News Release

The findings, using data from the Hopkins Ultraviolet Telescope (HUT) on NASA's Astro-2 observatory, confirm a critical prediction of the Big Bang cosmological theory -- that the chemical element helium should be widespread in the early universe. The data also enabled scientists to estimate the abundance of helium and hydrogen in the primordial universe; the calculations confirm predictions made by the standard Big Bang theory as to how much gas was produced at the beginning of the universe.

The observation has allowed astronomers to account for a portion of the invisible "dark matter" called baryonic matter, in the early universe, a discovery that might shed light on what constitutes some of the "missing mass" in today's universe.

The new findings also reveal the physical conditions that existed in intergalactic space at a time when the universe was only a fraction of its present age.

The data for the findings came from observations with HUT, one of three ultraviolet instruments on the Astro-2 observatory, which operated within the payload bay of the space shuttle Endeavour during a 17-day mission that ended March 18.

Arthur Davidsen, a Johns Hopkins University astrophysicist who heads the HUT project, will discuss the findings at a 9:30 a.m. news conference June 12, during a meeting of the American Astronomical Society in Pittsburgh. He will present a paper, "First Results from the Hopkins Ultraviolet Telescope on Astro-2," at 2 p.m. EDT on Tuesday, June 13.

Collaborating with Davidsen in this research are Johns Hopkins astrophysicists Gerard A. Kriss and Wei Zheng. A paper describing the results will be submitted for publication shortly in a scientific journal.

HUT's mission on Astro-2 was the culmination of Davidsen's 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 a range of light called the far ultraviolet spectrum. The Hubble Space Telescope observes a different portion of the spectrum.

"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 our theories about the origin of the universe in a Big Bang."

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 intergalactic space 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.

Hopkins astronomers were able to detect the helium by analyzing 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 -- about 10 billion years ago -- a time when most of the original hydrogen and helium gas produced by the Big Bang apparently had not yet condensed into stars and galaxies.

As ultraviolet light from the quasar shines through the vast intervening space, it also shines, like a headlight through fog, through the intergalactic medium of hydrogen and helium. Intense radiation from early galaxies and quasars apparently has completely ionized the hydrogen (stripped 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 of them have 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, scientists are able to calculate how much total intergalactic hydrogen and helium 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."

The degree of the helium's ionization also enabled the Hopkins astronomers to determine another important detail: while scientists have debated whether quasars or hot stars in young galaxies were more likely to have generated the ionizing radiation, the new HUT data show that quasar radiation is the most likely source, Davidsen said.

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 would have to focus on the helium instead.

A major obstacle in confirming the intergalactic medium's 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 far away that their light is heavily "contaminated" by clouds of gas and galaxies in the foreground, yet they are distant enough that their light is stretched into the proper redshift to be observed from within our galaxy. Hydrogen gas between the stars of our own galaxy makes the Milky Way opaque to ultraviolet light below a certain redshift. The more distant an object is in space, the faster it is moving, and the more its light has been stretched, or shifted, to longer wavelengths.

Ultraviolet measurements made with the Faint Object Camera on NASA's Hubble Space Telescope in 1994 uncovered a suspected spectral signature of primordial helium. Astronomers had to wait, however, for HUT's far-UV sensitivity and higher resolution to make definitive measurements.

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 1 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 amount of baryonic dark matter scientists believe exists, Davidsen said.

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