MG11 
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Abstract

Extreme Gravitational Lensing by Supermassive Black Holes

Photons grazing a supermassive black hole experience large deflections that cannot be described in the weak field approximation. Such photons give rise to very faint additional images of any given source, which would show up just outside the apparent shadow of the black hole. We investigate the properties of these images through an analytical expansion of the deflection angle around the minimal impact parameter, showing that their positions and brightnesses are related to the parameters of the black hole and the gravitational theory used to describe it. We present estimates for the position and the luminosity of images for several kinds of potential sources orbiting around Sgr A*, which indeed represents an ideal candidate lens, discussing the technical requirements for their detection. In particular, we suggest that future space interferometry missions will be able to detect these elusive witnesses of the strong gravitational fields generated by black holes. References: gr-qc/0208075, astro-ph/0503664, gr-qc/0507137, gr-qc/0604093

General solution for scalar perturbations in bouncing cosmologies

Bouncing cosmologies, suggested by String/M-theory, may provide an alternative to standard inflation to account for the origin of inhomogeneities in our universe. The fundamental question regards the correct way to evolve the scalar perturbations through the bounce. In this talk, we present the evolution of perturbations and the final spectrum for an arbitrary (spatially flat) bouncing cosmology, with the only assumption that the bounce is governed by a single physical scale. In particular, we find the condition for the pre-bounce growing mode of the Bardeen potential (which is scale-invariant in the Ekpyrotic scenario) to survive unaltered in the post-bounce. If some new physics acting at the bounce satisfies such condition, then bouncing cosmologies are entitled to become a real viable alternative to inflation for the generation of the observed inhomogeneities. References: hep-th/0502047, gr-qc/0506040, hep-th/0512066