J30 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



how far we can trace back the universe into the past, and it is 

 perhaps not surprising that the further we go the less certainty we 

 find. 



The big telescope at Mount Wilson shows us objects in space whose 

 light has taken 240 million years to reach us. Wlien we turn the 

 telescope on to these objects we see them, not as they are now, but 

 as they were 240 million years ago. These parts of the universe, 

 then, must have been in existence 240 million years ago, and we seem 

 justified in concluding that the universe as a whole is more than 

 240 million years old. Not only so, but these distant parts of space 

 are occupied by objects which do not differ in essentials from those 

 neai^r home, from which it seems safe to conclude that the universe 

 has not altered greatly in the past 240 million years ; in other words, 

 this period is only a small part of the evolutionary life of the 

 universe, so that the age of the universe is probably many times 

 240 million years. 



A study of our own earth confirms this conclusion. Geology can 

 reconstruct for us the physical conditions of 240 million years ago, 

 and we see that, broadly speaking, they were very similar to those 

 prevailing today. This not only shows that the earth is more than 

 240 million years old, but also that the sun has changed but little 

 in the past 240 million years. Thus the sun, and so also the universe 

 of which the sun forms part, must probably have an age of many 

 times 240 million years. 



By analyzing the radioactive properties of rocks of various kinds 

 in the crust of the earth, w^e can discover the length of time which 

 has elapsed since these various rocks solidified. The oldest rocks 

 of all show ages ranging up to 1,750 million years since solidification. 

 Thus we may safely conclude that the universe is at least 1,750 

 millions of years old. 



THE EXPANDING UNIVERSE 



For the next piece of evidence, we must return to the extreme 

 depths of space. We believe the great extragalactic nebulae to be 

 galaxies of stars generally similar to our own, and these are found 

 to be receding from our galaxy with immense speeds — the largest 

 speeds we encounter in astronomy, apart from the velocity of light. 

 The greatest so far observed is 42,000 kilometers a second, which 

 is one-seventh of the velocity of light. It is found to be a general 

 rule that the most distant nebulae are receding the most rapidly, 

 and the speeds of the various nebulae are proportional to their 

 distances from us. This is shown in figure 1, which embodies results 

 recently obtained by Hubble and Humason at Mount Wilson. The 

 abscissae represent the distances of various nebulae and groups of 



