Cosmology in (2 + 1) -Dimensions, Cyclic Models, and Deformations of M2,1

Do there exist Lorentzian manifolds all of whose light-like geodesics are periodic? A surprising fact is that such manifolds exist in abundance in (2 + 1)-dimensions (though in higher dimensions they are quite rare). This book deals with the deformation theory of M2,1 (which furnishes almost all the known examples of these objects).