Not There Yet
Based on the evidence collected so far, “MOND can claim an impressive number of correct predictions regarding the dynamics of galaxies,” says Anthony Aguirre, a cosmologist at the University of California, Santa Cruz. But there’s a reason it’s still not the prevailing theory.
MOND has not fared so well in describing the universe on larger scales, such as galaxy clusters, and particularly “rich clusters” that consist of dozens of bright galaxies and hundreds of fainter ones. The predictions MOND makes for rich clusters are off by a factor of 2, McGaugh concedes, meaning that you need twice as much mass as you see to explain galaxy motions.
To make up the deficit, one idea is that the clusters may harbor unexpectedly large quantities of neutrinos — another type of elusive invisible particle. But unlike dark matter particles, neutrinos are ordinary matter, known to exist in great numbers. The universe has enough concealed, ordinary stuff in it to potentially solve this problem without resorting to unknown dark matter, McGaugh says.
Another shortcoming of MOND is that, unlike general relativity, it offers no compelling physical reason as to why MONDian effects occur. There are theories explaining why MOND works, but no one yet knows which, if any, may be correct. Milgrom, for one, would like to see this deficiency addressed through the development of a broader theory of gravity that incorporates aspects of both general relativity and MOND while eliminating dark matter altogether. He came up with a relativity-friendly version of MOND in 2009, even though another such theory — called TEVIS — was devised five years earlier by Jacob Bekenstein of the Hebrew University in Jerusalem. “TEVIS works pretty well,” Bekenstein says, but like MOND, it can’t explain the behavior of truly large structures. “Maybe some smart guy will come along who can succeed, but that hasn’t happened yet.”
It’s a “really hard problem,” McGaugh acknowledges. “I can at least imagine a more general theory that encompasses general relativity and MOND, with MOND applying in one special case and general relativity applying for the rest.” He’s tried a hand at this himself but hasn’t gotten far, noting that “some people don’t think of it as a valid problem to work on.” Unfortunately, among cosmologists, that attitude extends to pretty much anything involving modified gravity.
Outside the Mainstream
McGaugh’s investigations of the countercultural MOND have exposed him to professional hardship and a barrage of criticism. “I’ve forgotten more slights than most people suffer,” he quips. “A new generation of students raised to believe in dark matter often assumes I must be some kind of crackpot.”
Still, he and Milgrom are not alone in taking MOND seriously. Other respected physicists have signed on, too, among them contemporary researchers in Belgium, France, the Netherlands, the U.K., the U.S. and elsewhere. All told, more than 100 astronomers have published scientific papers on the subject.
McGaugh is also buoyed by the fact that, unlike other alternative theories that have come and gone, MOND has held up surprisingly well. No one has unequivocally disproven it, despite concerted efforts over the past 30 years to do so. But McGaugh also recognizes that in the end, popular opinion — even among scientists — is largely irrelevant. “Ultimately,” he says, “science is not a consensus endeavor. The data rule.”
And that is what he has focused on — the data. He goes where it leads him, even though it has carried him on an unexpectedly tortuous journey. The majority of his research pertains to galaxies, which happen to be his specialty. He broadened his studies of low surface brightness galaxies to include “basically all kinds of galaxies,” he says, such as denser, high surface brightness galaxies, even more star-rich spirals and irregular galaxies — those that don’t come in spiral or ellipsoidal shapes. His conclusion? MOND works well in each of these cases.
On to Andromeda
McGaugh’s most recent research, undertaken with Milgrom and other collaborators, has focused on the undersized (“dwarf”) galaxies of Andromeda, the nearest large galaxy to the Milky Way. Astronomers have spotted among the outer fringes of Andromeda a few dozen small and roughly spherical galaxies with stars orbiting in random directions. The dwarfs’ low stellar densities (and correspondingly low gravitational forces) make them especially good places to look for MONDian effects.
The Pan-Andromeda Archaeological Survey (PANDAS) is exploring Andromeda in unprecedented detail, using the Canada-France-Hawaii Telescope in Hawaii. Shortly after the survey discovered and analyzed 10 new dwarf galaxies, McGaugh and Milgrom predicted in a 2013 paper how fast the stars should be moving, according to MOND. The stellar velocities were later measured, and their predictions were right on the mark for nine of the 10 (with the last one having too few stars to support a velocity measurement). Predictions for two additional nearby dwarf galaxies made by McGaugh and his postdoc Marcel Pawlowski, published in 2014, were also correct.
“When you make predictions and they come out right, that’s about as good as it gets,” McGaugh says. “As the Andromeda measurements have shown us, as the data improve, the agreement with MOND seems to keep getting better.”
He accepts the fact that MOND still faces many challenges. There is the cluster problem, which has not gone away, and the difficulty of tying in MOND with a broader description of gravity. Dark matter models, on the other hand, have not fared well in explaining star motions in galaxies, either, and in some cases the models are off by a factor of 100. McGaugh agrees with Bekenstein, who maintains that “dark matter models have problems of the same magnitude [as MOND].”
As to exactly where that leaves us, McGaugh is unsure, though he’s more convinced than ever that MOND warrants further investigation. “The formula has predictive power,” he says, “so it’s got to be telling us something.” If he’s right, other astronomers might find it worth their while to listen.