Cosmology
Things fall apart
Feb 5th 2004
From The Economist print edition
SourceWhat if the dark energy and dark matter essential to modern explanations of the universe don't really exist?
IT WAS beautiful, complex and wrong. In 150AD, Ptolemy of Alexandria published his theory of epicycles—the idea that the moon, the sun and the planets moved in circles which were moving in circles which were moving in circles around the Earth. This theory explained the motion of celestial objects to an astonishing degree of precision. It was, however, what computer programmers call a kludge: a dirty, inelegant solution. Some 1,500 years later, Johannes Kepler, a German astronomer, replaced the whole complex edifice with three simple laws.
Some people think modern astronomy is based on a kludge similar to Ptolemy's. At the moment, the received wisdom is that the obvious stuff in the universe—stars, planets, gas clouds and so on—is actually only 4% of its total content. About another quarter is so-called cold, dark matter, which is made of different particles from the familiar sort of matter, and can interact with the latter only via gravity. The remaining 70% is even stranger. It is known as dark energy, and acts to push the universe apart. However, the existence of cold, dark matter and dark energy has to be inferred from their effects on the visible, familiar stuff. If something else is actually causing those effects, the whole theoretical edifice would come crashing down.
According to a paper just published in the Monthly Notices of the Royal Astronomical Society by Tom Shanks and his colleagues at the University of Durham, in England, that might be about to happen. Many of the inferences about dark matter and dark energy come from detailed observations of the cosmic microwave background (CMB). This is radiation that pervades space, and is the earliest remnant of the Big Bang which is thought to have started it all. Small irregularities in the CMB have been used to deduce what the early universe looked like, and thus how much cold, dark matter and dark energy there is around.
Dr Shanks thinks these irregularities may have been misinterpreted. He and his colleagues have been analysing data on the CMB that were collected by WMAP, a satellite launched in 2001 by NASA, America's space agency. They have compared these data with those from telescopic surveys of galaxy clusters, and have found correlations between the two which, they say, indicate that the clusters are adding to the energy of the CMB by a process called inverse Compton scattering, in which hot gas boosts the energy of the microwaves. That, they say, might be enough to explain the irregularities without resorting to ghostly dark matter and energy.
Dr Shanks is not the only person questioning the status quo. In a pair of papers published in a December issue of Astronomy and Astrophysics, Sebastien Vauclair of the Astrophysics Laboratory of the Midi-Pyrénées, in Toulouse, and his colleagues also report the use of galaxy clusters to question the existence of dark energy. But their method uses the clusters in a completely different way from Dr Shanks, and thus opens a second flank against the conventional wisdom.
Cosmological theory says that the relationship between the mass of a galaxy cluster and its age is a test of the value of the “density parameter” of the universe. The density parameter is, in turn, a measure of just how much normal matter, dark matter and dark energy there is. But because the mass of a cluster is difficult to measure directly, astronomers have to infer it from computer models which tell them how the temperature of the gas in a cluster depends on that cluster's mass.
Even measuring the temperature of a cluster is difficult, though. What is easy to measure is its luminosity. And that should be enough, since luminosity and temperature are related. All you need to know are the details of the relationship, and by measuring luminosity you can backtrack to temperature and then to mass.
That has been done for nearby clusters, but not for distant ones which, because of the time light has taken to travel from them to Earth, provide a snapshot of earlier times. So Dr Vauclair and his colleagues used XMM-Newton, a European X-ray-observation satellite that was launched in 1999, to measure the X-ray luminosities and the temperatures of eight distant clusters of galaxies. They then compared the results with those from closer (and therefore apparently older) clusters.
The upshot was that the relationship between mass and age did not match the predictions of conventional theory. It did, however, match an alternative model with a much higher density of “ordinary” matter in it.
That does not mean conventional theory is yet dead. The Newton observations are at the limits of accuracy, so a mistake could have crept in. Or it could be that astronomers have misunderstood how galaxy clusters evolve. Changing that understanding would be uncomfortable, but not nearly as uncomfortable as throwing out cold, dark matter and dark energy.
On the other hand, a universe that requires three completely different sorts of stuff to explain its essence does have a whiff of epicycles about it. As Albert Einstein supposedly said, “Physics should be made as simple as possible, but not simpler.” Put Dr Shanks's and Dr Vauclair's observations together, and one cannot help but wonder whether Ptolemy might soon have some company in the annals of convoluted, discarded theories.
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