Acknowledgements

I would like to thank D. Brill, W. Burke, D. Chernoff, D. Chillingworth, J. Crutchfield. G. Gibbons. B.l Hu, J. Isenberg, A. Kaufman, E. Knobloch, J. Marsden, C. Misner, N. Packard and D. Rand for helpful discussions. This work was supported by a Miller Fellowship at Berkeley together with NSF grant AST 79-152^ 1 and was partially written whilst the author was a participant in the 'Early Universe' programme of the Institute for Theoretical Physics at Santa Barbara.