1. M. Ludvigsen: General Relativity. A Geometric Approach, (Cambridge University Press, Cambridge 1999)
2. R. d’Inverno: Introducing Einstein’s Relativity, (Oxford University Press, New York 1992)
3. E. P. Hubble: Pub. Yerkes Obs. 4, 69 (1920); Astrophys. J. 62, 409 (1925); Astrophys. J. 63, 236 (1926); Astrophys. J. 64, 321 (1926); Proc. Nat. Acad. Sci. U.S. 15, 169 (1929)
4. A. S. Eddington, The Nature of the Physical World, (The University of Michigan Press, Ann Arbor 1958) p. 165
5. A. S. Eddington, The Expanding Universe, (The University of Michigan Press, Ann Arbor 1958) Chapter III