Scientist look out at the universe and observe red shifts for galaxies that are roughly proportionate from their distance from us and attribute it to a velocity away from us. Also there is an indication that the universe's expansion is accelerating. My question is: Is or isn't a proportion of the red shifting attributed to gravity on a cosmic scale?

Here is my argument. As we go back in time, the observable universe was smaller, and locally, more dense. When we observe a galaxy that is 10 billion light years away, we are looking at it with light that left it 10 billion years ago. Excepting large scale cosmic structures and voids, generally the distant galaxy's local density should be roughly the density we had 10 billion years ago.

Now some might point out that from the perspective of that distant galaxy, mass distribution would be the same in all directions and would not effect photons emitted from that galaxy. But from our perspective, 10 billion light years into the future, we do see a mass distribution gradient. The photons that left that galaxy 10 billion years ago has gravity from in-front and behind working on it; that from our perspective, over its journey to us, it has collected more mass behind it than when it was first emitted, thus causing at least some of the red-shift we observe; that in a sense, it has climbed out of a gravity well.

Maybe I have it all wrong because I missed something. Maybe scientist already have factored this into their models. Or maybe I stumbled onto a something important that will change our understanding of the universe. Seeing as I am little more than a layman on the topic, I am inclined to believe one of the first two conditions probable likelihood. And, seeing as I am just a layman, a simple English explanation of what I missed would be appreciated. But if I am onto something, then maybe it might change our thoughts from the universe's accelerated expansion to an apparent accelerated expansion.


1 Answer 1


You are basically describing cosmological redshift. It is not a Doppler shift caused by galaxies moving away from us - it is the effect of the cosmological expansion of space. As photons travel across the universe their wavelength is "stretched" by the expansion of space. Those emitted from furthest away spend longer travelling and are stretched the most.

The physics that governs all this is entirely due to "gravity on a cosmic scale" and the way that it is understood and explained is by the application of General Relativity to the universe as a whole via the Friedmann-Lemaitre-Robertson-Walker metric, which has solutions known as the Friedmann equations. The Friedmann equations lead to a prediction of Hubble's law (or perhaps the Hubble-Lemaitre law), which has since been observationally confirmed.

In summary - the effects of gravity and the expanding universe on photons are already entirely described in the context of General Relativity by the Friedmann equations.

  • $\begingroup$ So the the redshift we observe is a combination of actual velocity away from us as well as gravity. And acording to the best minds, does not account for observations that indicate an aceleration in the expansion? $\endgroup$ Commented Sep 12, 2018 at 23:36
  • $\begingroup$ @KeithReynolds It is really not correct to think about cosmological redshift as due to a velocity. It is interpreted as I explain. Further essential reading arxiv.org/pdf/astro-ph/0310808.pdf The recession velocities are due to the expansion of space, not a movement through space. $\endgroup$
    – ProfRob
    Commented Sep 13, 2018 at 7:10
  • $\begingroup$ Yeah, when i mentioned velocity away from us, i was not thinking about a movement through space. Rather just the distance inbetween was growing because of an expansion of space. Now it is said the expansion rate is accelerating. I,m wondering if that could be apparent vs real due to changets in gravity acting on photons ovet time that reach us. $\endgroup$ Commented Sep 14, 2018 at 2:38

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