LARGE-SCALE STRUCTURE AND THE MEAN DENSITY OF THE UNIVERSE

The density parameter, Omega, is the most important single number in models of large scale structure. At present, the two best methods to estimate it are (i) comparison of the peculiar velocity field predicted by maps of the galaxy distribution with the measured peculiar velocities of a sample of galaxy clusters and (ii) comparison of the universal baryon fraction predicted by Big Bang nucleosynthesis theory with the measured baryon fraction in rich galaxy clusters. Taken at face value, these two methods give contadictory results, Omega = 1 in the first case, and Omega less than or equal to 0.5 in the second. These results can be reconciled if, either Omega = 1 and the standard nucleosynthesis constraints are incorrect, or Omega similar or equal to 0.4 and galaxies are substantially less clustered than the dark mass.