arXiv subjects

S. V. B. Goncalves

Publications and source records attributed to S. V. B. Goncalves.

At least 19 recordsLinked to original sources

arXiv2008

Newtonian Approach to the Matter Power Spectrum of the Generalized Chaplygin Gas

We model the cosmic medium as the mixture of a generalized Chaplygin gas and a pressureless matter component. Within a neo-Newtonian approach we compute the matter power spectrum. The 2dFGRS data are used to discriminate between unified models of the dark sector and different models, for which there is separate dark matter, in addition to that accounted for by the generalized Chaplygin gas. Leaving the corresponding density parameters free, we find that the unified models are strongly disfavored. On the other hand, using unified model priors, the observational data are also well described, in particular for small and large values of the generalized Chaplygin gas parameter $α$.

astro-ph↗

arXiv2008

Matter Power Spectrum for the Generalized Chaplygin Gas Model: The Newtonian Approach

We model the cosmic medium as the mixture of a generalized Chaplygin gas and a pressureless matter component. Within a neo-Newtonian approach (in which, different from standard Newtonian cosmology, the pressure enters the homogeneous and isotropic background dynamics) we compute the matter power spectrum. The 2dFGRS data are used to discriminate between unified models of the dark sector (a purely baryonic matter component of roughly 5 percent of the total energy content and roughly 95 percent generalized Chaplygin gas) and different models, for which there is separate dark matter, in addition to that accounted for by the generalized Chaplygin gas. Leaving the corresponding density parameters free, we find that the unified models are strongly disfavored. On the other hand, using unified model priors, the observational data are also well described, in particular for small and large values of the generalized Chaplygin gas parameter $α$. The latter result is in agreement with a recent, more qualitative but fully relativistic, perturbation analysis in Gorini et al.

astro-ph↗

arXiv2007

Instability of scalar perturbation in a phantomic cosmological scenario

Scalar perturbations can grow during a phantomic cosmological phase as the big rip is approached, in spite of the high accelerated expansion regime, if the equation of state is such that $\frac{p}ρ = α< - {5/3}$. It is shown that such result is independent of the spatial curvature. The perturbed equations are exactly solved for any value of the curvature parameter $k$ and of the equation of state parameter $α$. Growing modes are found asymptotically under the condition $α< - {5/3}$. Since the Hubble radius decreases in a phantom universe, such result indicates that a phantom scenario may not survive longtime due to gravitational instability.

gr-qc↗

arXiv2006

Scalar perturbations and the possible self-destruction of the phantom menace

Some analysis of the supernovae type Ia observational data seems to indicate that the Universe today is dominated by a phantom field, for which all energy conditions are violated. Such phantom field may imply a singularity in a future finite time, called big rip. Studying the evolution of scalar perturbations for such a field, we show that if the pressure is negative enough, the Universe can become highly inhomogeneous and this phantom menace may be avoided.

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arXiv2005

Late time accelerated Brans-Dicke pressureless solutions and the supernovae type Ia data

Cosmological solutions for a pressureless fluid in the Brans-Dicke theory exhibit asymptotical accelerated phase for some range of values of the parameter $ω$, interpolating a matter dominated phase and an inflationary phase. The effective gravitational coupling is negative. We test this model against the supernovae type Ia data. The fitting of the observational data is slightly better than that obtained from the $ΛCDM$ model. We make some speculations on how to reconcile the negative gravitational coupling in large scale with the local tests. Some considerations on the structure formation problem in this context are also made.

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arXiv2005

Bulk viscosity driving the acceleration of the Universe

The possibility that the present acceleration of the universe is driven by a kind of viscous fluid is exploited. At background level this model is similar to the generalized Chaplygin gas model (GCGM). But, at perturbative level, the viscous fluid exhibits interesting properties. In particular the oscillations in the power spectrum that plagues the GCGM are not present. Possible fundamental descriptions for this viscous dark energy are discussed.

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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.

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arXiv2004

Gravitational waves in the generalized Chaplygin gas model

The consequences of taking the generalized Chaplygin gas as the dark energy constituent of the Universe on the gravitational waves are studied and the spectrum obtained from this model, for the flat case, is analyzed. Besides its importance for the study of the primordial Universe, the gravitational waves represent an additional perspective (besides the CMB temperature and polarization anisotropies) to evaluate the consistence of the different dark energy models and establish better constraints to their parameters. The analysis presented here takes this fact into consideration to open one more perspective of verification of the generalized Chapligin gas model applicability. Nine particular cases are compared: one where no dark energy is present; two that simulate the $Λ$-CDM model; two where the gas acts like the traditional Chaplygin gas; and four where the dark energy is the generalized Chaplygin gas. The different spectra permit to distinguish the $Λ$-CDM and the Chaplygin gas scenarios.

gr-qc↗

arXiv2004

Troubles with quantum anisotropic cosmological models: loss of unitarity

The anisotropic Bianchi I cosmological model coupled with perfect fluid is quantized in the minisuperspace. The perfect fluid is described by using the Schutz formalism which allows to attribute dynamical degrees of freedom to matter. It is shown that the resulting model is non-unitary. This breaks the equivalence between the many-worlds and dBB interpretations of quantum mechanics.

gr-qc↗

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

Fitting the Supernova Type Ia Data with the Chaplygin Gas

The supernova type Ia observational data are fitted using a model with cold dark matter and the Chaplygin gas. The Chaplygin gas, which is characterized by a negative pressure varying with the inverse of density, represents in this model the dark energy responsible for the accelaration of the universe. The fitting depends essentially on four parameters: the Hubble constant, the velocity of sound of the Chaplygin gas and the fraction density of the Chaplygin gas and the cold dark matter. The best fitting model is obtained with H_0 = 65 km/Mpc.s, $c_s^2 \sim 0.92c$ and Omega_{c0} = 1, Omega_{m0} = 0, that is, a universe completely dominated by the Chaplygin gas. This reinforces the possibility that the Chaplygin gas may unify dark matter and dark energy, as it has already been claimed in the literature.

astro-ph↗

arXiv2002

Anisotropic quantum cosmological models: a discrepancy between many-worlds and dBB interpretations

In the isotropic quantum cosmological perfect fluid model, the initial singularity can be avoided, while the classical behaviour is recovered asymptotically. We verify if initial anisotropies can also be suppressed in a quantum version of a classical anisotropic model where gravity is coupled to a perfect fluid. Employing a Bianchi I cosmological model, we obtain a "Schrödinger-like" equation where the matter variables play de role of time. This equation has a hyperbolic signature. It can be explicitly solved and a wave packet is constructed. The expectation value of the scale factor, evaluated in the spirit of the many-worlds interpretation, reveals an isotropic Universe. On the other hand, the bohmian trajectories indicate the existence of anisotropies. This is an example where the Bohm-de Broglie and the many-worlds interpretations are not equivalent. It is argued that this inequivalence is due to the hyperbolic structure of the "Schrödinger-like" equation.

gr-qc↗

arXiv2001

Quantum cosmological perfect fluid model and its classical analogue

The quantization of gravity coupled to a perfect fluid model leads to a Schrödinger-like equation, where the matter variable plays the role of time. The wave function can be determined, in the flat case, for an arbitrary barotropic equation of state $p = αρ$; solutions can also be found for the radiative non-flat case. The wave packets are constructed, from which the expectation value for the scale factor is determined. The quantum scenarios reveal a bouncing Universe, free from singularity. We show that such quantum cosmological perfect fluid models admit a universal classical analogue, represented by the addition, to the ordinary classical model, of a repulsive stiff matter fluid. The meaning of the existence of this universal classical analogue is discussed. The quantum cosmological perfect fluid model is, for a flat spatial section, formally equivalent to a free particle in ordinary quantum mechanics, for any value of $α$, while the radiative non-flat case is equivalent to the harmonic oscillator. The repulsive fluid needed to reproduce the quantum results is the same in both cases.

gr-qc↗

arXiv2001

An analysis of helium primordial nucleosynthesis with a variable cosmological coupling

The synthesis of helium in the early Universe depends on many input parameters, including the value of the gravitational coupling during the period when the nucleosynthesis takes place. We compute the primordial abundance of helium as function of the gravitational coupling, using a semi-analytical method, in order to track the influence of $G$ in the primordial nucleosynthesis. To be specific, we construct a cosmological model with varying $G$, using the Brans-Dicke theory. The greater the value of $G$ at nucleosynthesis period, the greater the abundance of helium predicted. Using the observational data for the abundance of the primordial helium, constraints for the time variation of $G$ are established.

gr-qc↗

arXiv2001

Density perturbations in an Universe dominated by the Chaplygin gas

We study the fate of density perturbations in an Universe dominate by the Chaplygin gas, which exhibit negative pressure. We show that it is possible to obtain the value for the density contrast observed in large scale structure of the Universe by fixing a free parameter in the equation of state of this gas. The negative character of pressure must be significant only very recently.

gr-qc↗

arXiv2001

On the consistency of a repulsive gravity phase in the early Universe

We exploit the possibility of existence of a repulsive gravity phase in the evolution of the Universe. A toy model with a free scalar field minimally coupled to gravity, but with the "wrong sign" for the energy and negative curvature for the spatial section, is studied in detail. The background solutions display a bouncing, non-singular Universe. The model is well-behaved with respect to tensor perturbations. But, it exhibits growing models with respect to scalar perturbations whose maximum occurs in the bouncing. Hence, large inhomogeneties are produced. At least for this case, a repulsive phase may destroy homogeneity, and in this sense it may be unstable. A newtonian analogous model is worked out; it displays qualitatively the same behaviour. The generality of this result is discussed. In particular, it is shown that the addition of an attractive radiative fluid does not change essentially the results. We discuss also a quantum version of the classical repulsive phase, through the Wheeler-de Witt equation in mini-superspace, and we show that it displays essentially the same scenario as the corresponding attractive phase.

gr-qc↗

arXiv2000

Qualitative analysis of a scalar-tensor theory with exponential potential

A qualitative analysis of a scalar-tensor cosmological model, with an exponential potential for the scalar field, is performed. The phase diagram for the flat case is constructed. It is shown that solutions with an initial and final inflationary behaviour appear. The conditions for which the scenario favored by supernova type Ia observations becomes an attractor in the space of the solutions are established.

gr-qc↗

arXiv2000

Classical analogous of quantum cosmological perfect fluid models

Quantization in the mini-superspace of a gravity system coupled to a perfect fluid, leads to a solvable model which implies singularity free solutions through the construction of a superposition of the wavefunctions. We show that such models are equivalent to a classical system where, besides the perfect fluid, a repulsive fluid with an equation of state $p_Q = ρ_Q$ is present. This leads to speculate on the true nature of this quantization procedure. A perturbative analysis of the classical system reveals the condition for the stability of the classical system in terms of the existence of an anti-gravity phase.

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