The context of discovery: Lemaître and the origin of the primeval-atom universe

In spite of the paradigmatic status of the Big Bang model of the universe, the genesis of this idea has never been examined in detail. This paper investigates how the Belgian physicist and cosmologist Georges Lemaître in 1931 arrived at the hypothesis that the universe had begun in a Big Bang, or what he called a ‘primeval atom’. Four years earlier, he had suggested a closed expanding model in which the universe slowly inflated from an equilibrium Einstein state, but in 1931 he advocated an abrupt beginning from an initial, superdense concentration of nuclear matter. Why did Lemaître believe that the universe had a definite beginning a finite time ago? It turns out that the law of increase of entropy was one motivation, and that the existence of long-lived radioactive substances was another. Contrary to what is often stated, he most likely had the idea of an exploding universe before 1931. Among his chief inspirations to think about the origin of the universe, we draw attention to his persistent fascination of light as the primeval state of the world. Although this idea was originally seen in a theological perspective, religion played no direct role in Lemaître's hypothesis of 1931.