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The amplitude of density fluctuations at redshift z compared to their amplitude today, according to linear GI theory, is approximated to a few percent by [51]

Equation 11 (11)

A main discriminatory feature between low-Omegam and high-Omegam models is the effective freeze-out of the growth of fluctuations, which occurs when the universe enters its free expansion phase; roughly at 1 + z ~ Omegam-1 in an open model with OmegaLambda = 0, or later at 1 + z ~ Omegam-1/3 in a low density flat universe with a cosmological constant. Thus, structure of a given amplitude today form earlier in low-Omegam models than in Omegam = 1 models, and more so in an open model than in a flat model. This effect can be observed in several different ways.