Consider a small region of the Universe where the density of matter is slightly higher than elsewhere. The gravitational force holding this region together will be slightly greater than for comparable volumes elsewhere. Therefore, if gravity is the only force acting, the expansion of the overdense region will lag behind the expansion of the Universe as a whole. The contrast in density between this region and the rest of the Universe will be enhanced as time goes on. If the contrast becomes sufficiently great, the force of gravity may halt the expansion of the lump and cause it to collapse. The hope is that a lump of suitable mass would collapse to form a star, a galaxy or a cluster of galaxies. This is fine if no force other than gravity is acting, but if the Universe is hot enough, pressure forces may resist the compressing effect of gravity. This is a-particularly important point before the electrons and protons have combined to form neutral hydrogen, for then the radiation exerts a tremendous pressure by colliding with the free electrons. This pressure is so great that at that time, the force of gravity cannot dominate except for enormous lumps having masses in excess of 1018 solar masses. Consequently inhomogeneities on scales of clusters of galaxies or smaller cannot be gravitationally enhanced during the fireball phase of the Universe. Once the electrons and protons combine, however, there are no longer any free electrons and the
matter no longer feels the pressure of the radiation. Gravity then enhances all structure on scales exceeding 105 solar masses. On smaller scales, the pressure arising from the mutual collisions between atoms can resist gravitational amplification. Thus although gravity cannot form stars, it can certainly cause objects
of galactic size to condense out of the Universe.

The neutralization of the cosmic plasma takes place according to simple models when the Universe is some hundred thousand years old. This event, according to the gravitational instability theory, marks the beginning of the process of galaxy formation. It may take another hundred million years for gravity to halt the expansion of the lump, but once that happens, the lump (referred to as a PROTOGALAXY at this stage) goes into rapid collapse. It is not difficult to envisage the possible sequence of events during the collapse of the protogalaxy, though to establish the chain of events rigorously is another matter: this is a most complex prob¬lem in theoretical astrophysics. As the protogalaxy starts to collapse it may fragment, and the first generation. of stars will form within the fragments.-Some of these stars may evolve and explode before the protogalaxy has stopped collapsing. These will enrich the protogalactic material with elements such as carbon, oxygen and nitrogen. Strong gravitational interactions organize the stars into a stellar system of the kind we observe. Any gas not converted into stars may form a flattened disc by the time the collapse is halted by the build-up of pressure forces. Speculative ? Certainly, but this scenario is not totally without theoretical foundation. In this theory the galaxies form as a result of a succession of competitions between gravity and opposing pressure forces. First the pressures prevent gravitational enhancement,but at neutralization it is gravity that wins, only to be forest 11 A later by the build-up of pressure forces within the collapsing protogalaxy.

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