arXiv subjects

S. V. B. Gonçalves

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

At least 19 recordsLinked to original sources

arXiv2020

The effects of dark energy on the early universe with radiation and Bose-Einstein condensate

This work analyzes the effects of quantization on a Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) model with positive curvature and material content composed of a Bose-Einstein condensate, a radioactive fluid and a cosmological constant playing the role of the dark energy of the universe. The quantization of the model was performed using the finite difference method in the Crank-Nicolson scheme: solutions of the Wheeler-DeWitt equation are obtained, in the form of finite norm wave packets which are well defined in all space, even if the 3D-sphere is degenerate. The introduction of the Bose-Einstein condensate and cosmological constant preserves the existence of bounce solutions (for certain choices of parameters and initial conditions) with exits for inflation (de Sitter solutions). This occurs after the universe emerges from its quantum phase by a tunneling mechanism.

gr-qc↗

arXiv2019

Quantum Cosmology with many fluids and the choice of cosmological time

In this work we propose the quantization of a cosmological model describing the primordial universe filled with five barotropic fluids, namely: radiation, dust, vacuum, cosmic strings and domain walls. We intend to identify which fluid is best suited to provide phenomenologically the temporal variable in accordance with the observable universe. Through the Galerkin spectral method and the finite difference method in the Crank-Nicolson scheme (vacuum case), the quantum cosmological solutions are obtained and compared. We, also, compare the quantum cosmological solutions with the corresponding classical ones. The vacuum case is especially interesting because it provides a tunneling transition mechanism from the quantum to the classical phase and the possibility of calculating quantum tunneling probabilities.

gr-qc↗

arXiv2018

Primordial Universe with radiation and Bose-Einstein condensate

In this work we derive a scenario {in which} the early universe consists of {radiation fluid} and Bose-Einstein condensate. The possibility of gravitational self-interaction due to an attractive Bose-Einstein condensate is analyzed. The classical behavior of the scale factor of the universe is determined by a parameter associated with the Bose-Einstein fluid with bouncing or Big Crunch solutions. After we proceed to compute the finite-norm wave packet solutions to the Wheeler-DeWitt equation. The behavior of the scale factor is studied by applying the many-worlds interpretation of quantum mechanics. The quantum cosmological model is free from the singularities.

gr-qc↗

arXiv2016

Dynamics of a perfect fluid through velocity potentials with aplication in quantum cosmology

We review the Eulerian description of hidrodynamics using Seliger-Whitham's formalism (in classical case) and Schutz's formalism (in relativistic case). In these formalisms, the velocity field of a perfect fluid is described by scalar potentials. With this we can obtain the evolution equations of the fluid and its Hamiltonian. In the scenario of quantum cosmology the Schutz's formalism makes it possible to introduce phenomenologically a time variable in minisuperspace models.

gr-qc↗

arXiv2016

The Kantowski-Sachs quantum model with stiff matter fluid

In this paper we study the quantum cosmological Kantowski-Sachs model and solve the Wheeler-DeWitt equation in minisuperspace to obtain the wave function of the corresponding universe. The perfect fluid is described by the Schutz's canonical formalism, which allows to attribute dynamical degrees of freedom to matter. The time is introduced phenomenologically using the fluid's degrees of freedom. In particular, we adopt a stiff matter fluid. The viability of this model is analyzed and discussed.

gr-qc↗

arXiv2016

Classical and quantum cosmology with two perfect fluids: stiff matter and radiation

In this work the homogeneous and isotropic Universe of Friedmann-Robertson-Walker is studied in the presence of two fluids: stiff matter and radiation described by the Schutz's formalism. We obtain to the classic case the behaviour of the scale factor of the universe. For the quantum case the wave packets are constructed and the wave function of the universe is found.

gr-qc↗

arXiv2014

Polytropic equation of state and primordial quantum fluctuations

We study the primordial Universe in a cosmological model where inflation is driven by a fluid with a polytropic equation of state $p = αρ+ kρ^{1 + 1/n}$. We calculate the dynamics of the scalar factor and build a Universe with constant density at the origin. We also find the equivalent scalar field that could create such equation of state and calculate the corresponding slow-roll parameters. We calculate the scalar perturbations, the scalar power spectrum and the spectral index.

astro-ph.CO↗

arXiv2013

Gravitational lenses in the dark Universe

We discuss how different cosmological models of the Universe affect the probability that a background source has multiple images related by an angular distance $θ_E$ of the line of sight, \textit{i. e.}, the optical depth of gravitational lensing. We examine some cosmological models for different values of the density parameter $Ω_i$: i) the cold dark matter model, ii) the $Λ$CDM model, iii) the Bose-Einstein condensate dark matter model, iv) the Chaplygin gas model, v) the viscous fluid cosmological model and vi) the holographic dark energy model. We note that the dependence of the energy-matter content of the universe profoundly alters the frequency of multiple quasar image.

astro-ph.CO↗

arXiv2013

Cosmological perturbations during the Bose-Einstein condensation of dark matter

In the present work, we analyze the evolution of the scalar and tensorial perturbations and the quantities relevant for the physical description of the Universe, as the density contrast of the scalar perturbations and the gravitational waves energy density during the Bose-Einstein condensation of dark matter. The behavior of these parameters during the Bose-Einstein phase transition of dark matter is analyzed in details. To study the cosmological dynamics and evolution of scalar and tensorial perturbations in a Universe with and without cosmological constant we use both analytical and numerical methods. The Bose-Einstein phase transition modifies the evolution of gravitational waves of cosmological origin, as well as the process of large-scale structure formation.

astro-ph.CO↗

arXiv2011

Constraints on the Generalized Chaplygin Gas Model from Gamma-Ray Bursts

We study the Generalized Chaplygin gas model (GCGM) using Gamma-ray bursts as cosmological probes. In order to avoid the so-called circularity problem we use cosmology-independent data set and Bayesian statistics to impose constraints on the model parameters. We observe that a negative value for the parameter $α$ is favoured if we adopt a flat Universe and the estimated value of the parameter $H_{0}$ is lower than that found in literature.

astro-ph.CO↗

arXiv2011

Observational constraints of the Gravitational Waves in the Brans-Dicke Theory

We investigate the quantum origin of the primordial cosmological gravitational waves in the Brans-Dicke theory. We compute the number of gravitons $N_k$ produced during inflation and the observables: power spectrum $P_T$, spectral index $n_T$ and energy density $Ω_k$. By comparison with General Relativity we see that the results for both theories are the same for the case of the particles number $N_k$ and for the case of the power spectrum $P_T$ and the energy density $Ω$ only when $ω<10$. For the spectral index $n_T$ we found that when the Brans-Dicke coupling parameter $ω$ is bigger than unity the spectral index approximates the expression found in the General Relativity. This may awake us to the possibility that $ω$ varies with the cosmological scale.

astro-ph.CO↗

arXiv2007

Constraining a double component dark energy model using type Ia supernovae data

A two-component fluid representing dark energy is studied. One of the components has a polytropic form, while the other has a barotropic form. Exact solutions are obtained and the cosmological parameters are constrained using supernova type Ia data. In general, an open universe is predicted. A big rip scenario is largely preferred, but the dispersion in the parameter space is very high. Hence, even if scenarios without future singularities can not be excluded with the allowed range of parameters, a phantom cosmology, with an open spatial section, is a general prediction of the model. For a wide range of the equation of state parameters there is an asymptotic de Sitter phase.

astro-ph↗

arXiv2007

The dynamics of the early universe and the initial conditions for inflation in a model with radiation and a Chaplygin gas

The modeling of the early universe is done through the quantization of a Friedmann-Robertson-Walker model with positive curvature. The material content consists of two fluids: radiation and Chaplygin gas. The quantization of these models is made by following the Wheeler and DeWitt's prescriptions. Using the Schutz formalism, the time notion is recovered and the Wheeler-DeWitt equation transforms into a time dependent Schrödinger equation, which rules the dynamics of the early universe, under the action of an effective potential $V_{ef}$. Using a finite differences method and the Crank-Nicholson scheme, in a code implemented in the program OCTAVE, we solve the corresponding time dependent Schrödinger equation and obtain the time evolution of a initial wave packet. This wave packet satisfies appropriate boundary conditions. The calculation of the tunneling probabilities shows that the universe may emerge from the Planck era to an inflationary phase. It also shows that, the tunneling probability is a function of the mean energy of the initial wave packet and of two parameters related to the Chaplygin gas. We also show a comparison between these results and those obtained by the WKB approximation.

gr-qc↗

arXiv2006

Different faces of the phantom

The SNe type Ia data admit that the Universe today may be dominated by some exotic matter with negative pressure violating all energy conditions. Such exotic matter is called {\it phantom matter} due to the anomalies connected with violation of the energy conditions. If a phantom matter dominates the matter content of the universe, it can develop a singularity in a finite future proper time. Here we show that, under certain conditions, the evolution of perturbations of this matter may lead to avoidance of this future singularity (the Big Rip). At the same time, we show that local concentrations of a phantom field may form, among other regular configurations, black holes with asymptotically flat static regions, separated by an event horizon from an expanding, singularity-free, asymptotically de Sitter universe.

gr-qc↗

arXiv2005

Modeling the spectrum of gravitational waves in the primordial Universe

Recent observations from type Ia Supernovae and from cosmic microwave background (CMB) anisotropies have revealed that most of the matter of the Universe interacts in a repulsive manner, composing the so-called dark energy constituent of the Universe. The analysis of cosmic gravitational waves (GW) represents, besides the CMB temperature and polarization anisotropies, an additional approach in the determination of parameters that may constrain the dark energy models and their consistence. In recent work, a generalized Chaplygin gas model was considered in a flat universe and the corresponding spectrum of gravitational waves was obtained. The present work adds a massless gas component to that model and the new spectrum is compared to the previous one. The Chaplygin gas is also used to simulate a $Λ$-CDM model by means of a particular combination of parameters so that the Chaplygin gas and the $Λ$-CDM models can be easily distinguished in the theoretical scenarios here established. The lack of direct observational data is partialy solved when the signature of the GW on the CMB spectra is determined.

gr-qc↗

arXiv2003

Generalized Chaplygin gas with $α= 0$ and the $ΛCDM$ cosmological model

The generalized Chaplygin gas model is characterized by the equation of state $p = - \frac{A}{ρ^α}$. It is generally stated that the case $α= 0$ is equivalent to a model with cosmological constant and dust ($ΛCDM$). In this work we show that, if this is true for the background equations, this is not true for the perturbation equations. Hence, the mass spectrum predicted for both models may differ.

astro-ph↗

arXiv2003

Troubles with quantum anistropic 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. A Schrödinger-type equation is obtained where the matter variables play the role of time. However, the signature of the kinetic term is hyperbolic. This Schrödinger-like equation is solved and a wave packet is constructed. The norm of the resulting wave function comes out to be time dependent, indicating the loss of unitarity in this model. The loss of unitarity is due to the fact that the effective Hamiltonian is hermitian but not self-adjoint. The expectation value and the bohmian trajectories are evaluated leading to different cosmological scenarios, what is a consequence of the absence of a unitary quantum structure. The consistency of this quantum model is discussed as well as the generality of the absence of unitarity in anisotropic quantum models.

gr-qc↗

arXiv2003

Bayesian Analysis of the Chaplygin Gas and Cosmological Constant Models using the SNe Ia Data

The type Ia supernovae observational data are used to estimate the parameters of a cosmological model with cold dark matter and the Chaplygin gas. The Chaplygin gas model depends essentially on four parameters: the Hubble constant, the velocity of the sound of the Chaplygin gas, the curvature of the Universe and the fraction density of the Chaplygin gas and the cold dark matter. The Bayesian parameter estimation yields $H_0 = 62.1^{+3.3}_{-3.4} km/Mpc.s$, $Ω_{k0} = -0.84^{+1.51}_{-1.23}$, $Ω_{m0} = 0.0^{+0.82}_{-0.0}$, $% Ω_{c0} = 1.40^{1.15}_{-1.16}$, $\bar{A} = c_s^2 = 0.93^{+0.07}_{-0.21} c $, $t_0 = 14.2^{+2.8}_{-1.3} Gy$ and $q_0 = - 0.98^{+1.02}_{-0.62}$. These and other results indicate that a Universe completely dominated by the Chaplygin gas is favoured, at least as the type Ia supernovae data are concerned. A closed and accelerating Universe is also favoured. The Bayesian statistics indicates that the Chaplygin gas model is more likely than the standard cosmological constant ($ΛCDM$) model at 55.3% confidence level when an integration on all free parameters is performed. Assuming the spatially flat curvature, this percentage mounts to 65.3%. On the other hand, if the density of dark matter is fixed at zero value, the Chaplygin gas model becomes more preferred than the $ΛCDM$ model at 91.8% confidence level. Finally, the hypothesis of flat Universe and baryonic matter ($Ω_{b0}=0.04$) implies a Chaplygin gas model preferred over the $ΛCDM$ at a confidence level of 99.4%.

astro-ph↗