It is not difficult to understand the differences between 'Doppler redshift' and 'Cosmological redshift' conceptually. But how do astronomers distinguish them when doing observations?

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    $\begingroup$ My answer to this question probably covers how they remove doppler shift from cosmological shift $\endgroup$ – Jim May 28 '15 at 14:38
  • $\begingroup$ Patrick, is it a duplicate? physics.stackexchange.com/questions/113939/… $\endgroup$ – Fattie Dec 12 '15 at 3:45
  • $\begingroup$ Not quite. Gravitational redshift is something different to the doppler effect. $\endgroup$ – Rob Jeffries Oct 12 '16 at 7:38

I think there are duplicates of this, but couldn't immediately locate them.

The answer is you cannot tell observationally whether a single redshift measurement is caused by the expansion of the universe or by something moving away rapidly.

However, if one wished to interpret the ensemble of redshifts that we see in a non-expanding universe, then you must place us at the centre of the universe. You would then have to explain why more distant galaxies are moving faster and why at very large distances, the simple Hubble law breaks down (and the microwave background and the primordial elemental abundances and so on).

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    $\begingroup$ Also, you can interpretcosmological redshift as the result of the integrated doppler shifts between infinitely many local frames moving at infinitesimal velocitiy differences. $\endgroup$ – Thriveth May 28 '15 at 11:34

Cosmological redshift can be interpreted as the accumulated Doppler redshifts between infinitely many frames of reference, each locally at rest and moving away from us at a certain velocity. So you cannot distinguish the two because they are basically different cases of the same thing.

However, as it says in @Rob Jeffries' answer above:

  • when looking at many galaxies in the same region of the Universe, and they all show velocities distributed around a certain average velocity away from us, and
  • when we see that this velocity-distance relation holds true across the sky,

we conclude that this average velocity is the expansion velocity of the Universe at that distance, and the deviations are Doppler shifts due to local proper motion of the individual galaxies.

  • $\begingroup$ what do you mean by accumulated Doppler ? one analyzes redshift from light coming from one particular source. Huge or small is related to the scale of the observation. $\endgroup$ – user46925 May 28 '15 at 12:47
  • $\begingroup$ I mean that the light is Doppler-redshifted a little bit when shifting between these frames locally at rest in the expanding Universe. The many infinitesimal Doppler shifts due add up to one greater redshift which is what we measure. $\endgroup$ – Thriveth May 28 '15 at 13:37

Although I'm not an expert, I believe the simple answer is:

1) you can not tell them apart in individual cases


2) very simply, statistically you can pick up (apparently - quite clearly and obviously) the big picture. "Standard candles" can help and, very simply, overall averages set a picture.

I believe this is a dupe question .. Differentiating the gravitational redshift and the cosmological redshift?


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