If the talented Russian physicist Alexander Alexandrovich Friedmann had to be introduced with a single sentence, the most appropriate sentence would be the title of his biography translated from the Russian: Alexander A. Friedmann: The Man who Made the Universe Expand. Indeed, he was the first to realize in 1922 that Einstein's equations have solutions which describe not only a stationary Universe as Einstein initially believed, but also a non-stationary world. Friedmann won the debate with Einstein over the admissibility of such solutions, but his life was too short and he could not see the triumph of his views when the experimental evidence fully supported his predictions and demonstrated that the Universe was expanding. This book contains three papers by Friedmann - "On the Curvature of Space," "On the Possibility of a World with a Constant Negative Curvature of Space," and "On the Geometry of Curved Spaces." The third paper is a 28-page manuscript (dated 15 April 1922) which has not been published even in Russian. Unlike the existing two English translations of Friedmann's 1922 and 1924 papers (done from the German publications), now these papers are translated directly from the original Russian texts.