1. Vilenkin, A. & Field, G.B. Nature 326, 772 (1987).
2. Campbell, P. Nature 320, 679 (1986).
3. Witten, E. Nucl. Phys. B246, 557 (1985).
4. Chudnovsky, E.M., Field, G.B., Spergel, D.N. & Vilenkin, A. Phys. Rev. D34, 944 (1986).
5. Ostriker, J., Thompson, C. & Witten, E. Phys. Lett. (in the press).