Dark Matter and Structure Formation in the Universe

This chapter aims to present an introduction to current research on the nature of the cosmological dark matter and the origin of galaxies and large scale structure within the standard theoretical framework: gravitational collapse of fluctuations as the origin of structure in the expanding universe. General relativistic cosmology is summarized, and the data on the basic cosmological parameters ($t_o$ and $H_0 \equiv 100 h \kmsmpc$, $\Omega_0$, $\Omega_\Lambda$ and $\Omega_b$) are reviewed. Various particle physics candidates for hot, warm, and cold dark matter are briefly reviewed, together with current constraints and experiments that could detect or eliminate them. Also included is a very brief summary of the theory of cosmic defects, and a somewhat more extended exposition of the idea of cosmological inflation with a summary of some current models of inflation. The remainder is a discussion of observational constraints on cosmological model building, emphasizing models in which most of the dark matter is cold and the primordial fluctuations are the sort predicted by inflation. It is argued that the simplest models that have a hope of working are Cold Dark Matter with a cosmological constant ($\Lambda$CDM) if the Hubble parameter is high ($h \gsim 0.7$), and Cold + Hot Dark Matter (CHDM) if the Hubble parameter and age permit an $\Omega=1$ cosmology, as seems plausible in light of the data from the Hipparcos astrometric satellite. The most attractive variants of these models and the critical tests for each are discussed.