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Last Updated: Monday, 29 March, 2004, 09:32 GMT 10:32 UK
Earth on the 'Wimp highway'
By Roland Pease

Dark matter highway, David Low
Earth would be sitting in this stream of Wimps (Image by David Low)
Mysterious sub-atomic particles from another galaxy could be raining down on planet Earth, according to a collaboration of astronomers.

If so, it could explain controversial results from a particle-detection experiment deep inside mountains to the east of Rome.

The story concerns Wimps - standing for weakly interacting massive particles - which astronomers think may make up the bulk of the Universe.

For every kg of material made up from atoms like the ones we have in our bodies, or which make up the stars, there are up to 20kg of something completely different, whose principal quality is that it has never been actually observed directly by scientists.

Which is why they call it dark matter. But they know it is there because its effect on the movements of galaxies can be weighed.

If Wimps exist, they would fill the spaces between the stars, and would interact with normal matter so weakly that they would pass right the way through the Earth.

Seasonal variation

The Dama (DArk MAtter) experiment, at the underground Gran Sasso facility in Italy, is one of several around the world hoping to spot the tell-tale signs of rare collisions between Wimps and ordinary atoms.

Dama has been reporting possible detections, and what is more, it is seeing more events in summer than in winter.

Along with the stars being ripped out of the Sagittarius dwarf galaxy, there would be a large amount of dark matter and that would provide a Wimp highway that's coming right down on to the Earth
Prof Katherine Freese
This seasonal variation has been a particular puzzle. Should cosmic particles obey the cycles of the Earth? The solution could come from a completely different branch of science.

In 1994, UK astronomers discovered a small galaxy just on the other side of our Milky Way galaxy, about to swallowed up by its larger neighbour.

Since then it has become clear that this Sagittarius dwarf galaxy, as it is been called, is actually orbiting over the poles of our own galaxy, with long streams of stars preceding and trailing the main body.

It was when dark matter expert Katherine Freese, from the University of Michigan, heard astronomer Heidi Jo Newberg, from the Rensselaer Polytechnic Institute, talk about these so-called tidal streams that an explanation for the puzzling seasonal Dama Wimp result started to crystallise.

"Part of this stream of stars is coming past our part of the galaxy, close to the Solar System," explains Heidi Newberg.

Stream direction

And this is what excited Katherine Freese. "Along with the stars being ripped out of the Sagittarius dwarf galaxy, there would be a large amount of dark matter and that would provide a Wimp highway that's coming right down on to the Earth," she said.

The argument is that we are stuck in the middle of a fast-moving stream of Wimps, billions passing through every square metre of the Earth (and our bodies) each second at speeds of over a million km/h.

The seasonal variation in detection would then depend on whether the Earth's orbit around the Sun is taking us upstream or downstream in this flow of extragalactic debris.

Writing in the journal Physical Review Letters, the scientists say their theory should be provable, if dark-matter detectors could see a variation in the energy of atom-Wimp collisions from winter to summer.

But confirming that Wimps exist would only be the start of a bigger search - for the identity of what they are actually made from.

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