No abstract available

No abstract available

No abstract available

[en] An idealized spherically symmetric ralativistic model of an exploding object is constructed within the framework of the theory of surface layers in GR. A Vaidya solution for a radially radiating star is matched through a spherical shell of dust to a Schwarzschild solution. The (incomplete) equations for the motion of the spherical shell of dust and the radiation density of the Vaidya solution, as given by the matching conditions, are reduced for a first-order system and a general analysis of the characteristics of the motion is given. This system of differential equations is completed, adding a relation between the unknowns which represents the simplest way to avoid an unphysical signgularity in the motion. The results of a numerical integration of the equations are presented in two cases which the authors think may have some relationship to stellar explosions. A comparative set of results for other solutions is also given, and some possible generalizations of the model are pointed out. (Auth.)

[en] We analyse exact static solutions of the vacuum Einstein equation obtained from the Appell ring solution to the Laplace equation. We first perform a Newtonian analysis and show that the Appell solution may be interpreted as the field of a shell of infinite negative total mass bordered by a ring of infinite positive total mass. This situation can be reproduced in general relativity, but a different interpretation, not requiring the shell as a source, is also possible pasting two spacetimes through the ring. The superposition of a particle with an Appell ring is also considered and in this case the shell interpretation is found to be inconsistent. (author)

[en] We consider one-soliton perturbations of a flat Friedmann-Robertson-Walker (FRW) cosmological model, with an ideal fluid with pressure equal to the energy density (stiff fluid), in the case where the 'pole trajectory' parameter is negative, introducing thereby singularities along certain null hypersurfaces. Starting with a metric that approaches asymptotically the FRW background, we show that it is possible to construct an extension through these hypersurfaces such that the energy momentum tensor T_ab is finite and satisfies the energy conditions. The extension is only C¹, providing a sort of 'shock front' with continuity in T_ab, that has an associated phase transition from null dust to stiff fluid, the transition being of the form described by Chandrasekhar and Xanthopoulos. (author)

[en] In this paper it is shown that if Vaidya's radiation metric is considered from the point of view of kinetic theory in general relativity, the corresponding phase space distribution function can be generalized in a particular way. The new family of spherically symmetric radiation metrics obtained contains Vaidya's as a limiting situation. The Einstein field equations are solved in a ''comoving'' coordinate system. Two arbitrary functions of a single variable are introduced in the process of solving these equations. Particular examples considered are a stationary solution, a nonvacuum solution depending on a single parameter, and several limiting situations. (author)

[en] This paper analyzes the behavior of a family of finite exact perturbations of Friedmann-Robertson-Walker cosmologies constructed with the inverse scattering technique of Belinskii and Zakharov (1978). The application of this method in a five-dimensional spacetime with a massless scalar field and a subsequent Kaluza-Klein dimensional reduction makes it possible to construct models with perfect fluid material content in four dimensions. The behavior of the energy momentum and Weyl tensors are studied as characterizations of the gravitational and material behaviors. The important case of solitonic perturbations on de Sitter backgrounds is also treated. 45 refs

[en] The properties of a simples quantum mechanical model for the decay of two equal mass particles are studied and related to some recent work on complex energy eigenvalues. It consists essentially in a generalization of the Lee-Friedrichs model for an unstable particle and gives a highly idealized version of the K⁰-anti K⁰ system, including CP violations. The model is completely solvable, thus allowing a comparison with the well known Weisskopf-Wigner formalism for the decay amplitudes. A different model, describing the same system is also briefly outlined. (Author)

No abstract available