Left-invariant pseudo-Riemannian metrics on Lie groups: The null cone

We study left-invariant pseudo-Riemannian metrics on Lie groups using the moving bracket approach of the corresponding Lie algebra. We focus on metrics where the Lie algebra is in the null cone of the $G=O(p,q)$-action; i.e., Lie algebras μ where zero is in the closure of the orbits: $0\in \overline{G\cdot \mu }$. We provide examples of such Lie groups in various signatures and give some general results. For signatures $(1,q)$ and $(2,q)$ we classify all cases belonging to the null cone. More generally, we show that all nilpotent and completely solvable Lie algebras are in the null cone of some $O(p,q)$ action. In addition, several examples of non-trivial Levi-decomposable Lie algebras in the null cone are given.