TIME SCALE OF OUR UNIVERSE — OPIK 221 



time) . In that case the time of expansion from the state of greatest 

 density until today is insensitive to the precise value of world density, 

 and depends only upon the rate of expansion ; it is practically equal 

 to that of uncurved (Euclidean) space and, with the revised value 

 of the expansion constant, becomes 



^=4,500 million years. 



The figure is surprisingly close to the other estimates, although a 

 considerable uncertainty is involved, the extreme admissible values 

 being, perhaps, from 3 to 6 thousand million years. 



This would represent the age of the universe in a restricted sense, 

 or the time elapsed since it was in a highly condensed state. This 

 state cannot yet be described. Lemaitre's primeval atom is one of the 

 possibilities. The theory of the origin of the elements, as shown 

 above, does not provide a clue. The same is true of the cosmic rays, 

 which appear to be of stellar origin and whose connection with the 

 prestellar stage of the universe seems to be improbable (58, 59). 



SPACE REDDENING OF THE GALAXIES 



This phenomenon, announced by Stebbins and Whitford (60), and 

 consisting in an increase of the color index of distant galaxies, not 

 accounted for by the red shift, led to far-reaching speculations on 

 observable effects of stellar evolution. The effect seems to be restricted 

 to elliptical nebulae (purely Population II), whereas spirals (mixed 

 populations) do not show reddening (61). The distant nebulae are 

 observed at an earlier stage of evolution (on account of light time), 

 and it has been suggested that the effect could be accounted for by 

 the red giants of Population II disappearing with time (blowing up 

 or collapsing) , which would tend to make the population bluer. How- 

 ever, a multicolor study of the spectral-energy distribution of a dis- 

 tant elliptical nebula has shown that "the result is definitely not that 

 expected from the death of red giants" (62). The effect continues to 

 the greatest distances (63). 



Vaucouleurs (64) suggested that the effect is due to the depression 

 in the ultraviolet produced by absorption lines and bands. With the 

 red shift the ultraviolet depression is displaced into the blue, making 

 the blue-red color index redder. At least part of the effect can be 

 accounted for in such a manner (65). 



As to the spirals, they are known to contain a considerable amount 

 of nebulosity in emission (66) ; this, especially that due to hydrogen, 

 will fill the ultraviolet depression, comiterbalancing the absorption. 

 The absence of the reddening for spirals is thus explained without in- 

 voking stellar evolution. Over the time intervals involved, evolution 

 may well affect individual stars, but considerable effects upon the 

 entire Population II are unlikely. 



