A Relativistic Description of Gentry's New Redshift Interpretation

We obtain a new expression of the Friedmann-Robertson-Walker metric, which is an analogue of a static chart of the de Sitter space-time. The reduced metric contains two functions, $M(T,R)$ and $\Psi(T,R)$, which are interpreted as, respectively, the mass function and the gravitational potential. We find that, near the coordinate origin, the reduced metric can be approximated in a static form and that the approximated metric function, $\Psi(R)$, satisfies the Poisson equation. Moreover, when the model parameters of the Friedmann-Robertson-Walker metric are suitably chosen, the approximated metric coincides with exact solutions of the Einstein equation with the perfect fluid matter. We then solve the radial geodesics on the approximated space-time to obtain the distance-redshift relation of geodesic sources observed by the comoving observer at the origin. We find that the redshift is expressed in terms of a peculiar velocity of the source and the metric function, $\Psi(R)$, evaluated at the source position, and one may think that this is a new interpretation of {\it Gentry's new redshift interpretation}.Comment: 11 pages. Submitted to Modern Physics Letters

On the Nature of Singularities in Plane Symmetric Scalar Field Cosmologies

The nature of the initial singularity in spatially compact plane symmetric scalar field cosmologies is investigated. It is shown that this singularity is crushing and velocity dominated and that the Kretschmann scalar diverges uniformly as it is approached. The last fact means in particular that a maximal globally hyperbolic spacetime in this class cannot be extended towards the past through a Cauchy horizon. A subclass of these spacetimes is identified for which the singularity is isotropic.Comment: 7 pages, MPA-AR-94-

Black Hole Criticality in the Brans-Dicke Model

We study the collapse of a free scalar field in the Brans-Dicke model of gravity. At the critical point of black hole formation, the model admits two distinctive solutions dependent on the value of the coupling parameter. We find one solution to be discretely self-similar and the other to exhibit continuous self-similarity.Comment: 4 pages, REVTeX 3.0, 5 figures include

Classical Black Hole Production In Quantum Particle Collisions

The semiclassical picture of black hole production in trans-Planckian elementary particle collisions is reviewed.Comment: 5 pages, 7 figures; talk given at the 6th Alexander Friedmann International Seminar on Gravitation and Cosmology, Cargese, France, June 28-July 3, 2004; to appear in the proceedings (Int.J.Mod.Phys.A); v2: typos correcte

An Ontology Engineering Approach to User Profiling for Virtual Tours of Museums and Galleries

This paper describes a study of the development of a hierarchical ontology for producing and maintaining personalized profiles to improve the experience of visitors to virtual art galleries and museums. The paper begins by describing some of the features of virtual exhibitions and offers examples of virtual tours that the reader may wish to examine in more detail. The paper then discusses the ontology engineering (OE) approach and domain modelling languages (e.g. KACTUS, SENSUS and METHONTOLOGY). It then follows a basic OE approach to define classes for a cultural heritage virtual tour and to produce a Visitor Profile Ontology that is hierarchical and has static and dynamic elements. It concludes by suggesting ways in which the ontology may be automated to provide a richer, more immersive personalized visitor experience

Gamma-ray bursts as the birth-cries of black holes

The origin of cosmic gamma-ray bursts remains one of the most intriguing puzzles in astronomy. We suggest that purely general relativistic effects in the collapse of massive stars could account for these bursts. The late formation of closed trapped surfaces can occur naturally, allowing the escape of huge energy from curvature-generated fireballs, before these are hidden within a black hole.Comment: 4 pages Revtex, 1 figure. This essay received an honorable mention in the Gravity Research Foundation essay competitio

The role of the quantum properties of gravitational radiation in the dete ction of gravitational waves

The role that the quantum properties of a gravitational wave could play in the detection of gravitational radiation is analyzed. It is not only corroborated that in the current laser-interferometric detectors the resolution of the experimental apparatus could lie very far from the corresponding quantum threshold (thus the backreaction effect of the measuring device upon the gravitational wave is negligible), but it is also suggested that the consideration of the quantum properties of the wave could entail the definition of dispersion of the measurement outputs. This dispersion would be a function not only of the sensitivity of the measuring device, but also of the interaction time (between measuring device and gravitational radiation) and of the arm length of the corresponding laser- interferometer. It would have a minimum limit, and the introduction of the current experimental parameters insinuates that the dispersion of the existing proposals could lie very far from this minimum, which means that they would show a very large dispersion.Comment: 19 pages, Latex (use epsfig.sty

Estimates for the volume of a Lorentzian manifold

We prove new estimates for the volume of a Lorentzian manifold and show especially that cosmological spacetimes with crushing singularities have finite volume.Comment: 8 pages, a pdf version of the preprint can also be retrieved from http://www.math.uni-heidelberg.de/studinfo/gerhardt/LM-Volume.pdf v2: A further estimate has been added covering the case when the mean curvature is merely non-negative resp. non-positive (Theorem 1.1