Annu. Rev. Astron. Astrophys. 1991. 29: 325-362 Copyright © 1991 by Annual Reviews. All rights reserved

2.2.4. DOMAIN WALLS The GUTs phase transition produces certain characteristic discontinuities in the matter distribution. This happens during the spontaneous breakdown of symmetry, which leads the mediating scalar fields to have a discrete set of characteristic values. Thus one region or ``domain'' has one value of the scalar field, while its neighbor has another. The boundary wall between the domains would therefore be a region of discontinuity. We expect to see this as a sheet of inhomogeneity in the large scale matter distribution in the universe. The present data do not reveal any such inhomogeneity in the observable universe.