A spiral galaxy - but which?
It is shown that if the average density of the universe exceeds a certain critical value then the expansion of the universe will eventually cease and it will start to contract. Conversely, if the average density is less than or equal to the critical density, the expansion will go on forever. An expression for the critical density is derived from simple classical energy arguments in terms of G, the universal constant of gravitation, and H(t). The Hubble parameter, H(t), is essentially the reciprocal of the universe's age, 13.7 billion years. The critical density is found to be remarkably small, namely around 6 hydrogen atoms per cubic metre. This is a staggeringly low density, being 13 orders of magnitude smaller than the best vacuum ever achieved on earth.
Advantages: No relativity required.
Limitations: This opening Chapter ignores 'dark energy'.
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A spiral galaxy - but which?