“The surest and deepest way to study the geometry of the world and the structure of our Universe with the help of Einstein's theory consists in the application of this theory to the whole world and in the use of astronomical research.” These words are from the last page of The World as Space and Time. Friedmann is speaking here about the cosmological problem, about the application of the general theory of relativity to the study of the world as a whole, of the world considered as a single physical system. By September 1922, when these words were being written, three different attempts had been made to solve the cosmological problem: one by Einstein himself, another by the Dutch astronomer W. de Sitter, and the third by Friedmann in the first of his two cosmological works. Cosmology was taking its very first steps, and the results so far did not look very encouraging. As Friedmann says after the above-quoted phrase, “mathematical analysis lays down its arms, faced with the difficulties of the question, and astronomical investigations do not yet give a sufficiently reliable basis for experimental studies of the Universe.” And nevertheless: “these circumstances cannot be seen as more than temporary difficulties; our descendants will undoubtedly come to know the character of the Universe in which we are fated to live … ”

However, events developed much faster than could be expected from the initial predictions. And first of all thanks to Friedmann's work. His first study, as soon became clear, was already a major step in the right direction. Cosmology was to develop rapidly even in the 1920s.