A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-galactic Nebulae

As already pointed out in a previous Golden Oldie devoted to the Lemaitre’s short note of 1931 which can be considered as the true “Charter” of the modern big bang theory 1, although the Belgian scientist was primarily a remarkable mathematician and a theoretical physicist, he stayed closely related to astronomy all his life and always felt the absolute need for confronting the observational data and the general relativity theory. This basic fact explains why as soon as 1927, while still a beginner in cosmology, he was the first one to be able to understand the recent observations on the recession velocities of galaxies as a natural consequence of dynamical cosmological solutions of Einstein’s field equations.1 Before examining in detail the contents of his outstanding article, let us summarize the road which, in the few preceding years, led the young Lemaitre to the expanding universe (see e.g. 6). In 1923, the same year as he was ordained as a priest, Georges Lemaitre obtained a 3-year fellowship from the Belgian government, enabling him to study abroad.