We end up with one major problem: how could galaxies form so fast from the homogeneous background seen in the 2.7 K radiation? COBE and its successors give values of order 3 × 10-5 for temperature fluctuations on scales of several degrees. Following Zel'dovich and Novikov (p. 445), the angular scale of a fluctuation of mass M at contrast temperature 10-3 would be θ = 10 arcminutes × Ω 2/3 ( M /10 14 solar masses), setting the most appropriate angular scale. Silk (1968 ApJ 151, 459) gave the relation, also seen in the previous lecture , ΔT/T = Δ ρ / 3 ρ. There are unknowns in going from such small perturbations at z ~ 1400 to whole galaxies by about z =7. A crucial role was played by whatever the dark matter is. Longair devotes an entire chapter to setting out the "classical" scheme for galaxy formation in a purely baryonic Universe, solely to illustrate that it doesn't work. It is the perturbation spectrum of this dominant mass component that matters for galaxy formation. The effects on galaxy formation differ depending on whether
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