<p>I'm not sure how universal rotation is supposed to produce dark-energy-like effects in your scenario, but if there <em>was</em> a rotational effect, it would operate perpendicular to the axis of rotation but not parallel to it. This would produce a clear anisotropy in the expansion of the universe, which we simply don't see.</p>
<p>In fact, <a href="http://adsabs.harvard.edu/abs/1969MNRAS.142..129H" rel="noreferrer">Stephen Hawking pointed out back in 1969</a> that rotation of the (visible) universe would produce a detectable signature in the cosmic background radiation; the absence of any such signature put upper limits on any possible rotation. Subsequent studies have only strengthened this; <a href="http://adsabs.harvard.edu/abs/2016PhRvL.117m1302S" rel="noreferrer">a paper by Saadeh et al. in 2016</a> puts limits that are about <span class="math-container">$10^{8}$</span> times more stringent than Hawking's original paper.</p>

CC BY-SA 4.0
2021-07-19T19:19:16.357Z
What were the constraints provided in Hawking's original paper?
Daddy Kropotkin
CC BY-SA 4.0
2021-07-19T21:53:45.957Z
$\omega / H_{0} < 10^{-3}$, where $\omega$ is the vorticity (twice the angular velocity for a uniformly rotating system).
Peter Erwin
CC BY-SA 4.0
2021-07-19T23:56:43.990Z
Universe's three dimensional isotropy is just unproven assumption made by Einstein to provide simplicity (in lack of exact knowledge of Universe's actual structure, especially beyond the "visible" part of it).  If what I have read that the measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales is true, then this clearly contradicts Enstein's assumption that Universe is isotropic.
Alex
CC BY-SA 4.0
2021-07-20T05:53:04.540Z
@Alex What to you mean? The CMB measurements *confirm* the isotropy on the large scale. However, there *are* small variations in the CMB energy data, about 1 part in 100,000 (once you compensate for the anisotropies caused by our motion relative to the comoving frame of the CMB). [The famous CMB image](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Ilc_9yr_moll4096.png/800px-Ilc_9yr_moll4096.png) has been processed specifically to make those tiny deviations visible.
PM 2Ring
CC BY-SA 4.0
2021-07-20T12:46:08.223Z
@Alex "the Universe has a flat geometry on large scales" means the 3D spatial geometry is [Euclidian](https://en.wikipedia.org/wiki/Euclidean_space), as opposed to being spherical or [hyperbolic](https://en.wikipedia.org/wiki/Hyperbolic_space). It is orthogonal to the question of whether the universe is isotropic. (Although I suspect you might have prolems in GR reconciling a strongly anisotropic distribution of matter with a homogeneous geometry, flat or otherwise. So the fact that the large-scale geometry appears to be homogeneously "flat" may require istrophy.)
Peter Erwin
CC BY-SA 4.0
2021-07-20T14:55:42.840Z
@Peter Erwin Thanks for the clarification of what the notion "Universe flat geometry" means. Then my next question will be: if the Universe on larger scale indeed has flat Euclidian topology, how this correlates with the Einstein's suggestion of the space curvature caused by the objects mass gravitation - should we return back to Newton's concept of gravitational pull?
Alex
CC BY-SA 4.0
2021-07-20T18:39:00.207Z
@Alex It's the overall curvature of *spacetime* (the 4-dimensional manifold) that is caused by the distribution of mass-energy and defines geodesic trajectories and produces gravitational effects, not the curvature of a 3-dimensional sub-manifold (such as a spacelike subset of spacetime, which is what cosmologists are referring to when they talk about the universe being "flat").
Peter Erwin
CC BY-SA 4.0
2021-07-20T18:50:30.850Z
The distribution of mass-energy matters. If you have a massive object in empty space, then the spacetime nearby will be curved so as to allow for various orbits, etc.; there will be (I think) a tiny positive spatial curvature. If you have uniformly distributed mass-energy (large-scale cosmology), then the spatial curvature will depend on the amount of mass-energy. This is all part of Einstein's suggestion that *spacetime* curvature depends on the *distribution and amount* of mass-energy (more precisely, on the [stress-energy tensor](https://en.wikipedia.org/wiki/Stress–energy_tensor)).
Peter Erwin
CC BY-SA 4.0
2021-07-20T18:55:15.787Z
@Peter Erwin - what prevents from the assumption that 4-dimensional spacetime manifold is also flat as it's 3-dimensional sub-manifold?                                               Also what is your comment on https://www.sciencealert.com/astrophysicists-say-our-universe-could-really-be-a-giant-3d-donut ?
Alex
CC BY-SA 4.0
2021-07-20T20:48:20.030Z
@Alex The closest equivalent to a "flat" 4D spacetime would, i think, be [Minkowski spacetime](https://en.wikipedia.org/wiki/Minkowski_space), which is the spacetime of Special Relativity. (This is *not* the same as a 4D Euclidean space, because the time dimension does not behave like an ordinary space dimension.) But this contains no gravitating mass-energy and is static, so it doesn't match the cosmology of our universe...
Peter Erwin
CC BY-SA 4.0
2021-07-20T20:56:06.317Z
Re your "sciencealert" link: that's a discussion of *toplogy*, and as it says the argument is that the universe is (spatially) flat with a 3-torus topology. (This idea has been around for a while; I haven't looked at the actual paper, so I don't have a real opinion about it.
Peter Erwin
CC BY-SA 4.0
2021-07-22T13:36:12.557Z
@Ingolifs Here you go... https://www.quantamagazine.org/modified-gravity-theory-passes-a-critical-test-20200728/
Alex
CC BY-SA 4.0
2021-07-22T16:39:33.757Z
@ Ingolifs https://aeon.co/essays/we-should-explore-alternatives-to-the-standard-model-of-cosmology
Alex
CC BY-SA 4.0
2021-07-19T01:21:30.110Z
2021-07-19T01:21:30.110Z
Peter Erwin
<p>Any and all movement is with respect to something else. If you were the only thing in the Universe, you would not know if you’re moving or not, because you would not have any reference point to define this movement from.</p>
<p>The same holds for the Universe itself. It is expanding, that we know by looking at galaxies and seeing that most of them have a redshift. But it would be wrong to think of the Universe as an inflating balloon, because there’s an outside to the balloon—now <em>if</em> there is an outside to the Universe, we don’t and we can’t know about it, so it’s in the domain of metaphysics, not physics or astronomy.</p>
<p>So, then, <em>what</em> would the Universe be moving or rotating relative to? The mere notion of Universe “rotation” has no meaning, because there is no “exterior” to compare it with. And no, we can’t say that it’s moving relative to itself, because then, there’s no movement (just like we couldn’t know the Earth is turning if there were no objects outside of it to define rotation).</p>

CC BY-SA 4.0
2021-07-18T10:44:07.930Z
Even without external reference, the rotation of the Earth can be detected, and measured, through the precession of gyroscopes or Foucault pendulums.
notovny
CC BY-SA 4.0
2021-07-19T01:58:37.230Z
Because it rotates with respect to something else. It rotates with respect to the space around it. But there is nothing around the Universe.
Pierre Paquette
CC BY-SA 4.0
2021-07-20T05:42:35.647Z
There are rotating solutions to the Einstein Field Equations, the first one was found by Kurt Gödel. So a universe *could* have global rotation without the need for something external that it's rotating relative to. However, the Gödel solution has closed timelike curves, which are pretty weird. ;) FWIW, Einstein was rather displeased to learn of the Gödel solution, mostly because of the CTCs.
PM 2Ring
CC BY-SA 4.0
2021-07-18T05:29:58.953Z
2021-07-18T05:29:58.953Z
Pierre Paquette
<p>An old conundrum was why do rotating galaxies and galaxy clusters not fly apart due to centrifugal (or -petal as you prefer) force, as the gravity due to their observable mass is too small to prevent this. The advent of dark matter (and energy) helped to explain this. If the universe is rotating, their gravitational force may prevent rotation adding to its expansion, especially since they seem to be evenly distributed throughout the universe.</p>

CC BY-SA 4.0
2021-07-15T16:47:58.053Z
This answer seems to be more speculation than mainstream physics. Why do you think galactic rotation has any bearing on cosmological rotation?
Anders Sandberg
CC BY-SA 4.0
2021-07-15T18:44:37.127Z
@DeeGee May be with the assumption of rotating Universe and the centrifugal force caused by this rotation there will not be the need to introduce concepts of Dark Matter and Dark Energy?
Alex
CC BY-SA 4.0
2021-07-15T21:12:47.787Z
@Alex I really hadn't thought this through. The gravitational attraction of dark matter might be enough to retain mass at the rotation centre but as it is evenly distributed, it would have little effect in preventing expansion further out where the angular velocity would be huge.
DeeGee
CC BY-SA 4.0
2021-07-15T22:24:10.807Z
@DeeGee You are considering the existence of dark matter as a proven fact, while it is just a theoretical concept which has been introduced to explain gravitational lensing.                                                Likewise the dark energy is just a theoretical concept introduced to explain red shift acceleration...
Alex
CC BY-SA 4.0
2021-07-15T23:28:23.873Z
@Alex Dark matter is on much firmer ground than dark energy. We're pretty certain it exists, we just don't know what the hell it is.
Ingolifs
CC BY-SA 4.0
2021-07-16T12:12:56.823Z
@Ingolifs "We're pretty certain it exists, we just don't know what the hell it is." ;) :D
Alex
CC BY-SA 4.0
2021-07-16T22:30:01.763Z
https://researchoutreach.org/articles/hydrogens-second-flavour-solution-to-dark-matter/
Alex
CC BY-SA 4.0
2021-07-19T10:04:06.573Z
I did upvoted especially because some brilliant minds did analogues thoughts. This answer is no so speculative if some history is taken into account.
Alchimista
CC BY-SA 4.0
2021-07-21T21:53:59.233Z
@Ingolifs in https://theconversation.com/amp/why-do-astronomers-believe-in-dark-matter-122864 the keywords are "believe" and "I am betting"... 
Alex
CC BY-SA 4.0
2021-07-15T16:45:55.543Z
2021-07-15T16:45:55.543Z
DeeGee
<p>I have read that the measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales.
Could one assume then that the Universe is acceleratingly rotating around some axis and that the centrifugal force of this rotation produces Universe's accelerated expansion and could this suggestion possibly eliminate the need for introducing the concept of dark energy, which was suggested/hypothesized mostly to accommodate/explain accelerated Universe expansion, observable by spectral red shift of far away Galaxies.</p>

CC BY-SA 4.0
2021-07-15T12:55:45.487Z
But the expansion would then show anisotropy. Old and interesting question. I am sure posts already exists in SE, here or in Physics....
Alchimista
CC BY-SA 4.0
2021-07-15T13:09:01.123Z
@Alchimista I have recently posed a question (first on Physics Stack Exchange - https://physics.stackexchange.com/questions/648957/should-occams-razor-be-used-in-judging-hypothetical-cosmological-theories?- it was closed there as not belonging to the area of physics ... and later I posted the same on History of Science https://hsm.stackexchange.com/questions/13332/should-occams-razor-be-used-in-judging-hypothetical-cosmological-theories?) So I arrived to this question trying to apply Occam's Razor principle in this particular case.
Alex
CC BY-SA 4.0
2021-07-15T13:31:03.077Z
Related: https://astronomy.stackexchange.com/questions/32782/tycho-brahes-model/35055#35055
John
CC BY-SA 4.0
2021-07-17T15:24:33.000Z
https://www.livescience.com/65882-does-the-universe-rotate.html and https://astronomy.stackexchange.com/questions/6643/is-the-whole-universe-is-rotating-on-an-axis/30255#30255
Daddy Kropotkin
CC BY-SA 4.0
2021-07-17T18:16:55.703Z
@Daddy Kropotkin "All of these studies suggest that the universe is largely uniform and not spinning. This conclusion is one that's not likely to change. Future measurements of the polarization of the CMB may improve in the next few decades, but the new data is unlikely to challenge the previous findings."               
Will wait and see...
Alex
CC BY-SA 4.0
2021-07-17T18:23:51.057Z
@Daddy Kropotkin The assumption of the Universe uniformity can not be proven until there will be a better understanding what dark matter is and how it is distributed in the Universe...
Alex
<cosmology>
Could it be that Universe is rotating while expanding?
Alex
# Could it be that Universe is rotating while expanding?

(Asked by: username_0 on 2021-07-15)

I have read that the measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales.
Could one assume then that the Universe is acceleratingly rotating around some axis and that the centrifugal force of this rotation produces Universe's accelerated expansion and could this suggestion possibly eliminate the need for introducing the concept of dark energy, which was suggested/hypothesized mostly to accommodate/explain accelerated Universe expansion, observable by spectral red shift of far away Galaxies.

**Tags:** cosmology
> **Comments:**
> **username_1**: But the expansion would then show anisotropy. Old and interesting question. I am sure posts already exists in SE, here or in Physics....  
> **username_0**: @Alchimista I have recently posed a question (first on Physics Stack Exchange - https://physics.stackexchange.com/questions/648957/should-occams-razor-be-used-in-judging-hypothetical-cosmological-theories?- it was closed there as not belonging to the area of physics ... and later I posted the same on History of Science https://hsm.stackexchange.com/questions/13332/should-occams-razor-be-used-in-judging-hypothetical-cosmological-theories?) So I arrived to this question trying to apply Occam's Razor principle in this particular case.  
> **username_2**: Related: https://astronomy.stackexchange.com/questions/32782/tycho-brahes-model/35055#35055  
> **username_3**: https://www.livescience.com/65882-does-the-universe-rotate.html and https://astronomy.stackexchange.com/questions/6643/is-the-whole-universe-is-rotating-on-an-axis/30255#30255  
> **username_0**: @Daddy Kropotkin "All of these studies suggest that the universe is largely uniform and not spinning. This conclusion is one that's not likely to change. Future measurements of the polarization of the CMB may improve in the next few decades, but the new data is unlikely to challenge the previous findings."                Will wait and see...  
> **username_0**: @Daddy Kropotkin The assumption of the Universe uniformity can not be proven until there will be a better understanding what dark matter is and how it is distributed in the Universe...  

---

## 3 answers

### Answer by username_4 on 2021-07-19. Score: 6

I'm not sure how universal rotation is supposed to produce dark-energy-like effects in your scenario, but if there *was* a rotational effect, it would operate perpendicular to the axis of rotation but not parallel to it. This would produce a clear anisotropy in the expansion of the universe, which we simply don't see.

In fact, [Stephen Hawking pointed out back in 1969](http://adsabs.harvard.edu/abs/1969MNRAS.142..129H) that rotation of the (visible) universe would produce a detectable signature in the cosmic background radiation; the absence of any such signature put upper limits on any possible rotation. Subsequent studies have only strengthened this; [a paper by Saadeh et al. in 2016](http://adsabs.harvard.edu/abs/2016PhRvL.117m1302S) puts limits that are about $10^{8}$ times more stringent than Hawking's original paper.

> **Comments:**
> **username_3**: What were the constraints provided in Hawking's original paper?  
> **username_4**: $\omega / H_{0} < 10^{-3}$, where $\omega$ is the vorticity (twice the angular velocity for a uniformly rotating system).  
> **username_0**: Universe's three dimensional isotropy is just unproven assumption made by Einstein to provide simplicity (in lack of exact knowledge of Universe's actual structure, especially beyond the "visible" part of it).  If what I have read that the measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales is true, then this clearly contradicts Enstein's assumption that Universe is isotropic.  
> **username_5**: @Alex What to you mean? The CMB measurements *confirm* the isotropy on the large scale. However, there *are* small variations in the CMB energy data, about 1 part in 100,000 (once you compensate for the anisotropies caused by our motion relative to the comoving frame of the CMB). [The famous CMB image](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Ilc_9yr_moll4096.png/800px-Ilc_9yr_moll4096.png) has been processed specifically to make those tiny deviations visible.  
> **username_4**: @Alex "the Universe has a flat geometry on large scales" means the 3D spatial geometry is [Euclidian](https://en.wikipedia.org/wiki/Euclidean_space), as opposed to being spherical or [hyperbolic](https://en.wikipedia.org/wiki/Hyperbolic_space). It is orthogonal to the question of whether the universe is isotropic. (Although I suspect you might have prolems in GR reconciling a strongly anisotropic distribution of matter with a homogeneous geometry, flat or otherwise. So the fact that the large-scale geometry appears to be homogeneously "flat" may require istrophy.)  
> **username_0**: @Peter Erwin Thanks for the clarification of what the notion "Universe flat geometry" means. Then my next question will be: if the Universe on larger scale indeed has flat Euclidian topology, how this correlates with the Einstein's suggestion of the space curvature caused by the objects mass gravitation - should we return back to Newton's concept of gravitational pull?  
> **username_4**: @Alex It's the overall curvature of *spacetime* (the 4-dimensional manifold) that is caused by the distribution of mass-energy and defines geodesic trajectories and produces gravitational effects, not the curvature of a 3-dimensional sub-manifold (such as a spacelike subset of spacetime, which is what cosmologists are referring to when they talk about the universe being "flat").  
> **username_4**: The distribution of mass-energy matters. If you have a massive object in empty space, then the spacetime nearby will be curved so as to allow for various orbits, etc.; there will be (I think) a tiny positive spatial curvature. If you have uniformly distributed mass-energy (large-scale cosmology), then the spatial curvature will depend on the amount of mass-energy. This is all part of Einstein's suggestion that *spacetime* curvature depends on the *distribution and amount* of mass-energy (more precisely, on the [stress-energy tensor](https://en.wikipedia.org/wiki/Stress–energy_tensor)).  
> **username_0**: @Peter Erwin - what prevents from the assumption that 4-dimensional spacetime manifold is also flat as it's 3-dimensional sub-manifold?                                               Also what is your comment on https://www.sciencealert.com/astrophysicists-say-our-universe-could-really-be-a-giant-3d-donut ?  
> **username_4**: @Alex The closest equivalent to a "flat" 4D spacetime would, i think, be [Minkowski spacetime](https://en.wikipedia.org/wiki/Minkowski_space), which is the spacetime of Special Relativity. (This is *not* the same as a 4D Euclidean space, because the time dimension does not behave like an ordinary space dimension.) But this contains no gravitating mass-energy and is static, so it doesn't match the cosmology of our universe...  
> **username_4**: Re your "sciencealert" link: that's a discussion of *toplogy*, and as it says the argument is that the universe is (spatially) flat with a 3-torus topology. (This idea has been around for a while; I haven't looked at the actual paper, so I don't have a real opinion about it.  
> **username_0**: @Ingolifs Here you go... https://www.quantamagazine.org/modified-gravity-theory-passes-a-critical-test-20200728/  
> **username_0**: @ Ingolifs https://aeon.co/essays/we-should-explore-alternatives-to-the-standard-model-of-cosmology  

---

### Answer by username_6 on 2021-07-18. Score: -5

Any and all movement is with respect to something else. If you were the only thing in the Universe, you would not know if you’re moving or not, because you would not have any reference point to define this movement from.

The same holds for the Universe itself. It is expanding, that we know by looking at galaxies and seeing that most of them have a redshift. But it would be wrong to think of the Universe as an inflating balloon, because there’s an outside to the balloon—now *if* there is an outside to the Universe, we don’t and we can’t know about it, so it’s in the domain of metaphysics, not physics or astronomy.

So, then, *what* would the Universe be moving or rotating relative to? The mere notion of Universe “rotation” has no meaning, because there is no “exterior” to compare it with. And no, we can’t say that it’s moving relative to itself, because then, there’s no movement (just like we couldn’t know the Earth is turning if there were no objects outside of it to define rotation).

> **Comments:**
> **username_7**: Even without external reference, the rotation of the Earth can be detected, and measured, through the precession of gyroscopes or Foucault pendulums.  
> **username_6**: Because it rotates with respect to something else. It rotates with respect to the space around it. But there is nothing around the Universe.  
> **username_5**: There are rotating solutions to the Einstein Field Equations, the first one was found by Kurt Gödel. So a universe *could* have global rotation without the need for something external that it's rotating relative to. However, the Gödel solution has closed timelike curves, which are pretty weird. ;) FWIW, Einstein was rather displeased to learn of the Gödel solution, mostly because of the CTCs.  

---

### Answer by username_8 on 2021-07-15. Score: -5

An old conundrum was why do rotating galaxies and galaxy clusters not fly apart due to centrifugal (or -petal as you prefer) force, as the gravity due to their observable mass is too small to prevent this. The advent of dark matter (and energy) helped to explain this. If the universe is rotating, their gravitational force may prevent rotation adding to its expansion, especially since they seem to be evenly distributed throughout the universe.

> **Comments:**
> **username_9**: This answer seems to be more speculation than mainstream physics. Why do you think galactic rotation has any bearing on cosmological rotation?  
> **username_0**: @DeeGee May be with the assumption of rotating Universe and the centrifugal force caused by this rotation there will not be the need to introduce concepts of Dark Matter and Dark Energy?  
> **username_8**: @Alex I really hadn't thought this through. The gravitational attraction of dark matter might be enough to retain mass at the rotation centre but as it is evenly distributed, it would have little effect in preventing expansion further out where the angular velocity would be huge.  
> **username_0**: @DeeGee You are considering the existence of dark matter as a proven fact, while it is just a theoretical concept which has been introduced to explain gravitational lensing.                                                Likewise the dark energy is just a theoretical concept introduced to explain red shift acceleration...  
> **username_10**: @Alex Dark matter is on much firmer ground than dark energy. We're pretty certain it exists, we just don't know what the hell it is.  
> **username_0**: @Ingolifs "We're pretty certain it exists, we just don't know what the hell it is." ;) :D  
> **username_0**: https://researchoutreach.org/articles/hydrogens-second-flavour-solution-to-dark-matter/  
> **username_1**: I did upvoted especially because some brilliant minds did analogues thoughts. This answer is no so speculative if some history is taken into account.  
> **username_0**: @Ingolifs in https://theconversation.com/amp/why-do-astronomers-believe-in-dark-matter-122864 the keywords are "believe" and "I am betting"...   

---

