Select Historical Papers on Cosmology

These papers are relevant to the observational discovery of the expansion law, homogeneity, and isotropy of the universe.

C. Wirtz, 1924 "De Sitters Kosmologie und die Radialbewegungen der Spiralnebel" (Astronomische Nachrichten, volume 222, p.21)
K. Lundmark, 1924 "The determination of the curvature of space-time in de Sitter's world" (Monthly Notices of the Royal Astronomical Society, Vol. 84, p.747-770)
K. Lundmark, 1925 "Nebulae, The motions and the distances of spiral" (Monthly Notices of the Royal Astronomical Society, Vol. 85, p.865)
G. Stromberg, 1925 "Analysis of radial velocities of globular clusters and non-galactic nebulae" (Astrophys. J., 61, 353-362)
E. Hubble, 1926 "Extragalactic nebulae" (Astrophys. J., 64, 321-369)
G. Lemaitre, 1927 "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques" (Annales de la Societe Scientifique de Bruxelles, A47, p. 49-59)
E. Hubble, 1929 "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae" (Proceedings of the National Academy of Sciences of the United States of America, Volume 15, Issue 3, pp. 168-173)
E. Hubble, 1934 "The Distribution of Extra-Galactic Nebulae" (Astrophysical Journal, vol. 79, p.8)

Comments on expansion law

Everyone relied on the same set of velocities, a complete set of which are listed in Stromberg (1925) (~46 in total).

Wirtz (1924) assumed that galaxy diameters are "standard rulers" and found a loose correlation between velocity and diameter. He did not list his set of galaxies nor did he make any figures. He did not have any absolute distance calibration.

Stromberg (1925) assumed that galaxy absolute magnitudes are "standard candles". He did not find any significant correlation between velocity and apparent magnitude.

Lundmark (1924, 1925) combined both diameter and magnitude information to estimate galaxy distances, thus combining the "standard ruler" and "standard candle" assumptions. He used a variety of distance indicators (novae; Oepiks' method) to set the distance scale. He found a quadratic polynomial relation between distance and velocity with a large constant offset.

Lemaitre (1927) assumed that galaxy absolute magnitudes are "standard candles". He used Hubble's 1926 calibration but ignore Hubble's caveats about the standard candle assumption. He assumed a linear relationship between velocity and distance and made the first estimate of the slope (625 km/s/Mpc).

Robertson (1928) largely repeated Lemaitre's analysis and made the second estimate of the slope (463 km/s/Mpc).

Hubble (1929) assumed that the brightest stars in late-type spirals are "standard candles". He used his distances to 7 nearby galaxies to calibrate the brightest star intrinsic magnitude. He derived a linear relation between velocity and distance and made the third estimate of the slope (500 km/s/Mpc).

Comments on Homogeneity

Hubble (1926) made the first test of homogeneity by counting galaxies as a function of apparent magnitude and comparing with a Euclidean model.

Comments on Isotropy

It is difficult to find any paper that makes an express test of isotropy. Frederick Seares (1925, Ap. J., 62, 168) is a typical try - combining counts of galaxies and determining corrections for limiting mag., aberration, etc. The biggest confounding effect is dust extinction, but the effect of large-scale structure is also understood. Seares claims to find that counts in the South are 3/4 of those in the North. Hubble (1926) claims that this is due to the Virgo cluster in the North. Hubble (1934) is the first clear-cut test that I can find, although it is considerably more advanced than what could have been achieved earlier. Certainly isotropy was assumed well before 1934.