•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 --
as we shall see, 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 to
observations. COBE and WMAP show at z = 1000 the contrast
was ~10-6.
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