DeaneTR

Hi Everyone, I ended up here today while studying all the ideas that have come up about why James Webb has looked back to within 1/2 billion years of the big bang and found fully formed Galaxies that usually take longer than 1/2 billion years to form? I'll spare you all the crazy madness what that I've read going down this rabbit hole. I just want to ask something specific to this thread regarding the speed of light being invariant. Because in this article today: https://www.sciencealert.com/the-entire-universe-could-be-twice-as-old-as-we-thought they're dusting off Zwicky's tired light hypothesis. So my question is how can we be certain that the behavior of photons has always been the same (invariant) since the birth of the universe and are there any links I can read to help me better understand how the behavior of photon can be altered or has been altered in a way that would mess up our calculations for the age of the universe?