Physics of polarization fluctuations 229Figure 7.1.The temperature angular power spectrum for a cosmological model with mass
density 0 = 0 .3, vacuum energy density= 0 .7, Hubble parameterh= 0 .7, and a
scale-invariant spectrum of primordial adiabatic perturbations.
This prediction can then be compared with data from measured temperature
differences on the sky. Figure 7.1 shows a typical temperature power spectrum
from the inflationary class of models, described in more detail later. The
distinctive sequence of peaks arise from coherent acoustic oscillations in the fluid
during the tight coupling epoch and are of great importance in precision tests of
cosmological models; these peaks will be discussed in section 7.4. The effect
of diffusion damping is clearly visible in the decreasing power abovel=1000.
When viewing angular power spectrum plots in multipole space, keep in mind
thatl =200 corresponds approximately to fluctuations on angular scales of a
degree, and the angular scale is inversely proportional tol. The vertical axis
is conventionally plotted asl(l+ 1 )ClTbecause the Sachs–Wolfe temperature
fluctuations from a scale-invariant spectrum of density perturbations appears as
a horizontal line on such a plot.
7.3 Physics of polarization fluctuations
In addition to temperature fluctuations, the simple physics of decoupling
inevitably leads to non-zero polarization of the microwave background radiation