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Q: What is the angular momentum of the universe? ( Answered ,   7 Comments )
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 Subject: What is the angular momentum of the universe? Category: Science > Physics Asked by: yates9-ga List Price: \$20.00 Posted: 18 Nov 2004 16:43 PST Expires: 18 Dec 2004 16:43 PST Question ID: 430853
 ```I'm not an expert.. when I was studying physics in college I never got a satisfying answer to this question and to how one could go about proving it. I assume almost everyone will tell me that it is zero, but how could/can you demonstrate this is so?```
 Subject: Re: What is the angular momentum of the universe? Answered By: hedgie-ga on 18 Nov 2004 19:23 PST Rated:
 ```The answer given in comment by hfsha-ga has some merit - and is supported by this expert opinion: http://www.physlink.com/Education/AskExperts/ae230.cfm (This may be opinion of the same person, of course) but I think it does not fully answers your question. There is some debate between the experts on the issue: http://www.madsci.org/posts/archives/apr2002/1017882364.As.r.html and the topic had to do with a profound issue of the SEARCH TERM : Mach Principle ://www.google.com/search?q=Mach+principle which was so far not fully resolved to universal satisfaction. So - I will not give you a final, definitive answer, but I will comment on (possible) meaning of the question itself: The term 'angular momentum' is usually used on the context of classical physics - as a vector in 3D space. Defind e.g. here: http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html http://en.wikipedia.org/wiki/Angular_momentum This are two problems with answer given by hfshaw and others: 1) Vector quantity does not have a value. Only it's components in some frame of reference have value. Meaning: if you are a man, standing on the Earth, a spinning bullet passing by has a (non zereo) angular momentum. If you are a bug (or a microbe) sitting on that bullet , that same moment is zero. The value depends on the frame of reference (aka coordinate system). Hfshaw (and others) make an implicit assumption, that you are thinking of certain 'preferred' system of coordinates, implied by today's accepted (big bang) model of the universe - which is frame defined by microwave background and by the stars - and then the answer is sort of a tautology: with respect to the stars, the stars do not move (too much). (This is ignoring the question of the 'missing mass' or 'dark mass'- mass which is not visible (like stars) and is, according to some speculations, http://www.amazon.com/exec/obidos/tg/detail/-/0465037402/103-5666594-0955800?v=glance http://www.ebicom.net/~rsf1/missmass.htm making 90% of the total). 2) The second problem I see with glib answer to this question is that it shows a 3D thinking, while cosmological theories are inherently 4D, and the concept of angular momentum in relativistic (4D) theory is a topic for a PhD thesis in itself. The 3D thinking has some validity - since the big-bang moment defines a special frame - in which there is e.g. universal time, but that these do not have universal validity. There are (alternative) theories which try to explain red-shift (caused by expansion) by 'rotation of the whole universe' - however the question 'rotation with respect to what' needs to be answered before the concept of rotation has meaning - in relativistic (4D) universe. So - in conclusion: your question is touching issues which are not well defined and resolved. More simple questions: "For an observer on the Earth - what is the angular momentum of our galaxy?" can be answered. The visible stars clearly rotate around the Earth - and stars we see by naked eye are in the Milky Way. The questions about the whole universe, it's mass, energy, linear or angular momentum, require either relativistic appoach, or an assumption that one particular model (such as Big Bang) is valid, and also that you specify a frame of reference. For the 'special frame of reference implied by the Big-Bang' model - and assuming that the 'dark mass' behaves -- well - zero value is a good guess. So - real answers is: no one really knows, as yet. Hedgie``` Request for Answer Clarification by yates9-ga on 19 Nov 2004 01:09 PST ```Hedgie-ga, thank you for the work, but in a sense the line of thinking I was always interested in is more like racecar-ga's comment. This was my argument with my professors that perhaps the argument that the universe is isotropic is actually not really that strong. I don't know if this is possible, if it isn't I'll accept the answer, but can you tell me or conjecture if we did have a positive angular momentum, what consequences would appear? For one there would be a center of the universe, there would have to be... or there would have to be an intertial reference frame for the whole and the whole universe would have some kind of tails like galaxies do? Then the real tricky thing for me to imagine would be that probably the curvature of space would follow the outlines of this galaxy like structure? The thing that makes this question a bit relevant is that afterall most structure we see in the universe appears spinning around somehow...``` Clarification of Answer by hedgie-ga on 19 Nov 2004 01:53 PST ```yates9-ga To get a 'right' answer, one should disclose purpose and if possible, motive when first asking the question. By default (that is unless told otherwise) I assume that people ask about current mainstream theories. There are always alternative theories - and some are interesting: http://www.amazon.com/exec/obidos/tg/detail/-/0521662230/103-5666594-0955800?v=glance http://titles.cambridge.org/catalogue.asp?isbn=0521662230 and imaginative. For example guess that our universe is a black hole or that we are part of computer model .. (I like the last one as possible explanation of the QM weirdness :-) One alternative theory, speculates that red shift of remote galaxies is caused by rotation of the universe, not by expansion... the SEARCH TERM "rotating universe" ://www.google.com/search?hl=en&lr=lang_en&ie=ISO-8859-1&c2coff=1&q=rotating+universe&btnG=Search brings quite a few such theories, some of them quite interesting. However, as I said in the answer: Once you abandon (mainstream Big-Bang model, which fits our 3D intuition) you need to think in terms of General Relativity. Your question, and racecar's remark, appear to be based on 3D concepts. So I see an inherent conflict here. The question would have to be formulated in 4D terms, before it can be seriously debated. So answer is not 'NO' - but trather it is a different and even more complex question. To get a feel of how 4D reasoning looks (and how much - or how little can be explain without math) - please have a look at somewhat related question: http://answers.google.com/answers/threadview?id=351408 Hedgie``` Clarification of Answer by hedgie-ga on 19 Nov 2004 02:57 PST ```Correction: The link I wanted to give you (at the end of clarification) is this http://answers.google.com/answers/threadview?id=31905 The other one may be of interest too, but this one explains (or maintains) the 'strong principle of relativity' according to which laws of physics should be formulated in a way independent of the frame of reference - or as Einstein did put it --by equations which are invariant - not only with respect to Lorentz group - but for any transformation of coordinates. Some experts may disagree (racecar is not included in that group :-) but it is at least a program -- a worthy goal -- program which actually preceeds Einstein's relativity. If you accept that principle - for universe as whole as well - then the question 'does it rotates' always requires one to specify 'rotates with respect to what'. It makes sense to say 'with respect to me', or Earth - and the link above explains (or tries ) that such choice could be be a valid choice of the reference system. In that frame, universe rotates (around you - literaly) . However, what physics is really interested in, is not 'how universe appears in a given frame) but what are those invariant laws. Hedgie```
 yates9-ga rated this answer: `Interesting style of answering... a lot of different kinds of answers.`

 Subject: Re: What is the angular momentum of the universe? From: hfshaw-ga on 18 Nov 2004 16:55 PST
 ```Current models of the universe are based on the assumption that at large scales, it is isotropic -- that is, that it has no preferred directions. All observational evidence so far seems to support this assumption. Insofar as the universe *is*, in fact, isotropic the total angular momentum of the universe is zero. This is because angular momentum is a vector quantity, and if the universe had a net angular momentum vector, this would define a "special" direction, violating the assumption of isotropy.```
 Subject: Re: What is the angular momentum of the universe? From: racecar-ga on 18 Nov 2004 21:47 PST
 ```The (classical) angular momentum of an object about its center of mass is independent of the translational motion of the center of mass, and so does not depend on reference frame, as long as it's an inertial frame. For this reason, the center of mass is a common point about which to consider the angular momentum; so common that it is often unnecessary to state explicitly. For example, if you were to ask 'what is the angular momentum of the earth?' some sticklers might argue that the question is ambiguous, but really it's pretty clear that you mean the angular momentum about the earth's center of mass. The problem with the universe is that every point in it has equal right to call itself the center of mass.```
 Subject: Re: What is the angular momentum of the universe? From: yates9-ga on 19 Nov 2004 03:13 PST
 ```I resent a bit the statement that "what physics is interested in" is purely invariant laws. And actually overall it is odd how a something that might resemble one of einsteins simple thought experiements brings up so much resistance. But this'll become a forum discussion rather than an answer... I guess I was hoping it would be possible to physically prove based on some simple assumptions or specific measurements that it is either not true or highly unlikely. But it does not seem the case... Rather we would much prefer if it is the case because our formulas look nicer. Thank you for the work, though!```
 Subject: Re: What is the angular momentum of the universe? From: helpfulperson-ga on 19 Nov 2004 13:45 PST
 ```Surely to define an angular momentum for a body you need a frame of reference outside that body. For instance, to determine if the earth is rotating you need to be able to see outside the earth. This should be true for the universe too. Is there an error in the above logic?```
 Subject: Re: What is the angular momentum of the universe? From: racecar-ga on 19 Nov 2004 14:41 PST
 ```Helpfulperson: There is an error in the logic, if you're willing to concede that the laws of physics are basically known, and are the same in all inertial reference frames. So it is possible, armed with physical laws, to determine that the earth is spinning without looking outside. The reason is that spinning involves acceleration--directions of motions change. This gives rise to pseudo-forces, like the Coriolis force, or the centrifugal force which causes you to weigh less standing on the equator than at the poles. A pendulum does not oscillate back and forth in the same plane, but rotates at the Coriolis frequency (google Foucault pendulum). Observing these forces, we could conclude the earth is spinning without looking outside the earth. The same is not true of linear motion. There is no way to tell without looking outside the earth how fast the earth is moving in a straight line. Even if you do look outside, you still have to pick a (necessarily arbitrary) frame of reference. So it would be meaningless to ask what is the linear momentum of the universe.```
 Subject: Re: What is the angular momentum of the universe? From: yates9-ga on 20 Nov 2004 06:01 PST
 ```Racecar-ga, actually you are almost answering what I was hoping to learn... So even on the scale of galaxies turning around an axis there would be a theoretical coriolis effect on a galactical level? Would this kind of effect not interfere with signals coming from very distant galaxies? ... so the answer would be something like, it is unlikely that there is a positive angular momentum because we would have likely noted its effect on cosmological measures? hmm...```
 Subject: Re: What is the angular momentum of the universe? From: lyio-ga on 02 Dec 2004 22:51 PST
 ```Einstein's field equations permit an exact solution for a ROTATING universe, solved by Kurt Godel in the mid- 1940s. (whether or not such a universe would have an angular momentum is a trickier thought). Interestingly, EVERY point in this universe would appear to its' observers to be the center. (I'm the center of mine and you or your's- doesn't sound too bad). Time would move in 'circles'. check out: http://www.discovery.org/scripts/viewDB/index.php?command=view&program=CSC&id=1117 or search for "godel einstein rotating universe"```