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Distinguishing MOND from dark matter

  1. Nov 27, 2005 #1

    marcus

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    there must be a lot of ways already proposed to do this.

    can anyone list those considered most promising?
    has this been the subject of a thread already


    today I saw this paper which claims to suggest a way to
    observationally distinguish MOND from DM-----to tell which is right.
    I think the proposal depends on measurements within the solar system.

    Is this paper of interest? (I can't judge.) I think author is a grad student, maybe at Beijing Normal. It is being published.

    http://arxiv.org/abs/astro-ph/0511713
    Roche Lobe Sizes in Deep-MOND Gravity
    HongSheng Zhao
    4 pages, 2 figures, Astronomy and Astrophysics Letter, in press

    MOdified Newtonian Dynamics (MOND) is evolving from an empirical to a decent theory respecting fundamental physics after Bekenstein (2004) showed that lensing and Hubble expansion can be modeled rigourously in a Modified Relativity. The degeneracy of MOND with Dark Matter can be broken if we examine the non-linear MONDian Poisson's equation in detail. Here we study the effect of tides for a binary stellar system or a baryonic satellite-host galaxy system. We show that the Roche lobe is more squashed than the Newtonian case due to the anisotropic dilation effect in deep-MOND. We prove analytically that the Roche lobe volume scales linearly with the ``true" baryonic mass ratio in both Newtonian and deep-MOND regimes, insensitive to the modification to the inertia mass. Hence accurate Roche radii of satellites can break the degeneracy of MOND and dark matter theory. Globular clusters and dwarf galaxies of comparable luminosities and distances show a factor of ten scatter in limiting radii; this is difficult to explain in any ``mass-tracing-light" universe. The results here are generalizable to the intermediate MOND regime for a wide class of gravity modification function $\mu(g)$ (Zhao and Tian, astro-ph/0511754).
     
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  3. Nov 27, 2005 #2

    Chronos

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    I see nothing in this paper that is compelling in either direction.
     
  4. Nov 28, 2005 #3

    marcus

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    that's good isn't it? because he is not offering evidence in favor of one hypothesis or the other

    he is proposing a method to distinguish between the two by future observations, by future measurements of the
    Roche lobe of, say, a globular cluster that is orbiting a galaxy.

    We do not expect any bias one way or the other. What I am asking is DOES ANYBODY THINK THIS IS A GOOD WAY TO DISTINGUISH WHICH MODEL IS RIGHT? What do people think of this proposed method?

    So I go back to my original question, and I raise the ante by offering another paper by Zhao of the same type.

    Here is the new paper (remember Zhao is a grad student, going on postdoc but he may be a specially smart grad student)

    http://arxiv.org/abs/astro-ph/0511754
    Roche Lobe Shapes in MOND-like Modified Gravity
    HongSheng Zhao, LanLan Tian
    11p, 7 figs, submitted to Astronomy and Astrophysics

    "We consider how to break the near degeneracy between dark matter and baryonic MOdified Newtonian Dynamics (MOND). We show that the Roche Lobes of a two-body baryonic system (e.g., a globular cluster orbiting a host galaxy) are sensitive to modifications of the law of gravity. We generalise the analytical results obtained in the deep-MOND limit by Zhao (2005, http://arxiv.org/astro-ph/0511713), and consider in the framework of a general non-Newtonian law g proportional to R-zeta. We give analytical expressions for the inner Lagrange point and Roche lobe axis ratios. The Roche lobe volume is proven to scale linearly with the baryonic mass ratio which applies to any modification function mu(g) or power-law function zeta. The lobes are squashed, with the aspect ratio varying with mu(g) due to the anisotropic dilation effect in MOND-like gravity. We generalise these results for extended mass distribution. Precise measurement of Roche lobe shapes for different satellite-host separations could constrain the law of gravity."

    You can see in his abstract he cites the earlier paper that I mentioned in my post:

    Any opinions? Is this a good way to test between MOND vs DM?

    If MOND is wrong is this test good enough to shoot down MOND? Is this test actually technically performable? Are there better tests that have been proposed and studied already, which people plan to do?

    It would be interesting to get a SpaceTiger opinion on this.
     
    Last edited: Nov 28, 2005
  5. Nov 29, 2005 #4

    Chronos

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    You are absolutely right, marcus. I reflexively twitch every time I see the term MOND. Ever watch 'Planet of the Apes'? Every time I see MOND I think "... but, you're so damned ugly." Zhou appears to leave a way out for both sides no matter what outcome is achieved. I'm not saying that is a bad thing, I just question whether the test is very meaningful. I too would like to hear ST ring in on this.
     
    Last edited: Nov 29, 2005
  6. Nov 29, 2005 #5
    I don't really know if this helps, but I recently read an article about some problems that arise from the trajectory of some of the deep space satellites. Apparently their trajectories have to be changed because they don't follow the projected paths described by Newtonian mechanics and general relativity. They don't know if it is a problem with the satellite, if MOND exists, or if dark matter is pulling/pushing on it causing it's change in trajectory.
     
  7. Nov 30, 2005 #6

    Garth

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    Reference?

    Garth
     
  8. Nov 30, 2005 #7

    Labguy

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    Garth;
    This won't help much either, but I remember reading the same article about 6-8 months ago. I think that it was making the case for dark matter as opposed to MOND. But, I didn't keep it bookmarked so I don't know where it is either. Now it's going to bug me all day until I can find it...:mad:
     
  9. Nov 30, 2005 #8
    it is the pioneer anomaly and every crackpot and his dog thinks his alternate theory can solve the mystery!
     
  10. Nov 30, 2005 #9

    Garth

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    Including Self Creation Cosmology - but my dog isn't so sure! :rolleyes:

    Garth
     
  11. Nov 30, 2005 #10

    turbo

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    If someone does come up with a model that resolves the GR gravitation model with current observations without requiring DE, DM, Higgs and gravitational fields and their theoretical bosons, he or she will certainly be someone who the Standard Model "faithful" (at least those who are name-callers) have called a crackpot. That type of demeaning denunciation is not productive, and it points out the crying need for epistemology.


    "How does it happen that a properly endowed natural scientist comes to concern himself with epistemology? Is there no more valuable work in his specialty? I hear many of my colleagues saying, and I sense it from many more, that they feel this way. I cannot share this sentiment. ...Concepts that have proven useful in ordering things easily achieve such an authority over us that we forget their earthly origins and accept them as unalterable givens. Thus they come to be stamped as 'necessities of thought,' 'a priori givens,' etc. The path of scientific advance is often made impassable for a long time through such errors. For that reason, it is by no means an idle game if we become practiced in analyzing the long common place concepts and exhibiting those circumstances upon which their justification and usefulness depend, how they have grown up, individually, out of the givens of experience. By this means, their all-too-great authority will be broken."

    Einstein
     
  12. Nov 30, 2005 #11
    believe me, skepticism is alive and well in modern cosmology. I attended a talk last week by Peebles. The talk was about questioning the standard model.
     
  13. Nov 30, 2005 #12

    SpaceTiger

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    Sorry for the delay, I've been swamped with work. :yuck:

    It's interesting and I don't immediately see any problems with it. I doubt anyone that's not already convinced will be put over the edge by observations of globular cluster Roche lobes, but it's always nice to have different kinds of tests.

    What I find to be the most convincing evidence against this new relativistic variant of MOND is its failure to fit the third peak in the CMB:

    Did BOOMERANG hit MOND?
     
  14. Dec 1, 2005 #13

    EL

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    I have heard it is probably because of some engineering problem, some leakage which gives it an extra push in some direction.
     
  15. Dec 1, 2005 #14

    turbo

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    Leakage on both probes, with the leaks oriented in the right direction to provide an apparent Sunward acceleration, and with leaks of exactly the same magnitude so that the Sunward accelerations are the same, and the accelerations are smooth and constant? "Highly unlikely" is an understatement in this case.
     
  16. Dec 1, 2005 #15

    Garth

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    Others would agree with you.
    There have been several attempts to explain the Pioneer anomaly by conventional means, gas leakage, non-isotropic I-R radiation pressure, and a spin induced doppler effect. Such attempts have failed Can conventional forces really explain the anomalous acceleration of Pioneer 10/11?.
    The simplest explanation, for my money, recognises the near equality of the observed anomalous acceleration and the Hubble acceleration cH. This may also explain why it is not observed in the Keplerian dynamics of planetary orbits, the effect is an artifact of the cosmological solution to the gravitational field equations and not the spherically symmetric solution. It is cosmological in nature.

    As I have said elsewhere, one such cosmological effect that explains the anomaly is a cosmological clock drift between atomic and ephemeris clocks; although as Ostermann states in Relativity Theory and a Real Pioneer Effect this explanation would contradict GR. On the other hand it is predicted by Self Creation Cosmology - An Alternative Gravitational Theory .

    Garth
     
    Last edited: Dec 2, 2005
  17. Dec 6, 2005 #16
    my favourite is this one;

    Weak lensing mass reconstruction of the interacting cluster 1E0657-558: Direct evidence for the existence of dark matter
    Authors: Douglas Clowe (1), Anthony Gonzalez (2), Maxim Markevitch (3) ((1) Universitaet Bonn, (2) University of Florida, (3) CfA)
    Comments: 8 pages, 4 figure, accepted by ApJ
    Journal-ref: Astrophys.J. 604 (2004) 596-603
    We present a weak lensing mass reconstruction of the interacting cluster 1E0657-558 in which we detect both the main cluster and a sub-cluster. The sub-cluster is identified as a smaller cluster which has just undergone initial in-fall and pass-through of the primary cluster, and has been previously identified in both optical surveys and X-ray studies. The X-ray gas has been separated from the galaxies by ram-pressure stripping during the pass-through. The detected mass peak is located between the X-ray peak and galaxy concentration, although the position is consistent with the galaxy centroid within the errors of the mass reconstruction. We find that the mass peak for the main cluster is in good spatial agreement with the cluster galaxies and offset from the X-ray halo at 3.4 sigma significance, and determine that the mass-to-light ratios of the two components are consistent with those of relaxed clusters. The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster. This proof of the dark matter existence holds true even under the assumption of modified Newtonian gravity (MOND); from the observed gravitational shear to optical light ratios and mass peak - X-ray gas offsets, the dark matter component in a MOND regime has a total mass which is at least equal to the baryonic mass of the system.
    http://arxiv.org/abs/astro-ph/0312273
     
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