= Cosmological baryon density
{title2=$\rho_b\simeq m_p\eta_b n_\gamma$}
The <cosmological baryon density> is the mass density in baryonic matter, including hydrogen and helium. Up to nuclear binding-energy and composition corrections it is the proton mass times the baryon number density. The <baryon-to-photon ratio> links that number density to the <photon number density>. Larger baryon density increases nuclear reaction rates and permits the <deuterium bottleneck> to end earlier.
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