= Cluster baryon-fraction estimate of matter density
{title2=$\Omega_m=\Omega_b/f_{b,\rm cluster}$}
If a measured cluster baryon fraction represents the <cosmic baryon fraction> $\Omega_b/\Omega_m$, combining it with the <cosmological baryon density> inferred from <Big Bang nucleosynthesis> gives the displayed estimate. Gas and stellar baryons must both be counted, and all distance or <Hubble constant> dependence must be consistent. Baryon depletion or feedback modifies the identification: with $f_{b,\rm cluster}=b\Omega_b/\Omega_m$, the corrected answer is $\Omega_m=b\Omega_b/f_{b,\rm cluster}$. The method measures total matter rather than vacuum or radiation density.
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