Dwarf galaxies dancing around the spiral galaxy Andromeda have boosted a controversial alternative to the idea of dark matter – the invisible stuff that most astronomers think makes up about 80 per cent of the matter in the universe.
Researchers looking at the speeds of stars in Andromeda’s satellite galaxies have found that their motion is a near-perfect fit with what is predicted by modified Newtonian dynamics, or MOND. According to MOND, alterations to Newton’s laws of motion and gravity can explain at least some of the observed effects that have been attributed to dark matter.
The speed of visible matter in rotating galaxies had been one of the strongest arguments for dark matter. Stuff on the edges of galaxies is zooming around much too fast to be consistent with the gravitational pull of the mass we can see, and should therefore be flung off. Something must be adding enough gravity to hold galaxies together – and that’s where unseen dark matter comes in. The effect is particularly strong in tiny dwarf spheroidal galaxies, which are traditionally thought to be 99 per cent dark matter.
In the 1980s Mordehai Milgrom, then at Princeton University, tweaked Newton’s laws so that an object in a very weak gravitational field experiences a slightly stronger pull than Newton would have predicted. He showed that this revised version of gravity, now called MOND, can neatly describe the observed rotation of material in giant spiral galaxies without the need for dark matter.
Milgrom, now at the Weizmann Institute in Rehovot, Israel, and Stacy McGaugh of Case Western Reserve University in Cleveland, Ohio, have just shown that the formula works equally well for predicting the speeds of stars in dwarf spheroidal galaxies. They matched their computer models with new, independent observations of 17 Andromeda satellite galaxies.
“Our predictions are spot on. Just looking at the numbers for each dwarf, it is hard to distinguish the theory from the data,” McGaugh says. The pair also used MOND to predict the speeds of stars in ten more faint dwarfs that have yet to be measured.
However, MOND is not widely accepted because it is not clear why gravity should change in weaker fields, whereas multiple theories can provide candidate particles for dark matter, and dark matter helps explain the large-scale structure of the universe. In addition, although MOND works well for stars circling around in a galaxy, for over a decade it has failed to predict the speeds at which whole galaxies orbit around each other in clusters – something McGaugh himself describes as frustrating. To reproduce those observations requires unseen matter of some form to provide the gravity needed.
While clearly not a triumph for MOND, the cluster problem is not a disaster either, says James Binney, an astrophysicist from the University of Oxford. He believes that some sort of MOND-like behaviour may manifest itself on small scales, where dark matter is too rarefied to have an appreciable influence. On larger scales, like in galaxy clusters, dark matter would still hold sway.
For Avi Loeb of Harvard University, MOND is weakened by the continuing need for extra matter to account for the motion of orbiting galaxies. “It is not elegant any more because it leads to two speculations: first that you need to modify gravity, second that you still need some dark matter,” he says. But that doesn’t mean he wants to simply junk MOND. “The theory deserves a lot of respect.”