by Marcus Chown
However, a modification was necessary because the faint signal Wilson intended to look for would likely be drowned out by the radio waves emitted by the sky. Radio astronomers usually overcome this problem by rapidly switching their telescope between an astronomical source – a star or a galaxy – and a neighbouring patch of sky. By subtracting one signal from the other, they are able to neatly remove the emission from the sky.2 This would not work for observing the ‘galactic halo’ because we are inside the Milky Way; since the halo fills the sky, it is impossible to point away from it.
The solution hit upon by Penzias and Wilson was to compare the galactic halo with an artificial source of radio waves. Penzias built such a source, which he cooled with liquid helium to four degrees above absolute zero. He housed this ‘cold load’ in the shed strapped to the tapered end of the horn, which also contained the radio receiver.
Before doing any actual astronomy, Penzias and Wilson made sure their equipment was working by observing at a frequency where they expected there to be no radio glow from the galactic halo. It appeared daft but there was method in their madness: if the signal detected when they pointed the horn at a patch of empty sky was precisely zero, they would know their equipment was working as expected.
However, when they carried out the test, things did not go as planned. The signal they recorded was not zero; instead, there was a residual hiss of radio static. It was what would be emitted by a body at about –270 degrees Celsius, or three degrees above absolute zero.
At first, the two astronomers thought the hiss was coming from New York, which was just over the horizon from Holmdel, but when they pointed the horn away from the city, the static remained. Their next thought was that it might be coming from a source in the solar system – both the Sun and Jupiter broadcast radio waves – but as the months wore on and the Earth travelled around the Sun in its orbit, the static did not change. The astronomers next wondered whether the source might be a high-altitude explosion of a hydrogen bomb. On 9 July 1962, ‘Starfish Prime’ injected high-energy electrons into the Van Allen Belts, recently discovered regions of Earth’s magnetic field that trapped charged particles from the Sun. Spiralling around the magnetic field ‘lines’, such electrons would be expected to emit radio waves; such an effect would obviously decline with time, but the static did not.
Finally, Penzias and Wilson’s gaze settled on two pigeons that had made a nest inside the horn. At first sight, it did not appear to be a good place to make a home; they had to remake their nest every time the horn was turned to point in a new direction. However, the winters in New Jersey are cold, and the tapered end of the horn, where they had nested, was next to the refrigerator that cooled the electronics of the radio receiver. Anyone who has been around the back of a refrigerator knows it is warm there; the pigeons had chosen a cosy spot to raise their family. In doing so, however, they had coated the interior of the horn with what Penzias and Wilson referred to as a ‘white dielectric material’. To everyone else, it was pigeon shit, which, like everything else, glows with radio waves. Penzias and Wilson exchanged glances. Could they at last have found the source of the annoying hiss of radio static that had prevented them from doing any astronomy for so many months?
At a local hardware store, they bought a baited ‘Havahart’ trap. When a bird stepped on a finely balanced plate, a gate behind it dropped, trapping the bird. With the aid of the trap, the two astronomers caught the pigeons and posted them (!) in the company mail to another AT&T site at Whippany, New Jersey.3 With the pigeons gone, Penzias and Wilson climbed into the mouth of the horn to put in an hour of hard work with stiff brooms. For good measure, they stuck aluminium tape over the rivets holding together the horn’s metal sheets, just in case they were contributing to the radio hiss.
Back down on the ground, they changed back into their everyday clothes, full of hope that their problem was finally solved and that they would at long last be able to do some real astronomy. As the sixteen-tonne horn turned slowly on its axes, Penzias and Wilson’s eyes were glued to a pen-recorder that traced a jittery straight line across a paper roll. The horn’s opening arrived at the point where it was looking back at the sky. And the trace jumped.
The radio static was still there! Penzias and Wilson shook their heads in dismay. What in the world could it be?
Washington DC, Summer 1948
Ralph Alpher and Robert Herman stood for a while and admired their calculations. Written on the blackboard were the details they had painstakingly worked out during an evening of brainstorming. If they were right, the proof that the universe had been born rather than existed forever was literally fizzing in the air all around them, and it had an unmistakable signature.
The smoke from George Gamow’s cigarette was still hanging in the air. ‘Write it up, you two! Write it up!’ Gamow had ordered when he saw their calculations. Then he had left, firing off ideas like firecrackers, as was his way. By now, he was no doubt onto something else: galaxy formation, quantum theory, analogies to use in his popular series of ‘Mr Tompkins’ books – who knew what? The visits of Alpher’s larger-than-life supervisor were like drive-by shootings, leaving him and Herman stunned and overwhelmed. But to give him his credit, it was Gamow who had come up with the idea that set them on their path to discovery. Though he exasperated them – largely because his practical jokes and drunkenness made other physicists see him as more fly-by-night dilettante than serious scientist – they loved him dearly.4
Gamow had defected from Stalin’s Russia in 1933 with his wife, fellow physicist Lyubov Vokhminzeva. Unable to find a permanent academic post in Europe, he had headed for the US the following year, where he had ended up with a professorship at the George Washington University in Washington DC; this was where Alpher, who was studying at night while holding down a day job working on the theory of guided missiles, had become aware of him. Gamow was loud, enthusiastic, irreverent and larger than life in every way. He may not have been highly regarded by the physics community, but he knew all the greats personally – Albert Einstein, Niels Bohr, Werner Heisenberg – and he had made a major contribution to physics by being the first person to apply quantum theory to the nucleus of the atom, in the process explaining the long-standing mystery of radioactive ‘alpha decay’.
Alpher plucked up the courage to ask whether Gamow would take him on as a doctoral student, even though he was working at Johns Hopkins University in Baltimore, and Gamow said yes. It was only later that Alpher bumped into Robert Herman, a postdoctoral student who had an office a few doors down the corridor. Herman stopped by to introduce himself, and when Alpher told him about the calculations he was working on, he was instantly hooked.
The calculations had been triggered by Gamow, who had been thinking about the origin of the chemical ‘elements’. As pointed out earlier, by the 1940s it had become apparent that all ninety-two of the naturally occurring elements – from hydrogen, the lightest, to uranium, the heaviest – had not been put in the universe on day one by a Creator but had instead been made. The clue was in the correlation between the abundance of the elements and how strongly their nuclei were bound together. This was a powerful hint that nuclear reactions had been involved in the creation of the elements – that the universe started off with hydrogen, the simplest element, and the rest had subsequently been assembled from this basic nuclear Lego brick.
The problem with building up the elements like this was that all nuclei carry a positive electric charge; since like charges repel each other, this means they have a powerful aversion to each other. The only way for them to get close enough to stick together is for them to slam into each other at high speed, which is synonymous with high temperature. A temperature of many billions of degrees is required. But where in the universe is there such a blisteringly hot furnace?
The obvious place is deep inside stars, though Arthur Eddington had wrongly concluded that the interiors of stars were neither hot enough nor dense enough for ‘nucleosynthesis’. This was the sta
te of play in the mid-1940s, when Gamow began thinking about the problem of element-building.
Back in Russia, Gamow had been the student of Alexander Friedmann, who in 1922 was the first person to realise that Einstein’s general theory of relativity implies that we live in a restless universe that has to be in motion and cannot, as Einstein himself believed, be static and unchanging. Specifically, he reasoned, the universe must be either contracting down to or expanding from a superdense state. The term ‘Big Bang’ would not be coined for almost three decades, but Friedmann had discovered the Big Bang solutions to Einstein’s equations (Friedmann died prematurely in 1925, aged thirty-seven, which was another reason why Gamow was not tied to Russia).
In 1929, American astronomer Edwin Hubble, using the world’s biggest telescope on Mount Wilson in Southern California, discovered that the universe is indeed expanding, its constituent galaxies flying apart like pieces of cosmic shrapnel, just as Friedmann had predicted. But although an explosion in the distant past was likely responsible for cosmic expansion, nobody until Gamow thought seriously about it, because it seemed so remote from everyday experience.5
Gamow imagined the expansion of the universe running backwards, like a movie in reverse. When everything had been squeezed into a tiny, tiny volume, it would have been hot, for the same reason that air squeezed in a bicycle pump gets hot. The Big Bang, Gamow realised, would have been a blisteringly hot fireball. Could this have been the elusive furnace in which nature’s elements were forged? Gamow was not a details man – in fact, he was notorious for making errors in equations and adding things up incorrectly. He therefore gave the problem to his student, Alpher, to explore.
Neither Alpher nor Gamow knew the exact ingredients with which the universe had started, but they knew that they must have been simple. Alpher tried a number possibilities. One was a mix of protons and neutrons. Along with the proton, the neutron, discovered in 1932 by English physicist James Chadwick, is one of the two basic building blocks of all nuclei (apart from hydrogen, which contains a lone proton). By virtue of the fact that it carries no electric charge, a neutron can easily approach and stick to a nucleus. However, if too many neutrons embed themselves in a nucleus, it becomes unstable and one of its neutrons changes into a proton, a process known as ‘beta decay’.
Alpher very quickly realised that, because of the rapid expansion and cooling of the Big Bang fireball, there would have been only a brief opportunity for element-building, lasting from when the universe was about one minute old to when it was about ten minutes old. After that point, the expansion would have driven nuclei so far apart and they would have been moving so slowly that their collisions would have been too infrequent and low-impact to cause them to stick to each other. Another important effect was that free neutrons decay into protons in about ten minutes, so their supply would have been rapidly depleted.
Alpher duly carried out the calculations to see what would have been the result of the orgy of ‘nuclear reactions’ in the Big Bang fireball. He found that the furnace would have converted about 10 per cent of all nuclei into helium, leaving the remaining 90 per cent as hydrogen. This was a remarkable result – it was exactly what was observed in today’s universe.
Although this was an undoubted success that bolstered the case for the Big Bang having been the furnace in which the elements were forged, it was hard to see how any elements heavier than helium could have been made. Even if nuclear reactions had proceeded for longer than ten minutes, it would not have helped; the problem, as we know, was that nature has no stable nuclei with five or eight nucleons. Since helium has four nucleons (two protons and two neutrons), the route to building heavier nuclei – by either adding a nucleon (making a nucleus with five nucleons) or sticking two helium nuclei together (making a nucleus with eight nucleons) – is well and truly blocked.6
Alpher wrote a paper on his calculations, which was effectively his PhD dissertation. He co-authored it with Gamow, but his supervisor, ever the joker, decided to add Hans Bethe’s name to the paper. Although Bethe had contributed nothing to the work, it meant that the list of authors now read ‘Alpher, Bethe and Gamow’.7, 8 Alpher was dismayed. As a mere student, he needed maximum credit for his work in order to secure a permanent academic post, and Gamow had muddied the waters. Bethe was a big-name physicist who had worked on the Manhattan Project and had famously figured out a chain of nuclear reactions that might power the stars on a napkin in the dining car of a train travelling between Washington DC and New York; people were bound to assume he was the driving force behind the ‘cosmic nucleosynthesis’ calculations. Alpher’s worst fears were realised when, on turning up to defend his PhD thesis, he found himself confronted not only by Gamow and one or two of his colleagues but an audience of around 300 eager physicists.
However, nuclear reactions that built up elements were not the only consequence of a hot Big Bang. There was another one, and it was this that Alpher and Herman had been exploring, and which was the subject of the calculations Gamow had seen scrawled across the blackboard in Alpher’s office.
When the universe was about a minute old and its temperature about ten billion degrees, there would have been around ten billion photons per nucleon; they would have been utterly dominant and matter a very minor constituent of the universe.9 This prompted the question: Where did all those photons go? The answer, Gamow realised, was nowhere. Unlike the heat of the fireball of a nuclear explosion, which eventually dissipates into the surroundings, 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. Consequently, the photons of the ‘afterglow’ of the Big Bang must still be around us today. A quick back-of-the-envelope estimate revealed that the total energy of relic photons in any volume of space should be about the same as the total energy of starlight. Gamow concluded from this that they would be indistinguishable from starlight and that there was absolutely no chance of detecting them.
But Gamow, Alpher and Herman realised, was wrong. A crucial event in the history of the universe happened a few hundred thousand years after its birth, when the expanding fireball had cooled to about three thousand degrees. Nuclei and electrons were now flying around slowly enough that they could partner up and make the universe’s first ‘atoms’. This had a dramatic effect on the appearance of the universe. Whereas free electrons are very good at ‘scattering’, or redirecting, photons, electrons trapped in atoms are not. Consequently, before this ‘epoch of last scattering’, photons were forced to zigzag their way across space, like photons bouncing off water droplets in a fog. After this, the cosmic fog lifted and the universe became transparent. ‘Decoupled’ from matter, the Big Bang photons were able to fly unhindered in straight lines across space.
The relic photons of the Big Bang would no longer be the fiercely hot ones that began their journey 13.82 billion years ago. Greatly cooled by the expansion of the universe in the intervening aeons, they would today appear as short-wavelength radio waves, or ‘microwaves’. Furthermore, they would appear to be coming evenly from every direction in the sky.
This uniform glow of microwaves was the first of two unmistakable features that Alpher and Herman realised would make the ‘afterglow’ of the Big Bang distinguishable from starlight. The second feature was a little more technical.
In the fireball of the Big Bang, every time a photon bounced off a free electron, the pair exchanged energy. If an electron was moving fast, the photon gained energy; if it was moving slowly, it lost energy. The collisions were frequent, and the result of huge numbers of them was that the total available energy was shared out among the photons in a very special way; very few photons ended up with low energy and very few ended up with high energy, while a lot had energies between the extremes. Such a hump-shaped energy ‘spectrum’ is known as a ‘black body’ and is particularly simple because its shape depends on only one thing: the temperature.10 Despite the fact that the fireball of the Big Bang was expanding fast, photon–electron c
ollisions were a lot faster, so there was time for large numbers of them before the fireball expanded appreciably. Consequently, even as the temperature plummeted, the photons retained their characteristic black-body spectrum. This spectrum was the second property that Alpher and Herman realised would make the afterglow of the Big Bang fireball distinguishable from starlight. It was necessary only to know its temperature to know everything about it.
Alpher and Herman set about the task. They worked incredibly well together, having noticed from the moment they first met that they were of one mind. It was almost as if they had a telepathic connection. Eventually, they arrived at a temperature – the number Gamow saw on the blackboard that prompted him to order them to ‘write it up!’ It was a chilly five kelvin (–268 degrees Celsius). Today, the afterglow of the Big Bang would appear as microwaves coming from every direction in the sky, and its spectrum would be exactly the same as that from a body at five degrees above absolute zero.
*
Alpher and Herman did as Gamow had instructed and wrote up their prediction of the ‘cosmic background radiation’ as a short paper. If they were right, 99.9 per cent of all the photons in the universe are tied up in the afterglow of the Big Bang and a mere 0.1 per cent in the light from the stars and galaxies. It was a remarkable claim, but might they have made some mistake? They buried their doubts and sent their paper to the British science journal Nature.
The paper was published on 13 November 1948, and Alpher and Herman waited eagerly for the reaction of the scientific community,11 but it never came. Their prediction was met with a deafening silence. Not ones to give up without a fight, the two physicists mentioned their result in numerous talks over the following years. The talks were even attended by radio astronomers, whom they always made a point of buttonholing. ‘Can this relic radiation from the Big Bang be detected with radio telescopes?’ they asked. ‘No,’ came the unanimous (but incorrect) answer. And so no one embarked on a search for what, if it existed, was the single most striking feature of the universe: the afterglow of creation.
