58 research outputs found
An Analytic Model with Critical Behavior in Black Hole Formation
A simple analytic model is presented which exhibits a critical behavior in
black hole formation, namely, collapse of a thin shell coupled with outgoing
null fluid. It is seen that the critical behavior is caused by the
gravitational nonlinearity near the event horizon. We calculate the value of
the critical exponent analytically and find that it is very dependent on the
coupling constants of the system.Comment: 21pp., ReVTeX, 7 figures (postscript, compressed and uuencoded),
TIT/HEP-266/COSMO-4
Particle Creation in the Marginally Bound, Self Similar Collapse of Inhomogeneous Dust
We consider the evaporation of the (shell focusing) naked singularity formed
during the self-similar collapse of a marginally bound inhomogeneous dust
cloud, in the geometric optics approximation. We show that, neglecting the back
reaction of the spacetime, the radiation on \scrip tends to infinity as the
Cauchy Horizon is approached. Two consequences can be expected from this
result: (a) that the back reaction of spacetime will be large and eventually
halt the formation of a naked singularity thus preserving the Cosmic Censorship
Hypothesis and (b) matter attempting to collapse into a naked singularity will
radiate away energy at an intense rate, thereby possibly providing experimental
signatures of quantum effects in curved spacetimes.Comment: 18 pages, 4 figures. Version to appear in Nucl. Phys.
Collective magnetism at multiferroic vortex domain walls
Topological defects have been playgrounds for many emergent phenomena in
complex matter such as superfluids, liquid crystals, and early universe.
Recently, vortex-like topological defects with six interlocked structural
antiphase and ferroelectric domains merging into a vortex core were revealed in
multiferroic hexagonal manganites. Numerous vortices are found to form an
intriguing self-organized network. Thus, it is imperative to find out the
magnetic nature of these vortices. Using cryogenic magnetic force microscopy,
we discovered unprecedented alternating net moments at domain walls around
vortices that can correlate over the entire vortex network in hexagonal ErMnO3
The collective nature of domain wall magnetism originates from the
uncompensated Er3+ moments and the correlated organization of the vortex
network. Furthermore, our proposed model indicates a fascinating phenomenon of
field-controllable spin chirality. Our results demonstrate a new route to
achieving magnetoelectric coupling at domain walls in single-phase
multiferroics, which may be harnessed for nanoscale multifunctional devices.Comment: 18 pages, 10 figure
Numerical Approaches to Spacetime Singularities
This Living Review updates a previous version which its itself an update of a
review article. Numerical exploration of the properties of singularities could,
in principle, yield detailed understanding of their nature in physically
realistic cases. Examples of numerical investigations into the formation of
naked singularities, critical behavior in collapse, passage through the Cauchy
horizon, chaos of the Mixmaster singularity, and singularities in spatially
inhomogeneous cosmologies are discussed.Comment: 51 pages, 6 figures may be found in online version: Living Rev.
Relativity 2002-1 at www.livingreviews.or
Gestational Valproate Alters BOLD Activation in Response to Complex Social and Primary Sensory Stimuli
Valproic acid (VPA) has been used clinically as an anticonvulsant medication during pregnancy; however, it poses a neurodevelopmental risk due to its high teratogenicity. We hypothesized that midgestational (GD) exposure to VPA will lead to lasting deficits in social behavior and the processing of social stimuli. To test this, animals were given a single IP injection of 600 mg/kg of VPA on GD 12.5. Starting on postnatal day 2 (PND2), animals were examined for physical and behavior abnormalities. Functional MRI studies were carried out after PND60. VPA and control animals were given vehicle or a central infusion of a V1a antagonist 90 minutes before imaging. During imaging sessions, rats were presented with a juvenile test male followed by a primary visual stimulus (2 Hz pulsed light) to examine the effects of prenatal VPA on neural processing. VPA rats showed greater increases in BOLD signal response to the social stimulus compared to controls in the temporal cortex, thalamus, midbrain and the hypothalamus. Blocking the V1a receptor reduced the BOLD response in VPA animals only. Neural responses to the visual stimulus, however, were lower in VPA animals. Blockade with the V1a antagonist did not revert this latter effect. Our data suggest that prenatal VPA affects the processing of social stimuli and perhaps social memory, partly through a mechanism that may involve vasopressin V1a neurotransmission
Sepharadim/conversos and premodern Global Hispanism
Sepharadim participated in the Hispanic vernacular culture of the Iberian Peninsula. Even in the time of al-Andalus many spoke Hispano-Romance, and even their Hebrew literature belies a deep familiarity with and love of their native Hispano-Romance languages. However, since the early sixteenth century the vast majority of Sepharadim have never lived in the Hispanic world. Sepharadim lived not in Spanish colonies defined by Spanish conquest, but in a network of Mediterranean Jewish communities defined by diasporic values and institutions.
By contrast, the conversos, those Sepharadim who converted to Catholicism, whether in Spain or later in Portugal, Italy, or the New World, lived mostly in Spanish Imperial lands, were officially Catholic, and spoke normative Castilian. Their connections, both real and imagined, with Sephardic cultural practice put them at risk of social marginalization, incarceration, even death. Some were devout Catholics whose heritage and family history doomed them to these outcomes. Not surprisingly, many Spanish and Portugese conversos sought refuge in lands outside of Spanish control where they might live openly as Jews.
This exodus (1600s) from the lands formerly known as Sefarad led to a parallel Sephardic community of what conversos who re-embraced Judaism in Amsterdam and Italy by a generation of conversos trained in Spanish universities. The Sephardic/Converso cultural complex exceeds the boundaries of Spanish imperial geography, confuses Spanish, Portuguese, Catholic, and Jewish subjectivities, and defies traditional categories practiced in Hispanic studies, and are a unique example of the Global Hispanophone
Numerical hydrodynamics in general relativity
The current status of numerical solutions for the equations of ideal general
relativistic hydrodynamics is reviewed. With respect to an earlier version of
the article the present update provides additional information on numerical
schemes and extends the discussion of astrophysical simulations in general
relativistic hydrodynamics. Different formulations of the equations are
presented, with special mention of conservative and hyperbolic formulations
well-adapted to advanced numerical methods. A large sample of available
numerical schemes is discussed, paying particular attention to solution
procedures based on schemes exploiting the characteristic structure of the
equations through linearized Riemann solvers. A comprehensive summary of
astrophysical simulations in strong gravitational fields is presented. These
include gravitational collapse, accretion onto black holes and hydrodynamical
evolutions of neutron stars. The material contained in these sections
highlights the numerical challenges of various representative simulations. It
also follows, to some extent, the chronological development of the field,
concerning advances on the formulation of the gravitational field and
hydrodynamic equations and the numerical methodology designed to solve them.Comment: 105 pages, 12 figures. The full online-readable version of this
article, including several animations, will be published in Living Reviews in
Relativity at http://www.livingreviews.or
Exploring new physics frontiers through numerical relativity
The demand to obtain answers to highly complex problems within strong-field gravity has been met with significant progress in the numerical solution of Einstein's equations - along with some spectacular results - in various setups. We review techniques for solving Einstein's equations in generic spacetimes, focusing on fully nonlinear evolutions but also on how to benchmark those results with perturbative approaches. The results address problems in high-energy physics, holography, mathematical physics, fundamental physics, astrophysics and cosmology
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