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The cosmic microwave background is nearly isotropic thermal electromagnetic radiation. Its spectrum connects radio receiver measurements with the universe's thermal history: redshift preservation of a thermal photon spectrum cools a blackbody spectrum as space expands. The 1965 Princeton analysis described this relation.^[ref]

A radiometer cannot label a received photon as cosmic, atmospheric or instrumental. Its measured power combines the incoming radiation with the receiver response. The useful question is which comparisons and controls can isolate a sky contribution. This makes the Penzias-Wilson microwave background experiment an example of both observational cosmology and measurement calibration.

David Todd Wilkinson helped construct a receiving horn and radiometer for absolute microwave-background measurements. The 1965 hot-universe paper explicitly credited the Princeton instrument to him and P. G. Roll, and explained the 3-centimetre wavelength as a compromise between atmospheric absorption and longer-wavelength foregrounds. Its measurements were still pending when the paper interpreted the separately reported microwave excess. This separates the team's experimental preparation from the evidence already available for relic radiation.^[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.

Constructed, jointly with a coauthor, a radiometer and receiving horn for absolute thermal-radiation measurements at $3\mathrm{cm}$. The associated cosmic microwave background paper explained the wavelength choice as a compromise between atmospheric absorption and longer-wavelength foregrounds; results from this instrument were still pending.^[ref]

Coauthored the hot-universe interpretation of the cosmic microwave background. The analysis explained how cosmological redshift could cool primordial black-body radiation while preserving its thermal spectrum, and considered whether the reported microwave excess was this relic radiation.^[ref]