Jeremiah Birrell, Cheng-Tao Yang, Pisin Chen, Johann Rafelski
A deviation from chemical equilibrium in the primordial neutrino distribution arises naturally before BBN due to the drop in the deceleration parameter of the universe as the electron mass becomes an important scale after the neutrino chemical freeze-out at temperature $T_c$ and before the kinetic freeze-out at temperature $T_k$. This effect is described by a primordial neutrino fugacity $\Upsilon_\nu$ different from unity. We determine the magnitude of $\Upsilon_\nu$ as a function of $T_k$ and demonstrate a possible large modification of the primordial neutrino momentum distribution. $\Upsilon_\nu$ is preserved in the free-streaming neutrino momentum distribution and it is present at time of ion-electron recombination. It should thus serve as a new cosmological model parameter, hitherto presumed to be close to unity, in the evaluation of CMB fluctuations. We establish a modified reheating relation between the current photon and neutrino temperatures required by $\Upsilon_\nu\ne 1$.
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http://arxiv.org/abs/1303.2583
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