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What did George Gamow, Ralph Alpher, and Robert Herman predict about radiation from the early universe?

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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.

In 1948, George Gamow, Ralph Alpher, and Robert Herman proposed that the early universe was a hot, dense "primeval fireball" characterized by a thermal radiation field.1

They predicted that as the universe expanded, this radiation would cool and its photons would be redshifted to longer wavelengths. Consequently, they estimated that this remnant radiation should still exist in the present epoch, though it would have cooled to a very low temperature—approximately 5 kelvins above absolute zero—making it detectable by radio telescopes.12

Although this prediction was largely overlooked for many years, it was eventually confirmed in 1965 when Arno Penzias and Robert Wilson discovered the cosmic microwave background, which was found to have a temperature of about 3 K.13