© NASAThe answer to the riddle of dark matter could be found in our own solar system
The orbit of the innermost planet, Mercury, departs from what it should be under Newton's laws. A century ago, when Einstein explained this anomaly, it confirmed his theory of gravity - the general theory of relativity.
Now an Israeli physicist predicts that a similar but far more subtle anomaly in the orbits of the planets, if detected, might prove his own theory, known as modified Newtonian dynamics, or MOND. This provides an alternative theory to dark matter to explain why stars orbiting at the edge of spiral galaxies are not flung out into space. These stars are travelling at speeds too fast for conventional gravity from the mass at the heart of a spiral galaxy to hold them in their orbits, so something else must be keeping them on track.
One theory is that invisible dark matter provides that extra pull. But an alternative is MOND, devised in the early 1980s by Mordehai Milgrom, now at the Weizmann Institute of Science in Rehovot, Israel.
One of the suggestions behind MOND is that the gravity experienced by the galaxy's outer stars is somehow stronger than what would be expected under Newtonian physics. MOND has it that below a critical threshold acceleration, called a0, gravity switches from the conventional Newtonian form that weakens with the inverse-square of distance to a stronger form that declines merely with the inverse of distance.
In other words, Milgrom proposed that gravity was stronger than expected at the low accelerations experienced by the outermost orbiting stars.
By quantifying a0 at 10-10 metres per second per second, this single parameter makes it possible to explain stellar motion in hundreds of spiral galaxies. By contrast, the dark matter idea requires different amounts of the stuff with a different distribution in each galaxy.
But verification of MOND, like dark matter, has suffered from the fact that it manifests itself only on very large scales, comparable to the size of galaxies, and so is not amenable to local tests.
Not any more, says Milgrom. In a paper to be published in Monthly Notices of the Royal Astronomical Society, he claims there are forms of MOND that predict an effect on our cosmic doorstep: "It is the first time definite effects in the solar system are predicted by any version of MOND."
Milgrom reasons that if Newton's laws are correct, there will be a region between the sun and the centre of the galaxy where the gravity from both cancels out. But this is also where any MOND-based departure from Newtonian gravity will most clearly show up. In other words, if there is gravity in this region, where there should be none, then MOND exists.
If MOND exists, it will appear as if there is an anomalous, "phantom" mass in that region, exerting a gravitational force on the bodies in our solar system. And because this phantom force originates from a broad zone rather than a defined single point, it would exert a pull on the planets from two directions at the same time - a so-called "quadrupole" effect.
According to Milgrom, this force should cause the orbits of the planets to precess - that is, their elliptical orbits around the sun should slowly change their orientation, over time tracing out a pattern like the petals of a flower. This is similar to the effect predicted by Einstein in 1915. "The difference is it's far smaller and it actually gets bigger the farther a planet is from the sun - the opposite of the effect predicted by Einstein," says Milgrom.
However, we are not in a position to test this as we have not observed enough full orbits of some of the outer planets, like Neptune.
A question that immediately arises is could this new force be responsible for the Pioneer anomaly? NASA's two Pioneer space probes, launched in the early 1970s, are leaving the solar system slower than they should be. Milgrom says MOND would not be to blame. "In the outer solar system, the force is about 100 times too weak and of the wrong form to explain the Pioneer anomaly."
Milgrom says that so far reaction to his paper has been positive. "It's definitely interesting," says James Binney at the University of Oxford. "It's a test of MOND on a hitherto unexplored scale."
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