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Modified Gravity questions

  1. Feb 5, 2010 #1
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    I have been studying John Moffats Modified Gravity theory. I find it very interesting and I agree with him in that I think Dark Matter and Dark Energy are merely mediums to make our current equations work so we can predict the movements of the cosmos. However I am unclear on what modified gravity means to black holes, gravitational waves, and especially confused about how modified gravity explains the beginning point in time and space and the age of the universe.

    I am hoping someone here read his book and made more sense of it than I did. Or can it be that some of these things are yet to be explained under that model?
     
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  3. Feb 8, 2010 #2

    ohwilleke

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    A summation of the state of quantum gravity theory by the proponent of a conceptually similar theory, by a disciple of one of the founders of the theory, describes his theory, how he got there, and how it is fared in light of empirical research (see the conclusion).

    http://arxiv.org/PS_cache/arxiv/pdf/1001/1001.3876v1.pdf
     
  4. Feb 8, 2010 #3
    Yes I have read about MOND, TeVes, and quantum gravity. I don't see anything in this link you posted that answers my question above either. So if anyone has any info on relativities singularities in alternative models let me know I'd love to study that.

    I think MOG makes black holes kind of like gray holes... just really dense objects without a source of its own light.
     
  5. Feb 8, 2010 #4

    marcus

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    Ohwilleke you point to Bekenstein's recent article and say "see the conclusion".

    I didn't find a conclusion section in Bekenstein's article. And I didn't find a summing-up at the end. Maybe I'm overlooking some key passage. How about quoting for us some conclusions from Bekenstein? Make it easier, spell it out so to speak.

    I have to say that if I had to choose I would probably lean towards Bekenstein rather than Moffatt. Bekenstein impresses me as having more on the ball. But I could easily be wrong.

    Interest in MOND and improved MOND-like theories has greatly declined in the past 2 or 3 years, wouldn't you say? The weak-lens mapping of clouds of DM, and the bullet cluster pictures, took a lot of the wind out of MOND sails.

    If there still is reason to pursue MOND, then offhand I would say that Bekenstein (not Moffatt) is the most credible person to explain why and to assess the field's status. Just my two cents.
     
  6. Feb 9, 2010 #5

    ohwilleke

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    From Part 5, starting at page 16 (some citations omitted, emphasis added, paragraph breaks adjusted for ease of reading):



    Moffat, if I recall correctly, has claimed that he can overcome the bullet cluster issue (no, I don't have a cite and haven't seen it since shortly after the bullet cluster implications for MOND were first discussed).

    Bekenstein has the benefit of greater established respectability in the field and a small cadre of people working with his TeVeS theory that Moffat lacks. There are a fair number of TeVeS articles in the last two or three years (and the lack of 2009 articles in linked chapter may be a function of the gap between actually writing date and arvix publication).

    Bekenstein acknowledges the failures of TeVeS while pointing out the successes. He also points out that DM can serve as a post-hoc justification for just about anything. Notably, in Bekenstein's view the bullet cluster, which can conceivably be explained through simple neutrinos acting as DM, actually poses less of a threat to TeVeS than the overall difficulty that TeVeS has explaining weak lensing by galactic clusters generally.

    Of course, the other virtue of TeVeS is that it provides an analytic framework which very closely replicates lambda cold dark matter theories in a compact and constrained manner that can be used to make models that work. Even if modified gravity is not the correct mechanism, the success of MOND/TeVeS in so many areas suggests that it is useful (in much the way that the Tully-Fischer relation that inspired it is useful) in making predictions to test, and in understanding the deeper relationships that are driving lambda CDM in all but a few very extreme circumstances.

    Even if DM is right, there is still work to be done to explain why we have the quantities of DM in places that we do. Why, for example, is there a so much larger DM effect in galactic clusters than there is in mere galaxies? Are there, for example, multiple kinds of DM, with "type one" DM explaining most DM effects, and "type two" DM explaining what TeVeS fails to get right?
     
  7. Feb 10, 2010 #6
    If I good understand a difference between Beckenstein and Moffat propositions it is that Beckenstein proposes a modification of the Newton's law due a modification of the metric tensor and Moffat found a possible new massive field there.
    Here is an interesting link of Janssen and Prokopec
    http://arxiv.org/PS_cache/gr-qc/pdf/0611/0611005v1.pdf
    There is shown a problem with an instability close to Black Hole.

    I prefer Moffat idea and I would like to find a connection in vacuum fluctuation where the virtual particles do carry a relativistic mass. The virtual particle-antiparticle is not a stable real particle but it exists for a not defined time. If there is a fild with a set of the great amount virtual particles the mass could be defined then. So though the particle is instable the properties of the set of the instable particles may be stable.
     
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