NONTRIVIAL INTERACTIONS OF GRAVITATIONAL AND ELECTROMAGNETIC-WAVES WITH COSMIC STRINGS

The singular structure of the spacetime associated to a cosmic string interacting with either a plane-fronted gravitational wave or a pencil of electromagnetic radiation is analyzed. We find that depending on the value of the string constant the interaction can produce either a directional or an essential curvature singularity along the cosmic string.66326827

COSMIC WALLS FROM GRAVITATIONAL COLLAPSE

The formation of plane cosmic walls of negligible thickness from the collapse of smooth inhomogeneous plane-symmetric distributions of matter is considered. Two models with different asymptotic behaviors far from the wall in formation are constructed. In the first, the fluid far from the wall is anisotropic, with pressures proportional to density. The second model describes an asymptotically isotropic ideal gas in isentropic flow. Even though both models start from matter distributions with positive density and pressures everywhere, it is found that, during the collapse, negative pressures (tensions) appear within the wall in formation.49126500651

SPINNING STRINGS AND COSMIC DISLOCATIONS

It is shown that the 1+2 gravity spinning particle metric, when lifted to 1+3 dimensions in a boost-covariant way, gives rise to a chiral conical space-time which includes as particular cases the space-time of a spinning string and two space-times that can be associated with the chiral string with a lightlike phase and the twisted string recently discovered by Bekenstein. Some gravitational effects are briefly discussed and a possibility for a new type of anyon is mentioned.47104273427

Analytical potential-density pairs from complex-shifted Kuzmin-Toomre discs

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The complex-shift method is applied to the Kuzmin-Toomre family of discs to generate a family of non-axisymmetric flat distributions of matter. These are then superposed to construct non-axisymmetric flat rings. We also consider triaxial potential-density pairs obtained from these non-axisymmetric flat systems by means of suitable transformations. The use of the imaginary part of complex-shifted potential-density pairs is also discussed.398315631572Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

COSMIC STRINGS WITH CURVATURE CORRECTIONS

A generic model of string described by a Lagrangian density that depends on the extrinsic curvature of the string worldsheet is studied. Using a system of coordinates adapted to the string world sheet the equation of motion and the energy-momentum tensor are derived for strings evolving in curved spacetime. We find that the curvature corrections may change the relation between the string energy density and the tension. It can also introduce heat propagation along the string. We also find for the Polyakov as well as Nambu strings with a topological term that the open string end points can travel with a speed less than the velocity of light.4641721172

Potential-density pairs for bent bars

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A method is presented to bend a thin massive line when the curvature is small. The procedure is applied to a homogeneous thin bar with two types of curvatures. One of them mimics a galactic bar with two spiral arms at its tips. It is showed that if the bending function is a linear combination of Legendre polynomials, then the bent potential is an exact solution of the Laplace equation. A transformation is applied on the thin bent bars to generate three-dimensional potential-density pairs without singularities. The potentials of the thin bent bars are also used to generate non-axisymmetric planar distributions of matter.411423712382Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Chaos and fractals in geodesic motions around a nonrotating black hole with halos

We study the escape dynamics of test particles in general-relativistic gravitational fields generated by core-shell models, which are used in astrophysics as idealized models to observed mass distributions, such as the interior of galaxies. As a general-relativistic core-halo system, we use exact axisymmetric static solutions of Einstein's field equations which represent the superposition of a central Schwarzschild black hole (the core) and multipolar fields from external masses (the halo). We are particularly interested in the occurrence of chaos in the escape, which is characterized by a great sensitivity of the choice of escape by a test particle to initial conditions. The motion of both material particles and zero rest mass particles is considered. Chaos is quantified by the fractal dimension of the boundary between the basins of the different escapes. We find chaos in the motion of both material particles and null. geodesics, but its intensity depends strongly on the halo. We have found for all the cases we have considered that massless particles are less chaotic than massive particles.616A6506651

Space-time geometry and thermodynamic properties of a self-gravitating ball of fluid in phase transition

A numerical solution of Einstein field equations for a spherical symmetric and stationary system of identical and autogravitating particles in phase transition is presented. The fluid possesses a perfect fluid energy-momentum tensor, and the internal interactions of the system are represented by a van der Walls-like equation of state, able to describe a first order phase transition of the type gas-liquid. We find that the space-time curvature, the radial component of the metric, and the pressure and density show discontinuities in their radial derivatives in the phase coexistence region. This region is found to be a spherical surface concentric with the star, and the system can be thought of as a foliation of acronal, concentric and isobaric surfaces in which the coexistence of phases occurs in only one of these surfaces. This kind of system can be used to represent a star with a high energy density core and low energy density mantle in hydrodynamic equilibrium.70

THE INTERACTION OF OUTGOING AND INGOING SPHERICALLY SYMMETRICAL NULL FLUIDS

Using similarity methods, the Einstein field equations coupled to two oppositely directed null fluids for a spherically symmetric space-time are reduced to an autonomous system of three ordinary differential equations. The space of solutions is studied in some detail and solutions are found that represent: (i) the backscattering of an initially outgoing thick null fluid shell in a background gravitational held with a central naked singularity, (ii) the formation of strong space-time singularities by the interaction of thick null fluid shells, (iii) the interaction of a core of null radiation with an incoming shell of null fluid, and (iv) cosmological models of Kantowski-Sachs type with initial and final singularities clothed by apparent horizons. (C) 1995 American Institute of Physics.3673663367

Modeling the quantum evolution of the universe through classical matter

It is well known that the canonical quantization of the Friedmann-Lema\^itre-Robertson-Walker (FLRW) filled with a perfect fluid leads to nonsingular universes which, for later times, behave as their classical counterpart. This means that the expectation value of the scale factor $(t)$ never vanishes and, as $t\to\infty$, we recover the classical expression for the scale factor. In this paper, we show that such universes can be reproduced by classical cosmology given that the universe is filled with an exotic matter. In the case of a perfect fluid, we find an implicit equation of state (EoS). We then show that this single fluid with an implict EoS is equivalent to two non-interacting fluids, one of them representing stiff matter with negative energy density. In the case of two non-interacting scalar fields, one of them of the phantom type, we find their potential energy. In both cases we find that quantum mechanics changes completely the configuration of matter for small values of time, by adding a fluid or a scalar field with negative energy density. As time passes, the density of negative energy decreases and we recover the ordinary content of the classical universe. The more the initial wave function of the universe is concentrated around the classical big bang singularity, the more it is necessary to add negative energy, since this type of energy will be responsible for the removal of the classical singularity.Comment: updated version as accepted by Gen. Relativ. Gravi