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. 
J.S. Alcaniz (observatorio Nacional)