  Of the first two relativistiv world models, only the one by de Sitter
predicted redshifted spectra from far away astronomical objects. Slipher's
redshifts therefore seemed to arbitrate against Einstein's model which made no
such predictions. Both models were trying to describe a static universe.
However, Lemaitre found that de Sitter's construct resulted in a spatially
inhomogeneous universe. He then opted for a model that correspondes to
Einstein's closed, curved universe but allowed the radius of curvature to
change with time. Slipher's redshifts suggested to him that the universe is
dynamic and expanding. We also discuss the respective merits of Friedman and
Lemaitre in revealing the dynamic nature of the universe.
