SIZE AND AGE OF THE UNIVERSE— JEANS 



131 



nebulae at distances ranging up to about 40 million parsecs (130 

 million light-years) ; the ordinates represent the observed velocities 

 of recession of these nebulae expressed in kilometers per second. 

 It is at once seen that the velocities are verj^ approximately pro- 

 portional to the distances of the nebulae. 



The theory of relativity provides a ver}^ simple explanation of 

 these observed motions of the extragalactic nebulae, and of the law 

 obeyed by the speeds of the nebulae at different distances. It is, 

 in brief, that space itself is uniform!}^ expanding, and that the 



10 20 30 40 60 



Distance (megaparsecs) 



FiGDEE 1. — Extragalactic nebulae. Velocity -disturbance relation (Hubble). Isolated nebu- 

 lae (grouped). X, clusters of nebulae. 



nebulae embedded in it indicate the motions of space, much as 

 floating straws indicate the currents in a stream. If this is the true 

 explanation, then the nebular motions show that space is at present 

 expanding at such a rate that its linear dimensions double every 

 2,000 million years — which, let us notice in passing, is just about 

 the probable age of the earth. But the theory of relativity goes 

 further than this, and tells us that space is very unlikely to expand 

 continually at a uniform rate. Certain assumptions suggest that 

 the expansion increases, approximately at least, in geometrical pro- 

 gression with the time. If this is the true law, then the present 

 nebular motions show that space doubles its linear dimensions every 

 1,400 million years. In other words, 1,400 million years ago space 

 had only half its present linear dimensions; 2,800 million years ago 

 only a quarter of its present linear dimensions ; and so on. 



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