TIME SCALE OF OUR UNIVERSE OPIK 205 



lution — unless we could prove the total absence of any stars before 

 that date, and not only of those existing at present. 



A more drastic view preferred the concept of an absolute beginning, 

 perhaps identifiable with an act of creation. The definition of the 

 time scale remained the same as before, but additional meaning was 

 attached to it as that of the absolute age of the universe. The initial 

 stage, a singularity from which the universe started expanding, was 

 the limit of extrapolation not only from the present, but from any 

 state of the universe, however close to the initial stage. 



The difference between the two viewpoints is a matter of principle, 

 and not of how the initial state of the universe is pictured. Although 

 Eddington's primeval nebula, assumed to have preceded the present 

 expanding state of the universe, could have existed indefinitely, it 

 could equally well have been the first created object, called into being 

 in a peculiar state of almost exact equilibrium between gravitational 

 attraction and the hypothetical force of cosmic repulsion. On the 

 other hand, Lemaitre's primeval atom, "the egg from which the uni- 

 verse hatched," is most simply interpreted as the result of an act of 

 creation ; yet it could also have been the final outcome of collapse of 

 a previous universe, oscillating indefinitely in alternating expansion 

 and contraction. The choice between the two, continuous existence 

 or creation, will remain a matter for esthetic judgment, not for positive 

 science defined as theory verified by observation. 



There is no proof in purely esthetic matters. This does not mean 

 that esthetic methods of approach to scientific problems are worthless. 

 On the contrary, scientific theories are created by intuition, or by an 

 essentially esthetic process. However, without the flesh and bone of 

 experiment such theories remain mere shadows of possibilities. 



To remain on solid ground, in the following we will pay little atten- 

 tion to esthetic considerations, however important these might appear 

 from the standpoint of philosophy or religion. We will, further, be 

 guided by the principle of minimum hypothesis, or economy of 

 thought, which requires that new laws of nature must not be used for 

 the explanation of phenomena which can be accounted for by known 

 laws. This is a safeguard against becoming lost in the blind alleys 

 of guesswork. The chances are small that a theory not supported by 

 facts would prove to be correct. 



As already mentioned, the fundamental fact requiring a short time 

 scale was, and remains, the red shift of the extragalactic nebulae. 

 With the existing laws of nature this phenomenon is explained in the 

 most straightforward way as recession. There are yet no facts known 

 which would contradict this explanation. The Hubble-Humason 

 law (1928) of proportionality of the red shift, or velocity of recession 

 to distance, led to a time scale for the universe equal to a few thousand 



