About 10 years ago, observations of distant supernovas suggested that the universe is growing at an accelerating rate. Those stunning observations, and many corroborating studies since, determine the recessional velocity of objects as a function of the time when they emitted the light presently observed on Earth. Now Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics and colleagues have taken a completely different approach to probing the properties of the mysterious "dark energy" attributed with driving the cosmic acceleration. They have observed how the expansion of space retards the gravitational clumping together of mass to form clusters of galaxies. They began with detailed x-ray images, taken by the Chandra X-ray Observatory, of two sets of galaxy clusters. One set comprises clusters whose light was emitted roughly 5.5 billion years ago; the other includes nearby clusters. Recent advances in theoretical modeling enabled the group to compare cluster masses across the two sets. The mass observed in the nearby clusters, says Vikhlinin, is about one-fifth what it would be without the inhibiting effects of dark energy. Further, the Chandra observations allow the Vikhlinin team to estimate both the quantity of dark energy and the key equation of state parameter w. The Chandra-derived values agree well with those determined in earlier investigations. (A. Vikhlinin et al., Astrophys. J., in press; A. Vihlkinin et al., Astrophys. J., in press.) — Steven K. Blau