The two paths led Georges Lemaitre (1894-1966), the Belgian priest who became a founding father of modem cosmology, to the farthest reaches of the universe. The centenary of his birth fell on 17 July; his face, accompanied by the “primeval atom”, is currently featured on Belgium’s stamps.

Born in Charleroi, in the heart of French-speaking Belgium, he planned to become an engineer. But service in the First World War turned his thoughts to the priesthood, and he was ordained by Cardinal Mercier in 1923. After further mathematical studies in Cambridge and in the USA he returned to Louvain in 1927 as Professor of Astronomy, and remained there (apart from frequent travels) for the rest of his career.

The 1920’s were exciting years in astronomy. In 1919, the team led by Arthur (later Sir Arthur) Eddington, Professor of Astronomy at Cambridge, to the island of Principe had observed the bending of light near the sun that provided the first proof of Einstein’s general theory of relativity. In 1925, Hubble’s observations yielded the first evidence that the distant nebulae are separate galaxies, and that they are receding from us.

Lemaitre's achievement in those years was to show, in a famous paper of 1927, that the Einstein model of the universe could not be static: it must either expand or contract.