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There are two problems with the tag.

(1) The tag info is factually incorrect. [now corrected, thanks!]

Space expansion is a cosmological phenomenon wherein the proper distance between two spatial points for a given inertial reference frame increases from one moment of time to another. That is, space itself expands; the added distance is not due to relative motion of points or objects.

This is a common misconception, unfortunately propagated in many old answers on this site. The contents of the Universe are expanding, but there is not an objective sense in which space itself expands. Within the context of mainstream physics, "expansion of space" is not a physical phenomenon. It's just an artifact of common coordinate choices in cosmology.

This point has been discussed at length in many answers. See for example:

I am aware that I can submit an edit to the tag info without posting a meta thread about it. I did, and the edit was rejected. There is also a deeper problem:

(2) should be a synonym for , and not the reverse.

As a research cosmologist, I would never write "space expansion" or "expansion of space" in a paper. The standard terms for the expansion of the Universe are "cosmic expansion", "expansion of the Universe", or "Hubble expansion". This preference is borne out in the statistics.

"Cosmic expansion" appeared in the abstracts of 625 papers over the last decade.

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"Expansion of the Universe" appeared in the abstracts of 1747 papers over the last decade.

enter image description here

"Hubble expansion" appeared in the abstracts of 300 papers over the last decade.

enter image description here

"Space expansion" appeared in the abstracts of 131 papers over the last decade, and most of them are not even related to cosmology!

enter image description here

Just one paper from the most recent 10 is a cosmology paper: enter image description here

"Expansion of space" appeared in the abstracts of 152 papers over the last decade, and a significant fraction of them are not related to cosmology.

enter image description here

Just four papers from the most recent 10 are using "expansion of space" in a cosmology sense: enter image description here

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  • $\begingroup$ Uh, I'm pretty sure you can propose these changes since you've got >2k rep. Is a meta post necessary for this then? $\endgroup$
    – Kyle Kanos
    Commented Mar 30 at 20:03
  • $\begingroup$ @KyleKanos As noted in the post, "I am aware that I can submit an edit to the tag info without posting a meta thread about it. I did, and the edit was rejected." It seems the change is controversial enough to warrant a meta post... (Also, do you know how to propose swapping synonym with canonical tag?) $\endgroup$
    – Sten
    Commented Mar 30 at 20:32
  • $\begingroup$ This says it was approved & I don't see other tag wiki edits going back into Feb in the review. $\endgroup$
    – Kyle Kanos
    Commented Mar 31 at 12:49
  • $\begingroup$ @KyleKanos It was last year. I am happy that the new edit is approved! Still interested in the tag/synonym swap, which I am not sure it is possible to propose. $\endgroup$
    – Sten
    Commented Mar 31 at 17:25
  • $\begingroup$ It may be possible to do so, if one is created and approved by the community (via voting here) $\endgroup$
    – Kyle Kanos
    Commented Apr 1 at 1:04
  • $\begingroup$ The problem is that "cosmic-expansion" is the most natural choice, and that is already set as a synonym for space-expansion. From searching, my impression is that moderator intervention is required to undo a synonym. $\endgroup$
    – Sten
    Commented Apr 6 at 0:03

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