Artful Discovery Once upon a time, Hopkins astrophysicist Arthur Davidsen dreamed of proving that the Big Bang really happened. This week in Pittsburgh, Davidsen and his scientific team make that dream a reality... Emil Venere ------------------------------ Homewood News and Information It has been likened to the cosmic soup from which the stars, and ultimately everything else in the universe, formed--the "primordial intergalactic medium" of hydrogen and helium produced in the first three minutes after the birth of the cosmos. Finding it would provide a solid piece of evidence for the standard Big Bang theory, which proposes that the universe was born billions of years ago, exploding outward in a giant cosmic explosion of nearly infinite density and temperature. Using a telescope sensitive to ultraviolet light, a team of Hopkins astronomers led by Professor Arthur Davidsen has looked back in time, making the first definitive detection of the primordial helium. From that data, they were able to prove that the intergalactic medium exists. Also involved in the work were Hopkins astrophysicists Gerard A. Kriss and Wei Zheng. The discovery was announced June 12, during a meeting of the American Astronomical Society in Pittsburgh. The physicists used the Hopkins Ultraviolet Telescope on NASA's Astro-2 observatory, which flew aboard the space shuttle Endeavour in March. The findings confirm a critical prediction of the Big Bang theory--that helium should have been widespread in the early universe, long before the gas condensed to form the first stars. The heavier elements, such as carbon, nitrogen, oxygen, silicon and iron, came from nuclear reactions in the centers of stars, and thus didn't form until some time after the Big Bang. In addition to finding the helium, Hopkins astronomers have used HUT's data to estimate the total density of hydrogen and helium in intergalactic space; the calculations confirm predictions made by the Big Bang theory as to how much ordinary matter was produced at the beginning of the universe. The data also allowed astronomers to account for a portion of the invisible "dark matter" called baryonic matter, in the early cosmos, a discovery that might help to identify some of the "missing mass" in today's universe. The findings are the culmination of Davidsen's goal, conceived in the late 1970s, to find the hypothetical intergalactic medium. He reasoned that astronomers should be able to detect the helium portion of the gas mixture by using a spectrograph in space to measure a range of light called the far ultraviolet 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 Department of Physics and Astronomy. But detecting the primordial helium was never a certainty. The observations would depend on the existence of a quasar just the right distance from Earth. Light from the distant object would shine through the ancient helium, like a headlight through fog, enabling astronomers to detect its existence with a spectrograph. "When we started the project, we didn't even have a good quasar to look at to do this, but we thought we might find one," Davidsen said. "That was a leap of faith back in the late '70s, that there would be enough quasars bright enough." The gamble paid off. German astronomers in the late 1980s discovered a quasar just the right distance. It was named HS1700+64, and it's about 10 billion light-years away in the general direction of the constellation Draco. Quasars, the most luminous objects in the universe, are extremely distant objects that look like stars when viewed through a conventional telescope but actually emit more energy than an entire galaxy. They were common long ago, when the universe was young, and some scientists believe they played a role in the formation of galaxies. Astronomers had been searching for the intergalactic medium 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. 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, Davidsen decided to focus on the helium instead. The Big Bang theory proposes that a large amount of helium should have been present in the early universe, said Kriss, an associate research professor in the Department of Physics and Astronomy who has been with the HUT project for more than a decade. "Everyone was wondering, well, where is it?" he said. "And by golly, we finally see it." A major obstacle in confirming the intergalactic medium's existence has been the technical difficulty involved in detecting the helium. It requires a telescope sensitive to wavelengths of radiation falling within 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. Hopkins astronomers were able to detect the helium by analyzing ultraviolet light from HS1700+64. 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. Intense radiation from quasars apparently has completely ionized the hydrogen, stripping hydrogen atoms of their single electrons, which makes them 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 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." 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 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. Meanwhile, the HUT data 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 now show that the primordial hydrogen and helium are about equal to the amount of baryonic dark matter scientists believe exists in today's universe, Davidsen said.
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