2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/186702Recent data and new data analysis methods show that most probably the parameter $w$ in the equation of state of the dark energy is smaller than -1 at low redshifts. We briefly review some of the models with such a property and without violating null energy condition. We investigate the difference between the observables and predictions of these models, and how they can be explored to single out or constrain the origin of dark energy and its properties.4 papes, 2 figures. To appear in the proceedings of PASCOS-07 conference, Imperial College, London, UK, 2-7 July 2007AstrophysicsHigh Energy Physics - Phenomenologytext