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New light on dark matter in the galaxy

Latest research attempts make it easier to spot the WIMPs that possibly constitute dark matter

PARIS: Our galaxy has a disc of so-called dark matter that pervades it, a finding that could help unlock direct observation of this strange substance, a study published on Tuesday suggests.

Mooted as a theory 75 years ago but discovered only recently through indirect evidence, dark matter accounts for around 22 per cent of the mass of the Universe, scientists believe. An equally enigmatic phenomenon called dark energy accounts for 74 per cent, and visible matter only 4 per cent.

Until now, the main theory was that dark matter encircled the Milky Way like a lumpy halo.

But an international team of scientists on Tuesday said that such calculations are based only on the gravitational influence of dark matter alone, and not on the gravitational pull of other stuff, such as the stars and gas.

In a paper published by Britain’s Royal Astronomical Society (RAS), the team calculate that if the wider factors are taken into account, a thin but extensive disk of dark matter runs across our galaxy.

“The dark disc only has about half of the density of the dark matter halo, which is why no-one has spotted it before,” said lead researcher Justin Read of the University of Zurich, Switzerland. “However, despite its low density, if the disk exists, it has dramatic implications for the detection of dark matter.”

The existence of dark matter has been surmised indirectly, through the gravitational pull it exerts on light from distant stars and galaxies. But getting direct confirmation of it has been a headache.

One theory is that dark matter comprises particles. One candidate is called a WIMP, for Weakly Interacting Massive Particle. Several attempts have been made to spot WIMPs and other candidates for dark matter by placing exotic elements, such as germanium and xenon, as detectors in deep mines.

The idea is if the detector gets hit by a racing interstellar WIMP there will be a flash of light.

None, though, has thrown up anything. But the new work could help the hunt.

The earth and sun move at around 220 km a second along a nearly circular orbit relative to the centre of the Milky Way.

As the dark matter halo does not rotate, from our perspective, it feels as if there is a “wind” of dark matter flowing towards us at great speed.

In contrast, the dark matter disk moves slower than the halo. It co-rotates with the earth, with the result that its WIMPs are a lot slower when they hit our planet. This could be a boon, because slow-moving WIMPs — or whatever particle comprises dark matter — are likelier to cause a flash than fast-moving ones when the next generation of detectors comes on line.

Current detectors are unable to distinguish the slowcoaches from other background “noise.” If this hypothesis is right, dark matter could be directly detected “in the very near future,” the RAS said in a press release.

Another avenue of exploration has been thrown up by the Large Hadron Collider (LHC) which began operations last Wednesday near Geneva. Physicists hope that the LHC may show up novel sub-atomic particles, including so-called symmetric particles, when it smashes protons together at top speed from next year.

An alien planet

Meanwhile, astronomers have captured an image that they claim could be that of the first-known planet orbiting a star similar to the sun.

Using the Gemini North telescope in Hawaii, a Toronto University team discovered the potential planet that is eight times as massive, 10 times as hot and roughly 30,000 times as bright as Jupiter, near a star called 1RXS J160929.1-210524.

The star is 85 per cent as massive as the sun but less than 0.1 per cent its age, at an estimated five million years old. However, both objects are roughly 500 light years away from earth, New Scientist reported.

“This is the first time we have directly seen a planetary mass object in a likely orbit around a star like our sun. If we confirm this object is indeed gravitationally tied to the star, it’ll be a major step,” said team leader David Lafrenicre. — Agencies

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