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Lets say that we observe in a distant galaxy cluster, which in all the galaxies move far away from us. and here we see in the same cluster a galaxy that move away from us in a much lower velocity than the other galaxies. is this contradict the expansion of the universe?

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2 Answers 2

No, it doesn't contradict the paradigm of the universal inflation. It would merely mean that the example galactic cluster that you mention has its proper (own) motion at a smaller velocity relative to the proper motion of the Milky Way than the speed of the universal inflation at the distance between them. I.e. it would move towards us, if the Universe wasn't expanding. But since it does, galactic cluster's proper motion only managed to decrease relative velocity at which it moves away from us.

To put it in analogy, say you're on a speeding train that moves at 120 km/h relative to the station it just passed by. You then lean out of its window and throw an apple towards the station that the train is now moving away from, but you can only manage to throw the apple at a velocity of 40 km/h. That apple, since it came from the train moving at 120 km/h and shared its momentum, is now travelling away from the station's platform at 80 km/h, even though you threw it towards the station, because you only managed to reduce its velocity relative to the station for 40 km/h. So it would still move away from it, but for 40 km/h slower than the train does.

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Such a group is called a discordant redshift group. It could be the case that the exceptional galaxy is not a true member of the cluster. The famous example of this is Seyfert's sextet. Five of the members show very similar redshifts, from 4000 to 4500 km/s, while the sixth is measured at nearly 20000 km/s.

This is usually interpreted as being a compact cluster of 5 galaxies, and one background galaxy. It is not taken as evidence that contradicts the general expansion of the universe.

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