Ok, that's a "tired light" theory. It fails because that process just changes the frequency of photons, but not their density in space. The background radiation is (to extreme accuracy) blackbody radiation. Blackbody radiation has a spectrum, and an intensity, dictated by physics. The expansion of space preserves both aspects of "black bodiness"; tired light does not.
Tired light also doesn't explain the observed stretching in time of supernova light curves with distance.
> that process just changes the frequency of photons, but not their density in space.
Frequency is precisely the necessary thing that needs to be considered as an observed phenomenon that can have alternative explanations, since frequency relates to shifting toward red.
Are you saying that density/luminosity is an independent way of corroborating universal expansion? I don't believe that's the case.
Neither luminosity nor redshift seem, IMHO, to be strong enough evidence to uphold a "preposterous" (to use Sean Carroll's favorite descriptive term for the world) theory that massive things are not just traveling but accelerating away from oneanother.
I'm saying that tired light makes an incorrect prediction: that (absent some incredible coincidence) the CMBR will not have the density of blackbody radiation at the same spectral temperature.
It also falsely predicts that SN light curves are not stretched out at higher red shift.
Theories that make incorrect predictions are dead theories. We know for sure they are wrong.
This has nothing to do with the Big Bang, btw. Science works by killing theories, and tired light has been slain. It would be ruled out even if there wasn't an alternative theory available.
I don't doubt that the "tired light" theory has been slain. I can update my orientation from your informed response without hesitation. However, I don't think I'm truly talking about the "tired light" theory in a way that plugs into your understanding of it. Although I mention debris in the article, and then kind of took it off the table, what I still have left to posit is that light propagation looses energy over billions of light years, not some short distances that can be tested in a lab or verified via other corroborating observations that relate to shorter distances or parallax geometry. I've completely placed my theory and conjecture out of reach of the lab! Or have I? If I have it might not be worth much in the community of experts, but it still means something to me because I feel like, and nobody seems to be responding to this in any detail, cosmic speed limits are being broken all over the place, as my article states.
Yes, you are talking about a tired light theory. You are talking about a theory that robs energy from photons (never mind there's no known physical way to do this that doesn't scatter them, which isn't seen) but does not also stretch out the time of other physical processes, and stretch out the space containing the photons.
What you are proposing is ruled out by the generic argument against all such tired light theories.
Tired light also doesn't explain the observed stretching in time of supernova light curves with distance.
Ned Wright explains this at his web site:
https://www.astro.ucla.edu/~wright/tiredlit.htm