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Do e Observe Quantum Gravity Effects at Galactic Scales?

  1. Oct 1, 2005 #1

    marcus

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    Do We Observe Quantum Gravity Effects at Galactic Scales?

    http://www.arxiv.org/abs/astro-ph/0509163
    Do we Observe Quantum Gravity Effects at Galactic Scales?
    M. Reuter, H. Weyer
    6 pages, 1 figure. Talk given by M.R. at the 21st IAP meeting "Mass Profiles and Shapes of Cosmological Structures", Paris, July 4-9, 2005; to appear in the proceedings

    "The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that there could be strong renormalization effects at large distances, in particular a scale dependent Newton constant, which mimic the presence of dark matter at galactic and cosmological scales."

    the version of QEG referred to in this paper is that published by Reuter and Lauscher in 2002
    http://www.arxiv.org/abs/hep-th/0110021
    Is Quantum Einstein Gravity Nonperturbatively Renormalizable?
    O. Lauscher, M. Reuter
    18 pages, 3 figures
    Class.Quant.Grav. 19 (2002) 483-492

    "We find considerable evidence supporting the conjecture that four-dimensional Quantum Einstein Gravity is 'asymptotically safe' in Weinberg's sense. This would mean that the theory is likely to be nonperturbatively renormalizable and thus could be considered a fundamental (rather than merely effective) theory which is mathematically consistent and predictive down to arbitrarily small length scales. For a truncated version of the exact flow equation of the effective average action we establish the existence of a non-Gaussian renormalization group fixed point which is suitable for the construction of a nonperturbative infinite cutoff-limit. The truncation ansatz includes the Einstein-Hilbert action and a higher derivative term."

    here is an earlier paper about this, from October 2004. I started a PF thread about it when it came out, but that was a long time ago so I will add the abstract here

    http://arxiv.org/abs/hep-th/0410117
    Running Newton Constant, Improved Gravitational Actions, and Galaxy Rotation Curves
    M. Reuter, H. Weyer
    72 pages
    PhysRevD.70.124028
    "A renormalization group (RG) improvement of the Einstein-Hilbert action is performed which promotes Newton's constant and the cosmological constant to scalar functions on spacetime. They arise from solutions of an exact RG equation by means of a 'cutoff identification' which associates RG scales to the points of spacetime. The resulting modified Einstein equations for spherically symmetric, static spacetimes are derived and analyzed in detail. The modifications of the Newtonian limit due to the RG evolution are obtained for the general case. As an application, the viability of a scenario is investigated where strong quantum effects in the infrared cause Newton's constant to grow at large (astrophysical) distances. For two specific RG trajectories exact vacuum spacetimes modifying the Schwarzschild metric are obtained by means of a solution-generating Weyl transformation. Their possible relevance to the problem of the observed approximately flat galaxy rotation curves is discussed. It is found that a power law running of Newton's constant with a small exponent of the order 10^{-6} would account for their non-Keplerian behavior without having to postulate the presence of any dark matter in the galactic halo."

    I see this one has 11 citations, several from other researchers who have taken up the idea
     
    Last edited: Oct 2, 2005
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  3. Oct 1, 2005 #2

    wolram

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    Please forgive the intrusion, this seems interesting to me, but the language
    is to deep, i am sure this is important so will some one explain it in easy terms
    please.
     
  4. Oct 2, 2005 #3

    marcus

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    you are not intruding, it's an appropriate question wolram. It seems interesting to me too. I cannot provide an adequate discussion, nor can I evaluate-----there may be something wrong with Reuter's approach, and even if there is, it is still attracting attention and being tried out by other researchers. Even if there are serious flaws or details that remain to work out it still appears SUGGESTIVE to a number of people.

    Reuter has a scheme where the effective gravitational constant G is larger over very long distances

    (and also smaller at microscopic distances than what we measure at ordinary lab scale-----the apparatus to determine G is difficult to miniaturize so it has not been measured at scales much less than 1 centimeter)

    If G "runs", that is to say VARIES WITH DISTANCE, then maybe we dont need dark matter to explain galaxy rotation curves. Because if G is effectively larger at those large scales, maybe a smaller amount of matter (acting more strongly because of larger coupling constant G) is enough to explain the observed speeds and inferred accelerations.

    This is likely to be a gross oversimplification. I havent yet understood the real message. I suspect what he is talking about is not as simple as G varying with distance-----it could be something else (related to G) that is varying and it could be in response to something else (not distance). But at a superficial level that is roughly the idea.
     
    Last edited: Oct 2, 2005
  5. Oct 2, 2005 #4

    marcus

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    another paper of theirs seems to be easier to read

    http://arxiv.org/abs/hep-th/0410119
    Quantum Gravity at Astrophysical Distances?
    M. Reuter, H. Weyer
    46 pages, 4 figures, to appear in JCAP
    JCAP 0412 (2004) 001

    "Assuming that Quantum Einstein Gravity (QEG) is the correct theory of gravity on all length scales we use analytical results from nonperturbative renormalization group (RG) equations as well as experimental input in order to characterize the special RG trajectory of QEG which is realized in Nature and to determine its parameters. On this trajectory, we identify a regime of scales where gravitational physics is well described by classical General Relativity. Strong renormalization effects occur at both larger and smaller momentum scales. The latter lead to a growth of Newton's constant at large distances. We argue that this effect becomes visible at the scale of galaxies and could provide a solution to the astrophysical missing mass problem which does not require any dark matter. We show that an extremely weak power law running of Newton's constant leads to flat galaxy rotation curves similar to those observed in Nature. Furthermore, a possible resolution of the cosmological constant problem is proposed by noting that all RG trajectories admitting a long classical regime automatically give rise to a small cosmological constant."

    this paper is still not short but it is shorter than what they published in Physical Review series D. And it has fewer formulas and correspondingly more verbal explanation.
    But dont get your hopes up, it is still not sure it is RIGHT and it is still somewhat vague and speculative, and it is still hard (for me at least) to understand.

    the only thing objective I can point to is that other researchers are now jumping in on this----the citation numbers are rising on these Reuter papers, especially the ones that suggest some alternative to dark matter.

    ==========
    this JCAP paper, from October 2004, has 16 cites many quite recent
    the PhysRev paper, also from October 2004, has 11 cites, also several of them recent
    the LATEST paper, from September 2005? too soon
     
    Last edited: Oct 2, 2005
  6. Oct 2, 2005 #5

    wolram

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    Thank you MARCUS, it is all to coplex for me, i will wait for the tit bits, but
    i am sure a modifed gravity theory will work one day.
     
  7. Oct 3, 2005 #6

    marcus

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    I sympathise Wolram---it is our common plight.

    If you wouldn't mind telling me, it would save me doing a search: did you already start a thread in Astronomy about a paper by two Canadians I think they were, who claimed to get rid of the need for dark matter just using ordinary conventional vintage-1915 einstein gravity?

    http://arxiv.org/abs/astro-ph/0507619

    I hit this by accident and it is just the sort of thing you often come up with. they claim to have shown that dark matter isn't necessary just by ordinary general relativity. I find that quite puzzling.
     
    Last edited: Oct 3, 2005
  8. Oct 3, 2005 #7

    selfAdjoint

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    The velocity profile matches are amazing. As I recall, when this was first posted on the Astro board, there were demurs of the nature of "dark matter does other things than determine the velocity profile" and "So-and-so has modified the effect of dark mattter on the profile". Obviously what astronomers should do now is subtract this effect from their observations and see if there is any role left for DM. I don't think they will be able to brush this work off.
     
  9. Oct 4, 2005 #8

    marcus

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    I found the link to the thread you started about this:
    https://www.physicsforums.com/showthread.php?p=692372#post692372

    the Cooperstock and Tieu article

    I'd really like to see some follow up on this!
     
    Last edited: Oct 4, 2005
  10. Oct 4, 2005 #9

    wolram

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  11. Oct 4, 2005 #10

    marcus

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    sorry to hear that. I never lost a whole drive----several years email correspondence and things of that magnitude (it might feel a bit like electro-shock "therapy" would have, hopefully they dont do that any more) where a part of you is gone.
     
  12. Oct 4, 2005 #11

    ohwilleke

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    There are two ways of explaining galaxy rotation curves. You can have dark matter or you can modify traditional rules of gravity. This paper uses a gravity modification approach. But, it does so using a quantum mechanical approach the general relativity, rather than the theories such as MOND which have been used in the past.
     
  13. Oct 4, 2005 #12

    wolram

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    Come on guys, what is gravity should be so simple, i wish i never asked the question so long ago, give me a box of chocolates any day.
     
  14. Oct 4, 2005 #13

    selfAdjoint

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    What quantum mechanics? The paper has straight weak-field general relativity, and I sure didn't see anything else. Ohwilleke, on the other thread you had some technical objections to the developments in the paper; could you expand on that here?

    As for the other objection, basically that observation has detected "dark matter" closer to the core than was thought, well doesn't that mean that the Newtonian gravity of this observed matter doesn't serve to account for the rotation profiles? It was originally put out in the haloes because that's where it served just that purpose. If we now have (1)observation of matter elsewhere, and (2) a mechanism that can account for the profiles without dark matter, then it would follow that the newly observed matter is not "dark matter" wouldn't it?
     
  15. Oct 5, 2005 #14
    Just had a thought... :rolleyes: If I remember correctly, some think that spacetime itself has entropy/information associated with it, the probability of the configuration compared to all the possible states that spacetime can have, especially if it is discrete... or something like that.

    OK then, if there is entropy associated with spacetime, then isn't there also an energy associated with spacetime in general, maybe the cosmological constant or the Hawking radiation from the cosmological event horizon. If so, then could space itself be contributing to the gravitational field and give us more of an effect than we would otherwise think. And couldn't this explain all the effects of Dark Matter that we see at play on large scale objects? Thanks.
     
  16. Oct 5, 2005 #15
    And this is exactly what is reported in this new paper:http://arxiv.org/abs/gr-qc/0510015

    Read it through, you may find what your looking for?
     
  17. Oct 5, 2005 #16
    I searched the document and found no reference to dark matter.

    If the cosmological constant or dark energy contributes to the Einstein field equations, then wouldn't there be more of a gravitational effect than otherwise? In such a case, isn't dark matter the same as dark energy?

    I suppose that the wrapping due to the energy of the cosmological constant would pretty much cancel with no large gravitating bodies around. But in non-homogeneous regions, wouldn't the inhomogeneities become greater than otherwise?
     
  18. Oct 6, 2005 #17

    Kea

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    another NoDarkMatter paper

    A tensor-vector-scalar framework for modified dynamics and cosmic dark matter
    R.H. Sanders
    11 pages
    http://arxiv.org/abs/astro-ph/0502222

    Has PF mentioned this one? I just noticed it after a series of seminars here by Malcolm Longair, in particular discussing cosmological scale evidence for CDM.

    :smile:
     
  19. Oct 6, 2005 #18

    marcus

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    PF has not, to my knowledge, mentioned this one.
    Kea you could make a structured bibliography of all the Dark Matter-Huh? and Dark Energy-Hah! research papers. there is getting to be a bunch.
     
  20. Oct 6, 2005 #19

    selfAdjoint

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    Since this is a scalar-tensor theory, Garth may know something about it. Perhaps he will comment.

    Second thought, this is maybe like the joke: "I visited the Great Wall of China last month". "Oh did you meet my friend? He lives in Shanghai."
     
  21. Oct 7, 2005 #20

    Garth

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