Definition
The cosmic microwave background (CMB) is the bath of thermal radiation, now at a temperature of about 2.725 K, that fills the universe in every direction — the cooled afterglow of the hot, dense early universe.
It is the oldest light in the cosmos, released about 380,000 years after the Big Bang when the universe cooled enough for electrons and protons to combine into neutral hydrogen and photons could finally travel freely.
Why it matters
How it works
In the first 380,000 years the universe was an opaque plasma of electrons, protons, helium nuclei, and photons. Photons scattered constantly off free electrons, traveling no farther than a thermal mean free path before being absorbed and re-emitted. As space expanded the plasma cooled. Around 3,000 K, electrons combined with nuclei to form neutral atoms — a transition called recombination — and the universe abruptly became transparent. The photons set free at that moment have been streaming across the universe ever since.
Cosmic expansion has redshifted their wavelengths by a factor of about 1,100, cooling them from a 3,000 K visible-light spectrum to a 2.725 K microwave spectrum. Arno Penzias and Robert Wilson detected this radiation in 1964–65 as an unaccountable hiss in a radio antenna at Bell Labs — work that won the 1978 Nobel Prize.
Later satellite missions — COBE, WMAP, and Planck — mapped the CMB across the whole sky and revealed tiny temperature variations imprinted by acoustic oscillations in the primordial plasma. The angular size and amplitude of these features depend sharply on the geometry and composition of the universe, making the CMB a precision laboratory. Modern cosmology's headline parameters — age of 13.8 billion years, flat geometry, 5% ordinary matter, 27% dark matter, 68% dark energy — come largely from CMB analysis.