I've read and been told that, based on the distance between repeated irregularities in the microwave background radiation, we can now conclude that the universe is flat rather than open or closed. I heard this again at the excellent presentation given by Dr. Lawrence Krauss last night at the World Science Festival event. I understand this to mean that the curvature (average curvature?) of the universe is 0 rather than positive or negative. Is that the right interpretation? If so, I don't understand how this can be be determined by such a measurement. Every measurement has some possibility of error. I can see how one could definitively conclude that the curvature of the universe is positive or negative, but not that it is exactly 0. If anyone can help me out here, I'd appreciate it.