Image 1 Caption: Alexander Friedmann (1888--1925): Russian-Soviet mathematician, physicist, meteorologist, ballonist, and pioneering cosmologist (see Wikipedia: Alexander Friedmann; The MacTutor History of Mathematics archive: Aleksandr Aleksandrovich Friedmann).
Features:
The Friedmann equation is
Solutions of the Friedmann equation for the cosmic scale factor a(t) that are historically interesting are often called universes: e.g., the Einstein universe (1917) and the de Sitter universe (1917).
A zero or non-zero cosmological constant Λ can be included in the Friedmann equation, and so in the FE models.
However, a key point of the FE models is that in all cases they give NON-STATIC solutions---unless cosmological constant Λ to adjusted to exactly the right value to give the Einstein universe. Other than the Einstein universe case, the FE models must expand or contract---and they do this maintaining the cosmological principle.
Einstein is seen here, chalk in hand, lecturing in Vienna, Austria in 1921.
In the days when he was the supreme master of physics.
If either of them had discovered the Friedmann equation, it would have saved them a lot of work and would have clarified general relativistic cosmology several years earlier than in actual history.
v=H_0*r ,where "v" is recession velocity from the Milky Way, "H_0" is the Hubble constant H_0 = [(70 km/s)/Mpc]*h_70, h_70 = H_0/[(70 km/s)/Mpc], and "r" is proper distance (i.e., distance that can be measured with a ruler at one instant in cosmic time) from Milky Way. Note that in general "v" and "r" are NOT direct observables, but they become direct observables as asymptotically as r&arow;0, and so "H_0" can be directly measured.
Moreover, Lemaitre was able using published data on galaxies to find values for the Hubble constant (575 (km/s)/Mpc or 670 (km/s)/Mpc: Way 2013, p. 14) that were relatively close to what Hubble found in 1929 (500 (km/s)/Mpc: Hubble 1929, 3rd to last paragraph; Bo-39; Tamann 2005; Wikipedia: Timeline of Hubble constant values). In fact, both Lemaitre's and Hubble's Hubble constant values are too large by a large factor because of a large calibration error and other measurement uncertainties. Actually, Hubble constant = 70 (km/s)/Mpc (fiducial value accurate to within ∼ 10 %).
To emphasize, Lemaitre did NOT discover that the observable universe obeyed Hubble's law with his Hubble constant, but that if the observable universe obeyed Hubble's law and one used the data he used (which had as aforesaid a large calibration error and other measurement uncertainties), then the Hubble constant had the values he found.
Lemaitre did publish his results in 1927 as aforesaid, but they did NOT attract much interest and it seems likely that he did NOT wish to get into a piority dispute with Hubble, and so he did NOT emphasize to his pre Edwin Hubble (1889--1953) Hubble's law work.
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.