2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/7637The conceptual difficulties associated with a cosmological constant have led to the investigation of alternative models in which the equation of state parameter, $w=p/ρ$, of the dark energy evolves with time. We show that combining the supernova type Ia observations {\it with the constraints from WMAP observations} restricts large variation of $ρ(z)$ at low redshifts. The combination of these two observational constraints is stronger than either one. The results are completely consistent with the cosmological constant as the source of dark energy.Final version to appear in MNRAS (Letters); discussion enlarged and clarifications and references added; 6 pages; 3 figuresAstrophysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext