Angular momentum conservation in a collapsing universe

Join the discussion
Registration is free. Ask a follow-up in this thread, or start your own.
Sign up Log in
5 replies · 3K views
wolram
Gold Member
Dearly Missed
Messages
4,411
Reaction score
552
Last edited by a moderator:

Discussion

Astronomy news on Phys.org
I think i have found the ansewer, in that the universe has no (net) angular momentum?
 
wolram said:
I think i have found the answer, in that the universe has no (net) angular momentum?

Wolram, I really like this question. I haven't responded because I'm not sure enough. I'm hoping someone else will reply. My hunch is that at least according to the presently available theory the universe could have net angular momentum. I find the idea a curious one, almost bizarre. I would like to be told a reason that it could not have net angular momentum, or that the idea of a global angular momentum is meaningless. But I don't know of such a reason. So I remain intrigued, hoping some others will intervene.
 
marcus said:
Wolram, I really like this question. I haven't responded because I'm not sure enough. I'm hoping someone else will reply. My hunch is that at least according to the presently available theory the universe could have net angular momentum. I find the idea a curious one, almost bizarre. I would like to be told a reason that it could not have net angular momentum, or that the idea of a global angular momentum is meaningless. But I don't know of such a reason. So I remain intrigued, hoping some others will intervene.

If the universe did have an overall rotation, that would induce frame-dragging in the same direction. According to Mach's principle, that would be expected to have the effect that the rotation would be unobservable. GR only approximately satisfies Mach's principle, but it seems that within GR the frame-dragging effect would be of at least the right order of magnitude to make the rotation unobservable.
 
Jonathan Scott said:
If the universe did have an overall rotation, that would induce frame-dragging in the same direction. According to Mach's principle, that would be expected to have the effect that the rotation would be unobservable. GR only approximately satisfies Mach's principle, but it seems that within GR the frame-dragging effect would be of at least the right order of magnitude to make the rotation unobservable.

Johnathan, i am not sure if we are talking about the same thing, i was wondering about the angular momentum of galaxies ,and if this is conserved in a shrinking space down to the limits of the hypothetical bounce in qg theory.
 
wolram said:
Johnathan, i am not sure if we are talking about the same thing, i was wondering about the angular momentum of galaxies ,and if this is conserved in a shrinking space down to the limits of the hypothetical bounce in qg theory.

I haven't a clue what that bounce stuff assumes, but the general idea should hold; if the universe is described by GR, and appears to be rotating on average relative to some point in some frame of reference, then GR predicts that an observer at the origin of that apparent rotation will have their frame dragged around with it at a rate which increases with the fraction of the universe that is involved and is of the order of 1 to 1 when the whole universe is involved. That means that according to that observer, the universe will not appear to be rotating, as predicted by Mach's principle (although for GR the Mach's principle effect cannot possibly be exact - which seems somewhat irritating, when it could be so neat).