# Is the Universe rotating?

by utek1
Tags: rotating, universe
 P: 101 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.
 P: 101 ... 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.
 P: 5 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
P: 186
 Quote by wolram I think the answer will be , rotating in reference to what.
Where's the observer, and where's the test particle?
 P: 259 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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 Quote by Lino 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?
It's not clear to me what distinction you're making. If the universe is rotating, then a gyroscope changes its orientation relative to distant galaxies. If this experiment had this outcome, would you call it rotation of mass within the universe, or rotation of the universe itself?
 P: 259 The bottom line is that I don’t know! I also don’t know enough about the gyroscope experiments - although I am very glad that you reminded me of it – I’ll definitely follow up on it. However, I assume that if the object / mass is moving (in a circle) within a static medium (call it space-time, the brane, ...), then you’ll have (centrifugal?) forces operating, where-as if the object / mass is moving within a rotating medium, those forces would not be such ... I think. How this applies to a gyroscope ... I don’t know, but it does strike me that the forces would be different. Maybe my answer is that I need to spend sometime looking at info on gyroscopes and related experiements. As a basic starter, if you conducted the “gyroscope in a lab” experiment under water (large body of water with a rotation) would you expect this to impact on the results? Regards, Noel.
 PF Patron Sci Advisor Emeritus P: 5,311 Sorry, Noel, but I'm having a hard time figuring out what you're getting at in #83. It sounds to me like you probably need more background before you can tackle this successfully. If you want to post something about your current background in math and physics, I'd be happy to recommend a book on relativity that would be at the right level.
 P: 259 Apologies for #83 Ben, it made more sense in my head, before I started writting it! I don't have any formal background in Maths / Physical, but read whatever I can on the subject. Rather than a book on GR, can you recommend something (most likely an article / paper I presume) basic related to the workings of gyroscopes (as they apply to your posting), or the type of expirement that you mentioned? I can use that as the start of the spiders web for references and material. Regards and again, much appreciated, Noel.
 PF Patron Sci Advisor Emeritus P: 5,311 For gyroscopes, I guess you could read the WP article.
 P: 259 Thanks Ben. Appreciated. I have started on WP and a couple of other articles. One question, if I may; when the Gravity Prode B conducted it's experiment, it involved (approx) a year of observation as Earth rotated around the sun, would any equivilant test at a galactic or universal scale also require an (approx) 100% rotation before realistic results could be obtained? (If my reading on the subject will get to the answer to this, then please feel free to ignore the question.) Regards, Noel.
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