The bestselling author of The Elegant Universe and The Fabric of the Cosmos tackles perhaps the most mind-bending question in modern physics and cosmology: Is our universe the only universe?There was a time when "universe" meant all there is. Everything. Yet, a number of theories are converging on the possibility that our universe may be but one among many parallel universes populating a vast multiverse. Here, Briane Greene, one of our foremost physicists and science writers,