Hydrogen ionization fraction
= Hydrogen ionization fraction
{title2=$X_e=n_e/n_b$}
In hydrogen-only cosmology, charge neutrality gives $n_e=n_p$ and baryon conservation gives $n_b=n_p+n_H$. The fraction $X_e$ lies between zero and one, with $n_H=(1-X_e)n_b$. With helium present, <Electron> abundance per <hydrogen> nucleus and per total baryon are different conventions and must be specified. The <hydrogen-only Saha equation> gives the equilibrium fraction.