22,726 research outputs found
New Cosmic Accelerating Scenario without Dark Energy
We propose an alternative, nonsingular, cosmic scenario based on
gravitationally induced particle production. The model is an attempt to evade
the coincidence and cosmological constant problems of the standard model
(CDM) and also to connect the early and late time accelerating stages
of the Universe. Our space-time emerges from a pure initial de Sitter stage
thereby providing a natural solution to the horizon problem. Subsequently, due
to an instability provoked by the production of massless particles, the
Universe evolves smoothly to the standard radiation dominated era thereby
ending the production of radiation as required by the conformal invariance.
Next, the radiation becomes sub-dominant with the Universe entering in the cold
dark matter dominated era. Finally, the negative pressure associated with the
creation of cold dark matter (CCDM model) particles accelerates the expansion
and drives the Universe to a final de Sitter stage. The late time cosmic
expansion history of the CCDM model is exactly like in the standard
CDM model, however, there is no dark energy. This complete scenario is
fully determined by two extreme energy densities, or equivalently, the
associated de Sitter Hubble scales connected by , a result that has no correlation with the cosmological constant
problem. We also study the linear growth of matter perturbations at the final
accelerating stage. It is found that the CCDM growth index can be written as a
function of the growth index, . In this
framework, we also compare the observed growth rate of clustering with that
predicted by the current CCDM model. Performing a statistical test
we show that the CCDM model provides growth rates that match sufficiently well
with the observed growth rate of structure.Comment: 12 pages, 3 figures, accepted for publication by Phys. Rev. D. (final
version, some references have corrected). arXiv admin note: substantial text
overlap with arXiv:1106.193
Comportamento do tamarix (Tamarix sp. - Tamaricaceae) na recuperação de áreas degradadas por mineração.
As atividades de mineração alteram as áreas naturais pelo acúmulo de rejeitos finos e estéreis e pela retirada da camada superior de solo, causando impactos ambientais indesejáveis. Para reduzir estes danos, vem sendo realizado o plantio de espécies arbóreas e arbustivas para a recuperação das áreas degradadas. O presente trabalho teve como objetivo, avaliar o comportamento do tamarix (Tamarixsp.) em ensaio de seleção de espécies florestais na recuperação de área da barragem de rejeito da Mineração Caraíba S/A, situada no município de Jaguarari, Bahia, região semi-árida do Nordeste do Brasil. O tamarix foi introduzido do Peru, pela Embrapa Semi-Árido na década de 80, sendo utilizado na formação de quebra-ventos em regiões desérticas
Revegetação da barragem de rejeito da Mineração Caraíba.
A atividade de mineração tem causado alterações em áreas naturais, pelo acúmulo de rejeitos e estéreis e a retirada da camada superior de solo. Além de degradar solos, têm provocado desaparecimento de espécies vegetais e animais, causando impactos ambientais indesejáveis. A revegetação é o primeiro passo para recuperação dessas áreas. A escolha de espécies e técnicas de plantio é fundamental nesse processo. O presente trabalho, parceria entre a Mineração Caraíba e Embrapa Semi-Árido, visou estudar o comportamento de espécies arbóreas arbustivas plantadas na barragem de rejeito da Mineração Caraíba, em Jaguarari-BA
Constraints on Cold Dark Matter Accelerating Cosmologies and Cluster Formation
We discuss the properties of homogeneous and isotropic flat cosmologies in
which the present accelerating stage is powered only by the gravitationally
induced creation of cold dark matter (CCDM) particles (). For
some matter creation rates proposed in the literature, we show that the main
cosmological functions such as the scale factor of the universe, the Hubble
expansion rate, the growth factor and the cluster formation rate are
analytically defined. The best CCDM scenario has only one free parameter and
our joint analysis involving BAO + CMB + SNe Ia data yields
() where
is the observed matter density parameter. In particular, this implies that the
model has no dark energy but the part of the matter that is effectively
clustering is in good agreement with the latest determinations from large scale
structure. The growth of perturbation and the formation of galaxy clusters in
such scenarios are also investigated. Despite the fact that both scenarios may
share the same Hubble expansion, we find that matter creation cosmologies
predict stronger small scale dynamics which implies a faster growth rate of
perturbations with respect to the usual CDM cosmology. Such results
point to the possibility of a crucial observational test confronting CCDM with
CDM scenarios trough a more detailed analysis involving CMB, weak
lensing, as well as the large scale structure.Comment: 12 pages, 3 figures, Accepted for publication by Physical Rev.
Density-functionals not based on the electron gas: Local-density approximation for a Luttinger liquid
By shifting the reference system for the local-density approximation (LDA)
from the electron gas to other model systems one obtains a new class of density
functionals, which by design account for the correlations present in the chosen
reference system. This strategy is illustrated by constructing an explicit LDA
for the one-dimensional Hubbard model. While the traditional {\it ab initio}
LDA is based on a Fermi liquid (the electron gas), this one is based on a
Luttinger liquid. First applications to inhomogeneous Hubbard models, including
one containing a localized impurity, are reported.Comment: 4 pages, 4 figures (final version, contains additional applications
and discussion; accepted by Phys. Rev. Lett.
Awaking the vacuum with spheroidal shells
It has been shown that well-behaved spacetimes may induce the vacuum
fluctuations of some nonminimally coupled free scalar fields to go through a
phase of exponential growth. Here, we discuss this mechanism in the context of
spheroidal thin shells emphasizing the consequences of deviations from
spherical symmetry.Comment: 10 pages, 7 figures. Minor changes, version published on Phys. Rev.
Análise da estrutura de uma vegetação ciliar do rio São Francisco no Projeto de Irrigação Bebedouro, Petrolina-PE.
O presente trabalho foi realizado na vegetação ciliar do Rio S ão Francisco, no Projeto de I rrigação Bebedouro, em Petrolina-PE
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