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If my understanding of CMB and Hubble's Law is correct, then CMB photons emitted from more than ~14.4 Glyr during Recombination Epoch would not reach us. The reason is this would correspond to Hubble's Law velocity, v = Ho * D, between us and >14.4 Glyr (re-) combining electrons and protons exceeding the speed of light, which implies CMB photons emitted from >14.4 Glyr would never catch-up to us.

The CMB photons reaching us today have traveled 13.8 Glyr, but they were emitted from a sherical shell with radius ~6.9 Glyr, as distance increased by ~2x during transit due to Hubble's Law expansion. When the diameter of the emitting spherical shell reaches ~14.4 Glyr in ~600 million years, then the CMB photons from that distance will no longer reach us, but we would continue to see CMB photons emitted from ~6.9 Glyr to ~14.4 Glyr for another ~28.8 billion years.

Is this correct, or am I neglecting consideration of (relativistic) co-moving coordinates, or something else?

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All the photons of the CMB were created about 400,000 years after the big bang and have been propagating through the universe since then.

The CMB will not disappear in the future, it will simply become cooler and more redshifted as the universe continues to expand. Those photons are already on their way to us and are presently about 600 million light years away.

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  • $\begingroup$ So, this changes my question somewhat, Will cosmic microwave background become invisible in ~600 million plus 14.4 billion years? $\endgroup$
    – FritzS
    Commented Dec 14, 2023 at 18:26
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    $\begingroup$ "The CMB will not disappear in the future..." @FritzS I strongly suggest you explore other questions about the big bang. All parts of the universe were "in" the big bang; it did not happen at any point in space; the current observable universe has a radius of 46 billion light years. The CMB photons will just keep on arriving. $\endgroup$
    – ProfRob
    Commented Dec 14, 2023 at 19:22
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I would suggest reading up a bit on cosmological redshift. The CMB is the “leftover” photons we receive from when the universe was 380,000 years old (z ~ 1100). The photons are not going to disappear, but as time passes, the photons will be more redshifted and lose energy, and thus might not be detectable as photons anymore.

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From the comments of @PM 2Ring, I see my question is based on fundamental misconception. I have incorrectly assumed the expansion of space equates to the movement of objects within. Hence, the question is illogical and considered to be answered, but @PM 2Ring should get credit for the answer.

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