Does the Universe Rotate and Does it Have an Axis?

[bcrowell]

It's not meaningful to ask if the Universe is rotating or not. I think what the OP really means is whether the Universe as a whole has any angular momentum.

No, it's perfectly OK to talk about rotation. Note that four of the papers listed in #58 explicitly refer to rotation in the title. Actually, I think it would be problematic to discuss it in terms of angular momentum, since GR doesn't have any standard way to define the total angular momentum (or momentum, or energy) of a cosmologial solution.

"... Our universe has zero spin. Although for years Kurt Gödel tried to show that the universe was spinning by adding up the spins of the various galaxies, astronomers today believe that the total spin of the universe is zero." -Michio Kaku, Physics of the Impossible

I think Kaku is oversimplifying here in a popularization. It's not possible to prove that it has exactly zero rotation. One can only set an empirical upper limit. See the references in #58.

 

[Khashishi]

When I pirouette, the universe is rapidly rotating around me.

 

[Lino]

When I pirouette, the universe is rapidly rotating around me.

... and you are the centre of rotation!

 

[Ken G]

FAQ: Can we tell whether the universe is rotating?

It is possible according to general relativity to have cosmologies in which the universe is rotating. This is a nonMachian feature of GR, since the rotation is not relative to anything else.

Your post is clearing up a lot of misunderstandings, but there is also a kind of fine line to walk here. You are right that GR does not embed Mach's principle, it is possible to violate Mach's principle and still be GR. But this does not imply that we can have cosmologies that are rotating "according to GR." The fact that a solution to GR is possible does not mean that GR takes a stance on the physical allowability of that solution. We face physics solutions all the time that are not physical, and we don't say that our theories say the solutions are possible, we just don't look to the theory to tell us they are not possible.

Let me give an example to clarify. When I elastically scatter two spheres of unequal mass, I can use conservation of momentum and energy to calculate the possible solutions. But those equations allow a solution where the two spheres just pass right through each other. Do we then say that "according to conservation of energy and momentum, it is possible for two solid spheres to pass through each other"? No, we say that the conservation laws are moot on the point, so we need some other physical requirement to tell us what solutions are possible and what ones aren't. So someone who claims "the universe can't rotate" is not in contradiction with GR any more than someone who says "two solid spheres cannot pass through each other" is contradicting the conservation laws.

But I agree with you that we must recognize that someone claiming the universe can't rotate is going beyond GR to assert a physical truth, based on Machian philosophy, that we don't actually know is true. 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 to be a sign of support for the idea of adding Mach's principle to GR as a kind of additional postulate. It's a judgement call, and does not have practical ramifications but is interesting to ponder. It's ironic that Mach's principle, which was so instrumental in motivating Einstein's thinking, didn't end up in the formal machinery of GR.

 

[DavidMcC]

If a Kerr ring-"singularity" loses its angular momentum by gravitational radiation before fully collapsing to a non-rotating BH, that would explain the lack of rotation in a BH-based universe, wouldn't it?

 

[George Jones]

If a Kerr ring-"singularity" loses its angular momentum by gravitational radiation before fully collapsing to a non-rotating BH, that would explain the lack of rotation in a BH-based universe, wouldn't it?

But this doesn't happen. Observations indicate that many black at near the maximum rate.

http://news.bbc.co.uk/2/hi/science/nature/7184526.stm
http://arxiv.org/abs/astro-ph/0612354

There are more recent observations as well, but I don't have them at my fingertips. More direct methods for observing black hole spin should be available in few years,

https://www.cfa.harvard.edu/~loeb/sciam2.pdf.

 

[bcrowell]

If a Kerr ring-"singularity" loses its angular momentum by gravitational radiation before fully collapsing to a non-rotating BH, that would explain the lack of rotation in a BH-based universe, wouldn't it?

The universe isn't a black hole.

The Godel metric is the best known example of a rotating cosmology, and it doesn't contain gravitational radiation.

The Godel metric isn't realistic, but even in the case of more realistic rotating cosmological models, it seems implausible to me that the rotation would dissipate by radiation of gravitational waves. The reason is that there are only two time scales in such a model: the time scale corresponding to the Hubble constant, and the period of the rotation. Both of these are extremely long (billions of years), so the only gravitational waves you could get would be ones with periods of billions of years. But the efficiency of gravitational radiation typically goes like some high power of frequency, so it doesn't seem plausible to me that you could get strong gravitational radiation when the frequency is so incredibly low.

There is also the question of whether symmetry rules out such a process for a homogeneous cosmology. If I had to guess, I'd guess that it does.

There are also observational constraints. There are upper limits on the strength of the ambient gravitational radiation in our universe, and these upper limits are extremely low -- they say the radiation degrees of freedom of our universe are basically not activated at all (contrary to what would be expected on thermodynamic grounds).

 

[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 Earth's rotation. Round swirls of cloud migrate across the Earth due to rotation.

 

[bcrowell]

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 Earth's 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.

 

[Shenstar]

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/Copern...ent_of_cosmic_microwave_background_anisotropy

Or is this another issue?

 

[Chronos]

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.

 

[cph]

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.

 

[DavidMcC]

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.

 

[DavidMcC]

... 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.

 

[bcrowell]

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/physics/Relativity/BlackHoles/universe.html

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.

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...?

 

[bcrowell]

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?"

[...]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.

 

[Ken G]

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/Gödel_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.

 

[dougal217]

Doesn't w increase along with r in a bounded, rotating Universe (which by definition has a gravitational center)?

 

[bcrowell]

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/cosmology_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?" -- https://www.physicsforums.com/showpost.php?p=3348756&postcount=8

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

http://en.wikipedia.org/wiki/Gödel_metric
http://arxiv.org/abs/0902.4575
http://adsabs.harvard.edu/full/1985MNRAS.213..917B

-- \omega is constant everywhere on a surface of constant cosmological time (see "How are time and distance measured in cosmology?" -- https://www.physicsforums.com/showpost.php?p=3332515&postcount=7 ).

-Ben

 

[Imax]

I think the answer will be , rotating in reference to what.

Where's the observer, and where's the test particle?

 

[Lino]

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.

 

[bcrowell]

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?

 

[Lino]

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.

 

[bcrowell]

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.

 

[Lino]

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.

 

[bcrowell]

For gyroscopes, I guess you could read the WP article.

 

[Lino]

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.

 

[bcrowell]

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.)

The FAQ describes two types of tests. One is a test where you basically use the solar system as a gyroscope. In this type, I think the answer to your question is that we do want to use observations of the solar system over the longest possible time, in order to see a cumulative effect that is as big as possible, but the longest possible time is only 100 years or so. In both cases (GPB and solar system), what's working in your favor is that the effect is cumulative, so you want the longest possible period of observation.

The other type of test described in the FAQ is CMB tests, which are completely different.

 

[Imax]

If the Big Bang singularity had rotation, then it’s not unreasonable to say that the Universe could rotate. If that's the case, then there could be measurable differences in cosmic bckground radiation.

 

[Lino]

Thanks Ben.