[I wonder how many will get the title wording?]

The 499thAstronomy Castpodcast has just addressed the new forthcoming vote at the IAU in Vienna regarding the possibility of giving partial credit to Abbe G. Lemaitre for the discovery. [The voting method itself for the IAU has changed, too.]

So the following is my finger in this big pie:

The majority of this comes from a paper by Harry Nussbaumer (Inst. of Astronomy, Zurich).

1912 – Vesto Slipher obtains some of the first redshifts for galaxies. [I wonder how close he came to freezing while getting that data?]

1917 – Beginning of modern cosmology. Einstein publishes “Cosmological consideration on the general relativity). He applied GR to the entire universe. But he felt that theory should support the established Static Theory for the immutable universe. His cosmological constant was introduced to counter gravity.

1917 – A few months later, Willem de Sitter published his alternative model, also representing a static model. This was a simplified model that excluded matter, thus not acceptable to Einstein who in 1923 noted that two particles would rush apart from one another because of the cosmological constant, no doubt.

1922 – Friedman publishes his “About the curvature of Space” based on GR. He held to Einstein’s isotropic view but made R, the radius, variable with time, unlike Einstein and de Sitter’s model. Density, would, therefore, also be variable. ["He did not connect this finding to astronomical observations, and he did not spot the flaw in de Sitter's model." -- Nussbaumer and Bieri (2012).]

It is noteworthy that de Sitter's model had time varying throughout the universe, which explains his redshift result without motion (so no Doppler effect, which later came with others but now is seen differently). Einstein didn’t initially accept Friedman’s math due to an error on Einstein’s part, but he never accepted the physical significance of Friedman’s work. Thus, Friedman was taken as purely theoretical.

1925 – Lemaitre showed that de Sitter’s model violated homogeneity, as held by Einstein and himself. Also, Lemaitre treated universe as Euclidian, apparently unbeknownst to Einstein in 1932. [Also, Friedman dies of Typhus.]

1926 – Hubble determines distances to extragalactic nebulae, that he won't call "galaxies".

1927 – Independently of Friedman, Lemaitre also had radius and density to vary with time. Unlike Friedman, however, he spotted the weakness of de Sitter’s model that lacked spatial homogeneity.

Lemaitre concluded that extragalactic nebulae would exhibit redshifts in the form of v = H*d, though H wasn’t for Hubble but expansion, interestingly.

He knew of Hubble’s distances and he had Slipher’s redshifts. Though the uncertainties were great, Lemaitre postulated an expanding universe.

He published in 1927 in French and in a little known Belgian scientific journal.

A few months later, at the Solvay conference, he gave a reprint to Einstein, who considered it to be abominable. [I've seen some quotes where Einstein praised the math but thought the physics was abominable.] But, during the conference, Lemaitre visited with Einstein in a taxi ride to a laboratory and realized Einstein wasn’t informed about the astronomical facts. This didn’t sway Einstein, however.

I believe it was at this same conference where Hubble had time to visit with de Sitter and learn that redshift wasn't necessarily due to motion, at least in de Sitter's model from Einstein's field equations (but without mass).

1929 -- Hubble published findings that there was probably a linear relationship between nebular distances and redshifts. Apparently, he wasn’t aware of Lemaitre’s prior claim of this. But note that Hubble is not calling for an expanding universe.

1929 – Fritz Zwicky proposes the Tired Light explanation for redshift.

1930 – de Sitter, at the RAS meeting, confirmed Hubble’s work. Eddington and de Sitter were at a loss on how to explain the evidence. [de Sitter has redshift with no expansion, no homogeneity and no mass; Einstein has homogeneity and mass but no explanation for redshift.] Lemaitre read the minutes of the meeting, including the puzzlement, and immediately sent two reprints of his 1927 paper to Eddington, requesting he send the other copy to de Sitter.

Both Eddington and de Sitter quickly accepted Lemaitre’s model. Eddington publicly acknowledges this in 1930 in theMonthly Notices.

1930 (June) – Einstein visits Eddington and likely became updated on the observational and theoretical status of cosmology.

1931 – In Einstein’s trip to Pasadena, there is no mention of Hubble anywhere in his diary, though it is known they met. Einstein’s unifying field views seem to have been foremost on his mind. Seeing the spectral plates seems to have been more a tourist attraction for Einstein. Unlike Tolman, Hubble was not fluent in German and was also reluctant to talk theory. Hubble, in his letter to de Sitter, stated that interpretations “should be left to you and the very few others who are competent to discuss the matter (redshift and distance correlations) with authority.” Hubble avoided the cosmological implications of his findings.

It seems Einstein was already up to date on Slipher’s and Hubble’s work and may have felt little need to hear it directly as it would be redundant, possibly.

The age for the universe was approximated to about 10 billion years, but stars were soon determined to be about 100 times older than this; older than the universe. This added no encouragement for Einstein to fully accept any expanding model.

Lemaitre proposed an early stagnation period followed by an accelerated expansion due to the cosmological term of Einstein which would have greater effect as the universe became less dense.

1931 -- Hubble and Humason publish new results with 10x the amount of data.

1931 (March) -- Lemaitre translates his Belgium (French) paper but, with Hubble's great data, he leaves out his expansion calculations. But why? Lemaitre could have corrected his calculations with the new data, but this would be a revision and not a simple translation. He may have elected to honor Hubble for his great achievement in redshift results, perhaps realizing that Hubble never in the 38 page paper mentions the word expansion, leaving room for credit to the theorists like him.

1931 (March) – Einstein reviews Friedman work and, on the last pages, “Friedman presents his periodic model with lambda = 0. In addition, Lemaitre’s 1927 paper provided the connection between the increasing radius of curvature and the redshifts in the spectra.”

1931 (June) – Joined with Tolman in a paper that lambda is unnecessary in an expanding universe. Tolman seemed to want to hold on to the cosmological constant, however, and both Eddington and Lemaitre agreed that this constant might be an important force in nature.

1932 – At the IAU, Eddington and Lemaitre opposed the banishment of lambda.