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The Wikipedia entry on the Big Bang, in its section on "Abundance of primordial elements," cites the 1988 book by Kolb and Turner (The Early Universe), in saying that the ratio of photons to baryons

*"can be calculated independently from the detailed structure of CMB fluctuations."*

I assume by "structure" they mean its spatial Fourier transform peaks, as shown in the Wikipedia article on CMB, under "Primary anisotropy" - the graph is titled "The power spectrum of the CMB radiation temperature anisotropy in terms of the angular scale (or multipole moment)."

The question is, how do they determine that photon to baryon ratio? (I thought I might save myself the trouble of going to a library and consulting Kolb and Turner.)