Friday, June 22, 2018

Researchers Find Last of Universe's Missing Ordinary Matter

A simulation of the cosmic web, diffuse tendrils of gas that connect galaxies across the universe. (Credit: Illustris Collaboration)

Scientists have found what may be the universe’s lost sock at the back of the dryer—answering a long-running mystery that astrophysicists have dubbed the “missing baryon problem.”

In a new study, an international team of researchers, including scientists from CU Boulder, located the last reservoir of ordinary matter hiding in the universe. This matter, also called baryons, makes up all physical objects in existence, from stars to the cores of black holes. But until now, astrophysicists had found only about two-thirds of the matter that theorists predict was created by the Big Bang. 

To pin down the missing third, the researchers used the radiation emanating from a distant, ultra-bright black hole called a quasar. That lost matter exists as filaments of oxygen gas at temperatures of around 1 million degrees Celsius that lie in the space between galaxies, said CU Boulder’s Michael Shull, a co-author of the study.

Shull, of the Department of Astrophysical and Planetary Sciences (APS), said the results are important not just for completing the baryon problem but also for answering fundamental questions about how the universe began. 

“This is one of the key pillars of testing the Big Bang theory: figuring out the baryon census of hydrogen and helium and everything else in the periodic table,” he said. 

The new study, which appears today in Nature, was led by Fabrizio Nicastro of the Italian Istituto Nazionale di Astrofisica (INAF)—Osservatorio Astronomico di Roma and the Harvard-Smithsonian Center for Astrophysics. 

The results are the culmination of a 20-year search, Shull said. In the 1990s, astrophysicists came up with an estimate of how many hydrogen and helium atoms had been cooked up in the Big Bang. These baryons are distinct from the dark matter that makes up the bulk of the universe’s mass—and that scientists have yet to find. 

Researchers think they know where most of that ordinary matter wound up: about 10 percent in galaxies and close to 60 percent in diffuse clouds of gas filling the vast spaces between galaxies. But that still left the census a little more than 30 percent short.

That’s where Shull and his colleagues came in. In 2012, he and Charles Danforth, a research associate at CU Boulder, suggested that those baryons were likely in a web-like pattern in space called the warm-hot intergalactic medium (WHIM). It’s a wild terrain.

“This is where nature has become very perverse,” Shull said. “This intergalactic medium contains filaments of gas at temperatures from a few thousand degrees to a few million degrees.”

To search for missing atoms in that perverse territory, the international team pointed a series of satellites at a quasar called 1ES 1553—a black hole at the center of a galaxy that is consuming and spitting out huge quantities of gas. “It’s basically a really bright lighthouse out in space,” Shull said. 

Scientists can glean a lot of information by recording how the radiation from a quasar passes through space, like a sailor seeing a lighthouse through fog. First, the researchers used the Cosmic Origins Spectrograph on the Hubble Space Telescope to get an idea of where they might find the missing baryons in that fog. Next, they homed in on those baryons using the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton) satellite.

The team found the signatures of a type of highly ionized oxygen gas lying between the quasar and our solar system—and at a high enough density to, when extrapolated to the entire universe, account for the last 30 percent of ordinary matter.

“We found the missing baryons,” Shull said.

Shull said that the researchers will need to confirm their findings by pointing satellites at more bright quasars. He and Danforth, a co-author on the new study, will also explore how the oxygen gas got to these vast pockets of space. They suspect that it was blown out over billions of years from galaxies and quasars, but how is an open question.

“How does it get from the stars and the galaxies all the way out here into intergalactic space?” asked Danforth, who is also in APS. “There’s some sort of ecology going on between the two regions, and the details of that are poorly understood.”

Credit: colorado.edu

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