additional adjustment for each other, these 2 odds ratios stayed virtually unchanged. Plasma viscosity, tPA, and D-dimer showed no relationship with recurrent ischemic stroke, although tPA was significant for lacunar and large artery subtypes. Although each of these variables showed a negative relationship with recurrent hemorrhagic stroke, none of these relationships achieved statistical significance.</p>
<p><b>Conclusions:</b> Fibrinogen and CRP are risk predictors for ischemic but not hemorrhagic stroke, independent of potential confounders.</p>
Open Cosmic Strings in Black Hole Space-Times
We construct open cosmic string solutions in Schwarzschild black hole and
non-dilatonic black p-brane backgrounds. These strings can be thought to
stretch between two D-branes or between a D-brane and the horizon in curved
space-time. We study small fluctuations around these solutions and discuss
their basic properties.Comment: 11 pages, REVTex, 5 figures, a reference adde
Linking the trans-Planckian and the information loss problems in black hole physics
The trans-Planckian and information loss problems are usually discussed in
the literature as separate issues concerning the nature of Hawking radiation.
Here we instead argue that they are intimately linked, and can be understood as
"two sides of the same coin" once it is accepted that general relativity is an
effective field theory.Comment: 10 pages, 2 figures. Replaced with the version to be published in
General Relativity and Gravitatio
The black hole final state
We propose that in quantum gravity one needs to impose a final state boundary
condition at black hole singularities. This resolves the apparent contradiction
between string theory and semiclassical arguments over whether black hole
evaporation is unitary.Comment: 17 pages, harvmac, 1 figure, v2: comment about interactions and
references adde
String Thermalization at a Black Hole Horizon
Susskind has recently shown that a relativistic string approaching the event
horizon of a black hole spreads in both the transverse and longitudinal
directions in the reference frame of an outside observer. The transverse
spreading can be described as a branching diffusion of wee string bits. This
stochastic process provides a mechanism for thermalizing the quantum state of
the string as it spreads across the stretched horizon.Comment: 14 pages, latex, SU-ITP-94-4, NSF-ITP-94-1
Geometric Entropy of Nonrelativistic Fermions and Two Dimensional Strings
We consider the geometric entropy of free nonrelativistic fermions in two
dimensions and show that it is ultraviolet finite for finite fermi energies,
but divergent in the infrared. In terms of the corresponding collective field
theory this is a {\em nonperturbative} effect and is related to the soft
behaviour of the usual thermodynamic entropy at high temperatures. We then show
that thermodynamic entropy of the singlet sector of the one dimensional matrix
model at high temperatures is governed by nonperturbative effects of the
underlying string theory. In the high temperature limit the ``exact''
expression for the entropy is regular but leads to a negative specific heat,
thus implying an instability. We speculate that in a properly defined two
dimensional string theory, the thermodynamic entropy could approach a constant
at high temperatures and lead to a geometric entropy which is finite in the
ultraviolet.Comment: LaTex, 19 pages, no figures. Some references adde
Hawking Spectrum and High Frequency Dispersion
We study the spectrum of created particles in two-dimensional black hole
geometries for a linear, hermitian scalar field satisfying a Lorentz
non-invariant field equation with higher spatial derivative terms that are
suppressed by powers of a fundamental momentum scale . The preferred frame
is the ``free-fall frame" of the black hole. This model is a variation of
Unruh's sonic black hole analogy. We find that there are two qualitatively
different types of particle production in this model: a thermal Hawking flux
generated by ``mode conversion" at the black hole horizon, and a non-thermal
spectrum generated via scattering off the background into negative free-fall
frequency modes. This second process has nothing to do with black holes and
does not occur for the ordinary wave equation because such modes do not
propagate outside the horizon with positive Killing frequency. The horizon
component of the radiation is astonishingly close to a perfect thermal
spectrum: for the smoothest metric studied, with Hawking temperature
, agreement is of order at frequency
, and agreement to order persists out to
where the thermal number flux is ). The flux
from scattering dominates at large and becomes many orders of
magnitude larger than the horizon component for metrics with a ``kink", i.e. a
region of high curvature localized on a static worldline outside the horizon.
This non-thermal flux amounts to roughly 10\% of the total luminosity for the
kinkier metrics considered. The flux exhibits oscillations as a function of
frequency which can be explained by interference between the various
contributions to the flux.Comment: 32 pages, plain latex, 16 figures included using psfi
Consequence of Hawking radiation from 2d dilaton black holes
We investigate the CGHS model through numerical calculation. The behavior of
the mass function, which we introduced in our previous work as a ``local
mass'', is examined. We found that the mass function takes negative values,
which means that the amount of Hawking radiation becomes greater than the
initial mass of the black hole as in the case of the RST model.Comment: 17pages, 5 figures (three of them are attached, the other 2 figures
are available on request. Some mistakes including typographic errors have
been correcte
Frozen spatial chaos induced by boundaries
We show that rather simple but non-trivial boundary conditions could induce
the appearance of spatial chaos (that is stationary, stable, but spatially
disordered configurations) in extended dynamical systems with very simple
dynamics. We exemplify the phenomenon with a nonlinear reaction-diffusion
equation in a two-dimensional undulated domain. Concepts from the theory of
dynamical systems, and a transverse-single-mode approximation are used to
describe the spatially chaotic structures.Comment: 9 pages, 6 figures, submitted for publication; for related work visit
http://www.imedea.uib.es/~victo
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