H. P. de Oliveira

Non-linear charged black holes

Very strong electromagnetic fields are characterized by the appearance of non-linearities. In order to explore the consequences of such non-linearities we have solved exactly the combined gravitational non-linear electromagnetic field equations for a static and spherical symmetric spacetime, where the source of the curvature is a point electric charge. We have chosen a non-linear Lagrangian density in such a way that the Maxwell, the Born--Infeld, and the Heisenberg--Euler theories are included as particular cases. The general solution can represent a charged black hole with the same characteristics of the Reissner--Nordstro m one (two horizons and timelike singularity), or it can have only one horizon and a spacelike singularity, as in the Schwarzschild solution.

On perturbations in warm inflation

Warm inflation is an interesting possibility of describing the early universe, whose basic feature is the absence, at least in principle, of a preheating or reheating phase. Here we analyze the dynamics of warm inflation generalizing the usual slow-roll parameters that are useful for characterizing the inflationary phase. We study the evolution of entropy and adiabatic perturbations, where the main result is that for a very small amount of dissipation the entropy perturbations can be neglected and the purely adiabatic perturbations will be responsible for the primordial spectrum of inhomogeneities. Taking into account the COBE-DMR data of the cosmic microwave background anisotropy as well as the fact that the interval of inflation for which the scales of astrophysical interest cross outside the Hubble radius is about 50 e-folds before the end of inflation, we could estimate the magnitude of the dissipation term. It was also possible to show that at the end of inflation the universe is hot enough to provide a smooth transition to the radiation era.

Compactlike kinks and vortices in generalized models

This work deals with the presence of topological defects in k-field models, where the dynamics is generalized to include higher order power in the kinetic term. We investigate kinks in (1, 1) dimensions and vortices in (2, 1) dimensions, focusing on some specific features of the solutions. In particular, we show how the kinks and vortices change to compactlike solutions, controlled by the parameter used to introduce the generalized models.

Electrical properties of PVA/PPY blends

We present a dielectric study of polymeric blends composed by polypyrrole (PPY) incorporated into a matrix a polyvinyl alcohol (PVA) containing ferric chloride (FeCl 3 ). The PPY conducting regions are formed inside the matrix by in situ polymerization after the (PVA+FeCl 3 ) matrix is exposed to the vapor of the monomer. The mixed dielectric/conducting characteristics of the resulting blend is conveniently characterized by dielectric spectroscopy, and in this manner both the charge transport and charge storing processes can be followed. We have also examined how the temperature variation in the 300-24 K range affects the electrical behavior of the material.

Dynamical System Analysis for Inflation with Dissipation

We examine the solutions of the equations of motion for an expanding Universe, taking into account the radiation of the inflaton field energy. We then analyze the question of the generality of inflationary solutions in this more general setting of a dissipative system. We find a surprisingly rich behavior for the solutions of the dynamical system of equations in the presence of dissipational effects. We also determine that a value of dissipation as small as ∼ 10 −7 H can lead to a smooth exit from inflation to radiation.

Kolmogorov-Zakharov Spectrum in AdS Gravitational Collapse

We study black hole formation during the gravitational collapse of a massless scalar field in asymptotically D-dimensional anti-de Sitter AdS(D) spacetimes for D = 4, 5. We conclude that spherically symmetric gravitational collapse in asymptotically AdS spaces is turbulent and characterized by a Kolmogorov-Zakharov spectrum. Namely, we find that after an initial period of weakly nonlinear evolution, there is a regime where the power spectrum of the Ricci scalar evolves as ω(-s) with the frequency, ω, and s ≈ 1.7 ± 0.1.

Homoclinic chaos in the dynamics of a general Bianchi type-IX model

The dynamics of a general Bianchi IX model with three scale factors is examined. The matter content of the model is assumed to be comoving dust plus a positive cosmological constant. The model presents a critical point of saddle-center-center type in the finite region of phase space. This critical point engenders in the phase space dynamics the topology of stable and unstable four dimensional tubes

Non-singular inflationary cosmologies in Weyl integrable spacetime

R \times S^3

Critical phenomena of collapsing massless scalar wave packets

, where

Chaos and universality in the dynamics of inflationary cosmologies

R