12 The Evolution of the Universe 



The galaxies move in relation to each other but in a manner 

 quite different from orbital or rotational movements. Studying 

 peculiar shifts of red lines in the spectra of light emitted by celestial 

 bodies and galaxies, Slipher, in 1920, deduced that the galaxies 

 appear to be retreating from each other at rates proportional to 

 their distances from us, and, soon after, Hubble concluded that the 

 universe as a whole was expanding (Sandage, 1956). The optical 

 explanation of the "retreating galaxies" can be readily visualized 

 by the comparison made by Gamow ( 1951 ) . If we imagine that 

 the galaxies are black dots on a rubber balloon and that we are 

 sitting on any one of the dots as the balloon is inflated, all the other 

 dots will appear to be getting farther away from us. The more 

 distant dots will appear to "retreat" faster than the closer ones. 



This premise of the expanding nature of the universe is the basis 

 for the two current theories seeking to explain the present form and 

 content of the universe, the explosion theory (Gamow, 1951) and 

 the steady state theory (Hoyle, 1950; Bondi, 1952). 



According to the explosion theory, the beginning of our present 

 universe started at a time when all matter had collapsed from space 

 into a tremendous glob of primordial matter. In this presumed 

 glob, pressures and temperatures became so high (the temperature 

 is estimated to have been about a billion degrees) that matter was 

 dissociated into neutrons. Presumably at this point of greatest 

 contraction, the internal pressure literally blew the glob of neutrons 

 apart, or started it expanding. This matter has been expanding 

 ever since. As this expansion progressed, both temperature and 

 pressure decreased and soon reached a point at which neutrons 

 would disintegrate into protons and electrons which would com- 

 bine in the form of stable elements. Physicists believe that within 

 half an hour after the postulated expansion started, the entire mass 

 would have formed into atoms, mostly into hydrogen but possibly 

 some into helium as well. 



Under the influence of gravitational forces, this tremendous ex- 

 panding and turbulent mass of gas broke up into huge "gas balls," 

 each gas ball continuing its movement from the center of the ex- 

 plosion into space but also developing an axial rotation of its own. 

 Within each gas ball, the gases, again because of their turbulence, 

 gradually aggregated into smaller and more dense spheres which 

 condensed to become stars. Our sun is such a star. 



If the explosion theory is correct, the time since the actual ex- 

 plosion can be computed from the rate of expansion in the universe. 



