Belenkiy replies: As an applied scientist, Alexander Friedmann was keenly interested in testing his theories with available data. An interpretation of the right side of Einstein’s field equations as the “flow of matter” was particularly appealing to him as a meteorologist. Several of his recently discovered letters to Paul Ehrenfest in June 1922 (Physics Today, March 2013, page 9) clearly show Friedmann knew he had made an important discovery.

While Friedmann had some a priori estimates of Λ and Λ critical, he was wise enough not to include them in the final draft of his 1922 paper; he remained uncommitted to a particular scenario until the empirical data provided information.1 He was unaware of Vesto Slipher’s data on the nebulae’s high radial velocities, publicized by Arthur Eddington in 1923. As the letters to Ehrenfest testify, Russia was so devastated at the time Friedmann was writing his 1922 paper that he was still unable to obtain a copy of Willem de Sitter’s original 1917 paper, which first highlighted Slipher’s data. (Most likely, Friedmann learned of de Sitter’s solution of general relativity equations indirectly from a secondary source.)

Furthermore, in his 1923 book, Friedmann discussed the possibility of the universe’s birth from a “singularity.” Thus all components of the Big Bang cosmology were present in Friedmann’s writings long before Georges Lemaître entered the field.

As for Peter Hammerling’s comment, Friedmann’s metric, in contrast to the modern one, had the additional factor c–2, thus forcing renormalization of the relevant constants k and λ by the inverse factor c2.

1.
A.
Belenkiy
, in
Origins of the Expanding Universe: 1912–1932
, M. J. Way, D. Hunter, eds., ASP conf. ser. 471, Astronomical Society of the Pacific, San Francisco (
2013
), p.
71
; arXiv:1302.1498.