Dark Energy and Some Alternatives: a Brief Overview

The high-quality cosmological data, which became available in the last decade, have thrusted upon us a rather preposterous composition for the universe which poses one of the greatest challenges theoretical physics has ever faced: the so-called dark energy. By focusing our attention on specific examples of dark energy scenarios, we discuss three different candidates for this dark component, namely, a decaying vacuum energy or time-varying cosmological constant [$\Lambda(t)$], a rolling homogeneous quintessence field ($\Phi$), and modifications in gravity due to extra spatial dimensions. As discussed, all these candidates [along with the vacuum energy or cosmological constant ($\Lambda$)] seem somewhat to be able to explain the current observational results, which hampers any definitive conclusion on the actual nature of the dark energy.Comment: 8 pages, 6 figures, Invited talk at XXVI Brazilian National Meeting on Particles and Fields, Sao Lourenco, Brazil. To appear in Brazilian Journal of Physics (2006

Energy Conditions and Segre Classification of Phantom Fields

Recent discoveries in the field of observational cosmology have provided increasing evidence that the Universe is undergoing a late time acceleration, which has also stimulated speculations on the nature of the dark component responsible for such a phenomenon. Among several candidates discussed in the current literature, phantom fields, an exotic scalar field with a negative kinetic term and that violates most of the classical energy conditions, appear as a real possibility according to recent observational analysis. In this paper we examine the invariant characterization for the energy-momentum tensor of phantom fields through the Segre algebraic classification in the framework of general relativity. We also discuss some constraints which are imposed on the values of $V(\phi)$ from the classical energy conditions.Comment: 4 pages, 1 figure, to appear in Phys. Lett.