ABSTRACT
Following Einstein's general relativity of 1915, Friedmann used his theory in 1922 to construct models of the universe, assuming them to be homogeneous and isotropic. Although Hubble discovered the expansion of the universe in 1929, which is an immediate implication of the Friedmann models, he was unaware that it had been predicted theoretically that the velocity of recession of a galaxy is proportional to its distance. Considering the observational inaccuracies involved, I think it is one of the great triumphs of big-bang cosmology that one can fit the data with a narrow range of η's. Once η is known, one can calculate the number of baryons from the number of photons. This determines the contribution to Ω by baryons, Ωb. The distribution of galaxies shown here, which was presaged by work indicating that there are voids in the distribution of galaxies, shows that most of the galaxies lie in "sheets" or "shells" of some kind on the surfaces of voids.
