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]
A main discriminatory feature between
low-
m and
high-m
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 ~
m-1
in an open model with
= 0,
or later at
1 + z ~
m-1/3
in a low density flat universe with
a cosmological constant.
Thus, structure of a given amplitude today form earlier in
low-m models
than in
m = 1 models, and
more so in an open model than in a flat model.
This effect can be observed in several different ways.