A cosmic conundrum.

This article discusses how a new incarnation of Einstein's cosmological constant may point the way beyond general relativity. Quantum mechanics and relativity, combined with recent evidence of an accelerating universe, have led physicists to resurrect the cosmological term that Einstein introduced and later repudiated. But this term now represents a mysterious form of energy that permeates empty space and drives an accelerated cosmic expansion. The efforts to explain the origin of this energy may help scientists move beyond Einstein's theory in ways that are likely to change our fundamental understanding of the universe. The heart of Einstein's general theory of relativity is the field equation, which states that the geometry of spacetime (G subμν, Einstein's curvature tensor) is determined by the distribution of matter and energy (T subμν, the stress-energy tensor), where G is Newton's constant characterizing the strength of gravity. INSETS: A Change of Meaning;Models of the Cosmos: Then and Now;A Checkered History.