(X-ray) NASA/CXC/CfA/M. Markevitch et al.; (Optical) NASA/STScI; Magellan/U. Arizona/D. Clowe et al.; (Lensing) NASA/STScI; ESO WFI; Magellan/U. Arizona/D. Clowe et al.;.

Proof positive?
Astrophysicists point to this image of the colliding Bullet cluster of galaxies as revealing the existence of dark matter.

Dark Matter Not a Done Deal?

Some arguments will never end. A year ago, astronomers reported observations that were widely hailed as proof positive for the existence of dark matter, the mysterious stuff whose gravity holds the galaxies together. But now a different team says that the very same observation can be explained by a controversial theory that rejects dark matter and alters the rules of gravity.

The idea of dark matter arose nearly 75 years ago when Fritz Zwicky, an astronomer at the California Institute of Technology (Caltech) in Pasadena, noticed that some galaxies in the Coma cluster appeared to be moving so fast that the gravity from the rest of the cluster shouldn't have kept them from flying into space. Zwicky proposed that the cluster contained an additional but unseen form of "dark matter" that provided the extra gravity. That wild idea became more palatable in the 1960s, when astronomers found that stars zoom around the peripheries of individual galaxies too fast for the gravity of the other stars to hold them in. Most astrophysicists now assume that a typical galaxy consists of a vast clump, or "halo," of dark matter studded with stars and pervaded with gas, much as a bran muffin is laden with raisins and seasoned with cinnamon.

But there's a catch: No one has ever directly detected dark matter. Indeed, a few scientists argue that the mysterious stuff can be explained away entirely if gravity works slightly differently at galactic scales, a theory that would mean both Isaac Newton and Albert Einstein had gotten gravity wrong.

Not surprisingly, the idea has earned many detractors. It began to seem even more implausible last year, when a U.S. team led by Douglas Clowe, then of Steward Observatory in Tucson, Arizona, reported an observation that supposedly could be explained only by dark matter (ScienceNOW, 21 August 2006). Clowe and colleagues analyzed the head-on collision of the Bullet cluster, two massive groups of galaxies 3 billion light-years away. They found that the galaxies within the clusters, which weigh less than the superheated gas clouds that had been separated from them by the collision, actually exert a greater gravitational influence. Scientists hailed the discovery as proof of dark matter's existence.

Not so fast, says astrophysicist John Moffat of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. In a paper published online 25 October in the Monthly Notices of the Royal Astronomical Society, Moffat and co-author and Perimeter Institute colleague Joel Brownstein argue that their modified theory of gravity, which they call MOG, can also explain the Bullet cluster discrepancy. Einstein argued that gravity arises because mass warps space and time--he even came up with an equation for how the warping works. Moffat says he added "minimal" additional terms to Einstein's equation that subtly change how gravity behaves on galactic scales. The upshot is that gravity is stronger at these scales than Einstein predicted and that MOG can explain the gravity of the Bullet cluster without dark matter. Brownstein says he and Moffat have applied MOG to the behavior of more than 100 galaxies and more than 100 clusters, and in all cases, it has successfully predicted their motions "without the necessity of adding dark matter."

Astrophysicist Sean Carroll of Caltech is not convinced. "The Bullet cluster and related systems are beautifully consistent with the dark-matter hypothesis," he says, and "almost impossible to explain if ordinary matter is all there is."

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