Cosmological Neutrino Condensates

We investigate the possibility that neutrinos form superfluid-type condensates in background cosmological densities. Such condensates could give rise to small neutrino masses and splittings, as well as an important contribution, perhaps, to the cosmological constant. We discuss various channels in the context of the standard model. Many of these do not support a condensate, but some mixed-flavor channels do. We also suggest a new interaction, acting only among neutrinos, that could induce a neutrino Majorana mass of order 1 eV.