A peephole in time Analysis/Commentary If the universe was born in a showering explosion of heat and light, where's the afterglow? This question daunted scientists for centuries. In 1965, however, a land-based telescope confirmed the existence of a radiation throughout the known universe that conformed in appearance to this mythic afterglow. Then, in 1991, NASA's satellite, the Cosmic Background Explorer, confirmed the ever-present, ubiquitous shower of microwave rays now dubbed Cosmic Microwave Background. Last week, Nature magazine published the findings of the latest experiments seeking to measure and describe these cosmic debris. Knowing the large-scale structure of the CMB, scientists suspect they can discover the large structure of the universe, as well as trace back in time its early shape and make-up. In the beginning, as the Bible says, there was light -- a millisecond flash that swelled to enormous proportions, giving birth to matter, the stars and galaxies, and eventually to super-clusters that float on the surface of the universe. From a thin, hot soup, the rapid expansion expelled objects that transformed space into the waves Einstein described as the curvature of spacetime and Newton called gravity. Gravity, however, according to the new findings, was a late development. Even from the beginning, space was destined to be flat, to expand to a geometric shape that conformed to the calculations of Euclidean geometry. Overall, the universe presents a stable structure neither tending to collapse upon itself, nor to expand infinitely. The 12- to 15-billion year-old universe might be called entirely flat, if we didn't know there also existed such oddities as black holes, imploding stars, galactic clusters, and other Swiss-cheese-producing objects. The findings in Nature come from measurements made above the Earth's atmosphere by a large helium balloon, flying for ten days over Mount Erebus in Antarctica between December 1998 and January 1999. The balloon circled 5,000 miles to land only 31 miles from its launch site. The scientific team from NASA and the National Science Foundation appointed the Italian Paolo de Bernardis as their leader. May 3, 2000