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## Is the Universe rotating?

 Quote by Shenstar Where does the solar systems angular momentum come from? And is it possible that everything in the universe is rotating because the universe itself is rotating. Similar to how eddies and weather on the earth is affected by the earths rotation. Round swirls of cloud migrate across the earth due to rotation.
If that were the case, we'd see a preferential alignment of angular momentum vectors. We don't.
 Isn't there some data frm the CMB that shows there is some alignment to the solar plane or elliptic alignment? See here: http://en.wikipedia.org/wiki/Coperni...und_anisotropy Or is this another issue?
 Recognitions: Gold Member Science Advisor See the Sagnac Effect for more information. Observational constraints indicate the universe, if 'rotating', is doing so at a very leisurely rate - as noted by bcrowell. The solar system is obviously rotating - nothing new there. This is due to conservation of angular momentum from the original accretion disc from which it formed.
 Within a galactic black hole, one could seem to have evidence of rotation flow of stars, and hence the appearance of of rotation of their 'world within a world'. But no galaxies seen.
 George, bcrowell, there are two possibilities you seem to overlook: 1: our universe was formed by an unusually low spin BH; 2: the huge expansion of the universe since its birth had a spin-down effect by conservation of angular momentum combined with an enormous increase in moment of inertia. EDIT: Another issue is whether the time-scale of observation of cosmic BHs is relevant to the time-scale of events "inside" the space generated by a BH.
 ... yet another issue is how the spin of a BH in the parent universe affects the new space generated by it. I don't know what the physics for that would be, so it can't be used to simply rule it out. If BHs were purely classical objects, then it would be clear that the universe isn't a BH, but (IMO) they are not classical - their new space is not simply the region within their event horizon.

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 Quote by DavidMcC George, bcrowell, there are two possibilities you seem to overlook: 1: our universe was formed by an unusually low spin BH;
I didn't overlook that. In #67, I pointed out to you that the universe isn't a black hole. More on this topic: http://math.ucr.edu/home/baez/physic.../universe.html

 Quote by DavidMcC 2: the huge expansion of the universe since its birth had a spin-down effect by conservation of angular momentum combined with an enormous increase in moment of inertia.
You can't define the total angular momentum of the universe (see #61). Therefore it doesn't have a well-defined moment of inertia. But in any case, I think the Newtonian intuition that $\omega$ should decrease over time is probably correct in realistic cosmological solutions that include rotation. In the Godel metric, $\omega$ is the same at all points in spacetime. However, if you look at a more realistic rotating model, such as the one in this paper http://adsabs.harvard.edu/full/1985MNRAS.213..917B by Barrow et al., they state all their results in terms of the unitless ratio $\omega/H$ of the rotational velocity to the Hubble constant, and they explicitly state that this quantity changes over time. I believe p. 924, eq 4.8 gives the time variation. This is a solution that would apply after the time of last coupling. But I don't really see the relevance of this time variation if you want to explain why we observe a particular upper limit on the present value of $\omega$, since the model doesn't impose any constraint on the value of $\omega$ at earlier times. On the other hand, Barrow does argue that if you go back to the inflationary era, you should get an exponential fall-off of rotational velocity. This seems to me to be more relevant, since an exponential fall-off can kill off even an extremely large initial rotational velocity.

 Quote by DavidMcC EDIT: Another issue is whether the time-scale of observation of cosmic BHs is relevant to the time-scale of events "inside" the space generated by a BH.
When you say "cosmic BHs," it sounds like you're imagining that the universe is a black hole...?

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 Quote by Ken G Still, the fact that the unvierse is not observed to be rotating (which is indeed a meaningful statement as you point out) is taken by some[...]
Who is "some?"

 Quote by Ken G [...]to be a sign of support for the idea of adding Mach's principle to GR as a kind of additional postulate.
You can't add Mach's principle to GR as an additional postulate, because GR contradicts Mach's principle. It would be like adding an additional postulate to the laws of arithmetic saying that 2+2=5.

If one feels that the nonrotation of the universe requires explanation, then inflation is a good candidate, because inflation predicts zero rotation. This would be similar to the idea that if one feels that the flatness of the universe requires explanation, then inflation can do that.

Personally I don't feel that there is a strong case to be made that lack of rotation requires an explanation. The argument is much stronger in the case of flatness, because flatness is unstable, so to produce a flat universe without inflation, you need fine tuning.

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 Quote by bcrowell Who is "some?"
Well, searching for references is tedious, but it's not that much of a stretch to say "the universe is observed to not rotate" is a confirming instance for "the universe cannot rotate." But certainly one can find other reasons for that as well.

Interestingly, exactly what is "Mach's principle" gets debated, to the extent that it is not even clear if a universe that exhibits the Godel metric (speaking hypothetically) would be an example of Mach's principle or not. For example, http://en.wikipedia.org/wiki/G%C3%B6del_metric
states "Some have interpreted the Gödel universe as a counterexample to Einstein's hopes that general relativity should exhibit some kind of Mach principle, citing the fact that the matter is rotating (world lines twisting about each other) in a manner sufficient to pick out a preferred direction, although with no distinguished axis of rotation.

Others take Mach principle to mean some physical law tying the definition of nonspinning inertial frames at each event to the global distribution and motion of matter everywhere in the universe, and say that because the nonspinning inertial frames are precisely tied to the rotation of the dust in just the way such a Mach principle would suggest, this model does accord with Mach's ideas."

In other words, even if inertial forces associated with rotation were detected in the matter frame, if such forces were consistent with rotation of the matter it would still be viewed as Mach's principle. A refutation would require inertial forces that did not fit with rotation of the universe.
 You can't add Mach's principle to GR as an additional postulate, because GR contradicts Mach's principle. It would be like adding an additional postulate to the laws of arithmetic saying that 2+2=5.
Well that is just what I claimed is not true, so if you can support that claim, then what I said was wrong. Can you support your claim?
 If one feels that the nonrotation of the universe requires explanation, then inflation is a good candidate, because inflation predicts zero rotation. This would be similar to the idea that if one feels that the flatness of the universe requires explanation, then inflation can do that.
Yes, there certainly could be other reasons to expect a lack of rotation other than Mach's principle.
 Personally I don't feel that there is a strong case to be made that lack of rotation requires an explanation. The argument is much stronger in the case of flatness, because flatness is unstable, so to produce a flat universe without inflation, you need fine tuning.
I agree that lack of rotation does not really require explanation. But as long as we do not have a theory of gravity we can really be happy with, we will continue to want to wonder about whether or not we should be equipping our theory with a Mach's principle.
 Doesn't w increase along with r in a bounded, rotating Universe (which by definition has a gravitational center)?

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 Quote by dougal217 Doesn't w increase along with r in a bounded, rotating Universe (which by definition has a gravitational center)?
What do you mean by a "bounded" universe?

Standard cosmological models don't have boundaries: http://www.astro.ucla.edu/~wright/co...y_faq.html#XIN

They also don't have a center -- see the FAQ entry "Where did the Big Bang happen? Would that be the center of the universe?" -- http://www.physicsforums.com/showpos...56&postcount=8

In the rotating cosmological modes that I'm aware of, --

http://en.wikipedia.org/wiki/G%C3%B6del_metric
http://arxiv.org/abs/0902.4575

-- $\omega$ is constant everywhere on a surface of constant cosmological time (see "How are time and distance measured in cosmology?" -- http://www.physicsforums.com/showpos...15&postcount=7 ).

-Ben

 Quote by wolram I think the answer will be , rotating in reference to what.
Where's the observer, and where's the test particle?
 I've been reviewing the references / articles mentioned in this thread and the FAQ. I don't pretend to understand all the content, especially the maths, but I do have a question that you may be able to help with. All of the references / papers seem to discuss the potential for rotation of mass within the universe, as opposed to rotation of the universe itself. Is that correct? Regards, Noel.

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