ON EXPANSION IN THE WIDTH FOR DOMAIN-WALLS

The well-known idea to construct domain wall type solutions of field equations by means of an expansion in the width of the domain wall is re-examined. We observe that the problem involves singular perturbations. Hilbert-Chapman-Enskog method is used to construct the consistent perturbative expansion. We obtain the solutions to the second order in the width without introducing an effective action for the domain wall. We find that zeros of the scalar field in general do not lie on a Nambu-Goto trajectory. As examples we consider cylindrical and spherical domain walls. We find that the spherical domain wall, in contradistinction to the cylindrical one, shows an effective rigidity.