In 1931, Lemaître proposed in his "hypothèse de l'atome primitif" (hypothesis of the primeval atom) that the universe began with the "explosion" of the "primeval atom" — what was later called the Big Bang. Lemaître first took cosmic rays to be the remnants of the event, although it is now known that they originate within the local galaxy. Lemaître had to wait until shortly before his death to learn of the discovery of cosmic microwave background radiation, the remnant radiation of a dense and hot phase in the early universe.[6]

In his 1225 treatise De Luce (On Light), English theologian Robert Grosseteste explored the nature of matter and the cosmos. He described the birth of the universe in an explosion and the crystallization of matter to form stars and planets in a set of nested spheres around Earth. De Luce is the first attempt to describe the heavens and Earth using a single set of physical laws.[3]