This essay describes the life of the outstanding Soviet scientist A. A. Fridman (1888--1925), known in the western literature as Friedmann, in connection with the centenary of his birth. The main stages in the life of Fridman are discussed in chronological order, beginning with his years at high school and university. Archive files are used in an account of the first steps of Fridman's scientific activity dealing with the problems in mathematical physics and aerodynamics. His participation in the First World War (1914--1917) is then described, followed by his professorship in Perm' University (1818--1920) and finally by the last very fruitful part of his life in Petrograd (1920–1925) where he taught at leading higher educational establishments and worked at the Main Geophysical Laboratory of which he became Director during the last year of his life. Particular attention is given to Fridman's work on relativistic cosmology (1922--1925) and dispute on the subject between Fridman and Einstein in connection with Fridman's theory of an expanding universe. The full bibliography of Fridman's work (73 items) is included.