<p>The two effects are observationally indistinguishable for an individual galaxy. The redshift associated with universal expansion is often interpreted as a recession velocity and there is nothing in a spectrum that tells you what fraction of the redshift is contributed by which effect.</p>
<p>If a distant galaxy is associated with a cluster of galaxies, then you can make an attempt by assuming that the <em>average</em> recession velocity of the cluster represents the cosmological redshift and that the perturbations from that for individual galaxies are their peculiar velocities.</p>
<p>Nevertheless, the cluster <em>will</em> have it's own peculiar velocity as an additive term to all these individual peculiar velocities. Fortunately, that doesn't really matter when it comes to studying cluster dynamics.</p>
<p>Since the cosmological redshift increases with distance (Hubble's law), whereas peculiar velocities do not, then at large distances cosmological redshift can be assumed to dominate. See <a href="https://astronomy.stackexchange.com/questions/38635/what-does-located-in-the-hubble-flow-mean">What does &quot;located in the Hubble flow&quot; mean?</a></p>

CC BY-SA 4.0
2020-08-31T20:46:16.880Z
Thank you, great answer!
ParityViolator
CC BY-SA 4.0
2020-08-31T07:40:31.903Z
2020-08-31T07:47:15.213Z
2020-08-31T07:47:15.213Z
ProfRob
<p>Is there any way to distinguish the doppler redshift from a distant galaxy, caused by its initial recessional velocity relative to this galaxy, from the probably much greater, cosmological redshift, caused by the expansion of space?</p>

CC BY-SA 4.0
2020-08-31T07:42:56.353Z
See also https://astronomy.stackexchange.com/questions/12314/how-to-disentangle-a-very-distant-stars-relative-velocity-vs-redshift-distance and https://astronomy.stackexchange.com/questions/33392/cosmological-redshift-vs-doppler-redshift
ProfRob
CC BY-SA 4.0
2020-08-31T19:42:23.150Z
The alleged "duplicate" asks what the difference is, it doesn't ask how/whether they can be distinguished.
ProfRob
CC BY-SA 4.0
2020-08-31T20:31:25.047Z
Yes, I read that other question and answer to see if it covered my question. Mine is a technical question about measuring and distinguishing not the same question- thank you
ParityViolator
<redshift><doppler-effect>
Is there any way to distinguish the doppler redshift from a distant galaxy from the probably much greater cosmological redshift?
ParityViolator
# Is there any way to distinguish the doppler redshift from a distant galaxy from the probably much greater cosmological redshift?

(Asked by: username_0 on 2020-08-31)

Is there any way to distinguish the doppler redshift from a distant galaxy, caused by its initial recessional velocity relative to this galaxy, from the probably much greater, cosmological redshift, caused by the expansion of space?

**Tags:** redshift, doppler-effect

> **Comments:**
> **username_1**: See also https://astronomy.stackexchange.com/questions/12314/how-to-disentangle-a-very-distant-stars-relative-velocity-vs-redshift-distance and https://astronomy.stackexchange.com/questions/33392/cosmological-redshift-vs-doppler-redshift  
> **username_1**: The alleged "duplicate" asks what the difference is, it doesn't ask how/whether they can be distinguished.  
> **username_0**: Yes, I read that other question and answer to see if it covered my question. Mine is a technical question about measuring and distinguishing not the same question- thank you  

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## 1 answers

### Answer by username_1 on 2020-08-31. Score: 4 | Accepted answer

The two effects are observationally indistinguishable for an individual galaxy. The redshift associated with universal expansion is often interpreted as a recession velocity and there is nothing in a spectrum that tells you what fraction of the redshift is contributed by which effect.

If a distant galaxy is associated with a cluster of galaxies, then you can make an attempt by assuming that the *average* recession velocity of the cluster represents the cosmological redshift and that the perturbations from that for individual galaxies are their peculiar velocities.

Nevertheless, the cluster *will* have it's own peculiar velocity as an additive term to all these individual peculiar velocities. Fortunately, that doesn't really matter when it comes to studying cluster dynamics.

Since the cosmological redshift increases with distance (Hubble's law), whereas peculiar velocities do not, then at large distances cosmological redshift can be assumed to dominate. See [What does "located in the Hubble flow" mean?](https://astronomy.stackexchange.com/questions/38635/what-does-located-in-the-hubble-flow-mean)

> **Comments:**
> **username_0**: Thank you, great answer!  

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