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?

  • $\begingroup$ How much lower? How far away? Please be quantitative. $\endgroup$
    – ProfRob
    Jan 25, 2020 at 17:54
  • $\begingroup$ @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. $\endgroup$
    – Yos
    Jan 25, 2020 at 17:56
  • $\begingroup$ 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. $\endgroup$
    – ProfRob
    Jan 25, 2020 at 18:01
  • $\begingroup$ @RobJeffries because this would contradict the Hubble law right? Because according to the law speed is in direct proportion with distance? $\endgroup$
    – Yos
    Jan 25, 2020 at 18:05
  • 1
    $\begingroup$ 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). $\endgroup$
    – pela
    Jan 27, 2020 at 10:38


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