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Cosmic microwave background anisotropy is the direction dependence of the relic radiation's temperature and polarization. A temperature map can be written $T(\hat{\mathbf{n}})=\bar{T}[1+\Theta (\hat{\mathbf{n}})]$, separating a mean from its fractional angular variation. Comparing directions with a Differential Microwave Radiometer measures these variations without requiring the instrument to determine the absolute mean temperature.^[ref]

Intrinsic plasma fluctuations, motions and gravitational frequency shifts all contribute. The Cosmic microwave background power spectrum organizes the signal by angular scale; the Cosmic microwave background acoustic peaks record the evolution of the photon-baryon fluid. Frequency comparisons and instrumental checks help distinguish the relic signal from galactic foregrounds and noise before the map is used to infer cosmological parameters.

The Differential Microwave Radiometer measured differences in microwave power between pairs of sky directions to map Cosmic microwave background anisotropy. Comparing two directions suppresses a common absolute brightness offset; repeated comparisons during scanning are then combined into a sky map.

The COBE instrument used paired horn antennas at three frequencies, with independent channels that tested detector noise and frequency-dependent galactic foreground contamination. A differencing instrument does not by itself determine the absolute monopole temperature. Its measurement therefore complements the spectrum measured by the Far Infrared Absolute Spectrophotometer.^[ref]

The plane of a disk-shaped galaxy contains most of its stellar disk. In ordinary sky-coordinate usage the galactic plane refers to the Milky Way's disk plane. A concentration of interstellar dust near that plane produces a prominent component of galactic foreground emission.

Thermal interstellar dust produces a bright galactic foreground concentrated toward the Milky Way plane, with additional cloud structure. Its emission spectrum differs from the approximately $2.73\mathrm{K}$ blackbody spectrum of the cosmic microwave background, so comparisons across frequencies help separate it. In the sketch below this is the bright equatorial band.

After subtracting the mean, the motion-induced dipole and galactic foregrounds, intrinsic Cosmic microwave background anisotropy appears as angular hot and cold patches, typically at the $10^{-5}$ fractional scale. These spatial temperature fluctuations encode early density and velocity structure. The displayed patch pattern is illustrative, with no claim to reproduce an observed sky map.