Physical Foundations of Cosmology

9.10 Polarization of the cosmic microwave background 395

10 100 1000

0.01

0.1

1

10

100

1000

S+ T

T

l

l(l

+

l)C

(l

(^2) T
/2
π) [
μK
2 ]
0
Fig. 9.3.
As with scalar perturbations, the contribution of the tensor perturbations to the
CMB power spectrum also consists of a flat plateau at low multi-poles, due to
the superhorizon gravitational waves at last scattering. However, forl> 55 ,the
amplitudel(l+ 1 )ClT decreases quickly. Figure 9.3 was drawn using a precise
numerical code showing how the total spectrum is subdivided into scalar and tensor
components for the concordance model. Note that the tensor component dies off
rapidly where the acoustic peaks appear. Hence, detecting the tensor contribution
to the temperature autocorrelation function relies on comparing the height of the
plateau to the height of the acoustic peaks. It is difficult to separate this effect from
reionization or a spectral tilt. Polarization proves to be the better test for detecting
primordial gravitational waves.

9.10 Polarization of the cosmic microwave background

Thus far, we have focused on the temperature fluctuations in the cosmic microwave
background, because the temperature autocorrelation provides the single, most
powerful test for distinguishing cosmological models and determining cosmolog-
ical parameters. However, there is more information to be gained by measuring
the polarization and its correlation with the temperature fluctuations. In particu-
lar, polarization provides the cleanest and most sensitive method of detecting the