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Dark Matter Evidence

  1. Nov 21, 2006 #1

    Chronos

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    Here is the latest in the 'bullet cluster' series on evidence for the existence of dark matter:

    http://arxiv.org/abs/astro-ph/0611496
    Catching a bullet: direct evidence for the existence of dark matter
    Authors: D. Clowe (Ohio University), S. W. Randall, M. Markevitch (CFA)
    Comments: 4 pages, 1 figure, to appear in the proceedings of the 2006 UCLA Dark Matter Symposium. Accompanying the paper is a partial data release at this http URL

    We present X-ray and weak lensing observations of the merging cluster system 1E0657-556. Due to the recently collision of a merging subcluster with the main cluster, the X-ray plasma has been displaced from the cluster galaxies in both components. The weak lensing data shows that the lensing surface potential is in spatial agreement with the galaxies (~10% of the observed baryons) and not with the X-ray plasma (~90% of the observed baryons). We argue that this shows that regardless of the form of the gravitational force law at these large distances and low accelerations, these observations require that the majority of the mass of the system be some form of unseen matter.
     
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  3. Jan 9, 2007 #2

    SF

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    http://physicsweb.org/articles/news/11/1/3/1?rss=2.0
    Read the article and answer my question.

    They say:
    This was done by observing how light from distant galaxies is bent by the gravitational pull of dark matter in a process called gravitational lensing.

    Ok, great, but doesn't this mean that the only places where we can actually observe the lensing done by dark matter _is_ the area around visible galaxies, and we shouldn't be surprised that that's exactly where it was calculated to be?
    Sounds a bit like circular reasoning.
     
  4. Jan 10, 2007 #3
    hi SF,

    The idea of gravitational lensing is that the lens is in the foreground and the lensed objects are in the background. No matter where on the sky you look there should be enough background galaxies to make a measurement of the amount of matter in the foreground.

    The obstacle comes in resolving the distant background objects because the stretching of the images of these galaxies is what is used to measure the foreground matter distribution.
     
  5. Jan 12, 2007 #4
    The article by D. Clowe, S. W. Randall and M. Markevitch at
    http://arxiv.org/PS_cache/astro-ph/pdf/0611/0611496.pdf shows that the
    “bullet” galaxy cluster had three mass components.

    The first is the X-ray plasma component ( 90% of the visible matter) that was
    stripped out of the bullet galaxy cluster as it interacted (electromagnetically?) with
    the plasma in the “target” galaxy cluster. This component is displaced from the
    present position of the bullet, which is marked by:

    The second component (10% of the visible matter), is electrically neutral
    matter --- dust grains, molecular vapours and stellar stuff — as ordinarily
    observed in galaxies. In the same location is:

    A third invisible component is, ”dark matter”, “some type of matter (that) does
    not emit, absorb, or deflect light in any observed bandpass”. This component,
    unlike the plasma, is not displaced from the stellar stuff that marks the
    bullet’s present position. It is revealed by weak lensing, which if based on
    “ordinary” general relativity, shows that it is the most massive of the three
    components.

    The first two components are called “baryonic matter”. What observational
    evidence is there for restricting this classification to the first two
    components? Or is this done simply because the calculations of nucleosynthesis and elemental abundances cannot explain the now observed preponderance of dark matter?
     
    Last edited: Jan 12, 2007
  6. Jan 12, 2007 #5
    Could the gravitational field self gravitate? Can't the existence of the gravitational field be considered an energy source in itself which would cause a gravitational field too? Then it would be understandable that it would have its own momentum so that when the galaxy it's attached to suddenly stops the field would keep moving for a bit before recentering itself on the galaxy. Wouldn't this be just another form of gravitational wave where sudden changes in the momentum of the mass can cause its gravitational field to propagate out even further?
     
  7. Jan 13, 2007 #6

    Chronos

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    Self gravitating fields of such a nature would spiral into a feedback loop.
     
  8. Jan 13, 2007 #7
    Wouldn't the curvature due to the two galaxies cancel somewhere in between them?

    Also, doesn't the cuvature correspond to the density gradient rather than to the barycenter of mass? If so, why would one expect much lensing in the middle anyway?
     
  9. Jan 13, 2007 #8
    I have asked this question before in other threads with no answer. Is it possible (in theory) that the dark matter and dark energy result from a quantum interaction between "asymmetrical mass units" of matter + antimatter via an interaction involving gravity + antigravity ? Is this hypothesis in any way supported (or not supported) by the observation of "lensing" ?
     
  10. Jan 13, 2007 #9
    Another comment on this topic. Should it be called "dark mass" and not "dark matter" since Einstein with E = Mc^2 clearly was dealing with "mass" as M and not "matter" ? Perhaps only semantics, perhaps not if [matter ≠ mass].
     
  11. Jan 17, 2007 #10

    SF

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  12. Jan 18, 2007 #11

    Kea

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    This news is bad for DE, but on its own not enough to kill it. Of course, there is other evidence mounting ....
     
  13. Jan 18, 2007 #12

    Garth

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    You can't keep a good epicycle down..... :wink:

    Garth
     
  14. Jan 18, 2007 #13

    Kea

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    Indeed! Next we'll be hearing some argument that DE, despite being remarkably uniform over the entire history of the cosmos, somehow changes at small scales....hee, hee, this is all so funny.
     
  15. Jan 19, 2007 #14
    I find it hard ot believe that an accurate length scale for the detection of dark energy has been calculated. The evidence that I've seen for dark energy (supernovae type 1a and the integrated Sachs Wolfe Effect) seem to show that the universes expansion is accelerating BUT to make a measurement of how fast the expansion is accelerating and connect that to a laboratory length scale seems like overextending to me. I dont know who derived the length scale of 85 micrometers, but id like to see how it was done. No doubt the experiment was done well, but all i think it can do is put some weak constraints on dark energy.
     
  16. Feb 14, 2007 #15

    SF

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