A book about universes - expanding, contracting, oscillating, time-travelling - from the bestselling author of 100 Essential Things You Didn't Know You Didn't Know. This is a book about universes. It tells a story that revolves around a single extraordinary fact- that Albert Einstein's famous theory of relativity describes a series of entire universes. Not many solutions to Einstein's tantalising universe equations have ever been found, but those that have are all remarkable. Some describe universes that expand in size, while others contract. Some rotate like a top, while others are chaotically unpredictable. Some are perfectly smooth, while others are lumpy. Some permit time travel into the past. Only a few allow life to evolve within them; the rest, if they exist, remain unknown and unknowable to conscious minds. Here, in The Book of Universes, we are confronted with the most fantastic and far-reaching speculations within the entire realm of science.