So I learned recently that star systems are relatively flat due to the conservation of angular momentum.

On how large a scale does this degree of flattening apply. i.e will the universe eventually flatten out, or galaxy clusters, or is it just star systems. At a guess, it depends on the validity of this statement:

Given enough time flattening will occur regardless of how chaotic the initial conditions of the original system is.

This is related: Why don't stars in globular clusters all orbit in the same plane?


1 Answer 1


An infinite universe which is approximately homogeneous should not flatten out. Flattening out is a local process, but on the large scale of homogeneity the GR gravity dynamics overwhelm the local process. The scale of flattening is generally at the galaxy level, possibly also regarding galaxy clusters. I am curious about the quote in your question, and I would very much appreciate a reference for it.

  • $\begingroup$ Oh it's not a quote. I just used quotations to mark it as one statement in the post (I'll change it to bold). Its purpose in the context of the post was just to add stimulus in order to better understand what the question was asking. Could you provide some detail as to what GR effects counteract the flattening please? $\endgroup$
    – yolo
    Nov 22, 2020 at 21:30
  • $\begingroup$ Hi @yolo . If the universe is flat and infinite, the initial conditions can cause either of two things to happen. (1) The universe grows without limit. (2) The universe universe reaches a max size and then collapses. If (1) expansion does not permit anything but small sections to flatten. If (2) I am guessing that collapsing may possibly allow for more flattening, but probably not on any vast scale. I am not up to the math about (2) details. $\endgroup$
    – Buzz
    Nov 23, 2020 at 1:16

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