Caption: A cartoon of Georges Lemaitre (1894--1966).

Now for a fine point in the history of astronomy: Who discovered the expansion of the universe and Hubble's law?

1. There was some controversy in the early 2010s about how much credit Edwin Hubble (1889--1953) should get for these discoveries and who else should get some credit.

2. After a review of the literature (Hubble 1929, No-523--524, Livio 2011, Steer 2012, Way 2013, Trimble 2012, Trimble 2013, Elizalde 2018), yours truly follows the herd that says Hubble is justly credited as being the observational discoverer of both the expansion of the universe and Hubble's law in 1929.

3. A subtle point about the discoveries is that Hubble had to have the correct general-relativistic interpretation of the measured redshifts (mostly/all obtained by Vesto Slipher (1875--1969)). If Hubble had interpreted the measured redshifts as ordinary Doppler shifts (and NOT (general-relativistic) cosmological redshifts), he would NOT have had the right interpretation of the observations and would NOT himself have formally discovered the general-relativistic expansion of the universe and Hubble's law. But it seems likely that he did have the right interpretation though probably NOT himself a theoretical general relativistist. For example, Hubble does refer to the general-relativistic de Sitter universe as a possible interpretation of his observational discoveries in his discovery article (Hubble 1929, last paragraph).

   The sophisticated readers of Hubble (1929) (e.g., Willem de Sitter (1872--1934), Albert Einstein (1879--1955), Richard C. Tolman (1881--1948), Arthur Eddington (1882--1944), and Georges Lemaitre (1894--1966)) must have picked up the correct general-relativistic interpretation of the measured redshifts.

4. There were forerunners of the observational discoveries. However, they failed to put the pieces together or provide convincing evidence. The most notable of the forerunners is probably Knut Lundmark (1889--1958) who was somewhat close in an article he published in 1925. Hubble was aware of Lundmark's article and referenced it in Hubble (1929).

5. Now what of the theoretical discoveries?

   Famously, Einstein missed his chance of predicting the expanding universe from the Einstein field equations by assuming a static universe which he obtained by introducing the cosmological constant. His universe model is the static Einstein universe.

   However before 1929, Willem de Sitter (1872--1934), Alexander Friedmann (1888--1925), and Georges Lemaitre (1894--1966) all found expanding universe solutions to Friedmann equation (which in turn is derived from general relativity). These solutions all obey the theoretical Hubble's law and, in fact, the theoretical Hubble's law follows directly from the Friedmann equation without needing solutions (e.g., Li-37--38) and Lemaitre showed this explicitly first.

   Note Einstein and de Sitter derived their solutions to the Friedmann equation without having the Friedmann equation. Their derivations followed directly from general relativity by difficult paths.

   Note that the discoveries expanding universe solutions and the theoretical Hubble's law were theoretical discoveries, NOT discoveries that observable universe obeyed any of the solutions or Hubble's law.

6. However, Lemaitre in his article of 1927 in which he explicitly derived the theoretical Hubble's law also derived 2 possible values for the Hubble constant, 575 (km/s)/Mpc and 670 (km/s)/Mpc, based on published observations (Way 2013, p. 14). These Hubble constant values suffered from same overall systematic error that Hubble's Hubble constant (published 1929) suffered from. Hubble got 500 (km/s)/Mpc (Hubble 1929, 3rd to last paragraph; Bo-39; Tamann 2005; Wikipedia: Timeline of Hubble constant values). The modern Hubble constant = 70 (km/s)/Mpc fiducial value. So both Lemaitre and Hubble were off by large factors: i.e., approximately 8 or 10 and 7, respectively.

   Note that Lemaitre had NOT discovered Hubble's law was obeyed by the observable universe. What he had shown was that if Hubble's law were true for the observable universe, then existing observational data gave values for the Hubble constant.

   Nevertheless, Lemaitre had shown there was significant observational evidence for Hubble's law---which as aforesaid, he had theoretically discovered from the Friedmann equation. It seems likely that if Lemaitre's 1927 article had become well known, Hubble's law would have been called Lemaitre's law.

   Unfortunately, Lemaitre's 1927 article was in French and was published only in the obscure Annals of the Scientific Society of Brussels. This may be why the article was NOT much noticed.

   It seems likely that Lemaitre did NOT realize how important his Hubble's law work was in 1927 since it only appeared in a footnote in his article (Livio 2011). The lack of realizing the importance probably prevented Lemaitre from advertising his Hubble's law work.

   Lemaitre himself was, in fact, well known to other cosmologists---there were maybe 5 in the 1920s. So he could have advertised his Hubble's law work more than he did.

   When the article was translated into English by Lemaitre himself and published in Monthly_Notices of the Royal Astronomical Society (MNRAS) in 1931, Lemaitre omitted the footnote as of being of "no actual interest" (Livio 2011). Actually, "actuel" in French means "current" and that is probably what Lemaitre meant by "actual." It seems possible that Lemaitre did NOT wish to give the appearance of claiming priority for the observational discovery of the expansion of the universe and Hubble's law and get into a piority dispute with Hubble.

   Also, Lemaitre in 1931 was more interested in advertising his new primeval atom theory a forerunner of Big Bang theory. For a brief explication of primeval atom theory, see Astronomer file: georges_lemaitre.html: Lemaitre's Primeval Atom Theory.

7. Clearly, it would have been better for the advancement of science and Lemaitre's fame---though he's a famous enough cosmologist---if Lemaitre's 1927 article had become widely known in 1927.

   Yours truly thinks and probably Lemaitre himself thought that he'd simply missed the boat on getting partial credit for the observational discovery of the expansion of the universe and Hubble's law.

   However, in 2018, the IAU decided to give Lemaitre some credit and formally changed the name of Hubble's law from Hubble's law to the Hubble-Lemaitre law. Yours truly does NOT think the longer name will be much used. We've always called Hubble's law Hubble's law and it's shorter to say and write.

8. A brief chronology (with some recapitulation of the discussion above) of the discovery of the expansion of the universe and Hubble's law follows:
   1. Since 1917 some cosmological models by Willem de Sitter (1872--1934), Alexander Friedmann (1888--1925), and Georges Lemaitre (1894--1966) based on general relativity (via the Friedmann equation) had predicted the expansion of the universe and Hubble's law as an unnoticed implication.

   2. Knut Lundmark (1889--1958) had some inkling of Hubble's law in 1924, but was unable to clarify his thinking or present convincing evidence.

   3. Georges Lemaitre (1894--1966) published Hubble's law as a theoretical prediction (based on the Friedmann equation) in French (in the pretty obscure Annals of the Scientific Society of Brussels) in 1927. Based on published observations that he found in the literature, he determined 2 possible values for the Hubble constant, 575 (km/s)/Mpc and 670 (km/s)/Mpc (Way 2013, p. 14). This work constitutes significant evidence for Hubble's law though it is short of a full observational discovery of it. However, Lemaitre's work on Hubble's law was poorly advertised by Lemaitre and was alas little noticed.

   4. Edwin Hubble (1889--1953) in 1929 presented convincing observational evidence for Hubble's law (Steer, 2012). Actually, Hubble's data were rather poor compared to modern times and he had very large systematic errors. Nevertheless, he made the right deduction from his data, both in the light of astronomy history and in light of the data itself.

   5. By the by, Hubble was unaware of Lemaitre's earlier theoretical discovery of Hubble's law though he was aware of the general-relativistic de Sitter universe and perhaps other cosmological models predicting an expanding universe. But history as of 2018 has decided that Lemaitre needed some credit too and International Astronomical Union (IAU) renamed Hubble's law officially as the Hubble-Lemaitre law (see Wikipedia: Georges Lemaitre: Honours). Probably few people will use the new name since its longwinded and we are used to what we are used to.