The baryon to photon number density in the early universe plays a role during big bang nucleosynthesis as well as baryon acoustic oscillations, imprinted on the

The baryon to photon number density in the early universe plays a role during big bang nucleosynthesis as well as baryon acoustic oscillations, imprinted on the microwave background. We know that the total energy density must be (close to) the critical density (again from CMB measurements). I can't really provide more details without doing some reading myself first, though...