Physical Foundations of Cosmology

406 Cosmic microwave background anisotropies

0.0001 10 100 1000

0.001

0.01

0.1

1

10

100

contribution of reionization

no reionization

BB mode

EE mode

l

l(l

+

l)C

(l

T

2 /2

π) [

μK

2 ]

0

Fig. 9.7.

decreases on subhorizon scales. Hence, they do not contribute to the correlation of
theB-type polarization on angular scales corresponding tol>100. As a result,
the functionl(l+ 1 )ClBBreaches its maximum atl∼ 100 .In contrast withB-
polarization, due to scalar perturbations on subhorizon scales, there are substantial
correlations ofE-polarization forl> 100.
The correlation functionCETis the easiest to measure since it entails a cross-
correlation between the temperature fluctuation amplitude, which is large, and the
largest (Emode) polarization component. Measured atl> 50 ,CETwould supply
us with information about the history of the recombination.
TheBmode polarization is an especially important object of microwave back-
ground measurement, since it is the most decisive and probably the only realistic way
of detecting the nearly scale-invariant spectrum of gravitational waves predicted by
inflation. The cross-polarization multipolesCBTare the easiest moments to detect
if one is searching for signs of gravitational waves. The technological challenge of
detecting theBmode is considerable. We have already noted that the polarization
signal is small, but theBmode polarization component itself is a small fraction of
the total polarization in typical inflationary models, as shown in Figure 9.7. In addi-
tion, there are foregrounds to consider. For example, the lensing of the microwave
background by foreground sources distorts the background polarization pattern in