[neutralino]

Yes, one option is that the universe is infinite, however that picture does not rule out the possibility of the universe being finite-- the universe could have positive curvature and be closed. To picture this, let's throw away one dimension of space and imagine living on the surface of a sphere (so we don't see the third dimension in the radial direction of the sphere). In this case, we can continue the diagram you have in your last post ad infinitum without hitting any boundary, but the space is not infinite in its extent. To come back to "real life" the topology of the actual universe could then be equivalent to that of the 3-dimensional surface of a 4-d sphere (something that we can't really imagine!)