What your asking directly stems from two related open ended philosophy-of-science questions. These would be " Are universal constants actually constant?" and "Does the speed of light differ in speed at any point of time in its journey between two points of space in a continuous substrate?"

The answer to both like all philosophy questions is a long hit on the pot pipe and a "sure man, its possible but remains unlikely/over engineering the problem until we have justification through observing it" however I'll give my two cents.

"" Are universal constants actually constant?" " it probably depends on the constant. Fundamental math stuff that tie directly into computations logic and uncertainty precision limits like pi are eternal and unchanging. More physics type constants derived from statistical distribution like the cosmological constant might shift around a little especially at quantum precision error scales.

The speed of light probably is closer to the first one as its ultimately about mathematically derived logical boundaries on how fast any two points universe can interact to quantize a microstate. Its a computational limit and I don't see that changing unless the actual vaccum substrate of spacetime takes a sudden phase shift.

"Does the speed of light differ in speed at any point of time in its journey between two points of space in a continuous substrate?"

Veritasium did a good video about this one. The answer is its possible but currently unmeasurable . so if all hypothesis generate the same effective results then the simplest among them (light maintaining a constant speed during both ways of trip) is the most simple computationally efficient hypothesis among them.

So... I am not a scientist, just an enthusiast. But my understanding is that the speed of universe expansion doesn't correlate with the speed of light. The speed of light is still constant.

Instead, the universe expansion rate is measured via something called the "doppler effect". Scientists are able to use telescopes and take a snapshot of the night sky. Stars that tend to be brighter and more blue are closer to us. And stars that tend to be darker and more red are farther away from us. By taking snapshots and comparing it with previous snapshots over a long period of time, we are able to see a difference in color in each star which then shows us which stars are moving closer and which stars are moving further away.

Thus by measuring the speed at which the doppler effect changes, they can determine an estimate and compare whether the universe is expanding or shrinking and the speed at which it expands or shrinks over time without breaking the cosmic speed limit that is the speed of light.

Another analogy for the doppler effect is that it's similar to what happens when a train passes by us. But in the case of a train, the doppler effect is with sound. As the train gets closer, the sound gets louder and seems more higher pitched. Then when the train passes us and gets further away, the sound fades away and gets lower pitched. All the while though, the speed of the train is still constant.

Hope that makes sense. And anybody that knows more than me feel free to correct me. ;-)

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