An exploration of cutting-edge physics written for intelligent readers willing to pay close attention. Greene (Physics and Mathematics/Columbia Univ.; The Fabric of the Cosmos: Space, Time, and the Texture of Reality, 2004, etc.) points out that some astronomers believe that our universe is infinite. If so, the focus of his book, parallel universes, follows naturally. Mathematic calculations prove that an infinite universe contains innumerable regions with identical Milky Ways, solar systems and sentient life, plus many more regions differing in minor details. At this level, the existence of parallel universes is fairly simple. Thereafter, it becomes extremely complex. Essential to the current state of our cosmos is a momentary, massive expansion of the universe occurring an instant after the big bang: inflation. Some theorists maintain that this burst of expansion was not a one-time event but continues to occur, producing innumerable, parallel “bubble universes.” Near the beginning of the book, Greene pauses to recap string theory, the subject of his previous books. An unproven, complex but widely held system for explaining all forces and matter, it allows for half-a-dozen additional parallel universes such as the Cyclic, Brane and Landscape multiverses. The author makes imaginative use of charts, diagrams, photographs, analogies and anecdotes to describe difficult scientific ideas. His accounts of confirmed phenomena such as relativity, quantum theory and recent revelations in astronomy provide lucid explanations that will satisfy a lay audience. Where he delves into string theory, many readers will struggle, not always successfully. The latest cosmological discoveries and speculations, directed at a popular audience but definitely not oversimplified.