Professor, Department of Astronomy and Astrophysics
University of Chicago

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Peak Suppression

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 $\sim \tau_{{\rm ri}}$ (see Plate 5b, dotted line and negative, dashed line, contributions corresponding to $\vert\delta \Delta_T^2\vert^{1/2}$ between the $z_{\rm ri}=7$ and $z_{\rm ri}=0$ 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.