I was watching a Horizon program the other night on the BBC and was interested that they were able to repeat the "triangle on the surface of a sphere" experiment on the universe as a whole. When this experiment is performed on the surface of the Earth on a large enough scale the angles of the triangle don't add up to 180 degrees and shows that the earth is not flat to someone even though they may not be able to leave the surface. So , to recap, they were somehow able to draw these lines in the universe (I have no idea how!) and came back with the discovery that the angles of the triangle actually added up to180 degrees. They said that this showed that space was flat and not curved! My question is this. If space is flat why is space-time curved?