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Sandro Silva e Costa

Publications and source records attributed to Sandro Silva e Costa.

6 recordsLinked to original sources
arXiv2008

An entirely analytical cosmological model

The purpose of the present study is to show that in a particular cosmological model, with an affine equation of state, one can obtain, besides the background given by the scale factor, Hubble and deceleration parameters, a representation in terms of scalar fields and, more important, explicit mathematical expressions for the density contrast and the power spectrum. Although the model so obtained is not realistic, it reproduces features observed in some previous numerical studies and, therefore, it may be useful in the testing of numerical codes and as a pedagogical tool.

gr-qc↗
arXiv2008

Relations between the modified Chaplygin gas and a scalar field

In this work the modified Chaplygin gas is related to a cosmological scalar field. Analytical results, more general than the solutions previously shown in the literature, are presented for the case when the curvature is absent, and one entirely new particular result is shown for a case where there is curvature. Also, it is emphasized here that one interesting feature of the scalar field associated to the modified Chaplygin gas is the possible presence of two minima in its potential. Finally, it is argued that the scalar field representation of the Chaplygin gas can be another useful tool in the study of the evolution of perturbations and, as an example of this, a new analytical result for the density contrast is presented.

gr-qc↗
arXiv2007

A mathematical analysis of the evolution of perturbations in a modified Chaplygin gas model

One approach in modern cosmology consists in supposing that dark matter and dark energy are different manifestations of a single `quartessential' fluid. Following such idea, this work presents a study of the evolution of perturbations of density in a flat cosmological model with a modified Chaplygin gas acting as a single component. Our goal is to obtain properties of the model which can be used to distinguish it from another cosmological models which have the same solutions for the general evolution of the scale factor of the universe, without the construction of the power spectrum. Our analytical results, which alone can be used to uniquely characterize the specific model studied in our work, show that the evolution of the density contrast can be seen, at least in one particular case, as composed by a spheroidal wave function. We also present a numerical analysis which clearly indicates as one interesting feature of the model the appearence of peaks in the evolution of the density constrast.

gr-qc↗
arXiv2007

Connections among three roads to cosmic acceleration: decaying vacuum, bulk viscosity, and nonlinear fluids

We discuss the connection among three distinct classes of models often used to explain the late cosmic acceleration: decaying cosmological term, bulk viscous pressure, and nonlinear fluids. We focus on models that are equivalent at zeroth order, in the sense they lead to the same solutions for the evolution of the scale factor. More specifically, we show explicit examples where this equivalence is manifest, which include some well know models belonging to each class, such as a power law Lambda-term, a model with constant viscosity, and the Modified Chaplygin Gas. We also obtain new analytic solutions for some of these models, including a new Ansatz for the cosmic term.

astro-ph↗
arXiv2002

Casimir energy in finite expanding universes: a prelude

We present the Green's functions that are the solutions of the massive Klein-Gordon equation for a scalar field with non-minimal coupling to gravitation for several static and expanding cosmological models. An important feature of such functions is that they can be used to study the appearence of a Casimir energy in models of the universe with compact spatial sections.

astro-ph↗