Hi, As I understand things we exist on the 'surface' of a 3 sphere of radius R in the context of general relativity. The popular analogy is that our 3D space can be visualized as the surface of an expanding balloon I would like to ask if anything 'exists' on the interior/exterior of the balloon. More precisely I would like to ask if the int/ext contains the vacuum of quantum field theories. I am interested if zero-point energies can exist on the int/ext of the balloon and I am also interested in calculating casimir energies with our universe representing the boundary conditions (the plates in the popular casimir effect). Before one can do this clearly the question is can the int/ext of the balloon be considered tangible field theoretic manifolds. Thanks in advance.