= Cosmic microwave background anisotropy
{c}
{title2=CMB anisotropy}
<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.<Structure in the COBE Differential Microwave Radiometer First-Year Maps by George F. Smoot and others>[Differential observations and construction of anisotropy maps]{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.
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