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In 1964, two radio astronomers at Bell Telephone Laboratories spent most of a year trying to scrape pigeon droppings out of an antenna. Arno Penzias and Robert Wilson worked atop Crawford Hill in Holmdel, New Jersey, where Bell had built a six-meter horn-shaped reflector originally aimed at catching microwave signals bounced off the Echo balloon satellites — large aluminized mylar spheres NASA put into orbit at the start of the decade. By the time Penzias and Wilson took it over, the satellites were gone and the antenna was the most sensitive radio receiver in the world. They wanted to use it to measure the radio glow of the Milky Way at the seven-centimeter band.
To do that they had to subtract everything else. They cooled the receiver with liquid helium down to about 4 Kelvin — four degrees above absolute zero, the temperature at which a thermal noise floor effectively disappears. They pointed the antenna away from cities, away from the sun, away from the galactic plane. And still, at a wavelength of 7.35 centimeters — 4080 megahertz — there was a steady hiss. It was about a hundred times stronger than they had expected. It was the same in every direction. It was the same at midnight as at noon. It did not vary with the seasons.
The pigeons did not help. A pair of them had built a nest inside the throat of the horn and coated the inside with what Penzias diplomatically called "a white dielectric material." The two physicists climbed in, evicted the birds, scrubbed the antenna, and remounted the joints. The hiss came back at exactly the same level. They tried wrapping the seams in aluminum tape. They tried the second pigeon eviction. The hiss did not move.
Sixty kilometers down the road in Princeton, four people already knew what the hiss was. Robert H. Dicke, an experimentalist who had built early microwave radiometers during the war, had reasoned through a hot-Big-Bang scenario with his graduate student Jim Peebles. If the universe had begun in a dense, hot state, the radiation released as it cooled and became transparent should still be filling space — only stretched, by billions of years of cosmic expansion, into the microwave band. Peebles wrote the theoretical paper. Dicke had David Wilkinson and P. G. Roll building a small antenna on the Princeton campus to look for it. They had not yet pointed it at the sky.
The two teams met because of a phone call about a preprint. Bernard Burke, a radio astronomer at MIT, had heard the Princeton group describe the Peebles paper and had heard Penzias mention his unexplained noise. Burke told Penzias to call Dicke. Penzias called. Dicke listened, then turned to the Princeton room and reportedly said, "Boys, we've been scooped." Dicke sent over the unpublished Peebles manuscript. Penzias read it, called back, and invited the Princeton group to come listen to the noise themselves.
To avoid a priority fight, the two groups agreed to publish back to back. Two letters appeared in the July 1965 Astrophysical Journal Letters. The first, by Dicke, Peebles, Roll and Wilkinson, was titled "Cosmic Black-Body Radiation" and laid out the theoretical case. The second, by Penzias and Wilson, was titled "A Measurement of Excess Antenna Temperature at 4080 Megacycles per Second." It reported a residual background of about 3.5 Kelvin and noted, in one careful sentence, that "a possible explanation" was the one Dicke and his colleagues had given in the companion paper. Penzias and Wilson never even used the words Big Bang.
There was an earlier measurement, ignored at the time. In 1941, the Canadian astronomer Andrew McKellar, working with W. S. Adams's spectroscopic data on a B-type star, found that the rotational excitation of cyanogen molecules in interstellar space implied a background temperature of about 2.3 Kelvin. McKellar wrote that the temperature would have its own "perhaps limited significance." He did not connect it to cosmology. Nobody did, until decades later.
The discovery did two things at once. It killed the steady-state theory of the universe — the rival model defended throughout the fifties by Fred Hoyle, Hermann Bondi and Thomas Gold, in which matter is continuously created and the cosmos has no beginning. There is no clean way to manufacture a uniform 3-degree microwave glow in a steady-state universe. And it converted the Big Bang from an interpretive framework into a measurable physical scene: a moment, called recombination, about 380,000 years after the bang, when the cooling plasma let photons travel free for the first time.
Penzias and Wilson received the 1978 Nobel Prize in Physics for the measurement, sharing the year with the Soviet low-temperature physicist Pyotr Kapitsa, who had won for unrelated work on liquid helium. Jim Peebles, who had written the theory paper Penzias and Wilson confirmed, did not share that prize. He waited 41 more years. In 2019 Peebles was awarded a half share of the Nobel Prize in Physics for his theoretical work in physical cosmology. Dicke, Wilkinson and Roll never received one. The horn antenna on Crawford Hill is still standing, still maintained, listed on the National Register of Historic Places, with a small bronze plaque at its base recording what was found by accident inside it on a clear day in 1964.