Are there any cosmological models that say that the shape of the universe changes with time? As in it may oscillate between being a sphere, torus , saddle or a plane? I remember the course in Astrophysics that I took in my BSc studies, and we had it that the shape of the universe depends on the cosmological constant, ##k## either it's positive, negative or zero. And the options were if we had k>0 then it's a sphere if it's k<0 it's a saddle and for k=0 a plane. Why isn't there an option of a torus?