Image 1 Caption: Georges Lemaitre (1894--1966) circa 1933. See also St. Andrews: MacTutor: Georges Lemaitre (1894--1966).

Features:

1. Lemaitre was the co-discoverer of the Friedmann equation (AKA Friedmann-Lemaitre equation and the Friedmann equation (FE) models and the inventor in 1931 of the primeval atom theory. The primeval atom theory is now only of historical interest, but it is the theoretical ancestor of Big Bang cosmology (see No-525,530; Jean-Pierre Luminet, The Rise of Big Bang Models (4) : Lemaitre, 2015).

2. Yours truly does NOT know what Lemaitre is doing on that rather weird looking blackboard. Actually, it looks like he's chalking on wood panelling---yikes.

   

3. Image 2 Caption: Georges Lemaitre (1894--1966) between Robert Millikan (1868--1953) and Albert Einstein (1879--1955) at Caltech, 1933 Jan10.

4. Georges Lemaitre (1894--1966) videos (i.e., Georges Lemaitre (1894--1966) videos):
   1. Georges Lemaitre y la teoria del Big Bang | 1:30: Great---and in Spanish. Not suitable for the classroom.
   2. Au-dela du Big Bang 3 | 19:54: Scroll to 9:50 for Georges Lemaitre (1894--1966) et un lecon en francais. Not suitable for the classroom.
   3. (Lost Media) Interview with Georges Lemaitre (1964) | 19:47: Georges Lemaitre (1894--1966), the Master (it's his name), speaks to us from across the centuries---prospectively speaking. And with Flemish subtitles. See also Gontcho et al. 2023, "Resurfaced 1964 VRT video interview of Georges Lemaitre". NOT suitable for the classroom.
   4. Editorial note to "The beginning of the world from the point of view of quantum theory": Jean-Pierre Luminet 2011, May31, 16 pages: Popular: On Georges Lemaitre (1894--1966) and the Lemaitre universe and the primeval atom. For corrections, see Livio 2011 on Hubble's law omission and O'Raifeartaigh, 2019, p. 13 on dark energy and its negative pressure first use. See also Jeffery 2023, p. 43 for the radiation Lemaitre universe exact (analytic) solution. / keywords_cosmology.html

   

5. Image 3 Caption: The Lemaitre universe (1933).

6. The Lemaitre universe (1933)
   1. Lemaitre universe (1933) is a hyperspherical space like the Einstein universe, and so it is finite, but unbounded.

      Georges Lemaitre (1894--1966) only published cosmological models with hyperspherical space. Maybe he found them philosophically satisfying. Maybe he be believed that Friedmann equation models could NOT apply everywhere for all time and hyperspherical space naturally embodied that view point.

   2. The Lemaitre universe (1933) begins with a Big Bang singularity (as we would call it now), has a deceleration phase, then an Einstein universe phase (i.e., a nearly STATIC phase), and then an exponential expansion phase driven by the cosmological constant (AKA Lambda, AKA Λ).

      Explicaton:

      1. The Big Bang singularity allowed for Lemaitre primeval atom theory (see the below item Primeval Atom Theory).
      2. The deceleration phase was when matter (which we would now call baryonic matter) dominated the mass-energy of the Lemaitre universe before the expansion of the universe decreased the matter density importance. Lemaitre qualitatively hypothesized that galaxies could NOT form during an expanding universe phase. This is NOT correct, but was plausible in the 1930s long before computer simulations of large-scale structure formation were possible.
      3. The Einstein universe phase occurred when matter, the cosmological constant, and the curved spacetime parameter were in balance. This nearly STATIC phase allowed local contractions of matter to form galaxies. The Lemaitre universe free parameters could be tuned to make nearly STATIC phase as long as you would like. This allowed Lemaitre to solve the cosmic age problem of the 1920s/1930s: radioactive dating of the Earth gave it an age of 1.6--3 Gyr Wikipedia: Age of Earth: Arthur Holmes establishes radiometric dating), but the Hubble constant as measured then (∼ (500 km/s)/Mpc) interpreted via the Einstein-de Sitter universe (1932, standard model of cosmology c.1960s--c.1990s) or similar cosmological models gave an age of the universe of ∼ 1.3 Gyr (Wikipedia: Cosmic age problem; Wikipedia: Age of the Universe: History; Wikipedia: Hubble's law: Hubble time). How could the Earth be older than the observable universe? The Lemaitre universe solved the problem.
      4. The cosmological constant eventually broke the balance of "forces" and created the observed expansion of the universe.

7. Lemaitre's Primeval Atom Theory:
   1. The ancestor of Big Bang cosmology for the constituents of the observable universe was Georges Lemaitre's (1894--1966) primeval atom theory (1933). It is a historically interesting, but long discarded, theory (No-530).

   2. In brief, the primeval atom theory posited a cold "Big Bang" in which a giant mass of neutrons (i.e., the primeval atom itself) filling a small positive curvature universe (a hyperspherical universe: finite, but unbounded). This small universe expanded according to a the Lemaitre universe (1933). The primeval atom fragmented into smaller and smaller fragments of which the smallest are the hydrogen (H) and helium (He). Larger fragments continue to exist and some are radioactive isotopes. These radioactive isotopes are inside stars and planets and undergo radioactive decay: they power stars and provide radiogenic heat in planets.

   3. Lemaitre did NOT think of the primeval atom as the beginning of time. He thought of it as being as far back as speculative theorizing could reach.

   4. The primeval atom theory (1933) was a brilliant theory and was viable circa the 1930s. However, advances in nuclear physics and in the understanding of hydrogen burning in stars by the middle 1940s made it seem implausible and effectively ruled it out. Georges Lemaitre (1894--1966) himself continued to discuss the primeval atom theory (1933) in his later years while admitting he had become an old-fashioned cosmologist (Kr-58).

Images:

1. Credit/Permission: Anonymous photographer, circa 1933 / Public domain very probably.
   Image link: Universite Catholique de Louvain: Georges Lemaitre Archives.
   
2. Credit/Permission: Anonymous photographer, 1933 / Public domain very probably.
   Image link: Universite Catholique de Louvain: Georges Lemaitre Archives.
   
3. Credit/Permission: © Jean-Pierre Luminet, before or circa 2015 / No permission.
   Image link: Uhesitant.gif.
   Placeholder image link: alien_click_to_see_image.html.