<p>If we observe a distant galaxies cluster where all galaxies travel with the same speed away from us, except for one galaxy whose speed is significantly lower than of the neighboring galaxies. Does this necessarily contradict the principle of universe expansion?</p>

CC BY-SA 4.0
2020-01-25T17:54:20.240Z
How much lower? How far away? Please be quantitative.
ProfRob
CC BY-SA 4.0
2020-01-25T17:56:28.720Z
@RobJeffries the question asks if such a situation is possible in general, if there’s a specific distance or speed which allows for such situation then you can assume it.
Yos
CC BY-SA 4.0
2020-01-25T18:01:42.170Z
If you see a galaxy, *in the direction* of a galaxy cluster, with a very different velocity, then it isn't a member of the cluster.
ProfRob
CC BY-SA 4.0
2020-01-25T18:05:26.847Z
@RobJeffries because this would contradict the Hubble law right? Because according to the law speed is in direct proportion with distance?
Yos
CC BY-SA 4.0
2020-01-25T18:16:55.953Z
No, because a galaxy cluster is bound. A galaxy with a very different velocity cannot be bound to it.
ProfRob
CC BY-SA 4.0
2020-01-25T18:19:59.350Z
@RobJeffries what fo you mean by “bound”? Gravitationally bound? Does the definition of galaxy cluser say that all galaxies must have the same speed?
Yos
CC BY-SA 4.0
2020-01-27T10:38:05.847Z
Yes, a cluster is not just galaxies that are located in a small region on the sky, i.e. in a 2D projection. They're located in the same region in 3D, because they're gravitationally bound. Hence, their velocities cannot differ by more than roughly the escape velocity of the cluster, which is of the order of 1000 km/s (depending on the size of the cluster).
pela
<galaxy><big-bang-theory><galaxy-cluster><hubble-constant>
Must all galaxies in a galaxy cluster have the same expansion rate?
Yos
# Must all galaxies in a galaxy cluster have the same expansion rate?

(Asked by: username_0 on 2020-01-25)

If we observe a distant galaxies cluster where all galaxies travel with the same speed away from us, except for one galaxy whose speed is significantly lower than of the neighboring galaxies. Does this necessarily contradict the principle of universe expansion?

**Tags:** galaxy, big-bang-theory, galaxy-cluster, hubble-constant
> **Comments:**
> **username_1**: How much lower? How far away? Please be quantitative.  
> **username_0**: @RobJeffries the question asks if such a situation is possible in general, if there’s a specific distance or speed which allows for such situation then you can assume it.  
> **username_1**: If you see a galaxy, *in the direction* of a galaxy cluster, with a very different velocity, then it isn't a member of the cluster.  
> **username_0**: @RobJeffries because this would contradict the Hubble law right? Because according to the law speed is in direct proportion with distance?  
> **username_1**: No, because a galaxy cluster is bound. A galaxy with a very different velocity cannot be bound to it.  
> **username_0**: @RobJeffries what fo you mean by “bound”? Gravitationally bound? Does the definition of galaxy cluser say that all galaxies must have the same speed?  
> **username_2**: Yes, a cluster is not just galaxies that are located in a small region on the sky, i.e. in a 2D projection. They're located in the same region in 3D, because they're gravitationally bound. Hence, their velocities cannot differ by more than roughly the escape velocity of the cluster, which is of the order of 1000 km/s (depending on the size of the cluster).  

---

## 0 answers

