A perspective on totally geodesic submanifolds of the symmetric space $G_2/SO(4)$

Abstract: We provide an independent proof of the classification of the maximal totally geodesic submanifolds of the symmetric spaces $G_2$ and $G_2/SO(4)$, jointly with very natural descriptions of all of these submanifolds. The description of the totally geodesic submanifolds of $G_2$ is in terms of (1) principal subalgebras of $\mathfrak{g}_2$; (2) stabilizers of nonzero points of $\mathbb{R}^7$; (3) stabilizers of associative subalgebras; (4) the set of order two elements in $G_2$ (and its translations). The space $G_2/SO(4)$ is identified with the set of associative subalgebras of $\mathbb{R}^7$ and its maximal totally geodesic submanifolds can be described as the associative subalgebras adapted to a fixed principal subalgebra, the associative subalgebras orthogonal to a fixed nonzero vector, the associative subalgebras containing a fixed nonzero vector, and the associative subalgebras intersecting both a fixed associative subalgebra and its orthogonal. A second description is included in terms of Grassmannians, the advantage of which is that the associated Lie triple systems are easily described in matrix form.