Credits: Cover image courtesy of Pantheon Previous image Next image In 1929, astronomer Edwin Hubble, using data from the Mount Wilson Observatory in California, found that the universe is expanding. This was “probably the most important cosmic discovery of all time,” writes Alan Lightman in his new book, “Probable Impossibilities: Musings on Beginnings and Endings.” Certainly it is among the most thought-provoking. Hubble’s discovery complicates how we grasp space and time. Can you picture a universe that expands infinitely? And if it is expanding, it must have had a starting point in time. But what existed before that, and how did things get started?