The universe is not expanding. If it were, the competition between expansion and gravitational attraction would distort galaxies and galactic clusters - e.g. fringes only weakly bound by gravity would succumb to expansion and fly away. No distortions observed.
Sabine Hossenfelder: "The solution of general relativity that describes the expanding universe is a solution on average; it is good only on very large distances. But the solutions that describe galaxies are different - and just don't expand. It's not that galaxies expand unnoticeably, they just don't. The full solution, then, is both stitched together: Expanding space between non-expanding galaxies." http://backreaction.blogspot.bg/2017/08/you-dont-expand-just-because-universe.html
"Expanding space between non-expanding galaxies" is absurd of course, but in Hossenfelder's text part of the absurdity is omitted. What the Einsteinian cosmology actually assumes, implicitly, is
"Expanding space between non-expanding galactic clusters."
Super absurd, isn't it?
The idea that vacuum slows down light is largely discussed but only in terms of quantum gravity. The implication that the Hubble redshift might be due to gradually decreasing speed of starlight is persistently ignored (crimestop):
"...in some quantum-gravity models, the speed of photons in gamma rays would be affected by the grainy nature of spacetime..." https://fqxi.org/community/articles/display/255
Sabine Hossenfelder: "It's an old story: Quantum fluctuations of space-time might change the travel-time of light. Light of higher frequencies would be a little faster than that of lower frequencies. Or slower, depending on the sign of an unknown constant. Either way, the spectral colors of light would run apart, or 'disperse' as they say if they don't want you to understand what they say. Such quantum gravitational effects are miniscule, but added up over long distances they can become observable. Gamma ray bursts are therefore ideal to search for evidence of such an energy-dependent speed of light." http://backreaction.blogspot.fr/2017/01/what-burst-fresh-attempt-to-see-space.html
Nature: "As waves travel through a medium, they lose energy over time. This dampening effect would also happen to photons traveling through spacetime, the researchers found. Although the effect is small, high-energy photons traveling very long distances should lose a noticeable amount of energy, the researchers say. [...] If it is true that spacetime is a superfluid and that photons of different energies travel at different speeds or dissipate over time, that means relativity does not hold in all situations." http://www.nature.com/news/superfluid-spacetime-points-to-unification-of-physics-1.15437
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