577 research outputs found
Interacting quintessence and the coincidence problem
We investigate the role of a possible coupling of dark matter and dark
energy. In particular, we explore the consequences of such an interaction for
the coincidence problem, i.e., for the question, why the energy densities of
dark matter and dark energy are of the same order just at the present epoch. We
demonstrate, that, with the help of a suitable coupling, it is possible to
reproduce any scaling solution , where is the
scale factor of the Robertson-Walker metric and is a constant parameter.
and are the densities of dark energy and dark matter,
respectively. Furthermore, we show that an interaction between dark matter and
dark energy can drive the transition from an early matter dominated era to a
phase of accelerated expansion with a stable, stationary ratio of the energy
densities of both components.Comment: 3 pages, contribution to the Tenth Marcel Grossmann Meeting, Rio de
Janeiro, 20-26 July 200
Deuteron Matrix Elements in Chiral Effective Theory at Leading Order
We consider matrix elements of two-nucleon operators that arise in chiral
effective theories of the two-nucleon system. Generically, the short-distance
piece of these operators scales as 1/r^n, with r the relative separation of the
two nucleons. We show that, when evaluated between the leading-order wave
functions obtained in this effective theory, these two-nucleon operators are
independent of the cutoff used to renormalize the two-body problem for n=1 and
2. However, for n greater than or equal to 3 general arguments about the
short-distance behavior of the leading-order deuteron wave function show that
the matrix element will diverge.Comment: 7 pages, 5 .eps figure
On the stochastic mechanics of the free relativistic particle
Given a positive energy solution of the Klein-Gordon equation, the motion of
the free, spinless, relativistic particle is described in a fixed Lorentz frame
by a Markov diffusion process with non-constant diffusion coefficient. Proper
time is an increasing stochastic process and we derive a probabilistic
generalization of the equation . A
random time-change transformation provides the bridge between the and the
domain. In the domain, we obtain an \M^4-valued Markov process
with singular and constant diffusion coefficient. The square modulus of the
Klein-Gordon solution is an invariant, non integrable density for this Markov
process. It satisfies a relativistically covariant continuity equation
Controller Coordination Strategy for DC Microgrid Using Distributed Predictive Control Improving Voltage Stability
The paper presents the design and control strategy of an isolated DC microgrid, which is based on classical control techniques, predictive control and iterative algorithms. The design control parameters are maximum overshoot, settling time and voltage ripple. The strategy is designed to operate in two different modes, end-users minimum and maximum demand scenarios, and this is achieved through the incorporation of network dynamic loads. The control methodology developed allows to obtain a fast response of the design set points, and an efficient control for disturbance rejection. The simulation results obtained satisfy the proposed design guidelines by obtaining a maximum overshoot of 4.8%, settling time of 0.012 seconds and a voltage ripple of 0.1 percentage. The implemented system simulation was developed in Matlab-Simulink software
Associations between body composition and bone health in children and adolescents : a systematic review
More clarification on the associations between children's and adolescents' lean and fat mass (LM and FM) on the one hand and their bone health on the other hand is needed, given the rising prevalence of overweight and obesity in this population. This systematic literature review aimed to describe the current evidence on these associations. Data sources were Medline/PubMed, EMBASE, CINAHL and The Cochrane Library (up to November 2014). Search items included LM, FM, children and adolescents (0-18 years), bone health measured with dual-energy X-ray absorptiometry and peripheral quantitative computed tomography (pQCT) and search items concerning study design: observational and longitudinal studies. The study populations were healthy children and adolescents including obese children. Children with other diseases and clinical series of study subjects were excluded. Based on the studies included in this review (n = 19), there is a consensus that the contribution of LM to the variance of the different bone parameters is larger than the contribution of FM and that an increase in LM is associated with an increase in bone parameters. Most of the studies indicated that the increase in bone parameters seen in overweight and obese children and adolescents is due to an increase in LM and not to greater FM. The results on the association between body fat and bone parameters were contradictory and depended on children's age and sex. Still more data from studies with a longitudinal study design using (high resolution) pQCT and a representative sample are needed to get further insight in the associations between body fat and bone parameters in children, specifically concerning differences in sex, skeletal site and fat depots
Cosmological solutions with nonlinear bulk viscosity
A recently proposed nonlinear transport equation is used to model bulk
viscous cosmologies that may be far from equilibrium, as happens during viscous
fluid inflation or during reheating. The asymptotic stability of the de Sitter
and Friedmann solutions is investigated. The former is stable for bulk
viscosity index . New solutions are obtained in
the weakly nonlinear regime for . These solutions are singular and some of
them represent a late-time inflationary era.Comment: 16 pages Latex (IOP style); to appear Class. Quantum Gra
Stable Inflationary Dissipative Cosmologies
The stability of the de Sitter era of cosmic expansion in spatially curved
homogeneous isotropic universes is studied. The source of the gravitational
field is an imperfect fluid such that the parameters that characterize it may
change with time. In this way we extend our previous analysis for
spatially-flat spaces as well as the work of Barrow.Comment: 13 pages, LaTeX 2.09, 1 figure. To be published in International
Journal of Modern Physics
Lemaitre-Tolman-Bondi dust spacetimes: Symmetry properties and some extensions to the dissipative case
We consider extensions of Lemaitre-Tolman-Bondi (LTB) spacetimes to the
dissipative case. For doing that we previously carry out a systematic study on
LTB. This study is based on two different aspects of LTB. On the one hand, a
symmetry property of LTB will be presented. On the other hand, the description
of LTB in terms of some fundamental scalar functions (structure scalars)
appearing in the orthogonal splitting of Riemann tensor will be provided. We
shall consider as "natural" generalizations of LTB (hereafter referred to as
GLTB) either those metrics admitting some similar kind of symmetry as LTB, or
those sharing structure scalars with similar dependence on the metric.Comment: 13 pages RevTex. To appear in Phys. Rev. D. Some references corrected
and update
Thermodynamics of a black hole in a cavity
We present a unified thermodynamical description of the configurations
consisting on self-gravitating radiation with or without a black hole. We
compute the thermal fluctuations and evaluate where will they induce a
transition from metastable configurations towards stable ones. We show that the
probability of finding such a transition is exponentially small. This indicates
that, in a sequence of quasi equilibrium configurations, the system will remain
in the metastable states till it approaches very closely the critical point
beyond which no metastable configuration exists. Near that point, we relate the
divergence of the local temperature fluctuations to the approach of the
instability of the whole system, thereby generalizing the usual fluctuations
analysis in the cases where long range forces are present. When angular
momentum is added to the cavity, the above picture is slightly modified.
Nevertheless, at high angular momentum, the black hole loses most of its mass
before it reaches the critical point at which it evaporates completely.Comment: 27 pages, latex file, contains 3 figures available on request at
[email protected]
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
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