4,756 research outputs found
Causal Bulk Viscous Dissipative Isotropic Cosmologies with Variable Gravitational and Cosmological Constants
We consider the evolution of a flat Friedmann-Robertson-Walker Universe,
filled with a causal bulk viscous cosmological fluid, in the presence of
variable gravitational and cosmological constants. The basic equation for the
Hubble parameter, generalizing the evolution equation in the case of constant
gravitational coupling and cosmological term, is derived, under the
supplementary assumption that the total energy of the Universe is conserved. By
assuming that the cosmological constant is proportional to the square of the
Hubble parameter and a power law dependence of the bulk viscosity coefficient,
temperature and relaxation time on the energy density of the cosmological
fluid, two classes of exact solutions of the field equations are obtained. In
the first class of solutions the Universe ends in an inflationary era, while in
the second class of solutions the expansion of the Universe is non-inflationary
for all times. In both models the cosmological "constant" is a decreasing
function of time, while the gravitational "constant" increases in the early
period of evolution of the Universe, tending in the large time limit to a
constant value.Comment: 14 pages, 15 figure
An event-related potential study on the information flow during prospective memory interference
Poster PresentationBACKGROUND: Electroencephalography (EEG) can examine the temporal sequence of brain activity related to a specific event, which is referred to as event-related potential (ERP). Prospective memory (PM) interference presents the interference effect of an embedded PM task on other ongoing task. In the previous functional MRI study, we found that the fusiform gyrus, left inferior parietal lobe and left frontal lobe play an essential role in PM interference effect. However, little temporal information could be retrieved from the functional MRI …published_or_final_versio
Extremely Sub-wavelength Planar Magnetic Metamaterials
We present highly sub-wavelength magnetic metamaterials designed for
operation at radio frequencies (RFs). A dual layer design consisting of
independent planar spiral elements enables experimental demonstration of a unit
cell size (a) that is ~ 700 times smaller than the resonant wavelength
({\lambda}0). Simulations indicate that utilization of a conductive via to
connect spiral layers permits further optimization and we achieve a unit cell
that is {\lambda}0/a ~ 2000. Magnetic metamaterials are characterized by a
novel time domain method which permits determination of the complex magnetic
response. Numerical simulations are performed to support experimental data and
we find excellent agreement. These new designs make metamaterial low frequency
experimental investigations practical and suggest their use for study of
magneto-inductive waves, levitation, and further enable potential RF
applications.Comment: 5 pages, 4 figure
Calibrating rectangular interferometer meshes with external photodetectors
Multiport interferometer meshes can be used to implement unitary transformations on input vectors of light in both the classical and quantum domain. In practice, the phase-shifters in a mesh photonic circuit must be calibrated to compensate for phase errors due to fabrication variations. Calibration using photodetectors external to the mesh has been demonstrated for triangular meshes, but not rectangular meshes. Here, we propose an algorithm for the calibration of rectangular meshes using only external photodetectors and simulate it to evaluate its feasibility
Stochastic Cutoff Method for Long-Range Interacting Systems
A new Monte-Carlo method for long-range interacting systems is presented.
This method consists of eliminating interactions stochastically with the
detailed balance condition satisfied. When a pairwise interaction of a
-particle system decreases with the distance as ,
computational time per one Monte Carlo step is for
and for , where is the spatial
dimension. We apply the method to a two-dimensional magnetic dipolar system.
The method enables us to treat a huge system of spins with reasonable
computational time, and reproduces a circular order originated from long-range
dipolar interactions.Comment: 18 pages, 9 figures, 1 figure and 1 reference are adde
Renormalization Group Approach to Causal Viscous Cosmological Models
The renormalization group method is applied to the study of homogeneous and
flat Friedmann-Robertson-Walker type Universes, filled with a causal bulk
viscous cosmological fluid. The starting point of the study is the
consideration of the scaling properties of the gravitational field equations,
of the causal evolution equation of the bulk viscous pressure and of the
equations of state. The requirement of scale invariance imposes strong
constraints on the temporal evolution of the bulk viscosity coefficient,
temperature and relaxation time, thus leading to the possibility of obtaining
the bulk viscosity coefficient-energy density dependence. For a cosmological
model with bulk viscosity coefficient proportional to the Hubble parameter, we
perform the analysis of the renormalization group flow around the scale
invariant fixed point, therefore obtaining the long time behavior of the scale
factor.Comment: 19 pages. RevTeX4. Revised version. Accepted in Classical and Quantum
Gravit
Viscous Bianchi type I universes in brane cosmology
We consider the dynamics of a viscous cosmological fluid in the generalized
Randall-Sundrum model for an anisotropic, Bianchi type I brane. To describe the
dissipative effects we use the Israel-Hiscock-Stewart full causal thermodynamic
theory. By assuming that the matter on the brane obeys a linear barotropic
equation of state, and the bulk viscous pressure has a power law dependence on
the energy density, the general solution of the field equations can be obtained
in an exact parametric form. The obtained solutions describe generally a
non-inflationary brane world. In the large time limit the brane Universe
isotropizes, ending in an isotropic and homogeneous state. The evolution of the
temperature and of the comoving entropy of the Universe is also considered, and
it is shown that due to the viscous dissipative processes a large amount of
entropy is created in the early stages of evolution of the brane world.Comment: 13 pages, 5 figures, to appear in Class. Quantum Gra
Slow-light optical bullets in arrays of nonlinear Bragg-grating waveguides
We demonstrate how to control independently both spatial and temporal
dynamics of slow light. We reveal that specially designed nonlinear waveguide
arrays with phase-shifted Bragg gratings demonstrate the frequency-independent
spatial diffraction near the edge of the photonic bandgap, where the group
velocity of light can be strongly reduced. We show in numerical simulations
that such structures allow a great flexibility in designing and controlling
dispersion characteristics, and open a way for efficient spatiotemporal
self-trapping and the formation of slow-light optical bullets.Comment: 4 pages, 4 figures; available from
http://link.aps.org/abstract/PRL/v97/e23390
A solution for estimating the tensile yield strength from small specimens
The small punch test is an innovative test that utilises small disc-shaped specimens to assess the mechanical behaviour of materials. The main advantage is the relatively small specimen size. In this article, a modified analytical solution for the small punch maximum bend strength is proposed that is based on classical plate theory. A clear linear relationship is observed between the tensile yield strength σYS and the small punch maximum bend strength σy for both alloys and metal matrix composites. Copyright by ASTM Int'l (all rights reserved)
Cachexia and protein-energy wasting in children with chronic kidney disease
Children with chronic kidney disease (CKD) are at risk for “cachexia” or “protein-energy wasting” (PEW). These terms describe a pathophysiologic process resulting in the loss of muscle, with or without loss of fat, and involving maladaptive responses, including anorexia and elevated metabolic rate. PEW has been defined specifically in relation to CKD. We review the diagnostic criteria for cachexia and PEW in CKD and consider the limitations and applicability of these criteria to children with CKD. In addition, we present an overview of the manifestations and mechanisms of cachexia and PEW. A host of pathogenetic factors are considered, including systemic inflammation, endocrine perturbations, and abnormal neuropeptide signaling, as well as poor nutritional intake. Mortality risk, which is 100- to 200-fold higher in patients with end-stage renal disease than in the general population, is strongly correlated with the components of cachexia/PEW. Further research into the causes and consequences of wasting and growth retardation is needed in order to improve the survival and quality of life for children with CKD
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