"Many of the theoretical investigations of cosmology in the 1920s were examinations of De Sitter's model, which is particular because it can be understood both as a static model (as De Sitter did) and as an expanding model (as became the view after 1930). It is clearly problematic to read the pre-1930 literature in light of later knowledge. 'Expanding' versions of the De Sitter universe were found by in 1922, Hermann Weyl in 1923, and Lemaître in 1925, but were not conceived as expanding in any real sense. These works, as well as later works by Howard Percy Robertson and Richard Chase Tolman in the United States, consisted in transforming De Sitter's line element in such a way that it formally became static, that is, included a term F(t) referring to the time parameter. The metric, giving the distance in space-time between two neighboring points, would then be in the form ds^2 = c^2 dt^2 - F(t)(dx^2 = dy^2 +dz^2)."