  An interesting dynamical model for dark energy which does not require
extremely light scalar fields such as quintessence, and at the same time
explains the (near-) coincidence between the neutrino and dark energy densities
is the model of dark energy coupled to mass varying neutrinos (MaVaNs). Despite
the attractions of this model, we show that, generically, this model contains a
catastrophic instability which occurs when neutrinos become non-relativistic.
As a result of this instability, as neutrinos become non-relativistic, they
condense into neutrino nuggets which redshift away similar to cold dark matter,
and thus cease to act as dark energy. Any stable MaVaNs dark energy model is
extremely contrived, and is virtually indistinguishable from a cosmological
constant.
