TIME SCALE OF OUR UNIVERSE OPIK 223 



universe — is such as to suggest an age not exceeding 6,000 million years 

 for the universe in its present form and content. The extragalactic 

 nebulae, with our galaxy and its backbone of Population II, may have 

 been formed some 4,500 million years ago, the sun as a star of Popula- 

 tion I coming into being perhaps later. 



Cosmological repulsion is a theoretical superstructure which is not 

 necessarily required by the existing observational evidence. The 

 same is true of the continuous creation of matter and the alternative 

 interpretations of the nebular red shift ; these are mere possibilities, 

 serving the purely esthetic purpose of denying the universe a temporal 

 origin. 



The observed velocities of recession exceed one-fifth of the velocity 

 of light, the energy corresponding to a packing fraction (fraction of 

 mass converted into kinetic energy) of 0.02 per nucleon (proton or 

 neutron). Nothing short of an explosion from the densest-known 

 state of matter — nuclear fluid — could be advocated as the cause. Our 

 knowledge of the present density of matter in the universe is insufficient 

 to decide between the two possibilities : that of open space, in which 

 case the whole universe is an irreversible process of temporal origin, 

 and that of closed space, in which the universe may return to its 

 initial state, implying oscillations — the collapsing universe rebounding 

 from the elastic forces of the nuclear fluid at a state of maximum 

 compression, to begin a new phase of expansion. 



It may appear at first sight that, at an advanced stage of collapse, 

 when all individual bodies have melted into a imiform gaseous mass, 

 the gaseous universe may be prevented from further collapsing by the 

 elastic forces of the gas itself, like an oscillating gaseous star of which 

 the cepheids are examples. However, it is likely that, with the enor- 

 mous kinetic energy of contraction, the universe will first pass quickly 

 through the stage of building up of heavy elements from hydrogen 

 and helium, most of the hydrogen remaining unconverted before the 

 next stage, that of nuclear dissociation and formation of neutron gas, 

 begins — electrons being squeezed into and absorbed by the positively 

 charged atomic nuclei. This is the reverse of the process by which 

 Gamow and others visualized the origin of the elements after the ex- 

 plosion of the primeval atom. Formation of neutron gas absorbs 

 enormous amounts of energy, and this, so to speak, blows the bottom 

 off the resistance of the gas to compression. In such a case, the 

 so-called ratio of specific heats of the gas (mixed with strong radia- 

 tion) is less than 4/3 and, according to a well-known theorem on the 

 structure of gaseous spheres, the universe becomes intrinsically un- 

 stable and cannot cease collapsing while in a gaseous state. Only 

 when the perfect-gas laws no longer are valid, i. e., when the stage of 

 nuclear fluid is reached, will there develop enough resistance to stop 

 the collapse and invert the trend of events. 



