= Primordial helium response to electron-neutrino degeneracy
{title2=$n_n/n_p\simeq e^{-(m_n-m_p)/T-\xi_{\nu_e}}$}
Chemical equilibrium of $n+\nu_e\leftrightarrow p+e^-$, with negligible electron <chemical potential>, gives the stated <neutron-to-proton ratio>. Positive electron-neutrino degeneracy reduces the neutron fraction, whereas negative degeneracy increases it. The <primordial helium mass fraction> then responds through <neutron-limited helium synthesis> while protons outnumber neutrons; a sufficiently neutron-rich mixture instead becomes proton limited, so that trend must not be extrapolated indefinitely. Extra <neutrino> <energy density> also accelerates <cosmic expansion>, changing weak freeze-out and the time available for neutron decay. These effects can compete; a quantitative <Big Bang nucleosynthesis> calculation must account for the altered reaction rates as well as the altered expansion rate.
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