Relic neutrinos and cosmic background radiation: a new way of comparison
  A new equation of state is proposed in order to describe the thermal behavior of relic neutrinos. It is based on extensions of the MIT bag model to deal with the gravitational interaction and takes in account the fermionic character of neutrinos. The results for the temperature and entropy of relic neutrinos are compared with those of the cosmic background radiation, treated as a gas of photons at the temperature of 2.726 K. In particular, it is found that the temperature of the relic neutrinos is 3/4 of that of the photon gas. The ratio between the two entropies is also estimate. 
P. R. Silva
physics.gen-ph