http://philsci-archive.pitt.edu/9062/1/Popper_%26_cosmology_PhilSci.pdf "Popper's purpose with his letter in Nature was to discuss the generally accepted expansion of the universe in relation to theories that explained Edwin Hubble's redshift data on the assumption of special mechanisms operating in a static universe. According to one alternative the speed of light decreased with time, while another alternative (the class of "tired light" hypotheses) assumed that light gradually lost energy during its journey through empty space from the nebulae to the Earth. Clearly inspired by Milne, Popper examined two alternatives to the relativistic theory of cosmic expansion, arguing that they agreed in regard to observable effects, that is, they led to galactic redshifts of the kind predicted by the expansion theory. "The three theories are logically equivalent," he wrote, "and therefore do not describe alternative facts, but the same facts in alternative languages." But does the universe really expand? Or does the speed of light instead decrease? Or is it rather the case that the frequency of photons changes with the distance they travel? According to Popper, the question was "not more legitimate than, when prices of goods fall throughout the economic system, to ask whether 'in reality' the value of money has increased or the value of the goods has decreased."