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CMB anisotropies are variations in the spectrum of the cosmic microwave background (CMB) across the sky, essentially variations in the temperature associated with the black-body spectrums observed at various points in the celestial sphere. The variations are tiny, but they are clues, both to recombination (when the CMB was released) and what led up to it, and the effects on the CMB of passing through the universe since. Anisotropies can be classified as primary anisotropies, stemming from recombination and before, versus late time anisotropies (aka secondary anisotropies) from the effects of the universe that the photons have since passed through. Anisotropy contributors (beyond Earth atmosphere, and foreground microwave emissions from within the Milky Way):
(Diffusion damping is another primary contributor, but decreases the primary anisotropies.) The common analysis consists of a multipole expansion of the power spectrum (power meaning the square of the varying temperature-differences from the mean temperature), which is displayed as a graph of a function of power against the "l" (the degrees aka multipole moments aka multipole numbers of the spherical harmonic modes).
Anisotropies of high interest also include those of CMB polarization, which are considered a potential clue to the characteristics of primordial gravitational waves and inflation.