96 research outputs found
The Step-Harmonic Potential
We analyze the behavior of a quantum system described by a one-dimensional
asymmetric potential consisting of a step plus a harmonic barrier. We solve the
eigenvalue equation by the integral representation method, which allows us to
classify the independent solutions as equivalence classes of homotopic paths in
the complex plane. We then consider the propagation of a wave packet reflected
by the harmonic barrier and obtain an expression for the interaction time as a
function of the peak energy. For high energies we recover the classical
half-period limit.Comment: 19 pages, 7 figure
Renormalization Group approach to Gravity: the running of G and L inside galaxies and additional details on the elliptical NGC 4494
We explore the phenomenology of nontrivial quantum effects on low-energy
gravity. These effects come from the running of the gravitational coupling
parameter G and the cosmological constant L in the Einstein-Hilbert action, as
induced by the Renormalization Group (RG). The Renormalization Group corrected
General Relativity (RGGR model) is used to parametrize these quantum effects,
and it is assumed that the dominant dark matter-like effects inside galaxies is
due to these nontrivial RG effects. Here we present additional details on the
RGGR model application, in particular on the Poisson equation extension that
defines the effective potential, also we re-analyse the ordinary elliptical
galaxy NGC 4494 using a slightly different model for its baryonic contribution,
and explicit solutions are presented for the running of G and L. The values of
the NGC 4494 parameters as shown here have a better agreement with the general
RGGR picture for galaxies, and suggest a larger radial anisotropy than the
previously published result.Comment: 9 pages, 2 figs. Based on a talk presented at the VIII International
Workshop on the Dark Side of the Universe, June 10-15, 2012, Buzios, RJ,
Brazil. v2: typos removed, matches published versio
Is it possible to predict the success of non-invasive positive pressure ventilation in acute respiratory failure due to COPD?
AbstractThere is now sufficient evidence that non-invasive positive pressure ventilation (NIPPV) in selected patients with severe hypercapnic acute respiratory failure due to chronic obstructive pulmonary disease (COPD) is more effective than pharmacological therapy alone. The aim of this study was to identify prognostic factors to predict the success of this technique. Fifty-nine consecutive patients with COPD admitted to a respiratory ward for 75 episodes of acute respiratory failure treated with NIPPV were analysed: success (77%) or failure (23%) were evaluated by survival and the need for endotracheal intubation. There were no significant differences in age, sex, cause of relapse and lung function tests between the two groups. Patients in whom NIPPV was unsuccessful were significantly underweight, had an higher Acute Physiology and Chronic Health Evaluation (APACHE) II score, and a lower serum level of albumin in comparison with those in whom NIPPV was successful. They demonstrated significantly greater abnormalities in pH and P a CO2at baseline and after 2 h of NIPPV. The logistic regression analysis demonstrated that, when all the variables were tested together, a high APACHE II score and a low albumin level continued to have a significant predictive effect. This analysis could predict the outcome in 82% of patients. In conclusion, our study suggests that low albumin serum levels and a high APACHE II score may be important indices in predicting the success of NIPPV
Analysis and modelling of broad-band ferrite-based coaxial transmission-line transformers
The work presented in the paper focuses on accuracy of models for broad-band ferrite based coaxial transmission-line transformers. Soft-ferrites are largely used in VHF/UHF components allowing band enlargement on the low-edge side. Degradation of frequency performance on the high-edge side are produced both by ferrite losses, and by parasitic capacitance due to connection to the thermal and electrical ground in high power applications. Both a circuital model for low-power applications and a scalable e.m. model for high-power applications are presented and discussed. © 2010 EuMA
Bulk viscous cosmology with causal transport theory
We consider cosmological scenarios originating from a single imperfect fluid
with bulk viscosity and apply Eckart's and both the full and the truncated
M\"uller-Israel-Stewart's theories as descriptions of the non-equilibrium
processes. Our principal objective is to investigate if the dynamical
properties of Dark Matter and Dark Energy can be described by a single viscous
fluid and how such description changes when a causal theory
(M\"uller-Israel-Stewart's, both in its full and truncated forms) is taken into
account instead of Eckart's non-causal theory. To this purpose, we find
numerical solutions for the gravitational potential and compare its behaviour
with the corresponding LambdaCDM case. Eckart's and the full causal theory seem
to be disfavoured, whereas the truncated theory leads to results similar to
those of the LambdaCDM model for a bulk viscous speed in the interval 10^{-11}
<< c_b^2 < 10^{-8}. Tentatively relating such value to a square propagation
velocity of the order of T/m of perturbations in a non-relativistic gas of
particles with mass m at the epoch of matter-radiation equality, this may be
compatible with a mass range 0.1 GeV < m << 100 GeV.Comment: 23 pages, 7 figure
Dark matter effects in vacuum spacetime
We analyze a toy model describing an empty spacetime in which the motion of a
test mass (and the trajectories of photons) evidence the presence of a
continuous and homogeneous distribution of matter; however, since the
energy-momentum tensor vanishes, no real matter or energy distribution is
present at all. Thus, a hypothetical observer will conclude that he is immersed
in some sort of dark matter, even though he has no chance to directly detect
it. This suggests yet another possibility of explaining the elusive dark matter
as a purely dynamical effect due to the curvature of spacetime.Comment: 5 pages, 2 figures, expanded with comments about the exact motion and
curvature invariant
Scalar models for the generalized Chaplygin gas and the structure formation constraints
The generalized Chaplygin gas model represents an attempt to unify dark
matter and dark energy. It is characterized by a fluid with an equation of
state . It can be obtained from a generalization of the
DBI action for a scalar, tachyonic field. At background level, this model gives
very good results, but it suffers from many drawbacks at perturbative level. We
show that, while for background analysis it is possible to consider any value
for , the perturbative analysis must be restricted to positive values
of . This restriction can be circumvented if the origin of the
generalized Chaplygin gas is traced back to a self-interacting scalar field,
instead of the DBI action. But, in doing so, the predictions coming from
formation of large scale structures reduce the generalized Chaplygin gas model
to a kind of quintessence model, and the unification scenario is lost, if the
scalar field is the canonical one. However, if the unification condition is
imposed from the beginning as a prior, the model may remain competitive. More
interesting results, concerning the unification program, are obtained if a
non-canonical self-interacting scalar field, inspired by Rastall's theory of
gravity, is imposed. In this case, an agreement with the background tests is
possible.Comment: Latex file, 25 pages, 33 figures in eps format. New section on scalar
models. Accepted for publication in Gravitation&Cosmolog
Unified Dark Matter models with fast transition
We investigate the general properties of Unified Dark Matter (UDM) fluid
models where the pressure and the energy density are linked by a barotropic
equation of state (EoS) and the perturbations are adiabatic. The
EoS is assumed to admit a future attractor that acts as an effective
cosmological constant, while asymptotically in the past the pressure is
negligible. UDM models of the dark sector are appealing because they evade the
so-called "coincidence problem" and "predict" what can be interpreted as
, but in general suffer the effects of a non-negligible
Jeans scale that wreak havoc in the evolution of perturbations, causing a large
Integrated Sachs-Wolfe effect and/or changing structure formation at small
scales. Typically, observational constraints are violated, unless the
parameters of the UDM model are tuned to make it indistinguishable from
CDM. Here we show how this problem can be avoided, studying in detail
the functional form of the Jeans scale in adiabatic UDM perturbations and
introducing a class of models with a fast transition between an early
Einstein-de Sitter CDM-like era and a later CDM-like phase. If the
transition is fast enough, these models may exhibit satisfactory structure
formation and CMB fluctuations. To consider a concrete case, we introduce a toy
UDM model and show that it can predict CMB and matter power spectra that are in
agreement with observations for a wide range of parameter values.Comment: 30 pages, 15 figures, JHEP3 style, typos corrected; it matches the
published versio
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