Cosmological baryon density (${\rho }_b\simeq m_p{\eta }_b{n}_{\gamma }$)

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.