Papers by Sergio Vitorino de Borba Goncalves
Physical review, Nov 15, 1997
We study perturbatively a two-fluid model, where one of the fluids has an equation of state pϭϪ/3... more We study perturbatively a two-fluid model, where one of the fluids has an equation of state pϭϪ/3 ͑stringlike fluid͒, considering fluctuations in both fluids. Such a model can represent a closed universe whose dynamics is that of an open universe. We find strong instabilities at small scale in the perturbation of the stringlike fluid, whereas the ordinary matter fluid has a stable behavior. We discuss the consequences for the anisotropy of the cosmic microwave background. ͓S0556-2821͑97͒06122-5͔
... Este trabalho é dedicado integralmente a Gastão, Roberta e Igor, meu esposo e meus filhos que... more ... Este trabalho é dedicado integralmente a Gastão, Roberta e Igor, meu esposo e meus filhos queridos. ... A quantidade ds2, denominada elemento de linha, representa uma distância infinite-simal entre dois pontos xµ exµ + dxµ e é expressa em termos da métrica por meio da ...
EAS Publications Series, 2009
We model the cosmic medium as the mixture of a generalized Chaplygin gas and a pressureless matte... more 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.
International Journal of Modern Physics A, 2002
The quantization of gravity coupled to a perfect fluid model leads to a Schrödinger-like equation... more 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 = α ρ; 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. 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 fluid1,2. The existence of this universal classical analogue may imply that this perfect fluid coupled to gravity model is not a real quantum system. 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...
Classical and Quantum Gravity, 1999
We study the tensor modes of perturbations of a two-fluid model in general relativity, where one ... more We study the tensor modes of perturbations of a two-fluid model in general relativity, where one of the fluids has the equation of state p = -icons/Journals/Common/rho" ALT="rho" ALIGN="TOP"/>/3 (a variable cosmological constant, a cosmic string fluid, a texture) or p = -icons/Journals/Common/rho" ALT="rho" ALIGN="TOP"/> (a cosmological constant), while the other is ordinary matter (radiation, stiff matter, incoherent matter). In
General Relativity and Gravitation, 2002
We study the fate of density perturbations in an Universe dominate by the Chaplygin gas, which ex... more 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.
General Relativity and Gravitation, 2003
The anisotropic Bianchi I cosmological model coupled with perfect fluid is quantized in the minis... more 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 Schrödingerlike 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.
In the isotropic quantum cosmological perfect fluid model, the initial singularity can be avoided... more 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 "Schr\"odinger-like" equation where the
Physics Letters B, 2012
We investigate the quantum origin of the primordial cosmological gravitational waves in the Brans... more We investigate the quantum origin of the primordial cosmological gravitational waves in the Brans-Dicke theory in the two conformally related frames, the Jordan-Brans-Dicke frame and the Einstein frame. We calculated the theoretical observable in both frames and we compared both with General Relativity. 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. The comparison shows that for the case of the particles number N k the results are the same in both frames and in General Relativity when the Brans-Dicke parameter is much bigger than unity. For the spectral index n T we show that it is possible to get a scale invariant perturbation in the Jordan-Brans-Dicke frame when ω → ∞ and in the Einstein frame when ω → ±∞. In both frames, the results found for the power spectrum P T and the energy density Ω show that the preferred values of ω are different from that are found in the local tests.
Physics Letters A, 2001
Quantization in the mini-superspace of a gravity system coupled to a perfect fluid, leads to a so... more 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.
Physical Review D, 2002
The quantization of gravity coupled to a perfect fluid model leads to a Schrödingerlike equation,... more The quantization of gravity coupled to a perfect fluid model leads to a Schrödingerlike 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.
International Journal of Modern Physics A, 2001
A qualitative analysis of a scalar–tensor cosmological model, with an exponential potential for t... more 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 behavior appear. We discuss the possible existence of realistic scenarios and we generalize the results, adding ordinary matter to the scalar–tensor model.
EPL (Europhysics Letters), 2008
Scalar perturbations can grow during a phantomic cosmological phase as the big rip is approached,... more 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 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.
Astrophysics and Space Science, 1996
We give here the calculation of density perturbations in a gravitation theory with a scalar field... more We give here the calculation of density perturbations in a gravitation theory with a scalar field non-minimally coupled to gravity, i.e., the Brans-Dicke Theory of gravitation. The purpose is to show the influence of this scalar field on the dynamic behaviour of density perturbations along the eras where the equation of state for the matter can be put under the form p = αρ, where α is a constant. We analyse the asymptotic behaviour of this perturbations for the cases α = 0, α = −1, α = 1/3 and ρ = 0. In general, we obtain a decaying and growing modes. In the very important case of inflation, α = −1, there is no density perturbation, as it is well known. In the vacuum phase the perturbations on the scalar field and the gravitational field present growing modes at the beginning of the expansion and decaying modes at the end of this phase. In the case α = 0 it is possible, for some negative values of ω, to have an amplification of the perturbations with a superluminal expansion of the scale factor. We can also obtain strong growing modes for the density contrast for the case where there is a contraction phase which can have physical interest in some primordial era.
Uploads
Papers by Sergio Vitorino de Borba Goncalves