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Modifying Newtonian Gravitational Force

  1. Sep 13, 2008 #1

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    This Brownstein/Moffat paper (arXiv:astro-ph/0702146v3, 13 Sep 2007)) looks interesting to me but it's too deep for me to really understand. The gist of it seems to be:
    ===quote===
    The alternative to the dark matter paradigm is to modify the Newtonian 1/r2 gravitational force law so that the ordinary (visible) baryonic matter accounts for the observed gravitational effect.
    ===endquote===

    Does this paper have any merit?

    Frank
     
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  3. Sep 13, 2008 #2

    marcus

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    Frank, I don't have time to review the paper and give an honest itemized opinion but I can make some general observations.
    The paper is part of the MOND line of research started by Milgrom in the 1981. Bekenstein has contributed. Brownstein and Moffat are major contributors.
    I believe this line of research, as regards dark matter, is a noble effort but a lost cause.

    I have watched MOND research since 2004 and have a high opinion of Moffat et al, but I've lost interest lately in efforts to explain dark matter by modifying the law of gravity. I sense that a lot of people who were formerly interested in this research are now no longer. It is certainly something that should have been tried, and we can be glad that Moffat et al gave it their best shot.

    I also appreciate Moffat's diehard stubbornness. Also both Bekenstein and Moffat were technically adept and were able to carry Milgrom's idea of modifying gravity way past the original MOND idea.

    Also I think that the modified-GR work will carry over into continued efforts to explain the dark energy effect. It is always good to have people tinkering with the basic laws, probing to find out where there is some give and where you can tweak without causing utter havoc.

    These are just my personal opinions as an observer, for what they are worth. The main thing I want to get across is what killed off the interest in modified-GR as a way of explaining away dark matter.

    It was a worthy effort, but what killed interest was mainly weak lensing maps of dark matter clouds.

    An intervening cloud of dark matter distorts how we see the background of more distant galaxies by slightly elongating their images along an axis pointing towards the heart of the cloud. So even tho we cannot see the cloud we can tell where it is concentrated.

    Frank Wilczek in his illuminating book The Lightness of Being points out that we should rather call it transparent matter because it is not dark in the sense of absorbing light, as truly dark objects do. Clouds of this transparent matter have a couple of different gravitational lensing effects. This weak gravitational lensing effect allows to map out the density of the cloud---and show the contours of its concentration.

    As more and more clouds have been mapped, it has turned out there is quite some variety in the clouds, too much to explain using ordinary matter with modified-GR. There is variety in the amount of associated ordinary matter too. You can have two dwarf galaxies with the same total mass (counting both visible and dark) but with very different amounts of visible. The visible is not a fixed fraction of the total. If you would like a link to a recent article on that, let me know and i'll fetch one.
     
    Last edited: Sep 13, 2008
  4. Sep 13, 2008 #3

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    Marcus, thanks for your comments and thanks for the offer of the links, but no need, I'll dig them up myself now that I have your thoughts on this (which I value highly).

    Frank
     
  5. Sep 13, 2008 #4

    marcus

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    Frank, I'm grateful for the encouraging words. About digging up research papers about this, more power to you! I remembered one easy way I can help: we had this thread about dwarf galaxies and their dark matter clouds.
    https://www.physicsforums.com/showthread.php?t=253477
    some new observations. still good to be skeptical pending further confirmation, but something to know about. something published in Nature about this. the thread gives an arxiv preprint and an interpretive news item.
    apparently they observed variation in the proportion of dark matter to ordinary, and there are other interesting aspects as well
     
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