CMB anisotropies

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):

• The largest contributor is our peculiar velocity in relation to the Hubble flow, including Earth's orbital speed, the Sun's peculiar velocity, the Milky Way's rotation and peculiar velocity, and that of the Local Group, which combine to about 362 km/s (secondary). It is termed the cosmic microwave background dipole (CMB dipole, it is apparent in our view of the CMB but can be easily removed from observation data and for many purposes is treated as not really part of the CMB, i.e., merely a specific local effect).
• density differences due to initial fluctuations (primary).
• density differences due to baryon acoustic oscillations (primary).
• non-integrated Sachs-Wolfe effected (primary).
• integrated Sachs-Wolfe effect (secondary).
• gravitational lensing of the photons (secondary).
• Sunyaev-Zel'dovich effect (secondary).

(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.