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Physical basis for dark energy-I think Bojo found it

  1. Jun 19, 2007 #1

    marcus

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    Physical basis for dark energy--I think Bojo found it

    After reading his new paper over the course of a couple of weeks, I'm seeing strong indications that Bojowald is on the right track explaining the dark energy effect as a quantum geometrical correction.

    the math is straightforward, most of it quite easy to follow.
    the reasoning is robust---doesn't depend on a particular version of loop cosmology but is based on general features shared by a broad class of models.

    it explains the "coincidence" that the acceleration of expansion seems to have taken off recently

    we are talking an intrinsic "vacuum curvature" which arises as a quantum correction when the Gen Rel spacetime geometry is quantized and acquires aspects of discreteness.

    Anyone at all interested in these issues (cosmological constant, dark energy, observed expansive acceleration, testing different theories of dark energy) should read the paper.

    It has an excellent explanation of Loop quantum cosmology for newcomers.

    Bojowald's physical basis for dark energy is, as he says, eminently TESTABLE by measuring acceleration in deeper redshift with more precision. It is readily FALSIFIABLE, just like General Relativity itself was in 1915 when it first came out (all Eddington needed to do was observe the 1919 eclipse and he could have shot down Gen Rel).

    No negative pressure "inflaton" field needed here. No unicorns put in by hand. :smile: Comes right out of QG as a quantum correction to the inverse volume operator as it affects the kinetic term of the Hamiltonian. Good work Bojowald! It's really beautiful.


    http://arxiv.org/abs/0705.4398
    The Dark Side of a Patchwork Universe
    Martin Bojowald
    24 pages, 2 figures, Contribution to the special issue on Dark Energy by General Relativity and Gravitation

    "While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe."
     
    Last edited: Jun 19, 2007
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  3. Jun 19, 2007 #2

    Wallace

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    This model dosn't explain inflation, just DE (i.e. late time but not primordial inflation). Unless I missed something? The problem with this kind of theory is that it is retrodictive and any predictions it does make are only up to a constant paramter that the theory doesn't set the value of, i.e. see equation 14. This theory predicts that there will be an effective fluid with a negative pressure with an equation of state in the range -1/3 < w < -1 . Unfortunately there are a bazillion other theories that also make this prediction. Measuring the value of w then does not discriminate between theories, contrary to what is claimed by Marcus.

    It still could be right of course, but there are many other ways to see how an effective DE comes out 'naturally' from some theory or other. The fact that the theory does not predict the value of w makes it impossible to test, since any value of w you find is compatible with the theory!
     
  4. Jun 19, 2007 #3

    marcus

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    Did you read the paper?
     
  5. Jun 19, 2007 #4

    marcus

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    You have it backwards, Wallace. The old news (going back three or four years) is that in Loop cosmology you get primordial inflation naturally and generically from a certain quantum correction, without having to adjust or put in anything "by hand".

    there is still a question of whether you get ENOUGH e-foldings without fine-tuning anything or putting in something, but it is remarkable that you get as much as you do.

    for the past 3 or 4 years, the problem has been just the reverse of what you say: the model gave you primordial inflation but it did not give DE.

    Now late time DE behavior has been tentatively derived using A DIFFERENT kind of quantum correction proceedure. That is the new news.:smile:

    This is a preliminary result. We are seeing the first signs of it and I think it is very interesting, don't you?
     
    Last edited: Jun 19, 2007
  6. Jun 20, 2007 #5

    marcus

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    Wallace, you have misrepresented what I said, and then said that (which I didn't say) was wrong. I said

    I did not say the model was testable by measuring w, the DE equation of state.
    Indeed there is a simple equation (14) showing how w varies with an as yet undetermined parameter----so measuring w would not help test the theory, at least at present.
    Bojowald discusses the model's falsifiability, by further detailed observations of the history of expansion. You have to read the paper to see what he has in mind. He does not say falsifiable by measuring w.

    I would be glad if you would retract your unjustified critical remarks.

     
    Last edited: Jun 20, 2007
  7. Jun 20, 2007 #6

    Wallace

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    So there is a link in this theory between matter density, 'effective' DE density and the EOS of the effective DE w. This will confine the allowed range of models that fit this theory to a locus in the w-\Omega_m plane. The best fit for these parameters given the data may not lie on this line thus falsifying the theory. All well and good.

    There are several problems though. The first is that this paper has not stated what that locus is, just that there should be one. I realise it's preliminary and that you can't solve everything at once but until the definite prediction is made then it cannot be said that the theory is falsifiable. What values of these parameters would falsify this idea? Until this question can be answered then clearly we cannot test the theory. That's not to say it's a bad theory, just not testable yet. String theory is a lot older and gets a lot more press and it still hasn't made a testable prediction....

    The second issue is not so much a problem with this theory as such, but with the whole theoretical DE field. There are so many different models for the physical nature of DE, all with plausible origins, that even if this theory does make a definite prediction of the density - w locus and the best fit observational values lie on that line, then there will be innumerable other theories that will also give the same prediction. How then can we say which theory is 'right'?

    I don't know the answer to the above question, I'm just pointing out that it's a problem!

    On the issue of falsifiability my previous post was intended to point out that contrary to what was claimed, this theory is neither testable or falsifiable in its current form. If you took it as a personal criticism or I seemed to twist or misrepresent your words then I apologies, this was not intended. I did however feel that the overly ambitious promotion of this idea should be corrected. For that I make no apologies, though am happy to be shown to be wrong if I've misunderstood something. That is my reading of the paper presented. (How's that for a politician's response ;) )

    I slightly tangential issue is that you seem to think that this kind of origin for inflation is more 'natural' than a negative pressure scalar field. I think this is a dangerous type of argument. What if I in my gut felt the reverse? I'm not sure that we can say that an ad hoc addition of a field is any more of a 'unicorn' as you put it, than an ad hoc modification to some action engineered to give the same physical result? Neither were thought to exist before the data suggested it and both make the same predictions! Again I don't pretend the know the answer to the question of how we should decide which theory is right when they look exactly the same, but I'm curious as to why you seem so certain that you do know the answer?

    It's a similar debate to galaxy rotation curves. The theory doesn't fit the data so do you change the stuff (dark matter) or how the stuff interacts (MOND)? We now have enough data that points more strongly to DM so we can discriminate between these ideas. For inflation I don't know that this is true. We can change the stuff (Inflaton field) or change the how stuff interacts (LQC), but in this case if they both look the same how do we know which is true?
     
    Last edited: Jun 20, 2007
  8. Jun 20, 2007 #7

    Chronos

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    I agree with Wallace to the extent that Bojowald has not shown [or claimed] dark energy is an extension of primordial inflation.
     
  9. Jun 20, 2007 #8

    marcus

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    there is an essential difference which I wish you would acknowledge, Wallace.
    Bojowald is not CHANGING the basic quantum gravity model.
    Loop Quantum Gravity has been around since before Bojowald. the application to cosmology goes back to 2000 and work of Bojowald and Kastrup.

    He has only just now presented a preliminary argument that you can derive DE-like behavior from Loop Cosmology without putting anything in or changing anything
    Also the argument he uses is fairly robust and proceeds even without having worked out the details of a specific model. What we have here is a physically intuitive argument with some variable parameters which can be nailed down but which do not materially affect the main conclusion.

    There is a lot in the paper that is explicitly about potential for testing and falsifiability---if you haven't read and noted it by now, then perhaps I can quote what he says for you later, as time permits.

    Thanks for your reactions.
     
  10. Jun 20, 2007 #9

    Wallace

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    But Quantum Gravity is an untested hypothetical theory, therefore we cannot a priori decide that it's predictions are better than the predictions of a different theory than looks the same! If we could show that an established theory (such as GR for instance) predicts DE type behavior in a way not previously realized then that is very interesting. However pointing out that a hypothetical theory that is untested, not fully solved and containing free parameters also gives such behavior is less surprising.

    If you could quote those sections I'd appreciate it since I read them myself and failed to find any predictions that could be checked against observations. Remember that the suggestion of a link between two parameters is not testable if there is infinite freedom in that linking that allows any data to fit the model. What I can see in that paper is a suggestion that if the LQC theory is fully solved, then it looks as though it will make a definite prediction. What is achieved so far is an idea of the form the density - w locus of the theory would take. The parameters that specify that locus are not specified at this point though. Therefore your OP could have said that this work makes it appear that LCQ may in the future offer testable predictions, but it does not do that at present (at least the paper you linked doesn't).
     
  11. Jun 20, 2007 #10

    marcus

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    Wallace, here is the first sentence of my post

     
  12. Jun 20, 2007 #11

    Wallace

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    ..and here's some a later ones
    As I've said it's the claims that the theory is TESTABLE and FALSIFIABLE that I have issue with, and also that this theory is in some way less like a unicorn than a negative pressure scalar field. Clearly they are equally Pegasus like, but I don't know so much about unicorns ;)

    Look, to make it clear I'm not dismissing this as rubbish or of no value. It's clearly very interesting. I'm just trying to make it clear what it does and doesn't say in it's present form.
     
    Last edited: Jun 20, 2007
  13. Jun 20, 2007 #12

    marcus

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    Well, that's a constructive attitude. This is the first tentative appearance of this idea in LQG and there is enormous room for development.
     
  14. Jun 20, 2007 #13

    Wallace

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    Aye, just need to make sure we don't sensationalize new research to make it more profound than it is. We have New Scientist for that :tongue:
     
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