Updated: 16 Jun 2003

Hubble Law

Abstract

The Hubble Law, named after the American astronomer, Edwin Hubble, describes the relationship between the distance to far objects and their red-shift owing to universal expansion.

Keywords: Hubble Law, de Sitter effect, universal expansion

Introduction

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History

The American astronomer, Vesto M. Slipher, working at the Lowell Observatory in Arizona, had observed the red shift of some nearby galaxies as early as 1912. By 1928 he had amassed observations from more than 40 galaxies (Zeilik 1991, pp. 121 - 122). The Dutch astronomer Willem de Sitter interpreted Slipher’s red shifts as a Doppler effect, and incorporated the early observations in his 1917 cosmological model of an expanding universe based on Einstein’s (then new) general theory of relativity. For a time, the red shifts were known as the "de Sitter effect." This marked dominance of redshifts was presented by Curtis in the Shapley-Curtis debate as evidence that the "spiral nebulae" could not be ordinary galactic nebulae.

Then, in 1923, the German Carl Wirtz combined Slipher’s red shift measurements with rough estimates of the distances to galaxies based on their apparent size, and proposed a velocity-distance law, i.e. that redshift is proportional to distance (Pagels 1985, p. 97).

Six years later, in 1929, Hubble estimated the distance to 18 galaxies from the luminosity of their brightest stars and described a "roughly linear relation" with their red shifts. Not only was his conclusion a restatement of Wirtz’ earlier work, it was not justified by his data. According to Steven Weinberg (1993, p. 35) "a look at Hubble’s data leaves me perplexed how he could reach such a conclusion - galactic velocities seem almost uncorrelated with their distance. In fact, we would not expect any neat relation of proportionality between velocity and distance for these 18 galaxies - they are all much too close, none being further than the Virgo Cluster."

Later evidence, collected by Hubble and Milton L. Humason in 1931 using the 100 inch telescope at Mt. Wilson Observatory (Smoot & Davidson 1993, p. 49; Zeilik 1991, p. 122) extended the redshift observations out to 6.7% - far enough to be convincing - but this work, and Humason’s name, is usually overlooked in the rush to heap praise upon Hubble alone.

Hubble & Humason’s original plot of the distance vs. velocity relationship (Zeilik 1991, fig. 7.14) yielded too steep a slope ((H (170 km/s/Mly) to be consistent with modern data.

Application

The Hubble Law is most easily stated as the formula:

v = H₀r

(1)

References

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