Discovery & Blackbody Spectrum of the CMB: Physical Mechanisms

The CMB fills the sky in every direction with a temperature near 2.7 kelvins. Its spectrum follows the Planck blackbody curve with extraordinary precision, while tiny temperature differences across the sky encode density and velocity variations in the early universe.

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.

Before recombination, photons scattered repeatedly from free electrons. Once neutral atoms formed, scattering dropped sharply and photons streamed freely while cosmic expansion stretched them into microwaves.