Why did Arno Penzias and Robert Wilson receive the 1978 Nobel Prize in Physics?

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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.

Arno Penzias and Robert Wilson were awarded one-half of the 1978 Nobel Prize for Physics for their discovery of the cosmic microwave background (CMB) radiation.1

While conducting research at Bell Laboratories using an experimental antenna, the two scientists unexpectedly detected a uniform, faint electromagnetic radiation filling the entire sky. This radiation, measured at a temperature of approximately 3 K, provided critical empirical evidence for the big-bang model of the universe's origin. Most scientists now recognize this signal as the residual thermal energy left over from the primordial explosion that occurred billions of years ago.123