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Physicists can be so naive. The constants in physics equations are not necessarily "constant" They may represent unknown factors which do not appear to change, at least not in the short run. Constants are placed in equations because the known factors by themselves cannot produce an answer when specific values are placed in the equation. Placing the "constant" in the equation allows physicists to get an answer.
In the case of electromagnetic radiation over long distances, constants might not be the only factor which could be associated with differences in data. Conditions in space may change radiation between the time it leaves a distant galaxy and the time it reaches earth.
The values of many physical constants were determined with equipment that was not as sensitive as the equipment in use today. New equipment may produce different values or detect differences in potential factors that could not be determined even a few decades ago. Today's equipment may allow identification of factors that could not be identified before.