What is meant by "space is expanding"?

Question

Is it just the distance between galaxies that increases? Or is the distance increasing between the molecules in my body? Does expansion affect universal constants?

Various sources I have heard on the topic of expansion seem to contradict each other. (at least in my mind) If a physicist says: "There's not enough gravity to prevent expansion" then it makes it seem like a simple matter of Newtonian physics; as if galaxies were flying off under their own momentum. But if a physicist says: "Nothing can travel faster than light EXCEPT for the expansion of space itself" then this makes it seem like the invisible fabric of space-time is stretching.

Did you google the phrase? Did you check other relevant posts here? What is your effort? Show this in the question. — , Oct 21 '16 at 05:32
Possible duplicate of Why does space expansion not expand matter? — John Rennie, Oct 21 '16 at 05:34
If everything expands how would we know? There needs to be some reference, some measuring stick. — mattfitzgerald, Oct 21 '16 at 05:50
That could be interpreted as a duplicate. Note that that question has no answer. Also it was closed due to poor answers. I believe that my question is far more direct and concise and may attract higher quality answers. There are many many articles about red shift being proof of expansion. But I can't seem to find one that does a good job of explaining exactly what is expanding. I'll elaborate the question and re-open. Hopefully it will be good enough not to close. — Lorry Laurence mcLarry, Oct 21 '16 at 05:53
I think I get it now. The answer on this question is much shorter and more concise than the answers on the so called "duplicate". Too bad this one is closed. — Lorry Laurence mcLarry, Oct 21 '16 at 10:31

score 1 · Accepted Answer · answered Oct 21 '16 at 05:32

The space that you inhabit is expanding, but it is expanding slowly enough (~70 km/s/Mpc) that it doesn't have any significant effect at that scale. Even if you do think of this as "stretching" the bonds within molecules in your body, it's slow enough that everything can just go back to its equilibrium geometry.

To give you some idea of the rate of expansion, if molecules didn't go back to equilibrium geometry (they do), and everything within space also expanded (it doesn't), then over the course of a century, the distance between your head and feet would grow by about the length of a typical chemical bond (~$10^{-10} \text{m}$). A bond between two atoms would grow by ~$10^{-20} \text{m}$, which is a distance ~10 orders of magnitude smaller than the bond itself.

What is meant by "space is expanding"?

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