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How did gravity build astronomical objects that rotate?

  1. Sep 30, 2012 #1
    How did gravity build astronomical objects that rotate?

    It seems to me a safe bet, supported in part by observation, that the heterogeneous assembly of known astronomical objects comprising the observed universe, whose contents range from minor planets (one named for my mother) to the recently imaged remote galaxies observed by the Hubble telescope, and are all busily revolving or rotating. I’d like to understand, in broad terms, how this conglomerate, believed to have started out as a hot, dense, nearly homogeneous fluid some 13.7 billion years ago, became so heterogeneous and accumulated so much locally stored angular momentum.

    The short answer that “Gravitational condensation did it” doesn’t quite satisfy me, although it is of course correct.

    In struggling to gain a better understanding for myself of the fundamentals of structure formation, without relying on computer modelling done by others, I’ve had the thought
    that fluid shear must have been an important aspect of the process. The elastic or plastic shear of solids is reasonably well understood, but involves descriptions of shear appropriate only for uniform deformation, like pure shear (described by a symmetric second-rank tensor) and engineering or simple shear (described as the sum of a pure shear and a rotation). In an astronomical context the dynamic behaviour of fluids within larger host structures involves more complicated shearing deformations and motions, as in gravitating gases and gravitating particulate fluids; like the rings of Saturn, interstellar dust clouds (Orion Nebula) or, on a larger scale, the star clouds in the central Milky Way. I suppose that even galaxies in
    clusters (e.g. Fornax) are likely to be collectively sheared by gravity.

    The conclusion I’ve come to is that: “Gravitational condensation was aided in creating a heterogeneous universe of rotating and revolving structures, which on different scales store angular momentum, by its central-force character that generates Keplerian shear, which can causes matter to rotate and revolve.” Is this oversimplified guess anywhere near correct?

    And, finally, since tonight (30 September) is Full Moon, let me ask: does the Moon always
    present the same appearance to us because Keplerian shear stresses the Earth produces, acting on the Moon, partly compensate for the centripetal stresses generated by the Moon’s 28-day periods of axial rotation and Earth-orbit revolving?
    Last edited: Sep 30, 2012
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  3. Sep 30, 2012 #2


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    Re: How did gravity build astronomical objects that rotate?

    i think the key word is nearly homogeneous. because gravity attracts matter and more mass means more gravity, then gravity has a tendency to amplify any non-homogeneties. dense areas become denser. that's what gravity does. it's why we have planets and comets and asteroids in the solar system instead of a big cloud of material.

    also i believe the current thinking is that there were pockets of turbulence in the primordial universe. big swirls of turbulence eventually became galaxies, smaller swirls of turbulence became solar systems, and even smaller swirls became planets. all with a little spin.
  4. Sep 30, 2012 #3
    Re: How did gravity build astronomical objects that rotate?

    I think part of the reason is because when the "stuff" back then was falling together, it probably always hit at an angle. So that it would bounce off. If it's all random, it probably wouldn't be always hitting each other head-on.

    So then also if the stuff is attracted to each other, then it would hit at an angle, and then come back after a while in a big sort of figure eight, I guess?

    It's like... if you have two pieces of sticky stuff, and they hit each other off-center, they're going to stick together, but the whole glob at the end is going to be rotating.

    So then imagine A LOT of sticky stuff falling into one spot. Eventually, the different rotations sort of average out into one rotation.
  5. Sep 30, 2012 #4


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    Re: How did gravity build astronomical objects that rotate?

    Collisions are an important part of structure formation. During gravitational collapse, molecules shed kinetic energy via collisions with one another, creating turbulence in the cloud. A preferential direction of spin eventually emerges through this process which is further amplified by the increasing strength of the gravity well as particles spiral in to the center of gravity. As more particles are drawn in, they basically go with the flow. Here is a computer simulation of this phenomenon - http://blogs.discovermagazine.com/8...imulation-of-spiral-galaxy-formation-to-date/.
  6. Sep 30, 2012 #5
    Re: How did gravity build astronomical objects that rotate?

    okay, so the preferential direction of spin arises from the fact that the distribution of both the actual particles *and* their initial velocities are not isotropic, right?
  7. Sep 30, 2012 #6


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    Re: How did gravity build astronomical objects that rotate?

    It really does not matter. A preferred direction of rotation will emerge randomly from the chaos of all the collisions - even if the initial distribution and velocities are totally random. Of course, that kind of natural randomness is not all that natural. In most cases the initial distributions are not isotropic and that, of course, favors a particular outcome for the final direction of rotation, but, not the probability of rotation itself. The odds of no rotation emerging are worse than those of a coin flip coming up 'edges'.
  8. Sep 30, 2012 #7
    Re: How did gravity build astronomical objects that rotate?

    The rotation comes from the fact that there is only one angle you can hit an object and not make it rotate: straight in the direction of the center of mass. But an infinite amount of angles that will make it rotate: anywhere else.

    Rotation results.
  9. Oct 1, 2012 #8
    Re: How did gravity build astronomical objects that rotate?

    Thanks for these replies, all. I was a bit surprised at the near-unanimity and simplicity of your views along the lines of "Collisions between infalling lumps did it".

    Perhaps I confused the issue by mentioning Saturn's rings as exemplifying Keplerian shear. They do so because the rings are particulate rather than solid, not because lumps rotate due to collisions, as discussed in your replies. (Only rbj differed, suggesting "pockets of turbulence" in the early universe as the origin of circular motions.)

    An explanatory aside, about inhomogeneous Keplerian shear, which is a consequence of Newtons law of gravity:

    Circular orbital speeds about the sun, for example, are inversely proportional to the square root of the orbit radius. Wheras in a rotating rigid body the tangential speed at any point is proportional to the point's distance from the rotation centre. Hence shear, probably labelled Keplerian more than a hundred years ago by Maxwell, who first analysed the nature of Saturn's rings, or Keeler who spectroscopically verified his analysis. Another aside: the Roche limit is also a consequence of Keplarian shear. Or tidal stresses, if one wants to put it more generally.

    If "Collisions between infalling lumps did it" , why then does the sun rotate? Or spiral galaxies, for that matter? Not much of the universe is solid and fluid dynamics is a more appropriate tool in this context.
    Last edited: Oct 1, 2012
  10. Oct 1, 2012 #9


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    Re: How did gravity build astronomical objects that rotate?

    The primordial cloud that gravitationally condensed to ultimately form the sun and planets acquired spin via molecular collisions. See the video I referenced on computer simulation of galaxy formation. It is the cliff notes version of a lengthy supercomputer numerical analysis. Rest assured the laws of fluid dynamics are strongly considered. Gas/dust clouds in interstellar/intergalactic space are diffuse and, for all practical purposes, behave like a perfect fluid during gravitational collapse.
  11. Oct 1, 2012 #10
    Re: How did gravity build astronomical objects that rotate?

    Thanks for the reply, Chronos. Sadly, I'm unable to view the video you recommended. Bandwidth! Thanks also for reassuring me about a supercomputer analysis. But I would still like to know specifically if Keplerian shear is among the factors that promote the storing of angular momentum and if not, why not.

    The colliding of molecules and bigger lumps must also be important, as other replies make clear. I don't disagree. But: does that program reveal the relative importance of such easily understood factors? Or does it churn away at, say, the Navier-Stokes equations seasoned with a pinch of Newtonian gravity , so that in the end one just concludes: "The formation of observed structures can be successfully modelled with classical physics?" Period? Or does one just have to watch and believe a movie of it happening? Perhaps there's a voice-over that highlights such simplicities as shear and collisions. Is there, and what does it say?
  12. Oct 1, 2012 #11
    Re: How did gravity build astronomical objects that rotate?

    My view is that if an energetic system can move in a certain way then it will. This is sometimes summed up as "everything that is not forbidden is required."
  13. Oct 2, 2012 #12


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    Re: How did gravity build astronomical objects that rotate?

    well, as best as i can understand this is that if there were no swirls of turbulence in the primordial universe, i can see how spin can happen if two bodies fall toward each other, but because of the gravitational influence of other bodies, these two bodies do not fall directly toward their common center of mass. and if they hit each other slightly off center, then spin will result from that collision. but if angular momentum is conserved, there has to be some other bodies or groups of bodies that spin the other way in order for the spin-less initial conditions to be preserved in the overall universe. i don't know if that is the case or not. are there any studies that indicate if the universe as a whole has spin or not?

    how soon in the early universe that swirls of ostensible turbulence appear, i also do not know, but it seems to me that it would be very early. but WMAP tells us it wasn't isotropic, and it's not hard for me to believe that the curl of velocity vectors of "stuff" was also not isotropic. and if so, i would call these "swirls of turbulence".
  14. Oct 3, 2012 #13
    Re: How did gravity build astronomical objects that rotate?

    Maybe the universe had spin, maybe it didn't, but local effects dominate so it doesn't really matter. Let's say that you have a collapsing ball of gas. That ball is never going to be perfectly symmetrical and the center of mass is never going to be in the center. There is always going to be some asymmetry, and that's where the spin comes from.

    The vortices come from Rossby waves and the Rossby instability. It is a very general property of rotating liquids and gasses. That's just what they do, form vortices.
  15. Oct 3, 2012 #14
    Re: How did gravity build astronomical objects that rotate?

    You sound very sure, Imalooser, about the origin of spin. Do you then include with spin all other circular motions --- of galaxies, stars, planets etc?

    About Rossby waves; they are fascinating example of fluid motion in the atmospheres of rotating planets; waves driven by Coriolis forces, which are in turn are caused by thermally-driven motions of fluids away from or towards the poles, in rotating planets that are heated by stars. Do they occur in other places?

    Interesting but not fundamental in the context of this thread; Rossby waves may well be a specific consequence of shear in a fluid, under special circumstances, but I don’t see how they can be the fundamental reason for the storage of angular momentum in the universe, which is what I’m trying to understand.

    In fact it seems to me that none of the kindly folk who have so far contributed to this thread fundamentally understand shear, particularly Keplerian shear; let alone its general consequences in gravitating fluids. Come on, folk, this is not even real gravity stuff like General Relativity!
    It’s simply gravitating matter ruled by Newton’s old law of gravity, and shear as in a greasy old pack of cards.
    Last edited: Oct 3, 2012
  16. Oct 3, 2012 #15
    Re: How did gravity build astronomical objects that rotate?

    Yes, as you say, the early universe was not quite homogeneous, and the observed Cosmic Microwave Background reveals it. At the 1 part in 10,000 level it's patchy hot and cold compared to its mean,very low Black Body spectrum temperature of a few degrees Kelvin. The angular spectrum of patches matches very well what is expected from primordial quantum fluctuations manifesting themselves as "sound" waves in the then-plasma universe. I don't think there is any evidence one way or the other that would justify describing this as turbulence with vortices, although common sense suggests that fluid flow in the early universe can't have been laminar at the ruling extreme temperatures. Computer models of structure formation use the observed inhomogeneities to account pretty satisfactorily for the present day structures we observe with telescopes. But these models don't illuminate for me the fundamental physics -- they just tell us that "Gravity did it" which for a lot of physicists may be satisfactory, but it's not so far from the possibly equally unhelpful "God did it" approach. That's why I started this thread. I'm curious.
  17. Oct 3, 2012 #16
    Re: How did gravity build astronomical objects that rotate?

    Yes, spiinning planets, suns, galaxies, black holes. I'm not at all an expert on this stuff though. It is a simple fact that just about everything in space rotates, and I think the basic reason is "because it can."

    Rossby waves occur in the oceans of Earth. They are believed to be crucial in stellar magnetospheres during the formation of planets. They are easy to create in experimental setups. I've read about them in superfluid helium. I'd like to know what the requirements are. I think all you need is a rotating gas or liquid and some sort of change in density.
    As far as I know they don't have anything to do with "storage of angular momentum" other than that conservation of angular momentum is fundamental. Rossby waves are unstable and spawn vortices that usually dissipate angular momentum.

    I know zero about Keplerian shear and a cursory search didn't help much. Have any references?
  18. Oct 3, 2012 #17
    Re: How did gravity build astronomical objects that rotate?

    Try Wikipedia and Google. Or search for info about Saturn's rings. and why they are particulate and not solid If you can access it, Van Nostrand's Scientific Encyclopedia will explain it. Or, if all else fails, try reading post #8 in this thread.

    I still don't think your Rossby waves are appropriate.
  19. Oct 4, 2012 #18


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    Re: How did gravity build astronomical objects that rotate?

    well, if there is turbulence in some fluid, there is some scale, where you can zoom to, in which some region of turbulence is spinning, like eddy currents. if there is spin, there is an axis of rotation. along that axis, gravity will just pull the pancake flat. but in the two dimensions that are perpendicular to that axis of spin, collapse from gravity has opposition so it does not flatten in those two dimensions. this is why these spiral galaxies are sorta flat. same for solar systems.

    as far as comparing Gravity to God, even if the fundamental interaction has no currently known source (why does matter curve spacetime?), there is at least a mathematically described interaction that can be used to predict physical outcomes. that is different from saying "God did it". it is not equally unhelpful.
  20. Oct 5, 2012 #19
    Re: How did gravity build astronomical objects that rotate?

    Not equally, perhaps. You're right, rbj, I was overstating the case. But still unhelpful; the "mathematically described interaction" so far seems to have been unhelpful for explaining the "physical outcome" of ubiquitous astronomical circular motions. I'm trying to provide an explanation, rather than attributing it all to primeval turbulence. Old Henry Ford said "History is bunk" and maybe he had a point!

    In a gravitational context a simple mathematical treatment of turbulence is doubly impossible because of the non-linearity of both the Navier Stokes equations and Einstein's field equations, but the effect of the shearing character of Newton's law on gravitational collapse doesn't seem to have been noticed.

    Is it too simple? But then it probably has (there's nothing new under the sun). If so, does anyone know where and by whom?
  21. Oct 5, 2012 #20
    Re: How did gravity build astronomical objects that rotate?

    Rotate in what way?
    If it's only down to collision interactions wouldn't you get a rotation in more than one axis?
    By applying a gravity well to an interacting cloud of molecules, surely your predetermining the results, since the gravity well is essentially planar, axis driven.
    Applying a gravity well seems like circular logic unless you use a spherically symmetric gravity well, but then you don't get the shear forces.
    The observations show that most, if not all, rotations are planar, the moons, planets, galaxies, nebulae.
    So I'm going to assume you mean 'highly planar' rotations?
    If you want to use Newtonian mechanics then I think Newton answered this already, in a round about way, but ask this question, what caused the gravitational collapse in the first place?
    If you take the three descriptions of rotations described by Newton and apply each of them to a gas/dust cloud (using the Bernoulli principle), the only real conclusion you can make is that the shear forces have to be there in the first place to cause the gravitational collapse.
    So where do shear forces come from? At the point other rotations intersect? Where did the other rotations come from?
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