In 1964 and 1965, two radio astronomers at Bell Telephone Laboratories spent roughly a year trying to track down a faint, persistent noise in their antenna. They checked the electronics, they considered the sky, and at one point they cleaned out a pair of nesting pigeons and the mess the birds had left behind. The noise stayed. It turned out to be the cosmic microwave background, the radiation left over from the hot, dense early universe.
The one thing worth adjusting is the emphasis. The pigeons make a good anecdote, but they were one item on a list of things ruled out, not the heart of the work.
An instrument built for something else
The two astronomers were Arno Penzias and Robert Wilson, and the instrument was the Holmdel Horn Antenna, a large horn-shaped microwave antenna at the Bell Labs site on Crawford Hill in Holmdel, New Jersey. It had not been built for cosmology. It was built for Project Echo, an early experiment in bouncing radio signals off satellites.
With Project Echo finished, Penzias and Wilson set out to use the antenna as a radio telescope. That meant accounting for every source of noise in their receiver, because radio astronomy depends on measuring faint signals against a quiet background. They needed to know exactly how much noise the instrument itself produced before they could trust anything it detected.
The hiss that would not go away
When they did the accounting, a faint excess of noise remained that they could not explain. It was small, but it was real, and it had two features that made it stubborn.
It was the same in every direction they pointed the antenna. And it did not change with the time of day, the season, or where the antenna was aimed in the sky. A noise source on the ground, or in a particular patch of sky, would not behave that way. Something that looked the same everywhere and at all times was hard to attribute to any local cause.
They worked through the possibilities methodically. They considered radio interference from nearby New York City. They considered radiation from the Milky Way. They checked the antenna’s own electronics, cooling the receiver with liquid helium to reduce the noise it generated itself. They found a pair of pigeons nesting in the antenna’s throat, removed the birds, and cleaned out the droppings, which Penzias is said to have referred to, drily, as a white dielectric material. They taped down riveted joints in the metal in case those were contributing. As the Nobel Foundation’s account of the discovery puts it, the pair spent a year persistently working through every possible cause for the unwanted noise. None of it removed the signal.
What the noise turned out to be
The interpretation came from outside Bell Labs. About 60 kilometres away, a group at Princeton University led by Robert Dicke had been working on the theory of an early hot universe, and had reasoned that such a beginning should have left behind a faint bath of radiation, cooled by billions of years of cosmic expansion into the microwave range. The Princeton group was preparing to look for it.
Through a chain of conversations, Penzias was put in touch with Dicke, and the two efforts met. The unexplained noise at Holmdel had the characteristics the Princeton theory predicted. The signal that Penzias and Wilson could not eliminate was not a fault in their instrument. It was the thing the Princeton group had been about to go looking for.
In 1965 the two teams published together in the Astrophysical Journal Letters. Penzias and Wilson contributed a deliberately understated paper titled “A Measurement of Excess Antenna Temperature at 4080 Megacycles per Second,” and the Princeton group published a companion note supplying the cosmological interpretation. In his 1978 Nobel lecture, Wilson set out the measurement plainly: an excess antenna temperature of about 3.5 kelvin that remained after every known instrumental and atmospheric contribution had been subtracted. Penzias and Wilson shared the 1978 Nobel Prize in Physics for the discovery of the cosmic microwave background radiation.
Why the noise mattered
The radiation Penzias and Wilson detected does not date from the very beginning of the universe. It comes from roughly 380,000 years afterward, the point at which the cooling universe first became transparent and allowed light to travel freely. It is the oldest light there is to detect, and it arrives from every direction, which is why the antenna found it to be the same wherever it pointed.
Its discovery is usually described as the observation that decisively shifted cosmology toward the Big Bang model and away from steady-state theories. A faint background of microwave radiation, even in temperature across the whole sky, was a natural prediction of the first picture and not of the second. The detection did not by itself prove the Big Bang model, but it was the piece of evidence that moved the weight of opinion decisively toward it.
What is worth keeping from the Holmdel story is less the pigeons than the method. Penzias and Wilson were not looking for the origin of the universe. They were doing the unglamorous work of characterising their instrument properly, and they declined to dismiss a small residual signal as someone else’s problem. The discovery came out of refusing to round the noise down to zero.
