If the dominant components of the universe were radiation and baryons, and the primeval baryon distribution had a roughly flat spectrum normalized to galaxy clustering on scales ≡20 Mpc, and young stars were able to keep the bulk of the matter ionized at redshifts z ⪆ 20, then several encouraging results would follow. The first generation that starts to form when Compton drag becomes unimportant would have masses and radii comparable to galaxies. Mass fluctuations on scales ≡200 Mpc could be relatively large and so perhaps favorable for development of large-scale structure. And the residual fluctuations in the background temperature would have a coherence length of 3°-5° and a standard deviation δT/T ≈ 10⁻⁵, close to but below the observational bounds.
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P. J. E. Peebles (1987) studied this question.