“We now have the best picture of how galaxies like our own formed their stars.” -Casey Papovich
The expanding Universe is counterintuitive in a lot of ways: on the largest scales, the distances between galaxies are increasing at an alarming rate, yet the distances between the stars in our own galaxies or the atoms in our bodies don’t appear to change. It makes one wonder whether there’s a size limit to how small a piece of the Universe can be and still expand?
If everything expanded as the Universe did, then the coins would need to be replaced by paint. Image credit: “Fun with Astronomy” by Mae and Ira Freeman, via http://amzn.to/2aKd9qD.
Oddly enough, General Relativity doesn’t really care whether you’re talking about subatomic distances or billions of light years, but it does care about whether the object you’re talking about is gravitationally bound to itself or not. This means an atom in deep space won’t expand, but also that a large cluster of galaxies (and everything in it) will only see the space beyond it expand, not the space within.
The various groups and clusters we can see here — including our local group — are all individually bound, ...