24,869 research outputs found
Newtonian Perturbations on Models with Matter Creation
Creation of Cold Dark Matter (CCDM) can macroscopically be described by a
negative pressure, and, therefore, the mechanism is capable to accelerate the
Universe, without the need of an additional dark energy component. In this
framework we discuss the evolution of perturbations by considering a
Neo-Newtonian approach where, unlike in the standard Newtonian cosmology, the
fluid pressure is taken into account even in the homogeneous and isotropic
background equations (Lima, Zanchin and Brandenberger, MNRAS {\bf 291}, L1,
1997). The evolution of the density contrast is calculated in the linear
approximation and compared to the one predicted by the CDM model. The
difference between the CCDM and CDM predictions at the perturbative
level is quantified by using three different statistical methods, namely: a
simple -analysis in the relevant space parameter, a Bayesian
statistical inference, and, finally, a Kolmogorov-Smirnov test. We find that
under certain circumstances the CCDM scenario analysed here predicts an overall
dynamics (including Hubble flow and matter fluctuation field) which fully
recovers that of the traditional cosmic concordance model. Our basic conclusion
is that such a reduction of the dark sector provides a viable alternative
description to the accelerating CDM cosmology.Comment: Physical Review D in press, 10 pages, 4 figure
Critical behavior of the spin-3/2 Blume-Capel model on a random two-dimensional lattice
We investigate the critical properties of the spin-3/2 Blume-Capel model in
two dimensions on a random lattice with quenched connectivity disorder. The
disordered system is simulated by applying the cluster hybrid Monte Carlo
update algorithm and re-weighting techniques. We calculate the critical
temperature as well as the critical point exponents , ,
, and . We find that, contrary of what happens to the spin-1/2
case, this random system does not belong to the same universality class as the
regular two-dimensional ferromagnetic model.Comment: 5 pages and 5 figure
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
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