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This question already has an answer here:

Asking purely out of curiosity:

Is there any research or publication in Modern Physics that considers the possibility that the speed of light has changed since the Big Bang, and not remained constant (as what we know it today)?

By changed, I mean either gradually or abruptly, though I guess that the former makes more sense.

Now, I am guessing that the answer to this question might be more of "a matter of terminology".

For example:

We can say that the speed of light hasn't changed in the perspective of everyone who is within the given space, but since this space by itself has expanded, anyone theoretically "outside it" will "see" that light is in fact covering a larger distance per time unit (larger than what it used to when the given space was smaller)... Or we can say that it has slowed significantly, while the universe itself hasn't expanded at all.

On the other hand, I might be speaking nonsense here, so I'd be happy if one of you out there (or here) could enlighten me on this...

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marked as duplicate by John Rennie, Jon Custer, Qmechanic Nov 21 '17 at 18:04

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate of Has the speed of light changed over time? $\endgroup$ – user121330 Nov 21 '17 at 17:12
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  • $\begingroup$ I don't think "a change in the speed of light" has a physical meaning. A change relative to what? Say, relative to the size of the Earth orbit. However, the size of the orbit depends on the mass and the mass depends on the energy as in $E=Mc^2$, so a change in the speed of light would also change the size of the orbit. The speed of light simply is a convention of how we measure things, but ultimately it a 1:1 symmetry relation between space and time: $c=1$ as in 1 light second per second where 1 meter is defined as 1/299,792,456th part of 1 light second. $\endgroup$ – safesphere Nov 21 '17 at 18:20
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    $\begingroup$ I personally found several papers on cosmology where changing c is discussed. At least they were from professionals. If I recover my pdf I will post the refs here. $\endgroup$ – Alchimista Nov 21 '17 at 19:11
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Joao Magueijo published a non-fiction account of research into trying to remove the need for inflation in Big Bang cosmology by allowing the speed of light to be a function of cosmological time. The title is Faster Than the Speed of Light: The Story of a Scientific Speculation.

It doesn't go into the nitty-gritty calculational details of the research but it does give the names of people who were investigating the idea. It also talks about the resistance to the idea within the larger research community of cosmologists, along with the resulting rejection by referees of scientific journals.

In his book, Magueijo talks about how fascinating the general relativity work was, but that it couldn't be professionally rewarding because of the rejection of the speculation of the idea by the mainstream community.

Whether you agree or disagree that the approach has merit, the book is a good read and piques the conscience about which biases in science are needed (conservation laws) and which are pure traditionalism (e.g., steady state vs expanding universe, or circular vs elliptical orbits).

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