Now, in the past few years our view of the universe as a whole has changed radically. Fifteen years ago, a writer could be comfortable with one of three plausible choices: the universe was expanding, and the expansion would never slow down; or, second choice, the universe was expanding, but the expansion would proceed slower and slower, to produce a universe that was ultimately flat in a geometrical sense; or, the third alternative, the universe was expanding, but would eventually stop that expansion, reverse direction, and ultimately collapse back again in a “Big Crunch” fireball beyond which no information from our present universe could possibly survive. Even ten years ago, no scientist was in a position to rule out any one of the three choices.

These three options are no longer equally plausible. There is good evidence that the universe is not merely expanding, but the expansion is running faster than ever. Unfortunately, I earlier opted for the Big Crunch model. With an accelerating universe, any new edition would need a lot more than the fixing of small errors or minor inconsistencies.

How much more? Well, this book is now twenty-five percent longer than the first version, with a final new section undreamed of in the original. The universe in which the book exists has changed; and therefore, in what is in some ways the most important change of all, the ending must follow a quite different cosmology.

Suppose, five or ten years from now, there is another radical change in our understanding of the origins, nature, and future of the universe. Does this mean another major revision of the text?

The one thing that would change this book so profoundly that I am not sure that it could survive, even with the most radical revisions, would be the discovery that the speed of light is not a physical velocity limit for objects and signals.