$\alpha$-associated Metric On Rigged Null hypersurfaces

Abstract

Let $x:M\to\Bm$ be the canonical injection of a Null Hypersurface $(M,g)$ in a semi-Riemannian manifold $(\overline{M},\bar g)$. A rigging for $M$ is a vector field $L$ defined on some open set of $\overline{M}$ containing $M$ such that $L_p\notin T_pM$ for each $p\in M$. Such a vector field induces a null rigging $N$. Let $\bar \eta$ be the 1-form which is $\bar g$-metrically equivalent to $N$ and $\eta=x^\star\bar\eta$ its pull back on $M$. We introduce and study for a given non vanishing function $\alpha$ on $M$ the so-called $\alpha$-associated (semi-)Riemannian metric $ g_{\alpha}=g+\alpha\eta\otimes \eta$. For a closed rigging $N$ we give a constructive method to find an $\alpha$-associated metric whose Levi-Civita connection coincides with the connection $\nabla$ induced on $M$ by the Levi-Civita connection $\overline{\nabla}$ of $\overline{M}$ and the null rigging $N$. We relate geometric objects of ${g}_{\alpha}$ to those of $g$ and $\overline{g}$. As application, we show that given a null Monge hypersurface $M$ in $\R_q^{n+1},$ there always exists a rigging and an $\alpha$-associated metric whose Levi-Civita connection coincides with the induced connection on $M$.