From the observations both of the lack of of a Gunn-Peterson trough [Gunn & Peterson, 1965] in quasar spectra
and its preliminary detection [Becker et al, 2001], we know that hydrogen
was reionized at . This is thought to occur through the
ionizing radiation of the first generation of massive stars (see e.g. [Loeb & Barkana, 2001] for a review). The
consequent recoupling of CMB photons to the baryons causes a few percent of
them to be rescattered. Linearly, rescattering induces three changes to the
photon distribution: suppression of primordial
anisotropy, generation of large angle polarization,
and a large angle Doppler effect. The latter two
are suppressed on small scales by the cancellation highlighted in §4.2.1.
Non-linear effects can counter this suppression; these are the subject of active
research and are outlined in §4.3.4.
Plate 5b: Secondary anisotropies: (b) Scattering secondaries: rescattering suppression, Doppler, modulated Doppler (density and ionization), and SZ effects.