Astronomers have known for some time that they can see only a fraction of the matter that must exist in the universe. New observations with NASAs Chandra X-ray Observatory have detected at least the shadows of some of the rest, astronomers announced today.
Four separate research groups used Chandra to detect hot intergalactic gas that connects galaxies and clusters of galaxies. Astronomers described the filamentary structures as being like fog in river channels, and the gas is thought to represent more matter than all the stars of the universe combined.
The researchers said the cosmic web of hot gas probably also contains exotic dark matter, which is theorized to exist but has never been detected or defined. Most theorists believe that dark matter and dark energy comprise more of the mass/energy budget of the cosmos than the things we can readily observe.
"We had strong suspicions from the Big Bang theory and observations of the early universe that this gas exists in the present era, but like a stealth aircraft it had eluded our detection," said Claude Canizares of the Massachusetts Institute of Technology (MIT), who along with Taotao Fang, led of one of the teams.
The research will be reported in four separate papers in the Astrophysical Journal.
"The Chandra observations, together with ultraviolet observations, are a major advance in our understanding of how the universe evolved over the last 10 billion years," said Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Ma. Nicastro is head of one of the teams of scientists involved in the discovery.
The hot gas can be used to trace the presence of the more massive dark matter component, the researchers say, and the new work may eventually enable astronomers to map the distribution of dark matter in the universe and perhaps understand its origin.
Scientists think dark matter exists because the regular matter they can see is not enough to keep galaxies from flying apart. The presence of dark energy is inferred because the universe is accelerating at an ever-faster pace, and some repulsive force is needed to explain that.
In the work announced today, some of the observations were done by watching what happens as light passes through the foggy cosmic rivers. On their way to Earth, X-rays from a distant galaxy dim as they pass through the rivers of intergalactic gas. By measuring the amount of dimming due to oxygen and other elements in the cloud, astronomers estimated the temperature, density and mass of the absorbing gas.
In previous research, ultraviolet telescopes had detected cooler components of the gas, but because of its high temperatures most of it is detectable only with an extremely sensitive X-ray telescope, astronomers said.
During the first few billion years of the universe, theory holds, about 20 percent of the matter came together under the influence of gravity to form groups and clusters of galaxies. Scientists think most of the remaining normal matter and dark matter formed an immense filamentary web connecting the clusters of galaxies. It is predicted to be so hot that it would be invisible to optical, infrared, and radio telescopes.
"Computer simulations have been telling us for several years that most of the missing gas in the universe should be in hot filaments," said Smita Mathur, an Ohio State University researcher and leader of one of the new studies. "Most of those filaments are too faint to see, but it looks like we are finally finding their shadows."
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