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Big Bang models result from applying Relativity equations to expanding space; however, as Einstein noted, Relativity was not established in expanding space. A basic question arises: is Relativity valid in expanding space? Its validity in expanding space has implications on the properties of space, matter, time, light speed? Has anyone analyzed this? Or the validity of Relativity in expanding space is irrelevant because Big Bang models are just mathematical models and the only thing that matters is to obtain equations that fit cosmic data?