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Universal expansion and the speed of light?
How do we know the speed of light is constant throughout the universe. This seems to be an accepted law as researchers are using it to calculate our distance from other celestial bodies.
If the universe is expanding at an increasing rate so that its density is decreasing, and black holes are so dense that their immense gravity can alter the dynamics of light, then why wouldn't the constant change in the density of the universe also cause variations in the speed of light?
This was first observed with radio waves (radio photons are electromagnetic waves, just like light, except that they have much longer wavelengths). Regardless of the relative motion of the observer or the source (including early airplanes), the speed of the photons was always the same.
Maxwell wrote a series of equations to explain the behavior of electromagnetic radiation. The equations did explain all the observed behaviors. And these equations "predict" that the speed should always be the same for any observer, which did come as a surprise. Under Newtonian Relativity, it was expected that an observer moving towards the source should detect the waves coming in at a higher relative speed.
Lorentz explained that by supposing that time itself flows differently when an observer moves and/or that the distances are not measured equally by a moving observer, versus a stationary observer. [actually, this is the same statement, expressed in two different ways]
When Einstein came along, what he supposed is: what if this speed of light (in a vacuum) were constant everywhere in the universe, always measured the same for all observers AND happens to be a "speed limit" for information what would the universe look like.
That is how he came up with his theory of Relativity. The theory does not "state" that the speed of light is the same everywhere, it "supposes" it. Based on this supposition, it makes predictions. We then test these predictions against our observations... and they match what we see.
The problem with a theory is that you can never prove it to be completely right (as you say, maybe there is a corner of the universe that we have never observed and where the speed of light is different). However, it is possible to prove that the theory is false.
All you need is an observation that contradicts the theory.
For example, if someone creates a theory saying that, for example, red light kills bacteria, it might be difficult to prove that it works on ALL bacteria.
However, all you need to do is find some bacteria that survive exposure to red light and then you would have proven the theory to be wrong.
That is why whenever a theory comes out, all scientists begin by trying to prove it wrong. For decades, scientists have tried to prove Relativity wrong... and they have failed. It looks more and more like the universe really behaves AS IF the hypotheses are correct (or, at least, useful).
We DO NOT KNOW that the speed of light is constant throughout the universe, but so far, the universe appears to behave AS IF the speed of light is constant throughout the universe.
We are not using the speed of light to calculate distances. We do use it to set up a unit of distance. We then use tricks, devices and comparison to measure the distance to an object and, once we have done that, we estimate how long it would take light from this object to travel here, in order to express the distance.
When we say: This galaxy is 2.5 million light-years away, it is NOT because we measured how long light takes to get here. It is because we will have observed Cepheid variables (a specific type of variable stars) and we will have measured their apparent brightness. We then compared this apparent brightness with the brightness that this Cepheid would have had at a known distance.
This allows us to calculate the distance. Once we have this distance, we ask ourselves: how long would light take to reach here from that distance? 2.5 million years.
We already know how black holes change the dynamics of light. In fact, any gravity field (including Earth's) changes the characteristics of light... but not its speed. This has been tested almost half a century ago.
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