2,723 research outputs found
Gravitational radiation reaction in compact binary systems: Contribution of the quadrupole-monopole interaction
The radiation reaction in compact spinning binaries on eccentric orbits due
to the quadrupole-monopole interaction is studied. This contribution is of
second post-Newtonian order. As result of the precession of spins the magnitude
of the orbital angular momentum is not conserved. Therefore a proper
characterization of the perturbed radial motion is provided by the energy
and angular average . As powerful computing tools, the generalized
true and eccentric anomaly parametrizations are introduced. Then the secular
losses in energy and magnitude of orbital angular momentum together with the
secular evolution of the relative orientations of the orbital angular momentum
and spins are found for eccentric orbits by use of the residue theorem. The
circular orbit limit of the energy loss agrees with Poisson's earlier result.Comment: accepted for publication in Phys. Rev.
Friedmann branes with variable tension
We introduce brane-worlds with non-constant tension, strenghtening the
analogy with fluid membranes, which exhibit a temperature-dependence according
to the empirical law established by E\"otv\"os. This new degree of freedom
allows for evolving gravitational and cosmological constants, the latter being
a natural candidate for dark energy. We establish the covariant dynamics on a
brane with variable tension in full generality, by considering asymmetrically
embedded branes and allowing for non-standard model fields in the 5-dimensional
space-time. Then we apply the formalism for a perfect fluid on a Friedmann
brane, which is embedded in a 5-dimensional charged Vaidya-Anti de Sitter
space-time.Comment: 12 pages, to appear in Phys. Rev.
Do 18-month-olds really attribute mental states to others? A critical test
In the research reported here, we investigated whether 18-month-olds would use their own past experience of visual access to attribute perception and consequent beliefs to other people. Infants in this study wore either opaque blindfolds (opaque condition) or trick blindfolds that looked opaque but were actually transparent (trick condition). Then both groups of infants observed an actor wearing one of the same blindfolds that they themselves had experienced, while a puppet removed an object from its location. Anticipatory eye movements revealed that infants who had experienced opaque blindfolds expected the actor to behave in accordance with a false belief about the object's location, but that infants who had experienced trick blindfolds did not exhibit that expectation. Our results suggest that 18-month-olds used self-experience with the blindfolds to assess the actor's visual access and to update her belief state accordingly. These data constitute compelling evidence that 18-month-olds infer perceptual access and appreciate its causal role in altering the epistemic states of other people
Gravitational radiation reaction in compact binary systems: Contribution of the magnetic dipole-magnetic dipole interaction
We study the gravitational radiation reaction in compact binary systems
composed of neutron stars with spin and huge magnetic dipole moments
(magnetars). The magnetic dipole moments undergo a precessional motion about
the respective spins. At sufficiently high values of the magnetic dipole
moments, their interaction generates second post-Newtonian order contributions
both to the equations of motion and to the gravitational radiation escaping the
system. We parametrize the radial motion and average over a radial period in
order to find the secular contributions to the energy and magnitude of the
orbital angular momentum losses, in the generic case of \textit{eccentric}
orbits. Similarly as for the spin-orbit, spin-spin, quadrupole-monopole
interactions, here too we deduce the secular evolution of the relative
orientations of the orbital angular momentum and spins. These equations,
supplemented by the evolution equations for the angles characterizing the
orientation of the dipole moments form a first order differential system, which
is closed. The circular orbit limit of the energy loss agrees with Ioka and
Taniguchi's earlier result
Spectrum of stochastic evolution operators: Local matrix representation approach
A matrix representation of the evolution operator associated with a nonlinear
stochastic flow with additive noise is used to compute its spectrum. In the
weak noise limit a perturbative expansion for the spectrum is formulated in
terms of local matrix representations of the evolution operator centered on
classical periodic orbits. The evaluation of perturbative corrections is easier
to implement in this framework than in the standard Feynman diagram
perturbation theory. The result are perturbative corrections to a stochastic
analog of the Gutzwiller semiclassical spectral determinant computed to several
orders beyond what has so far been attainable in stochastic and
quantum-mechanical applications.Comment: 7 pages, 2 figures, Third approach to a problem we considered in
chao-dyn/9807034 and chao-dyn/981100
Asymmetric Swiss-cheese brane-worlds
We study a brane-world cosmological scenario with local inhomogeneities
represented by black holes. The brane is asymmetrically embedded into the bulk.
The black strings/cigars penetrating the Friedmann brane generate a
Swiss-cheese type structure. This universe forever expands and decelerates, as
its general relativistic analogue. The evolution of the cosmological fluid
however can proceed along four branches, two allowed to have positive energy
density, one of them having the symmetric embedding limit. On this branch a
future pressure singularity can arise for either (a) a difference in the
cosmological constants of the cosmological and black hole brane regions (b) a
difference in the left and right bulk cosmological constants. While the
behaviour (a) can be avoided by a redefinition of the fluid variables, (b)
establishes a critical value of the asymmetry over which the pressure
singularity occurs. We introduce the pressure singularity censorship which
bounds the degree of asymmetry in the bulk cosmological constant. We also show
as a model independent generic feature that the asymmetry source term due to
the bulk cosmological constant increases in the early universe. In order to
obey the nucleosynthesis constraints, the brane tension should be constrained
therefore both from below and from above. With the maximal degree of asymmetry
obeying the pressure singularity censorship, the higher limit is 10 times the
lower limit. The degree of asymmetry allowed by present cosmological
observations is however much less, pushing the upper limit to infinity.Comment: v2: considerably expanded, 19 pages, 8 figures, many new references.
Pressure singularity censorship introduced, strict limits on the possible
degree of asymmetry derived. v3: model independent analysis shows that the
asymmetry bounds the brane tension from above. Limits on the maximal tension
set. Version published in JCA
Semi-transparent brane-worlds
We study the evolution of a closed Friedmann brane perturbed by the Hawking
radiation escaping a bulk black hole. The semi-transparent brane absorbes some
of the infalling radiation, the rest being transmitted across the brane to the
other bulk region. We characterize the cosmological evolution in terms of the
transmission rate . For small values of a critical-like
behaviour could be observed, when the acceleration due to radiation pressure
and the deceleration induced by the increasing self-gravity of the brane
roughly compensate each other, and cosmological evolution is approximately the
same as without radiation. Lighter (heavier) branes than those with the
critical energy density will recollapse slower (faster). This feature is
obstructed at high values of , where the overall effect of the
radiation is to speed-up the recollapse. We determine the maximal value of the
transmission rate for which the critical-like behaviour is observed. We also
study the effect of transmission on the evolution of different source terms of
the Friedmann equation. We conclude that among all semi-transparent branes the
slowest recollapse occurs for light branes with total absorption.Comment: 15 pages, 8 figure
Linear Einstein equations and Kerr-Schild maps
We prove that given a solution of the Einstein equations for the
matter field , an autoparallel null vector field and a solution
of the linearized Einstein equation on the
given background, the Kerr-Schild metric ( arbitrary constant) is an exact solution of the Einstein equation for the
energy-momentum tensor . The mixed form of the Einstein equation for
Kerr-Schild metrics with autoparallel null congruence is also linear. Some more
technical conditions hold when the null congruence is not autoparallel. These
results generalize previous theorems for vacuum due to Xanthopoulos and for
flat seed space-time due to G\"{u}rses and G\"{u}rsey.Comment: 9 pages, accepted by Class. Quant. Gra
SeqX: a tool to detect, analyze and visualize residue co-locations in protein and nucleic acid structures
BACKGROUND: The interacting residues of protein and nucleic acid sequences are close to each other â they are co-located. Structure databases (like Protein Data Bank, PDB and Nucleic Acid Data Bank, NDB) contain all information about these co-locations; however it is not an easy task to penetrate this complex information. We developed a JAVA tool, called SeqX for this purpose. RESULTS: SeqX tool is useful to detect, analyze and visualize residue co-locations in protein and nucleic acid structures. The user a. selects a structure from PDB; b. chooses an atom that is commonly present in every residues of the nucleic acid and/or protein structure(s) c. defines a distance from these atoms (3â15 Ă
). The SeqX tool detects every residue that is located within the defined distances from the defined "backbone" atom(s); provides a DotPlot-like visualization (Residues Contact Map), and calculates the frequency of every possible residue pairs (Residue Contact Table) in the observed structure. It is possible to exclude +/- 1 to 10 neighbor residues in the same polymeric chain from detection, which greatly improves the specificity of detections (up to 60% when tested on dsDNA). Results obtained on protein structures showed highly significant correlations with results obtained from literature (p < 0.0001, n = 210, four different subsets). The co-location frequency of physico-chemically compatible amino acids is significantly higher than is calculated and expected in random protein sequences (p < 0.0001, n = 80). CONCLUSION: The tool is simple and easy to use and provides a quick and reliable visualization and analyses of residue co-locations in protein and nucleic acid structures. AVAILABILITY AND REQUIREMENTS: SeqX, Java J2SE Runtime Environment 5.0 (available from [see Additional file 1] ) and at least a 1 GHz processor and with a minimum 256 Mb RAM. Source codes are available from the authors
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