Sometimes our understanding of our universe is given a huge boost by one insightful thinker. Such a boost came in the first half of the twentieth century, when an obscure Belgian priest put his mind to deciphering the nature of the cosmos. Is the universe evolving to some unforeseen end, or is it static, as the Greeks believed? The debate has preoccupied thinkers from Heraclitus to the author of the Upanishads, from the Mayans to Einstein. The Day Without Yesterday covers the modern history of an evolving universe, and how Georges Lemaitre convinced a generation of thinkers to embrace the notion of cosmic expansion and the theory that this expansion could be traced backward to the cosmic origins, a starting point for space and time that Lemaitre called “the day without yesterday.” Lemaitre’s skill with mathematics and the equations of relativity enabled him to think much more broadly about cosmology than anyone else at the time, including Einstein. Lemaitre proposed the expanding model of the universe to Einstein, who rejected it. Had Einstein followed Lemaitre’s thinking, he could have predicted the expansion of the universe more than a decade before it was actually discovered.