The picture described above provides one explanation of why the Universe may have become lumpy. We are, however, a long way m. from being able to verify such a theory by observation: galaxies ay bear few birthmarks that might provide rather indirect clues to their origins. It is not surprising, therefore, that cosmologists have been able to dream up a variety of theories for the origin of galaxies. The cosmic turbulence theory provides an important viable alternative to the gravitational instability picture. Ever since the discovery of spiral galaxies, it has been remarked that they resemble cosmic whirlpools. This is just what the cosmic turbulence theory proposes: that galaxies are fossilized eddies left over from a previous era when the Universe was turbulent. Prior to the time when the Universe became neutral, the intense radiation pressure would have caused the hypothetical turbulent whirls and eddies to bounce off one another, creating small eddies. Immediately after neutralization, the radiation can n longer affect the motion of matter, and when the eddies collide and mix there is nothing to prevent them from generating enormous shock waves and lumps which are later to be identified with galaxies. Unfortunately, turbulence is poorly understood and consequently it is rather difficult to calculate precisely what should be expected on the basis of the cosmic turbulence theory. Never¬theless, the theory has intuitive appeal in producing the cosmic whirlpools directly. An objection to the theory is that at the very early times when nucleosynthesis takes place , such a turbulent Universe represents a radical departure from the simple models. It seems unlikely that a turbulent Universe would provide the required quantities of deuterium and helium.

We see in the galaxy formation theory an important application of the Big Bang hypothesis. The hypothesis tells us that in the past there was a time before which the matter was ionized and after which the matter was neutral. This EPOCH OF NEUTRALIZATION marks the onset of the galaxy formation process in both theories we described. The hot Big Bang hypothesis tells us also that at very early times the synthesis of deuterium and helium took place. The requirement that all the observed deuterium and helium be synthesized cosmically allows us to impose constraints on the various theories of galaxy formation without even looking at galaxies!

Comments are closed.