Cosmological large scale structure

Abstract

The interrelated cosmological problems of large scale structure and dark matter are reviewed. Contrary to Willy's initial hopes, it appears that the Big Bang Nucleosynthesis constraint requiring the baryon density to be sub-critical remains valid. This Big Bang Nucleosynthesis constraint on the baryon density is compared with the densities implied by visible matter, dark halos, dynamics of clusters, gravitational lenses, large scale velocity flows, and the Ω = 1 flatness/inflation argument. It is shown that (1) the majority of baryons are dark; and (2) non-baryonic dark matter is probably required on large scales. It is also noted that halo dark matter could be either baryonic or non-baryonic. Discrimination between “cold” and “hot” non-baryonic candidates is shown to depend on the assumed “seeds” that stimulate structure formation. Gaussian density fluctuations, such as those induced by quantum fluctuations, favor cold dark matter, whereas topological defects, such as strings, textures or domain walls, may work equally well or better with hot dark matter. A possible connection between cold dark matter, globular cluster ages and the Hubble constant is mentioned. Recent large scale structure measurements, coupled with microwave anisotropy limits, are shown to raise some questions for the previously favored density fluctuation picture.