In Brian Greene's 'Fabric of the Cosmos', he describes three possible curvatures that space may have: positive curvature (like a ball or torus), negative curvature (like a saddle) or zero curvature (like an infinite flat tabletop, or like a Pacman video game screen). In his analogy to a video game screen, he demonstrates how, as in Pacman, if you exit side of the screen, you reappear at the other side, same with top/bottom, so it is with a flat universe - if you continue in one direction long enough in a zero-curvature universe, you will eventually wrap around, and arrive back where you started. He says that, mathematically, this is called a "2 dimensional torus". ??? Colour me hogtied, but I thought that was the quintessential closed, curved universe (be it spherical or toroidal). i.e.: the way you get 3 dimensional space to loop back on itself is to bend it in the 4th dimension so that it is a 4D sphere or torus. A zero-curvature universe would very definitely NOT loop around like a video game screen, it would continue on forever. Am I misunderstanding?