<p>The <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="nofollow noreferrer">cosmological microwave background (CMB)</a> is the oldest electromagnetic radiation in the universe. How do we know how far away the CMB is?</p>

CC BY-SA 4.0
2020-11-13T08:37:00.203Z
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?"
usernumber
CC BY-SA 4.0
2020-11-13T09:13:29.843Z
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": https://en.wikipedia.org/wiki/Core%E2%80%93mantle_boundary :)
Jean-Marie Prival
CC BY-SA 4.0
2020-11-13T10:23:40.067Z
@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
usernumber
<distances><cosmic-microwave-background>
How did we determine the distance of the CMB?
usernumber
# How did we determine the distance of the CMB?

(Asked by: username_0 on 2020-11-12)

The [cosmological microwave background (CMB)](https://en.wikipedia.org/wiki/Cosmic_microwave_background) is the oldest electromagnetic radiation in the universe. How do we know how far away the CMB is?

**Tags:** distances, cosmic-microwave-background

> **Comments:**
> **username_0**: 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?"  
> **username_1**: 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": https://en.wikipedia.org/wiki/Core%E2%80%93mantle_boundary :)  
> **username_0**: @username_1 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  

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