Two teams in New Jersey were, in the same months, working on opposite ends of the same problem without knowing it. About thirty miles apart, one was building an antenna to find a faint signal from the birth of the universe. The other had already found it, and was doing everything it could to make it go away.
The Team Trying to Kill the Signal
A twenty-foot horn-shaped antenna sat on a hill in Holmdel, pointed at empty sky, picking up a sound that would not go away. It was a faint, steady hiss, the same in every direction the antenna turned, the same by day and by night, the same in winter as in summer. The two men assigned to the instrument were trying to do careful radio astronomy, and the hiss was ruining it.
Arno Penzias and Robert Wilson worked for Bell Telephone Laboratories. The antenna had been built for a practical purpose — bouncing signals off the Echo balloon satellites — and Bell had let the two of them use it for pure research between jobs. They wanted to measure faint radio emissions from the edges of the Milky Way, which meant they first had to account for every other source of noise the antenna could possibly hear: the receiver’s own electronics, heat from the metal, the atmosphere, the ground. Only then could they trust whatever was left.
What was left was the hiss. Translated into the language radio astronomers use, it amounted to an excess temperature of about 3.5 degrees above absolute zero — small, but stubbornly, impossibly present. It corresponded to no star, no galaxy, no city, no machine. It came from nowhere and everywhere.
The Team Trying to Find It
About thirty miles to the southwest, at Princeton, a physicist named Robert Dicke had been thinking about a very old idea. If the universe began hot and dense and had been expanding and cooling ever since, the fireball of that beginning should still be detectable — not as light anymore, but as faint microwave radiation soaking the whole sky, cooled by billions of years of expansion to just a few degrees above absolute zero. The prediction was decades old; Ralph Alpher and Robert Herman had calculated something like it back in 1948 and it had been largely forgotten. Dicke had rediscovered the reasoning on his own and set a small team — Jim Peebles, Peter Roll, David Wilkinson — to build an antenna to look for the signal.
They were still building it. The thing the Princeton men were straining to detect, the Holmdel men were already drowning in — and trying to scrub out.
Ruling Out the World
Penzias and Wilson went down the list. New York City, low on the horizon, might be leaking radio noise; they pointed away from it, and the hiss held. The receiver itself might be the culprit; they had already cooled it with liquid helium to within four degrees of absolute zero to keep its own noise down, and the excess survived that. They checked the seams in the antenna’s aluminum skin, taping over the riveted joints in case signals were leaking in. Nothing changed.
Then they found the pigeons.
A pair of them had moved into the throat of the horn, where the dish narrows toward the receiver, and had coated the inside with what Penzias would later describe, with deadpan precision, as “white dielectric material.” Droppings. Bird droppings inside a precision instrument could, in principle, absorb and re-radiate heat and produce exactly the kind of low, even hum they were hearing. It was a real lead.
The Pigeon Campaign
They evicted the birds with a Havahart trap, the humane kind that catches an animal without harming it. They mailed the pigeons off in the company’s interoffice mail to a man elsewhere in the Bell system who kept pigeons as a hobby, hoping that was the end of it. The birds had a homing instinct. Within days they were back in the antenna.
Then the men climbed inside the horn and cleaned it out by hand, scrubbing away the dielectric material and taping over the spots where the pigeons had roosted. It was unpleasant, undignified work for two physicists, and they did it because the alternative was admitting they could not explain their own data.
The pigeons returned again. This time the solution was a shotgun, a decision Penzias remembered without pride. The birds were gone for good, the antenna was clean, the seams were sealed.
The hiss remained, exactly as loud as before.
The Two Tracks Meet
Penzias had mentioned his nagging noise to Bernard Burke, a radio astronomer friend, who had heard about a preprint by Peebles and pointed him back to Dicke. Penzias picked up the phone and called Princeton. Dicke was in his office with his team, in a meeting about the very detector they were racing to finish, when the call came in describing a 3.5-degree excess that came uniformly from all over the sky.
Dicke listened, asked a few questions, and hung up. He turned to the room and said, “Boys, we’ve been scooped.”
What the Hiss Was
The noise Penzias and Wilson had spent a year trying to scrub out of their antenna was the afterglow of the beginning of the universe. About 13.8 billion years ago everything that exists was packed into a state hot and dense beyond imagining; as it expanded, it cooled, and roughly 380,000 years in it cleared enough for light to travel freely for the first time. That light has been stretching and cooling ever since. By the time it reached a horn antenna in New Jersey it had dropped to a temperature of just under three degrees above absolute zero, a uniform glow in microwave wavelengths filling every corner of the sky. The cosmic microwave background. The literal leftover heat of creation.
In July 1965 the two groups published side by side in the Astrophysical Journal. The Princeton team laid out the theory of what such radiation would mean. Penzias and Wilson’s paper was titled, with characteristic restraint, “A Measurement of Excess Antenna Temperature at 4080 Mc/s.” It ran a little over a page and barely mentioned the Big Bang. It simply reported the noise they could not get rid of, and noted that someone next door had an explanation.
The signal that had nearly been blamed on bird droppings turned out to be the strongest evidence yet that the universe had a beginning. In 1978 Penzias and Wilson shared half the Nobel Prize in Physics for it — the other half went to Pyotr Kapitsa that year, for unrelated work in low-temperature physics. The Princeton team, who had known what they were looking for and gone looking, got the citation in the footnotes and not the medal. The men who found it had spent a year trying to make it stop.