
#55
Mar108, 09:48 PM

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#56
Mar1808, 05:34 PM

P: 8

I would propose that the universe is no rotating for the following reason.
If you have a laser fired at you it does not matter whether you are stationary or moving closer or further from the laser the light will strike you at the same time. The same effect means that everywhere appears to be the centre of the universe. If everywhere is the centre then everywhere is expanding away from us therefore it cannot be spinning. We appear to be at the centre of an expanding bubble but then so does everywhere else. This is my working assumption and I would be happy if someone could put me right if this is not so. Ed Joyce 



#57
May1411, 10:31 PM

P: 1

If the universe is rotating, then we have no possible way in which to observe its rotation, since we are observers from within the metric which is dependent upon the observable structure. The observable universe could be doing starjumps, or stretching like a tired bear, but from who's perspective? We are within a spacetime phenomenon and, it must follow, that all methods of measurement remain inside the same metric. From an observer outside of our metric, a metre "here" may well be a billion kilometres long "elsewhere", or a second "here" may as well be Planck time "there". We ourselves have no possible choice, but to use our universe within its given metric.
However: Godel solved a nonmetrical solution to spacetime. We need to leap forward a few years and then come back again: Penrose et al developed the idea of a light cone within spacetime. For simplicity, let's flatten it out. You have an event (planet, fart, whatever) as a single point. You know that light has a definite speed, so run light escaping up the yaxis. You also know that it can't escape sideways any faster than usual, so run distance up along the xaxis. What you end up with is a cone of events that can be influenced by the event at the origin. There remains outside of the cone events that cannot be observed, nor can they influence actions within the cone. So: We have so far talked about a flat universe  or at least one that follows geodesic principles  under those circumstances, there is no physical possibility of a photon from outside the light cone impinging within our own reality. Godel's maths suggested that we could rotate the universe and skew the light cones, so that events which we should see in the future, we see now: so we observe future history. Or, if bent the other way, we can observe events who's light cone is separated, but by bending spacetime we can observe and manipulate previous events. My maths isn't good enough to run through the above. Perhaps someone can help. 



#58
May1511, 12:45 PM

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PF Gold
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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. There does not have to be a center of rotation, and such solutions can be homogeneous. One of the earliest cosmological solutions to the Einstein field equations to be discovered was the Gödel metric, which rotates and has closed timelike curves. Solarsystem observations[Clemence 1957] put a modelindependent upper limit of 10^7 radians/year on the rotation, which is an order of magnitude too lax to rule out the Gödel metric. Observations of the cosmic microwave background's anisotropy impose a limit that is tighter (perhaps 10^9 rad/yr[Su 2009] or 10^15 rad/yr[Barrow 1985]), but modeldependent. Because all of the present observation are consistent with zero rotational velocity, it is not possible to attribute any prominent cosmological role to rotation. In particular, centrifugal forces cannot contribute significantly to cosmological expansion. Clemence, C.M. (1957). 'Astronomical Time', Rev. Mod. Phys. Vol. 29, p. 2 Hawking, S.W. (1969). 'On the Rotation of the Universe', Mon. Not. R. astr. Soc. Vol. 142, p. 529. Collins, C.B., and Hawking, S.W. (1973). 'The Rotation and Distortion of the Universe', Mon. Not. R. astr.Soc. Vol 162, p. 307. Barrow, J. D., Juszkiewicz, R., & Sonoda, D. H., "Universal rotation: how large can it be?," 1985  http://adsabs.harvard.edu/full/1985MNRAS.213..917B Su and Chu, "Is the universe rotating?," 2009, http://arxiv.org/abs/0902.4575 



#59
May1511, 01:01 PM

P: 150

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.




#60
May1511, 01:45 PM

P: 43

"... 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




#61
May1511, 03:16 PM

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#62
May2411, 07:34 PM

P: 832

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




#63
Jun111, 02:58 PM

P: 259





#64
Jun711, 05:55 PM

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P: 3,071

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. 



#65
Jun811, 07:55 AM

P: 101

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




#66
Jun811, 08:22 AM

Mentor
P: 6,037

http://news.bbc.co.uk/2/hi/science/nature/7184526.stm http://arxiv.org/abs/astroph/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. 



#67
Jun811, 12:14 PM

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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). 



#68
Jun811, 12:50 PM

P: 29

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.




#69
Jun811, 03:32 PM

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#70
Jun811, 05:58 PM

P: 29

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? 



#71
Jun811, 11:15 PM

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P: 9,173

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.




#72
Jun811, 11:25 PM

P: 45

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.



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