2006 年 39 巻 2 号 p. 1-15
Einstein began the relativistic cosmology by his 1917 paper. But what is his most important contribution to the cosmology? I will argue that the answer is that he changed the notion of gravity so as to imply both the attractive and the repulsive force. Beginning with the elements of the general relativity, I will briefly review de Sitter's objection against Einstein's spherical universe, Lemaitre's and Eddington's works, the classic version of the expanding universe, the big-bang cosmology, and the inflationary cosmology. The seminal ideas emerged from the Einstein-de Sitter controversy culminated in the fruitful results of the inflationary cosmology, and the key idea was the gravity as a repulsive force.