Bulk of Missing "Normal" Matter Found in Cosmic Web

May 21, 2008

Much of the missing "normal" matter in the cosmos has been found clustered around wispy ropes of invisible matter spanning the space between galaxies.

The filaments form part of the vast weblike superstructure of the universe, within which galaxies are embedded like sparkling sequins.

But even with billions of visible galaxies, astronomers have not been able to account for the majority of normal, or baryonic, matter believed to have been created by the big bang.

"We always had a suspicion that the missing normal matter was hidden in the space between galaxies, but we couldn't prove it," said study co-author Mike Shull, an astrophysicist at the University of Colorado at Boulder.

In a new study, Shull and colleague Charles Danforth probed this region, called the intergalactic medium, using ultraviolet light emitted by distant galaxies with radiation-spewing black holes at their centers.

These galaxies, dubbed quasars, act like lighthouses piercing a fog, revealing gases that are too hot to be detected by optical scans but too cool to be seen by x-ray probes.

The researchers found evidence that about 40 percent of the missing baryonic matter is concentrated around filaments that crisscross the intergalactic medium.

Altered Light

Regular visible matter is made up of protons, neutrons, and other subatomic particles collectively called baryons.

Baryonic matter only accounts for about 5 percent of the universe, and galaxies, stars, planets, and all life-forms are thought to represent about a tenth of that 5 percent.

The rest of the universe is in the form of a mysterious invisible substance called dark matter and an unknown force that is causing the universe's expansion to accelerate called dark energy.

(Related: "At Ten, Dark Energy 'Most Profound Problem' in Physics" [May 16, 2008].)

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