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Measurement of the expansion of the universe is based, in part, on the red shift of light. The speed of light is a constant, although some people argue that it may be changing over time, I am going to ignore that argument for now. But light can lose energy to gravity. In other words, if you look light from a large mass, it will be red shifted by the gravity of the mass. So, while light from our sun travels at the same speed all across the solar system, it is red shifted at the edge of our solar system.

So, does the red shift used by Hubble come from expansion or from gravity?

Are we measuring expansion by assuming that light does not lose energy to the gravity it has escaped?

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There are two ways I can answer this question. I'm pretty sure one of the ways is, while technically correct, mostly worthless to you because it ignore the question you're trying to ask and focuses on the question you did ask. So let's start with that one.

The redshift used by Hubble comes from expansion only and from gravity only. In general relativity, gravity is a description of how spacetime is curved and how it behaves. The expansion of space can be written into the Einstein field equations as reacting to gravity; being driven by it, influencing it, and being influenced by it. Therefore, it is valid for us to say that redshift due to expansion is "technically" due to gravity (the LHS of the EFEs, for all you technical enthusiasts). So it is both.

Okay, the second way to answer this is more helpful to what you wanted to ask, so I thought I'd save the best for last.

The redshift Hubble measures is due to the expansion of space. One of the primary ways it measures expansion is by measuring the redshift from Type-1A supernovae, which has a spectrum and intensity that is always the same. Let me present two strong reasons why you can be sure that Hubble isn't measuring redshift due to gravity from the star or other objects along the way. Firstly, we do the math. We calculate how much redshift we should see purely due to the source star, which always has the same mass because of how these supernovae are produced. This allows us to remove it from the measurements. Second, even were we to not do this, the amount of redshift is roughly consistent for all Type-1A supernovae. So we can look for one fairly close to us, where the redshift due to expansion would be expected to be minimal, and we can then use that as a base line to eliminate gravitational redshift from ones that are further away. Any left over redshift must be due to expansion. Additionally, we know that other objects on route do not provide much redshifting because if something of large mass was in the middle of the light's path, it would gravitationally blueshift as it approached the object and redshift by (almost) exactly the same amount as it left. A net cancelling effect.

So what Hubble uses is the redshift due to expansion. And we do not assume there is no gravitational redshift, we just have very easy ways of nullifying its effects

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  • $\begingroup$ Passing by a mass has a zero net shift as stated. But should we consider the aggregate of all masses around the point of emission as affecting the red shift too. In this case, the further the source is the more masses affects the red shift from that source. $\endgroup$
    – Riad
    Commented Dec 16, 2021 at 11:08
  • $\begingroup$ @Riad But wouldn't the reverse also be true? If we consider all the mass surrounding the source as pulling it back, shouldn't we consider all of the mass surrounding the destination as pulling it forward? Given the homogeneity of the universe at large scales, wouldn't that tend to have a net cancelling effect, leaving just the source mass and destination mass as the two unequal contributors? In fact, I'd expect this to make it less subject to uncertainty the farther away the source is because the homogeneity would increasingly dominate any perturbations. $\endgroup$
    – Jim
    Commented Jan 10, 2022 at 19:20
  • $\begingroup$ Jim; Yo could very well be correct.. I was only pointing to other possibilities to consider. It also matters if the universe is finite or not and where are we situated in it. The great-attractor and the dipole in the microwave background radiation seem to suggest in-homogeneity. regards . $\endgroup$
    – Riad
    Commented Jan 11, 2022 at 20:28
  • $\begingroup$ @Riad the great attractor is the gravitational center of our supercluster. Those will exist necessarily. The dipole in the CMB is due to our peculiar motion - that relative to the comoving frame. Neither suggests inhomogeneity at cosmological scales $\endgroup$
    – Jim
    Commented Jan 12, 2022 at 13:57
  • $\begingroup$ See this quote ''Thus we are currently unable to prove homogeneity of the Universe on large-scales '' arxiv.org/pdf/1104.1300.pdf. $\endgroup$
    – Riad
    Commented Jan 13, 2022 at 17:46
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From expansion of the universe, the expansion of the universe is a repulsive force. Why it is a force? Because just like gravity, it is from spacetime, eventually it pushes atoms apart

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    $\begingroup$ It wasn't my vote, but I'd still say the answer is a stub and that expansion is not a force at all $\endgroup$
    – Jim
    Commented Apr 18, 2015 at 15:24
  • $\begingroup$ A mechanism that alters proper distance between points. A force changes momentum of objects over time; it accelerates mass. Expansion doesn't do this (okay, it changes the momentum of photons but that isn't the point). $\endgroup$
    – Jim
    Commented Apr 18, 2015 at 15:41
  • $\begingroup$ I personally may not like the word repulsive either, but it gets the point across, so I can't argue with your using of it $\endgroup$
    – Jim
    Commented Apr 18, 2015 at 15:54

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