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Chalnoth case against Loop Quantum Cosmology

  1. Aug 24, 2010 #1

    marcus

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    Loop Quantum Cosmology could be interesting to discuss because
    1) it has been radically transformed since 2007 (with what Ashtekar calls "improved dynamics")
    2) it has begun a phase of rapid growth (new crop of researchers , publication rate more than doubled since 2006)
    3) it now includes research into ways to test models by measurements of the microwave background.

    On the other hand there are arguments against LQC
    Does anyone have comments? Anything they want to add? It's always good to raise doubts where there are grounds for skepticism. Among other benefits, it can help clarify the issues.
     
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  3. Aug 24, 2010 #2

    wolram

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    Is there information from Planc yet?
     
  4. Aug 24, 2010 #3

    marcus

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    Here is a European Space Agency (ESA) source on the Planck mission:
    http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=47333
    ==quote from ESA source==
    A first batch of astronomy data, called the Early Release Compact Source Catalogue, is scheduled for public release in January 2011. To calibrate the data to the exquisite precision required to extract the main cosmology results will require about two years of data processing and analysis. The first set of processed data will be made available to the worldwide scientific community towards the end of 2012.
    ==endquote==

    According to this, one would expect the first Planck report with implications for cosmology to be issued a bit over 2 years hence---late 2012.
     
    Last edited: Aug 24, 2010
  5. Aug 24, 2010 #4

    marcus

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    If I start giving links to technical papers it will make everyone's eyes glaze. But maybe I will do that quickly at the start, and more or less get it over with, and then get back to regular non-technical talk a few posts later down the thread.

    Of course LQC does not consistently assume a uniform universe. The field began by making some assumptions of uniformity, to make it easier to get results. And now the main thrust of research is the gradual removal of those assumptions (of isotropy and homogeneity).

    So for instance one sees LQC papers about Bianchi I, Bianchi II, and Bianchi IX models. In other words, some models studied are not isotropic, not the same in all directions. And one sees LQC spin foam papers---these depart from the original LQC math framework and begin preliminary use of the full LQG theory, with its spin foam path integral. This was of primary concern all along---how to bring the application to cosmology into firm contact with the full theory---so it is important that progress is being made on that front as well.

    http://arxiv.org/abs/1003.3483
    Towards Spinfoam Cosmology
    Eugenio Bianchi, Carlo Rovelli, Francesca Vidotto
    (Submitted on 17 Mar 2010)
    "We compute the transition amplitude between coherent quantum-states of geometry peaked on homogeneous isotropic metrics. We use the holomorphic representations of loop quantum gravity and the Kaminski-Kisielowski-Lewandowski generalization of the new vertex, and work at first order in the vertex expansion, second order in the graph (multipole) expansion, and first order in 1/volume. We show that the resulting amplitude is in the kernel of a differential operator whose classical limit is the canonical hamiltonian of a Friedmann-Robertson-Walker cosmology. This result is an indication that the dynamics of loop quantum gravity defined by the new vertex yields the Friedmann equation in the appropriate limit."

    http://arxiv.org/abs/0909.4221
    Loop Quantum Cosmology and Spin Foams
    Abhay Ashtekar, Miguel Campiglia, Adam Henderson

    http://arXiv.org/abs/0911.2653
    Triangulated Loop Quantum Cosmology: Bianchi IX and inhomogenous perturbations
    Marco Valerio Battisti, Antonino Marciano, Carlo Rovelli

    http://arxiv.org/abs/1006.2369
    Hybrid Quantization: From Bianchi I to the Gowdy Model
    Mercedes Martín-Benito, Guillermo A. Mena Marugán, Edward Wilson-Ewing

    http://arxiv.org/abs/1005.5565
    Loop quantum cosmology of Bianchi type IX models
    Edward Wilson-Ewing
    (Submitted on 30 May 2010)
    "The loop quantum cosmology 'improved dynamics' of the Bianchi type IX model are studied. The action of the Hamiltonian constraint operator is obtained via techniques developed for the Bianchi type I and type II models, no new input is required. It is shown that the big bang and big crunch singularities are resolved by quantum gravity effects. We also present the effective equations which provide modifications to the classical equations of motion due to quantum geometry effects."

    http://arxiv.org/abs/0910.1278
    Loop quantum cosmology of Bianchi type II models
    Abhay Ashtekar, Edward Wilson-Ewing

    http://arxiv.org/abs/0903.3397
    Loop quantum cosmology of Bianchi I models
    Abhay Ashtekar, Edward Wilson-Ewing

    http://arxiv.org/abs/0805.3511
    The covariant entropy bound and loop quantum cosmology
    Abhay Ashtekar, Edward Wilson-Ewing

    http://arxiv.org/abs/0911.3097
    On the spinfoam expansion in cosmology
    Carlo Rovelli, Francesca Vidotto

    http://arxiv.org/abs/0805.4585
    Stepping out of Homogeneity in Loop Quantum Cosmology
    Carlo Rovelli, Francesca Vidotto
     
    Last edited: Aug 24, 2010
  6. Aug 24, 2010 #5

    marcus

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    I'll give you some of my impressions of LQC: I've began watching it before 2003, when it was still primarily an invention of Martin Bojowald's. Since then it has radically changed its model every three years or so. In 2003 with the ABL paper (Ashtekar, Bojowald, Lewandowski).
    Then again around 2006 with "improved dynamics", and with increasingly extensive computer modeling of the bounce under a range of assumptions.
    Starting around 2009 it has been rapidly shedding restrictive assumptions of the "uniformity" type, as the papers cited above indicate, and transitioning to a spinfoam formulation.

    Impressionistically, around the time of the bounce quantum corrections cause gravity to be a repellent force. Violently repellent, triggering an episode of superinflation.
    To the extent that individual particles could exist, each particle would essentially be everywhere in the portion of the universe being simulated.
    The bounce occurs at a substantial fraction of Planck density. At that density one can assume any pre-existing spatially separated angular momenta have been summed and largely canceled. In effect, spatial separation ceases to exist and spatially separated angular momentum that might once have existed (one of Chalnoth's concerns earlier) ceases to exist.

    We cannot say very much, as yet, about the previous contracting phase. Should we even imagine it as a universe resembling our own? LQC does provide us with provisional tools with which to run time back before the start of expansion. But we can only "see" back in time with these tools to the extent that we can remove simplifying assumptions (which is work in progress) and test, by comparing model to data.

    One expects matter to be included in the spinfoam formulation and that in some sense matter and geometry become indistinguishable in the quantum regime around the bounce.
    Matter would become separate from geometry by a kind of spontaneous symmetry breaking as the universe expands and cools. The current state of LQG is given in Rovelli's survey paper http://arxiv.org/abs/1004.1780 which indicates how matter might be included by enlarging the group labels carried by the spinfoam/network. Fermions on network nodes, Yang-Mills fields on network links. Since the inclusion of matter under normal conditions is still to be carried out, we won't know for some time how this looks during the quantum regime around the bounce.

    In any case the "clumping" that Chalnoth worried about earlier seems rather more of a classical consideration. To the extent that we imagine particles existing, their locations become governed by quantum uncertainty and difficult to pin down. The concept of spatially separate clumps seems inappropriate or difficult to define in regimes near Planck density.
     
    Last edited: Aug 24, 2010
  7. Aug 24, 2010 #6

    marcus

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    Here's a little sidebar note on entropy: Thanu Padmanabhan, a recognized expert on general relativity and its relation to thermodynamics, has pointed out that entropy is observer-dependent.For example the entropy we associate with a black hole event horizon is certainly from the point of view of an observer outside the horizon!

    At the level of logical detail, the observer is the one who determines what the distinguishable states of the system are---he defines what are macrostates and microstates. Likewise the Second Law, that entropy tends to increase, requires an observer to be meaningful. Unless you imagine an Observer outside the universe somehow looking down from Eternity, there would seem to be no absolute entropy and no absolute second law.

    So we, looking back towards the start of our universe's expansion may see a beginning which has (for us, by our measures) low entropy. But if the contracting phase happened to be in some respects recognizably similar and also had observers---they might look ahead to the collapse of their universe and see (by their measures) only increasing entropy. Essentially something similar to that of a black hole horizon into which they were falling.

    The law would say that no one observer can expect to witness an increase in entropy (as he measures it). With the before/after shift in observer viewpoint, indeed the shredding of any pre-bounce observers, that law would be scrupulously obeyed.
     
    Last edited: Aug 24, 2010
  8. Aug 24, 2010 #7

    marcus

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    To indicate the gradual move, in observational cosmology, towards testing LQC, here's a sample of papers by one of the researchers involved with that, Aurelien Barrau (he used to be more concerned with stringy cosmology, but his interest has shifted just in the past 2 years). http://arxiv.org/find/grp_physics/1/au:+Barrau_A/0/1/0/all/0/1

    http://arxiv.org/abs/1003.4660
    Inflation in loop quantum cosmology: Dynamics and spectrum of gravitational waves
    Jakub Mielczarek, Thomas Cailleteau, Julien Grain, Aurelien Barrau
    Comments: 11 pages, 14 figures. Matches version published in Phys. Rev. D
    Journal-ref: Phys.Rev.D81:104049,2010
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)

    http://arxiv.org/abs/0911.3745
    Loop quantum gravity and the CMB: toward pre-Big Bounce cosmology
    Aurelien Barrau
    Comments: Proceedings of the 12th Marcel Grossman Meeting on General Relativity. 3 pages, no figure
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

    http://arxiv.org/abs/0910.2892
    Fully Loop-Quantum-Cosmology-corrected propagation of gravitational waves during slow-roll inflation
    J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
    Comments: 9 pages, no figure, minor corrections
    Journal-ref: Phys.Rev.D81:024040,2010
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

    http://arxiv.org/abs/0902.3605
    Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation
    J. Grain, A. Barrau, A. Gorecki
    Comments: 15 pages, 5 figures, published version with minor modifications, results unchanged
    Journal-ref: Phys.Rev.D79:084015,2009
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

    http://arxiv.org/abs/0902.0145
    Cosmological footprints of loop quantum gravity
    J. Grain, A. Barrau
    Comments: Accepted by Phys. Rev. Lett., 7 pages, 2 figures
    Journal-ref: Phys.Rev.Lett.102:081301,2009
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
     
    Last edited: Aug 24, 2010
  9. Aug 24, 2010 #8

    marcus

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    More grist for the mill :biggrin:

    == http://arxiv.org/abs/1003.3483 ==
    From the point of view of cosmology, the system we have described opens in principle the way to the description of inhomogeneous degrees of freedom at the bounce, circumventing the difficulties of the model given in [23].

    In particular, the covariant dynamics used here can readily be extended to larger graphs. Coherent states have been largely used in loop quantum cosmology (see for instance [43–46]) in particular in relation to the problem of finding effective equations or in numerical simulations [47–49].

    Here, however, homogeneous and isotropic states appear naturally as states peaked on homogeneous and isotropic mean values of the quantum states, in the context of a formalism which -–we stress–- is not a reduction of the dynamics to homogeneous and isotropic degrees of freedom. In physical terms, these states represent a universe where inhomogeneous and anisotropic degrees of freedom are taken into account but fluctuate around zero. This provides also an elegant solution of the problem of having to choose between coordinate or momenta in imposing a symmetry reduction in cosmology [50–52].

    Ideally, this formalism could describe inhomogeneous and anisotropic quantum fluctuations of the geometry at the bounce.
    ==endquote==
     
  10. Aug 25, 2010 #9

    Chalnoth

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    Science data from Planck which will have an impact on inflation will be released in about two years' time. Even then, however, it is rather unlikely that Planck will be capable of detecting B-mode polarization, and as such it's somewhat unlikely to have anything to say about LQC.
     
  11. Aug 25, 2010 #10

    Chalnoth

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    Well, yes, absolutely, it is a classical consideration because I would expect it to prevent the collapsing universe from getting anywhere near the Planck density in the first place. That's sort of why I raise the objection.
     
  12. Aug 25, 2010 #11
    Hi, Marcus, these are quite interesting comments about entropy, could you please provide me with some reference from Padmanabhan where he points out this entropy "observer-dependency".
    Thanks.
     
  13. Aug 25, 2010 #12

    marcus

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    Here are his preprints
    http://arxiv.org/find/grp_physics/1/au:+padmanabhan_t/0/1/0/all/0/1
    What I saw was in 2009, I think.
    Most likely it was http://arxiv.org/abs/0910.0839 (A Dialogue on the Nature of Gravity).
    If you don't see it there, please get back to me and I will help look.

    Sorry that at the moment I can't point you to a definite page of a definite article. Some related Padmanabhan articles are http://arxiv.org/abs/0903.1254 and http://arxiv.org/abs/0911.5004
    But I suspect the "Dialogue" has it.
     
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  14. Aug 25, 2010 #13

    Chalnoth

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    Another classical problem with collapse that would need to be resolved is the problem of peculiar velocities.

    In an expanding universe, an object moving with respect to the background tends to "catch up" to a region of the universe that is moving in the same direction. This produces an effective friction on all matter, which means that as time goes on, peculiar velocities tend to get closer and closer to the expansion rate (until objects fall into some local gravitational potential well and are trapped).

    But in a collapsing universe, the exact opposite of this effect occurs, indicating that the universe becomes less and less FRW with time. Thus a collapsing universe model doesn't need to be able to deal with just small perturbations around uniformity, but huge deviations from it. Ideally it should first be shown that in the non-quantum regime, collapse is actually allowed to continue towards the Planck density.
     
  15. Aug 25, 2010 #14

    bapowell

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    Planck has a projected detection sensitivity of r \sim 0.01 at 95% CL, where r is the tensor/scalar ratio. I don't think it's quite fair to say that such a value of r is as likely or unlikely as any other.
     
  16. Aug 25, 2010 #15

    Chalnoth

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    The difficulty is that the 95% confidence limit isn't enough to really make any strong statements about the science. The primary problem here is that if the measurement depends critically on the polarization of the CMB, then the measurement is also going to depend critically upon how good the foreground removal is, as well as the errors in the foreground estimation. This systematic difficulty means that one needs much better than 95% confidence limit to really make statements about the science.

    It's an indication, really, that if Planck has anything to say about this, it will probably be at the level of, "Interesting, but we need much better observations to be sure."

    Maybe I'm wrong, I don't know. I certainly hope so. But I also wouldn't hold my breath.

    I think the main discoveries with regard to Planck will be:

    1. Much better measurement of E-mode polarization on large angular scales than was available previously. In addition to placing better constraints on reionization, we may also be able to get some good information about lensing of the CMB from large scale structure.
    2. Due to the improved frequency range of Planck over earlier CMB experiments, we should have much better measurements of the foregrounds, including our own galaxy. The early-release compact source catalog that should be coming out relatively soon should be quite useful to a number of people studying such objects, for instance.
    3. The improved frequency coverage of Planck also will allow us to get significantly better measurement of the small-scale CMB fluctuations than has been available previously. In particular, ground and balloon-based experiments that do measure the CMB in this range have difficulties removing foregrounds due to limited frequency coverage, have poor calibration relative to what is available in space, and are very limited in terms of sky coverage. Thus Planck will image a much larger fraction of the CMB at higher resolution than has been available previously. In particular, this should significantly impact the measurement of [itex]n_s[/itex].

    There may be other significant benefits as well, but I'm a bit skeptical about anything that requires a good measurement of the B-mode polarization signal (side comment: the tensor to scalar ratio actually impacts temperature and E-mode polarization anisotropies as well, but the constraints aren't great).
     
  17. Aug 25, 2010 #16

    bapowell

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    Fair enough.

    Good list, although I would think the improved resolution of the small scale fluctuations would improve our knowledge of the running of [itex]n_s[/itex], not our knowledge of [itex]n_s[/itex] at the pivot scale.

    I would also add improved knowledge of non-Gaussianities to this list. With the few recent 'detections' of non-Gaussianities found in CMB/LSS analyses, Planck should be able to weigh-in with some quality data. Local non-Gaussianities should be detected above [itex]f_{NL} \sim 5[/itex], with the tantalizing possibility of ruling out single field inflation. Planck's constraints on equilateral NGs will be weaker, but given that models which generate this kind of NG (eg DBI inflation) typically tend to generate lots of it, I would view even a null detection of equilateral non-Gaussianities as an interesting result.

    True -- we'll probably get a refined upper limit out of Planck.
     
  18. Aug 25, 2010 #17

    Chalnoth

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    It does that too, but it reduces the degeneracy between [itex]n_s[/itex] and other cosmological parameters (I forget offhand which ones, specifically, tend to be rather degenerate).

    Ah, yes, that's another good one.
     
  19. Aug 25, 2010 #18

    marcus

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    Just checking in. LQG does not so far say very much about the contracting phase, what it is, what it consists of. The model just resolves the singularity and goes back in time to a contracting phase.

    When either analytical (equation) models or numerical (computer) models are run they tend to show that contraction proceeds to some substantial fraction of Planck density (like 40 %).

    Are you still claiming that whatever pre-bounce contraction would be prevented from "getting anywhere near the Planck density in the first place."?

    I'm curious why you think you know this, if you do.

    Also I'm wondering if this is still the basic reason you "raise the objection"---i.e. say that LQC will not work.

    Have to go but interested in your or others' (like Powell's!) speculations about this.
    My attitude is we cannot at present say very much about what LQC model will or won't work according to Nature. So I wonder when I hear people making apparently confident statements about that.
     
    Last edited: Aug 25, 2010
  20. Aug 25, 2010 #19

    Chalnoth

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    I haven't claimed to know this. I've only claimed to remain skeptical about it. And you haven't yet directly addressed this particular statement. Especially when simply extrapolating backwards in time doesn't tell us anything about how likely a realistic contracting phase moving forward in time would be to collapse this far.

    The problem is that there is a fundamental asymmetry between looking backward in time in our own universe (where anisotropies tend to get smoothed out more and more) and looking forward in time in some contracting universe (where any small anisotropies that existed would get amplified more and more).
     
  21. Aug 25, 2010 #20

    marcus

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    What I want to ask is do still expect this? And if so why? I can't imagine any reason to expect it so I'm curious as to what you have in mind.
     
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