Came across this article on Futurism: https://futurism.com/

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Came across this article on Futurism:

https://futurism.com/theory-challenging-einsteins-physics-can-soon-be-tested/

Something about this seems really fucking obvious to me. They're complaining about the speed of light being variable, but the constant value has always been the speed of light IN A VACUUM and they make no mention of that here.

There have been many experiments where scientists have slowed light down by passing it through various mediums. They're talking about light being slower just after the Big Bang, but with so much dense matter existing, surely it wouldn't have been travelling through a pure vacuum?

On top of that, we know that light is affected by gravity, given the immense gravitational forces of the Big Bang, couldn't that have simply slowed light down?

Am I missing something here? Can someone enlighten me?