If light can slow down in vacuum, then we have a static-universe explanation of both the cosmological (Hubble) redshift and Halton Arp's intrinsic redshift. The idea of slow speed of light in vacuum is not too preposterous:
"Some physicists, however, suggest that there might be one other cosmic factor that could influence the speed of light: quantum vacuum fluctuation. This theory holds that so-called empty spaces in the Universe aren't actually empty - they're teeming with particles that are just constantly changing from existent to non-existent states. Quantum fluctuations, therefore, could slow down the speed of light." https://www.sciencealert.com/how-much-do-we-really-know-about-the-speed-of-light
Can light slow down in vacuum? Yes:
"The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently..."But once that pattern has been imposed - even now the light is no longer in the mask, it's just propagating in free space - the speed is still slow," Padgett added." http://www.upi.com/Science_News/2015/01/23/Scientists-slow-down-light-particles/1191422035480
"Glasgow researchers slow the speed of light"
View: https://www.dailymotion.com/video/x2ffsym
"Researchers at the University of Ottawa observed that twisted light in a vacuum travels slower than the universal physical constant established as the speed of light by Einstein’s theory of relativity...If it’s possible to slow the speed of light by altering its structure, it may also be possible to speed up light. The researchers are now planning to use FROG to measure other types of structured light that their calculations have predicted may travel around 1 femtosecond faster than the speed of light in a vacuum." https://www.optica.org/en-us/about/newsroom/news_releases/2016/slowing_down_light_with_a_twist/
"Some physicists, however, suggest that there might be one other cosmic factor that could influence the speed of light: quantum vacuum fluctuation. This theory holds that so-called empty spaces in the Universe aren't actually empty - they're teeming with particles that are just constantly changing from existent to non-existent states. Quantum fluctuations, therefore, could slow down the speed of light." https://www.sciencealert.com/how-much-do-we-really-know-about-the-speed-of-light
Can light slow down in vacuum? Yes:
"The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently..."But once that pattern has been imposed - even now the light is no longer in the mask, it's just propagating in free space - the speed is still slow," Padgett added." http://www.upi.com/Science_News/2015/01/23/Scientists-slow-down-light-particles/1191422035480
"Glasgow researchers slow the speed of light"
"Researchers at the University of Ottawa observed that twisted light in a vacuum travels slower than the universal physical constant established as the speed of light by Einstein’s theory of relativity...If it’s possible to slow the speed of light by altering its structure, it may also be possible to speed up light. The researchers are now planning to use FROG to measure other types of structured light that their calculations have predicted may travel around 1 femtosecond faster than the speed of light in a vacuum." https://www.optica.org/en-us/about/newsroom/news_releases/2016/slowing_down_light_with_a_twist/