Alternative dark energy models: An overview

A large number of recent observational data strongly suggest that we live in a flat, accelerating Universe composed of similar to 1/3 of matter (baryonic + dark) and similar to 2/3 of an exotic component with large negative pressure, usually named Dark Energy or Quintessence. The basic set of experiments includes: observations from SNe la, CMB anisotropies, large scale structure, X-ray data from galaxy clusters, age estimates of globular clusters and old high redshift galaxies (OHRG's). It is now widely believed that such results provide the remaining piece of information connecting the inflationary flatness prediction (Omega(T) = 1) with astronomical observations. From a theoretical viewpoint, they have also stimulated the current interest for more general models containing an extra component describing this unknown dark energy, and simultaneously accounting for the present accelerating stage of the Universe. In this review we present a simplified picture of the main results and discuss briefly some difficulties underlying the emerging dark energy paradigm.