People tell me that we cannot extend our inertial frame, as defined in special relativity, across the Universe because that would be in violation of general relativity. The problem is that general relativity says that space-time can be curved whereas a global inertial frame assumes flat space-time. However it has been observationally verified that the Universe is spatially very close to flat. Thus only the time-component can be curved leading to the Universe's expansion rate either accelerating or decelerating. If the Universe's expansion rate is constant then space-time is indeed flat. But if one is only considering a small interval of cosmological time then the Universe's expansion rate is almost constant. Thus it seems to me that one could extend our inertial frame globally provided that one is only considering a small interval of cosmological time. Is that correct?