The cosmological microwave background (CMB) is the oldest electromagnetic radiation in the universe. How do we know how far away the CMB is?

  • $\begingroup$ Perhaps a better phrasing for the question I have in mind might be "How do we know how far away the source of the CMB is?" $\endgroup$
    – usernumber
    Nov 13, 2020 at 8:37
  • 2
    $\begingroup$ For me the answer is: "Thanks to seismology, we know the distance is ~2900 km". But only because, as a geologist, for me CMB stands for "core–mantle boundary": en.wikipedia.org/wiki/Core%E2%80%93mantle_boundary :) $\endgroup$ Nov 13, 2020 at 9:13
  • $\begingroup$ @Jean-MariePrival It's a good thing I expanded the acronym then. Otherwise, the question would have been marked as "Questions about Earth science [...] is off-topic." and closed :p $\endgroup$
    – usernumber
    Nov 13, 2020 at 10:23


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