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Office of News and Information
212 Whitehead Hall / 3400 N. Charles Street
Baltimore, Maryland 21218-2692
Phone: (410) 516-7160 / Fax (410) 516-5251

February 9, 1995
CONTACT: Emil Venere

Astrophysicists Devise New Method for
Investigating Dark Matter

(Editors: This release on recent Johns Hopkins research on the dark matter question also is relevant to coverage of the Astro-2 space shuttle flight, an ultraviolet astronomy mission now scheduled for a March 2, 1995, launch)

Technique will be used with data from March shuttle flight Two Johns Hopkins astrophysicists have devised a novel method for calculating how much of the original hydrogen and helium produced in the Big Bang remains diffused in the vast space between galaxies, knowledge that might help cosmologists identify a portion of the hypothetical "dark matter."

The technique could lead to a greater understanding of how the universe mysteriously evolved from its original state of smoothly distributed matter to its present clumpy condition in which clusters of galaxies are distributed unevenly. Although the well-accepted Big Bang theory suggests that the primordial "intergalactic medium" of hydrogen and helium should exist, it has not actually been detected.

Proving its existence and calculating its mass would provide vital clues in attempts to learn how the cosmos developed from its explosive birth, said Hopkins astrophysicists Arthur F. Davidsen and Wei Zheng.

"We're looking for much more detail about how the universe got from there to here," said Dr. Davidsen, a professor in the Hopkins Department of Physics and Astronomy. "Given that the Big Bang did what we believe it did, how did the galaxies form? The intergalactic medium is like a missing link in that chain of events that connects the Big Bang to what we have today."

Their innovative technique is outlined in a scientific paper to be published Feb. 20 in The Astrophysical Journal: Letters. The technique adds a new dimension to a well-known concept conceived 30 years ago by astrophysicists James P. Gunn and Bruce Peterson. The Gunn-Peterson effect postulated that scientists should be able to detect the hydrogen originally created in the Big Bang by analyzing the light from quasars, the most luminous objects in the universe. As the light passes through the presumed medium, the gas should absorb a portion of the ultraviolet spectrum, producing a distinct spectrographic signature.

But scientists, using a variety of telescopes and instruments, have not detected the primordial hydrogen. One theory is that the hydrogen has been ionized -- intense radiation has stripped hydrogen atoms of their single electrons -- rendering them unable to absorb light. But helium has two electrons, making it possible that some of the helium atoms have not been fully ionized. If a portion of the helium atoms have retained an electron, they should absorb some of the ultraviolet light from quasars, making the gas detectable with the right kind of ultraviolet instrument.

The Hopkins Ultraviolet Telescope, sensitive to a portion of the spectrum best suited for the research, is scheduled for a March 2 launch aboard the space shuttle Endeavour, on the Astro-2 mission. Dr. Davidsen and members of his science team plan to use HUT to search for signs of the primordial helium in the intergalactic medium.

But even with data from HUT, without the new method devised by Dr. Davidsen and Dr. Zheng, scientists would only be able to confirm the presence of helium. They would not be able to calculate how much total helium and hydrogen are present.

The Hopkins scientists have determined that the closer the helium is to a quasar, the more ionized it should be. That means researchers should be able to construct a "proximity profile" of changing ultraviolet absorption in the space near quasars. By analyzing the curving profile's shape, scientists plan to calculate the total mass of the hypothetical helium. The computation takes into account the theoretical ratio of hydrogen to helium in the universe. So, once the mass of helium is known, the total mass of gas can be determined.

Current theories propose that the intergalactic medium of gas is diffused throughout the universe between the galaxies. At the birth of the cosmos the two simplest elements, hydrogen and helium, were produced and evenly dispersed. The primordial gas subsequently condensed to form the stars and galaxies we see today. But some, or perhaps much of that gas may still remain. A major stumbling block in confirming its existence has been the technical difficulty involved in detecting the helium. HUT is sensitive to a range of ultraviolet light called the far ultraviolet spectrum. That spectral range is best suited to the search for the intergalactic medium because it enables astronomers to study quasars that are just the right distance from Earth: they are not so distant that their light is heavily "contaminated" by clouds of gas and galaxies in the foreground, yet their redshift is high enough to bring the helium absorption into HUT's view.

However, because even "nearby" quasars are very distant from Earth, astronomers will be looking back in time. HUT's target, a quasar known as HS1700+64, is located at such a distance that its light now reaching Earth would be about 75 percent of the age of the universe -- roughly 10 billion years old if the universe was born 12 billion to 15 billion years ago.

"We are looking for some evidence of this smooth intergalactic medium that was the original product of the Big Bang, trying to see how much was there, directly," Dr. Davidsen said. The intergalactic medium could help to provide an answer to a major riddle in cosmology: where is the "missing mass" in the universe. The observable universe adds up to no more than 1 percent of the mass required to produce the gravitational force that seems to be present throughout the universe. The standard Big Bang theory predicts that a portion of the remaining, unseen mass is in the form of normal, or baryonic matter -- the stuff people and planets are made of. Theories suggest that up to 10 percent of the missing mass is baryonic, and the rest is possibly some form of exotic matter -- perhaps a variety of unknown subatomic particles that are difficult to detect. The intergalactic medium is a popular candidate for at least a portion of the baryonic matter making up the missing mass.

Calculating the intergalactic medium's mass also could give scientists valuable clues about the radiation sources that existed in the early universe.

"What is the cause of the ionization?" Dr. Zheng asked. "Most of the diffuse matter in the early universe is highly ionized. The electrons are removed from the atoms. That means in the early universe there is very strong radiation that ionized everything, and we would like to know how strong are these radiation fields in the early universe, and what could have produced them."

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