1,181 research outputs found
Partially locally rotationally symmetric perfect fluid cosmologies
We show that there are no new consistent cosmological perfect fluid solutions
when in an open neighbourhood of an event the fluid kinematical
variables and the electric and magnetic Weyl curvature are all assumed
rotationally symmetric about a common spatial axis, specialising the Weyl
curvature tensor to algebraic Petrov type D. The consistent solutions of this
kind are either locally rotationally symmetric, or are subcases of the Szekeres
dust models. Parts of our results require the assumption of a barotropic
equation of state. Additionally we demonstrate that local rotational symmetry
of perfect fluid cosmologies follows from rotational symmetry of the Riemann
curvature tensor and of its covariant derivatives only up to second order, thus
strengthening a previous result.Comment: 20 pages, LaTeX2.09 (10pt), no figures; shortened revised version,
new references; accepted for publication in Classical and Quantum Gravit
Fluid moment hierarchy equations derived from quantum kinetic theory
A set of quantum hydrodynamic equations are derived from the moments of the
electrostatic mean-field Wigner kinetic equation. No assumptions are made on
the particular local equilibrium or on the statistical ensemble wave functions.
Quantum diffraction effects appear explicitly only in the transport equation
for the heat flux triad, which is the third-order moment of the Wigner
pseudo-distribution. The general linear dispersion relation is derived, from
which a quantum modified Bohm-Gross relation is recovered in the long
wave-length limit. Nonlinear, traveling wave solutions are numerically found in
the one-dimensional case. The results shed light on the relation between
quantum kinetic theory, the Bohm-de Broglie-Madelung eikonal approach, and
quantum fluid transport around given equilibrium distribution functions.Comment: 5 pages, three figures, uses elsarticle.cl
Fluid moment hierarchy equations derived from gauge invariant quantum kinetic theory
The gauge invariant electromagnetic Wigner equation is taken as the basis for
a fluid-like system describing quantum plasmas, derived from the moments of the
gauge invariant Wigner function. The use of the standard, gauge dependent
Wigner function is shown to produce inconsistencies, if a direct correspondence
principle is applied. The propagation of linear transverse waves is considered
and shown to be in agreement with the kinetic theory in the long wavelength
approximation, provided an adequate closure is chosen for the macroscopic
equations. A general recipe to solve the closure problem is suggested.Comment: 12 pages, 1 figur
Photon-graviton pair conversion
We consider the conversion of gravitons and photons as a four-wave mixing
process. A nonlinear coupled systems of equations involving two gravitons and
two photons is obtained, and the energy exchange between the different degrees
of freedom is found. The scattering amplitudes are obtained, from which a
crossection for incoherent processes can be found. An analytical example is
given, and applications to the early Universe are discussed.Comment: 5 pages, slightly modified as compared to v1, to appear in Class.
Quantum Grav. as a Letter to the Edito
Fermat's principle and variational analysis of an optical model for light propagation exhibiting a critical radius
Fermat's principle and variational analysis is used to analyze the
trajectories of light propagating in a radially inhomogeneous medium with a
singularity in the center. It is found that the light trajectories are similar
to those around a black hole, in the sense that there exists a critical radius
within which the light cannot escape, but spirals into the singularity.Comment: 5 pages from tex file + 3 figures (eps files
Axistationary perfect fluids -- a tetrad approach
Stationary axisymmetric perfect fluid space-times are investigated using the
curvature description of geometries. Attention is focused on space-times with a
vanishing electric part of the Weyl tensor. It is shown that the only
incompressible axistationary magnetic perfect fluid is the interior
Schwarzschild solution. The existence of a rigidly rotating perfect fluid,
generalizing the interior Schwarzschild metric is proven. Theorems are stated
on Petrov types and electric/magnetic Weyl tensors.Comment: 12 page
Oral Appliance Treatment for Obstructive Sleep Apnea: An Update
Oral appliances (OA) have emerged as an alternative to continuous positive airway pressure (CPAP) for obstructive sleep apnea (OSA) treatment. The most commonly used OA reduces upper airway collapse by advancing the mandible (OAm). There is a strong evidence base demonstrating OAm improve OSA in the majority of patients, including some with more severe disease. However OAm are not efficacious for all, with approximately one-third of patients experiencing no therapeutic benefit. OAm are generally well tolerated, although short-term adverse effects during acclimatization are common. Long-term dental changes do occur, but these are for the most part subclinical and do not preclude continued use. Patients often prefer OAm to gold-standard CPAP treatment. Head-to-head trials confirm CPAP is superior in reducing OSA parameters on polysomnography; however, this greater efficacy does not necessarily translate into better health outcomes in clinical practice. Comparable effectiveness of OAm and CPAP has been attributed to higher reported nightly use of OAm, suggesting that inferiority in reducing apneic events may be counteracted by greater treatment adherence. Recently, significant advances in commercially available OAm technologies have been made. Remotely controlled mandibular positioners have the potential to identify treatment responders and the level of therapeutic advancement required in single night titration polysomnography. Objective monitoring of OAm adherence using small embedded temperature sensing data loggers is now available and will enhance clinical practice and research. These technologies will further enhance efficacy and effectiveness of OAm treatment for OSA
Higher-order nonlinear electron-acoustic solitary excitations in partially degenerate quantum electron-ion plasmas
Propagation of dressed solitary excitations are studied in a partially
degenerate quantum plasma in the framework of quantum-hydrodynamics (QHD) model
using multiple scales technique. The evolution equation together with a linear
inhomogeneous differential equation is solved using Kodama-Taniuti
renormalizing technique. It is shown that the type of solitary excitations
(bright or dark) is defined by two critical plasma parameter values.Comment: To appear in Indian Journal of Physic
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