Only birdbrains would bring up a family inside a microwave antenna built for satellite communications. But that’s just what they were, the pair of pigeons which had set up home in the throat of the giant “horn” antenna at Holmdel in northern New Jersey. Every time the antenna swung round the sky, it upended the nest and the hapless pigeons had to rebuild it. But rebuild it they did. Soon the interior of the antenna was completely coated with pigeon droppings, or “a white dielectric material”, as radio astronomers Arno Penzias and Robert Wilson liked to call it. Could this be the source of the maddening radio static that had been driving them crazy for months? Or was it something else?
SWEEPING pigeon droppings wasn’t what Arno Penzias and Robert Wilson had in mind when they answered the call from AT&T Bell Telephone Laboratories for radio astronomers to come and share their skills with an industrial research lab. But one spring morning in 1965, there they were, clambering into the mouth of the giant microwave horn, armed with stiff brooms ready for the big sweep.
The Holmdel antenna had been built to bounce radio signals off Echo 1, the giant silvered beach ball that was the ancestor of modern communications satellites. Designed to pick up the very faint reflected signals, the antenna had a unique ability to screen out spurious microwaves from the environment – emitted by everything from trees to people to buildings. This allowed it to detect very faint signals from the sky, which was what had attracted Wilson, who was hoping to pick up the faint glow of ultra-cold hydrogen gas in the halo around the Milky Way.
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When Penzias and Wilson finally got their hands on the antenna, Bell’s satellite engineers had finished their tests with Echo 1’s successor, Telstar, the first truly modern communications satellite. For a while things seemed to be going well. Then they noticed the anomalous static.
Penzias and Wilson checked the radio receiver – but that was working fine. They put aluminium tape over the rivets holding together the antenna’s aluminium sheets but that had no effect. And the static could not be from the Milky Way because the signal was the same wherever the antenna was pointed. Nor could it be from a source in the Solar System because, as the Earth moved in its orbit and 1964 turned into 1965, the signal remained doggedly constant.
The two astronomers briefly thought it might be New York City, just over the horizon to the north, but like the Milky Way that would be a localised source. They thought it might be electrons injected into the upper atmosphere by a high-altitude nuclear test in 1962, but those should have faded within a year. Finally, their gaze alighted on the pigeons. Everything hotter than absolute zero gives off radio waves – including pigeon droppings.
It took no more than an hour for Penzias and Wilson to sweep clean the interior of the antenna and clamber back down to the ground. In the shed-like control room strapped to the tapered end of the horn, they turned on the receiver. Their hearts sank: the anomalous signal was still there. It was exactly what would be expected from a body at around -270 °C, 3.15 degrees above absolute zero. What could it be?
An extraordinary possibility came up during a phone call Penzias made to fellow radio astronomer Bernie Burke. Burke had heard that a team at nearby Princeton University led by Robert Dicke was saying that if the Universe had begun in a hot, dense state – a big bang – then the heat would have had nowhere to go, since the Universe, by definition, was all there was. Greatly cooled by the expansion of the Universe, the heat would still be around today as a ubiquitous glow of microwaves at a temperature of less than 10 degrees above absolute zero. Dicke’s team was constructing a telescope to look for it.
Penzias phoned Dicke, and Dicke’s people drove over the next day. They inspected the equipment, pored over the pen-recorder data, and quizzed Penzias and Wilson. Finally, they were sure of it: they’d been pipped to the post. The persistent static, which Penzias and Wilson had seriously thought might be the microwave glow of pigeon droppings, had to be the afterglow of the big bang fireball. Rarely in the history of science can a scientific discovery so profound have been mistaken for something so mundane.
Inadvertently, Penzias and Wilson had made the greatest cosmological discovery since the 1920s when Edwin Hubble discovered that the Universe was expanding. The afterglow was strong evidence that the Universe had indeed been born in a big bang, while most astronomers at the time, including Wilson, believed it had existed forever.
But the discovery prompted a serious question: why had the “cosmic microwave background”, as it is now known, not been found before? The radiation accounts for an astonishing 99 per cent of the light in today’s Universe, with a mere 1 per cent contributed by starlight. And the technology to detect it existed in the 1940s.
As it turns out, the radiation was detected earlier. In fact, it has the peculiar distinction of being both predicted and detected before it was “discovered”. In 1948, the American physicists Ralph Alpher and Robert Herman pointed out in the journal Nature that, if the Universe began in a hot big bang, then the residual heat should still be around today, cooled to about 5 degrees above absolute zero. The radiation should have two distinctive features: it should be coming equally from all directions and it should have the spectrum of a so-called black body.
Alpher and Herman even asked radio astronomers whether this leftover radiation might be detectable, but they were wrongly told that it would not. A few years later, the Russians Igor Novikov and I. D. Doroshkevich came to the same conclusion as Alpher and Herman. They even identified the Holmdel antenna as the one instrument capable of detecting the afterglow. But when they got hold of a paper by the Echo engineers – which contained a strong hint of the static picked up by Penzias and Wilson – they misread a crucial sentence and concluded that there was no evidence for a big bang afterglow.
Perhaps the most ridiculous twist in the whole story is that evidence for the cosmic background radiation had been found still earlier, in 1938. At Mount Wilson Observatory in California, astronomer Walter Adams had been observing a star whose light was being partially absorbed by an interstellar gas cloud. The absorbing gas, he concluded, was a molecule called cyanogen. The odd thing was that in the deathly cold of space, cyanogen molecules should be in their lowest possible energy state. But they weren’t. To be absorbing the starlight, they had to be in the next lowest state.
Andrew McKellar, an astronomer at the Dominion Observatory in Canada, pointed out that the only explanation was that the cyanogen molecules were being buffeted by radiation at 2.3 degrees above absolute zero. Penzias and Wilson’s discovery explained it all. The cyanogen molecules were interstellar thermometers, floating in space and quietly taking the temperature of the afterglow of the big bang.
For their serendipitous discovery of the cosmic background radiation, Penzias and Wilson were awarded the 1978 Nobel Prize in Physics. And the pigeons? Well, there was no such happy ending for them. They were caught and sent through the company mail to Whippany, another Bell Labs site about 60 kilometres away. But within two days they had found their way back to Holmdel, and instead of a welcoming committee they found only a man with a shotgun. These two unsung heroes of cosmology paid the ultimate price for science.