THE PROBLEM OF THE EXPANDING UNIVERSE 



By Edwin Hubbt^e 

 Mount Wilson Observatory 



1 propose to discuss the problem of the expanding universe from the 

 observational point of view. The fact that such a venture is permis- 

 sible is emphatic evidence that empirical research has definitely en- 

 tered the field of cosmology. The exploration of space has swept out- 

 ward in successive waves, first, through the system of the planets, 

 then, through the stellar system, and, finally, into the realm of the 

 nebulae. Today we study a region of space so vast and so homo- 

 geneous that it may well be a fair sample of the universe. At any 

 rate, we are justified in adopting the assumption as a working hypothe- 

 sis and attempting to infer the nature of the universe from the ob- 

 served characteristics of the sample. One phase of this ambitious 

 project is the observational test of the current theory of the expanding 

 universes of general relativity. 



I shall briefly describe the observable region of space as revealed 

 by preliminary reconnaissance with large telescopes, then sketch the 

 theory in outline, and, finally, discuss the recent more accurate ob- 

 servations that were designed to clarify and to test the theory. 



THE OBSERVABLE REGION 



The sun, as you know, is a star, one of several thousand million stars 

 which together form the stellar system. This system is a great swarm 

 of stars isolated in space. It drifts through the universe as a swarm 

 of bees moves through the summer air. From our position near the 

 sun we look out through the swarm of stars, past the borders, and 

 into the universe beyond. 



Until recently those outer regions lay in the realm of speculation. 

 Today we explore them with confidence. They are empty for the 

 most part, vast stretches of empty space. But here and there, sep- 

 arated by immense intervals, other stellar systems are found, com- 

 parable with our own. We find them thinly scattered through space 



' Reprinted by permission from the Sigma XI quarterly, vol. 80, No. 2, April 1942. In- 

 cluded in Science in Progress, Series III, fall, 1942. 



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