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Arno Allan Penzias and Robert Woodrow Wilson discovered an unexplained microwave antenna signal at Bell Laboratories. Their measurement compared a horn-reflector antenna with a cooled reference and separately estimated atmospheric emission, instrumental losses and ground pickup.

The remaining excess was approximately isotropic, unpolarized and seasonally stable. Those checks made a purely local instrumental explanation less plausible. Interpreted as cosmic microwave background, the signal connected a calibrated radio-noise measurement to the thermal history of the cosmos; the original single-frequency result did not yet establish the full blackbody spectrum.^[ref]

One half went to Pyotr Leonidovich Kapitza for inventions and discoveries in low-temperature physics. Arno Allan Penzias and Robert Woodrow Wilson received one quarter each for discovering the cosmic microwave background.^[ref]

In 1965, Arno Penzias and Robert Woodrow Wilson reported measurements with a 20-foot horn-reflector antenna at $4.080\mathrm{GHz}$. They manually switched a traveling-wave maser receiver between the horn and a helium-cooled reference. From $6.7\mathrm{K}$ zenith antenna temperature, atmospheric and antenna/ground contributions left an excess $3.5\pm 1.0\mathrm{K}$.^[ref]

Elevation scans estimated the atmospheric term. Taping horn seams produced no measurable change; a nearby transmitter tested ground response. These controls addressed distinct possible contaminations.^[ref]

The comparison does not make all nuisance contributions disappear. A reference can establish a response scale, but atmospheric emission and pickup through unwanted antenna directions are also real incident radiation. In his later account, Robert Woodrow Wilson describes these competing contributions as central to a radio astronomy measurement.^[ref]

The Princeton group proposed primordial thermal radiation as an interpretation and called for more wavelength measurements. Their separate $3\mathrm{cm}$ radiometer had not yet produced results.^[ref] A single-frequency excess therefore does not by itself establish a complete Planck spectrum. Reproducibility across independent instruments, observing directions and frequencies tests the interpretation further.

Robert Woodrow Wilson and Arno Allan Penzias measured an unexplained microwave signal using a horn-reflector antenna and a receiver compared with a cooled reference. Their 1965 paper reported excess antenna noise that was approximately isotropic, unpolarized and seasonally stable.

They tested contributions from the atmosphere, instrumental losses and ground pickup before identifying the residual. Its interpretation as cosmic microwave background connected a carefully calibrated radio measurement to the thermal history of the Universe. The initial excess antenna temperature was not itself a full measurement of the background's blackbody spectrum.^[ref]