Like scattering before recombination, scattering at late times suppresses anisotropies
in the distribution that have already formed. Reionization therefore suppresses
the amplitude of the acoustic peaks by the fraction of photons
rescattered, approximately the optical depth (see Plate 5b,
dotted line and negative, dashed line, contributions corresponding to
between the
and
models). Unlike the plasma before recombination, the medium
is optically thin and so the mean free path and diffusion length of the photons
is of order the horizon itself. New acoustic oscillations cannot form. On scales
approaching the horizon at reionization, inhomogeneities have yet to be converted
into anisotropies (see §3.8) and so
large angle fluctuations are not suppressed. While these effects are relatively
large compared with the expected precision of future experiments, they mimic a
change in the overall normalization of fluctuations
except at the lowest, cosmic variance limited, multipoles.