Galaxy distribution in a cold dark matter universe

Abstract

The observed galaxy distribution is almost our sole source of information about the distribution of matter on very large scales. Most efforts to estimate the mean density of the Universe from the dynamical behaviour of observed inhomogeneities assume the galaxy distribution to trace that of the mass1–4. If galaxies are assumed to be a statistically fair sample from the overall mass distribution, the Universe must be open, but if galaxies are systematically overrepresented in high density regions then the observational data may be compatible with a closed or flat universe5,6. There are many reasons to suspect that the galaxy distribution may indeed be biassed in this way with respect to the mass7–10, and, with a particular model for the biasing, a flat universe dominated by cold dark matter (CDM) provides a remarkably good description of observed structure6,11,12. Here we show that the gravitational growth of structure by hierarchical clustering leads automatically to a bias in the distribution of galaxies. The strength of this bias is such that if the Universe does indeed conform to the CDM model then its density must approach the closure value. The mechanism predicts a significant dependence of the strength of galaxy clustering on the depth of the potential well of the galaxies considered.