Don Savage Headquarters, Washington, DC June 12, 1995 (Phone: 202/358-1547) Emil Venere Johns Hopkins University, Baltimore, MD (Phone: 410/516-7160) RELEASE: 95-87 ASTRO-2 PROVIDES FIRST DEFINITIVE DETECTION OF PRIMORDIAL HELIUM Astronomers using NASA's Astro-2 observatory today announced the first definitive detection of one of the two original building blocks of the universe -- the element helium created in the Big Bang explosion. The finding that the chemical element helium is widespread in the early universe confirms a critical prediction of the Big Bang cosmological theory and provides clues to several other major mysteries in astronomy. The announcement was made by Dr. Arthur Davidsen of Johns Hopkins University, Baltimore, MD, at a meeting of the American Astronomical Society in Pittsburgh, PA. Davidsen is the Principal Investigator for the Hopkins Ultraviolet Telescope (HUT), one of three ultraviolet (UV) instruments on the Astro-2 observatory which was operated in the payload bay of the Space Shuttle Endeavour during a 17-day mission in March of this year. "It's a very rewarding feeling to find that we actually have achieved what we set out to do at the beginning of the project 17 years ago," said Davidsen, a professor in the Johns Hopkins Department of Physics and Astronomy. "It certainly helps confirm the Big Bang." "This long-sought primordial helium represents a major milestone in astronomy and is the most significant achievement for the very successful Astro-2 mission," said Dr. Daniel Weedman, Director of NASA's Astrophysics Division in Washington, DC. The data enabled scientists to estimate the abundance of helium and hydrogen in the primordial universe, confirming predictions made by the standard Big Bang theory as to how much gas was produced at the beginning of the universe. The observation also has allowed astronomers to detect a portion of the invisible "dark matter" in the early universe, a discovery that might shed light on what constitutes some of the "missing mass" in today's universe. Confirming the Big Bang Theory The findings matched an important prediction of the Big Bang theory -- that a primordial mixture of helium and hydrogen was created at the birth of the universe. By showing that significant amounts of helium existed in the early universe, the discovery reaffirms the theory that the chemical elements hydrogen and helium were formed in the first three minutes after the Big Bang. The heavy elements, (carbon, nitrogen, oxygen, silicon, iron, etc.) come from nuclear reactions in the centers of stars, and thus didn't form until some time after the Big Bang. Davidsen said HUT's mission on Astro-2 was the culmination of his goal, conceived 17 years ago, to find the hypothetical "primordial intergalactic medium" created by the Big Bang. He reasoned that astronomers should be able to detect the helium gas by using a spectrograph in space to measure within a range of light called the far ultraviolet spectrum. Hopkins astronomers were able to detect the helium by analyzing far ultraviolet light from a distant quasar called HS1700+64, about 10 billion light years away. By observing such a remote object, astronomers were essentially looking back to a time when the universe was less than a quarter of its present age, a time when most of the original hydrogen and helium gas produced by the Big Bang had not yet condensed into stars and galaxies. As ultraviolet light from the quasar shines through the vast intervening space, it also shines through the intergalactic medium of hydrogen and helium, like a headlight through fog. Intense radiation from early galaxies and quasars completely ionized the hydrogen (stripped the hydrogen atoms of their single electrons), making hydrogen atoms invisible to detection by spectroscopy because they cannot absorb any of the quasar's light. But helium atoms in their natural state have two electrons; some helium atoms retained an electron despite the ionizing radiation, and HUT was able to detect the small portion of helium atoms that were not fully ionized. From the data collected, astrophysicists are able to calculate how much total intergalactic helium and hydrogen may exist. The degree of helium absorption detected by the spectrograph suggests that a massive amount of gas was present in the intergalactic medium about 10 billion years ago. "We are only seeing the tail of the dog," Davidsen said. "It's enough of a tail to know that it's a very big dog." Astronomers have been searching for the primordial gas for 30 years, ever since astrophysicists James P. Gunn and Bruce Peterson first 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. But scientists, using a variety of telescopes and instruments, were not able to detect the primordial hydrogen and concluded that it may have been completely ionized by intense radiation. To detect the primordial medium, astronomers decided to focus on the helium instead. A major stumbling block in confirming the intergalactic medium's existence has been the technical difficulty involved in detecting the helium. The far ultraviolet 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 far away that their light is heavily "contaminated" by galaxies in the foreground, yet they are distant enough that their light is stretched into the proper redshift to be observed. The HUT data also appear to have provided a partial answer to the puzzle of dark matter. The observable universe adds up to no more than one percent of the mass required to produce the gravitational force that seems to be present. 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. Calculations based on HUT's data show that the primordial hydrogen and helium are about equal to the total amount of baryonic dark matter scientists believe exists, Davidsen said. - end - NASA press releases and other information are available automatically by sending an Internet electronic mail message to domo@hq.nasa.gov. In the body of the message (not the subject line) users should type the words "subscribe press-release" (no quotes). The system will reply with a confirmation via E-mail of each subscription. A second automatic message will include additional information on the service. Questions should be directed to (202) 358-4043.