1. For early work see the review papers ofY. B. Zeldovich: inConfrontation of Cosmological Theories with Observation Data, edited byM. S. Longair (Dordrecht, 1974);T. Criss, R. A. Matzner, M. Ryan andL. Shepley: inGeneral Relativity and Gravitation, edited byG. Shaviv andN. Rosen (New York, N. Y., 1975).
2. J. R. Gott, J. E. Gunn, D. N. Schramm andB. M. Tinsley:Astrophys. J.,194, 543 (1974).
3. C. B. Collins andS. W. Hawking:Astrophys. J.,180, 317 (1973);Mon. Not. R. Astron. Soc.,462, 307 (1973).
4. N. Caderni andR. Fabbri:Phys. Lett. A (in press).
5. N. Caderni andR. Fabbri:Nuovo Cimento B,44, 228 (1978);Phys. Lett. A,68, 144 (1978).