Early-universe constraints on dark energy

In the past years ``quintessence'' models have been considered which can produce the accelerated expansion in the universe suggested by recent astronomical observations. One of the key differences between quintessence and a cosmological constant is that the energy density in quintessence, _, could be a significant fraction of the overall energy even in the early universe, while the cosmological constant will be dynamically relevant only at late times. We use standard big bang nucleosynthesis and the observed abundances of primordial nuclides to put constraints on _ at temperatures near T1 MeV. We point out that current experimental data do not support the presence of such a field, providing the strong constraint _ (MeV) <0. 045 at 2 C. L. and strengthening previous results. We also consider the effect a scaling field has on cosmic microwave background (CMB) anisotropies using the recent data from BOOMERANG and DASI combined with SNIa data, providing the CMB constraint _<~0. 39 at 2 during the radiation dominated epoch.