system
Given the following post, respond as if you're commenting on Reddit:
human
What does the Universe's constant expansion mean for matter? Are distances at atomic level expanded, too?
gpt
Long story short, no.

Don't think of metric expansion as stuff being stretched apart. It's a phenomenon of *geometry,* not of matter. Distances between fixed coordinate points grow larger over time, at a rate proportional to the distance between them. But *bound systems* are not fixed points. They're objects held together by whatever — electrostatics, gravity, molecular bonds, whatever it may be. So metric expansion has no effect there.

You don't see the effect of metric expansion until you look at things which are sufficiently far apart that nothing is holding them together. Individual solid objects are held together; solar systems and galaxies are held together. Whole *galactic clusters* are held together. But clusters of galaxies which are separated by hundreds of millions of light-years aren't held together by anything. So the distances between them increases over time.