waynehu
Professor,
Department of Astronomy and Astrophysics
University of Chicago
The rescattered radiation becomes polarized since, as discussed in §3.8
temperature inhomogeneities, become anisotropies by projection, passing through
quadrupole anisotropies when the perturbations are on the horizon scale at any
given time. The result is a bump in the power spectrum
of the 
-polarization on angular scales corresponding to the
horizon at reionization (see Plate 1).
Because of the low optical depth of reionization and the finite range of scales
that contribute to the quadrupole, the polarization contributions are on the order
of tenths of 
K on scales of 
few. In a perfect, foreground free world, this is not beyond
the reach of experiments and can be used to isolate the reionization epoch [Hogan et al, 1982,Zaldarriaga et al, 1997]. As in the ISW
effect, cancellation of contributions along the line of sight guarantees a sharp
suppression of contributions at higher multipoles in linear theory. Spatial modulation
of the optical depth due to density and ionization (see §4.3.4)
does produce higher order polarization but at an entirely negligible level in
most models [Hu, 2000a].
