Media Memo

With the Astro-2 mission coming to an end with the shuttle Endeavour's scheduled landing Friday, astronomers working with the Hopkins Ultraviolet Telescope believe they have collected information that may enable scientists to answer a fundamental question of cosmology: was an "intergalactic medium" of hydrogen and helium created in the Big Bang of cosmic creation?

"Well, I can tell you now that we succeeded in getting the data we need to try to answer that question," said Arthur F. Davidsen, who heads the HUT project. "It's too early to tell what the answer will be exactly, but either way, it's going to be very interesting. The data are just spectacular."

"We are searching for the stuff that was created in the Big Bang itself, the original hydrogen and helium gas that was created in the first few minutes after the explosion of the universe, that created everything we see today," he said during a news conference Thursday at Marshall Space Flight Center in Huntsville, Ala.

The search for the intergalactic medium was one of many exciting science programs pursued by HUT. Astronomers used the telescope for a wide range of observations, peering into distant reaches of space and around our own solar system.

"We feel that we are likely to uncover some tremendous new secrets of the universe through those observations," said Dr. Davidsen, a professor in the Johns Hopkins Department of Physics and Astronomy.

Astro-2 was a followup to the Astro-1 mission, on the space shuttle Columbia in 1990.

"We eventually accomplished some considerable success with Astro- 1, but this Astro-2 mission is really in another category altogether," Dr. Davidsen said. "I feel like I'm floating on Cloud 9 now."

HUT astronomers collected four to five times as much data as they did with Astro-1. And the data were better quality because HUT had been improved since its debut, making it about two and a half times more sensitive than it was on Astro-1.

Many characteristics about the universe can be studied best by analyzing ultraviolet light. HUT detects a portion of the ultraviolet spectrum that is rich in such data, promising to reveal a wealth of astrophysical information.

The search for the primordial intergalactic medium has potential to help shape the future of cosmology, Dr. Davidsen noted. Scientists will take a few days off and then start analyzing data when they return to Johns Hopkins next week, he said.

The standard Big Bang theory says that hydrogen and helium, the two most simple elements, were created within the first few minutes after the explosion that marked the birth of the universe.

"That gas is supposed to have expanded and cooled and eventually condensed here and there to form the galaxies, including our own galaxy, and all the stars and planets we have," Dr. Davidsen said.

After the stars were formed by this original gas, the heavier elements were manufactured inside stars and distributed in space by stellar explosions. Therefore, everything that exists is, according to theory, a product of the original hydrogen and helium that HUT was used to search for.

"We are looking back in time to try to find that gas, that original material out of which all of us are now made," Dr. Davidsen said. "We want to see if it's really there, and if so, how much is there."

Astronomers used HUT to make 14 observations of two distant quasars, billions of light years away. The astronomers concentrated on one of the quasars, called HS1700+64, about 10 billion light years away. Because the objects are billions of light years away, observing their light is like looking back in time. The universe is an estimated 12 billion to 15 billion years old.

HUT's spectrograph was used to study the ultraviolet light shining through the void between galaxies. As the light passes through the medium presumed to fill the void, the gas should absorb a portion of the ultraviolet spectrum, producing a distinct spectrographic signature.

No other ultraviolet telescopes can detect the range of ultraviolet radiation that is ideal for the intergalactic medium research. HUT is sensitive to a range of ultraviolet light called the far ultraviolet spectrum. The telescope's 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, they are just the right distance that their light has been shifted to the proper frequency to bring the helium absorption into HUT's view. The farther away an object is in space, the faster it is receding from our galaxy, so the more its light is being stretched into longer wavelengths.

Scientists have been searching for the intergalactic medium for 30 years, since astrophysicists James P. Gunn and Bruce Peterson first postulated that the hydrogen could be detected by analyzing ultraviolet light from distant quasars, the most luminous objects in the universe. 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. However, helium has two electrons, making it possible that some of the helium atoms have not been fully ionized. If some of the helium atoms have retained an electron, they should absorb a portion of the ultraviolet light from quasars, making the gas detectable with the right kind of ultraviolet instrument.

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