ABSTRACT
5.1 Gravitational instability
No cosmological model is complete without a credible theory for the formation of the structure that is observed in the Universe. This structure, fully described in Chapter 3, ranges from very small scales (e.g., globular clusters and dwarf galaxies), through bigger scales (e.g., galaxies and clusters of galaxies) to the largest scale (e.g., large voids, superclusters, clusters of superclusters, great walls). Although a wide variety of structure formation models can and have been considered, the basic idea that has persisted since the time of Newton is gravitational instability, which amplifies the growth of density fluctuations. Other scenarios are possible and will be considered here briefly, but gravitational instability has two great virtues: (1) it is the only known long-range force or process than can aggregate matter and (2) it is physically well understood. Gravitational instability is therefore the dominant paradigm for understanding structure formation, and this section is devoted to its detailed consideration. Subsequent sections focus on statistical methods of characterizing the distribution of structure, possible scenarios for structure formation, and the available observational constraints on competing cosmogenic scenarios.
