Hubble's diagram and cosmic expansion

doi:10.1073/pnas.2536799100

Hubble's diagram and cosmic expansion

Robert P. Kirshner*

Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138

Contributed by Robert P. Kirshner, October 21, 2003

Abstract

Edwin Hubble's classic article on the expanding universe appeared in PNAS in 1929 [Hubble, E. P. (1929) Proc. Natl. Acad. Sci. USA 15, 168–173]. The chief result, that a galaxy's distance is proportional to its redshift, is so well known and so deeply embedded into the language of astronomy through the Hubble diagram, the Hubble constant, Hubble's Law, and the Hubble time, that the article itself is rarely referenced. Even though Hubble's distances have a large systematic error, Hubble's velocities come chiefly from Vesto Melvin Slipher, and the interpretation in terms of the de Sitter effect is out of the mainstream of modern cosmology, this article opened the way to investigation of the expanding, evolving, and accelerating universe that engages today's burgeoning field of cosmology.

Footnotes

↵ * E-mail: kirshner{at}cfa.harvard.edu.
This Perspective is published as part of a series highlighting landmark papers published in PNAS. Read more about this classic PNAS article online at www.pnas.org/misc/classics.shtml.
↵ † There are just 73 citations of Hubble's original paper in NASA's Astrophysics Data System. There are 1,001 citations of ref. 7.
↵ ‡ The conventional units of the Hubble constant are a bit obscure: 1 megaparsec (Mpc) = 10⁶ parsec = 3.26 × 10⁶ light years = 3.086 × 10¹⁶ m. A Hubble constant of 70 km/s/Mpc corresponds to 2.27 × 10^–18 s^–1. Then, the Hubble time is 1/2.27 × 10¹⁸ s or 13.9 × 10⁹ yr.
↵ § Kirshner, R. P., Aguilera, C., Barris, B., Becker, A., Challis, P., Chornock, R., Clocchiatti, A., Covarrubias, R., Filippenko, A. V., Garnavich, P. M., et al. (2003) Bull. Am. Astron. Soc. 202, 2308 (abstr.).