GRAVITATIONAL SHOCK-WAVES GENERATED BY EXTENDED SOURCES - ULTRARELATIVISTIC COSMIC STRINGS, MONOPOLES AND DOMAIN-WALLS

We study the gravitational shock waves generated by ultrarelativistic extended sources, mainly when cosmic strings (local, global, spinning and superconducting) and other topological defects (monopoles and domain walls) are boosted and become ultrarelativistic. In this limit, the sources travel at the speed of light and the physical parameters (mass, charges and spin) must be taken gamma-dependent [gamma = (1 - upsilon-2)-1/2] and must vanish in an appropriated way. The scattering matrices of Klein-Gordon fields in these geometries are computed and the low- and high-energy behaviour analysed. Comparison with the scattering by point-like sources (Aichelburg-Sexl geometry for which the S-matrix has coulombian type poles) is made. For extended as well as point-like sources, the ultrarelativistic limit and the weak-field limit (or large-distance behaviour) of the geometry give the same scattering matrices.