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The Dark Side of a Patchwork Universe

  1. Jun 3, 2007 #1

    jal

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    WOW!!!! wow!!! What a paper!!!!
    Tell Martin Bojowald (and those that he cited), that I’m throwing a party and supplying the refreshments and photo ops. (They can get an expense account from their depts.).
    Marcus, you can forget all the other papers …. This is the most influential paper … and it will be for years to come.
    I want to tell all the “seekers” about this paper.
    I want to tell the whole world!
    Contrarily to Martin Bojowald, I can take a definite position and say that his paper presents a strong argument as to why the “inflaton” is not needed.
    http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.4398v1.pdf
    The Dark Side of a Patchwork Universe
    Martin Bojowald
    30 may 2007
    A complete understanding of the universe currently faces several problems, most of which are occasionally expected to be solved by some version of quantum gravity. This also applies to the dark energy problem.
    Schematically, one has a picture where space is presented as a discrete structure building up from a small state at the big bang to a highly refined, nearly continuous fabric today. The evolution picture is thus that of an irregular lattice structure which changes in internal time by elementary changes of geometry.

    Note: just add one more unit every once in a while

    From the point of view of quantum field theory on curved space-times one can effectively view the finiteness of vacuum energy in loop quantum gravity as a cut-off provided by the underlying discrete structure of loop quantum gravity. On the grounds of dimensional arguments one would expect that the cut-off occurs at Planckian values of energy or length, which would certainly result in the well known mismatch between the predicted and observed cosmological constants.

    Note: I would like to see arguments why the cutoff cannot be at 10^18 (gluon interaction sizes/length)

    It is to be expected that vacuum energy in this formalism does not only depend on the matter state but also on quantum geometry.

    In fact, such a quantum geometry epoch of inflation typically does not last long enough to provide all 60 e-foldings required for successful structure formation.
    Moreover, such an isotropic model with only inverse volume corrections is not
    very accurate at large volume because it does not fully take into account the dynamical discreteness of space manifesting itself in lattice refinements determined by the elementary moves of a Hamiltonian constraint.
    Rather, during expansion the discrete structure of space subdivides as described in Sec. 2 which can be modeled by adding new small, discrete patches resulting from new vertices of graphs. When the number of patches increases with volume, their size stays nearly constant or could even decrease.

    francesca
    I did not even look at the other papers that you mentioned. Martin Bojowald’s paper was just tooooo much!
    jal
     
  2. jcsd
  3. Jun 3, 2007 #2
    it was just for the chronicle :-)
    Bojowald's paper is the only one I'm going to read in dept, by now...
    Otherwise I notice in Bojowald's references a Engle's paper that could be interesting,
    I don't think that it was mentioned yet in PF:

    http://arxiv.org/abs/gr-qc/0701132
    On the physical interpretation of states in loop quantum cosmology
    Authors: Jonathan Engle
    Abstract: In this paper we address the physical meaning of states in loop quantum cosmology (LQC). A first step in this is the completion of the program begun in "Quantum field theory and its symmetry reduction", applied to LQC. Specifically, we introduce a family of (what are called) b-embeddings of isotropic loop quantum cosmology (LQC) into full loop quantum gravity. As a side note, we exhibit a large class of operators preserving each of these embeddings, and show their consistency with the LQC quantization. Embedding at the gauge and diffeomorphism invariant level is discussed in the conclusion section.
     
  4. Jun 4, 2007 #3

    Chronos

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    An engaging paper, thank francesca. The idea seems sound and possibly confirmable with the right geometry. I'm just not sure where to look for the right geometry . . . there's hunerds of em. Seriously though, Bojowald took the idea well beyond my vague notion of the next logical step.
     
  5. Jun 4, 2007 #4

    jal

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    Chronos
    As you can see from my reaction .... it was inevitable.
    francesca
    I have difficulties following the presentation.

    question

    Can I interpret this to mean that we are not considering anything within the proton, neutron drip-line?
    Are we considering the gluon/quark sea!!!
    http://www.fynu.ucl.ac.be/librairie/ocde/rapportocde/lr_oecd_vs16_2.htm
    Study Group on Radioactive Nuclear Beams
    Nuclei with an excess or deficiency of neutrons relative to the valley of stability are unstable and they decay [e.g. via the emission of  particles (e- or e+)]. The existence of nuclei is limited by frontiers in three directions of the nuclear chart: i) the neutron rich limit for fixed Z; ii) the proton rich limit for fixed N; iii) the large mass limit. We have not fully reached any of these limits except for the very light nuclei. (proton drip-line, neutron drip-line)
    --------------
    http://www.jlab.org/highlights/phys.html
    Jefferson Lab
    Strange Quarks in the Proton
    For instance, strange quark/anti-quark pairs are present in this quark sea even though the proton has, on average, no overall strangeness.
    Actually, even in the lower energy range, one should keep the quarks in mind, because their motion inside a nucleon may change when it resides in a nucleus
    -------------
    jal
     
  6. Jun 4, 2007 #5
    Sorry I'm dull :uhh: I don't understand what's the connection with Bojowald's paper...
    Well, if you are considering inflation, it has to happen before baryogenesis, so also before nucleosynthesis...
    anyway: what's your problem with nuclear physics? Can you put again the question?
    :confused:
     
  7. Jun 6, 2007 #6

    Chronos

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    Not at all dull, francesca, more like enlightened. I think inflation can be disregarded. Baryogenesis might be ruled out . . . not so certain about nucleosynthesis. I see a problem with that.
     
  8. Jun 6, 2007 #7

    jal

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    I was waiting for comment from the more informed and critical members of this forum and I was also searching for clarifications and so as to improve my knowledge.
    Since no one has posted any other comments …. here are my comments. (for a general audience)
    Martin Bojowald in “The Dark Side of a Patchwork Universe” is also proposing that quantization could be an approach for solving the Casimir Effects, which is outside of the proton, neutron drip-line and proposes an intuitive understanding of the “quark sea” which is inside the drip-line.
    -----------------
    http://arxiv.org/PS_cache/hep-th/pdf/0406/0406024v1.pdf
    REVIEW ARTICLE
    The Casimir effect: Recent controversies and Progress
    Kimball A. Milton
    02 june 2004
    ---------------
    p. 61 This promises to add another bit of understanding to our knowledge of Casimir forces, knowledge that seems to grow only incrementally based on specific calculations, since a general understanding is still not at hand.
    p. 62
    6. Dynamical Casimir Effects
    Dynamically,
    photons indeed should be produced by QED by a rapidly oscillating bubble, but to produce the requisite number (106 per flash) necessitated, if not superluminal velocities at least macroscopic collapse time scales of order 10−15 s, rather than the observed 10−11 s scale [80].
    -----------
    The casmir effect has been observed down to 10nm. The similarity with the "quark sea" at 0.1 fm is only that... a similarity. The two are different.
    --------------
    This is the only game in town.
    http://www.phys.psu.edu/~cteq/handbook/v1.1/handbook.pdf
    Handbook of perturbative QCD
    --------------
    Doing quantization (LQG) is much more intuitive that “dipping” into an unknown “quark sea” and picking out “particles” that make the parton model work.
    -------------

    --------------
    http://cerncourier.com/main/article/44/5/13/1
    … so lattices 2.5 fm across or larger are thought to be sufficient for calculations at present.
    The development of higher order, "improved" discretizations of QCD has allowed calculations to be performed that give answers close to continuum QCD, with values for the lattice spacing of around 0.1 fm.
    (Note: size of proton approx. 1.0 fm)
    Two different values of the lattice spacing have been simulated to check discretization errors and two different volumes (2.5 and 3.5 fm across) to check finite volume errors.
    -----------------
    (Note: this is still within the nucleus/drip-line.)
    --------------

    Confusion reigns in the only game in town.
    The naming of the processes/action and the naming of the particles are all mixed up.
    (Let me use a coin for an example. It could be representing a quark/gluon in the “quark sea”)

    To me it would be like turning a coin, front (+), side (zero/quark sea/Z.P.E.), back (-) and then renaming those actions as well as renaming the front, side, back when all along you forgot that it’s a coin that you are turning over. Then, renaming all the ways (x,y,z) that the coin can be turned. (Even if you have 3 coins.) I don’t see how the transformation from an action to a particle or any transformation from a particle to an action can change the coins. How can re-naming of the position or re-naming the momentum change the coins?
    What would you do if you had 12 coins? Call it a quark sea? )
    Would one more coin (13) or 4 more (16) be the entry point into the parton model in the drip-line?
    -------------
    http://hyperphysics.phy-astr.gsu.edu/hbase/particles/haddia.html#c1
    hadron diagram
    ----------------

    I would bet that when CERN goes fishing in the “quark sea” that their anchor will reach bottom at 10^-18.
    If I got it all wrong then I’ll take my place in a long line up of people who know more than me.

    Maybe someone else has comment for the specialized audience?
    [​IMG]
    (The image may not show due to overload)
    ----------------
    jal
     
  9. Jun 7, 2007 #8
    Cosmologists use to cite Casimir when they talk about the contribute of vacuum energy to negative pressure... I don't like this habit because while I have no problem with pair production and Casimir theory, I think that what has been measured in Casimir experiments is just Van Der Waals forces: this is what I've understood talking informally with a friend of mine working on an experiment of these.
    Otherwise I'm convinced that QG will lead to a better comprehension to our present notion of vacuum, so...
     
  10. Jun 7, 2007 #9

    jal

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    Latest test/find

    http://arxiv.org/abs/0705.3793v1
    Precision measurement of the Casimir-Lif****z force in a fluid
    Authors: Jeremy N. Munday, Federico Capasso
    (Submitted on 25 May 2007)
    Abstract: The Casimir force, which results from the confinement of the quantum mechanical zero-point fluctuations of the electromagnetic fields, has received significant attention in recent years for its effect on micro- and nano-scale mechanical systems. With few exceptions, experimental observations have been limited to conductive bodies interacting separated by vacuum or air. However, interesting phenomena including repulsive forces are expected to exist in certain circumstances between metals and dielectrics when the intervening medium is not vacuum. In order to better understand the effect of the Casimir force in such situations and to test the robustness of the generalized Casimir-Lif****z theory, we have performed the first precision measurements of the Casimir force between two metals immersed in a fluid. For this situation, the measured force is attractive and is approximately 80% smaller than the force predicted by Casimir for ideal metals in vacuum. We present experimental results and find them to be consistent with Lif****z's theory.
    ------------
    heheh the pf does not like **** heheh
    jal
     
  11. Jun 8, 2007 #10

    Chronos

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    It's not surprising [to me at least] the Casimir force appears to diminish at sufficiently short distances. Something had to give, don't you think? I would be curious to know 'where' [exactly] the departure from theory becomes apparent. It is similarly unclear 'where' [exactly] GR and QM become mutually exclusive. Designing an experiment to isolate the scale at which these 'breaks' occur is surprisingly difficult.
     
  12. Jun 8, 2007 #11

    marcus

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    Jal, francesca and others, this is a really interesting paper!
    Thanks for spotting it and starting a thread for discussion. I have been reading it since i returned from vacation and will try to have some comments soon.
     
  13. Jun 14, 2007 #12

    marcus

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    He does seem to have an explanation for the dark energy effect, as a LQC quantum correction to the matter Hamiltonian, which is testable by further precise observation of cosmic acceleration.

    this quantum correction is extremely small in any given local region ("patch") of space, but adds up cumulatively so as to become significant.

    he seems to be working with a finite-volume universe, which is in agreement with analysis of recent WMAP and other data which is tending to center the confidence intervals for Omega a bit on the plus-side of 1. Ned Wright's recent LCDM "best fit" Omega was 1.011

    Sean Carroll's slides, which recently attracted a lot of attention, also appear to suppose that a universe "like ours" would have finite spatial volume. Carroll is prominent and even a bit trendy if a cosmologist can be that---so perhaps the spatially finite universe model is coming into style
     
  14. Jun 14, 2007 #13

    marcus

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    something that is remarkable

    he has an explanation of the "dark energy" acceleration effect which does not require assuming any dark energy particle or extra field---it comes out of the LQC machinery he already has,

    and rather robust: insensitive to details of quantization

    so there is no need to imagine new paraphernalia and gizmos. it is Occamy.

    and he seems to have the beginnings of an explanation of the coincidence that dark energy density is about same order magnitude as matter energy density at the present historical era.
    I don't fully see how this comes out but I see suggestions of it in the paper.
    that in itself is pretty remarkable.
     
  15. Jun 19, 2007 #14

    marcus

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    Hi francesca,
    this paper by Engle is quite interesting.
    Actually it WAS mentioned here at PF
    https://www.physicsforums.com/showpost.php?p=1222749&postcount=554
    on page 37 of the links thread. But we didn't have any discussion AFAIK.

    It came out in January 2007. I'm surprised we didn't have discussion back then. Maybe it was because of the KITP workshop on "Quantum Nature of Spacetime Singularities" (with Ashtekar, Thiemann, Bojowald plus other loop and string people) which was in January and took up a lot of my attention.

    What Jonathan Engle does is both important and mathematically very nice.

    He explores the root idea of Loop Cosmology discovered by Bojowald and his mentor Hans Kastrup, and he looks how to EMBED Loop Cosmology into the full Loop Gravity theory. I like his style.

    Engle got his PhD at Penn State (Ashtekar's group) in 2006 and then went to join Rovelli's group at Marseille. I think he is American, but I don't know. If so it shows again what happens when there is a smart American who wants to go into Quantum Geometry/Gravity. Eventually the new postdoc will probably have to be "expatriated" and go to work in Europe. The US establishment is too stupid to make sufficient funding and room available. Eventually people go to Canada or France.
     
  16. Jun 19, 2007 #15
    I don't know, I'm in Europe but I'm going to espatriate too...
    Finding a good place is a spread problem, everytime and everywhere!

    Jonathan is doing a good job in Marseille, I look forward to know more about this new vertex amplitude...

    ps: it was impossible that you had missed the Engle's paper :wink: sorry
     
  17. Jun 19, 2007 #16

    marcus

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    I know you are joking, but no sorries, francesca. I appreciate your often reminding me about things and depend on your taste and judgment about what is interesting. In fact I had forgotten about this paper. It's very interesting!

    Just now I looked at his PhD thesis (Ashtekar was his advisor) and will also take a look at his previous paper, of which this one is a sequel.

    Engle's thesis
    Black Hole Entropy, Constraints, and Symmetry in Quantum Gravity
    http://igpg.gravity.psu.edu/archives/thesis/2006/englethesis.pdf

    his paper with Rovelli with the new spinfoam formula
    http://arxiv.org/abs/0705.2388
    The loop-quantum-gravity vertex-amplitude

    his January 2007 paper that you flagged
    http://arxiv.org/abs/gr-qc/0701132
    On the physical interpretation of states in loop quantum cosmology

    the "prequel" to that one
    http://arxiv.org/abs/gr-qc/0511107
    Quantum field theory and its symmetry reduction
    (published by CQG in 2006)

    In case anyone hasn't met the guy, here's a Penn State newsletter snapshot
    http://www.phys.psu.edu/images/nuggets/100.jpg
    after PhD from Penn he won an NSF fellowship to postdoc with Rovelli. Overseas fellowships are a great idea--our tax support
    enables smart US physicists to study abroad since we don't have adequate QG education and research programs here at home.
     
    Last edited: Jun 19, 2007
  18. Jun 20, 2007 #17

    Chronos

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    Ouch. QG programs may be less prolific [and less publicized] in the US, but I think they have produced their fair share of gifted researchers.
     
  19. Jun 20, 2007 #18

    marcus

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    I said US institutional QG resources are inadequate, not that they don't exist. We have only one physics department in the country which has a QG group (more than just one faculty member).

    And I mean inadequate given the present stage of research in the field: the rate of new development and expansion of frontiers. Things are happening in quantum gravity.

    Another way to look at it is to ask: of Ashtekar's last five Loop PhD's, how many have had to leave the USA to find good places to work?
    There's Jon Engle, Josh Willis, Kevin Vandersloot, Yi Ling, and Olaf Dreyer.
    Josh Willis is with Christensen's group in London Ontario
    (where Jeff Morton, one of Baez recent PhDs, also went).
    Jon Engle went to Marseille with Rovelli's group.
    Olaf Dreyer went to Perimeter and then to University of London in UK
    Yi Ling also went to Perimeter.
    Kevin Vandersloot won a Curie fellowship and went to the UK (an American getting European research funding.)

    Here's the announcement about Kevin
    "Kevin Vandersloot has won the prestigious Marie Curie Post-doctoral Fellowship of the European Union. This has been a coveted honor in Europe and non EU citizens became eligible for nominations only recently. Because the number of awards to non EU citizens is restricted, the competition is even fiercer than for the regular Marie Curie Fellowships. Kevin will take this Fellowship at the Institute of Cosmology and Gravitation in Portsmouth, UK."
    http://www.gravity.psu.edu/news.shtml

    So the total score is zero. US academic institutions got NONE of that trained talent. All five most recent Ashtekar Loop PhDs left the country to join QG research groups outside the US.
     
    Last edited: Jun 20, 2007
  20. Jun 21, 2007 #19
    Marcus, the coincidence comes from the fact that (p. 17) the factor between vacuum and matter energy density is C/2 No^2/3 where
    C = 0,742 d^2 (with an Immirzi parameter of 0,2375 as derived from black hole entropy calculations). "No" must logically be of order one (number of patches at planck scale), so remains d^2. It corresponds to the quantization ambiguity, which he says must also be of order one ( he refers to a 2006 paper of his on inhomogenities that I cannot find...).

    By the way, the most recent measurements give a factor 2 between vacuum and matter density, so that assuming that No=1 one would get d between 2 and 2.5.
     
  21. Apr 8, 2008 #20

    jal

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