  We show that mass varying neutrinos (MaVaNs) can behave as a negative
pressure fluid which could be the origin of the cosmic acceleration. We derive
a model independent relation between the neutrino mass and the equation of
state parameter of the neutrino dark energy, which is applicable for general
theories of mass varying particles. The neutrino mass depends on the local
neutrino density and the observed neutrino mass can exceed the cosmological
bound on a constant neutrino mass. We discuss microscopic realizations of the
MaVaN acceleration scenario, which involve a sterile neutrino. We consider
naturalness constraints for mass varying particles, and find that both ev
cutoffs and ev mass particles are needed to avoid fine-tuning. These
considerations give a (current) mass of order an eV for the sterile neutrino in
microscopic realizations, which could be detectable at MiniBooNE. Because the
sterile neutrino was much heavier at earlier times, constraints from big bang
nucleosynthesis on additional states are not problematic. We consider regions
of high neutrino density and find that the most likely place today to find
neutrino masses which are significantly different from the neutrino masses in
our solar system is in a supernova. The possibility of different neutrino mass
in different regions of the galaxy and the local group could be significant for
Z-burst models of ultra-high energy cosmic rays. We also consider the cosmology
of and the constraints on the ``acceleron'', the scalar field which is
responsible for the varying neutrino mass, and briefly discuss neutrino density
dependent variations in other constants, such as the fine structure constant.
