Degenerate relic neutrino
= Degenerate relic neutrino
A degenerate relic neutrino has $\xi_\nu\gg1$. In the ultrarelativistic limit its <Fermi-Dirac distribution> approaches a filled momentum sphere, giving for one internal state
$$
\rho_\nu\simeq\frac{\mu_\nu^4}{8\pi^2},
$$
while the antineutrino density is exponentially suppressed.