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This question already has an answer here:

Let's suppose that the speed of light would not be a constant but a function of something. As is quite clear now, universe is expanding exponentially. If the speed of light would actually be a function of this expansion, maybe in terms of cosmological constant... We would never find this out because the measure of distance (definition of meter) is now locked; meaning it is defined in terms of the speed of light! Also, in a way we know that the speed of light may indeed be a function of cosmological constant because very distant targets are escaping us faster than the speed of light? Is it hence indeed so that the speed of light just can't be measured, or defined currently by any independent means?

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marked as duplicate by honeste_vivere, ZeroTheHero, Jon Custer, Kyle Kanos, John Rennie cosmology Aug 29 '17 at 7:12

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The speed of light is now fixed due to the current definition of the metre and the second. Ir is now exactly 299,792,458$ms^{-1}$.

See Speed of light at Wikipedia

So, today, if you perform an experiment to measure the speed of light, you are really calibrating your equipment rather measuring the speed of light.

The speed will not change again until we redefine the metre or the second.

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    $\begingroup$ While this answer is true, and I upvoted it, the real physics problem behind the question cannot/is not solved by mere definitions. $\endgroup$ – lalala Aug 28 '17 at 11:11
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    $\begingroup$ @lalala We have to decide what we mean by time, length, speed, etc. With our current definitions, the effect is that the speed of light is fixed. This is because our current understanding suggests that this is reasonable. This could change; maybe in the future, special relativity will be superseded and we will change the definitions and, after that, the speed of light might change again. For the moment, that is just speculation or science fiction. $\endgroup$ – badjohn Aug 28 '17 at 11:14

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