Galaxy orbital velocities explained without 'dark matter' halos.

Discussion in 'Pseudoscience' started by nebel, Dec 19, 2014.

  1. Hayden Registered Senior Member

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    It is not in dispute, observed fact, that the outer stars are moving at a faster speed than the Keplerian maths suggest.

    In an orbital motion, the orbital speed is determined by the central pull, so any higher speed observation would call for either new physics or increased mass. MOND explains but changes Newtonian Physics a bit, other option which is widely accepted is dark matter. The distribution profile of dark matter still remains a subject matter of further research.
     
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  3. nebel Valued Senior Member

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    Keplerian math is based on a huge central mass, a star in a planetary system mostly void of matter, and galaxies of various configurations are far from that easy arrangement.
    In those cases of more even mass distribution, the shell theorems are more applicable and they show that here is more gravity on the outside than the inside every time. more gravity on the outside would show up in larger orbital velocity. For these small curvature outer orbits, acceleration by deviating from the tangent requires more distance travelled.
    It is just possible that dark matter might turn out to be a red herring.
     
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  5. nebel Valued Senior Member

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    Talking about Black Holes that have no inside:
    "--The gravity is in an enfolding surface tension--"


    Tracing the singularity from the beginning, in an " even " molecular cloud, all particles there would be in a balanced gravitational condition, but a lesser density with distance would start a squeezing by gravity from the outside, the area of lesser density. This warping of spacetime would propagate at the speed of light, and matter would be pushed by that pressure into the center, where gravity would always be zero, be balanced.
    Would the arrival of a singularity condition change that ? . The compression of matter leaves behind the gravitational field, long before the arrival of the EH or the singularity condition,
    gravity is a surface tension.
    There is much more gravity on the outside than any inside. orbital velocities show that, dark matter or not.
     
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  7. nebel Valued Senior Member

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    If you underestimate the required velocities, commensurate with an overestimated gravity, because of ignoring the near zero effective gravity in the interior, you are bound to over - estimate the required escape velocities on the outside, tempting you to add dark matter to explain what kept the "outriders" from spiralling away.
    Is not mass calculated by comparing inside and outside velocities?
     
  8. RainbowSingularity Valued Senior Member

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    how do gravity fields that over lap entire planets get zeroed out ?
    is this what some phrase as "collapsing the wave function" ?
     
  9. RainbowSingularity Valued Senior Member

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    like fungi ?
    they digest their food on the outside
     
  10. nebel Valued Senior Member

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    That is my question too. Not about "lapping over planets?" but
    It has been stated by others, that at the center of a body, where there is no effective gravity, because the surrounding mass balances out, pulling equally in all directions,-- at the centre, there really is double gravity, just that the balancing hides that fact, makes it un detectible, except through dime delation, if you could get at it. ( not my understanding) so:
    There is either no gravity, or double gravity that cancels out, at the centre. anyway,
    An equal density, perfect disk of loose matter, like a dream galaxy, would rotate like a solid disk, because gravity would exist in conformity with the shell theorem, zero at centre, no movement there, maximum velocity at the perimeter.
     
    Last edited: Sep 16, 2018
  11. nebel Valued Senior Member

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    1,451
    The Observer
    Astronomy
    Robin McKie from the article in The Guardian science:

    Sat 20 Oct 2018 21.37 BST:
    "--Vast haloes of material that cannot be detected by traditional methods must be surrounding galaxies and are holding them together, say astronomers. These are believed to be made up --"

    The reporter might have misunderstood the astronomers, but if the halos are surrounding the galaxies, outside the perimeter, ( talking about ellipticals too) and are not inside, they will not contribute to the velocities
    The shell theorem bars all external matter to have an influence on the enclosed portion.
    Orbital velocities of bodies can not be boosted by matter outside their orbits, Dark matter, if any must be on the inside, not surrounding, on the outside. or?
     
  12. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    10,578
    As usual you read something, misunderstand it and go off on some absurd tangent.

    When the astronomers say a halo they mean a sphere (roughly) of dark matter that is larger than the visible galaxy. They do not mean some sort of shell of dark matter outside the galaxy.
     
  13. nebel Valued Senior Member

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    1,451
    If the halo is larger than the inclosed galaxy, spiral, bared or elliptical , and it is a sphere, it could be called a shell, if the dark matter predominated where there is no baryonic, shining matter inside that body.
    Is there dark matter everywhere, even inside galaxies, stars, black holes? in addition to the visible matter we are aware to be made of ?
    The point to consider was:
    The dark matter, beyond the visible perimeter of a galaxy or other body would act like a shell in the shell theorem. It has absolutely no gravitational influence on velocities inside that shell, that represents the galaxy itself. Besides:
    perhaps the higher velocity measured are of transient features, coming in or leaving, just visiting that region. or? origin?
     
    Last edited: Oct 24, 2018
  14. James R Just this guy, you know? Staff Member

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    No, it doesn't. That result applies specifically to spherical shells.
     
  15. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    10,578
    Not true. There is no shell.
     
  16. nebel Valued Senior Member

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    The offshoot of the shell theorem is that
    In a stack of shells, onion-stile, the outer shell has no gravitational influence on the inner, as a general principle. The shells do not have to be perfect in shape, or even be concrete entities. (Faraday and Gauss)
    To what degree dark matter halos are symmetrical, spherical, or wether dark matter permeates the inside of galaxies too, I do not know. does anybody?
    The Guardian reporter wrote that the external halo would influence the inside velocities, that would hardly happen in ellipticals or globular clusters.
     
  17. James R Just this guy, you know? Staff Member

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    Yes.

    I have no idea what you mean by "perfect in shape" here. The shell theorem applies to spherical shells, as I said. Waving your hands vaguely at Faraday and Gauss won't change that.

    Yes. Astrophysicists know. I'm also fairly confident that a google search would quickly turn up the relevant information. The halos can't be symmetrical spherical shells, that's for sure.
     
  18. nebel Valued Senior Member

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    The shell theorem is a mathematical model, that for simplicity's sake assumes a perfect "same radius" situation, In real life, small variation from that model will not change the fact that the inside of a shell has no gravity present, and the outside surface gravity, (projected out) of a shell is the only one that counts, and
    an entity, perhaps even a dark matter halo can be considered a series of shells. but
    I realize, a spiral galaxies case is different from an elliptical one. . thank you.
     
  19. Janus58 Valued Senior Member

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    1,930
    Yes, DM permeates inside galaxies. For the Milky way, a common estimate for DM density in the Milky way is p(r) = p(0) (a^2+r0^2)/(a^2+r^2)

    p(r) is the density at radius r from the center
    p(0) is the density of dark matter in the region of the Sun (0.3 Gev/cm^3)
    a is the core radius of the DM halo
    r0 is the distance of the Sun from the galactic center

    This assumes a spherical distribution for the DM (if it turned out that the Milky way DM halo was elliptical, this estimate would have to be adjusted accordingly. The point is that the DM model has never assumed that DM was confined to regions exterior to the galaxies. This view of DM only existing outside of galaxies comes solely from a sloppy layman interpretation of the word "halo".
     
  20. nebel Valued Senior Member

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    If dark matter co - existed with baryonic stuff since the beginning, and perhaps clustered before, or with it, of course it would contribute to the high peripheral velocities.
    Perhaps assuming that dark matter, strange as it seems, might also not generate gravity the way the visible matter does, could enhance that.
    The topic was started to draw attention to how even ordinary matter acts so much more at the rim and just outside, as is popularly assumed.
     
  21. nebel Valued Senior Member

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    1,451
    Any entity can be considered for analysis to be composed of discreet shells, layers.
    You posted this interesting chart, thank you, showing extraordinary high velocities outside the galaxies. Each one of these green point positions would own their velocity from mass nearer to the centre, an inside "shell" not outside.
    Barring alternate theories, enough of them readable, it would have to be DM "halo"matter, reaching beyond the realm of shiny matter, creating them, if the observed objects, gas, are permanent not transient.** (It might take a million years for one orbit).green dots might represent matter with higher than escape velocity. for example. thank you.
    ** question, is the dark matter revolving like the green dots too DM=LGM?

    Please Register or Log in to view the hidden image!

     
  22. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    More nonsense.
     
  23. James R Just this guy, you know? Staff Member

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    Thanks Janus58.

    So, my previous post should be corrected to: wherever the dark matter halos are, they aren't just spherical shells surround the visible matter in a galaxy.
     

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