Modern cosmology began in 1917 with Einstein and de Sitter. For the first few
years it was mainly a matter for the theoreticians with, however, Slipher's
redshifts as strong support for de Sitter. Eddington's book on general
relativity, published in 1923, got the observers going: in 1924/25 Wirtz,
Silberstein, Lundmark and Stromgren were inspired be de Sitter's redshift
prediction; also Hubble in 1929 referred to de Sitter. In 1925 Lemaitre found
the snare in de Sitter's theory; it violated the principle of homogeneity.
In 1927 he developed his theory of the dynamical universe, with the linear
velocity-distance relationship v=Hr. Based on Slipher's nebular redshifts and
Hubble's distances he showed the universe to be expanding, and calculated the
coefficient H. With Lemaitre's work Slipher's  redshifts were given a completely
different meaning to what they had in de Sitter's theory. The path from de Sitter's 
universe and his redshifts to Lemaitre's universe and his redshifts, combined 
with Slipher's crucial role are described and documented in Harry Nussbaumer 
and Lydia Bieri "Discovering the Expanding Universe", Cambridge University 
Press, 2010, foreword by Allan Sandage.