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Why is the cosmic microwave background also called three-degree radiation?

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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) is often referred to as "three-degree radiation" because its temperature is approximately 3 kelvins (K) above absolute zero.1

This terminology originated from early theoretical predictions made by George Gamow and his colleagues, who estimated that the thermal radiation left over from the big bang would have cooled to a very low temperature by the present epoch. When the radiation was discovered in 1965, it was characterized as having a temperature of about 3 K.12

More precise measurements, such as those conducted by the Cosmic Background Explorer (COBE) satellite, have since refined this value to approximately 2.735 K. As the universe continues to expand, this radiation has been redshifted to longer wavelengths, causing the temperature of this primordial "first light" to drop significantly from its initial state of 3,000 K at the time of recombination.13