Cosmic microwave background

The cosmic microwave background is relic electromagnetic radiation released when the early universe became transparent. Its nearly uniform glow, now at about 2.7 kelvin, carries small variations that reveal the universe’s early structure.

Recombination: The era when electrons combined with atomic nuclei, allowing photons to travel freely through the universe. Photons released as the universe became transparent now form the background radiation.
Big Bang: The early hot, dense phase from which the observable universe has expanded and evolved. The background is leftover radiation from the Big Bang’s hot early stages.
Penzias–Wilson discovery: The 1965 identification of an isotropic microwave signal by Arno Penzias and Robert Wilson. This observation provided the first widely accepted detection of the background.
Lambda-CDM model: The standard cosmological model combining a cosmological constant with cold dark matter and ordinary matter. Its parameters are tightly constrained by the background’s measured patterns.
Cosmic infrared background: The accumulated infrared emission from distant galaxies, integrated across cosmic history. Unlike the microwave background, it comes mainly from later generations of galaxies.
Photon decoupling: The transition when photons stopped scattering frequently from matter and began traveling largely unhindered. The background’s photons have traveled freely since this transition.
George Gamow: A theoretical physicist who developed early accounts of nucleosynthesis and relic radiation in a hot expanding universe. His work helped establish the expectation of a cooled cosmic radiation background.
COBE: NASA’s Cosmic Background Explorer satellite, which measured the background’s spectrum and mapped its large-scale temperature variations. COBE confirmed the black-body spectrum and detected primordial anisotropies.
Cosmological parameter: A numerical quantity that specifies a property of a cosmological model, such as matter density or expansion rate. The background provides precise estimates of several fundamental cosmological parameters.
Galactic foreground: Emission and other signals produced within the Milky Way that contaminate observations of the distant universe. Foregrounds overlap the background’s frequencies but do not share its primordial origin.

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