272 research outputs found
Cosmological model with viscosity media (dark fluid) described by an effective equation of state
A generally parameterized equation of state (EOS) is investigated in the
cosmological evolution with bulk viscosity media modelled as dark fluid, which
can be regarded as a unification of dark energy and dark matter. Compared with
the case of the perfect fluid, this EOS has possessed four additional
parameters, which can be interpreted as the case of the non-perfect fluid with
time-dependent viscosity or the model with variable cosmological constant. From
this general EOS, a completely integrable dynamical equation to the scale
factor is obtained with its solution explicitly given out. (i) In this
parameterized model of cosmology, for a special choice of the parameters we can
explain the late-time accelerating expansion universe in a new view. The early
inflation, the median (relatively late time) deceleration, and the recently
cosmic acceleration may be unified in a single equation. (ii) A generalized
relation of the Hubble parameter scaling with the redshift is obtained for some
cosmology interests. (iii) By using the SNe Ia data to fit the effective
viscosity model we show that the case of matter described by plus with
effective viscosity contributions can fit the observational gold data in an
acceptable levelComment: 9 fig
Static stretching time required to reduce iliacus muscle stiffness
Static stretching (SS) is an effective intervention to reduce muscle stiffness and is also performed for the iliopsoas muscle. The iliopsoas muscle consists of the iliacus and psoas major muscles, among which the former has a greater physiological cross-sectional area and hip flexion moment arm. Static stretching time required to reduce muscle stiffness can differ among muscles, and the required time for the iliacus muscle remains unclear. The purpose of this study was to investigate the time required to reduce iliacus muscle stiffness. Twenty-six healthy men participated in this study. A 1-min hip extension SS was performed five times. Shear elastic modulus, an index of muscle stiffness, of the iliacus muscle was measured using ultrasonic shear wave elastography before SS and immediately after each SS. One-way repeated analysis of variance showed a statistical effect of time on the shear elastic modulus. A paired t-test with Holm adjustment revealed that the shear elastic moduli after 1â5 SS were statistically lower than that before SS. In addition, the shear elastic modulus after 5 SS was statistically lower than that after 1 SS. The results suggested that the stiffness of the iliacus muscle decreased with 1-min SS and further decreased with 5-min SS
Nonlocal Gravitational Models and Exact Solutions
A nonlocal gravity model with a function , where is
the d'Alembert operator, is considered. The algorithm, allowing to reconstruct
, corresponding to the given Hubble parameter and the state
parameter of the matter, is proposed. Using this algorithm, we find the
functions , corresponding to de Sitter solutions.Comment: 5 pages, v2: refs. added, to appear in the proceedings of the
International Workshop "Supersymmetries and Quantum Symmetries" (SQS'2011),
Dubna, Russia, July 18-23, 2011, http://theor.jinr.ru/sqs/2011
On bouncing solutions in non-local gravity
A non-local modified gravity model with an analytic function of the
d'Alembert operator is considered. This model has been recently proposed as a
possible way of resolving the singularities problem in cosmology. We present an
exact bouncing solution, which is simpler compared to the already known one in
this model in the sense it does not require an additional matter to satisfy all
the gravitational equations.Comment: 5 pages; v2: matching the jounral versio
Two-dimensional effective action for matter fields coupled to the dilaton
We revise the calculation of the one-loop effective action for scalar and
spinor fields coupled to the dilaton in two dimensions. Applying the method of
covariant perturbation theory for the heat kernel we derive the effective
action in an explicitly covariant form that produces both the conformally
invariant and the conformally anomalous terms.For scalar fields the conformally
invariant part of the action is nonlocal. The obtained effective action is
proved to be infrared finite. We also compute the one-loop effective action for
scalar fields at finite temperature.Comment: LaTeX, 25 page
Can universe exit from phantom inflation due to gravitational back reaction?
The effects of the gravitational back reaction of cosmological perturbations
are investigated in a phantom inflation model. The effective energy-momentum
tensor of the gravitational back reaction of cosmological perturbations whose
wavelengths are larger than the Hubble radius is calculated. Our results show
that the effects of gravitational back reaction will counteract that of the
phantom energy. It is demonstrated in a chaotic phantom inflation model that if
the phantom field at the end of inflation is larger than a critical value
determined by the necessary e-folds, the phantom inflation phase might be
terminated by the gravitational back reaction.Comment: 9 pages, Revtex4, to appear in JCA
Running coupling: Does the coupling between dark energy and dark matter change sign during the cosmological evolution?
In this paper we put forward a running coupling scenario for describing the
interaction between dark energy and dark matter. The dark sector interaction in
our scenario is free of the assumption that the interaction term is
proportional to the Hubble expansion rate and the energy densities of dark
sectors. We only use a time-variable coupling (with the scale factor
of the universe) to characterize the interaction . We propose a
parametrization form for the running coupling in which the
early-time coupling is given by a constant , while today the coupling is
given by another constant, . For investigating the feature of the running
coupling, we employ three dark energy models, namely, the cosmological constant
model (), the constant model (), and the time-dependent
model (). We constrain the models with the current
observational data, including the type Ia supernova, the baryon acoustic
oscillation, the cosmic microwave background, the Hubble expansion rate, and
the X-ray gas mass fraction data. The fitting results indicate that a
time-varying vacuum scenario is favored, in which the coupling crosses
the noninteracting line () during the cosmological evolution and the sign
changes from negative to positive. The crossing of the noninteracting line
happens at around , and the crossing behavior is favored at about
1 confidence level. Our work implies that we should pay more attention
to the time-varying vacuum model and seriously consider the phenomenological
construction of a sign-changeable or oscillatory interaction between dark
sectors.Comment: 8 pages, 5 figures; refs added; to appear in EPJ
Angle-resolved photoemission spectroscopy with an tunable magnetic field
Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for
probing the momentum-resolved single-particle spectral function of materials.
Historically, magnetic fields have been carefully avoided as
they are detrimental to the control of photoelectron trajectory during the
photoelectron detection process. However, magnetic field is an important
experimental knob for both probing and tuning symmetry-breaking phases and
electronic topology in quantum materials. In this paper, we introduce an easily
implementable method for realizing an tunable magnetic field
at the sample position in an ARPES experiment and analyze magnetic field
induced artifacts in ARPES data. Specifically, we identified and quantified
three distinct extrinsic effects of a magnetic field: Fermi surface rotation,
momentum shrinking, and momentum broadening. We examined these effects in three
prototypical quantum materials, i.e., a topological insulator (BiSe),
an iron-based superconductor (LiFeAs), and a cuprate superconductor
(BiSrCaCuO), and demonstrate the feasibility of ARPES
measurements in the presence of a controllable magnetic field. Our studies lay
the foundation for the future development of the technique and interpretation
of ARPES measurements of field-tunable quantum phases.Comment: 23 pages, 6 figure
Tensor perturbations of -branes
We analyze the tensor perturbations of flat thick domain wall branes in
gravity. Our results indicate that under the transverse and traceless
gauge, the metric perturbations decouple from the perturbation of the scalar
field. Besides, the perturbed equation reduces to the familiar Klein-Gordon
equation for massless spin-2 particles only when the bulk curvature is a
constant or when . As an application of our results, we consider the
possibility of localizing gravity on some flat thick branes. The stability of
these brane solutions is also shortly discussed.Comment: 9 pages, 3 figures, improved version, accepted by PL
Dark energy problem: from phantom theory to modified Gauss-Bonnet gravity
The solution of dark energy problem in the models without scalars is
presented. It is shown that late-time accelerating cosmology may be generated
by the ideal fluid with some implicit equation of state. The universe evolution
within modified Gauss-Bonnet gravity is considered. It is demonstrated that
such gravitational approach may predict the (quintessential, cosmological
constant or transient phantom) acceleration of the late-time universe with
natural transiton from deceleration to acceleration (or from non-phantom to
phantom era in the last case).Comment: LaTeX 8 pages, prepared for the Proceedings of QFEXT'05, minor
correctons, references adde
- âŠ