Cosmic microwave background

The cosmic microwave background was released when the expanding universe cooled enough for electrons and protons to combine into neutral hydrogen. Before that transition, photons were repeatedly scattered by free electrons and could not travel far; afterward, they moved mostly unimpeded through space. The radiation has since been stretched by cosmic expansion from visible and infrared wavelengths into the microwave band.

Its discovery in 1965 by Arno Penzias and Robert Wilson provided decisive evidence for a hot, dense early universe. They detected persistent microwave noise with a horn antenna at Bell Telephone Laboratories, while a nearby theoretical group led by Robert Dicke recognized that the signal matched a predicted relic radiation field.¹ The discovery earned Penzias and Wilson the 1978 Nobel Prize in Physics.²

The cosmic microwave background is extraordinarily uniform, but tiny temperature variations contain the seeds of later cosmic structure. Anisotropies at roughly one part in 100,000 reflect small differences in density and gravitational potential that eventually grew into galaxies, clusters, and the large-scale cosmic web.

Its spectrum is an almost perfect blackbody, measured with exceptional precision by the COBE satellite. The characteristic temperature is approximately 2.725 kelvins, far colder than the radiation’s original state because expansion has greatly redshifted it.³ The pattern of acoustic peaks in its angular power spectrum constrains the universe’s age, geometry, composition, expansion history, and primordial fluctuation spectrum. These measurements support a universe dominated today by dark energy and containing substantial dark matter.

Space missions have progressively sharpened the microwave map. NASA’s Cosmic Background Explorer established the blackbody spectrum and detected the first large-scale anisotropies; the Wilkinson Microwave Anisotropy Probe refined cosmological parameters; and ESA’s Planck mission produced high-resolution temperature and polarization maps of the microwave sky.³

Polarization supplies a second, independent record of early conditions. Its patterns distinguish E-mode polarization, generated mainly by density fluctuations, from the much weaker B-mode signal that can arise from gravitational lensing or, if detected at primordial scales, from gravitational waves produced during inflation. Foreground emission from the Milky Way, including synchrotron radiation and thermal dust, must be modeled and removed before such signals can be interpreted.⁴

The cosmic microwave background is not observed from a single physical shell in an ordinary sense: the “surface of last scattering” is the farthest region from which photons can reach us without later scattering, and its apparent distance depends on the cosmological model. Because the universe was opaque before recombination, microwave observations cannot directly image earlier electromagnetic events; they instead offer an indirect window onto that era.

Small secondary distortions and temperature shifts carry information beyond the primary map. The Sunyaev–Zeldovich effect changes CMB photons as they pass through hot gas in galaxy clusters, helping locate clusters and probe their thermal pressure. Gravitational lensing by intervening structure also subtly remaps the background, allowing researchers to infer the distribution of matter between Earth and the last-scattering surface.⁵ Persistent anomalies on the largest angular scales remain topics of statistical investigation rather than established failures of the standard cosmological model.

Recombination

The early-universe transition in which electrons combined with atomic nuclei to form mostly neutral atoms, allowing photons to travel freely.

Last-scattering surface

The apparent spherical region from which the cosmic microwave background photons reaching an observer were last scattered.

Anisotropy

A directional variation in a measured property; in the CMB, usually a tiny temperature or polarization fluctuation.

Blackbody spectrum

The characteristic thermal radiation spectrum determined by an object’s temperature.

Sunyaev–Zeldovich effect

A change in CMB photon energy caused primarily by scattering from energetic electrons in hot, ionized gas.