Physical Foundations of Cosmology

9.11 Reionization 409

of the higher multipoles are suppressed by about 20% compared with their original
value, while the lower multipoles are untouched. As we have already mentioned,
this can imitate the tilting of the spectral index to a certain extent and lead to an
extra degeneracy and further cosmic confusion.
This degeneracy can be easily resolved if we consider the influence of the reion-
ization on polarization spectra. In fact, reionization leads to distinct features in
these spectra. The temperature fluctuations in the scattered fraction of photons are
not completely washed out on the scales corresponding to the reionization horizon.
As a result, there is a net contribution of the rescattered photons to the total tem-
perature fluctuations for the multipoles∼lion, and it is polarized. It is obvious
that the extra polarization induced by reionization is proportional to the fraction
of the rescattered photons, 1−exp(−μ(zion)), and to the quadrupole anisotropy of
the rescattered photons at the beginning of reionization. Because this quadrupole
anisotropy is mostly due to the perturbations with scales of order of the reionization
horizon, an extra contribution to the polarization correlation functions should have
a local maximum atl∼lion.This explains the behavior of the correlation func-
tions in Figure 9.7 where the results for the polarization in the concordance model
withzion20 are presented. We would like to stress that because of the presence
of the long-wavelength gravitational waves, bothEandBmodes of polarization
will be generated. Thus, we see how measuring the polarization at low multipoles
can reveal details of the reionization history and help us to resolve the degeneracy
problem.