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As far as I can understand, Hubble's law has the form of V(to)=HoDo at a close distance. The meaning of this equation is as follows. For example, when z=0.01, the emitted photons arrive at us and tell us the speed and distance measured at now. That is, the distance to this galaxy is a(to)X.

My question is as follows: When observing a distant galaxy, is its proper distane a(to)X or a(t)X? ( X is comoving distance)

As far as I understand, I think we should use a(to) because we see photons arriving now, whether it's a galaxy far away.

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