Discovery & Blackbody Spectrum of the CMB: Structures & Vocabulary

The cosmic microwave background, or CMB, is relic thermal radiation released when the universe became transparent roughly 380,000 years after the hot early phase began. Its nearly perfect blackbody spectrum is one of the strongest pieces of evidence that the universe was once hot, dense, and in thermal equilibrium.

Before recombination, free electrons scattered photons frequently, coupling radiation to ionized matter. As the universe cooled, neutral atoms formed, the scattering rate dropped sharply, and photons began traveling freely. Cosmic expansion then stretched their wavelengths from visible/infrared energies into today's microwave band.

Penzias and Wilson discovered an unexplained microwave background in 1965, while later satellite missions measured its spectrum and sky distribution with increasing precision. The thermal spectrum and angular anisotropies match predictions of the hot Big Bang model.

The CMB has a smooth Planck spectrum plus tiny directional temperature variations. The average spectrum and anisotropy map answer different questions: thermal history versus initial structure.