  The equation of state of the hypothetical dark energy component, which
constitutes about two thirds of the critical density of the universe, may be
very different from that of a cosmological constant. Employing a
phenomenological model, we investigate semi-analytically the constraints
imposed on the scalar quintessence by supernovae observations, and by the
acoustic scale extracted from recent CMB data. We show that a universe with a
quintessence-dominated phase in the dark age is consistent with the current
observational constraints.
