Cosmic baryon fraction
= Cosmic baryon fraction
{title2=$f_b=\Omega_b/\Omega_m$}
The <cosmic baryon fraction> is the ratio of mean baryonic matter density to total matter density. Assigning this fraction to a halo is a simplifying assumption; cooling, ejection and other baryonic processes can change the halo's retained gas and stellar fractions. A baryon conversion efficiency becomes a stellar-to-total-halo mass ratio only after multiplying by the applicable baryon fraction.