Astronomers claim dark matter breakthrough

19:00 01 October 03

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The identity of the Universe's dark matter may finally have been discovered. In what seems to be the most convincing claim for dark matter so far, researchers in England and France say gamma rays coming from the centre of our galaxy show hallmarks of these ghostly particles.

The research has only just been made public, so the team is still waiting for a response from other dark matter experts. But though the researchers are cautious, there is no hiding their excitement. "I've dropped everything else to work on this," says Dan Hooper of the University of Oxford. "We're really excited," adds his colleague Céline Boehm, also of Oxford. "I'm cautious but it's surprising everything fits so well."

The identity of the Universe's dark matter, which outweighs the visible stuff by at least a factor of seven, is the outstanding mystery of modern astronomy. Scientists think it must exist because its gravity affects the way galaxies hold together. But the particles do not emit any electromagnetic radiation so they have never been detected directly. No one knows what the particles are like, or exactly how they are distributed.

However, because dark matter "feels" gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy. So the team turned their attention to a distinctive pattern of gamma rays coming from the centre of the Milky Way (see graphic). The sharp signal, which has an energy of 511 kiloelectronvolts (keV), is believed to be due to the annihilation of electrons and positrons ­ the antimatter equivalent of electrons.

Virtual standstill

But where did the electrons and positrons come from? People have speculated that the source is anything from the blast waves of a "hypernova" ­ a super-powerful supernova ­ to a neutron star or black hole. "But none of the explanations have seemed satisfactory," says Hooper.

The researchers wondered whether the electrons and positrons might in fact come from the annihilation of dark matter particles and their antiparticles at the centre of the galaxy. But to produce a sharp line at 511 keV ­ which is the "rest energy" of an electron ­ the electrons and positrons must be slowed to a virtual standstill before they annihilate each other, ruling out dark matter at the large masses most researchers expect.

"Heavy dark matter particles would produce high-energy electrons," says Hooper. "Since it's difficult to imagine how they could be slowed to a standstill, we were forced to consider a surprisingly light dark matter particle."

By "light", the researchers mean one to 100 megaelectronvolts, which is between 1000 and 10 times lighter than a proton. Such a light particle is surprising because particle accelerators routinely create particles of this mass, so the particle should have revealed itself.

"To have escaped detection, it must be very weakly interacting," says Hooper. "A particle in [this] range could have been missed," agrees Nigel Smith, head of the UK Dark Matter Collaboration Experiment.

If dark matter really is made up of such light particles, every cubic centimetre of space in the vicinity of the Earth must contain a few tens of them. So you should be able to detect them in lab-based experiments.

"The claim would become much more interesting if a particle or nuclear physics experiment finds a new particle with the properties the team suggest," says Ben Allanach of CERN, the European centre for particle physics.

Teams hunting for dark matter on Earth usually focus on much more massive particles ­ bigger than 10 gigaelectronvolts ­ by trying to detect the recoil of an atomic nucleus hit by a dark matter particle.

Hooper, Boehm and their colleagues are now looking into whether any existing experiment might show evidence of the new particle or whether any could be easily modified to detect it. The researchers plan to submit their paper to Physical Review Letters.

Marcus Chown

Closest asteroid yet flies past Earth

18:17 02 October 03

NewScientist.com news service

An asteroid about the size of a small house passed just 88,000 kilometres from the Earth by on Saturday 27 September - the closest approach of a natural object ever recorded. Geostationary communication satellites circle the Earth 42,000km from the planet's centre.

The asteroid, designated 2003 SQ222, came from inside the Earth's orbit and so was only spotted after it had whizzed by. The first sighting was on Sunday 28 by the Lowell Observatory Near-Earth Object Search program in Arizona, US.

Amateur astronomer Peter Birtwhistle of Great Shefford, Berkshire, UK, then photographed it on Monday 29. This provided data that helped Brian Marsden, of the Harvard-Smithsonian Center for Astrophysics, to calculate its orbit.

The asteroid's 1.85-year orbit is quite eccentric, indicating it cannot be a man-made object, Marsden says. He estimates the asteroid measured less than 10 metres. This is too small to have posed a danger to Earth, although it would have made a spectacular fireball had it entered the atmosphere.

House fires

The passage came at about 2300 GMT, only 10 hours after a bright fireball streaked over the Orissa region of India. Indian villagers have found pieces of the meteorite, which reportedly cause two house fires. However, this event was not connected to the fly past of 2003 SQ222, says Marsden.

The previous record for closest approach of an asteroid - 108,000km measured from the centre of the Earth - was set in 1994 by another 10m object named 1994 XM1.

But the third-closest approach - at 120,000km - was object 2002 MN, which was about 80m in diameter. If on target, that could have exploded in the Earth's lower atmosphere and devastated a couple of thousand square kilometres on the ground.

Another small asteroid, 2003 SW130, missed the Earth by 160,000km on 19 September, making it a busy month for asteroid watchers.

Jeff Hecht