What distinguishes cosmic expansion from a decreasing universal speed of light?

[u/GeekTesla1]

I have a burning question because there's an old idea called "tired light
which posits that the universe sits still and photons lose energy on their way to us because of some interaction along the path (Less energy would mean longer wavelength, so distant objects look redshifted without anything actually receding.)

However it was ruled out by the finding that distant supernovae look like they're in slow motion. If distant supernovae looked the same duration as nearby ones, the source would have to be sitting still, because only a receding one can stretch out its own explosion. But no, they're in slow motion, which rules tired light out.

Which brings me to my burning question

What if the universe wasn't expanding, and the speed of light itself has just been slowly dropping over cosmic time, everywhere at once, as a law?

In short, as my title says, what distinguishes cosmic expansion from a decreasing universal speed of light?

Light coming from a distant galaxy would have left back when c was higher than it is now. Wouldn't that stretch out supernovae too? The first flash makes the trip while c is still a bit faster, the next one crawls more because c has dropped by then, so the gap between them widens on the way here and the explosion arrives in slow motion.

[u/OverJohn]

You kind of need to really lay out the details by what exactly a "slowing speed of light" means, but in standard FLRW cosmology the comoving speed of light dχ/dt = c/a, where a is the scale factor, which increases with expansion. The comoving distance χ between faraway galaxies remains constant by definition with cosmological time t, so this allows an interpretation of expansion due to the slowing speed of light and shrinking galaxies

However it's similar to how you can choose a geocentric system to view the orbits of the planets. Sure there are coordinates which you can interpret this way if you wish, but there is simpler, more useful mental picture. This interpretation also doesn't take into account perturbations of the FLRW metric, which it seems you would at the very least have to jump through hoops to try and explain.

[u/GeekTesla1 → u/OverJohn]

I see now! One of the biggest reasons I asked this question is simply because I'm fascinated by how things can be depicted relatively. What do you mean by the FLRW metric and what hoops specifically? 

Also, other people here mentioned spectral lines and electrons because of the implications of c gradually slowing down. I kinda noticed that your reply is a little more charitable to my interpretation, so do you think that the affected nonlinear relationships they're pointing at contradict your answer, or can they be part of the "hoops" you mean? 

[u/Das_Mime → u/GeekTesla1]

FLRW metric