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              Celestial trick of the light reveals invisible dark matter
Wednesday, May 16th, 2007 Ring of gravity in outer space lets scientists ‘see’ exotic material
The ring, which is 2.6 million light years across, has been detected by astronomers using the Hubble Space Telescope, providing some of the strongest evidence yet for the existence of the enigmatic material. Dark matter does not shine like stars, or reflect visible light like planets, so it cannot be directly seen. Nevertheless, scientists have calculated that it makes up almost a quarter of the mass of the Universe. The ordinary matter that can be seen through telescopes – stars, galaxies and dust – can account for no more than 4 per cent of the Universe as a whole, as it does not generate sufficient gravity for the cosmos to be stable. Models suggest that about 23 per cent of the total mass is made up of dark matter, while the remaining 73 per cent is something even odder: a dark energy that drives the Universe’s expansion. The nature of both dark matter and dark energy remains elusive, though scientists suspect the former is made up of as yet unknown particles that pervade the entire Universe, but which have little effect on the ordinary particles we can see. These are sometimes known as weakly interacting massive particles, or “wimps”. The new observations of the galaxy cluster ZwCI0023+24, which lies five billion light years away from Earth, promise to offer fresh clues to the nature of the substance. Though the ring cannot be seen physically, its presence has been confirmed by observing how its gravity bends the light of more distant galaxies that lie behind it, as seen from Earth. James Jee, of Johns Hopkins University in Baltimore, Maryland, who led the discovery team, said: “This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster. “Although the invisible matter has been found before in other galaxy clusters, it has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters. “By seeing a dark matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter.” The discovery, which is published in the Astrophysical Journal, was made unexpectedly when Dr Jee’s team was attempting to map the distribution of dark matter within the galaxy cluster. At first, the astronomers were convinced that they had made a mistake when a set of concentric rings, like ripples in a pond, appeared from their data. “I was annoyed when I saw the ring because I thought it was an artefact, which would have implied a flaw in our data reduction,” Dr Jee said. “But the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this.” The likely explanation for the ring comes from previous research into the same cluster, published in 2002 by Oliver Czoske, of Bonn University in Germany, which indicated that it had collided with another galaxy cluster between one billion and two billion years ago. Computer simulations of this cosmic collision suggest that the dark matter fell towards the centre of the cluster and then rippled back out, creating the characteristic pattern. Holland Ford, also of Johns Hopkins University, said: “By studying this collision, we are seeing how dark matter responds to gravity. Nature is doing an experiment for us that we can’t do in a lab, and it agrees with our theoretical models.” Dr Jee said: “The collision between the two galaxy clusters created a ripple of dark matter which left distinct footprints in the shapes of the background galaxies. It’s like looking at the pebbles on the bottom of a pond with ripples on the surface. “The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.” Points of light — The points of light in the background of the image above are galaxies observed in visible light with the Hubble Space Telescope, in the galaxy cluster ZwCl0024+1652 — The concentric blue dusty rings show the distribution of dark matter in the galaxy cluster. Though the dark matter itself is invisible, its position has been calculated from its gravitational effect on light reaching Earth from the background galaxies — The existence of dark matter was proposed in 1933 by the astrophysicist Fritz Zwicky, whose studies of galaxy clusters found they had far more mass than could be predicted from visible light alone — One theory about dark matter is that it is made of weakly-interacting massive particles or “Wimps” — Billions of Wimps are thought to be passing through the body every instant, but they are almost impossible to detect — An Anglo-American team is currently engaged in a search for Wimps in a disused salt mine 3,500ft under the cliffs of the North Yorkshire coastline Source: Johns Hopkins University, Times database Have Your Say: Celestial trick of the light reveals invisible dark matter Please read our posting guidelines before posting. Alternatively you can discuss this report here. Related News
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A rippling halo of dark matter, the mysterious substance thought to account for much of the mass of the Universe, has been discovered around a distant cluster of galaxies.
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