Chiral, you write : " If "time" had been going more slowly before, then light emitted longer ago could appear to have lower frequency " But I'm not sure relative what? light?
Then light need one invariant 'clock rate', or 'wavelength/Frequency' that then gets manipulated by this otherwise 'universal time change' you're wondering about, wouldn't it?
Or you're thinking of early light then being misinterpreted as it reach us in 'fast time'?
I'm not sure, but it's a interesting idea.
Let me translate it to this. Isn't that the same as to suggest that 'c' is a variable of sorts, although at all times keeping its 'proportion' relative all frames of reference, meaning that 'c' is 'c', both then and now. Seems a hard thing to test.
The second one (misinterpretation by us now in "fast time") is how I am trying to think about it.
I am definitely struggling with what it would mean for time to change. Would the frequency of the light shift as well, keeping up with time, and ultimately giving no difference? How can I reconcile this theory with the photon's point of view, in which its journey is instantaneous?
I think I am out of my league!