On the higher order mean curvatures of spacelike hypersurfaces in pp-wave spacetimes

ct. We study some geometric aspects of the higher order mean curvatures (or, more simply, the so-called r-th mean curvatures) of a spacelike hypersurface immersed in a pp-wave spacetime, namely, in a connected Lorentzian manifold admitting a parallel and lightlike vector field. Initially, we develop general Minkowski-type integral formulas for compact (without boundary) spacelike hypersurfaces and we apply them to the study of the uniqueness and nonexistence of compact spacelike hypersurfaces in terms of their r-mean curvatures. Next, we obtain a characterization of r-stability for r-maximal compact spacelike hypersurfaces through of the analysis of the first nonzero eigenvalue of an differential operator naturally attached to the r-th mean curvature. For the noncompact case, by applying new forms of maximum principles on complete noncompact Riemannian manifolds due to Caminha [17] and Al´ıas, Caminha and Nascimento [3], we obtain sufficient geometric conditions involving some r-th mean curvature and the volume growth that allow us to establish some nonexistence results or to guarantee that a complete noncompact spacelike hypersurface is either totally geodesic, or totally umbilical, or maximal, or r-maximal. We also obtain estimates for the index of minimum relative nullity of spacelike hypersurface