4,126 research outputs found
Spectrum and Statistical Entropy of AdS Black Holes
Popular approaches to quantum gravity describe black hole microstates
differently and apply different statistics to count them. Since the
relationship between the approaches is not clear, this obscures the role of
statistics in calculating the black hole entropy. We address this issue by
discussing the entropy of eternal AdS black holes in dimension four and above
within the context of a midisuperspace model. We determine the black hole
eigenstates and find that they describe the quantization in half integer units
of a certain function of the Arnowitt-Deser-Misner (ADM) mass and the
cosmological constant. In the limit of a vanishing cosmological constant (the
Schwarzschild limit) the quantized function becomes the horizon area and in the
limit of a large cosmological constant it approaches the ADM mass of the black
holes. We show that in the Schwarzschild limit the area quatization leads to
the Bekenstein-Hawking entropy if Boltzmann statistics are employed. In the
limit of a large cosmological constant the Bekenstein-Hawking entropy can be
recovered only via Bose statistics. The two limits are separated by a first
order phase transition, which seems to suggest a shift from "particle-like"
degrees of freedom at large cosmological constant to geometric degrees of
freedom as the cosmological constant approaches zero.Comment: 14 pages. No figures. Some references added. Version to appear in
Phys. Rev.
Reflection and Transmission at the Apparent Horizon during Gravitational Collapse
We examine the wave-functionals describing the collapse of a self-gravitating
dust ball in an exact quantization of the gravity-dust system. We show that
ingoing (collapsing) dust shell modes outside the apparent horizon must
necessarily be accompanied by outgoing modes inside the apparent horizon, whose
amplitude is suppressed by the square root of the Boltzmann factor at the
Hawking temperature. Likewise, ingoing modes in the interior must be
accompanied by outgoing modes in the exterior, again with an amplitude
suppressed by the same factor. A suitable superposition of the two solutions is
necessary to conserve the dust probability flux across the apparent horizon,
thus each region contains both ingoing and outgoing dust modes. If one
restricts oneself to considering only the modes outside the apparent horizon
then one should think of the apparent horizon as a partial reflector, the
probability for a shell to reflect being given by the Boltzmann factor at the
Hawking temperature determined by the mass contained within it. However, if one
considers the entire wave function, the outgoing wave in the exterior is seen
to be the transmission through the horizon of the interior outgoing wave that
accompanies the collapsing shells. This transmission could allow information
from the interior to be transferred to the exterior.Comment: 19 pages, no figures. To appear in Phys. Rev.
The Quantum Stress-Tensor in Self-Similar Spherical Dust Collapse
We calculate the quantum stress tensor for a massless scalar field in the 2-d
self-similar spherical dust collapse model which admits a naked singularity. We
find that the outgoing radiation flux diverges on the Cauchy horizon. This may
have two consequences. The resultant back reaction may prevent the naked
singularity from forming, thus preserving cosmic censorship through quantum
effects. The divergent flux may lead to an observable signature differentiating
naked singularities from black holes in astrophysical observations.Comment: Latex File, 19 page
Model systems, lipid rafts, and cell membranes
Views of how cell membranes are organized are presently changing. The lipid bilayer that constitutes these membranes is no longer understood to be a homogeneous fluid. Instead, lipid assemblies, termed rafts, have been introduced to provide fluid platforms that segregate membrane components and dynamically compartmentalize membranes. These assemblies are thought to be composed mainly of sphingolipids and cholesterol in the outer leaflet, somehow connected to domains of unknown composition in the inner leaflet. Specific classes of proteins are associated with the rafts. This review critically analyzes what is known of phase behavior and liquid-liquid immiscibility in model systems and compares these data with what is known of domain formation in cell membranes
O acervo raro da biblioteca Milton de Albuquerque, da Embrapa Amazônia Oriental.
bitstream/item/63592/1/Oriental-Doc187.PD
Técnicas laboratoriais para isolamento de Campylobacter em material avícola.
Projeto/Plano de Ação: 03.08.06.004
Uso de tomógrafos de raios-X em resolução nanométrica e micrométrica para investigar, em duas e três dimensões, o espaço poroso e a água no interior de amostras de solo.
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