How can we distinguish between the metric expansion of space and the speed of light slowing over time?
I understand that the universe is expanding. But it seems that our main measurement techniques involve assuming the speed of light is constant over time. I know that we define length in terms of a constant speed of light and our unit of time, the second. Does this mean the two are indistinguishable - that is, the expansion and the slowing of the speed of light are the same thing?