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Fifty years ago, two young radio astronomers at AT&T Bell Labs were just about at their wits’ end. Arno Penzias and Robert Wilson had been hoping to use a giant radio horn, built for pioneering satellite communications, to detect the halo of cold gas they believed surrounded our Milky Way. But for six months, they had done not the slightest bit of science. This was because of an anomalous hiss of radio static their horn, at Holmdel, New Jersey, was picking up.
Initially, Penzias and Wilson wondered whether the source was New York City, which was just over the horizon. But, when they pointed the horn – basically a giant funnel for radio waves – away from the Big Apple, they still registered the signal. Next, they wondered whether the static was coming from a recent nuclear bomb test, which had injected radio-emitting electrons high into the atmosphere. But the signal did not fade with time, as would be expected of such a source. They wondered whether the static might be coming from a radio source among the planets and moons of the Solar System. But, as the months passed and the Earth travelled in its orbit around the Sun, the static did not vary.
Finally, the gaze of Penzias and Wilson alighted on two pigeons which had nested inside the railway-carriage-sized horn. Winters in New Jersey are cold, and it was warm and snug in the neck of the horn because of heat from the refrigerator that cooled the horn’s electronic detectors. Everything glows with radio waves: people, trees, buildings – and pigeon droppings, which now coated the interior of the Holmdel horn. Thinking, bizarrely, that this might be the source of the persistent and annoying radio static, Penzias and Wilson trapped the pigeons, sent them – in the company mail! – to another AT&T site, and got into the horn with stiff brooms to clean out the “white dielectric material”. But the anomalous static remained. It was the signal one would expect from a body whose temperature was a chilly three degrees above “absolute zero”, or -270° Celsius.
At this point, Penzias happened to phone a scientific colleague. During the call, he complained bitterly about not being able to do any astronomy because of a persistent hiss of radio static coming from every direction in the sky. The scientist on the other end of the line sat up. He had heard that a physicist called Jim Peebles had recently given a seminar about the possibility of there being leftover heat from the early universe. If he recalled correctly, a radio horn was being built to search for just such a signal at Princeton University, barely 25 miles from Holmdel.
Penzias immediately phoned Peebles’ boss, Bob Dicke. There was a short exchange of information. Dicke later recalled putting down the phone and saying to his team, who were sitting about in his office eating a packed lunch: “Well, boys, we’ve been scooped.”
Penzias and Wilson had inadvertently stumbled on the leftover heat “afterglow” of the Big Bang fireball. A whopping 99.9 per cent of all the “photons”, or particles of light, are tied up in this “cosmic background radiation” and a mere 0.1 per cent in the light from stars and galaxies. The afterglow of creation is the single most striking feature of our universe. If our eyes, instead of being able to see visible light, could see radio waves – technically, short-wavelength radio waves known as “microwaves” – we would see the whole of the universe glowing brilliantly white like the inside of a lightbulb.
This is the neat account you will read in many astronomy books. But this is not the whole story. The discovery of the cosmic background radiation is a classic example of the chaotic way in which much of science is actually done. The cosmic background radiation was predicted before it was discovered – but nobody took any notice. Bizarrely, it was also discovered before it was discovered – but nobody realised.
The story begins with the puzzle of the origin of the atoms in our bodies. By the 1930s, there was clear evidence that these had not been put in the universe on day one by a creator but every atom, from oxygen to uranium, had been assembled from the simplest atomic Lego-brick, hydrogen. The problem is that the cores, or “nuclei”, of atoms repel each other ferociously. The only way they can get close enough to stick via the tractor-beam-like “nuclear force” is for them to slam into each other at high speed. Since microscopic motion is synonymous with temperature, this means an extremely high temperature – in fact, a super-hot furnace at billions of degrees.
The obvious site of such an element-forging furnace was the interior of stars. But the English astronomer Arthur Eddington had calculated – wrongly, it turned out – that stars could never reach such high temperatures. This prompted the Russian physicist George Gamow to look for an alternative furnace that could have forged the elements. In 1929, the American astronomer Edwin Hubble had discovered that the universe is expanding, its constituent galaxies – including our Milky Way – flying apart from each other like pieces of cosmic shrapnel. If this expansion were imagined running backwards, like a movie in reverse, there would come a time when all of creation would be squeezed into the tiniest of tiny volumes. This was the moment of the universe’s birth: the Big Bang.
Crucially, Gamow realised that if the universe were squeezed into a small volume, it would be hot for the same reason that air squeezed in a bicycle pump gets hot. In other words, the Big Bang would have been a hot Big Bang – a fireball not unlike the fireball of a nuclear explosion. But whereas the heat of an explosion dissipates into the environment after a few minutes or a few hours, the heat of the Big Bang fireball had nowhere to go. It was bottled up in the universe, which, by definition, is all there is. It should therefore still be around today, greatly cooled by the expansion of the universe since the Big Bang, and appearing not as high-energy visible light but as low-energy radio waves.
Gamow mistakenly believed that the afterglow of creation would be impossible to distinguish from other astronomical sources of radio waves. However, his students, Ralph Alpher and Robert Herman, realised it would have an unmistakable fingerprint. First, it would come equally from every direction in the sky; and, secondly, the way in which its brightness varied with energy would be that of a so-called black body. Alpher and Herman published their prediction in the international science journal Nature in 1948. But nobody took any notice. In fact, when the two physicists asked radio astronomers whether the afterglow of the Big Bang fireball might be detectable with radio telescopes, they were told – wrongly – that it would not be.
Fast forward to 1964. Bob Dicke at Princeton believed in a bouncing universe, one that repeatedly expanded and contracted, going through an infinite series of big bangs and big crunches like a giant beating heart. By now, Eddington had been shown to be wrong about stars – as they grow old, the cores of massive stars do indeed attain temperatures of billions of degrees – and Fred Hoyle and his colleagues had shown in 1957 that all atoms except the very lightest ones are forged in the furnaces of stars. Dicke therefore faced a problem.
If each cycle of a bouncing universe were to start out like the last, something must destroy the elements forged by stars in the previous cycle. Heat, Dicke realised, would do the trick. The Big Bang or big crunch would be like the fireball of a nuclear explosion. Thus Dicke came to the same conclusion as Gamow, but for completely the opposite reason.
Dicke, unlike the radio astronomers Alpher and Herman had talked to, realised that the relic fireball radiation should still be around in today’s universe and, furthermore, that it should have an unmistakeable observational fingerprint. He set two young physicists, David Wilkinson and Peter Roll, the task of building a horn to detect the relic radiation from the roof of Princeton’s geology building. It was while the experiment was still under construction, in the spring of 1965, that Wilkinson and Roll were unfortunately scooped by Penzias and Wilson.
The Bell Labs and Princeton groups announced the discovery in a joint paper in Astrophysical Journal Letters with the distinctly unexciting and obscure title “A Measurement of Excess Antenna Temperature at 4080 megacycles per second.”
With the publication came other farcical twists to the story. It turned out that two Russian theorists, Andrei Dorochkevitch and Igor Novikov, knew of Alpher and Herman’s Nature prediction that the universe should contain heat radiation left over from the Big Bang. They even identified the Bell Labs horn at Holmdel as the only radio telescope in the world capable of detecting the Big Bang afterglow. But they fell at the last hurdle, misreading a Bell Labs technical paper. When they relayed what they had found to the eminent Russian cosmologist Yakov Zel’Dovich, he wrongly concluded that the Holmdel horn showed no sign of picking up the signal from the beginning of time.
If this was not farcical enough, it turned out that the afterglow had not only been predicted in 1948, it had actually been detected a decade earlier in 1938. Water Adams, the director of the world’s biggest telescope, the 100-inch on Mount Wilson, north of Los Angeles, observed starlight being absorbed by cyanogen molecules, spinning in space. For some reason these tiny molecular dumbbells were spinning faster than they ought to be.
Andrew McKellar, an astronomer at the Canadian Dominion Observatory, suggested that the anomalous motion of the cyanogen molecules could be explained if they were being buffeted by radio waves corresponding to temperatures of about 3 degrees above “absolute zero” (-270 °C). With Penzias and Wilson’s discovery, it suddenly became clear what the cyanogen molecules were being buffeted by: the leftover heat of the Big Bang.
The discovery of the afterglow of creation, along with the discovery of the expansion of the universe, was one of the biggest cosmological discoveries of the 20th century. It was very strong evidence that the universe was born in a hot Big Bang.
Furthermore, imprinted on the cosmic background radiation is a “baby photo” of the universe as it was when it was just 380,000 years old. For decoding this picture, and seeing the “seeds” of structures such as galaxy clusters in today’s universe, John Mather and George Smoot won the 2006 Nobel Prize for Physics.
The cosmic background radiation continues to be a goldmine of information about our universe. It is responsible for turning cosmology – the science of the origin, evolution and fate of the universe – into a precision science. Because of the cosmic background radiation, astronomers can now say that the universe is precisely 13.82 billion years old and that it contains 4.9 per cent ordinary “atomic” matter, 26.8 per cent “dark matter” and 68.3 per cent “dark energy” (As for what the dark matter and dark energy are, that is where astronomers are stuck!)
As for Penzias and Wilson, for two years they did not admit publicly that they had found the afterglow of the Big Bang. Ironically, they believed in the Steady State theory, which maintained that, as the universe expands, new material fountains out of the vacuum and congeals into new galaxies, so the universe looks the same at all times. The appeal of such a universe, which would have existed for ever, is that is not necessary to confront the awkward Big Bang question: What happened before?
Despite their extreme caution, Penzias and Wilson won the 1978 Nobel Prize for Physics. One reason was that the 1965 paper announcing the discovery of the cosmic background radiation had failed to mention Gamow and Alpher and Herman, creating bitterness, and a problem for the Nobel committee about whom to award the prize to. A simple solution was to give the prize to two astronomers who made their discovery by accident. But this does not do Penzias and Wilson justice. The main reason they won the prize was that they discovered something fundamental and important about the universe and they were first-rate experimenters who eliminated every possible spurious source of radio waves until there was only one thing left – the afterglow of the Big Bang.
Marcus Chown is the author of Afterglow of Creation, published by Faber & Faber
