1. Robertson, H. P., Rev. Mod. Phys., 5, 62 (1953). Bondi, H., Cosmology (Cambridge University Press, Cambridge, England, 1960). Departures from homogeneity and isotropy apparently do not prevent the occurrence of singularities in cosmology as shown by Hawking, S. W., Proc. Roy. Soc., A, 294, 511 (1966); A, 295, 490 (1966).
2. Rindler, W., Mon. Not. Roy. Astro. Soc., 116, 662 (1956).
3. Harrison, E. R., Astron. J. (in the press).
4. Wigner, E. P., Rev. Mod. Phys., 29, 255 (1957). Bergmann, P. G., and Komar, A. B., Recent Developments in General Relativity (Pergamon, London, 1962). Anderson, J. L., Relativity and Gravitation (edit. by Chiu, H. Y., and Hoffmann, W. F.) (Benjamin, New York, 1964).
5. Landau, L. D., Niels Bohr and the Development of Physics (edit. by Pauli, W.) (Pergamon, London, 1955).