Scientists find missing baryons -- half the universe's missing matter

Two separate teams of researchers have found half of the universe's hidden matter, partially solving a mystery that has long troubled astronomers.

When it comes to the search for missing matter, dark matter -- the mysterious, invisible material accounting for roughly 80 precent of the mass of the universe -- hogs the headlines. But astronomers have also struggled to find a lot of visible matter, too.

Models predict there should be roughly twice as much visible matter as is routinely observed by surveys of the cosmos.

Now, a pair of research teams have found the missing matter. The matter is made up of particles called baryons, heavy subatomic particles made up of three quarks. Astronomers discovered the missing baryons among strands of hot, diffuse gas linking the universe's galaxies together -- the faintest portions of what's known as the cosmic web.

"The missing baryon problem is solved," astronomer Hideki Tanimura told New Scientist.

Tanimura, a researcher at the Institute of Space Astrophysics in Orsay, France, headed one of the scientific teams responsible for discovering the missing particles. Anna de Graaff, a researcher at the University of Edinburgh, led the other team.

Because these isolated gas filaments aren't very hot, they don't give off much energy, making them difficult to observe with X-ray telescopes. But researchers were able to confirm their existence using a phenomenon known as the Sunyaev-Zel'dovich effect, which describes the behavior of light left over from the Big Bang as it travels through hot gas.

As the light passes through the filaments, some of it scatters off the electrons in the gas. The ionized gas causes a distortion in the cosmic microwave background.

Astronomers have previously attempted to map the phenomenon using observations from the Planck satellite. And scientists have successfully identified missing baryons among extremely hot gas filaments.

But many of the cosmic web's filaments are too scattered to be detected. Using data from the Sloan Digital Sky Survey, scientists identified galaxies that should be connected by filaments of diffuse gas.

They then overlaid maps of Planck signals corresponding with the regions of interest. The technique revealed masses of strands too faint to been seen on their own.

One group found filaments six times denser than the other, but further analysis suggests the discrepancy can be explained by the fact that some Planck renderings were recorded at varying distances.

The works by the two teams were published online this week. The two papers are both being considered for publication in the Monthly Notices of the Royal Astronomical Society.

"Everybody sort of knows that it has to be there, but this is the first time that somebody -- two different groups, no less -- has come up with a definitive detection," said Ralph Kraft at the Harvard-Smithsonian Center for Astrophysics. "This goes a long way toward showing that many of our ideas of how galaxies form and how structures form over the history of the universe are pretty much correct."

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