Carl Wilhelm Wirtz
As already Vesto Slipher in 1912, Wirtz in 1918 observed a systematic redshift of nebulae, which was difficult to interpret in terms of a cosmological model in which the Universe is filled more or less uniformly with stars and nebulae. Wirtz additionally used the equivalent in German of K correction. The term continues to be used in present-day observational cosmology, but Wirtz's observational evidence that the Universe is expanding is not often mentioned. He wrote:
It is remarkable, that our system of fixed stars shall have such a very strong displacement of 820 km/s, and equally strange is the interpretation of the systematic constant k = + 656 km. If we ascribe a verbatim interpretation to this value, then this means that the system of spiral nebulae is drifting apart by a velocity of 656 km with respect to the momentary location of the solar system as the center.
In 1922, he wrote a paper where he argued that the observational results suggest, that the redshifts of distant galaxies are becoming higher than more closer ones, which he interpreted as an increase of their radial velocities with distance, and that larger masses have smaller redshifts than smaller ones. In another note of the same year, he argued that counter-clockwise spiraling galaxies have smaller redshifts than clockwise spiraling ones.  In 1924 he obtained more precise results, and interpreted them both as a confirmation of an increase of radial velocities with distance, but also as confirmation of a de Sitter universe, in which the increase of redshift is seen as caused by an increased time dilation in distant parts of the universe.
In 1936, Wirtz wrote a short paper alluding to the priority for his 1922-conclusion that the radial velocities of galaxies are increasing with their distance.
- Kragh, H.; Smith, R. W. (2003). "Who discovered the expanding universe". History of Science 41: 141–62. Bibcode:2003HisSc..41..141K.
- Kinney, A. C.; Calzetti, D.; Bohlin, R. C.; McQuade, K.; Storchi-Bergmann, T.; Schmitt, H. R. (1996). "Template ultraviolet spectra to near-infrared spectra of star-forming galaxies and their application to K-corrections". The Astrophysical Journal 467: 38–60. Bibcode:1996ApJ...467...38K. doi:10.1086/177583.
- Wirtz, C. W. (1918). "Über die Bewegungen der Nebelflecke. Vierte Mitteilung". Astronomische Nachrichten 206 (4933): 109–112. Bibcode:1918AN....206..109V. doi:10.1002/asna.19182061302.
- van den Bergh, Sidney (2011). "Discovery of the Expansion of the Universe". arXiv:1108.0709 [physics.hist-ph].
- Wirtz, C. W. (1922). "Einiges zur Statistik der Radialbewegungen von Spiralnebeln und Kugelsternhaufen". Astronomische Nachrichten 215 (5153): 349–354. Bibcode:1922AN....215..349W. doi:10.1002/asna.19212151703
- Wirtz, C. W. (1922). "Notiz zur Radialbewegung der Spiralnebel". Astronomische Nachrichten 215 (5184): 451. Bibcode:1922AN....215..451H.
- Wirtz, C. W. (1924). "De Sitters Kosmologie und die Radialbewegungen der Spiralnebel". Astronomische Nachrichten 222 (5306): 21–26. Bibcode:1924AN....222...21W. doi:10.1002/asna.19242220203.
- Wirtz, C. W. (1936). "Ein literarischer Hinweis zur Radialbewegung der Spiralnebel". Zeitschrift für Astrophysik 11: 261. Bibcode:1936ZA.....11..261W.