Using the baryon-photon ratio to calculate the primordial abundances of baryonic matter:
The photon density = 3.7 x 102/cm-3 Baryon density = (Universe mass density)/Mass_Proton = 4.2 x 10-31 g cm-3 / 1.66 x 10-24 g = ~2.5 x 10-7 Baryon density / photon density = 2.5 x 10-7 / 3.7 x 102 = ~6.7 x 10-10 For every baryon there are ~1010 photons.
