Talk detail

Participant	Lemos, Jose Sande
Institution	Physics Department, Instituto Superior Tecnico, University of Lisbon - Avenida Rovisco Pais 1 - Lisboa - Lisboa - Portugal

Session	BH4	Accepted		Order	Time
Talk	Oral abstract	Title	Compactness of relativistic charged spheres
Coauthors
Abstract	The Buchdahl bound states that the radius to mass ratio of a star is equal or greater than 9/4, on the assumption that the star is made of a perfect fluid, the density is a nonincreasing function of the radius and the exterior is the Schwarzschild solution. The bound is saturated by the infinite central pressure Schwarzschild interior solution. A generalization of this bound to electrically charged stars has been given by Andreeasson. This Buchdahl-Andreasson bound is found through the assumption that the radial pressure plus twice the tangential pressure of the matter is less than the energy density. For zero electric charge one recovers the Buchdahl bound. A class of configurations that saturate the electrically charged Buchdahl-Andreasson bound are electrically charged shells. Another class of configurations is found here. Indeed, Guilfoyle's electrically charged stars which have a very stiff equation of state, the Cooperstock-de la Cruz-Florides equation of state, also saturate the bound. When the electric charge is zero Guilfoyle's stars reduce to the Schwarzschild incompressible stars. It remains to find a proof in Buchdahl's manner such that these configurations are also thelimiting configurations of the Buchdahl-Andreasson bound
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Session	ES1	Accepted		Order	Time
Talk	Oral abstract	Title	General Relativity in d->infinite dimensions: The Shapiro Delay Effect
Coauthors
Abstract	General Relativity (GR) in more than four spacetime dimensions has been the subject of increasing attention in recent years. In the limit of very many dimensions, GR simplifies dramatically since it reduces to a theory of non-interacting particles, of a given radius and with negligible. In this limit, although the particle dynamics becomes almost trivial, the study of the theory in an 1/d expansion presents nevertheless very interesting features. We discuss the Shapiro delay effect for a photon orbit emitted from a point and reflected back from another point in a central gravitational field in the d->infinity limit. We also show through explicit examples that this higher d expansion can be a very efficient method for analytic calculations, achieving surprisingly accurate results even for relatively low dimensions.
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