Cosmic microwave background anisotropy

Cosmic microwave background anisotropy is the small variation in the relic radiation’s temperature and polarization across the sky. Its patterns encode early-universe conditions and the growth of cosmic structure.

Last scattering surface: The early-universe boundary from which cosmic microwave background photons traveled freely toward observers. Its density and velocity variations imprint temperature and polarization patterns on the background.
Cosmic microwave background: Relic electromagnetic radiation released when the early universe became transparent, now observed as a nearly uniform microwave glow. Anisotropy is the small-scale departure from this radiation’s near-uniform sky brightness.
Cosmic microwave background discovery: The 1965 identification of a pervasive microwave signal as relic radiation from the early universe. The discovery established the radiation whose anisotropies later became measurable.
Lambda-CDM model: The standard cosmological model, with a cosmological constant and cold dark matter in an expanding universe. Its parameters are tightly constrained by the observed anisotropy patterns.
Primordial gravitational waves: Gravitational waves generated in the early universe, potentially during cosmic inflation. Their predicted polarization signature has not yet been securely detected in the microwave background.
Sachs–Wolfe effect: A gravitational redshift or blueshift of cosmic microwave background photons as they cross changing gravitational potentials. It connects large-angle temperature variations to gravitational potentials near photon decoupling.
Angular power spectrum: A statistical description of how the variance of a sky signal is distributed across angular scales. Its peaks summarize temperature and polarization anisotropy as a function of angular size.
COBE: NASA’s Cosmic Background Explorer satellite, which measured the cosmic microwave background spectrum and large-scale anisotropy. Its Differential Microwave Radiometer detected primordial temperature variations in 1992.
Cosmic inflation: A hypothesized period of accelerated expansion in the universe’s earliest moments. Anisotropy statistics test inflationary predictions for primordial density fluctuations.
BICEP2: A South Pole experiment that reported a possible primordial B-mode polarization signal in 2014. The claimed signal was later explained largely by polarized Milky Way dust.

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