Dark riddles november 2007 Scientific American

Dark matter, the substance no one has ever seen, continues to baffle cosmologists. New observations of the distribution of dark matter in a distant cluster of galaxies may even force scientists to propose a fifth force of nature—or to rewrite the basics of Newtonian gravity. Little wonder that many researchers hope that the unsettling result will turn out to be an observational fluke.

Giant clusters of galaxies consist of two observable components. The galaxies themselves can be seen with large optical telescopes. The hot, tenuous gas in between the galaxies can be spied and mapped by X-ray satellites like NASA’s Chandra X-ray Observatory. But according to current wisdom, galaxy cluster have a third, invisible component: mysterious dark matter that pulled in the atoms from which stars and galaxies formed.

Dark matter can only be charted by the subtle way its gravity slightly bends light, altering the shapes of faint galaxies in the distant background. The ability to detect such "weak lensing" has seen major improvements during the past decade.

Last year, researchers hailed observations of the "Bullet Cluster" as the first definitive proof of the existence of dark matter. The Bullet is really two clusters in the process of merging. As expected, the colliding gas in the two clusters is dragged to the common center of gravity, while the individual galaxies move on relatively unimpeded. Crucially, weak lensing observations revealed the existence of huge amounts of dark matter coinciding with the galaxies, not with the gas. That’s exactly what’s predicted by popular theories, in which dark matter hardly interacts with itself and in which galaxies form and reside in regions where the dark matter density is highest.

But now, another cluster spoils the party. At a distance of 2.4 billion light years in the constellation of Orion, Abell 520 also consists of two colliding clusters. However, according to a team led by Andisheh Mahdavi and Henk Hoekstra of the University of Victoria, British Columbia, the dark matter in Abell 520 doesn’t appear to be tied to the galaxies. Instead, the lensing observations – carried out with the 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea in Hawaii – indicate that huge amounts of dark matter are concentrated in the core of the colliding pair, where most of the hot gas is found but few galaxies are seen. As the team writes in their October 20 Astrophysical Journal paper, this dissociation between dark matter and galaxies "cannot be easily explained within the current…dark matter paradigm."

"It’s a remarkable result," says cosmologist David Spergel of Princeton University. "A conservative explanation would be that not all dark matter concentrations are efficient in the formation of stars and galaxies. The alternative is that dark matter interacts with itself in response to an unknown, fifth force of Nature, which only involves dark matter." Under the influence of such an attractive force, two clouds of dark matter could no longer pass through each other unimpeded but would eventually be dragged like the hot cluster gas, ending up in the common center of gravity of the colliding clusters.

Robert Sanders of the University of Groningen in the Netherlands says there’s a third solution to the problem: modified Newtonian dynamics (MOND). Invented in the early 1980’s by Mordehai Milgrom of the Weizman Institute in Rehovot, Israel, MOND proposes that the observed signatures of dark matter really result from a different behavior of the force of gravity. In particular gravity in low-acceleration regions (like the outskirts of galaxies) would weaken linearly with distance, not exponentially. Even in a MOND universe, some dark matter has to exist, but it could consist of "normal" particles, such as neutrinos, instead of mysterious, undetected stuff. Sanders says he and Milgrom are writing a paper on how MOND can accommodate the cluster observations. "These new results—if they are real—could be an outstanding success for MOND," he says.

If they are real, indeed. Douglas Clowe of the University of Arizona has his doubts. Last year, Clowe was the lead author of the paper on the Bullet Cluster. "We have separate [weak lensing] data on Abell 520," he notes, "and our results do not agree with those of Mahdavi’s team." Clowe says the statistical significance of Mahdavi’s results is not particularly impressive. Spergel agrees that the Abell 520 results are "suggestive, but not yet convincing." Luckily, new Hubble Space Telescope observations of Abell 520 are in the pipeline. As Clowe puts it: "If they confirm the ground-based observations, it’s time to start worrying."

© Govert Schilling