Andrew Lange, an astrophysicist whose balloon-borne measurements of light left over from the Big Bang played a key role in elucidating the shape and nature of the universe, was found dead on Jan. 22 in Los Angeles. He was 52. His death was announced by Jean-Lou Chameau, the president of the California Institute of Technology, where Lange had until recently been chairman of the division of physics, mathematics and astronomy. Chameau said Lange appeared to have taken his own life. Lange devoted his career to a haze of faint microwaves that pervade the sky, providing a whispery ghost image of the universe when it was only 400,000 years old, before there were stars, galaxies or even atoms. In the late 1990s, this work put him in the middle of one of the grandest quests in science, the effort to discern the fate of the universe -- whether it would keep expanding forever or collapse one day into the fire from which it had emerged 14 billion years ago. According to Einstein, that fate was determined by the large-scale geometry of space time, which in turn is determined by the amount of matter and energy in the universe. Theorists had long favored the Euclidean, or 'flat,' geometry that students learn in school -- a triangle between three distant quasars would have internal angles that add up to 180 degrees -- but astronomers could not find enough matter in the universe to make it flat. Lange and his collaborators found that it was energy that was making it flat. Lange was co-leader, with Paolo de Bernardis of the University of Rome, of an experiment called Boomerang, short for Balloon Observations of Millimetric Extragalactic Radiation and Geophysics. During a 10-day flight at 120,000 feet over Antarctica in 1998, the gadget was able to detect and map faint ripples in the microwave glow, which could be used like the ticks on a ruler to gauge the cosmos and read off its geometry and other parameters. The universe, they reported in 1999, is indeed flat. These results not only suggested that it would expand forever; they also provided crucial support for a bold theory that the universe had undergone an explosive expansion, called inflation, when it was only a fraction of a fraction of a second old. Michael S. Turner, a cosmologist at the University of Chicago, said of Lange, 'He and his Boomerang team got one of the biggest prizes in cosmology: They were the first to show that the geometry of the universe is flat.' Two similar experiments reported similar results at the same time. Lange was part of one of them, the Maxima experiment, which was led by his former professor Paul Richards of the University of California, Berkeley. His former student Mark Devlin of the University of Pennsylvania was co-leader of the other, known as the Microwave Anisotropy Telescope. But astronomers believed that Boomerang produced the most convincing data. Marc Kamionkowski, a Caltech cosmologist, said, 'Boomerang was just crystal clear; it was the shot heard around the world.' The results were subsequently confirmed by the NASA's Wilkinson Microwave Anisotropy Probe, a satellite that has been mapping the entire microwave sky since 2001. These results also dovetailed with the strange discovery in 1998 that the expansion of the universe was speeding up, a discovery that many astronomers and physicists had viewed with suspicion. Most of the universe turned out to be not stars and galaxies or even the mysterious as-yet-unidentified dark matter surrounding them, but the so-called dark energy driving the acceleration of the universe. 'So there are two very different lines of evidence indicating that the universe is accelerating,' Lange said in an interview with the European Space Agency in 2001. 'I believe it now.' Andrew E. Lange was born in Urbana, Ill., on July 23, 1957, the oldest son of Joan Lange, a school librarian, and Albert Lange, an architect, and grew up in Easton, Conn. He attended Princeton University and then the University of California, Berkeley, where he received a Ph.D. in 1987 and was promptly hired as a professor. While he was at Berkeley as a Packard fellow, Lange met another Packard fellow, Frances Arnold, a chemical engineer, who had attended Princeton and Berkeley when he did, but whom he had never met before then. The pair held a wedding ceremony for family and friends in 1994, but never made it official with the state of California, Arnold said. That same year Lange joined Caltech. By then the discovery of the cosmic microwave background was transforming cosmology. Lange, working with James Bock, now at NASA's Jet Propulsion Laboratory, built new detectors, known as spider web bolometers, designed for studying the cosmic microwave work. The detectors were used on both Maxima and Boomerang and a series of subsequent cosmic experiments, including Planck, a satellite launched last year by the European Space Agency. Lange was part of the Planck team but, always eager to be in the forefront, was already working on ideas for new experiments. 'What will we discover with them?' Lange asked rhetorically in his 2001 interview. 'We don't know. The great thing here is that you cannot predict what's going to come next. Five years ago no one took the idea of the dark energy seriously!' His friends and colleagues described Lange as a warm man who always had a sparkle in his eye. 'He was one of those guys who had an aura of utter confidence and competency,' Devlin said. But he also struggled with depression. 'He cared immensely about other people and not enough about himself,' said Arnold, who is now a professor of chemical engineering at Caltech. At the time of Lange's death, they were separated. Arnold survives him, along with three children: James Bailey, her son from a previous relationship, whom Lange raised as his own; William A. Lange; and Joseph I. Lange. He is also survived by his parents and a sister, Karen Lange, of Washington, and a brother, Adam Lange, of St. Louis. 