Two of the three main ingredients for understanding the universe during the first half of the twentieth century were observational: its immense size and its expansion. The third was Einstein's general theory of relativity. It related the force of gravity to the structure of spacetime. Two years after his definitive account of the theory, Einstein (1917) applied it to cosmology. His first model, introducing the cosmological constant,was static – matter without motion. Shortly afterward deSitter (1917a,b) discovered an expanding but empty solution of Einstein's equations – motion without matter. Then Friedmann (1922) found the intermediate solutions with both expansion and matter, which Lemaître (1927) independently rediscovered. Eddington (1930, 1931a) was about to publish them independently yet again when Lemaître, who had formerly been his student, gently reminded him that they were already known. So Eddington publicized these solutions more widely and also showed that Einstein's static universe would become unstable if condensations formed within it.

A small fraction of cosmological thought during this period strayed from the homogeneous models to the nature and origin of structure in the universe.