arXiv subjects

G. A. Monerat

Publications and source records attributed to G. A. Monerat.

30 records · Page 2Linked to original sources

arXiv2009

A canonical transformation and the tunneling probability for the birth of an asymptotically DeSitter universe with dust

In the present work, we study the quantum cosmology description of closed Friedmann-Robertson-Walker models in the presence of a positive cosmological constant and a generic perfect fluid. We work in the Schutz's variational formalism. If one uses the scale factor and its canonically conjugated momentum as the phase space variables that describe the geometrical sector of these models, one obtains Wheeler-DeWitt equations with operator ordering ambiguities. In order to avoid those ambiguities and simplify the quantum treatment of the models, we introduce new phase space variables. We explicitly demonstrate that the transformation leading from the old set of variables to the new one is canonical. In order to show that the above canonical transformations simplify the quantum treatment of those models, we consider a particular model where the perfect fluid is dust. We solve the Wheeler-DeWitt equation numerically using the Crank-Nicholson scheme and determine the time evolution of the initial wave function. Finally, we compare the results for the present model with the ones for another model where the only difference is the presence of a radiative perfect fluid, instead of dust.

gr-qc↗

arXiv2007

Explorando Sistemas Hamiltonianos II: A Natureza de Pontos de Equilibrio Degenerados

Neste segundo artigo sobre sistemas Hamiltonianos, apresentamos o metodo da explosao para a determinacao da natureza de pontos fixos (pontos de equilibrio degenerados. Aplicamos o metodo a dois modelos hamiltonianos com um e dois graus de liberdade, respectivamente. Primeiramente, analisamos um sistema formado por um pendulo simples submetido a um torque externo constante T. Em seguida, consideramos um sistema formado por um pendulo duplo com segmentos de comprimentos e massas iguais, tambem submetidos a torques externos constantes e nao nulos. A presenca de pontos de equilibrio degenerados nos casos dos pendulos simples e duplo ocorre para certos valores dos torques externos. In this second article on Hamiltonian systems, we present the blow-up method for the determination of the nature of degenerate fixed points (equilibrium points). We apply the method to two hamiltonian models with one and two degrees of freedom, respectively. Firstly we study a system formed by a simple pendulum submitted to a constant external torque T. Then we consider a system formed by a double pendulum of segments with equal lengths and masses, also submitted to non-vanishing constant external torques. The presence of degenerate equilibrium points in both cases of simple and double pendulums occurs for some values of the external torques.

math-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↗

arXiv2007

Tunneling probability for the birth of an asymptotically DeSitter universe

In the present work, we quantize a closed Friedmann-Robertson-Walker model in the presence of a positive cosmological constant and radiation. It gives rise to a Wheeler-DeWitt equation for the scale factor which has the form of a Schrödinger equation for a potential with a barrier. We solve it numerically and determine the tunneling probability for the birth of a asymptotically DeSitter, inflationary universe, initially, as a function of the mean energy of the initial wave-function. Then, we verify that the tunneling probability increases with the cosmological constant, for a fixed value of the mean energy of the initial wave-function. Our treatment of the problem is more general than previous ones, based on the WKB approximation. That is the case because we take into account the fact that the scale factor ($a$) cannot be smaller than zero. It means that, one has to introduce an infinity potential wall at $a = 0$, which forces any wave-packet to be zero there. That condition introduces new results, in comparison with previous works.

gr-qc↗

arXiv2007

Reply to "Comment on 'Quantization of FRW spacetimes in the presence of a cosmological constant and radiation'"

The Comment by Amore {\it et al.} [gr-qc/0611029] contains a valid criticism of the numerical precision of the results reported in a recent paper of ours [Phys. Rev. D {\bf 73}, 044022 (2006)], as well as fresh ideas on how to characterize a quantum cosmological singularity. However, we argue that, contrary to what is suggested in the Comment, the quantum cosmological models we studied show hardly any sign of singular behavior.

gr-qc↗

arXiv2005

Quantization of Friedmann-Robertson-Walker spacetimes in the presence of a negative cosmological constant and radiation

We quantize three Friedmann-Robertson-Walker models in the presence of a negative cosmological constant and radiation. The models differ from each other by the constant curvature of the spatial sections, which may be positive, negative or zero. They give rise to Wheeler-DeWitt equations for the scale factor which have the form of the Schroedinger equation for the quartic anharmonic oscillator. We find their eigenvalues and eigenfunctions by using a method first developed by Chhajlany and Malnev, and use the eigenfunctions in order to construct wave packets for each case and evaluate the time-dependent expected value of the scale factors. We find for all of them that the expected values of the scale factors oscillate between maximum and minimum values. Since the expectation values of the scale factors never vanish, we conclude that these models do not have singularities.

gr-qc↗

arXiv2002

Qualitative Study of a Robot Arm as a Hamiltonian System

A double pendulum subject to external torques is used as a model to study the stability of a planar manipulator with two links and two rotational driven joints. The hamiltonian equations of motion and the fixed points (stationary solutions) in phase space are determined. Under suitable conditions, the presence of constant torques does not change the number of fixed points, and preserves the topology of orbits in their linear neighborhoods; two equivalent invariant manifolds are observed, each corresponding to a saddle-center fixed point.

cs.RO↗

arXiv2002

A Quantum Cosmological Model With Static and Dynamic Wormholes

Quantization is performed of a Friedmann-Robertson-Walker universe filled with a conformally invariant scalar field and a perfect fluid with equation of state $p=αρ$. A well-known discrete set of static quantum wormholes is shown to exist for radiation ($α=1/3$), and a novel continuous set is found for cosmic strings ($α= -1/3$), the latter states having throat radii of any size. In both cases wave-packet solutions to the Wheeler-DeWitt equation are obtained with all the properties of evolving quantum wormholes. In the case of a radiation fluid, a detailed analysis of the quantum dynamics is made in the context of the Bohm-de Broglie interpretation. It is shown that a repulsive quantum force inversely proportional to the cube of the scale factor prevents singularities in the quantum domain. For the states considered, there are no particle horizons either.

gr-qc↗

arXiv2002

Quantum cosmological perfect fluid models

Perfect fluid Friedmann-Robertson-Walker quantum cosmological models for an arbitrary barotropic equation of state $p = αρ$ are constructed using Schutz's variational formalism. In this approach the notion of time can be recovered. By superposition of stationary states, finite-norm wave-packet solutions to the Wheeler-DeWitt equation are found. The behaviour of the scale factor is studied by applying the many-worlds and the ontological interpretations of quantum mechanics. Singularity-free models are obtained for $α< 1$. Accelerated expansion at present requires $- 1/3 > α> - 1$.

gr-qc↗

arXiv2001

Bianchi type IX world with torsion and shear

Oscillating phases in Bianchi type IX anisotropic universes are obtained as linearized solutions of Einstein-Cartan theory of gravity.Instead of making use of Weyssenhoff spinning fluids we addopt axionic torsion matter where the torsion tensor is totally skew-symmetric as in Kalb-Rammond type theories.Spin-torsion,metric and matter density oscillate at different phases.

astro-ph↗

arXiv1998

Chaos in Preinflationary Friedmann-Robertson-Walker Universes

The dynamics of a preinflacionary phase of the universe, and its exit to inflation, is discussed. This phase is modeled by a closed Friedmann-Robertson-Walker geometry, the matter content of which is radiation plus a scalar field minimally coupled to the gravitational field. The simple configuration, with two effective degres of freedom only, presents a very complicated dynamics connected to the existence of critical points of saddle-center type and saddle type in phase space of the system. Each of these critical points is associated to an extremum of the scalar field potential. The Topology of the phase space about the saddle-center is characterized by homoclinic cylinders emanating from unstable periodic orbits, and the transversal crossing of the cylinders, due to the non-integrability of the system, results in a chaotic dynamics. The topology of the homoclinic cylinders provides an invariant characterization of chaos. The model exhibits one or more exits to inflation, associated to one or more strong asymptotic de Sitter attractors present in phase space, but the way out from the initial singularity into any of the inflationary exits is chaotic. We discuss possible mechanisms, connected to the spectrum of inhomogeneous fluctuations in the models, which would allow us to distinguish physically the several exits to inflation.

gr-qc↗

arXiv1997

Chaotic exit to inflation: pre-inflationary Friedmann-Robertson-Walker universes

We show that homogeneous and isotropic cosmological models with radiation and scalar field have, in general, two possible chaotic exits to inflation. These models may describe the early stages of inflation. The central point of our analysis is based on the possible existence of one or two saddle-centers points in the phase space of the models.

gr-qc↗