THE EXPANDING UNIVERSE — HUBBLE 123 



If these systematic red shifts are interpreted as the familiar Dopp- 

 ler shifts, it follows that the nebulae are receding from us in all 

 directions at velocities that increase directly with the momentary 

 distances. The rate of increase is about 100 miles per second per 

 million light-years of distance, and the observations have been car- 

 ried out to nearly 250 million light-years where the red shifts cor- 

 respond to velocities of recession of nearly 25,000 miles per second 

 or 1/7 the velocity of light. 



On this interpretation the present distribution of nebulae could 

 be accounted for by the assumption that all the nebulae were once 

 jammed together in a very small volume of space. Then, at a certain 

 instant, some 1,800 million years ago, the jam exploded, the nebulae 

 rushed outward in all directions with all possible velocities, and they 

 have maintained these velocities to the present day. Thus the nebu- 

 lae have now receded to various distances, depending upon their 

 initial velocities, and our observations necessarily uncover the law 

 of red shifts. 



This pattern of history seems so remarkable that some observers 

 view it wdth pardonable reserve, and try to imagine alternative ex- 

 planations for the law of red shifts. Up to the ])resent, they have 

 failed. Other ways are known by which red shifts might be pro- 

 duced, but all of them introduce additional effects that should be 

 conspicuous and actually are not found. Red shifts represent Doppler 

 effects, physical recession of the nebulae, or the action of some 

 hitherto unrecognized principle in nature. 



COSMOLOGICAL THEORY 



The preliminary sketch of the observable region was completed 

 about 10 years ago. It was not necessarily a finished picture, but it 

 furnished a rough framework within which precise, detailed investi- 

 gations could be planned with a proper understanding of their relation 

 to the general scheme. Such new investigations, of course, were 

 guided when practical by current theory. Let me explain the sig- 

 nificance of this procedure. 



Mathematicians deal with possible worlds, with an infinite number 

 of logically consistent systems. Observers explore the one particular 

 world we inhabit. Between the two stands the theorist. He studies 

 possible worlds but only those which are compatible with the infor- 

 mation furnished by observers. In other words, theory attempts to 

 segregate the minimum number of possible worlds which must include 

 the actual world we inhabit. Then the observer, with new factual 

 information, attempts to reduce the list still further. And so it goes, 

 observation and theory advancing together toward the common goal 

 of science, knowledge of the structure and behavior of the physical 

 universe. 



