> I don't think we have conclusively proven c to be a constant, right? We obviously can't directly test what the speed of light was 1.5 billion years after the

> I don't think we have conclusively proven c to be a constant, right? We obviously can't directly test what the speed of light was 1.5 billion years after the Big Bang. But we can test for changes in the fine structure constant back then, which would have to have been different back then if the speed of light were different. No such changes have been found.