Domain Walls and the Universe

D=11 supergravity possesses D=5 Calabi-Yau compactified solutions that may be identified with the vacua of the Horava-Witten orbifold construction for M--theory/heterotic duality. The simplest of these solutions naturally involves two 3-brane domain walls, which may be identified with the orbifold boundary planes; this solution also possesses an unbroken $\Z_2$ symmetry. Consideration of nearby excited solutions, truncated to the zero-mode and $\Z_2$ invariant sector, yields an effective D=4 heterotic theory displaying chirality and N=1, D=4 supersymmetry.