Cosmic Microwave Background

The cosmic microwave background (CMB) is the oldest electromagnetic radiation in the universe: the thermal afterglow of the hot, dense early cosmos, released about 380,000 years after the Big Bang and observed today as a near-uniform glow in the microwave band. Before that epoch the universe was an opaque plasma of free electrons and nuclei that scattered light incessantly; as it expanded and cooled below about 3000 K, electrons and protons combined into neutral atoms (recombination) and the radiation streamed free. Those photons have traveled essentially undisturbed ever since, stretched by cosmic expansion from the visible/infrared into the microwave.

The CMB has a blackbody (Planck) spectrum to extraordinary precision, with a present-day temperature of T₀ = 2.72548 ± 0.00057 K — making it the most perfect blackbody ever measured, with deviations below 50 parts per million. Its temperature scales with redshift as T(z) = T₀(1+z), so the radiation was hotter in the past in exact proportion to how much the universe has since expanded. This thermal character is direct evidence that the early universe passed through a state of thermal equilibrium at high density.

Although remarkably uniform, the CMB carries temperature anisotropies of about one part in 100,000. These tiny ripples, mapped by COBE, WMAP, and the Planck satellite, are the imprint of density fluctuations in the early universe — the seeds from which galaxies and clusters later grew. The detailed pattern of the anisotropies (the acoustic peaks) encodes the geometry, composition, and age of the universe, making the CMB the single richest observational dataset in modern cosmology.

Predicted in the 1940s by George Gamow, Ralph Alpher, and Robert Herman as a consequence of a hot early universe, the CMB was discovered serendipitously in 1964-1965 by Arno Penzias and Robert Wilson at Bell Labs, who detected an unremovable isotropic 3 K excess noise. A Princeton group led by Robert Dicke supplied the cosmological interpretation, and Penzias and Wilson received the 1978 Nobel Prize.