This stuns me for some reason. I was not explicitly aware of this. Why? Just left over momentum from something? Is there a bias in direction (overall or among subsets)?
tl;dr parent and parent's link are not wrong, just very simple. Unfortunately it does not make explicit that "at least a little" does not mean the entirety of the rotation, or even its dominant component.
In the context above gravitational shear is the first of the empirical https://en.wikipedia.org/wiki/Oort_constants (differential rotation shears the non-solid disc). Differential rotation means gas in the direction of the galactic centre drags a bit compared to gas further from the centre, effectively acting as a small torque on a large gas cloud. If the gas cloud has no net rotation at all, this torque will develop it. That's what I take the sentence you focus on to be trying to say.
Unevenness is related to the Jeans instability, which describes the failure of internal gas pressure to prevent gravitational collapse. Chandrasekhar developed a more complicated treatment useful for when the angular momentum of the bulk gas cloud is non-negligible.
Roughly, collisions within the gas cloud convert kinetic energy to light which carries energy out of the collapsing cloud. The cloud consequently cools and contracts gravitationally.
However, still roughly, superimposing a net rotation on random motions within the gas prevents collapse in the direction perpendicular to the net rotation's spin axis. So contraction is mainly along the spin axis. Result: a disc, which is essentially what parent's link says.
The coupling of the galaxy's spin to the solar system's spin is at best weak; the solar system's spin axis is tilted about 60 degrees from the galactic plane; the plane is perpendicular to the galaxy's spin axis, in line with the previous paragraph.
The sun's path around the centre of the galaxy is a bit messy compared to a kinematically hot star (which will feel perturbations like the bar or spiral arms less). The sun tends to bob up and down relative to the midplane of the galactic disc. The Oort constants are local and position-dependent, so the solar system's migration -- and any migration of its precursor -- means wandering into regions with different gravitational shear.
Questions that afaik are still open (but I would be happy for a galaxy or solar system dynamics person to correct me, particularly if the correction includes magnetics and chemodynamics/metallicity-dependent pressure!): has the solar system's spin axis tilt to the galactic disc evolved since formation? Accepting the Coatlicue hypothesis, was the remnant of the explosion of the heavy star that was the precursor of the solar system (and others) aligned with the star's rotation? Was that precursor star's rotational axis aligned with the galactic disc? The remnant was almost certainly not of uniform density. Alternatively accepting the Wolf-Rayet bubble Giant Molecular Cloud (GMC) nebular hypothesis, did the GMC fragmentize under galactic shear, and if so was "our" fragment's spin dominated by that initially, with spin perpendicular to the disc but evolving to the present tilt, or was our fragment's spin axis initially tilted close to the present approx. 60 degrees to the galactic disc where it has since remained?
Finally, also afaik, the spins (more broadly the angular momentum vectors) of stars in the local bubble are essentially random. So repeat the questions in the preceding paragraph for each of those...
I didn't study physics past A-Level and none of this stuff really came up, what little understanding I have is as an interested bystander (I loved physics at school but wasn't the career for me), if I had multiple life times I'd have loved to have studied it at university though, so much stuff to learn in a short life time.
It addresses a different question (Why is the Solar System Flat?) but it touches on the question of why things spin when they clump together from gravity.
An answer through the lens of my own understanding: it's just more difficult NOT to have angular momentum.
If you have a lot of particles falling toward each other due to gravity, imagine how difficult it would be to set it up such that all of them fall straight into their collective center of mass. They'd have to be in a precise, orderly configuration (e.g. equally spaced apart on a unit sphere). Note that each particle affects each other particle -- if any one particle gets too close to another, their gravitational interaction will cause them to move toward each other, and add an angular component to their motion with respect to the center of mass.
There are so much more disorderly configurations that will result in the particles moving with at least SOME angular momentum about their center of mass. Vector sum them all together, and it'd again be difficult for that sum -- the total angular momentum -- to be zero; they'd have to cancel each other out exactly, and there's just way more configurations where that isn't the case.
That's why it's simply much more likely for anything made out of particles in space to be spinning than not.
Additionally, as they fall toward the center of mass, the radius lowers, too, which means to conserve angular momentum you'll see their angular velocities increase. Helps make it more subjectively noticeable to us that everything is spinning.
Your intuition about Boltzmann entropy (disguised as the probability of finding a cloud with no net rotation in its bulk vs one with some) interested me. I think it highlights a "past hypothesis" problem in the wake of your link to the minutephysics video.
Your description is pretty much entirely gravitational. When you add matter-matter collisions, especially inelastic ones, you get changes in internal energy of the grains, and radiation outwards, and that's what drives collapse of gas clouds in timescales shorter than the age of the universe.
Changes in internal energy (e.g. internal rotational degrees of freedom in large dust grains, large being at least the size of a hydrogen molecule) alters the "bounce" from a collision making it easier for collisional grains to clump together. See <https://news.ycombinator.com/item?id=36418364> for a neat example involving partially filled water bottles being dropped to the floor. When the water is swirled, the bounce is reduced. Analogously, one can swirl or flex an isolated molecule around some axis.
For much smaller grains (e.g. atomic hydrogen) that feel electromagnetism, the light emitted by collisions ("scattering") is even more important, as it carries away internal pressure from the cloud, rather than just shifting it around internally to the cloud. In other words, for each in a pair of collided grains, subsequent collisions will be at lower energies, and so have smaller recoils.
Looking at this through the lens of General Relativity (it being a gravitational problem, after all), we want to consider a covariant angular momentum quantity. In suitable coordinates (~ Cartesian, with the origin at the centre of momentum of the cloud), this angular momentum is best associated with the off-diagonal spatial shear components of the stress-energy tensor. That's the upper blue triangle in this diagram:
In the diagram the 0-3 are the four Lorentzian spacetime dimensions, and 0 is the timelike one. Roughly, for T^{mn}, m is the (signed) "goesinto" direction and n is the (signed) "goesoutof" one, or if you prefer, the flux of m-momentum in the n-direction.
Essentially the goal in the early collapse of a cloud is to shuffle the nonzeros out of the pressure diagonal (in green) into the energy (T^00 in red, which dominates a tensor contraction to a Newton-like mass) and the components above the pressure diagonal, which one can think of as heat, angular momentum, and radiation.
Matter-matter interactions enable this shuffling much faster than for matter that only interacts gravitationally. A cloud of cold collisionless, non-radiating, non-interacting (except via gravity) dust with few internal degrees of freedom has trouble collapsing gravitationally. "Their gravitational interaction will cause them to move towards each other" is true, but if they don't interact non-gravitationally they'll mostly just slide right past each other. We see this in galaxy-galaxy collisions (like in the famous Bullet cluster) where stars are spaced far enough apart that they're essentially as non-colliding as dark matter; it's the less-compacted interstellar gas and dust clouds which smack into each other and throw off lots of X-ray radiation, which helps the gas swirl around near the site of the collision and collapse into star forming regions.
> [many] disorderly configurations ... with at least some angular momentum
Sure, but the thrust of the minutephysics video (if not exactly answering the question your comment's parent asked) is evolving from a system with relatively little angular momentum to a system with a lot, and contracting from a blob to an arrangement with a clear axis and eventually towards a thin disc. I would think that initial conditions of condensed objects moving on roughly circular Keplerian orbits roughly constraned to a plane is less typical than initial conditions of a cloud with random internal motion. From a Boltzmann perspective, there are a lot more microstates which can describe the cloud-blob macrostate than the star system macrostate. But the "past hypothesis" problem is that if the latter evolves from the former, then surely the primordial gas-cloud must be of lower entropy than the nice orderly-looking star system, especially if one finds cats, cars, computers, and teapots in it?
So the question from my second paragraph becomes: does a cloud with random internal motions and no bulk rotation have more entropy than a cloud that has a clear rotational axis? As short-duration snapshots, the answer appears to be yes, and for reasons very similar to the ones you gave in your comment. But because of the evolution of the former to the latter, which is evidently common (we see stars and galaxies everywhere in our sky, and have good models for star formation and ok ones for galaxy formation), the answer appears to be no.
https://en.wikipedia.org/wiki/Past_hypothesis (2nd last paragraph) is a terse starting point if you want more. There are bound to be youtube videos about it too, hopefully at least as good as the minutephysics video at the top.
PS: for experts who want to think about angular momentum differently, e.g. the J parameter in a Kerr black hole or some other vacuum spacetime, are directed to §5.11 of Misner Thorne & Wheeler vs e.g. the problems at the end of Wald's chapter on asymptotic flatness.
Wait, debate ? Or am I failing to understand what you mean ?
\Lambda-CDM at the linear level requires the decay of vector-mode perturbations associated with vorticity; at the very least, the small scalar perturbations must totally dominate. This motivated a couple decades of searches for vector (and tensor) perturbations encoded in the CMB. Post-WMAP/Planck polarization data, aren't vector modes dead as a doornail in the linear theory ? (e.g. <https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...> | <https://link.aps.org/accepted/10.1103/PhysRevLett.117.131302> aka astro-ph/1605.07178 which kills off early-time vorticity imho, conflicting with your final paragraph.)
I also know of some work on late-time vorticity, proposing studies of CMB lensing curl, kinetic SZ and moving-lens tomography, galaxy rotation planes and so forth, but isn't practically all of this work explicitly generating null-tests of \Lambda-CDM, rather than a cosmology where "the whole universe is rotating", Bianchi or otherwise ?
Finally, I also know of some work trying to use large N-body simulations to look at scales where predictions from the linear theory become unreliable (~ Mpc). However, I don't think this is what you mean.
If there is a published review of "a debate over if the whole universe is rotating" that you could direct me to, I would be grateful. I got nowhere with "rotational anisotropy" as a search term, and seem to lack the capacity to imagine possible synonyms other than as above. Don't spare me, I'm prepared to admit and embrace the consequences of my ignorance.
I'm sorry to say that none of these is close to the review I asked for, and afaics your first result explicitly quotes from and your fourth is preceded by an image taken from the PRL letter (Sadeeh et al) I supplied in my comment's second paragraph. The former is a summary of Sadeeh et al's results: the constraints on vorticity are unforgivingly strong and vanishingly small. The latter does not even discuss vorticity or rotation or spin outside the caption of said image. Search-fu failure. That the bigthink article is simply pop-sci (Ethan Siegel is very well known) and not a literature review is something I think you should have noticed.
As to your last link, the conjecture about a preferred handedness of spiral galaxies went nowhere (you found a link from more than a decade ago; the preprint (and a visit to google scholar and citeseerx) does not match a peer-reviewed publication <https://arxiv.org/abs/0812.3437>), and additionally says nothing about galaxy clusters, ellipticals, and so forth. Even more importantly it's only for z < 0.085 which is in the region where the linear theory of cosmology is not expected by anyone to apply, c.f. my comment's penultimate paragraph.
Finally, your second result to a 2013 paper is different and interesting, and at last has evidence of engagement by other authors <https://scholar.google.co.uk/scholar?cites=10702549710128965...> however considerably more than half of those are by U V Satya Seshavathram et al talking about their very non-standard hypotheses in many fields <https://www.researchgate.net/scientific-contributions/U-V-S-...> (note the cold fusion stuff, and the at least four very different rotating cosmologies, quantum and not, dark foam and not). Also, the first (by GS ordering at the link above) citation of the 2011 paper you found, amusingly, is a self-citation (Chechin, Astronomy Reports 2016). Another Chechin self-cite, in a 2014 paper, also looks interesting (although it's only cited twice).
So it's pretty clear on your supplied evidence that there is no debate about the rotation of the universe. That's not too surprising, even BOOMERaNG data fails to support (and even somewhat undermines) early-time (pre-CMB) relevance of anything but scalar perturbations.
Thanks anyway. I'll enjoy reading these two wild Kazakh ideas I didn't see when they were fresh last decade, so your efforts weren't wasted.
(ETA: to be clear, Chechin and his coauthors are doing physics. Their math is sensible, their argument based on physical principles. They demonstrate professional familiarity with the standard cosmology. They advance a hypothesis (well a couple hypotheses, they too have multiple spinning universe models; their Generalized Jeans one is the most interesting) which is probably in conflict with observations available not long after publication date. They aren't pretending to be physicists, and they're not cranks. But the idea that there is an angular velocity proportional to the square root of the (dark) energy density is ... pretty wild! As in nobody's put it in a zoo of models.)
Yes, it is left over momentum from the big bang. It's crazy to think, but objects are still in the process of "settling down" 13.5 billion years later. They will probably continue to do it since there is no other force acting upon them.
