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Has Dark Matter Gone Missing?

19 April 2012 5:41 pm
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If a new study is true, then the search for dark matter just got a lot weirder. Our little corner of the Milky Way contains no observable concentration of the mysterious stuff whose gravity binds the galaxy, claims one team of astronomers. That finding would present a major problem for models of how galaxies form and may undermine the whole notion of dark matter, the researchers claim. But some scientists doubt the reliability of the team's method for measuring the elusive substance.

"This is not just some piddling little detail," says Frederic Hessman, an astronomer at the University of Göttingen in Germany who was not involved in the work. "If this is right, it turns everything totally upside-down." But that's a big if, says Julio Navarro, an astrophysicist at the University of Victoria in Canada: "The argument is provocative, but it remains inconclusive, in my opinion."

According to standard cosmology, we should be swimming in dark matter. Measurements of the afterglow of the big bang—the so-called cosmic microwave background—and of the distribution of the galaxies suggest that 85% of all matter in the universe is dark matter. What's more, decades of astronomical observations show that the stars within galaxies swirl about faster than they could if only the gravity of the others stars were holding them in. In fact, the speed with which the sun goes around the center of our galaxy suggests that dark matter ought to be about as abundant as ordinary matter at our distance from the galactic center, about 27,000 light-years.

But that's not what Christian Moni Bidin, an astronomer at the University of Concepción in Chile, and colleagues find. Using data gathered with several telescopes, they studied old stars called red giants in a cylindrical region a couple of light-years wide and extending 13,000 light-years above the plane of the galaxy. Treating the stars a bit like atoms in a gas, researchers assumed that they were trapped in the gravitational "well" of the galaxy. So by studying distributions of the stars' speeds in three dimensions, they could deduce the well's shape and hence the total distribution of mass from both dark and ordinary matter along the cylinder. Subtracting the distribution of ordinary matter as determined from star counts would then reveal the distribution of dark matter.

When Moni Bidin and colleagues did the analysis, however, they found that no dark matter was needed to explain the stars' speeds. The researchers had expected to detect a complicated mass distribution with a contribution from the galaxy's disk of stars and gas and the presumably spherical "halo" of dark matter surrounding the disk. Instead, they found that the disk alone neatly explained their data, as they report in a paper in press at The Astrophysical Journal.

The data don't disprove the existence of dark matter, Moni Bidin is quick to say. Astrophysicists still need the stuff to explain the speed of the stars in the galaxy. However, the data do suggest that there isn't any dark matter in our neck of the woods. "We're not saying that there isn't any dark matter," Moni Bidin says. "We're just saying that there isn't any dark matter here."

But that could lead to a major problem with the whole idea of dark matter. For example, one way to explain why there is no dark matter 27,000 light-years from the center of the Milky Way would be to assume it's all in one tall cigar-shaped lump that sticks through the center of the galaxy. But simulations show that such a shape for the halo is unlikely, Moni Bidin says.

Another possibility is that dark matter is made not of "cold," massive particles moving very slowly, but rather of "warm," lightweight particles moving much faster. In that case, the galactic halo would be larger and more uniform, producing an even and therefore undetectable background, Hessman says. But that inference would fly in the face of standard cosmology, which assumes that galaxies start to form as cold dark matter starts to condense in massive clumps. "Basically," Hessman says, "the cosmologists should say, 'Oh my God!' because you're taking away the one thing that makes everything work and they're going to have to go back to square one."

Or not. The new result may say more about the method than the distribution of dark matter, Navarro says. To get that distribution, at each position in space Moni Bidin and colleagues must subtract one large quantity (the amount of ordinary matter) from another large quantity (the amount of total mass) to get a small quantity. That process is likely to suffer from large uncertainties, Navarro says. "I applaud them for trying," he says. "I just don't think this method will ever give a conclusive answer." Moni Bidin says the method is robust and that larger surveys to come will pin down the dark matter distribution more precisely.

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