Moduli Spaces and Brane Solitons for M-Theory Compactifications on Holonomy G_2 Manifolds

We investigate the local geometry on the moduli space of G_2 structures that arises in compactifications of M-theory on holonomy G_2 manifolds. In particular, we determine the homogeneity properties of couplings of the associated N=1, D=4 supergravity under the scaling of moduli space coordinates. We then find some brane solitons of N=1, D=4 supergravity that are associated with wrapping M-branes on cycles of the compact space. These include cosmic strings and domain walls that preserve half of supersymmetry of the four-dimensional theory, and non-supersymmetric electrically and magnetically charged black holes. The geometry of some of the black holes is that of non-extreme M-brane configurations reduced to four-dimensions on a seven torus.