how the cosmic microwave background was discovered

The Mechanism

*Arno Allan Penzias* (born Munich, *26 April 1933*; died San Francisco, *22 January 2024*, aged 90) and *Robert Woodrow Wilson* (born Houston, *10 January 1936*; now 90) were two early-career radio astronomers at *Bell Telephone Laboratories' Crawford Hill site* outside *Holmdel, New Jersey*. The instrument they had been assigned was a *20-foot horn-reflector antenna* — a 20-foot-square aperture funnelled into a Cassegrain receiver, originally built in 1959 for *Project Echo*, NASA's experimental satellite-bounce communication programme, and reconfigured by 1962 to study microwave emission from the Milky Way at *4080 megahertz* (7.35 cm wavelength). When Penzias and Wilson began joint observations in *June 1964* they found that the antenna registered an unexpected *excess background noise* corresponding to a sky temperature of about *3.5 kelvin* — uniform in every direction, present day and night, present at every elevation above the horizon, present whether the antenna pointed at the Milky Way or at empty sky. They spent the next *eleven months* trying to eliminate it. They pointed the antenna at New York City; the noise did not change. They calculated the contribution from atmospheric emission and found it could not account for more than 2.3 K. They discovered that a pair of *pigeons* had taken up residence inside the horn and were depositing what Penzias politely called *"a white dielectric material"* on the throat; they trapped and removed the pigeons, scrubbed out the droppings, and remeasured. The noise was unchanged. By early 1965 Penzias and Wilson had exhausted every terrestrial explanation and were preparing to write up the result as an unexplained measurement when Penzias, on a separate phone call about pulsar work, learned from the radio astronomer *Bernard Burke* that a group at *Princeton University* under *Robert H. Dicke* — Dicke, P.J.E. Peebles, P.G. Roll, and D.T. Wilkinson — was building a *3-cm-wavelength radiometer on the roof of the Princeton physics building* with the explicit purpose of detecting a *relic blackbody radiation field* predicted theoretically as the cooled-down afterglow of an early hot dense phase of the universe. Penzias telephoned Dicke that day. Dicke listened, hung up, turned to his colleagues in the room, and said: *"Well, boys, we've been scooped."* The two groups arranged to publish their results back-to-back in *The Astrophysical Journal*, *volume 142*, *1 July 1965*: Dicke, Peebles, Roll, Wilkinson, *"Cosmic Black-Body Radiation,"* pages 414-419, with the theoretical interpretation; Penzias and Wilson, *"A Measurement of Excess Antenna Temperature at 4080 Mc/s,"* pages 419-421, with the measurement. The measurement paper, just over 600 words long, made no mention of the Big Bang and ended with a single sentence pointing to the Dicke companion paper for "a possible explanation." The discovery established the cosmic microwave background — the redshifted electromagnetic glow released when the universe became transparent to its own radiation, about *380,000 years after the initial expansion* — as the strongest single piece of evidence for hot-Big-Bang cosmology, and effectively ended the *Steady-State* model that had been the alternative since the late 1940s. The *1978 Nobel Prize in Physics* was awarded jointly to Penzias and Wilson "for their discovery of cosmic microwave background radiation," and to Pyotr Kapitsa for unrelated low-temperature work. The Crawford Hill horn antenna was designated a U.S. *National Historic Landmark* in 1990; it still stands at Holmdel, New Jersey. The CMB has since been mapped at progressively higher resolution by COBE (1989-1993, Mather and Smoot Nobel 2006), WMAP (2001-2010), and Planck (2009-2013), producing the all-sky temperature maps that now anchor every measurement of the universe's age (13.8 billion years), composition, and geometry.