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I was told that photons lose energy (= increase their wave length) when moving away from mass and gain energy moving towards mass. Could this increased red shift of photons, originated a long time ago, come from the fact that they started in an old, dense universe and end up in a young, less dense universe?

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  • $\begingroup$ But that is essentially what explains cosmological redshift. The expansion of the universe is what causes the redshift. "Gravitational redshift" isn't some separate phenomenon. $\endgroup$
    – ProfRob
    Commented Mar 12, 2021 at 20:07
  • $\begingroup$ @ProfRob I agree, but it could produce even more redshift due to the difference in density of the source region and the target region, which could look like the universe is accelerating the expansion. $\endgroup$
    – Cosinus
    Commented Mar 12, 2021 at 20:45
  • $\begingroup$ Obviously if there is a violation of the cosmological principle then that will impact on redshift. There are small differences that result in the Sunyaev-Zel'dovich effect for example. Barely detectable. It's a fair question though and I'm sure there is work that suggest the "acceleration" is due to inhomogeneity in the universe. NB. To explain the apparent acceleration (nothing to do with expansion and the Hubble law, which you seem to be confusing with acceleration). $\endgroup$
    – ProfRob
    Commented Mar 12, 2021 at 21:07
  • $\begingroup$ Perhaps I am naive but I interpret the shifts as always due to a combination of their various sources. In the sense that photons leaving a big mass would appear red shifted to us, and even more so if in addition the distance keep increasing. Though the red shift of very far objects is described by the cosmological shift. The latter should incorporate the effects. $\endgroup$
    – Alchimista
    Commented Mar 13, 2021 at 9:33
  • $\begingroup$ @fasterthanlight, only partially because it's seams to be based on the assumption that objects them self were heavier in the past. My question is about the density of the source region versus the density of the target region. $\endgroup$
    – Cosinus
    Commented Mar 13, 2021 at 11:13

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