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Thanks. That caution makes sense to me (I'm glad).It looks to be related, but there's only so far you can push the comparison. Bell's Spaceship paradox is at heart a relativity of simultaneity problem within a flat local region; this cosmological question is about comparisons of coordinates across non-locally curved regions.

Hope it's okay to clarify my understanding in context. Is it correct to say the curvature between coordinates in the cosmological question is described, accounted for, by the second term of the FLRW solution to the GR field equations, an equation taken as a model of the shape of the our 4d universe?

[itex]{ -c }^{ 2 }d{ \tau }^{ 2 }={ -c }^{ 2 }d{ t }^{ 2 }+a{ \left( t \right) }^{ 2 }d{ \Sigma }^{ 2 }[/itex]

And is t in that FLRW equation the coordinate time of a frame co-moving (at rest) with respect to the CMB? Or rather, what is that t?

[Edit] Sorry, maybe that should be a separate thread. I just wouldn't have the question if I wasn't trying to follow this one.

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