# Is the Universe rotating?

by utek1
Tags: rotating, universe
P: 1,253
 Quote by Ulysees Are the most distant objects ever observed, all over the sky? Or do they tend to be in one direction?
Observations of every type of object observed, whether near or far, are isotropic (the same in all directions). This is a very fundamental observation that is at the heart of our current cosmological model.
 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
 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 star-jumps, or stretching like a tired bear, but from who's perspective? We are within a space-time 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 non-metrical solution to space-time. We need to leap forward a few years and then come back again: Penrose et al developed the idea of a light cone within space-time. 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 y-axis. You also know that it can't escape sideways any faster than usual, so run distance up along the x-axis. 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 space-time we can observe and manipulate previous events. My maths isn't good enough to run through the above. Perhaps someone can help.
Emeritus
PF Gold
P: 5,598
 Quote by stiv 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.
This is incorrect.

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 non-Machian 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.

Solar-system observations[Clemence 1957] put a model-independent 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 model-dependent.

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
 P: 154 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.
 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
Emeritus
PF Gold
P: 5,598
 Quote by Flatland 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.

 Quote by MACHO-WIMP "... 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.
 P: 901 When I pirouette, the universe is rapidly rotating around me.
P: 297
 Quote by Khashishi When I pirouette, the universe is rapidly rotating around me.
... and you are the centre of rotation!
PF Gold
P: 3,136
 Quote by bcrowell 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.
 P: 101 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?
Mentor
P: 6,248
 Quote by 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?
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.
Emeritus
PF Gold
P: 5,598
 Quote by 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?
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).
 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.
Emeritus