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We show that if all fundamental observers measure the cosmic microwave background radiation to be almost isotropic in a region of an expanding universe, then that universe is almost spatially homogeneous and isotropic in that region. This demonstrates the stability of the important exact result of this type previously proved by Ehlers, Geren and Sachs (1968), and formalizes the way in which almost-isotropy of the background radiation gives evidence that the universe is almost a Friedmann-Lema\^{i}tre-Robertson- Walker (FLRW) space-time. This is the needed foundation for all recent analyses, based on the Sachs-Wolfe effect, of how background anisotropies relate to growing density inhomogeneities in an almost-FLRW expanding universe.
By using this approach, through which we can consider directly how matter imposes anisotropies on freely propagating background radiation, we also show how background anisotropy measurements place limits on the deviations of the universe from an FLRW geometry. This limits are not as detailed as those determined by the usual Sachs-Wolfe approach, but they are more model- independent. \\
Ehlers, J., Geren, P., and Sachs, R. K. 1968, J. Math. Phys., 9, 1344.
