When light from an object takes longer to reach us, how do we know that it’s spacetime that’s “moving”, and not the object itself?

Question

I guess that it’s easier to tell when the shift is happening faster than the speed of light, but what about when it’s not that fast?

In: Physics

Answer 1

If you know how far away something is from us, then you know essentially how much of it’s apparent movement relative to us is due to universal expansion. Any difference from that is the objects velocity within spacetime at the precise moment the light that we see was emitted.