The baryon density of the universe is equal to the product of the baryon-to-light ratio , M _ { b } / L , and the luminosity density , j .
We estimate M _ { b } / L as the sum of the masses of the X-ray gas and the visible stars in a rich cluster of galaxies divided by the luminosity of the cluster galaxies in precisely the same sky aperture .
We evaluate the gas-to-light ratio derived from the EMSS detect cell flux and the CNOC cluster redshift survey galaxies .
After making an aperture correction to an effective overdensity of 500 \rho _ { c } , we find that \Omega _ { gas } = 0.012 - 0.016 h ^ { -3 / 2 } , depending on the galaxy fading correction .
Adding in the galaxy baryons at a mass-to-light ratio of 5 { M _ { \odot } } / { L _ { \odot } } , equivalent to \Omega _ { \ast } = 0.003 h ^ { -1 } , we find that \Omega _ { b } = 0.015 - 0.019 for H _ { 0 } = 100 { km s ^ { -1 } Mpc ^ { -1 } } ( or 0.040 - 0.051 for H _ { 0 } = 50 ) .
Expressed as the baryon to photon ratio , \eta , this corresponds to \eta = 4.0 - 5.2 \times 10 ^ { -10 } ( H _ { 0 } = 100 ) and is in the mid-range of values from other methods .
The individual clusters have a dispersion about the mean \Omega _ { gas } of 40 % , and the \chi ^ { 2 } of the 14 clusters is consistent with the hypothesis that the gas-to-light ratio is a universal constant .
If we ignore the light of the cD , the variance increases by a factor of three .
After the radial segregation of gas and light within a cluster is taken into account , these statistics indicate that there is little variation of the gas-to-light ratio from cluster to cluster over the 0.2 to 0.55 range in redshift .