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I can readily accept the theory that the universe is expanding as a mathematical model to explain the fact that all galaxies are moving away from each other, but I have difficulty understanding intuitively how the space itself can physically expand. How can one tell the difference between these two scenarios:

  1. the space (carrying all galaxies in it) is expanding;

  2. all galaxies happen to be indeed moving away from each other within the space as a fixed framework.

The consequence of either of the two scenarios seems to be the same to an observer on Earth, who sees all galaxies are moving away from the Earth (redshift observed by Hubble). Based on this observation, how can one tell it is due to scenario 1 instead of 2?

Again, how can the empty space expand (or "more space is created") as a piece of physical stuff can?

I realize some similar questions were asked before (e.g., by Doc in March 2020) with a few answers. I am asking this question again for the following reasons:

(1) I feel (so did Doc) those previous answers were not necessarily to the point to address specifically why the observed recession of galaxies is due to scenario 1 instead of 2 stated above.

(2) General Relativity (GR) was mentioned in some of the previous answers. But the issue here concerns the expansion of 3-D space, while GR describes the curvature of 4-D Spacetime due to mass/energy/momentum in the space. According to Wikipedia, "Although cosmic expansion is often framed as a consequence of general relativity, it is also predicted by Newtonian gravity". To simplify the issue, is there a way to address the question without involving GR?

(3) Professor Leonard Susskind claims that the two scenarios mentioned above are "identical, there is no difference" in his online Lecture 1 on Cosmology (https://www.youtube.com/watch?v=P-medYaqVak). He further said: "You either take the position that the galaxies are moving away from each other, or you take the position that they are embedded in this grid, and the grid is expanding." If these two explanations are identical, why do most people emphasize that it is the space that is expanding instead of the galaxies moving away from each other?

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    $\begingroup$ Does this answer your question? How do we know that the space metric is expanding as opposed to objects moving relative to one another? $\endgroup$
    – John Rennie
    Commented Jan 18 at 10:10
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    $\begingroup$ See also: physics.stackexchange.com/q/442986 $\endgroup$
    – D. Halsey
    Commented Jan 18 at 13:49
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    $\begingroup$ Importantly, "expansion of space" is not different from galaxies moving apart from each other. The answer linked by @D.Halsey makes this point accurately; see also physics.stackexchange.com/a/771021/180843. Answers in the marked duplicate question unfortunately miss this point. $\endgroup$
    – Sten
    Commented Jan 18 at 15:54
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    $\begingroup$ The expansion of space is a concept that comes from GR, so no there is no way to address this question without involving GR. $\endgroup$
    – John Rennie
    Commented Jan 19 at 7:29
  • $\begingroup$ They used to play a practical joke on apprentices in the construction business in German speaking countries by telling them to go to the foreman and to ask for a "Siemens Lufthaken", a hypothetical device that could anchor heavy loads "in the air" without any support. All attempts to construct something like an imaginary or virtual grid in the physical vacuum that would allow definition of motion in relation to that grid are basically just that... somebody is still insecure about relativity. There is no physics that allows us to "anchor" to the vacuum. The vacuum is non-intuitive, though. $\endgroup$
    – FlatterMann
    Commented Jan 20 at 11:01

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