The cosmological matter density

The status of observational cosmology is a subject that David Schramm followed intently. As spokesman for the entire field of particle astrophysics, David was interested in the full picture. He was always conversant with the latest developments in observations of the light elements, as they directly impacted his work on primordial nucleosynthesis and the resulting predicted abundances of deuterium, helium, and lithium. He was especially keen on knowing the status of the latest measurements of the cosmic density parameter, Ωm, as a sufficiently high value, higher than that predicted for primordial nucleosynthesis, motivates the case for a non-baryonic component of dark matter. He had a deep interest in the phenomenology of large-scale structure, as this provides a powerful clue to the nature of the dark matter and the initial fluctuations generated in the early Universe. This review briefly summarizes current techniques for estimation of the density of the Universe. These estimates on a variety of physical scales yield generally consistent results, suggesting that the dark matter, apart from a possible smooth component, is well mixed with the galaxy distribution on large scales. A near consensus has emerged that the matter density of the Universe, Ωm, is a factor of 3–4 less than required for closure. Measures of the amplitude and growth rate of structure in the local Universe are dependent on a degenerate combination of Ωm and the bias b in the observed galaxy distribution. The unknown bias in the galaxy distribution has been a persistent problem, but methods for breaking the degeneracy exist and are likely to be widely applied in the next several years.