Sunday, October 23, 2005

Could dark matter just simply be figmentary?

Much ado about nothing - Cosmology

779 words
22 October 2005
The Economist
(c) The Economist Newspaper Limited, London 2005. All rights reserved.

Dark matter is no longer needed. Perhaps.

A controversial new calculation suggests dark matter might not exist

THE adage “what you see is what you get” could be thought to ring true for a group of people who dedicate their lives to collecting tiny flickers of light from very distant objects. But astronomers and cosmologists, who do exactly that, have long held that the universe is pervaded by far more than that which can be seen. Since the 1930s, they have postulated the existence of “dark matter”, an ethereal and, as yet, undetected form of matter.

Physicists claim to need dark matter to explain why the stars in the outermost reaches of rotating galaxies are moving at such great speeds. If these galaxies consisted only of the stars that can been seen, their gravity would be insufficient to hold on to the outermost stars. The individual stars would simply fly out of the galaxy, like a doll thrown from a rapidly spinning merry-go-round. Thus, the galaxy must contain some mysterious matter that makes it massive enough to keep hold of these stars.

Such matter would pervade the entire universe. According to standard cosmological models, 20-30% of the mass of the universe must consist of the stuff. Such matter does not absorb or give off light, and is extremely difficult to detect. None of the large underground experiments that have been designed to catch a glimpse of it has yet seen anything.

Now, in a controversial paper that has recently appeared on arXiv, an online collection of physics papers, Fred Cooperstock and Steven Tieu of the University of Victoria in Canada claim that one of the key pieces of evidence for the existence of dark matter is not really there.

Instead of using Newton's theory of gravity to examine why fast-moving stars remain within their galaxies, the pair applied general relativity, Einstein's theory of gravity, to the problem. Einstein's theory holds that matter and energy distort space, and massive bodies such as planets and stars travel in this warped space. The theory supersedes Newton's law in situations where gravity is very strong, such as next to a black hole, or where energies and speeds are very high. But neither applies to the case of stars orbiting the outer reaches of a galaxy.

The key, Dr Cooperstock claims, is a “non-linearity” that arises in Einstein's theory. If one body is much larger than its neighbours, such as the sun compared with the planets of the solar system, its effect dominates the others. But when all the bodies are very massive, such as stars in a galaxy, this non-linearity becomes important. Each star strongly influences—and is influenced by—the rest.

To their surprise, when he and Mr Tieu did the calculation with general relativity, they found that they were able to reproduce the observed speeds at which individual stars are orbiting the centre of a galaxy, without requiring the galaxy to contain dark matter. Indeed, the distribution of mass through the galaxy roughly followed the distributions of visible matter, with no need for exotic new particles.

However, most physicists and astronomers remain unconvinced. They point to a body of other evidence for the existence of dark matter. The way distant light is bent around galaxies hints that they should have more mass than can be seen. So do measurements of how the largest-scale structures in the universe formed over time and of how clusters of galaxies are held together. Furthermore, a follow-up paper by Mikolaj Korzynski of Warsaw University in Poland has claimed that there is a fatal flaw that may make Dr Cooperstock's model a mathematical but not a physical possibility.

Dr Cooperstock says that he is preparing a detailed response. After giving the scientific community a chance carefully to consider his model, he plans to submit the work to the Astrophysical Journal.

“We just cut off this piece. It is part of the puzzle,” Dr Cooperstock says. At worst, the piece will not fit and his model will turn out to be incorrect. In that case, though, physicists will have gained a better understanding of how the predictions of general relativity mesh with Newtonian gravity in the case of galaxies. The assumptions on which the existence of dark matter are based will thus rest on stronger foundations. At best, Dr Cooperstock's model will stand up to scrutiny. Then, for galaxies at least, it will turn out that there is not much more to them than meets the eye. In either case, physics will be the better for it.


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