The predictions of a theory proposed as an alternative to dark matter have been verified in a new class of objects, according to a study currently in press. The results seem unlikely to convince astrophysicists to abandon dark matter as one of the cornerstones of the standard model of cosmology; but suggest a direction in which new work is urgently needed.
The alternative theory is Modified Newtonian Dynamics, or MOND, proposed in 1983 by astrophysicist Moti Milgrom of the Weizmann Institute of Science in Israel. Like dark matter, MOND was hypothesized to explain why galaxies remain in one piece when observations of the rotation speeds and estimates of the mass of their luminous contents suggest they should fly apart. Rather than assuming the universe contains 5/6 invisible dark matter, Milgrom proposed a modification to Newton’s laws that kicked in at the low acceleration scales of stars in galaxies, and that strengthened the attractive force of gravity, having the effect of dark matter without imagining a mysterious extra substance. MOND successfully predicted the observed rotation speeds of galaxies and gained supporters but its adherents dropped off dramatically after observations of the cosmic microwave background (CMB) – the so-called “echo of the Big Bang” — first by the balloon experiment BOOMERanG and then by NASA’s Wilkinson Anisotropy Probe (WMAP), provided stunning confirmation of predictions from the dark matter theory.
One astronomer who still hasn’t given up on MOND is Stacy McGaugh of the University of Maryland in College Park. In his latest paper, in press at Physical Review Letters, McGaugh compares the prediction of the theory to the rotation speeds of spiral galaxies that are gas-rich. While estimates of the mass of most galaxies are uncertain; because it’s not known exactly how to convert measurements of the light from the stars into the mass of the galaxy, the mass of gas-rich galaxies can be measured precisely from the intensity of a radio emission from the hydrogen atoms that make up the gas. That enabled McGaugh to test MOND more rigorously than has ever been done before. He found that, in detail, the galaxies obey the Tully-Fischer relation, the observation that more massive galaxies rotate proportionally faster than less massive ones. “MOND predicted this naturally ahead of time,” says McGaugh. In contrast a dark matter model that assumed the proportion of dark matter in the galaxies matched the cosmic value of 5/6 gave a prediction that was some way off the observations.
Cosmologist Anthony Aguirre of the University of California, Santa Cruz, says the result is no surprise given earlier work by McGaugh and others showing that the relation was predicted by MOND for other classes of galaxies. But the application to gas-rich galaxies improves the result, he says. “The basic result was already in place but the errors are smaller,” he says. The work is unlikely to lead to the acceptance of MOND, because that still isn’t confirmed by observations of the CMB, but it should motivate further work on more sophisticated dark matter models that would also match the observed Tully-Fischer relation, Aguirre says.
Cosmologist Priyamvada Natarajan of Yale University agrees. She emphasizes that, MOND is unable to explain observations of clusters of galaxies, including the bullet cluster (pictured), a collision of two clusters with a pattern of bright matter that matches exactly what would be expected in the dark matter model. But she agrees McGaugh’s work highlights a regularity in the behavior of galaxies that hasn’t been explained yet by processes involving dark matter. “It reveals there are physical processes we don’t understand,” she says. These almost certainly involve interaction between dark matter and normal matter that can result in a deviation of the proportion of dark matter to ordinary matter from the cosmic average value of 5/6. “We don’t fully understand how baryons [ordinary matter] and dark matter interplay. I see this as a challenge. It points the way forward,” she says.
McGaugh accepts that his work apparently verifying a prediction of MOND is now most likely to be interpreted within the dark matter paradigm, but says he’s OK with that. “I think it’s telling us something about nature. Maybe it’s telling us something about the nature of dark matter,” he says.
Astrophysicist Danilo Marchesini of Tufts University says he thinks McGaugh’s approach is very clever, and that the next step is to put a bigger effort into estimating the masses of galaxies containing stars rather than gas, to see if the result still holds.
Image: The Bullet Cluster/ NASA