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Thread: Lemaitre and the expanding Universe

  1. #1
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    Lemaitre and the expanding Universe

    Lemaitre is credited with deriving an expanding Universe from Einstein's field equations (EFTs), well before Hubble's observations of galaxy distance and receding velocity correlation. Can anyone offer a layperson's guide to show how Lemaitre derived his conclusions from the EFTs? Thanks!

  2. #2
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    Quote Originally Posted by Cheap Astronomy View Post
    Lemaitre is credited with deriving an expanding Universe from Einstein's field equations (EFTs), well before Hubble's observations of galaxy distance and receding velocity correlation. Can anyone offer a layperson's guide to show how Lemaitre derived his conclusions from the EFTs? Thanks!
    My understanding is he did it pretty much like Hubble, but his original paper didnít get wide distribution so was unknown in the US and Hubble came to a similar conclusion independently.

    Essentially, he worked out that an Einstein model universe had to expand or contract. Einstein introduced his fudge factor to allow a static universe (the cosmological constant which came back into discussion with the discovery of dark energy/accelerating expansion) because there had been a general assumption that the universe was static. Lemaitre determined that if the universe was expanding, there should be a red shift seen for more distant galaxies and found that in data, much like Hubble did a couple years later.

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  3. #3
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    The Soviet Friedmann first came up with the Big Bang from Einstein's equation.

  4. #4
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    I had a quick read of his 1931 paper (you can find a PDF of it online at https://academic.oup.com/mnras/article/91/5/490/985169). I believe (happy to be corrected by people more comfortable with GR and 1930s language!) that what he showed was that if you had any inhomogeneity in the universe then you found that under the GR equations the shell dividing it from the rest of the homogenous universe was not stable. It would tend to expand, it would not stay still. From this he inferred that for a universe with any variations in density the size of the universe would increase over time.

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