A matter of scale
The "angular power spectrum" of the unpolarized temperature fluctuations in the cosmic microwave background (CMB) shows the relative strength of the temperature variations as a function of their size (extent), like comparing the height of chops to swells on a stormy sea. The "base level" anisotropy of about 1000 μK2 arises from variations in the gravitational potential energy at the epoch of decoupling (the slope of the base level provides a measure of the spectral index, ns). The peaks arise from oscillating sound waves produced by the primordial inflationary fluctuations. This oscillatory signal is suppressed by the scattering of CMB photons off reionized electrons, which polarizes them (not shown) and can mimic a change in ns.