# Movement of galaxies vs expansion of space

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?

• Does this answer your question? How do we know that the space metric is expanding as opposed to objects moving relative to one another? Commented Jan 18 at 10:10