= Electron-positron equilibrium pair abundance below the electron mass
For $T\ll m_e$ and negligible <chemical potentials>, the combined equilibrium density of <electrons> and positrons divided by the <photon number density> is
$$
\frac{n_{e^-}+n_{e^+}}{n_\gamma}
=\frac{2\pi^2}{\zeta(3)}
\left(\frac{m_e}{2\pi T}\right)^{3/2}e^{-m_e/T}.
$$
The exponential suppression eventually makes pair equilibrium incompatible with the residual <electrons> required by <charge neutrality>.
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