Cosmological neutrino entropy changes due to flavor statistical mixing
  Entropy changes due to delocalization and decoherence effects should modify
the predictions for the cosmological neutrino background (C$\nu$B) temperature
when one treats neutrino flavors in the framework of composite quantum systems.
Assuming that the final stage of neutrino interactions with the $\gamma
e^{-}e^{+}$ radiation plasma before decoupling works as a measurement scheme
that projects neutrinos into flavor quantum states, the resulting
free-streaming neutrinos can be described as a statistical ensemble of
flavor-mixed neutrinos. Even not corresponding to an electronic-flavor pure
state, after decoupling the statistical ensemble is described by a density
matrix that evolves in time with the full Hamiltonian accounting for flavor
mixing, momentum delocalization and, in case of an open quantum system
approach, decoherence effects. Since the statistical weights, $w$, shall follow
the electron elastic scattering cross section rapport given by $0.16\,w_{e} =
w_{\mu} = w_{\tau}$, the von-Neumann entropy will deserve some special
attention. Depending on the quantum measurement scheme used for quantifying the
entropy, mixing associated to dissipative effects can lead to an increasing of
the flavor associated von-Neumann entropy for free-streaming neutrinos. The
production of von-Neumann entropy mitigates the constraints on the predictions
for energy densities and temperatures of a cosmologically evolving isentropic
fluid, in this case, the cosmological neutrino background. The effects of
entropy changes on the cosmological neutrino temperature are quantified, and
the {\em constraint} involving the number of neutrino species, $N_{\nu} \approx
3$, in the phenomenological confront with Big Bang nucleosynthesis parameters
is consistently relieved.
