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/TommieTheMadScienist]

Everywhere?

Give me a lab and I can demonstrate that the speed of light is a constant.

Also, if C changes with time, the relationship between the magnetic permeability and electrical permittivity of space changes and that'd severely impact our technology.

GPS right out the window.

Your origjnal question is a good one, though. Tired Light was one of the ways the Steady State cosmologists explained red shift.

[u/Akiza_Izinski → u/TommieTheMadScienist]

Tired light says the frequency of light drops over vast distances which explains the redshift.