Abstract
In his well-known problem section appearing in [Yl], s.-T. Yau posed the problem of establishing a Lorentzian analogue of the Cheeger-Gromoll [CG] splitting theorem of Riemannian geometry. Although stated in [Yl] as a problem in pure Lorentzian geometry, there is a physical, as well as mathematical, motivation for considering this problem [Y2). The purpose of this paper is to discuss the physical motivation, and to survey the progress that has been made on this and related problems. We will also briefly describe some of the methods which have been employed to attack such problems.
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