A new accelerating cosmology driven only by baryons plus cold dark matter (CDM) is proposed in the framework of general relativity. In this model the present accelerating stage of the Universe is powered by the negative pressure describing the gravitationally-induced particle production of cold dark matter particles. This kind of scenario has only one free parameter and the differential equation governing the evolution of the scale factor is exactly the same of the ΛCDM model. For a spatially flat Universe, as predicted by inflation (Ω dm + Ω baryon = 1), it is found that the effectively observed matter density parameter is Ω mef f = 1 − α, where α is the constant parameter specifying the CDM particle creation rate. The supernovae test based on the Union data (2008) requires α ∼ 0.71 so that Ω mef f ∼ 0.29 as independently derived from weak gravitational lensing, the large scale structure and other complementary observations. PACS numbers: 98.80.-k, 95.35.+d,95.30.Tg * Electronic address: limajas@astro.iag.usp.br † Electronic address: jfernando@astro.iag.usp.br ‡ Electronic address: foliveira@astro.iag.usp.br [1] P. Astier et al., Astron. Astrophys. 447, 31 (2006); A. G. Riess et al., Astrop. J. 659, 98 (2007). [2] M. Kowalski et al., Astrophys. J. 686, 749 (2008),

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 291, L1, 1997). The evolution of the density contrast is calculated in the linear approximation and compared to the one predicted by the ΛCDM model. The difference between the CCDM and ΛCDM predictions at the perturbative level is quantified by using three different statistical methods, namely: a simple χ 2 -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 analyzed 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 ΛCDM cosmology.

A component of dark energy has been recently proposed to explain the current acceleration of the Universe. Unless some unknown symmetry in Nature prevents or suppresses it, such a field may interact with the pressureless component of dark matter, giving rise to the so-called models of coupled quintessence. In this paper we propose a new cosmological scenario where radiation and baryons are conserved, while the dark energy component is decaying into cold dark matter (CDM). The dilution of CDM particles, attenuated with respect to the usual $a^{-3}$ scaling due to the interacting process, is characterized by a positive parameter $\epsilon$, whereas the dark energy satisfies the equation of state $p_x=\omega \rho_x$ ($\omega < 0$). We carry out a joint statistical analysis involving recent observations from type Ia supernovae, baryon acoustic oscillation peak, and Cosmic Microwave Background shift parameter to check the observational viability of the coupled quintessence scenario here proposed.Comment: 7 pages, 7 figures. Minor corrections to match published versio

We analyze the interaction between Dark Energy and Dark Matter from a thermodynamical perspective. By assuming they have different temperatures, we study the possibility of occurring a decay from Dark Matter into Dark Energy, characterized by a negative parameter Q. We find that, if at least one of the fluids has non vanishing chemical potential, for instance µx < 0 and µ dm = 0 or µx = 0 and µ dm > 0, the decay is possible, where µx and µ dm are the chemical potentials of Dark Energy and Dark Matter, respectively. Using recent cosmological data, we find that, for a fairly simple interaction, the Dark Matter decay is favored with a probability of ∼ 93% over the Dark Energy decay. This result comes from a likelihood analysis where only background evolution has been considered.

We study the effect of shear and rotation on results previously obtained dealing with the application of the spherical collapse model (SCM) to generalized Chaplygin gas (gCg) dominated universes. The system is composed of baryons and gCg and the collapse is studied for different values of the parameter $\alpha$ of the gCg. We show that the joint effect of shear and rotation is that of slowing down the collapse with respect to the simple SCM. This result is of utmost importance for the so-called unified dark matter models, since the described slow down in the growth of density perturbation can solve one of the main problems of the quoted models, namely the instability described in previous papers [e.g., H. B. Sandvik {\it et al.}, Phys. Rev. D {\bf 69}, 123524 (2004)] at the linear perturbation level.Comment: 10 pages, 4 figures. Matches published versio

This paper aims to put constraints on the transition redshift zt, which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to do that, we use the non-parametric Gaussian Process method with H(z) and SNe Ia data. The deceleration parameter reconstruction from H(z) data yields zt=0.59+0.12−0.11. The reconstruction from SNe Ia data assumes spatial flatness and yields zt=0.683+0.11−0.082. These results were found with a Gaussian kernel and we show that they are consistent with two other kernel choices.

A Friedmann like cosmological model in Einstein-Cartan framework is studied when the torsion function is assumed to be proportional to a single φ(t) function coming just from the spin vector contribution of ordinary matter. By analysing four different types of torsion function written in terms of one, two and three free parameters, we found that a model with φ(t) = −αH(t) ρ m (t)/ρ 0c n is totally compatible with recent cosmological data, where α and n are free parameters to be constrained from observations, ρ m is the matter energy density and ρ 0c the critical density. The recent accelerated phase of expansion of the universe is correctly reproduced by the contribution coming from torsion function, with a deceleration parameter indicating a transition redshift of about 0.65.

This paper aims to put constraints on the transition redshift z t , which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to perform our analyses, we consider a flat universe and assume a parametrization for the comoving distance D C (z) up to third degree on z, a second degree parametrization for the Hubble parameter H(z) and a linear parametrization for the deceleration parameter q(z). For each case, we show that type Ia supernovae and H(z) data complement each other on the parameter space and tighter constrains for the transition redshift are obtained. By combining the type Ia supernovae observations and Hubble parameter measurements it is possible to constrain the values of z t , for each approach, as 0.806 ± 0.094, 0.870 ± 0.063 and 0.973 ± 0.058 at 1σ c.l., respectively. Then, such approaches provide cosmological-model independent estimates for this parameter.