arXiv subjects

R. Colistete Jr.

Publications and source records attributed to R. Colistete Jr..

7 recordsLinked to original sources

arXiv2007

Bulk Viscous Cosmology

We propose a scenario in which the dark components of the Universe are manifestations of a single bulk viscous fluid. Using dynamical system methods, a qualitative study of the homogeneous, isotropic background scenario is performed in order to determine the phase space of all possible solutions. The specific model which we investigate shares similarities with a generalized Chaplygin gas in the background but is characterized by non-adiabatic pressure perturbations. This model is tested against supernova type Ia and matter power spectrum data. Different from other unified descriptions of dark matter and dark energy, the matter power spectrum is well behaved, i.e., there are no instabilities or oscillations on small perturbation scales. The model is competitive in comparison with the currently most popular proposals for the description of the cosmological dark sector.

astro-ph↗

arXiv2006

BETOCS using the 157 gold SNe Ia Data : Hubble is not humble

The type Ia supernovae observational data is one of the most important in observational cosmology nowadays. Here we present the first public version of BETOCS (BayEsian Tools for Observational Cosmology using SNe Ia), which is a powerful and high productivity tool aimed to help the theoretical physicist community investigate cosmological models using type Ia supernovae (SNe Ia) observational data. BETOCS is applied to the generalized Chaplygin gas model (GCGM), traditional Chaplygin gas model (CGM) and $Λ$CDM, ranging from 5 to 3 free parameters, respectively. The ``gold sample'' of 157 supernovae data is used. It is shown that the Chaplygin gas scenario is viable (in most cases the $Λ$CDM is disfavoured) and the quartessence scenario (that unifies the description for dark matter and dark energy) is favoured. The Hubble parameter ($H_0$) is important and should not be fixed and it can be estimated or marginalized with or without the Hubble Space Telescope prior.

astro-ph↗

arXiv2005

Bayesian Analysis of the (Generalized) Chaplygin Gas and Cosmological Constant Models using the 157 gold SNe Ia Data

The generalized Chaplygin gas model (GCGM) contains 5 free parameters, here, they are constrained through the type Ia supernovae data, i.e., the ``gold sample'' of 157 supernovae data. Negative and large positive values for $α$ are taken into account. The analysis is made by employing the Bayesian statistics and the prediction for each parameter is obtained by marginalizing on the remained ones. This procedure leads to the following predictions: $α= - 0.75^{+4.04}_{-0.24}$, $H_0=65.00^{+1.77}_{-1.75}$, $Ω_{k0} = - 0.77^{+1.14}_{-5.94}$, $Ω_{m0} = 0.00^{+1.95}_{-0.00}$, $Ω_{c0} = 1.36^{+5.36}_{-0.85}$, $\bar A = 1.000^{+0.000}_{-0.534}$. Through the same analysis the specific case of the ordinary Chaplygin gas model (CGM), for which $α= 1$, is studied. In this case, there are now four free parameters and the predictions for them are: $H_0 = 65.01^{+1.81}_{-1.71}$, $Ω_{k0} = - 2.73^{+1.53}_{-0.97}$, $Ω_{m0} = 0.00^{+1.22}_{-0.00}$, $Ω_{c0} = 1.34^{+0.94}_{-0.70}$, $\bar A = 1.000^{+0.000}_{-0.270}$. To complete the analysis the $Λ$CDM, with its three free parameters, is considered. For all these models, particular cases are considered where one or two parameters are fixed. The age of the Universe, the deceleration parameter and the moment the Universe begins to accelerate are also evaluated. The quartessence scenario, is favoured. A closed (and in some cases a flat) and accelerating Universe is also preferred. The CGM case $α= 1$ is far from been ruled out, and it is even preferred in some particular cases. In most of the cases the $Λ$CDM is disfavoured with respect to GCGM and CGM.

astro-ph↗

arXiv2004

Bayesian Statistics and Parameter Constraints on the Generalized Chaplygin Gas Model using SNe Ia Data

The type Ia supernovae (SNe Ia) observational data are used to estimate the parameters of a cosmological model with cold dark matter and the generalized Chaplygin gas model (GCGM). The GCGM depends essentially on five parameters: the Hubble constant, the parameter $\bar{A}$ related to the velocity of the sound, the equation of state parameter $α$, the curvature of the Universe and the fraction density of the generalized Chaplygin gas (or the cold dark matter). The parameter $α$ is allowed to take negative values and to be greater than 1. The Bayesian parameter estimation yields $α= - 0.86^{+6.01}_{-0.15}$, $H_0 = 62.0^{+1.32}_{-1.42} km/Mpc.s$, $Ω_{k0}=-1.26_{-1.42}^{+1.32}$, $Ω_{m0} = 0.00^{+0.86}_{-0.00}$, $Ω_{c0} = 1.39^{+1.21}_{-1.25}$, $\bar A =1.00^{+0.00}_{-0.39}$, $t_0 = 15.3^{+4.2}_{-3.2}$ and $q_0 = -0.80^{+0.86}_{-0.62}$, where $t_0$ is the age of the Universe and $q_0$ is the value of the deceleration parameter today. Our results indicate that a Universe completely dominated by the generalized Chaplygin gas is favoured, what reinforces the idea that the this gas may unify the description for dark matter and dark energy, at least as the SNe Ia data is concerned. A closed and accelerating Universe is also favoured. The traditional Chaplygin gas model (CGM), $α= 1$ is not ruled out, even if it does not give the best-fitting. Particular cases with four or three independent free parameters are also analysed.

astro-ph↗

arXiv2002

Dark energy, dark matter and the Chaplygin gas

The possibility that the dark energy may be described by the Chaplygin gas is discussed. Some observational constraints are established. These observational constraints indicate that a unified model for dark energy and dark matter through the employement of the Chaplygin gas is favored.

gr-qc↗

arXiv2002

Higher-order geodesic deviations applied to the Kerr metric

Starting with an exact and simple geodesic, we generate approximate geodesics by summing up higher-order geodesic deviations within a General Relativistic setting, without using Newtonian and post-Newtonian approximations. We apply this method to the problem of closed orbital motion of test particles in the Kerr metric space-time. With a simple circular orbit in the equatorial plane taken as the initial geodesic we obtain finite eccentricity orbits in the form of Taylor series with the eccentricity playing the role of small parameter. The explicit expressions of these higher-order geodesic deviations are derived using successive systems of linear equations with constant coefficients, whose solutions are of harmonic oscillator type. This scheme gives best results when applied to the orbits with low eccentricities, but with arbitrary values of $(GM/Rc^2)$, smaller than 1/6 in the Schwarzschild limit.

gr-qc↗

arXiv2000

Gaussian superpositions in scalar-tensor quantum cosmological models

A free scalar field minimally coupled to gravity model is quantized and the Wheeler-DeWitt equation in minisuperspace is solved analytically, exhibiting positive and negative frequency modes. The analysis is performed for positive, negative and zero values of the curvature of the spatial section. Gaussian superpositions of the modes are constructed, and the quantum bohmian trajectories are determined in the framework of the Bohm-de Broglie interpretation of quantum cosmology. Oscillating universes appear in all cases, but with a characteristic scale of the order of the Planck scale. Bouncing regular solutions emerge for the flat curvature case. They contract classically from infinity until a minimum size, where quantum effects become important acting as repulsive forces avoiding the singularity and creating an inflationary phase, expanding afterwards to an infinite size, approaching the classical expansion as long as the scale factor increases. These are non-singular solutions which are viable models to describe the early Universe.

gr-qc↗