E. S. Moreira

Classical self-forces in a space with a topological defect

We derive the gravitational and electrostatic self-energies of a particle at rest in the background of a cosmic dispiration (topological defect), finding that the particle may experience potential steps, well potentials or potential barriers depending on the nature of the interaction and also on certain properties of the defect. The results may turn out to be useful in cosmology and condensed matter physics.

Remarks on vacuum fluctuations around a spinning cosmic string

We apply the point-splitting method to investigate vacuum fluctuations in the nonglobally hyperbolic background of a spinning cosmic string. Implementing renormalization by removing the Minkowski contribution it is shown that although the Green function in the literature satisfies the usual Green function equation, it leads to pathological physical results. 04.70.Dy, 04.62.+v

Hot scalar radiation setting bounds on the curvature coupling parameter

This paper addresses the interplay between vacuum and thermal local averages for massless scalar radiation near a plane wall of a large cavity where the Dirichlet boundary condition is assumed to hold. The main result is that stable thermodynamic equilibrium is possible only if the curvature coupling parameter is restricted to a certain range. In more than three spacetime dimensions such a range contains the conformal coupling, but it does not contain the minimal coupling. Since this same range for possible values of the curvature coupling parameter also applies to massive scalar radiation, it may be relevant in settings where arbitrarily coupled scalar fields are present.

Spinning strings, cosmic dislocations, and chronology protection

Instituto de Ciˆencias Exatas, Universidade Federal de Itajub´a,Av. BPS 1303 Pinheirinho, 37500-903 Itajub´a, Minas Gerais, Brazil(Dated: September, 2003)A massless scalar field is quantized in the background of a spinning string with cosmic dislocation.By increasing the spin density toward the dislocation parameter, a region containing closed timelikecurves (CTCs) eventually forms around the defect. Correspondingly, the propagator tends to the or-dinary cosmic string propagator, leading therefore to a mean-square field fluctuation, which remainswell behaved throughout the process, unlike the vacuum expectation value of the energy-momentumtensor, which diverges due to a subtle mechanism. These results suggest that back reaction leadsto the formation of a “horizon” that protects from the appearance of CTCs.

Semiclassical backreaction around a cosmic dislocation

The energy-momentum vacuum average of a conformally coupled massless scalar field vibrating around a cosmic dislocation--a cosmic string with a dislocation along its axis--is taken as source of the linearized semiclassical Einstein equations. The solution up to first order in the Planck constant is derived. Motion of a test particle is then discussed, showing that under certain circumstances a helical-like dragging effect, with no classical analogue around the cosmic dislocation, is induced by backreaction.

Vacuum stress around a topological defect

Abstract We show that a dispiration (a disclination plus a screw dislocation) polarizes the vacuum of a scalar field giving rise to an energy momentum tensor which, as seen from a local inertial frame, presents non vanishing off-diagonal components. The results may have applications in cosmology (chiral cosmic strings) and condensed matter physics (materials with linear defects).

Hot scalar radiation around a cosmic string setting bounds on the coupling parameter ξ

A bstractIn this work, by investigating the low temperature behaviour of a scalar field around a cosmic string, and assuming stable thermodynamic equilibrium, it is shown that the coupling parameter of the field with curvature must be restricted to a certain range of values whose bounds are determined by the deficit angle of the associated conical geometry.

Local thermal behaviour of a massive scalar field near a reflecting wall

A bstractThe mean square fluctuation and the expectation value of the stress-energy-momentum tensor of a neutral massive scalar field at finite temperature are determined near an infinite plane Dirichlet wall, and also near an infinite plane Neumann wall. The flat background has an arbitrary number of dimensions and the field is arbitrarily coupled to the vanishing curvature. It is shown that, unlike vacuum contributions, thermal contributions are free from boundary divergences, and that the thermal behaviour of the scalar field near a Dirichlet wall differs considerably from that near a Neumann wall. Far from the wall the study reveals a local version of dimensional reduction, namely, corrections to familiar blackbody expressions are linear in the temperature, with the corresponding coefficients given only in terms of vacuum expectation values in a background with one less dimension. It is shown that such corrections are “classical” (i.e., not dependent on Planck’s constant) only if the scalar field is massless. A natural conjecture that arises is that the “local dimensional reduction” is universal since it operates for massless and massive fields alike and regardless of the boundary conditions.

Gravitational spectral shift caused by Casimir stresses

The linearized Einstein field equations with the renormalized stress tensor of a massless quantum scalar field as source are solved in the 4-dimensional spacetime near an infinite plane boundary. The motion of particles and light is investigated disclosing a variety of effects. For instance, it is shown that light rays initially parallel to the boundary do not bend. On the other hand, under the same initial conditions, particles will be affected by the presence of the quantum field, being accelerated toward the plane boundary or departing from it depending on the value of the curvature coupling parameter {\xi}. It is suggested that {\xi} can be set by measuring the spectral deviation of a light ray travelling perpendicularly to the boundary.

Vacuum polarization around a spinning cosmic string

Abstract We investigate vacuum polarization of a scalar field in the gravitational background of a spinning cosmic string. The behavior of the vacuum fluctuations is confronted with the chronology protection conjecture concerning the formation of time machines.