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Astronomers 'see' strands of cosmic web

The gas that binds Astronomers say they have for the first time seen the gas strands theorised to hold the universe together in a "cosmic web".

Cosmologists believe that matter between galaxies is distributed in a network of strands known as the cosmic web.

The vast majority of atoms in the universe are thought to reside in this web as hydrogen left over from the Big Bang, and galaxies are believed to form at network nodes.

Until now, this web had never been seen. Small sections of it had been detected by its absorption of light from bright background sources, but those results didn't reveal its overall structure.

In their paper, published today in the journal Nature, the researchers had used the intense radiation generated by a quasar — a by-product of a supermassive black hole — to act as a type of cosmic flashlight to illuminate part of the vast filament network.

They focused on massive nebula, or deep-space cloud, where the strands intersect.

"This is the first time anyone has been able to capture an image of the cosmic web, demonstrating its filamentary structure," says astronomy doctoral student Fabrizio Arrigoni Battaia, who took part in the observations at the Keck Observatory in Hawaii.

The researchers detected a very large, luminous nebula of gas extending about 2 million light-years across intergalactic space.

'We were lucky'

They could study the nebula thanks to illumination provided by a quasar — radiation generated by cosmic matter falling into a galaxy's central supermassive black hole — with the aid of computer light filters.

Quasars are the most luminous objects in the universe.

"In this case, we were lucky that the flashlight is pointing right at the cosmic web, making some of its gas glow," says researcher Sebastiano Cantalupo of the University of California, Santa Cruz .

"We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission from the quasar."

The researchers didn't have everything go their way. They estimate the amount of gas in the nebula to be at least ten times more than expected from the results of computer simulations.

"We think there may be more gas contained in small dense clumps within the cosmic web than is seen in our models. These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models," says Cantalupo.