Newtonian Universes and the Curvature of Space

The authors demonstrated that all the basic achievements of theoretical cosmol-ogy up to the 1930s could have been exactly deduced from Newtonian hydrody-namics in the 18th century or even earlier. The equations governing the motion of a homogeneous and isotropic cosmic medium in the Newtonian theory are exactly the same as in the Robertson–Walker models. Relativity theory would become significant at distances much larger than those within the field viewed by observers up to that time. The difference between the k 0, k c 0 and k 0 Robertson–Walker models has a very simple explanation in the Newtonian mod-els: the matter moves with a velocity smaller than, equal to or greater than the escape velocity from its own gravitational field, respectively. This is what the authors have proved. Had this all been done 200 years earlier, cosmology would have had a chance to become, already at that time, a natural science rather than part of philosophy and metaphysics. It is tempting to wonder: if the Milne–McCrea model was so simple, why had nobody even tried to find it earlier? The embarrassing idea that suggests itself most naturally is this: