Nearby dark-matter-free zone poses cosmic conundrum

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Invisible dark matter is supposed to make up over 80 per cent of the universe's mass but a new survey of nearby stars and galaxies suggests our patch of the cosmos could be totally free of the stuff. "The claim is that in this volume, there is no dark matter," says Christian Moni Bidin at the University of Concepci?n in Chile, who led the study.

The surprising finding contradicts otherwise successful predictions about the distribution of dark matter in the universe, leaving many puzzled about how else to explain the universe's history. It also fits with new observations by a fiery minority of physicists who dispute whether the mysterious matter, which has never been observed directly, is even necessary.

Rather than seeing it directly, physicists first deduced the existence of dark matter from the way that our galaxy rotates. If the only matter in the Milky Way is the visible stuff like stars and planets, then stars at the edge are moving too quickly to be held by our galaxy's gravity.

In order to keep the stars on the fringes from flying away, there must be some additional mass creating the extra gravity needed to hold them. A similar situation has been observed in other galaxies and physicists have used this calculate that this must amount to about 83 per cent of the total mass of the universe.

Ant on vinyl

This has in turn provided predictions for the way that dark matter is distributed in the universe, and fed into our understanding of how the universe formed. For example, supercomputer simulations of the entire history of the universe that include these distributions of dark matter do an excellent job of reproducing the structures we actually see in the universe now.

However, until now most velocity measurements considered only stars zipping around the Milky Way radially, like an ant sitting on a vinyl record. By contrast, Moni Bidin's team looked outward from the plane of the galaxy, perpendicular to the galactic disc. Using historical survey data and new observations from telescopes at the La Silla Observatory and the Las Campanas Observatory, both in Chile, the researchers mapped the motions of more than 400 stars up to 13,000 light-years from the sun.

They used those measurements to calculate the mass of matter in a volume four times larger than had been considered before at this level of precision. Under the standard dark matter theory, there should be at least as much dark matter as visible matter in this region. "Contrary to our expectations, there is none," Moni Bidin says. "The result matches the visible mass strongly." The work will be published in the Astrophysical Journal.

That might explain why experiments on Earth hoping to catch particles of dark matter have turned up confusing results, but it's too soon to give up on dark matter, says dark matter theorist Dan Hooper at Fermilab in Batavia, Illinois. He says we still need that extra mass to explain why the galaxy holds together ? and how all the structures in the universe, from dwarf galaxies to superclusters, formed at all.

Snowballing clusters

"We have many independent lines of reasoning that lead us to the conclusion that we have substantial amounts of dark matter in the local part of our galaxy," says Hooper. "This is not going to be easily abandoned as an idea. I'm not saying they're wrong, just that you're going to have to work really hard to convince me."

Smith himself agrees that dark matter is still needed. "It explains an enormous number of things really famously, and puts them together into one framework," Smith says. "That's a really powerful theory."

However, the new survey is not the only example of observations that do not fit with our current picture of dark matter. In a recent study, to be published in Astrophysical Bulletin, Igor Karachentsev calculated that much less dark matter than expected was required to explain the amount of mass in the local universe, a region 163 million light years from the sun in every direction. That takes the headache posed by Moni Bidin's work and "makes it even worse", Moni Bidin says.

Meanwhile, Pavel Kroupa at the University of Bonn in Germany has looked at the way dark matter around the Milky Way might be distributed. In the standard view, dark matter drew together under its own gravity to form small clusters shortly after the big bang. Those clusters snowballed in size, and galaxies as we see them today grew up inside massive, near-spherical haloes of dark matter.

Einstein alternative

If that were true, the streams of stars, clusters and small galaxies that orbit the Milky Way should be distributed randomly in a sphere around the main disc. But Kroupa reports in a paper to be published in Publications of the Astronomical Society of Australia that most of them are clustered in an enormous disc that rotates in a plane perpendicular to that of the Milky Way. That disc could be the remnants of another galaxy that collided with the Milky Way some 11 billion years ago, but it could not be the result of dark matter, Kroupa says.

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