839 research outputs found

    Gravitational Effects in Supersymmetric Domain Wall Backgrounds

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    A recent study of supersymmetric domain walls in N=1N=1 supergravity theories revealed a new class of domain walls interpolating between supersymmetric vacua with different non-positive cosmological constants. We classify three classes of domain wall configurations and study the geodesic structure of the induced space-time. Motion of massive test particles in such space-times shows that these walls are always repulsive from the anti-deSitter (AdS) side, while on the Minkowski side test particles feel no force. Freely falling particles far away from a wall in an AdS vacuum experience a constant proper acceleration, \ie\ they are Rindler particles. A new coordinate system for discussing AdS space-time is presented which eliminates the use of a periodic time-like coordinate.Comment: 13 pages + 4 figures (not included

    Relativistic Diskoseismology

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    We will summarize results of calculations of the modes of oscillation trapped within the inner region of accretion disks by the strong-field gravitational properties of a black hole (or a compact, weakly-magnetized neutron star). Their driving and damping will also be addressed. The focus will be on the most observable class: the analogue of internal gravity modes in stars. Their frequencies which corrrespond to the lowest mode numbers depend almost entirely upon only the mass and angular momentum of the black hole. Such a feature may have been detected in the X-ray power spectra of two galactic `microquasars', allowing the angular momentum of the black hole to be determined in one case.Comment: To be published in Physics Reports, proceedings of the conference Astrophysical Fluids: From Atomic Nuclei to Stars and Galaxies; 10 pages, 5 postscript figure

    Relativistic Stellar Pulsations With Near-Zone Boundary Conditions

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    A new method is presented here for evaluating approximately the pulsation modes of relativistic stellar models. This approximation relies on the fact that gravitational radiation influences these modes only on timescales that are much longer than the basic hydrodynamic timescale of the system. This makes it possible to impose the boundary conditions on the gravitational potentials at the surface of the star rather than in the asymptotic wave zone of the gravitational field. This approximation is tested here by predicting the frequencies of the outgoing non-radial hydrodynamic modes of non-rotating stars. The real parts of the frequencies are determined with an accuracy that is better than our knowledge of the exact frequencies (about 0.01%) except in the most relativistic models where it decreases to about 0.1%. The imaginary parts of the frequencies are determined with an accuracy of approximately M/R, where M is the mass and R is the radius of the star in question.Comment: 10 pages (REVTeX 3.1), 5 figs., 1 table, fixed minor typos, published in Phys. Rev. D 56, 2118 (1997

    Puncture of gravitating domain walls

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    We investigate the semi-classical instability of vacuum domain walls to processes where the domain walls decay by the formation of closed string loop boundaries on their worldvolumes. Intuitively, a wall which is initially spherical may `pop', so that a hole corresponding to a string boundary component on the wall, may form. We find instantons, and calculate the rates, for such processes. We show that after puncture, the hole grows exponentially at the same rate that the wall expands. It follows that the wall is never completely thermalized by a single expanding hole; at arbitrarily late times there is still a large, thin shell of matter which may drive an exponential expansion of the universe. We also study the situation where the wall is subjected to multiple punctures. We find that in order to completely annihilate the wall by this process, at least four string loops must be nucleated. We argue that this process may be relevant in certain brane-world scenarios, where the universe itself is a domain wall.Comment: 13 pages REVTeX, 3 .ps figures, added some references - version to appear in Physics Letters

    The relationship between impulsivity, affect and a history of psychological adversity: a cognitive-affective neuroscience approach

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    There is increasing evidence that trauma exposure is associated with impulsive behaviour and difficulties regulating affect. The findings of recent studies implicate the disruption of neurobiological mechanisms, particularly those involving the neurotransmitter serotonin, in both impulsivity and affect regulation

    Inducing a Concurrent Motor Load Reduces Categorization Precision for Facial Expressions

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    Motor theories of expression perception posit that observers simulate facial expressions within their own motor system, aiding perception and interpretation. Consistent with this view, reports have suggested that blocking facial mimicry induces expression labeling errors and alters patterns of ratings. Crucially, however, it is unclear whether changes in labeling and rating behavior reflect genuine perceptual phenomena (e.g., greater internal noise associated with expression perception or interpretation) or are products of response bias. In an effort to advance this literature, the present study introduces a new psychophysical paradigm for investigating motor contributions to expression perception that overcomes some of the limitations inherent in simple labeling and rating tasks. Observers were asked to judge whether smiles drawn from a morph continuum were sincere or insincere, in the presence or absence of a motor load induced by the concurrent production of vowel sounds. Having confirmed that smile sincerity judgments depend on cues from both eye and mouth regions (Experiment 1), we demonstrated that vowel production reduces the precision with which smiles are categorized (Experiment 2). In Experiment 3, we replicated this effect when observers were required to produce vowels, but not when they passively listened to the same vowel sounds. In Experiments 4 and 5, we found that gender categorizations, equated for difficulty, were unaffected by vowel production, irrespective of the presence of a smiling expression. These findings greatly advance our understanding of motor contributions to expression perception and represent a timely contribution in light of recent high-profile challenges to the existing evidence base
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