So in a closed universe which has the possibility that travelling far enough across the universe you would end up where you started. The close universe infers curvature and could be described by an extra dimension. Is this dimension just abstract. If this could be verified then how can it be abstract. RE: "It's just a way to express coordinates" you can have a w,x,y,z coordinate system where w is an extra spacial dimension. So what you are saying is the curvature of space would NOT be "w" in a w,x,y,z system where xyz refer to classical three-dimensional space.