843 research outputs found
Gravitational Effects in Supersymmetric Domain Wall Backgrounds
A recent study of supersymmetric domain walls in 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
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
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
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
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
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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