WHAT do we know about our Universe? That it is 13.7 billion years old. That its geometry is flat and its texture smooth. That just over 4 per cent of its content is ordinary matter, while 23 per cent is mysterious dark matter and 73 per cent the even more mysterious dark energy.
These are just some of the cornerstones of what is now known as the standard model of cosmology. For years, critics accused cosmologists of raising great towers of theory on slight foundations. But a year ago that changed. Using a satellite called the Wilkinson Microwave Anisotropy Probe (WMAP) stationed millions of kilometres from Earth, scientists built up the most detailed map of the sky to date, showing the temperature variations in microwave radiation from the young universe. From this they extracted information about the cosmos that was far more precise and supportive of the standard model than anything seen before. Cosmology, it seemed, was all sewn up.
But is it? Effective though the standard model is at explaining key observations, it still has a soft underbelly of assumptions 鈥� not least that we know how to accurately interpret those famous microwave maps. One puzzle that my colleagues and I have uncovered is that nearby galaxy clusters appear to lie in regions of the sky where the temperature of the microwave radiation is low. Why should this be?
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One answer is that the gases within these galaxy clusters may be distorting the echoes from the big bang by squeezing energy out of microwave radiation as it passes through their region of space. This is not a far-fetched idea. Stirred up by gravity, gas atoms inside galaxy clusters will break up into protons and electrons, and the electric charges such particles carry would allow them to scatter and 鈥渃ool鈥� microwave radiation.
Nor is the basic observation new. The WMAP team itself has reported microwave cooling close to the centres of galaxy clusters, and taken this into account. But our study goes further: it detects microwave cooling so far out from cluster centres that it raises the possibility that even clouds of gas adrift in the space between galaxy clusters may distort the microwave background.
If more distant galaxy clusters produce the same cooling effects, what might this mean for the famous ripples in the microwave maps? These tiny temperature variations were imprinted into the microwave background when the early universe was 1/1000th its present size, and contain the key to the contents of the universe. For example, measuring the size of the ripples should allow physicists to calculate how much ordinary matter, cold dark matter and dark energy it contains. It was calculations of this sort that last year prompted the headlines suggesting that cosmology was all wrapped up. How reliable are those calculations?
The WMAP team has already reported evidence that radiation emitted by the first stars may have modified the microwave signal when the universe was 10 to 20 times smaller than it is today. Our results suggest that the microwave echo of the big bang may have had to come through even more 鈥渢raffic鈥� on its journey to Earth. At best this complicates the calculation; at worst it could undermine the evidence for dark energy and cold dark matter.
That wouldn鈥檛 dismay everyone. Dark energy is a particularly troublesome component of the standard model: it is hard to square with theories of quantum gravity and its observed density is so small that it may be quantum mechanically unstable.
Despite last year鈥檚 triumphant headlines, many theorists would in fact like an escape route from the standard model. It remains to be seen how far our observations will go in this direction, but if they are correct, the rumours that we are living in a 鈥渘ew era of precision cosmology鈥� may prove premature.