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 feautures 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 inportant only when neutrinos become non-relativistic. Some speculations on features of this scenario which might be present in a more realistic theory are also presented. 
R. D. Peccei (Department of Physics and Astronomy, University of   California at Los Angeles)