4,646 research outputs found
Perturbations in the relaxation mechanism for a large cosmological constant
Recently, a mechanism for relaxing a large cosmological constant (CC) has
been proposed [arxiv:0902.2215], which permits solutions with low Hubble rates
at late times without fine-tuning. The setup is implemented in the LXCDM
framework, and we found a reasonable cosmological background evolution similar
to the LCDM model with a fine-tuned CC. In this work we analyse analytically
the perturbations in this relaxation model, and we show that their evolution is
also similar to the LCDM model, especially in the matter era. Some tracking
properties of the vacuum energy are discussed, too.Comment: 18 pages, LaTeX; discussion improved, accepted by CQ
Cosmologies with a time dependent vacuum
The idea that the cosmological term, Lambda, should be a time dependent
quantity in cosmology is a most natural one. It is difficult to conceive an
expanding universe with a strictly constant vacuum energy density, namely one
that has remained immutable since the origin of time. A smoothly evolving
vacuum energy density that inherits its time-dependence from cosmological
functions, such as the Hubble rate or the scale factor, is not only a
qualitatively more plausible and intuitive idea, but is also suggested by
fundamental physics, in particular by quantum field theory (QFT) in curved
space-time. To implement this notion, is not strictly necessary to resort to ad
hoc scalar fields, as usually done in the literature (e.g. in quintessence
formulations and the like). A "running" Lambda term can be expected on very
similar grounds as one expects (and observes) the running of couplings and
masses with a physical energy scale in QFT. Furthermore, the experimental
evidence that the equation of state of the dark energy could be evolving with
time/redshift (including the possibility that it might currently behave
phantom-like) suggests that a time-variable Lambda term (possibly accompanied
by a variable Newton's gravitational coupling G=G(t)) could account in a
natural way for all these features. Remarkably enough, a class of these models
(the "new cosmon") could even be the clue for solving the old cosmological
constant problem, including the coincidence problem.Comment: LaTeX, 15 pages, 4 figure
Spontaneous and Stimulated Raman Scattering near Metal Nanostructures in the Ultrafast, High-Intensity regime
The inclusion of atomic inversion in Raman scattering can significantly alter
field dynamics in plasmonic settings. Our calculations show that large local
fields and femtosecond pulses combine to yield: (i) population inversion within
hot spots; (ii) gain saturation; and (iii) conversion efficiencies
characterized by a switch-like transition to the stimulated regime that spans
twelve orders of magnitude. While in Raman scattering atomic inversion is
usually neglected, we demonstrate that in some circumstances full accounting of
the dynamics of the Bloch vector is required
Effective growth of matter density fluctuations in the running LCDM and LXCDM models
We investigate the matter density fluctuations \delta\rho/\rho for two dark
energy (DE) models in the literature in which the cosmological term \Lambda is
a running parameter. In the first model, the running LCDM model, matter and DE
exchange energy, whereas in the second model, the LXCDM model, the total DE and
matter components are conserved separately. The LXCDM model was proposed as an
interesting solution to the cosmic coincidence problem. It includes an extra
dynamical component, the "cosmon" X, which interacts with the running \Lambda,
but not with matter. In our analysis we make use of the current value of the
linear bias parameter, b^2(0)= P_{GG}/P_{MM}, where P_{MM} ~
(\delta\rho/\rho)^2 is the present matter power spectrum and P_{GG} is the
galaxy fluctuation power spectrum. The former can be computed within a given
model, and the latter is found from the observed LSS data (at small z) obtained
by the 2dF galaxy redshift survey. It is found that b^2(0)=1 within a 10%
accuracy for the standard LCDM model. Adopting this limit for any DE model and
using a method based on the effective equation of state for the DE, we can set
a limit on the growth of matter density perturbations for the running LCDM
model, the solution of which is known. This provides a good test of the
procedure, which we then apply to the LXCDM model in order to determine the
physical region of parameter space, compatible with the LSS data. In this
region, the LXCDM model is consistent with known observations and provides at
the same time a viable solution to the cosmic coincidence problem.Comment: LaTeX, 38 pages, 8 figures. Version accepted in JCA
Dilution versus pollution in watercourses affected by acid mine drainage: a graphic model for the Iberian Pyrite Belt (SW Spain)
The aim of this study was to chemically characterize
the water quality impacts of the 88 acid mine drainage
(AMD) generating mines in the Spanish sector of the Iberian
Pyrite Belt (IPB). This was necessary because the Water
Framework Directive of the European Union and the hydrological
plans of the Tinto, Odiel, and Piedras river basins
require that water quality be improved enough to allow at
least some of the rivers in the IPB to sustain healthy fish
populations by 2027. The results indicate a clear decrease in
metals, arsenic, and sulfate concentrations and increased pH
between the AMD-sources and the river channels.info:eu-repo/semantics/publishedVersio
Cosmological models with interacting components and mass-varying neutrinos
A model for a homogeneous and isotropic spatially flat Universe, composed of
baryons, radiation, neutrinos, dark matter and dark energy is analyzed. We
infer that dark energy (considered to behave as a scalar field) interacts with
dark matter (either by the Wetterich model, or by the Anderson and Carroll
model) and with neutrinos by a model proposed by Brookfield et al.. The latter
is understood to have a mass-varying behavior. We show that for a very-softly
varying field, both interacting models for dark matter give the same results.
The models reproduce the expected red-shift performances of the present
behavior of the Universe.Comment: 8 pages, 5 figures, to be published in Gravitation and Cosmolog
The J_1-J_2 antiferromagnet with Dzyaloshinskii-Moriya interaction on the square lattice: An exact diagonalization study
We examine the influence of an anisotropic interaction term of
Dzyaloshinskii-Moriya (DM) type on the groundstate ordering of the J_1-J_2
spin-1/2-Heisenberg antiferromagnet on the square lattice. For the DM term we
consider several symmetries corresponding to different crystal structures. For
the pure J_1-J_2 model there are strong indications for a quantum spin liquid
in the region of 0.4 < J_2/J_1 < 0.65. We find that a DM interaction influences
the breakdown of the conventional antiferromagnetic order by i) shifting the
spin liquid region, ii) changing the isotropic character of the groundstate
towards anisotropic correlations and iii) creating for certain symmetries a net
ferromagnetic moment.Comment: 7 pages, RevTeX, 6 ps-figures, to appear in J. Phys.: Cond. Ma
Water quality in the Municipality of São Domingos (Santiago island, Cape Verde)
O presente estudo tem como objetivo avaliar a qualidade da
água e potencialidades de uso no concelho de São Domingos, na ilha de
Santiago (Cabo Verde). Os resultados obtidos mostram, o carácter
mineralizado destas águas, com valor médio de condutividade elétrica de
1361 µS/cm. A classificação hidroquímica, de acordo com o diagrama de
Piper, conduziu à discriminação dos seguintes tipos: águas mistas
(bicarbonatadas, cloretadas) e mistas sódicas (cloretadas e
bicarbonatadas). Considerando o que está estabelecido na legislação
Cabo-verdiana e Portuguesa relativamente à qualidade da água, verificouse
que apenas 36 % das amostras têm qualidade suficiente para consumo
humano. No que respeita à utilização da água para rega, cerca de 68 %
das amostras apresentam evidências de risco de salinização alto a muito
alto.This research aims the evaluation of water quality and potential
uses at São Domingos municipality, at Santiago Island (Cape Verde). The
results show the mineralized nature of these waters, with an average
electrical conductivity of 1361 μS/cm. The hydrochemical classification,
according to the Piper diagram, led to the following types: mixed
(bicarbonate and chloride) and mixed with sodium (chloride and
bicarbonate). Considering what is established for water quality in the
Cape Verde and Portuguese legislation, only 36 % of the water samples
are able for human consumption. Regarding irrigation use, approximately
68 % of the samples present high-to-very-high salinization risk.(undefined)info:eu-repo/semantics/publishedVersio
Tissue Engineered Meniscus Repair: Influence of Cell Passage Number, Tissue Origin, and Biomaterial Carrier
Objective. Studies have shown that meniscal repairs have better outcomes over both partial and total meniscectomies. Tissue engineering strategies to repair meniscus tears have been explored using cell sources that involve a donor as well as a period of in vitro cell expansion before use. This study explored cell sources that could be easily harvested and rapidly isolated by enzymatic digestion and cannulated delivery. Methods. Bovine menisci were used to create a bucket handle tear. Cell lines were established from meniscus, synovium, and adipose tissue and fluorescently labeled. At passages P2, P4, and P8, cells were added to the defect from the following experimental groups: cells alone, collagen gel, collagen scaffold, or hyaluronic acid. Menisci constructs were xenografted subcutaneously onto the dorsum of athymic rats and incubated for 3, 6, and 9 weeks, at which time they were retrieved and processed for histology. Results. Meniscal cells were able to repair defects faster and significantly better than adipose or synovium derived cells. Adipose cells were the least effective in comparison. Repair was significantly better at 9 weeks compared with 6 and 3 weeks. Macroscopic examination of menisci that received cell implants showed the thickest tissue in menisci that had collagen implants, and the thinnest fill occurred in menisci treated with cells alone. Histology confirmed no cells or integrative repair in the control specimens. Conclusions. Delivery of cells alone outperformed the additional use of biomaterials. Our results suggest a strategy that would use both meniscus and synovial cells for arthroscopic meniscal repair
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