A MODIFIED theory of gravity could explain one of astronomy's big mysteries: how spiral galaxies rotate without flying apart.

The favourite theory is that dark matter in the galaxies provides the required extra gravitational pull. But Israeli physicist Mordechai Milgrom had a different idea. In the early 1980s he came up with a theory called modified Newtonian dynamics or MOND, in which extra-strong gravity takes the place of dark matter.

The big problem with MOND has been that it contradicts the well-tested predictions of relativity, and so does not work in systems in which objects are moving at close to the speed of light or under large accelerations. This has meant that it could not make predictions about pulsars, black holes and, most importantly, how the universe came into being in the big bang. But now Jacob Bekenstein of the Hebrew University in Jerusalem has found a way round the problem.

Proponents of MOND believe theirs is a far more elegant explanation than dark matter. While the familiar, conventional force of gravity gets weaker in proportion to the square of distance, Milgrom suggested that there is also another form of gravity whose strength diminishes more slowly, declining linearly with distance.

In MOND, a single parameter explains the motion of stars in hundreds of spiral galaxies. This parameter is the acceleration, a0, below which gravity switches from its normal form to the stronger form. The dark matter theory requires different amounts of the stuff with a different distribution in each galaxy.

To fix MOND's clash with general relativity, Bekenstein has introduced twin fields. One gives rise to the conventional gravitational phenomena, while the second serves as the arena for the phenomena involving the other fundamental forces of nature - the electromagnetic force and the strong and weak nuclear forces.

Bekenstein says his theory is consistent with general relativity, gravity and MOND. "The theory reduces to Einstein's theory of gravity at high speeds and accelerations well above a0, to Newtonian gravity at low speeds and accelerations above a0, and to MOND at accelerations below a0."

The new theory fixes a number of problems with MOND, including its failure to explain the "lensing" of light as it is bent by the gravity of galaxy clusters. "Light is fundamentally relativistic, which is why MOND fails to predict its behaviour," Bekenstein says. "With the new relativistic theory, the bending turns out to be much the same as observed."

Bekenstein says his theory is consistent with observations of the solar system and beyond. Intriguingly, it predicts some subtle new effects that might modify gravity in the outer solar system. "It's too early to say, but it might just have something to do with the Pioneer anomaly - the unexpected acceleration being experienced by NASA's Pioneer probes," he says.

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