It does not. It expands at every point in spacetime. This effect is quite literally negligible on all length scales smaller than far extra-galactic distances.

It is equivalent to asking why a car driving towards me does not turn blue due to doppler shifting. Except even more negligible than that.

I never said it is only expanding outside. Also moving relative to space is not nonsense. Motion is not just relative to other objects in space. This what Newton's bucket thought experiment showed.

Edit: the pearl is made from around bound by electric forces, so you can repeat the same argument for inside the pearl. You can repeat it all the way down until you have to admit that the approximations inherent in the concepts useful to describe the world at distances from nano meters to mega parsec eventually break down.

> Also, why would space limit itself to only expanding outside a physical body?

It doesn't, it's just that the forces within the body (the spring in the analogy) are stronger than the expansion.

The "relative to space" bit is the nonsense part of their analogy, but I'm not sure of a clearer way to describe things than what they did. It's not my field.

Maybe a better way to phrase it would have been moving over the rubber/space? Or through the rubber/space? It really is motion relative to the underlying manifold though. Let's say there was such a thing as a gravitational soliton, that is, a stable ripple in spacetime, then if you are on the same spot as this wrinkle as expansion starts you will move away from it. You move relative to a structure made from spacetime. This was my field for many years. :-)

The mathematically precise statement is that you are not moving on a geodesic. You are therefore experiencing acceleration (the objects that are not bound and have increasing distance are on geodesics and do not experience acceleration).

You can't make an electron bigger, so it can't be stretched out. Other point particles are the same. The forces that hold physical stuff together are much more powerful than the gentle expansion of space, so we don't see people and planets dissolving.

Indeed you can, if you realize that the place occupied by the "point particle" is described by its wavefunction.

I know! But when you poke it, it still acts like a particle and the wavefunction localizes again, right? It doesn't exactly "puff up" like a billiard ball. Electrons don't really get bigger when they are put into states where their position is less localized.

But the expansion of space does "stretch" light- or at least, the wavelength gets longer, so it's not not happening.