3,306 research outputs found
An analytic model for the transition from decelerated to accelerated cosmic expansion
We consider the scenario where our observable universe is devised as a
dynamical four-dimensional hypersurface embedded in a five-dimensional bulk
spacetime, with a large extra dimension, which is the {\it generalization of
the flat FRW cosmological metric to five dimensions}. This scenario generates a
simple analytical model where different stages of the evolution of the universe
are approximated by distinct parameterizations of the {\it same} spacetime. In
this model the evolution from decelerated to accelerated expansion can be
interpreted as a "first-order" phase transition between two successive stages.
The dominant energy condition allows different parts of the universe to evolve,
from deceleration to acceleration, at different redshifts within a narrow era.
This picture corresponds to the creation of bubbles of new phase, in the middle
of the old one, typical of first-order phase transitions. Taking today, we find that the cross-over from deceleration to acceleration
occurs at , regardless of the equation of state in the very
early universe. In the case of primordial radiation, the model predicts that
the deceleration parameter "jumps" from to at . At the present time and the equation of state of the
universe is , in agreement with observations and some
theoretical predictions.Comment: The abstract and introduction are improved and the discussion section
is expanded. A number of references are adde
Spatial variations of the fine-structure constant in symmetron models
We investigate the variation of the fine-structure constant, {\alpha}, in
symmetron models using N-body simulations in which the full spatial
distribution of {\alpha} at different redshifts has been calculated. In
particular, we obtain simulated sky maps for this variation, and determine its
power spectrum. We find that in high-density regions of space (such as deep
inside dark matter halos) the value of {\alpha} approaches the value measured
on Earth. In the low-density outskirts of halos the scalar field value can
approach the symmetry breaking value and leads to significantly different
values of {\alpha}. If the scalar-photon coupling strength {\beta}{\gamma} is
of order unity we find that the variation of {\alpha} inside dark matter halos
can be of the same magnitude as the recent claims by Webb et al. of a dipole
variation. Importantly, our results also show that with low-redshift symmetry
breaking these models exhibit some dependence of {\alpha} on lookback time (as
opposed to a pure spatial dipole) which could in principle be detected by
sufficiently accurate spectroscopic measurements, such as those of ALMA and the
ELT-HIRES.Comment: 11 pages, 9 figure
The cosmological behavior of Bekenstein's modified theory of gravity
We study the background cosmology governed by the Tensor-Vector-Scalar theory
of gravity proposed by Bekenstein. We consider a broad family of potentials
that lead to modified gravity and calculate the evolution of the field
variables both numerically and analytically. We find a range of possible
behaviors, from scaling to the late time domination of either the additional
gravitational degrees of freedom or the background fluid.Comment: 10 pages, 8 figures, A few typos corrected in the text and figures.
Version published in PR
CVD of CrO2 Thin Films: Influence of the Deposition Parameters on their Structural and Magnetic Properties
This work reports on the synthesis of CrO2 thin films by atmospheric pressure
CVD using chromium trioxide (CrO3) and oxygen. Highly oriented (100) CrO2 films
containing highly oriented (0001) Cr2O3 were grown onto Al2O3(0001) substrates.
Films display a sharp magnetic transition at 375 K and a saturation
magnetization of 1.92 Bohr magnetons per f.u., close to the bulk value of 2
Bohr magnetons per f.u. for the CrO2.
Keywords: Chromium dioxide (CrO2), Atmospheric pressure CVD, Spintronics.Comment: 5 pages, 6 figure
Chameleons with Field Dependent Couplings
Certain scalar-tensor theories exhibit the so-called chameleon mechanism,
whereby observational signatures of scalar fields are hidden by a combination
of self-interactions and interactions with ambient matter. Not all
scalar-tensor theories exhibit such a chameleon mechanism, which has been
originally found in models with inverse power run-away potentials and field
independent couplings to matter. In this paper we investigate field-theories
with field-dependent couplings and a power-law potential for the scalar field.
We show that the theory indeed is a chameleon field theory. We find the
thin-shell solution for a spherical body and investigate the consequences for
E\"ot-Wash experiments, fifth-force searches and Casimir force experiments.
Requiring that the scalar-field evades gravitational tests, we find that the
coupling is sensitive to a mass-scale which is of order of the Hubble scale
today.Comment: 17 pages, 20 figure
Von Meyenburg Complex or Liver Metastasis? Case Report and Literature Review
Hepatic lesions represent a common finding in clinical practice. Bile ducts hamartomas, also known as Von Meyenburg complex, are benign hepatic malformations composed of small dilated cystic bile ducts lined by fibrous stroma. They represent a rare and asymptomatic clinic-pathological entity. Imagiological findings are variable, and may present as multiple small scattered lesions, or rarely as a single nodule. These findings may resemble secondary lesions and, although benign, there are cases of progression to colangiocarcinoma. Thus it is important to include this complex in the differential diagnoses of focal hepatic lesions, being necessary a detailed investigation for their differentiation. This case represents an atypical presentation of a rare and not frequently considered entity in clinical practic
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