I was talking to my daughter about space-time, and about the theory that it is like a 4-D sphere. I mentioned that, for me, it's a big conceptual problem because as you look back in time towards the big bang in any direction you are looking away from where we find ourselves - yet a circle should come back to where it started. Then I thought that this is like what happens with the spherical earth: at sea level it looks flat and you can see about 3 miles until the curvature makes the view 'disappear over the horizon'. The actual circle round the earth is 25,000 miles (8,333.3-recurring times longer than 3 miles). The three-mile figure, I suppose, depends on the height of a human being at sea level. Still, I thought you could use the same sort of logic with the universe. We can see 10,000 billion light years back to the 'early universe', but maybe this is just the 'horizon' before the curvature goes round and comes back to where it started (here). Using the same calculation as above, that would make the full circumference of the universe 8.3 trillion light years. It would actually depend on our 3-D 'height' which I suppose would consist in the volume (or the mass?) of the earth. I'm an editor, not a physicist, so of course there might be a huge hole in my logic, but I thought I should share it with you. If I'm wrong, tell me (in layman's terms), why I must be wrong.