13,134 research outputs found
CCDM Model with Spatial Curvature and The Breaking of "Dark Degeneracy"
Creation of Cold Dark Matter (CCDM), in the context of Einstein Field
Equations, leads to a negative creation pressure, which can be used to explain
the accelerated expansion of the Universe. Recently, it has been shown that the
dynamics of expansion of such models can not be distinguished from the
concordance CDM model, even at higher orders in the evolution of
density perturbations, leading at the so called "dark degeneracy". However,
depending on the form of the CDM creation rate, the inclusion of spatial
curvature leads to a different behavior of CCDM when compared to CDM,
even at background level. With a simple form for the creation rate, namely,
, we show that this model can be distinguished from
CDM, provided the Universe has some amount of spatial curvature.
Observationally, however, the current limits on spatial flatness from CMB
indicate that neither of the models are significantly favored against the other
by current data, at least in the background level.Comment: 13 pages, 5 figure
Bayesian analysis of CCDM Models
Creation of Cold Dark Matter (CCDM), in the context of Einstein Field
Equations, leads to negative creation pressure, which can be used to explain
the accelerated expansion of the Universe. In this work we tested six different
spatially flat models for matter creation using statistical tools, at light of
SN Ia data: Akaike Information Criterion (AIC), Bayesian Information Criterion
(BIC) and Bayesian Evidence (BE). These approaches allow to compare models
considering goodness of fit and number of free parameters, penalizing excess of
complexity. We find that JO model is slightly favoured over LJO/CDM
model, however, neither of these, nor model can be
discarded from the current analysis. Three other scenarios are discarded either
from poor fitting, either from excess of free parameters.Comment: 16 pages, 6 figures, 6 tables. Corrected some text and language in
new versio
Thermodynamic constraints on matter creation models
Entropy is a fundamental concept from Thermodynamics and it can be used to
study models on context of Creation Cold Dark Matter (CCDM). From conditions on
the first ()\footnote{Throughout the present work we will use
dots to indicate time derivatives and dashes to indicate derivatives with
respect to scale factor.} and second order () time derivatives of
total entropy in the initial expansion of Sitter through the radiation and
matter eras until the end of Sitter expansion, it is possible to estimate the
intervals of parameters. The total entropy () is calculated as sum of
the entropy at all eras ( and ) plus the entropy of the
event horizon (). This term derives from the Holographic Principle where
it suggests that all information is contained on the observable horizon. The
main feature of this method for these models are that thermodynamic equilibrium
is reached in a final de Sitter era. Total entropy of the universe is
calculated with three terms: apparent horizon (), entropy of matter
() and entropy of radiation (). This analysis allows to
estimate intervals of parameters of CCDM models.Comment: 16 pages, 11 figures. Replaced in order to match accepted versio
CCDM model from quantum particle creation: constraints on dark matter mass
In this work the results from the quantum process of matter creation have
been used in order to constrain the mass of the dark matter particles in an
accelerated Cold Dark Matter model (Creation Cold Dark Matter, CCDM). In order
to take into account a back reaction effect due to the particle creation
phenomenon, it has been assumed a small deviation for the scale
factor in the matter dominated era of the form .
Based on recent data, the best fit values for the mass of dark matter
created particles and the parameter have been found as
GeV, restricted to a 68.3\% c.l. interval of
() GeV and at
68.3\% c.l. For these best fit values the model correctly recovers a transition
from decelerated to accelerated expansion and admits a positive creation rate
near the present era. Contrary to recent works in CCDM models where the
creation rate was phenomenologically derived, here we have used a quantum
mechanical result for the creation rate of real massive scalar particles, given
a self consistent justification for the physical process. This method also
indicates a possible solution to the so called "dark degeneracy", where one can
not distinguish if it is the quantum vacuum contribution or quantum particle
creation which accelerates the Universe expansion.Comment: 16 pages, 5 figures. Major modifications have been done, following
the referee suggestions. The deduction of the treatment is now more
transparent, figures have been added showing the statistical limits over the
dark matter mass, and the best fit for DM mass has been slightly modifie
"A Theory of Production" The Estimation of the Cobb-Douglas Function: A Retrospective View
The Cobb-Douglas production function is still today the most ubiquitous form in theoretical and empirical analyses of growth and productivity. The estimation of the parameters of aggregate production functions is central to much of today's work on growth, technological change, productivity, and labor. This paper has taken up Samuelson's [1979] invitation to verify empirically his claim that all the regression of the Cobb-Douglas [1928] production function does is to reproduce the income accounting identity according to which value added equals the sum of the wage bill plus total profits. This paper concludes that Samuelson was right, and believes that this argument has very serious implications for today's work in macroeconomics.
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