95 research outputs found
Observational constraints on late-time Lambda(t) cosmology
The cosmological constant, i.e., the energy density stored in the true vacuum
state of all existing fields in the Universe, is the simplest and the most
natural possibility to describe the current cosmic acceleration. However,
despite its observational successes, such a possibility exacerbates the well
known cosmological constant problem, requiring a natural explanation for its
small, but nonzero, value. In this paper we study cosmological consequences of
a scenario driven by a varying cosmological term, in which the vacuum energy
density decays linearly with the Hubble parameter. We test the viability of
this scenario and study a possible way to distinguish it from the current
standard cosmological model by using recent observations of type Ia supernova
(Supernova Legacy Survey Collaboration), measurements of the baryonic acoustic
oscillation from the Sloan Digital Sky Survey and the position of the first
peak of the cosmic microwave background angular spectrum from the three-year
Wilkinson Microwave Anisotropy Probe.Comment: Some important revisions. To appear in Physical Review
Active galactic nuclei, gravitational redshifts, and cosmological tensions
Gravitational redshift is a classical effect of Einstein's General
Relativity, already measured in stars, quasars and clusters of galaxies. We
here identify the signature of gravitational redshift in the emission lines of
active galaxies due to supermassive black holes and discuss their impact on
cosmological inference from type Ia supernovae. Firstly, from the full width at
half maximum of lines of 75 Seyfert type I galaxies of the AGN
Black Hole Mass Database, we derive a gravitational redshift . Expanding this analysis to 86755 quasars from DR14 of
SDSS we have a mean value . Then, by comparing
the redshifts of 34 lines measured at the central and outer regions of LINER
galaxies in the SAMI survey we obtain .
These numbers are compatible with central black holes of solar
masses and broad line regions of ~pc. For non-AGN galaxies the
gravitational redshift is compatible with zero and, as they constitute most of
SNe Ia host galaxies, the impact on the cosmological parameters is negligible.Comment: 5 pages, 5 figure
Inhomogeneous vacuum energy
Vacuum energy remains the simplest model of dark energy which could drive the
accelerated expansion of the Universe without necessarily introducing any new
degrees of freedom. Inhomogeneous vacuum energy is necessarily interacting in
general relativity. Although the four-velocity of vacuum energy is undefined,
an interacting vacuum has an energy transfer and the vacuum energy defines a
particular foliation of spacetime with spatially homogeneous vacuum energy in
cosmological solutions. It is possible to give a consistent description of
vacuum dynamics and in particular the relativistic equations of motion for
inhomogeneous perturbations given a covariant prescription for the vacuum
energy, or equivalently the energy transfer four-vector, and we construct
gauge-invariant vacuum perturbations. We show that any dark energy cosmology
can be decomposed into an interacting vacuum+matter cosmology whose
inhomogeneous perturbations obey simple first-order equations.Comment: 8 pages; v2 clarified discussion of Chaplygin gas model, references
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Evolution of density perturbations in decaying vacuum cosmology
We study cosmological perturbations in the context of an interacting dark
energy model, in which the cosmological term decays linearly with the Hubble
parameter, with concomitant matter production. A previous joint analysis of the
redshift-distance relation for type Ia supernovas, baryonic acoustic
oscillations, and the position of the first peak in the anisotropy spectrum of
the cosmic microwave background has led to acceptable values for the
cosmological parameters. Here we present our analysis of small perturbations,
under the assumption that the cosmological term, and therefore the matter
production, are strictly homogeneous. Such a homogeneous production tends to
dilute the matter contrast, leading to a late-time suppression in the power
spectrum. Nevertheless, an excellent agreement with the observational data can
be achieved by using a higher matter density as compared to the concordance
value previously obtained. This may indicate that our hypothesis of homogeneous
matter production must be relaxed by allowing perturbations in the interacting
cosmological term.Comment: Revised version. To appear in Physical Review
Supernova constraints on decaying vacuum cosmology
There is mounting observational evidence that the expansion of our Universe
is undergoing a late-time acceleration. Among many proposals to describe this
phenomenon, the cosmological constant seems to be the simplest and the most
natural explanation. However, despite its observational successes, such a
possibility exacerbates the well known cosmological constant problem, requiring
a natural explanation for its small, but nonzero, value. In this paper we
consider a cosmological scenario driven by a varying cosmological term, in
which the vacuum energy density decays linearly with the Hubble parameter. We
show that this model is indistinguishable from the standard one in that the
early radiation phase is followed by a long dust-dominated era, and only
recently the varying cosmological term becomes dominant, accelerating the
cosmic expansion. In order to test the viability of this scenario we have used
the most recent type Ia supernova data, i.e., the High-Z SN Search (HZS) Team
and the Supernova Legacy Survey (SNLS) Collaboration data. In particular, for
the SNLS sample we have found the present matter density and Hubble parameters
in the intervals [0.27, 0.37] and [0.68, 0.72], respectively (at 95% c.l.),
which is in good agreement with the currently accepted estimates for these
parameters.Comment: Version accepted for publication in Physical Review
Observational tests for \Lambda(t)CDM cosmology
We investigate the observational viability of a class of cosmological models
in which the vacuum energy density decays linearly with the Hubble parameter,
resulting in a production of cold dark matter particles at late times.
Similarly to the flat \Lambda CDM case, there is only one free parameter to be
adjusted by the data in this class of \Lambda(t)CDM scenarios, namely, the
matter density parameter. To perform our analysis we use three of the most
recent SNe Ia compilation sets (Union2, SDSS and Constitution) along with the
current measurements of distance to the BAO peaks at z = 0.2 and z = 0.35 and
the position of the first acoustic peak of the CMB power spectrum. We show that
in terms of statistics both models provide good fits to the data and
similar results. A quantitative analysis discussing the differences in
parameter estimation due to SNe light-curve fitting methods (SALT2 and MLCS2k2)
is studied using the current SDSS and Constitution SNe Ia compilations. A
matter power spectrum analysis using the 2dFGRS is also performed, providing a
very good concordance with the constraints from the SDSS and Constitution
MLCS2k2 data.Comment: Revised version, to appear in JCA
Adaptation of life-cycle inventories of sawn timber used in the structure of roofs in the state of São Paulo
Este trabalho teve como objetivo desenvolver o inventário de ciclo de vida para madeira empregada na estrutura de telhado de edificações populares do Estado de São Paulo, com base na adaptação de inventários existentes na base de dados ecoinvent (versão 3.1) para a realidade da produção madeireira no Brasil. Considerou-se como sistema de produto a produção de 1m3 de madeira serrada de cambará proveniente de manejo florestal sustentável na Amazônia, desde a extração das toras até a peça de madeira serrada estocada em São Paulo. As informações nacionais foram obtidas da literatura. A despeito das semelhanças entre o sistema de produto da base ecoinvent e a realidade nacional na extração das toras na floresta, há diferenças referentes ao transporte até a serraria, ao desdobro das toras e à geração e destinação de resÃduos: no Brasil, todas as etapas são executadas próximas à região de exploração, resultando em alterações no modelo de transporte, máquinas empregadas e resÃduos gerados. Além disso, os valores de consumo de diesel e eletricidade apresentaram ordens de grandeza diferentes dos ICVs de referência. Com base nessa análise, conclui-se que a estrutura de dados dos ICVs existentes na base ecoinvent auxilia a construção de inventários nacionais e que a adaptação dos inventários é imprescindÃvel para a realização de estudos de avaliação do ciclo de vida condizentes com as condições do Brasil. Entretanto, considerando as diferenças observadas, é recomendada a apuração dos Ãndices de consumo nacionais por meio de verificação e coleta de dados inloc
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