Measuring the spectrum of the microwave background was just one of the goals of COBE. Once that was achieved, all attention focused on the experiment which was surveying the sky for any sign of unevenness in the Big Bang radiation: the Holy Grail of cosmology, as it was often called.
Shortly after the satellite’s launch, the COBE team had taken a ‘quick look’ at a small portion of the sky. But that had revealed nothing but unbroken blandness.
By April 1991, COBE had surveyed the entire sky. It had confirmed that one half of it was marginally brighter than the other because of the motion of our Galaxy through space. But, once this effect was ignored, there were no other hot spots in the microwave background. The COBE team concluded that 380,000 years after the Big Bang, no region of the Universe was denser than any other by more than one part in 10,000.1
It was now nearly 30 years since Partridge and Wilkinson had begun the first serious search for variations in the Big Bang radiation, and no one had found the slightest trace – apart from the distortion due to the motion of our Galaxy. Where was the imprint of lumpiness in the early Universe? Where were the ‘seeds’ from which galaxies such as our Milky Way formed after the Big Bang?
The matter in the aftermath of creation was spread throughout space amazingly smoothly and yet the Universe we live in, replete with stars and galaxies, is remarkably uneven. The smoothness of the cosmic microwave background seemed to be contradicting the very fact that we are here at all.
There were mutterings that perhaps the Big Bang might be wrong. But very few astronomers would go that far. What was wrong was our understanding of galaxy formation. That was something tagged onto the Big Bang theory. It was an important addition, but an addition nonetheless. The Big Bang itself was pretty incontrovertible. After all, no one could deny that the Universe was expanding and that it was suffused with relic heat radiation. Both observations strongly indicated that in the distant past a titanic explosion had occurred in the Universe.
Nevertheless, the scientific community was beginning to get nervous. ‘If COBE gets to one part in a million and still sees the sky completely smooth, Big Bang theories will be in a lot of trouble,’ said Dave Wilkinson.
The instrument on board COBE that was searching for any variation in the background radiation was called the Differential Microwave Radiometer (DMR). It was a direct descendant of the one used by the Princeton and Berkeley groups in the late 1970s, when they had both discovered that the microwave background was a fraction of a degree hotter in the direction the Earth was flying through space.
There was nothing very complicated about the DMR. Apart from a bunch of electronics, all it really contained was a pair of microwave horns arranged in a sort of ‘V’ shape. The angle of the ‘V’ was 60 degrees, which meant the horns pointed at patches of the sky 60 degrees apart. Each patch was about seven degrees across, equivalent to 14 times the apparent diameter of the Moon.
The electronics would compare the signal picked up by each horn. In this way it would be possible to measure the tiny temperature difference between the two patches of sky. In fact, the DMR was so sensitive that it could detect a difference in temperature of only 0.00001 of a degree.
Measuring the temperature difference between two patches of the sky is a long way from making a map of how the temperature changes over the whole sky. But COBE was moving, and that made all the difference. Not only was it spinning on its axis so that the horns saw patches of sky around a circle, but the satellite was orbiting the Earth. The orbit changed gradually in such a way that in the course of a year the twin horns would measure the temperature differences not between two patches of sky, but between millions of patches.
The DMR completed its first map of the entire sky in December 1991, after it had been operating for a year. Each of its horns had made a staggering 70 million measurements. The COBE team began to look for fluctuations in brightness.
Each of the measurements was like one piece of an enormous jigsaw puzzle. It was only when they were all put together to make a map of the temperature of the whole sky that patterns started to emerge. This was the hard bit, and it could be done only with the aid of a powerful computer.
The first person to see something was Ned Wright. At the time, the computer at Goddard was still crunching methodically through the data. But Wright had got impatient and had devised a way to take a quick peek at the data. He made a rough map and took it to the rest of the team. It had hot blobs and cold blobs on it. Was it really a picture of the Universe as it was 380,000 years after the Big Bang?
At first, everyone was cautious. ‘There were a dozen things other than the background radiation that could have caused that signal,’ says Wilkinson.
The biggest worry was that the signal was not coming from the microwave background at all but from our Galaxy. The Galaxy is known to glow at microwave wavelengths, so the COBE team had to estimate how bright this glow was and subtract it. It was for this reason that they had included not one pair of microwave horns in the DMR but three.
The three pairs operated at different wavelengths: 3.3, 5.7 and 9.5 millimetres. There were two independent receivers at each wavelength, allowing the team to make six maps of the sky. COBE picked up confusing radiation from the Milky Way at all three of these wavelengths. But the Milky Way was brightest at the longest wavelength. The COBE team used these observations to subtract the Galaxy’s emission from the maps they made at the two shorter wavelengths.
When the effect of the Galaxy had been subtracted, the team did indeed have a map of the sky which contained bright blobs and cold blobs. They made a colour photograph which showed the whole sky with just what COBE had seen. Mauve patches showed bits of the sky that seemed to be hotter than the average, with blue showing colder patches.
Some have called this a ‘baby photo’ of the Universe. Unfortunately, it is not really a photograph of the Universe 13.7 billion years ago. The team knew that most of the blobs were not caused by the microwave background but by electrons jiggling about in their highly sensitive detectors.
After all this incredible effort, the team had a map whose features were partly caused by the sky and partly caused by their detectors, and it was impossible to distinguish the effect of one from the other.
But the COBE team did not despair. They had known all along that this would be the case. After all, they were attempting one of the most difficult measurements in science, one that had defied the best efforts of dozens of astronomers over the past quarter of a century.
The only way to be sure they were real hot spots and cold spots was to compare the maps at wavelengths of 3.3 and 5.7 millimetres. The team projected the maps onto the same screen so they could see them superimposed on each other. They then switched them on and off alternately. Disappointingly, most of the blobs changed. If these had been real structures in the Universe, they would have stayed in the same place. Since blobs caused by electrons in the detectors would be spread about entirely randomly, they would change. The team therefore concluded that what they were seeing was essentially caused by electrons in the detectors.
But the team did not give up here, either. They had never thought it would be easy. With the aid of a computer they carefully analysed the two maps. What they found was that a significant amount of the structure did appear the same in the maps at 3.3 and 5.7 millimetres. In fact, it was more than you would have expected by mere chance.
The COBE team had at last found evidence of lumps in the early Universe.
Unfortunately, they could not say precisely where they were. It was impossible to point to any single blob and say, ‘That is a real blob in the early Universe.’ Instead, a ‘statistical’ analysis let the team say how large the fluctuations – or ripples – are at different scales, even though they could not produce a map showing exactly where the bright spots were.
The bright spots were typically 30 millionths of a degree hotter than the average temperature. They occurred on all scales, from the smallest COBE could detect – 14 times the apparent diameter of the Moon – up to the largest – one quarter of the entire sky.
Ironically, the Soviet Relict I experiment had just missed them when in 1983 it orbited on board the Soviet satellite Prognoz-9. But even if it had found anything, it is arguable whether anyone would have believed its result. Relict I’s detectors operated at only a single wavelength of 8 millimetres and it picked up unwanted radiation from the Earth because it was badly shielded.
But there were still other possible confusing signals that COBE might have picked up. For nine months, the team considered every other possibility. But one by one they eliminated them. No individual signal contributed more than a tenth of the size of the signal spotted by Wright.
‘We argued all spring,’ says Wilkinson. But, by April 1992, the COBE team was as sure as it was ever going to be that something was lurking in the data. It was time to make an announcement.
A press release was drafted and bounced back and forth between the COBE team and the NASA press office. Finally, everyone was satisfied.
The team also decided to issue a photograph along with the press release. It was the one showing the whole sky, with mauve patches showing bits of the sky that seemed to be hotter than the average and blue showing colder patches.
The date and venue for the announcement were fixed. It was going to be at the American Physical Society on 24 April 1992. George Smoot was the frontman, though Ned Wright, Charles Bennett and Al Kogut would also be up on the podium explaining aspects of the experiment.
The lecture hall was unusually packed that day. Already a lot of excitement had been whipped up. The scientists themselves were tense with anticipation. There had been rumours for at least six months that COBE had found something, and to some it had seemed that the Big Bang theory was in trouble.
But there was another reason why the hall was unusually packed. Unknown to the COBE team, the Lawrence Berkeley Laboratory, which is managed by the University of California at Berkeley, had put out its own press release in advance of NASA. It had gone to privileged newspapers, which had been fired with excitement about the story.
George Smoot introduced the work in a 20-minute talk. He presented the result and tried to give the people some idea of what it all meant. Asked by someone in the audience just how important it was, he said: ‘Well, if you are a religious person, it’s like seeing the face of God.’
Nobody was prepared for what happened next as the story raced around the world at the speed of light.
At The Guardian, in England, the paper’s science correspondent, Tim Radford, watched as his fellow journalists were utterly transformed. ‘Everyone in the office who knew even a little about science was rushing about like a mad thing, saying this was the greatest story ever,’ he says.
At first, Radford wasn’t convinced. ‘But when I got to the end of writing the story, even I was beginning to get excited.’
The story reached the front page of virtually every major newspaper in the world. You could not turn on a television without hearing that scientists had discovered the secret of the Universe.
This was one story that the journalists could not be accused of over-hyping; it was over-hyped by the scientific community itself. The fires were fuelled by famous scientists, and it was impossible to get more famous than the British theoretical physicist Stephen Hawking. When he said of the COBE finding, ‘It’s the greatest discovery of the century – if not of all time,’ there was no stopping the story.
The Independent newspaper in the UK ran the story across its front page with the banner headline: ‘How the Universe began.’ Exploding out of the page was a graphic showing the entire history of the Universe, from the moment of creation to the present day, with the missing step – the birth of galaxies – now filled in by COBE.
As the British astrophysicist George Efstathiou commented, only major disasters and the marriage of Princess Diana have generated comparable media coverage.
‘They have found the Holy Grail of cosmology,’ claimed Michael Turner of the University of Chicago.
‘It is a discovery of equal importance to the discovery that the Universe is expanding, or the original discovery of the background radiation,’ said Hawking in The Daily Mail. ‘It will probably earn those who made it the Nobel Prize.’
The COBE team was taken aback. ‘I was flabbergasted by the media coverage,’ says Wilkinson. ‘We had expected to get some media interest – but nothing like this.’
Robert Wilson was also amazed. ‘There was more publicity than when Arno and I actually discovered the radiation,’ he says.
1. Actually, the 380,000 years used throughout this book is the modern estimate. At the time of COBE, the epoch of last scattering was most often cited as happening about 300,000 years after the beginning of the Universe.