The Stuff of the Big Bang
Star crossed" doesn't begin to describe Arthur Davidsen's space odyssey. His pride and joy -- the Hopkins Ultraviolet Telescope (HUT) -- moved into the on-deck circle at Cape Canaveral in late 1985, already seven years after he had proposed flying the $16 million instrument aboard a space shuttle. The HUT would do no less than detect the oldest stuff in the universe: helium created in the inferno of the big bang. But in January 1986 the Challenger blew up. It killed all seven crew members, put the brakes on the shuttle program and bumped Davidsen,s telescope, scheduled for the next shuttle, to the back of the line. The Johns Hopkins University astrophysicist got another shot in 1990, but on that shuttle ground controllers couldn't get the HUT to point the right way. Finally, the HUT got a ticket on last March's Endeavor flight.
It was worth the wait. Last week, at a :meeting of the American Astronomical Society in Pittsburgh, Davidsen announced that the HUT had detected individual helium atoms made less than three minutes after the big bang. The HUT snared ultraviolet light from a quasar, in the constellation Draco, some 10 billion light-years away. The bright quasar shines like a flashlight in a Turkish bath. Just as steam absorbs light, so does the helium: the light spectrum that the HUT measured left no doubt that worlds upon worlds of helium fill intergalactic space. Because there is so much of it, and because it is spread out so uniformly and diffusely between galaxies, Davidsen concludes that the helium was not created in the nuclear furnaces of stars but was made when the world was new.
The finding shores up the much-maligned big-bang theory by showing that the universe is permeated with the kind of atoms, in the right amount, that the theory predicts. It also addresses the nagging question of what the universe is made of. Both theory and observation suggest that only 10 percent of the universe is made of normal matter like that forming atoms, galaxies, planets and people. (The rest is some exotic, undiscovered "dark matter.") The huge quantities of intergalactic helium, says Davidsen, account for so much of the normal matter that the mass of all the trillions of stars and billions of galaxies amounts to no more than a rounding error.
Another missing member of the cosmos turned up at last week's AAS meeting: the solar system's comet reservoir. Astronomers led by Anita Cochran of the University of Texas at Austin reported that they had used the Hubble Space Telescope to spy the cosmic equivalent of a taxicab holding pen. Just beyond Neptune, and stretching for at least 40 billion miles, they glimpsed a disc containing at least 200 million frozen, comet-size bodies. The most crowded region of the solar system, it appears to be the long-theorized Kuiper Belt, filled with icy chunks that are sometimes kicked out of orbit by the gravity of Uranus, Neptune, Saturn or Jupiter and sent hurtling toward the sun in orbits lasting 20 years or less. Comet Shoemaker-Levy, which bombarded Jupiter so spectacularly last summer, is one of these short-period comets; long-period comets like Halley's come from a belt called the Oort Cloud, much farther away. If Cochran is right, the solar system is many times bigger than anyone knew. And there are enough comets-in-waiting to put on sky shows, like the Jupiter pelting, until long after the sun flickers out.