The formation of cosmic structure

The database for cosmic structure formation is broad in scope and growing rapidly. This paper reviews the nature of the beasts in the cosmic zoo, their abundances, their hierarchical nature, how they cluster and how they flow. The author pays special attention to microwave background experiments which offer the deepest and cleanest probe. Although the author sketches the main theoretical ideas for the origin of structure, the author primarily focuses on the most popular paradigm: (scalar) density perturbations that initially formed a homogeneous and isotropic Gaussian random field characterized by a spectrum of spatial eigenmodes, dσρ/d nl k, which gives the contribution of waves of comoving wavenumber k to the rms fluctuation level σρ. The dynamical evolution of the density field is described, from the generation phase in an inflationary epoch through the linear evolution when the waves enter causal contact into the nonlinear regime when mode-mode coupling allows the proto-structures to turn away from the general expansion and collapse. Special attention is paid to the minimal CDM model, with the dark matter being cold elementary particle relics of the Big Bang having nearly the closure density and with a nearly scale invariant post-inflation spectrum for the gravitational potential. However, although most natural, neither scale invariance nor Gaussian-ness is a necessary outcome if more than one scalar field is important during inflation. Mechanisms which operate when the waves come into causal contact do break scale invariance. and determine the nature of the subsequent nonlinear era