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To make a picture of what I am asking I have to use a comparission...So let say Adam walks on a 10km long bridge and wants to reach Sara at the end of the bridge but as he walks the Sun heats the bridge to gain length so it is then 10km and 10 meters long. For Sara waiting for Adam it seems he is a little bit slower than she expected assuming the velocity of Adam and his distance from her at the start. Is the same applied to a photon as Adam, space as bridge and planet Earth as Sara?

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    $\begingroup$ The speed of light to a local observer is always c, but that's not necessarily true for a non-local observer. There are a few different ways to measure cosmological distance, and the speed you get depends on which distance (and time) measure you use. $\endgroup$
    – PM 2Ring
    Dec 31 '20 at 8:32
  • $\begingroup$ But it's fair to say that expansion does slow down light. From physics.stackexchange.com/a/477761/123208 the region that emitted the CMB photons we receive today was 41.6 million lightyears (in proper distance) from our region when those photons were emitted around 13.7 billion years ago, but that region is now about 45 billion lightyears from us. $\endgroup$
    – PM 2Ring
    Dec 31 '20 at 8:35
  • $\begingroup$ There are several posts here on this & related topics, eg physics.stackexchange.com/q/26549/123208 & physics.stackexchange.com/a/268237/123208 $\endgroup$
    – PM 2Ring
    Dec 31 '20 at 8:59
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The answer of your question is simply No. Speed of causality in universe is constant (C) and hence, the photon doesn't "apparently slow down", but rather it's wavelength changes (in an expanding space time). See https://en.m.wikipedia.org/wiki/Redshift

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The answer to this question is pretty obvious. The speed of light does not change. It will still travel the same amount of distance at the same time in the same medium. Just there will be one amazing thing going on. If you send a light beam from one planet to other planet which are moving far apart then the distance between them will be increasing so the light would take more time than expected. But this is not due to the decreasing speed of light but because of the increasing distance between planets. The same thing applies to this question. The wavelength of the photons tend to increase due time and this phenomenon is called "wavelength shifting" . You can look for wavelength shifting on Google and you can understand this phenomenon also.

AMAZING FACT: The speed of expansion of universe is continuously increasing and a time will come when this will be even greater than the speed of light.

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  • $\begingroup$ If a single wavelength wave streches during trajectory the tail of the wave will apparently reach the target with more delay regarding the head of that wave differently than if the wave haven't been streched... $\endgroup$ Dec 31 '20 at 7:10
  • $\begingroup$ When you use the word "APPARENTLY SLOW" this means that "THE SPEED SEEMS TO TAKE MORE TIME THAN EXPECTED" but this does not mean that the speed has changed, right? $\endgroup$ Dec 31 '20 at 9:23

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