Neutrino models of dark energy

I consider a scenario proposed by Fardon, Nelson, and Weiner where dark energy and neutrinos are connected. As a result, neutrino masses are not constant but depend on the neutrino number density. By examining the full equation of state for the dark sector, I show that in this scenario the dark energy is equivalent to having a cosmological constant, but one that "runs" as the neutrino mass changes with temperature. Two examples are examined that illustrate the principal features of the dark sector of this scenario. In particular, the cosmological constant is seen to be negligible for most of the evolution of the Universe, becoming important only when neutrinos become nonrelativistic. Some speculations on features of this scenario which might be present in a more realistic theory are also presented.