Torbjorn,

I will certainly, willingly plead ignorance and I would appreciate someone explain what I am getting wrong, but I still see the same issue in your explanation;

“ in vacuum, is a constant. But the expansion of space pulls out EM field ripples, “wave packets”, of photons so that their frequency goes down (redshift) as the wavelength increases.”

There is the “vacuum,” through which light travels at C. Then there is this “space” that is expanding, causing redshift, ie, the frequency/wavelength to be stretched. These appear to be two, distinct metrics, being related to one another.

Go to conventional redshift, say the source of the light actually moving away in space. This is not due to “space expanding,” but distance increasing. The result is redshift for the reason you describe; “their frequency goes down (redshift) as the wavelength increases.” The wavelength has to increase, because the source is moving away in space, not that the metric of the space is expanding. More lightyears, not “expanded” lightyears.

To use the analogy of sound and a train going away, so that its whistle sounds at a lower frequency. The train tracks are not being stretched. It is because there are more tracks between you and the train.

Similarly when it is said that the universe expands, it is argued that light takes longer to cross. That is why they say that while the universe is only supposed to be 13.8 billion years old, it is supposed to be 93 billion lightyears across. From wikipedia;

The proper distance — the distance as would be measured at a specific time, including the present — between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs).

So lightyears are not being stretched by this expansion, but are the units this expansion is denominated in.

What is the basis of the speed of intergalactic light, if it is not intergalactic space? What is this “vacuum?”

Einstein said that space is what you measure with a ruler. The speed of light measures the vacuum, but that’s not space????