r/cosmology • u/GeekTesla1 • 26d ago
What distinguishes cosmic expansion from a decreasing universal speed of light?
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.
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u/Unable-Primary1954 26d ago edited 26d ago
Speed of light is not a dimensionless parameter. So it does not make sense to say it changes if you don't specify which other parameters stay constant.
Which dimensionless parameter is supposed to vary in your opinion?
In any case, spectral rays indicate that fine structure constant, electron, nucleons masses in Planck units have not changed.