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Space is expanding, but as I understand it, galaxies are not since gravity keeps them together.

If you think of space becoming bigger, your galaxies just become further apart. But taking the size of space as a constant it appears as though the galaxies are shrinking.

But no matter how you look at it (coins on a balloon if you want), the space at the edges of a galaxy is moving away under it, because the space is expanding but the galaxy is not.

And since the expansion is faster than the speed of light, would that mean that the edges of a sufficiently large galaxy will move inwards at a speed higher than that of light?

Or is space just "thicker" around galaxies, and only expanding between them?

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  • $\begingroup$ Space is expanding, but as I understand it, galaxies are not since gravity keeps them together. - It's more that the expansion of space is only relevant on much larger scales, often larger than galaxy clusters. $\endgroup$
    – HDE 226868
    Commented Oct 25, 2015 at 18:04

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It means nothing to say "space is expending at the speed of light", since expansion measure in 1/s (i.e., m/s/m). You can measure velocity of expansion only for a given distance. Speed is c billions year-light far away. So the expansion at the scale of a galaxy would be very small, anyway.

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