This background radiation corresponds to a temperature of less than 3K above absolute zero - very cold indeed, but exactly what one would expect to find from a fireball that has been expanding - and cooling - for 15 billion years.
This sea of radiation appears, at first sight, to be absolutely uniform - the same no matter in which direction we look - but therein lies a snag. If it is perfectly uniform, how did the present stars and galaxies, which are far from uniform, arise?
The most natural idea is that these lumps of matter started as slight fluctuations in the density of matter and radiation in the primordial gas. And in that case, the background radiation should still show some faint traces of that past irregularity.
This is what radio astronomers such as Rod Davies of Manchester University and John Beckman of Tenerife observatory have been searching for over the past dozen years.
As Professor Beckman says: 'In the early days there were predictions that these fluctuations would set in at a level of one part in 1,000, and people started to look, but they were not found at this level. So the theorists went back to their books, and said, 'Oh yes, how could we have been so silly; we meant one part in 10,000.' So the astronomers beavered away - and it was getting quite difficult to measure such small differences, at levels of one part in 10,000 - and they still didn't find anything. So then the theorists began to say, 'Well, what we really meant to say was one part in 100,000.' So the observers really had to push things to the limit.'
Remarkably, two groups have now succeeded in this. Two years ago an American team published the results from its satellite, Cobe, the Cosmic Background Explorer. The variations from perfect uniformity that it found were small - about one part in 100,000 - and difficult to measure, but statistically there seemed little doubt that Cobe had discovered the 'ripples in space'.
But Cobe was only able to show that there were fluctuations across the sky, slightly greater than the random noise generated by the instrument itself. What was needed was a more sensitive experiment, and this is what professors Davies and Beckman and their colleagues set out to provide almost 10 years ago.
They used no multi-million-pound satellite but a simple yet powerful ground-based experiment set up at the 8,000ft-high Mount Teide observatory in Tenerife. Here a team under Professor Beckman has been running the equipment 24 hours a day.
'The technique is to look at two patches in the sky using two antenna horns side by side. These look at the sky reflected in a flat metal mirror, and the electronics subtracts the signal of one horn from the other, very rapidly, to minimise the effect of atmospheric fluctuation. The idea is to measure the temperature difference between these two patches of sky.
'But also the reflector wobbles every 10 seconds so that the patches switch places - effectively we look at three patches of sky and it's the difference between the centre patch and the two outside ones that we measure.'
Every 24 hours the equipment sweeps out a circular strip around the sky; and by looking for months on end at the same strip of sky the observers have been able to increase the sensitivity of their measurements several times above that of Cobe. Every few months they realign their instruments to search another strip of sky.
In this way the temperatures of different sky areas can be fairly compared, any local fluctuations due to weather or interference being cancelled out. The idea is that if there are actual 'hot spots' in the sky, some areas will give consistently higher readings than others in studies carried out over a long time. And this is exactly what the investigators have found, as they report in Nature.
For the first time, the Tenerife team has been able to start drawing a map of 'hot' and 'cool' spots about five degrees across which can be assigned to specific directions in the sky.
The Tenerife team's work is strong confirmation of the Cobe results, but it also takes them a vital step further. Professor Beckman says: 'The Cobe result is very, very good - it deserved the attention it received. They had the advantage of being above the atmosphere, but the sensitivity is less than in our experiment.
'They looked over the whole sky, and, by averaging, they could say, correctly, that they had found a ripple and this ripple didn't change from one frequency to another - so that enabled them to say it was true background fluctuation.
'What they couldn't say is exactly where the ripples are. Now what we've been able to do, looking at a smaller piece of sky, is actually detect a certain number of individual ripples and measure their sizes.'
The Tenerife work is certain to cause much excitement among cosmologists, who are still struggling to understand the mechanism of the very early expansion of the Universe, and how it led to our present situation. These cosmic-
background measurements are looking at primordial fluctuations - traces of what happened in the first billionth of a billionth of a billionth of a second of time.
And, as the team says, there aren't many ways of doing that.
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