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Given my new understanding of this (Thanks!), a few questions come to mind:
Consider a cosmic background photon which was originally emitted someplace
which I'll call 'there', and is just now getting 'here'. Say this photon
comes from an event that was 10^10 years ago. How far apart were 'here'
and 'there' when the photon was emitted? How far apart are 'here' and
'there' now? If space has been expanding, it's not clear to me that the
answer to either question is 10^10 lightyears.
Seems like the answer is tied up in general relativity and the
geometry of the Universe. Let me pose the same question a little
differently.
If about 12 billion years ago our friend 'Q' (from STNG) turned on a
flashlight for 1 second while standing at the 'center' of the Universe
(from previous Paul Camp writings I suspect there is no such
place--but--), where are the photons from that flashlight today? In a
Euclidian geometry the photons would be 12 billion lightyears away
(provided they didn't pass too near any large massive bodies and
provided the Universe itself isn't inside a black hole). But what
does GR say about these photons? Without knowing the mass and size of
the Universe, do we really know enough about it's geometry so as to be
able to answer the question?