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From my understanding, the speed of light is reduced when traveling in a medium. Also, space is the closest thing we have to a vacuum, but it is not a perfect vacuum.

So I wonder if the almost vacuum effect of space is meaningful enough to be taken into account when calculating the location of galaxies & stars billions of light-years away from us?

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  • $\begingroup$ I believe that the density is so low that the refractive index is taken as one, but could not find by google any calculations. $\endgroup$
    – anna v
    Oct 18, 2021 at 4:01
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    $\begingroup$ Does this answer your question? Is light actually faster than what our present measurements tell us? $\endgroup$
    – John Rennie
    Oct 18, 2021 at 4:19
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    $\begingroup$ Yes you are right!! Space is the closest we have to a vacuum. It has 30 particles/$m^3$ and some voids in space have 4-5 particles/$m^3$. But according to me that does not effect the speed of light that much as these particles are too small to increase the optical density of space. $\endgroup$
    – user297948
    Oct 18, 2021 at 4:25
  • $\begingroup$ Found this hypertextbook.com/facts/2000/DaWeiCai.shtmlvfor density in outer space. $\endgroup$
    – anna v
    Oct 18, 2021 at 4:35

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