A model for time-dependent cosmological constant and its consistency with the present Friedmann universe

We use a model where the cosmological term can be related to the chiral gauge anomaly of a possible quantum scenario of the initial evolution of the universe. We show that this term is compatible with the Friedmann behavior of the present universe.Comment: 5 pages, Revtex 4, twocolumn (minor corrections and improved reference list. To appear in Classical and Quantum Gravity

Can Non Gravitational Black Holes Exists?

We claim that the existence of a mechanism such that photons may be trapped in a compact domain is not an exclusive property of gravitational forces. We show the case in which a non-linear electrodynamics allows such effect. In this latter case we should call this region an Electromagnetic Black Hole (EBH).Comment: 11 pages, LaTex, no figures, revised versio

Effective geometry in Astrophysics

The effective metric is introduced by means of two examples (non-linear electromagnetism and hydrodynamics),along with applications in Astrophysics. A sketch of the generality of the effect is also given.Comment: 9 pages, contributions for the proceedings of the First International Workshop on Astronomy and Relativistic Astrophysics (IWARA 2003), Olinda (Brazil