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Dark matter and weighing galaxies

  1. Apr 26, 2014 #1

    I have a question (well its 2 questions)... I'm not hugely knowledged on the subject so i tend to question things in a more ignorant way but i wanted to know two things:

    Firstly, how do astronomers weight a galaxy at an accurate level to be confident enough to say there is more stuff that is invisible to us (dark matter) than what we visibly see? How is it done and done accurately that we can be confident on that?

    Secondly, with this invisible matter aka dark matter that we cannot see... is it possible that due to the billions of light sources throughout the universe, light phase cancellation causes the matter to go dark....so really its ordinary matter but we cannot see it because the light is cancelled out by other light sources ?
  2. jcsd
  3. Apr 26, 2014 #2
    We measure mass by its gravitational influence, We can say with confidence that their is "dark mass" due to the gravitational influence of Baryonic matter (visible matter) and the gravitational influence of dark matter. The gravitational influence of dark matter and baryonic matter affects a galaxies rotation curve.

    Based on observations if you look at the rotation curve of a galaxy, and the distribution of visible matter the rotation of stars further from the galactic center should be slower the further you get from the center. However stars are moving near the same speed the further from the center you get, as compared the stars near the center. This told scientists that there is missing mass distributed around the galaxy where there are no stars. Also gravitational lensing where there is no visible influence is another indicator of DM

    Dark matter is a weakly interactive particle with similar properties as neutrinos. It doesn't interact with the electromagnetic or strong force. We know it interacts with gravity, may or may not interact with the weak force. However it may or may not interact with other weakly interact with other weakly interactive particles such as neutrinos. One candidate is that dark matter is a sterile neutrino. A neutrino is so weakly interactive that it can travel through a 1000 light years of lead without and interaction. here is a recent thread with a recent paper on it


    here is another thread with a similar paper.


    this article I recommend reading as it explains in a FAQ style what we have learned from observational cosmology, its coverage of dark matter and dark energy is decent and in a layman format.

  4. Apr 29, 2014 #3
    Regarding the 2nd part of the question - "could it be ordinary matter in which cancellations render it 'dark' to us"

    Don't forget that it appears that Dark Matter and Dark Energy comprise roughly 96% of our Universe, with Dark Matter at 70%. If that amazing amount were ordinary matter, do you suppose 4% could block/cancel 70%. See? not likely.

    Dark Matter may be hard to swallow as it is so humbling but the only solid competitor is MOND and it has many problems and will continue to until we understand gravity better since it depends on a non-linear, non-constant gravity contrary to Newton and everything we can measure close at hand.
  5. May 19, 2014 #4
    The universe may not be smooth. The distribution of matter could be a result of topology, as opposed to most matter being imperceptible.
  6. May 19, 2014 #5


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    This is the second post in which you have suggested that. Can you cite any work on any topology theories that would explain dark matter halos, the Bullet Cluster, and so forth or is this just some personal theory of your own?
  7. May 19, 2014 #6
  8. May 20, 2014 #7
    While there certainly is room for some speculation, not only is that frowned upon in these forums without some evidence, and even though it is extremely difficult to wrap ones head around sizes smaller than Planck Length, recently chance plus ESA's Integral Gamma Ray Observatory observations deny "graininess", apparently accurate to one-trillionth the size of Planck Length.

    Pretty exciting stuff at
  9. May 20, 2014 #8
    None of the papers I linked to concern graininess.

    Dark matter is an etiology of our observations. It's one of many. We have great data to evidence, describe and predict phenomena. However, I'm aware of no evidence that supports any singular etiology. This is analogous to Newton's gravity. He had great data, but lacked etiology.
  10. May 20, 2014 #9
    actually the last paper does,

    "Basically we are considering a symplectic geometry as a commutative limit of noncommutative geometry which is regarded as a microscopic structure of spacetime, like as the classical mechanics in a mundane scale is simply a coarse graining of quantum mechanics at atomic world. Riemannian geometry thus appears at a macroscopic world as a coarse graining of the non commutative geometry"

    you must have missed that detail.


    page 9 section 1.4

    I'll bet you also didn't know that MOND uses Dipolar dark matter DDM.

    page 12 section 4.5
    Last edited: May 20, 2014
  11. May 20, 2014 #10
    It seemed to me that if cancellations can't be responsible for "ordinary matter appearing dark" what is left? Then you brought up "smoothness" and it seemed to me this can only be some sort of blocking since afaik the overwhelming evidence points to a remarkably flat topology, unless I am missing your drift altogether.... or are you referring to this purported discovery of immense structures of Quasars that apparently has a few running around worrying that "the sky is falling"? At the very least it seems a little early for that.
  12. May 20, 2014 #11
    For me it just means we can't rule out any possibility, lovely nature about science. There is strong evidence to support dark matter and strong evidence to show the universe is extremely smooth. Doesn't necessarily mean either statement is proven. Just that research and observations currently favors DM and smoothness. As well as dark energy. The LCDM model is extremely strong, but then again the same could be said about LQC.

    I try to follow the rule of thumb study everything, you always learn something. I've even studied some utterly ridiculous models.
  13. May 20, 2014 #12
  14. May 20, 2014 #13
    I'm not concerned with the new observations. The idea of a homogeneous, isotropic universe is too convenient. It's inconsistent with basic ontology and abstract math, which frequently fails to remain abstract.

    I think topology makes more sense than DM for simple reasons. We've observed topological deformations. We understand some fluid dynamics. If we apply it to space-time, wave saturation can account for the distribution of matter and expansion without the need for DM or DE.

    Last edited: May 20, 2014
  15. May 20, 2014 #14
  16. May 21, 2014 #15
    I'm somewhat skeptic but nonetheless an interesting interpretation that might work. It is mathematically distributed/integrated as part of a whole dynamic system(math) instead of objectively separating all constituent to account for the missing 'thing' -as a mathematical model( fun for mathematicians ^^)not as much as observational/experimental solution. However, DM is more in line with observation as more of a solution and offer success in giving observation data a positive relation and sense (for large structure). Besides, We already have non/-nuetrino's DM as a go-go starting line..
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