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(tau)], 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.