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I’ve always wondered whether there might be some other large scale effect happening that causes light frequencies to shift. What if photons travelling through a vacuum just slowly lose energy and their frequency drops? Would that be distinguishable from expansion by any of the measurements we can do here?

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This is known as the “tired light” hypothesis, and there are a number of problems with it; see https://www.astro.ucla.edu/~wright/tiredlit.htm .

Oh cool, thanks! I’ve asked a few people about it but never received a satisfactory answer why it wasn’t a viable theory. This covers it nicely.

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