22 - Gravitational graininess initiates clustering

Summary

The single atoms each to other tend,

Attract, attracted to, the next in place

Form'd and impell'd its neighbour to embrace.

Alexander Pope

Parallel to the comparison developed in Sections 15 and 16, we next turn to new phenomena which graininess introduces into the growth of perturbations. In a major application of this theory, the grains are galaxies. Despite our ignorance of their formation, we can ask how gravity causes galaxies to cluster. Does the clustering we expect explain what we see?

The simplest result can even be anticipated from our analysis in the last section. Consider a gas of galaxies. Let it be uniform except for √N fluctuations. If we treat it, for a moment, as a fluid, then the growth rate of perturbations in a standard cosmology, ρ₁(t)∝ R(t)∝t^⅔, tells us that observed clusters with ~ 10⁴ galaxies should be able to form easily starting at redshifts of 10² - 10³. So galaxy clustering promises to be more understandable than galaxy formation, although it still has its mysteries.

Realizing the relative ease of galaxy clustering, it becomes natural to push the process back a step. Could galaxies themselves be the result of an earlier clustering? Suppose that isothermal perturbations of the Jeans mass at decoupling, ~ 10⁶ M_⊙, started (somehow) with large amplitudes and formed bound systems. It would take about 10⁴ of these to form a small galaxy.