The problem with basic doppler shift is that redshift increases proportional to distance in all directions. So either we are at the exact center of this expansion, or redshift is an optical effect.

The one possible source would be some form of medium interfering with the light. A "tired light" theory. Yet there didn't seem to be any other distortions, so it was dismissed. Then it was decided space itself must be expanding, making every point appear as the center, became the solution.

Which does overlook the central premise of spacetime, that the speed of light is constant in any frame. So that if it did expand, the speed of the light crossing it should increase.

Originally spacetime was based on the fact that in a moving frame, both the speed of light and any units to measure distance reduced equally, given the motion of the frame and any activity within it can't add up to more than the speed of light. Such that in a frame moving at the speed of light, time "stops." Basically no motion can be measured.

So the math used to describe this was inverted to say that space could also expand, not just contract. But if the space were to expand, then the light crossing it should increase proportionally.

What gets overlooked in the focus on the math, is there has to be some underlaying equilibrium against which the moving frame is measured.

Given that in a frame moving at the speed of light, both distance and duration shrink to zero, if we were to go the opposite direction and look at the frame with the longest ruler and fastest clock, it would be closest to the equilibrium of the vacuum. The unmoving void of absolute zero.

Which then means the real qualities of space are infinity and equilibrium. Like a number line, from zero to infinity.

Apparently there is one way light does redshift over distance alone, as multi spectrum packets. As the higher frequencies dissipate faster. Yet that would mean our measuring devices are sampling a wave front, not detecting individual photons traveling billions of lightyears. So the quantification of light would be an artifact of measurement, not integral to the light itself. A "loading," or "threshold" theory.

Which would cause serious trauma to the larger field of Quantum theory and the shrieks of crackpottery would go parabolic.

https://worrydream.com/refs/Mead_2001_-_Interview_(American_Spectator).html