What Cosmologists “Do”
	• Density contrast depends on size scale: – Galaxies a million times higher than average – Clusters a thousand times higher than average – Superclusters a few times higher than average • Average density varies as (1+z)3, so at z=100 (a matter-dominated epoch), galaxies had ∆=∂ρ/ρ=1 and could not have formed at higher z.  Clusters and superclusters had to form at much lower z.  Formation of structure is in principle accessible.