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Flat in this context applies only to the curvature in the "assumed" space time grid which is something to be looked at in a three dimentional view plus another (t) ... and flatness must then apply to a notion of what the universe is really doing...is it expanding forever or set to colapse one day...I think that is their flatness as we are talking about a geometric layout which somehow includes time and has little to do with the reality of the shape of the universe...or at least that is my take upon the matter.
\alex
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