Georges-Henri Lemaitre was a Belgian Catholic priest and physicist who first proposed what became known as the Big Bang theory of the origin of the universe. He studied physics and mathematics after World War I, where he served in the Belgian army. In his 1927 paper, he proposed that the universe began as a single primordial atom which exploded, supporting the findings of Edwin Hubble that the universe is expanding. Lemaitre's theory became the foundation of modern cosmology and earned him numerous honors, including being named the first president of the Pontifical Academy of Sciences.