Vast filaments of hot gas tracing the web of dark matter that underpins the large-scale structure of the Universe have been seen for the first time.
Astronomers using NASA's X-ray satellite observatory, Chandra, viewed the filaments stretching for millions of light years through space, with one passing through our own galaxy. They calculate the filaments contain five times more mass than all the stars in the Universe.
The gas was long-suspected to exist but was revealed by its effect on the passage of X-rays from distant celestial objects known as quasars. The gas has temperatures between 300,000 and three million Kelvin, making it some of the hottest gas in the Universe.
"The discovery confirms the existence of an extremely large fraction of the mass of the Universe," says Robert Warwick at the University of Leicester. This is "extremely important", because 90 per cent of the matter believed to exist in the Universe has never been seen - the so-called "dark matter".
The filaments signpost the location of dark matter because the hot gas sits in its gravitational influence like mist in a valley. Once astronomers discover the nature of the dark matter, they hope to understand the creation of the Universe in much more detail.
Michael Merrifield, University of Nottingham, says, "Astronomers are always looking for ways to couple the behaviour of normal matter to dark matter."
The new discovery looks perfect, because gas at millions of degrees centigrade has a natural tendency to spread. To find it confined into filaments means that a very strong gravitational field must be pulling it into place. According to theory, only dark matter can do the trick.
"In effect, the large gravitational field of the dark matter is acting like a cosmic pressure cooker, holding the gas together," says Merrifield.
Warwick also believes the discovery gives the field of X-ray astronomy a new urgency.
"The existing instrumentation had to work hard to see this," he says. "What we want is an X-ray telescope that can see it in every direction, so we can finally map the true extent and distribution of matter in the Universe."
The European Space Agency is currently considering one such instrument, called Xeus. It could also investigate the composition of the gas to see if it had ever been inside stars, or whether it was fixed in place shortly after the Big Bang.
The new research is published in four papers in the Astrophysical Journal.
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10:45 10 October 2008
