= Reception-event dependence of intergalactic redshift
{title2=$1+z_{BA}=a(t_A)/a(t_B)$}
A <cosmological redshift> relates a particular emission event to a particular reception event along a <null geodesic>. Two sources' redshifts on Earth's past light cone do not specify the light exchanged between those sources. If a photon from B passes through A before reaching Earth, and A's comparison emission lies at that passage event, then $1+z_{BA}=(1+z_B)/(1+z_A)$. Without this causal and temporal specification the ratio need not give the redshift measured in A.
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