New Matter Effects and BBN Constraints for Mass Varying Neutrinos

The presence of light (m_a ~ 10^-6 ev) scalar fields in the early universe can modify the cosmology of neutrinos considerably by allowing their masses to vary on cosmological times. In this paper, we consider the effect of Planck-suppressed couplings of this scalar to electrons and show that such couplings can easily make new sterile states thermally inaccessible in the early universe, preserving the successes of big bang nucleosynthesis predictions. We consider the circumstances under which these effects give the proper initial conditions for recently considered models of neutrino dark energy, and consider limits from tests of the equivalence principle. The parameters which satisfy cosmological constraints naturally give rise to interesting signals in terrestrial neutrino oscillation experiments.