TOPOLOGICAL DEFECTS IN GRAVITATIONAL THEORIES WITH NONLINEAR LAGRANGIANS

The gravitational field of monopoles, cosmic strings, and domain walls is studied in the quadratic gravitational theory R + alphaR2 with alp ha\R\ much less than 1, and is compared with the result in Einstein's theory. The metric acquires modifications which correspond to a short- range ''Newtonian'' potential for gauge cosmic strings, gauge monopole s, and domain walls, and to a long-range one for global monopoles and global cosmic strings. In this theory the corrections turn out to be a ttractive for all the defects. We explain, however, that the sign of t hese corrections in general depends on the particular higher-order der ivative theory and topological defect under consideration. The possibl e relevance of our results to the study of the evolution of topologica l defects in the early Universe is pointed out.