1. J. D. North, The Measure of the Universe ( Oxford: Oxford University Press, 1965 ).
2. I shall, as discussed later, assume that General Relativity is the correct theory describ¬ing space-time and gravitation.
3. The way this can be done locally has been carefully described by Kristian and Sachs, Astrophysical Journal 143 (1966), p. 379.
4. A very readable discussion of recent observations and their interpretation is in D. W. Sciama, Modern Cosmology (Cambridge: Cambridge University Press, 1971). A more detailed discussion of the physics involved is in S. Weinberg, Gravitation and Cosmology (New York: Wiley and Sons, 1972); a detailed review of cosmology and its philosophy is E. R. Harrison’s Cosmology (Cambridge: Cambridge University Press).
5. In certain such universes, there could be two centres. The argument then proceeds unchanged; there are two galaxies whose situation is completely different from that of all other galaxies in the universe.