Fritz Zwicky (so called father of dark matter) had proposed various alternative mechanisms to explain redshifts in those early days but he himself accepted that they did not work as expected.
Since all other ‘mechanisms’ failed … so only the ‘velocity’ mechanism survived.
But that was not just the survival of fittest thing.
Hubble was skeptical of velocity interpretation of redshifts but … actually he was also alone. He did not openly disregard velocity meanings. He cited de-Sitter in his 1929 paper that in the de-Sitter Cosmology, displacement of spectra arise from reasons other than motion.
But … de-Sitter also left him alone. Mainly because perhaps the specific point of de-Sitter was not having direct link with General Relativity.
After 1929, de-Sitter and Arthur Eddington both were anxious how to explain Hubble type redshift-distance relationship within GR framework.
Hubble was thinking that de-Sitter already had the prediction of cosmological redshifts in terms other than motion.
de-Sitter knew that he did have prediction but only by way of speculation or expectation out of observed data of few redshifted galaxies by time 1917.
de-Sitter did speculate that redshift could be systematically linked with distance and he had speculate this thing in year 1917.
Above is de-Sitter (1917) paper. The speculation about redshift-distance can be seen at the end of page No.26.
He is saying that distance may cause redshifts , giving rise to spurious positive radial velocity.
Meanings of ‘Spurious’ can be seen here:
On the next page (27), he is saying that Helium stars actually show this type of displacement of (spectra) … On same page, he is using term ‘apparent velocities’ for galactic redshifts.
Who was Willem de Sitter – Wikipedia?
After development of GR equation in 1916 … Einstein was perhaps the first who developed a model of whole universe based on GR equations in year 1917.
de-Sitter was the second person who also developed a model of whole Universe based on GR equations in same year 1917.
The 1917 de-Sitter paper is like a ‘solution’ to GR equations or at least regarded as solution to GR equations.
In this 1917 paper, de-Sitter is saying that distance may be linked to ‘spurious’ velocity (redshift).
In this paper … he did develop solution to GR equations … but there is lot of general discussion as well … like various developments in astronomy etc. including redshifts.
Now whether had ‘Spurious Velocity’ come from GR or from general speculation?
If it came from GR, then why de-Sitter and Eddington … after 1929 … were sitting together to find a solution of Hubble type redshifts from within GR equations and were not reaching at satisfactory conclusion…???
I do not know answer to this question. Only justification is that de-Sitter knew that page 26–28 of his 1917 paper were mere wild speculations and thus they did not relate to the current problem of how to account for Hubble type redshift-distance relationship within the framework of GR.
de-Sitter and Arthur Eddington were actually worrying in year 1930 regarding how to account for Hubble type redshift-distance relationship within the framework of GR equations. Following reference tells the story that they were worrying on this point:
The story can be found on page 8 of this PDF file (PDF page).
The rest of the story is that Lemaitre approached Eddington that he already had published similar solution in year 1927.
Then Eddington got a plan. He advised Lemaitre to publish the translation but with modifications.
The English translation (1931) of Lemaitre paper changed the game. That was a modified and deceptive translation … showing as if Lemaitre had already (in 1927) derived Hubble Law from GR equations.
In this way, Hubble was left alone. de-Sitter also adopted Lemaitre explanation. Zwicky proposed alternatives but then withdrew his proposals not because he was satisfied with expansionism … only because he had not found viable alternative.
About Latest developments:
It is often stated that latest developments have confirmed expansion of universe. Actually there is no latest development with regards to providing direct evidence that galaxies are in fact in motion. Even the so called ‘inflation theory’ has been ‘derived’ from de-Sitter model where cause of redshifts was other than motion.