The discovery of the cosmic microwave background provides a wonderful example of the way science is really done. Though the writers of textbooks – and very often scientists themselves – would like us to believe that science progresses in a series of logical steps, taken coolly and calmly, one after another, this is patently not so. Far from being orderly, the progress of science is more like that of a drunkard staggering two steps backward for every three in the forward direction, and making the odd sideways lurch just for good measure. Consider again the story of how the fireball radiation came to light …
In the late 1940s, George Gamow and his co-workers guessed that if the Universe had begun in a Big Bang, the early Universe would have been filled with intense radiation, and the pale afterglow should still be around 13.7 billion years later. (They were right, but for the wrong reason.) But though they investigated the possibility of looking for the fireball radiation, they were told by radio astronomers that it was undetectable. Everyone forgot about the relic radiation because Gamow’s theory was discredited.
But a decade and a half later Bob Dicke rediscovered the fireball radiation – for an entirely different reason. He decided that a search for the pale afterglow of creation was feasible and gave two young radio astronomers the job of looking for it. But on the eve of their attempt – and here the story descends into farce – another pair of astronomers working barely an hour’s drive away stumbled on the cosmic background radiation entirely by accident, after first thinking they might be seeing the faint radio-glow of pigeon droppings.
It’s a strange tale – and it gets stranger. Consider the baffling question of why the cosmic microwave background was not discovered earlier. After all, it had been predicted a full 17 years before that fateful phone call from Arno Penzias at Bell Labs to Bob Dicke at Princeton.
The question has long puzzled Dave Wilkinson. ‘I’ve often wondered why nobody in that 17 years put two and two together,’ he says. ‘Not only was the microwave radiometer a standard instrument in radio astronomy, but Gamow’s group had publicised this idea that there ought to be microwave radiation in the Universe with a temperature of just a few degrees. Gamow had even written popular articles about it in Scientific American. To go and look for the radiation, all you needed were two things – a good microwave horn and a cold load.’
Robert Wilson also thinks it was amazing that no one carried out a search for the relic radiation earlier. ‘At any time after Alpher and Herman made their prediction, it could have been checked,’ he says. ‘If Bob Dicke had decided to look for the fireball radiation, he could have done it with World War Two equipment. In fact, a radio receiver like Wilkinson’s could probably have been built not too long after the war.
‘Of course, Alpher and Herman did go and talk to some radio astronomers, who sort of said, no, the measurement is impossible. But I’m sure that if Dicke had thought of doing it, he would have done it and succeeded.’
The reason Dicke did not do it was because it simply never occurred to him, something that he kicks himself for today. ‘On a number of occasions during and after the war, I could have used my microwave receiver to do some interesting astronomy,’ says Dicke. ‘But I missed them all. I was kind of stupid. You see, at the time I didn’t quite realise what astronomy was. I’d only ever done one course on the subject.’
But not everyone overlooked the prediction of Alpher and Herman. In the Soviet Union, a couple of alert astronomers, Andrei Doroshkevich and Igor Novikov, very nearly put two and two together. ‘They knew about Alpher and Herman’s prediction of the fireball radiation’, says Wilkinson, ‘and they had also identified the antenna at Bell Labs as the one antenna in the world that was capable of verifying it.’
In 1964, Doroshkevich and Novikov, like their counterparts at Princeton, were poring over Ed Ohm’s papers in the Bell System Technical Journal (it seems Russians read the American scientific literature more thoroughly than the Americans). And they had focused their attention on Ohm’s 1961 paper – the one that contained the first reference to the mysterious radio hiss.
But, having put nearly all the jigsaw pieces together, Doroshkevich and Novikov made a heartbreaking mistake just as they were about to complete the puzzle: they misread Ohm’s paper.
Ohm stated that he had measured the ‘sky’ temperature to be a little over three degrees. By this he meant that when he pointed the 20-foot antenna at the sky and accounted for every source of unwanted radio waves, he was still left with an unexplained residue of three degrees. But the Russian astronomers thought that, in calculating his sky temperature, Ohm had not removed the temperature of the atmosphere. By a coincidence this was also about three degrees, so when the two astronomers subtracted this from the sky temperature, they ended up with essentially nothing. They therefore concluded that there could be no appreciable background glow in the Universe.
‘When we looked at the very same paper, we thought there was a very good chance that fireball radiation was in there,’ says Wilkinson. ‘But when Doroshkevich and Novikov looked at it, they came to completely the opposite conclusion.’
The two Russian astronomers relayed their conclusion to their senior colleague, Yakov Boris Zel’dovich, one of the world’s most eminent cosmologists. He took it as proof that the hot Big Bang was wrong, and in 1965 published a paper in which he said precisely this.
Ironically, another prominent cosmologist, Fred Hoyle, had the previous year concluded that the Universe must certainly have gone through a hot dense phase at some time in the distant past. It was particularly significant that Hoyle should have come to this conclusion because it was his theory that the elements were cooked inside stars which had been responsible for sinking Gamow’s idea that they were made in a hot Big Bang.
But, by the early 1960s, it had become abundantly clear to Hoyle that although his theory was enormously successful in explaining the origin of the huge majority of the elements, there was far too much helium around. Since the beginning of the Universe there had not been enough time for stars to have made it all.
Hoyle and a colleague, Roger Tayler, concluded that the helium must have been made in either a Big Bang or else a lot of ‘little bangs’ spread all over the Universe. Nature was not simple; the elements had not been made in a single place. They had been cooked inside stars and also during a hot dense phase which the Universe had gone through. An obvious consequence of such a hot dense phase, Hoyle and Tayler realised, would be fireball radiation, and its cooled remnant should still be around today.1
So now there were three independent teams in the world that had realised there ought to be a universal microwave background permeating the Universe.
But, in 1964, when Hoyle and Tayler submitted for publication a paper on the origin of the Universe’s helium, they unaccountably left out the prediction of the cosmic background radiation – despite the fact that they had included it in an early draft. The story of the cosmic background radiation has its missed opportunities on the theoretical side as well as on the observational side.
One person who has thought long and hard about why the discovery of the cosmic background radiation – one of the most important discoveries of the twentieth century – had to be made by accident and why there was no earlier systematic search for it is the Nobel Prize-winning physicist Steven Weinberg. In his excellent popular account of the Big Bang, The First Three Minutes, he gives three main reasons why. First, says Weinberg, the prediction of the fireball radiation came out of a theory which was later discredited. By the 1950s, it was clear that most elements could not have been made in the Big Bang, as George Gamow had hoped. Secondly, says Weinberg, the theorists who first predicted the Big Bang radiation were told by radio astronomers that it was quite undetectable. But the most important reason why the Big Bang theory did not lead to a search for the fireball radiation, says Weinberg, was that before 1965 it was extraordinarily difficult for any physicist really to take seriously any theory of the early Universe. It was a failure of imagination again. The temperature and density of matter in the first few minutes of the Universe would be so extreme and far removed from everyday experience that it was hard for anyone to really believe that such a state had ever existed. As Weinberg says, the mistake of physicists is not to take theories too seriously but not to take them seriously enough.
There is a further bizarre twist to this tale. It turns out that not only had the Big Bang radiation been predicted long before that fateful phone call from Penzias to Dicke, but it had actually been observed as well. In fact, evidence for the cosmic background radiation had been around for more than 25 years. It had even been published in the scientific literature, but nobody had taken any notice.
In 1938, a full decade before Alpher and Herman made their prediction of the fireball radiation, Walter Adams, director of the Mount Wilson Observatory in southern California, turned a telescope on a nearby star in the constellation of Ophiuchus, the serpent holder. He immediately noticed an unusual dip in the star’s spectrum. The dip was just what would be expected if some of the light was being absorbed by molecules of a gas called cyanogen.
Now molecules are fragile things. They are easily broken apart by extreme heat, so they tend not to be found close to stars. Adams therefore concluded that the cyanogen molecules he was seeing were in an invisible cloud of interstellar gas suspended in space somewhere between the star and the Earth.
Such clouds of gas are scattered all over the Galaxy – they are the places where stars like the Sun are born – so finding one in front of this nearby star was not much of a surprise. But the pattern of the cyanogen absorption was. The only way Adams could make any sense of the pattern was if most of the cyanogen molecules – which are like little atomic dumb-bells – were spinning, tumbling end over end as they drifted through space.
But this was impossible. Interstellar space is mind-numbingly cold, within a whisker of absolute zero, the temperature at which all movement slows to a standstill.
Something had to be driving the tiny cyanogen molecules, causing them to rotate. Andrew McKellar, an astronomer at the Dominion Observatory in Canada, calculated what that something had to be. It was radiation at a temperature of about 2.3 degrees above absolute zero and at a wavelength of 2.64 millimetres. But what that radiation was and where it was coming from he had no idea.
Several other stars were found which also revealed cyanogen molecules rotating faster than they should. So the radiation that was buffeting the tiny cyanogen molecules had to be widespread in the Galaxy – if not universal.
No other astronomers considered the anomaly worth losing sleep over, so, like so many discoveries made before their time, it was forgotten. Until 1965, that is. Then, several people, including the Russian astronomer Iosef Schlovski, suddenly remembered the work of Adams and McKellar. They pointed out that the tiny molecules that had no right to be spinning in the dead cold of space were spinning because they were being buffeted by the afterglow of the Big Bang. They were made-to-order interstellar thermometers, sitting in space and quietly taking the temperature of the Universe.
At last, the mystery of the cyanogen molecules was solved.
In June 1965, Jim Peebles gave a public lecture on the fireball radiation at a meeting of the American Physical Society in New York. It was destined to bring home to him for the first time just what a risk the Princeton team had taken in claiming from just one observation that the radiation from the Big Bang had been found.
To illustrate his talk, Peebles had prepared a lantern slide showing Penzias and Wilson’s background measurement at 7.35 centimetres – a single point. Through it he had drawn the distinctive humped curve of a black body, showing how the Big Bang radiation varied with wavelength. When Peebles projected the slide onto the screen, he was startled by the audience’s reaction. ‘People began to giggle,’ he says.
Frowning, Peebles looked up at the graph on the screen. And, for the first time, he saw it through the eyes of other people, realising with a sudden shock just how ridiculous it must seem. With total confidence, he had drawn a complicated curve through a single data point. He had joined the dots when there was only one dot to join. Even schoolchildren knew that you could draw any curve whatsoever through a single point and all would be equally correct.
‘Immediately people saw the graph, they saw how ridiculous it was,’ says Peebles. ‘They were thinking, “This guy is really sticking his neck out.”
‘I was aware it was pretty speculative to think that this detection was the background radiation. But what I hadn’t reflected on was just how dramatic a prediction we had made and how much room there was to be wrong.’
But though he and Wilkinson and the rest had well and truly stuck their necks out, Peebles did not think it had required any particular courage to do so. ‘If we had been wrong, it would have rolled off us like water off a duck’s back,’ he says.
‘You do science by making bold guesses. They’re not bold in the sense that you’re putting your physical neck on the line. So as long as you don’t make too many bold guesses that turn out to be wrong, you’re not even compromising your reputation particularly.’
The jury was still out on whether Peebles was right or wrong. A decision would be made when more data points had been collected. As the affair of the giggling audience had emphasised, a lot more proof was needed before anyone could be that certain the signal Penzias and Wilson had picked up had really come from the Big Bang.
The Big Bang idea passed its first major test in December 1965, when Wilkinson and Roll finally got their rooftop antenna working, nearly a year after they had begun building it. They successfully measured the temperature of the sky at a wavelength of 3.2 centimetres and found that it was around three degrees above absolute zero, in perfect agreement with what had been found by Penzias and Wilson.
‘Getting that result was a great relief for us, because our neck was stuck pretty far out,’ says Wilkinson. ‘Our paper in Astrophysical Journal Letters got a lot of ridicule. Most people thought our interpretation was pretty wild. I mean, we only had one point!’
But relief was not the only emotion Wilkinson felt when he and Roll finally made that measurement. ‘I have to admit it was a bit of an anticlimax,’ he says.
Bell Labs had effectively stolen Princeton’s thunder. Back in the spring, Peebles had felt no real disappointment that Roll and Wilkinson had been pipped at the post. But it was different now, seeing that the measurement the Princeton team had made did indeed agree with the Big Bang interpretation and that Princeton could easily have been first. ‘That’s when I started to think it was a great shame that Dave and Peter were scooped,’ says Peebles.
So now there were two observations of the background radiation at two wavelengths. It was not much perhaps, but it was a start. Both were consistent with the spectrum of the radiation being a black body with a single temperature, precisely what would be expected if the radiation came from the Big Bang.
But there was a second test of whether the background radiation really came from the Big Bang. As well as having a black body spectrum, it should be equally bright in all directions in the sky. ‘We knew that this stuff had better be smoothly distributed around us,’ says Peebles.
As soon as the Princeton group had made the second detection of the cosmic background radiation, Dicke thought it would be a good idea to modify the apparatus and try to measure how smooth the radiation was over the sky. Wilkinson was now joined by a new partner.
In the summer of 1965, while he and Peter Roll had been fiddling with their rooftop experiment, Dicke had hired a young physicist called Bruce Partridge. When Partridge joined the ‘gravity group’ from Oxford, his first job was to choose an experiment to work on. Dicke showed him the two which were under way at the time.
The first was an experiment to measure the oblateness of the Sun. This was another of Dicke’s pet ideas. If the Sun were found to be oblate – slightly squashed in shape – then Dicke’s own theory of gravity would be just as effective as Einstein’s at explaining the orbit of Mercury.
But when Partridge looked at the solar oblateness experiment, he was dismayed. ‘The whole thing looked horrendously complicated,’ he says. ‘There were racks of electronics everywhere.’ He asked weakly if he could see the second experiment. Dicke took him to see Wilkinson and Roll’s microwave background experiment, and Partridge immediately breathed a sigh of relief. ‘It looked so much simpler,’ he says. ‘So that’s the experiment I chose.’
So it was that Bruce Partridge came to be working with Dave Wilkinson on the experiment to measure the smoothness of the microwave background.
In practice the measurement would involve pointing an antenna at different parts of the sky and comparing the temperature it registered. There were certain advantages in comparing the background radiation with itself. For a start, you did not have to worry so much about sources of unwanted radio waves since they would often be the same when the antenna was looking in two directions. When one temperature was subtracted from the other to see what was left, these signals would simply cancel out.
Penzias and Wilson had already shown that the background radiation varied in temperature by less than 10 per cent as their antenna swung around the sky. ‘We realised that we could do a hell of a lot better than that,’ says Partridge.
‘We didn’t even take our antenna off the roof,’ says Wilkinson. ‘Instead of having it pointing straight up, we simply tipped it over to 45 degrees.’
The idea was to point the trumpet-shaped horn at the ‘celestial equator’. This was an imaginary circle in the sky, essentially where the Earth’s equator would be if it were extended out to meet the sky. So, every 24 hours, the rotation of the Earth would swing the antenna through a complete circle. If the background radiation varied around this circle, then the temperature the antenna registered should vary slowly over the course of 24 hours.
It sounded straightforward, but in practice there were complications. Other mundane things could also make the temperature recorded by Wilkinson and Partridge’s antenna vary every 24 hours. For instance, during the day the Sun would heat the horn, causing it to produce more unwanted radio waves than at night. Somehow they would have to distinguish this temperature variation from a real effect in the cosmic background radiation.
In the earlier experiment, Wilkinson and Roll had managed to remove such unwanted effects by occasionally making the antenna look at an artificial source of radio waves kept at constant temperature – a cold load. But this was deemed too cumbersome for the smoothness experiment. Instead, Wilkinson and Partridge periodically slid a vertical metal mirror in front of the horn so that instead of looking at the celestial equator, it looked at a spot in the sky known as the north celestial pole.
Essentially, this is where the Earth’s North Pole would be if it were extended upwards to meet the sky. The celestial pole never moves: if you had the patience to watch the sky all night, you would see all the stars slowly circle round it.
In their smoothness experiment, Wilkinson and Partridge constantly subtracted the temperature of the sky along the celestial equator from the temperature of the sky at the celestial pole. In both cases, the day–night heating would cause the signal to vary in the same way, so this unwanted effect cancelled out. Also, in both cases, the horn would be looking through the same amount of atmosphere, so the unwanted signal from the atmosphere cancelled out.
But the most important thing about looking in the direction of the celestial pole was that the temperature of the background was constant there. In effect, the patch of the sky at the north celestial pole was a natural cold load. By constantly looking back and forth between the equator and the North Pole, Wilkinson and Partridge were able to map the true temperature variations in the background radiation around the celestial equator.
By 1966, Wilkinson and Partridge had found that the background radiation varied in temperature by less than about 0.1 per cent around the celestial equator. ‘We improved Penzias and Wilson’s result by nearly a hundred times,’ says Partridge.
The Big Bang radiation had passed its second major test with flying colours. It was coming equally from all directions, at least as far as the technology of 1966 could tell.
After the discovery of the cosmic background radiation, people began to take the early Universe seriously. Gamow had shown how a knowledge of nuclear physics could help us understand what was going on in the Universe a few minutes after the Big Bang, when the temperature was billions of degrees. But what about even earlier times, when the temperature was even higher, the conditions even more extreme? Insight into these remote times would come from a curious marriage between the science of the very small and that of the very large – between particle physics and cosmology.
Particle physicists want to find out what makes up all of matter. At one time, they thought it was atoms, but then they found that atoms are made of smaller things – protons, neutrons and electrons. Later, to their dismay, they found that even protons and neutrons are made of smaller things – quarks. Nobody has isolated a quark and nobody is sure whether these particles really are the end of the line. Perhaps the particles of matter are like Russian dolls, and we will constantly find new ones as we probe deeper and deeper beneath the surface reality.
The early Universe and particle physics are intimately connected because, at the high temperatures that existed in the Big Bang, particles flew about so fast that when they struck each other they disintegrated into their constituents. Particle physicists mimic this inside giant particle accelerators, whirling the microscopic components of matter at great speed and slamming them into each other. For a fleeting instant, they can create conditions that have not existed in the Universe since the first split second after the Big Bang.
Gamow knew about nuclear physics – the physics at temperatures of millions and billions of degrees – and applied that knowledge to the early Universe. Today’s physicists have learnt about the physics at temperatures of trillions of degrees and hotter. Whereas Gamow probed the era a few minutes after the Big Bang, today’s physicists confidently predict the conditions in the first thousandths of a second. In fact, they have gone much further back, although with less confidence.
It may seem audacious for us sitting here on Earth to claim we know what the Universe was like at such a remote time. After all, the Big Bang theory is largely based on three pieces of observational evidence: the expansion of the Universe, the existence of the fireball radiation and the abundance of helium. But we can say so much because the early Universe was so simple. It gets hotter and hotter in a predictable way the further back we probe, but at any time we only have to know the temperature and we have completely described the entire Universe. It remains only to put in the physics of particles that would have existed at that temperature and we know everything.
The problem, of course, is that sooner or later our knowledge of particle physics gets shaky. We simply cannot achieve comparable temperatures on Earth to test it. We are in unknown country. But even here there is now a guide. For the marriage between particle physics and cosmology has shown how they are interdependent, how many of the features of the Universe must have been determined by the physics of the very small in the earliest moments after the Big Bang. Whatever physics we use, it cannot have consequences in the greater Universe that conflict with what astronomers observe all around us.
This is the legacy of Gamow. For we now see that the ultimate questions of where the Universe came from can be answered only by particle physics.
George Gamow died in 1968, so he did not live to see his ideas vindicated. They received the ultimate seal of approval in 1978, when Arno Penzias and Robert Wilson were awarded the Nobel Prize for their discovery of the cosmic background radiation.
Wilson got his first hint about the prize in early 1978. ‘Some guy published a prediction of future Nobel Prizes – I think it was in the magazine Omni – and he listed us,’ says Wilson. ‘But he’d been wrong on a bunch of things, so Arno and I didn’t take it seriously.’ In the summer of 1978, there was another hint, this time from Jerry Rickson, an Irishman who had worked for a while at Bell Labs before returning to Europe. While visiting Sweden, Rickson had been buttonholed by one of the country’s leading radio astronomers. ‘Jerry got asked some very detailed questions about Arno and me and our relationship,’ says Wilson. ‘Who did what – that sort of thing.’
Later, a Swiss colleague of Wilson’s called Martin Schneider dropped an even more blatant hint. Schneider was overdue handing Wilson a progress report on an experiment, so, when the two ran into each other in a corridor at Bell Labs, Wilson mentioned the report, asking whether he could have it on his desk the next day. To Wilson’s amazement, Schneider said no. ‘You won’t want it tomorrow’, he said, gleefully, ‘because they’re going to announce your Nobel Prize!’
‘I must admit I didn’t take that too seriously,’ says Wilson. But the next day he was woken by the phone jangling at 7 a.m. It was another of Wilson’s colleagues at Bell Labs. He had heard a news item on WCBS and wanted to know was it true what people were saying, that he and Arno Penzias had won the Nobel Prize?
Wilson could not say for sure. But finally he received a telegram saying that the Swedish Royal Academy of Sciences had awarded the 1978 Nobel Prize for Physics to Penzias and Wilson for discovering the three-degree cosmic background radiation. ‘It wasn’t a complete surprise after all the hints,’ says Wilson.
‘I still don’t know where Schneider got his information. But he’s the sort who would dig around and investigate things. It was nice there was no hint until the last year. I think it was lucky not to have people saying year after year, “You’re going to get the Nobel Prize.”’
The Nobel Committee had decided to award its prize to the discoverers of the microwave background rather than to those who had predicted its existence. In this way, they neatly avoided the sticky problem of deciding who in fact deserved the credit.
‘I was deeply disappointed that Bob Dicke didn’t get part of the prize,’ says Peebles. ‘I think a good solution would have been Penzias, Wilson and Dicke.’ Gamow had died in 1968, and one of the rules is that Nobel Prizes are never given posthumously. ‘I suppose that with all these awards the Nobel people have to make some sort of semi-arbitrary decision. And that’s what they did in the case of the microwave background.’
Another consideration of the Committee may have been that theories are more quavery than experimental results. Certainly, the two physicists who won the prize for discovering high-temperature superconductors got it within a couple of years, whereas for the theory of relativity, one of the towering achievements of twentieth-century science, Einstein never got the prize.
Wilkinson is absolutely clear on why Penzias and Wilson got the award. ‘They discovered something fundamental and important about the Universe,’ he says. ‘Also, they were first-rate experimenters.’
The pair underlined this in the late 1960s, when they made another major astronomical discovery. ‘They discovered large amounts of the molecule carbon monoxide floating out in space,’ says Wilkinson. After molecular hydrogen, carbon monoxide turned out to be the most common molecule in the Universe.
1. Hoyle and Tayler were well aware of Alpher and Herman’s prediction of the afterglow of creation. Their helium-abundance argument had simply led them to the same conclusion.