When the idea of dark matter first pushed its way into astronomers’ consciousness a few decades ago, the primary reaction was: “Seriously? There’s a mysterious, invisible substance out there, with a mass six or more times greater than that of the visible stars and galaxies, only we have no way of detecting it, but really, it’s there? OK then.” Or something like that, albeit in more formal scientific language.
These days, dark matter is a firmly established principle of cosmology; most of the questions now focus on how the stuff is distributed through the universe, and which of many possible subatomic particles it’s made of.
Most of the questions, but not all. Ever since the early 80’s, a competing theory has been struggling for acceptance. Known as MOND, for Modified Newtonian Dynamics, it posits that dark matter’s main effect — allowing galaxies to spin faster than they should — isn’t caused by extra stuff, but instead by a change in how gravity works under certain conditions.
That underdog theory has now gotten a boost: using MOND-based models, and assuming no dark matter whatever, astronomers have successfully predicted the orbital speeds of stars in 15 faint dwarf galaxies that hover around the nearby Andromeda spiral galaxy. MOND can already explain galaxies that spin like the Milky Way — not surprisingly, since the theory was invented to do just that. But this is its first test in galaxies that aren’t spinning as a whole, but whose individual stars are instead following their own random orbits. MOND predicted how fast those stars should be moving, and, says Stacy McGaugh of Case Western Reserve University, lead author of a paper on the predictions, “It’s spot-on.”
Whether this will change the minds of mainstream cosmologists about the existence of dark matter is another question entirely. McGaugh himself was completely dismissive about MOND when he first heard about it. “Who wants to waste their time hearing about that crap,” he recalls thinking when MOND’s creator, the Israeli astrophysicist Mordehai Milgrom, showed up to give a presentation many years ago. McGaugh went to listen anyway. His reaction afterward? “This is crazy talk.”
But maybe not. Gravity weakens rapidly the further you move from the attracting body — the Earth say. Milgrom’s idea is that at very low intensity, like out at the edges of galaxies, that weakening should slow. If that were true, the attraction produced by visible matter at those great distances would be greater than current estimates suggest — perhaps enough to allow the edges of galaxies to rotate unexpectedly fast, without the help of any invisible, unknown matter holding them together. The same phenomenon would allow galaxies in clusters to orbit one another other at surprisingly high speed without the distances among them steadily widening.
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This alone wouldn’t make MOND a competitor to conventional dark-matter theory, which explains those high speeds equally well. But the mainstream theory does have some shortcomings that MOND avoids. Conventional theory predicts, for example, that there should be thousands of small clumps of dark matter floating around in the so-called Local Group of galaxies, and that these clumps should have formed the seeds for thousands of dwarf galaxies. But in fact, the Local Group has only a few dozen dwarfs — at least, that anyone has found so far.
Numbers aside, the theory says that any dwarf galaxies we do see should have dense knots of dark matter at their cores. But as far as anyone can tell by carefully measuring the orbital speeds of individual stars, they don’t. MOND, on the other hand is agnostic on how many dwarf galaxies there should be; it’s fine with just a dozen. It’s also better than conventional dark-matter theory at explaining the mass-to-light relation in so-called low-surface-brightness galaxies, another class of objects that are dim, like dwarfs, but not especially small.
That’s what eventually won McGaugh over. “I bothered to learn about MOND,” he says. “Many of the more vocal critics choose to remain willfully ignorant.”
But at least one subscriber to the mainstream theory who has taken a serious look at MOND remains unconvinced. “If MOND were purely a theory of modified gravity, that would be one thing,” says Avi Loeb, chair of the astronomy department at Harvard. But even MOND requires some sort of dark matter to explain such crucial phenomena as evolution of structure in the universe — why and how galaxies have been pulled into huge clusters, for example, rather than being smoothly distributed through space. MOND enthusiasts, he says, call on the gravity exerted by giant clouds of neutrinos — a type of fleet, lightweight elementary particle so ethereal that it could zip through a chunk of lead a trillion miles thick without even noticing – to accomplish that feat.
While neutrinos are known to exist, however, it’s not clear they can do what MOND supporters claim. Dark matter, by contrast, explains both cosmic structure and the behavior of individual galaxies in a much simpler way. “With [the extra factor of neutrinos], MOND loses its appeal,” says Loeb, “because it is no longer purely a modified theory of gravity.” For his money, that’s just too speculative.
That doesn’t make MOND crazy, though. As physicist Anthony Aguirre, of the University of California, Santa Cruz, wrote a few years ago, “MOND is out of the mainstream, but it is far from wacky.” That’s more than many astrophysicists would have said for dark matter a generation ago.