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How does Planck’s radiation law describe the spectrum of the cosmic microwave background?

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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.

Planck’s radiation law describes the intensity of electromagnetic radiation emitted by a blackbody as a function of wavelength and temperature. In the context of the cosmic microwave background (CMB), this law characterizes the spectrum of the radiation as a near-perfect blackbody distribution.123

The CMB represents the relict thermal radiation from the early, hot universe. As the universe expanded, this radiation cooled and its wavelengths were stretched, resulting in the current observed temperature of approximately 2.73 K. Precise measurements, most notably by the Cosmic Background Explorer (COBE) satellite, confirmed that the CMB spectrum matches the intensity distribution predicted by Planck’s law for a blackbody at this specific temperature.324

Because the radiation is isotropic—meaning it appears the same in every direction—it serves as a fundamental piece of evidence for the big bang theory, reflecting the state of the universe when it first became transparent to radiation.342