Bulk of Missing "Normal" Matter Found in Cosmic Web

Ker Than
for National Geographic News
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].)

Using NASA's Hubble and FUSE space observatories, Shull and Danforth examined light from 28 distant quasars scattered across the night sky.

As some of the quasars' light travels through space, it pierces filaments of dark matter and gas.

Atoms of neutral hydrogen and charged oxygen clustered around the filaments absorb portions of the quasar's ultraviolet light, creating dark bands in the spectrum that reaches Earth.

By analyzing this altered light, scientists can determine the position of a filament and the amount of normal matter gathered around it.

The work is described in the May 20 issue of The Astrophysical Journal.

Ken Sembach is an astrophysicist at the Space Telescope Science Institute in Maryland who was not involved in the study.

"This is a truly amazing achievement of the FUSE and Hubble observatories," Sembach said.

"Until those two observatories started looking, we had no way to test whether these [dark matter] models that people take for granted are right."

Half Still Missing

Scientists think dark matter filaments are part of a larger cosmic web connecting vast dark matter isles.

Together the filaments and isles form a hidden support structure for galaxies that could be likened to a dense cluster of brain cells connected by gangly appendages.

Normal matter is drawn toward this dark cosmic web by gravity and flows within and around it like electric impulses coursing through neurons.

Where normal matter concentrates within the web, galaxies and galaxy clusters form.

The new observations confirm that much of the universe's baryons flow through the filaments.

"What we're seeing is stuff that hasn't fallen into galaxies yet but is strewn out along the web," study co-author Shull told said.

But even with the new discovery, more than half of the universe's baryonic matter is still unaccounted for.

Scientists think the remaining missing matter most likely exists in the form of extremely hot gas that also floats between galaxies.

This gas is heated to millions of degrees and will require future x-ray telescopes to detect.

"That's where the theoretical models tell us it should be," Shull said.

"If we build a bigger telescope in the next decade for x-rays and we don't see it, that will be very difficult to understand."

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