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?
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General relativity was written to describe all behaviour of spacetime. And expanding universe is one small subset of the whole general class of spactimes in the phase space of general relativity.
So yes, relativity does apply to an expanding universe. At least general relativity does.
Zo the Relativist
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@ Jerry Schrimer since GR applies in the expanding space of inter galaxy cluster voids, where gravity is not as strong as near earth, we could compare the speed of light to that. Now as per Shapiro, near the Sun (where gravity is stronger then near the earth), speed of light is less then c (when viewed from earth). Now the speed of light in the void (inter galaxy cluster expanding space) should be more then c (when viewed from earth), because gravity there is even less strong then near earth? – Árpád Szendrei Jul 05 '18 at 00:11
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@ÁrpádSzendrei: all that is important is that the speed of light is constant when measured locally. If the local speed of light varied, you would have numerous consequences for chemistry and spectroscopy that would be detectible unless you really futzed with your model about how it varied to get those things undetectable. – Zo the Relativist Jul 05 '18 at 19:18