On the fundamental group of a complete globally hyperbolic Lorentzian   manifold with a lower bound for the curvature tensor
  In this paper, we study the fundamental group of a certain class of globally hyperbolic Lorentzian manifolds with a positive curvature tensor. We prove that the fundamental group of lightlike geodesically complete parametrized Lorentzian products is finite under the conditions of a positive curvature tensor and the fiber compact. 
Jun-ichi Mukuno
math.DG