Stars in the outer regions of spiral galaxies orbit so fast that, by rights, they ought to fly off into intergalactic space. Until now, astronomers have explained their motion by claiming that the stars are gripped by the gravity of dark matter. Theories suggest that the dark matter, be it in the form of black holes or planets or even undiscovered subatomic particles, forms a galactic "halo" 10 to 100 times as massive as all the stars put together.

Now Steven Whitehouse and George Kraniotis of London University's Royal Holloway College are challenging that view. They selected five nearby spiral galaxies with well-known "rotation curves" measurements of how the orbital speed of stars varies with distance from the centre of the galaxy. They subtracted the rotation curve that would have been expected if the gravity of the visible material was the only force keeping the whirling stars in check.

What Whitehouse and Kraniotis were left with was a rotation curve in which the velocity increased linearly, in direct proportion to radial distance. "It was exactly the effect you would expect from dark energy," says Kraniotis. Two years ago, a team led by Saul Perlmutter of Lawrence Berkeley Laboratory in California discovered that empty space appeared to contain energy--dark energy--which was pushing galaxies apart and speeding up the expansion of the universe (New Scientist, 3 April, p 28).

This energy, identified with the "cosmological constant" in big bang models of the universe, is tiny and its effect is only apparent over very large scales. "Oddly enough, nobody considered whether it might have an effect on the scale of individual galaxies," says Kraniotis.

Whitehouse and Kraniotis claim that the linear contribution to the rotation curves of their five galaxies can only be explained by Perlmutter's dark energy. Since the dark energy in galaxies has an equivalent mass to dark matter, it exerts a gravitational pull on the stars. "It is this that prevents them from flying off," says Kraniotis. "You don't need to postulate large amounts of dark matter."

While other researchers are cautious about the claim, they nevertheless believe it is worth taking seriously. "I think it is very important to seek alternatives to the dark matter hypothesis in explanations of galactic rotation curves," says Miloslav Svec of McGill University in Montreal. "Unobservable dark matter in a completely transparent universe is a very unsatisfying idea."

Kraniotis and Whitehouse hope to persuade astronomers to examine more spiral galaxies and spur theorists to find an explanation for the dark energy. They have submitted a paper on their work to the journal Monthly Notices of the Royal Astronomical Society. This article will appear in the December 18 issue of New Scientist New Scientist. Copyright 1999 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.