  Before the discovery of the expanding universe, one of the challenges faced
in early relativistic cosmology was the determination of the finite and
constant curvature radius of space-time by using astronomical observations.
Great interest in this specific question was shown by de Sitter, Silberstein,
and Lundmark. Their ideas and methods for measuring the cosmic curvature
radius, at that time interpreted as equivalent to the size of the universe,
contributed to the development of the empirical approach to relativistic
cosmology. Their works are a noteworthy example of the efforts made by modern
cosmologists towards the understanding of the universe as a whole, its
properties, and its content.
