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Why does the cosmic microwave background have a blackbody spectrum?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The cosmic microwave background (CMB) exhibits a blackbody spectrum because it originated as thermal radiation in the hot, dense early universe.1

During the early stages of the big bang, the universe was in a state of thermodynamic equilibrium, where matter and radiation were tightly coupled at extremely high temperatures. This environment naturally produced a radiation field with a unique distribution of intensity across wavelengths, known as Planck’s radiation law, which is a function solely of the temperature.231

As the universe expanded, this radiation cooled, and the photons were redshifted to longer wavelengths. Because this expansion preserved the characteristic shape of the original thermal distribution, the CMB remains a near-perfect blackbody spectrum today, albeit at the much lower temperature of approximately 2.735 K. This spectrum serves as a fundamental piece of evidence for the big bang theory, confirming the universe's evolution from a hot, primordial state.4153