The idea that the universe is expanding was one of the most revolutionary and unsettling findings of modern astronomy. But the discovery was not made by Edwin Hubble at the Mount Wilson Observatory in California in 1929, as so many textbooks suggest. The germ of the idea actually arose in the halls of MIT and Harvard, a few years before Hubble initiated his historic measurements of galaxy motions. It hatched in the mind of a Jesuit priest then studying at the Institute's physics department.

A military hero, Georges Lemaître had received the Croix de Guerre for his service in the Belgian artillery after Germany invaded his homeland in World War I. He went on to earn a doctorate in mathematics at the Catholic University of Louvain; afterward, perhaps affected by the horrors he observed from the trenches, he enrolled in a seminary. Although he was ordained in 1923, the Church permitted him to continue his scientific pursuits. Captivated by the beauty of Einstein's new general theory of relativity, the abbé proceeded to the University of Cambridge to broaden his understanding of the theory's equations under the guidance of the astrophysicist Arthur ­Eddington, who deemed his student "exceptionally brilliant."

In 1924, after a year in England, Lemaître traveled to the United States to study at Harvard's observatory and enroll in MIT's PhD program in physics. His dark hair combed straight back and his cherubic face adorned with round glasses, he could easily be spotted on the college campuses by his attire--a black suit or an ankle-length cassock, set off by a stiff white clerical collar. Some could find him just by following the sound of his full, loud laugh, which was readily aroused.

In pursuit of his second PhD, Lemaître became interested in applying general relativity to the universe at large, which many in the 1920s believed to consist entirely of our own galaxy. By then totally absorbed by astronomy, he made sure to attend the 1925 meeting of the American Astronomical Society in Washington, DC, where a crucial discovery was announced: Edwin Hubble had proved that certain spiral nebulae, previously thought to be gaseous clouds within the Milky Way, were actually separate galaxies far beyond its borders.

While others in the room were focused on Hubble's revelations about the true nature of these long-perplexing nebulae, ­Lemaître was two jumps ahead. Though new to astronomy, he quickly realized that the newfound galaxies could be used to test certain predictions that general relativity made about the universe's behavior. Soon after the meeting, Lemaître began formulating his own cosmological model.