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. Lemaitre 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.