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Jimmy87

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1915 - Einstein's theory of General Relativity was published.

1923 - Hubble discovered a Cepheid variable in the Andromeda nebula and was able to conclude there are other galaxies out there.

1927 - The Fifth Solvay Conference on physics. Arguably the greatest meeting of world leading physicists ever. George Lemaitre who recently did his PhD at MIT attended to ask Einstein about work he was doing on his new theory of GR. He used Einstein's theory to show the universe must be expanding. Einstein acknowledged the maths but said something along the lines of 'The physics and conclusion however are abominable" thus rejecting his idea.

1929 - Hubble announced his finding on red shift of galaxies showing evidence for the expansion of the universe.

The confusion I had, that I was hoping someone would clarify, is what the cosmological constant Einstein inserted was actually intended for? Some web sources say he introduced it so his equations represented a universe that wasn't expanding i.e. to halt the expansion. Other sources say that because Einstein thought the universe was static (and therefore has always been static) then the gravitational attraction of matter would cause the universe to collapse in on itself. Therefore the cosmological constant served as a kind of 'anti gravity'. Both of these justifications for including the cosmological constant at the time seem the opposite; one saying its used as 'anti gravity' to stop the collapse of the universe and one to stop the expansion of the universe. They both seem like opposite effects?

Any interesting facts anyone has about the history of cosmology around this time would be greatly appreciated. Thanks.