Actually there were 6 men who contributed to the
Expanding Universe model, during the 1920s, Ben…not one person.
Lemaître was just one of 3 theoretical physicists working on physical cosmology…
- Alexander Friedmann (1922),
- Howard Percy Robertson (1924-25); and with Arthur Geoffrey Walker (1936), mathematician,
- Georges Lemaître (1927), author of Hypothesis of the Primeval Atom
All 3 independently formulated the Expanding Universe model (it was called the “Big Bang“ model until Fred Hoyle coined the misnomer name in BBC interview, 1949).
All three (4, counting Walker with Robertson, in 1936) came up with the same metric that provide the exact solution to Einstein’s field equations for General Relativity. This metric is known today as the
FLRW Metric (named after the 4 men - Friedmann, Lemaître, Robertson & Walker).
Although, Einstein have his own cosmology - the Static Universe model (1917) - it was his GR field equations that provide the framework for 3 independent models of the same concept…so even though Einstein disagree with Friedmann & Lemaître, he was in part, played a pinnacle role in the Expanding Universe model.
When the FLRW Metric is used with Einstein’s field equations, it became known as the
Friedmann Equation.
Robertson’s (1924) & Lemaître’s (1927) biggest contributions to the budding hypothesis, was incorporating Vesto Slipher’s Doppler Effect into the models, predicting that the observations of distant galaxies would appear to be stretching or lengthening towards the red of the EM spectrum, moving away from the Earth’s observer - the (Gravitational) Redshift would indicate the Universe to be “expanding”.
I did say “6 men”, didn’t I?
Not only did Edwin Hubble discovered in 1919, the Universe was larger than the Milky Way, there were many nebulae that were actually galaxies. Hubble‘s greatest contribution was in 1929, where he discover the
Gravitational Redshift, of the distant galaxies, which was the very first piece of evidence for the Expanding Universe (Big Bang) model...that verified the independent predictions of Lemaître & Robertson.