The Bryant-Salamon G₂ manifolds and hypersurface geometry

We show that two of the Bryant-Salamon G_2-manifolds have a simple topology ; homeomorphic to the complement of some submanifolds of the 7-dimensional sphere. In this connection, we show there exists a complete Ricci-flat (non-flat) metric on the complement of an m-dimensional sphere in an n-dimensional sphere for some n-1>m. We also give many examples of special Lagrangian submanifolds of the cotangent bundle of the sphere with the Stenzel metric. Hypersurface geometry is essential in the argument.