Abstract. On a Lorentzian manifold the existence of a parallel null vector field implies certain constraint conditions on the induced Riemannian geometry of a space-like hypersurface. We will derive these constraint conditions and, conversely, show that every real analytic Riemannian manifold satisfying the constraint conditions can be extended to a Lorentzian manifold with a parallel null vector field. Similarly, every parallel null spinor on a Lorentzian manifold induces an imaginary generalised Killing spinor on a space-like hypersurface. Then, based on the fact that a parallel spinor field induces a parallel vector field, we can apply the first result to prove: every real analytic Riemannian manifold carrying a real analytic, imaginary generalised Killing spinor can be extended to a Lorentzian manifold with a parallel null spinor. Finally, we give examples of geodesically complete Riemannian manifolds satisfying the constraint conditions.
Background and main resultsThis paper is a contribution to the research programme of studying global and causal properties of Lorentzian manifolds with special holonomy. A Lorentzian manifold has special holonomy if the connected component of its holonomy group is reduced from the full group SO 0 (1, n), but still acts indecomposably, i.e., without non-degenerate invariant subspaces. In this situation the Lorentzian manifold admits a bundle of tangent null lines that is invariant under parallel transport. The possible special Lorentzian holonomy groups were classified in [7] and [14], all of them can be realised by local metrics [12], but many questions about the consequences of special holonomy for global and causal properties of the manifold are still open. A special case of this situation is when the parallel null line bundle is spanned by a parallel null vector field. This is the case we will study in this paper. It is motivated by the question which Lorentzian manifolds admit a parallel spinor field, which in turn draws its motivation from mathematical physics. Since a parallel spinor is invariant under the spin representation of the holonomy group, indecomposable Lorentzian manifolds with parallel spinors have special holonomy. However, since SO 0 (1, n) has no proper irreducible subgroups, the situation is very different from the Riemannian case, where we have several irreducible holonomy groups that admit an invariant spinor. In fact, a spinor field φ on any Lorentzian manifold (M, g) induces a causal vector field V φ , its Dirac current, which is defined by g(X, V φ ) = − X · φ, φ , 2010 Mathematics Subject Classification. Primary 53C50, 53C27; Secondary 53C44, 35A10, 83C05.