= Penzias-Wilson microwave background experiment
{c}
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 $3.5\pm1.0\,\mathrm K$.<A Measurement of Excess Antenna Temperature at 4080 Mc per Second by Arno Allan Penzias and Robert Woodrow Wilson>[pp.419–420: apparatus, temperature contributions and residual]{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.<A Measurement of Excess Antenna Temperature at 4080 Mc per Second by Arno Allan Penzias and Robert Woodrow Wilson>[p.420: elevation, seam and back-lobe tests]{ref}
\Image[/cmb-horn-radiometer-and-reference-load.png]
{title=Comparing a sky horn with a cold reference separates receiver response from incident radiation}
{description=Original functional diagram inspired by the reported comparison, not a reconstruction or photograph of the historical apparatus. The added warm reference illustrates a generic <two-load radiometer calibration>; it is not claimed to have been switched in this form in the original experiment. Radiation from the sky, atmosphere, ground and antenna can contribute at the receiver input.}
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.<The Cosmic Microwave Background Radiation by Robert Woodrow Wilson>[1978 lecture, introduction and sectionII: sky, ground, atmosphere and instrument contributions]{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.<Cosmic Black-Body Radiation by R. H. Dicke and others>[pp.415–416: interpretation, Princeton apparatus and need for further wavelengths]{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.
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