Extended quintessence and the primordial helium abundance - art. no. 123504

In extended quintessence models, a scalar field which couples to the curvat ure scalar R provides most of the energy density of the universe. We point out that such models can also lead naturally to a decrease in the primordia l abundance of helium-4, relieving the tension which currently exists betwe en the primordial helium-4 abundance inferred from observations and the amo unt predicted by standard big bang nucleosynthesis corresponding to the obs erved deuterium abundance. Using negative power-law potentials for the quin tessence field, we determine the range of model parameters which can lead t o an interesting reduction in the helium-ii abundance, and we show that it overlaps with the region allowed by other constraints on extended quintesse nce models.