In the spring of 1964, two young radio astronomers in New Jersey went to war with a pair of pigeons. Arno Penzias and Robert Wilson had been given the use of a six-metre horn-shaped antenna at Bell Laboratories' Crawford Hill site in Holmdel, built to catch signals bounced off the Echo balloon satellite and now surplus to requirements. They wanted it for radio astronomy, which meant first accounting for every scrap of noise the instrument produced. One scrap refused to be accounted for: a faint, steady microwave hiss, equivalent to a temperature of about 3.5 degrees above absolute zero, arriving from every direction in the sky, day and night, in every season.
They checked everything. It was not New York City's radio clatter over the horizon, not military transmitters, not a fault in the amplifiers or the riveted joints. Suspicion finally fell on the pigeons roosting in the horn's throat, which had coated the interior with what Penzias, in one of science's great euphemisms, logged as "white dielectric material". The birds were trapped and posted to another Bell Labs site miles away; being pigeons, they promptly homed straight back, and eventually had to be dealt with more permanently. The droppings were scrubbed out. The hiss dipped slightly — and stayed. For the better part of a year the two men lived with a noise they could not explain, too careful to ignore it and too baffled to publish.
Fifty kilometres away in Princeton, meanwhile, the physicist Robert Dicke and his group — including a young Jim Peebles — were preparing to search for precisely that signal. Peebles's calculations suggested that if the universe had begun in a hot, dense state, the light of that era should survive today as a faint, all-sky bath of microwaves. In 1965 Penzias heard of the Princeton work through a colleague and telephoned Dicke, who listened, put down the receiver and told his team: "Boys, we've been scooped." The groups agreed to publish back-to-back papers in the Astrophysical Journal. Penzias and Wilson's ran to about 600 words under the studiously boring title "A Measurement of Excess Antenna Temperature at 4,080 Megacycles per Second", leaving the cosmic interpretation entirely to their neighbours.
The oldest light there is
What they had stumbled on was the afterglow of the Big Bang — the cosmic microwave background. For its first 380,000 years or so, the universe was a plasma too hot for atoms to hold together, and light could not travel freely through the glare. When everything cooled enough for electrons and nuclei to combine, the fog lifted and the trapped light escaped, everywhere at once. Expanding space has been stretching it ever since, by a factor of more than a thousand, so that what began as a searing glow now registers as microwaves at just 2.7 degrees above absolute zero. It comes from every direction because it fills the universe; an old analogue television tuned between stations picked up a small fraction of its dancing static from the CMB itself, which means generations of viewers had already watched the Big Bang without knowing it.
The bitter footnote is that the discovery had been predicted, then forgotten. Back in 1948 Ralph Alpher and Robert Herman, working alongside George Gamow on how the first elements were forged in the primordial furnace, had calculated that a hot early universe should leave a relic glow of roughly five degrees — and nobody went looking, because almost nobody realised the whisper of creation might actually be measurable. The measurement also ended the great cosmological feud of the century. The rival steady-state theory, championed by Fred Hoyle — who had coined the very phrase "Big Bang" on BBC radio in 1949, insisting ever after that no sneer was intended — had no natural way to produce such a glow, and never recovered.
Penzias and Wilson shared the 1978 Nobel Prize in Physics for the find. The signal became cosmology's most productive quarry: in 1992 the COBE satellite mapped ripples in it of just one part in 100,000 — the seeds of future galaxies — earning John Mather and George Smoot the 2006 Nobel, and the later WMAP and Planck missions read from it the universe's age, about 13.8 billion years, and its recipe. Peebles collected a Nobel of his own in 2019, and the Holmdel horn antenna now stands as a US National Historic Landmark.
The pigeons, it turned out, had been innocent all along. The noise that would not go away was the oldest light in existence — the universe's baby photograph, delivered unbidden to a hilltop in New Jersey and very nearly written off as droppings.
Quiz nuggets
- Arno Penzias and Robert Wilson found the cosmic microwave background in 1964 using a horn antenna at Bell Labs in Holmdel, New Jersey.
- Penzias logged the pigeon droppings coating the antenna as "white dielectric material".
- Ralph Alpher and Robert Herman predicted the relic glow in 1948; today it measures about 2.7 degrees above absolute zero.
- The CMB is light released about 380,000 years after the Big Bang, when the universe first became transparent.
- Penzias and Wilson took the 1978 Nobel Prize in Physics; COBE's ripple map brought Mather and Smoot the 2006 prize.