The cosmic microwave background (CMB) is nearly uniform electromagnetic radiation filling the universe, observed today primarily at microwave wavelengths. It is the remnant of the hot early phase described by the Big Bang model. Its radiation last interacted strongly with matter about 380,000 years after the beginning of cosmic expansion. Its spectrum, temperature variations, and polarization provide central evidence for modern cosmology and a detailed record of conditions before stars and galaxies formed. (esa.int)
Physical origin
The early universe contained a hot plasma in which photons repeatedly scattered from free electrons. Frequent scattering prevented radiation from travelling freely over long distances. Expansion cooled the plasma until electrons could combine with atomic nuclei, principally protons, to form neutral atoms. This transition, called recombination, occurred when the temperature was approximately 3,000 kelvin. The declining abundance of free electrons made the universe largely transparent, allowing radiation to propagate almost unhindered. (esa.int)
The CMB therefore records a transition in the behaviour of pre-existing radiation, rather than a single flash produced at the beginning of the universe. Recombination and photon decoupling are closely related: the former describes neutral-atom formation, while the latter describes radiation ceasing to interact frequently with matter. Subsequent expansion stretched photon wavelengths through cosmological redshift, converting the originally much hotter radiation into the microwave background observed today. (esa.int)
Spectrum and temperature
The CMB has an exceptionally accurate blackbody spectrum, the characteristic distribution of thermal radiation. Its mean temperature is approximately 2.725 kelvin. Measurements by the Far Infrared Absolute Spectrophotometer, or FIRAS, aboard the Cosmic Background Explorer satellite established this close agreement across a broad range of frequencies. The spectrum strongly supports an early hot, thermalized universe rather than an origin through accumulated emission from ordinary astronomical objects. (lambda.gsfc.nasa.gov)
The mean spectrum and the angular temperature pattern contain different information. The former tests the radiation’s thermal history; the latter reveals small departures from uniformity. Although the background appears almost identical in every direction, its intrinsic temperature fluctuations are typically only about one part in 100,000. These tiny variations are measurable with sensitive microwave instruments and reflect conditions in the primordial matter–radiation distribution. (esa.int)
Discovery and major observations
Theoretical work in the 1940s connected a hot early universe with the production of light elements through nucleosynthesis and predicted surviving background radiation. George Gamow, Ralph Alpher, and Robert Herman contributed to this development. In 1964, Arno Penzias and Robert Wilson detected an unexplained microwave signal with a horn antenna in New Jersey; the discovery was published in 1965. Their identification of the background earned them the Nobel Prize in Physics in 1978. (esa.int)
NASA launched Cosmic Background Explorer (COBE) in 1989. Its instruments established the blackbody spectrum and detected primordial temperature anisotropies, announced in 1992. John Mather and George Smoot received the 2006 physics Nobel Prize for these results. NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, measured the fluctuations in greater detail. ESA’s Planck observatory, launched in 2009, extended measurements of temperature and polarization with improved angular resolution and broader frequency coverage. (nobelprize.org)
Anisotropies and cosmological information
Temperature anisotropies encode the combined effects of density differences, gravitational fields, and motion in the early universe. Before decoupling, ordinary matter and radiation behaved approximately as a coupled fluid. Gravity pulled matter into denser regions, while radiation pressure opposed compression. Their competition generated acoustic oscillations, which left a sequence of peaks in the CMB’s angular power spectrum—the distribution of fluctuation strength across angular scales. (esa.int)
Peak positions and relative heights constrain cosmic geometry, the abundance of ordinary baryonic matter, and the density of dark matter. Dark matter contributes gravitationally but does not participate in the same tightly coupled photon–matter oscillations. The characteristic acoustic scale acts as a standard ruler, linking physical processes before decoupling with distances across the expanding universe. (esa.int)
Interpretation requires a cosmological model. Under the spatially flat Lambda–Cold Dark Matter model, Planck’s 2018 analysis inferred an expansion rate of kilometres per second per megaparsec. This is a model-dependent inference from early-universe observations, not a direct measurement of nearby galaxies’ recession. The data also constrain primordial fluctuations and the properties of relativistic particles, including neutrinos. (arxiv.org)
Polarization and later modifications
A small fraction of the CMB exhibits polarization, generated when radiation scatters from electrons. Its sky pattern can be separated into E modes and B modes. E-mode measurements provide additional information about primordial fluctuations and later scattering. Gravitational lensing by intervening matter changes photon trajectories and converts part of the E-mode pattern into B modes. (esa.int)
Some models of cosmic inflation predict primordial gravitational waves that would also generate B-mode polarization. Distinguishing this signal from lensing and polarized Galactic dust requires observations at multiple frequencies. A joint Planck–BICEP2–Keck analysis reported in 2015 showed that the earlier claimed primordial signal was not robust after dust emission was accounted for. (esa.int)
The observed background also carries information about later cosmic history. Radiation from early stars and galaxies reionized gas, allowing free electrons to scatter some CMB photons again. Hot electrons in galaxy clusters transfer energy to background photons through inverse Compton scattering, producing the Sunyaev–Zel’dovich effect. These modifications make the CMB a probe of intervening matter as well as of the early universe. (esa.int)