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In our science class we have a question to answer, which is 'explain how redshift supports the idea of an expanding universe'.

I am not sure how to answer this question, as I know that there are 'different types' of redshifts. The one we (my class) were focusing on for this topic was the Doppeler effect. We were told that the galaxies were moving away from us, so that causes the light to shift. I have explained it like this in the past and have gotten it correct, however, I also know that on a scale so large, that space itself expansing is causing the redshift. So I am not sure how I should answer this question because I feel like the teacher is asking us to explain the doppeler effect, not cosmological redshift. So if I explain it using the Doppeler effect example, would the answer generally be incorrect because it is cosmological redshift that supports an expanding universe?

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    $\begingroup$ It doesn't matter. In the Milne model, space does not expand, so the redshift is Doppler. In the "official" Friedmann model, space does expand, so the redshift is cosmological. Either way, the redshift proves the expansion of the universe. $\endgroup$ – safesphere May 29 at 2:57
  • $\begingroup$ @safesphere, if "space does not expand" in the Milne model, then how in that model does "redshift prove the expansion of the universe" - where can the expansion occur, other than in space? Or is the implication that the concept of "the universe" in this context encompasses only non-empty space, and the contents of the universe are thus spilling out into a larger area of previously empty space? In other words, "space" in the Milne model extends beyond the limits of the universe? If so, redshift does not prove "expansion" according to any particular sense in which the word is used. $\endgroup$ – Steve May 29 at 10:59
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    $\begingroup$ @Steve In the Milne model, the expansion of the universe can be viewed as an explosion of matter into an infinite static space. However, this model is also compliant with the FLRW cosmology as a special case. In this view, the universe in this model expands as an expansion of space. Both views are equivalent thus confirming that the Doppler redshift is equivalent to the cosmological redshift. $\endgroup$ – safesphere May 29 at 15:47
  • $\begingroup$ @safesphere, ah. I would only say then that when two models have an inconsistent definition of expansion, and the evidence is consistent with both, neither can be said to be "proved". $\endgroup$ – Steve May 29 at 15:54
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    $\begingroup$ @Steve The definition of the expansion of the universe is consistent in all models. It is simply the fact that galaxies are receding from each other. Whether they are receding by flying apart in a static space or by being carried away by an expanding space depends on the model and its interpretation, but doesn't change the definition of the expansion of the universe. You seem to think of the expansion of the universe as of the expansion of space. However these are different concepts. The latter is one of possible reasons of the former. $\endgroup$ – safesphere May 29 at 16:13
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Cosmological and Doppler redshifts are equivalent.

We usually use the "Cosmic expansion stretches light wavelengths" explanation, because it is intuitive, and a completely valid explanation.

However, we could equally well see cosmological redshift as the accumulated Doppler redshifts between the local rest frames of every set of neighboring points in the Universe.

As a photon travels through expanding space, it travels through a series of spatial points, which each has their own frame of being "at rest", slightly different from that of the neighboring points. As the photon travels, this difference builds up, and redshift accumulates.

If I am at point A, and a photon is emitted at point B some cosmological distance from me, an observer at rest at an intermediate point C will see the light from point B as less redshifted than I do at point A. But from my point of view, I might as well say -- and equally true -- that tpoint C is also moving relative to me, and that the difference in perceived redshifts is due to their velocity, not because anybody is stretching their light waves.

Of course, the danger of using the simple Doppler Shift framework of explanation is that it may lead to wrong conclusions like e.g. the widespread misconception that the redshift approaces infinity as the recession velocity approaches $c$, that either galaxies can never recede form us faster than light (they can) or that we can never see galaxies receding faster than light (we can). These are usually based on misunderstandings about how Relaytivity works (which is not so strange, it can be quite difficult to understand and I certainly drew all these wrong conclusions myself until I was taught that they were wrong).

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  • $\begingroup$ "As a photon travels through expanding space..." Doesn't that beg the very question asked? The OP asserts that the observed redshift is consistent with at least two explanations: (a) the recessing of the source; and (b) the expansion of the space between the source and observer. These would seem to be subtly but fundamentally different explanations (one attributes movement to the source, the other attributes an expansionary behaviour to the empty space itself), and the OP doesn't seem sure that the redshift supports either explanation against the other. $\endgroup$ – Steve May 29 at 10:48
  • $\begingroup$ "the danger of using the simple Doppler Shift framework of explanation is that it may lead to wrong conclusions like e.g. the widespread misconception that the redshift approaces infinity as the recession velocity approaches $c$" - I think by "wrong" you actually mean "non compliant with the official model". These conclusions are not necessarily wrong in other models. For example, if I interpret the Milne model as an explosion in the infinite Minkowski space, then these conclusions would be correct and not "misconceptions", would they not? $\endgroup$ – safesphere May 29 at 19:50

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