Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Six themes for QG in 2015 (developments to watch for)

  1. Jan 10, 2015 #1

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Not in any particular order (all are active important research initiatives).

    1. Algebraic geomatter (google "geometry and the quantum: basics")
    2. LQG with constant curvature simplices (embodying the cosmological curvature constant Λ)
    3. Flux formulation LQG
    4. LambdaCDM bounce
    5. Projective LQG
    6. Starbounce (google "planck star")

    1. refers to Sep and Dec 2014 papers by Chamseddine Connes Mukhanov where quanta of both GR geometry and StdMdl matter seem to grow from the same algebraic root
    2. refers to December 2014 paper by Haggard Han Kaminsky Riello (google "4D loop quantum gravity with a cosmological constant", part of the title of their paper) and February 2015 paper by Rovelli Vidotto ("compact phase space, cosmological constant, discrete time")
    3. google "flux formulation LQG" for Dittrich&Geiller new formulation of LQG, likely opening to the GR limit.
    4. google "LambdaCDM bounce" for standard cosmology combined with matter bounce/Loop bounce.
    5. google "projective LQG" for massive work by Lanery&Thiemann reformulating LQG.
    6. google "planck star" for work by Barrau, Rovelli, Vidotto on prospects of seeing collapse rebound explosions

    I will get some links. These are research topics that I think may see some development in 2015. One wants to have looked at some of the relevant 2014 papers, or at least know of their existence, and be on the look-out for further progress this year. BTW nobody says "geomatter" or "starbounce". Those are just concise descriptive verbal handles I use for convenience and concreteness sake.

    The wording of theme #2 was edited to make it less technical and more directly understandable.
    EDIT: in the case of theme#2 a 2009 paper by Bahr&Dittrich proposing simplices with constant curvature should be mentioned (was this a "sleeper" in the sense of not getting sufficiently noticed and followed-up in the intervening 5 years?)
    http://arxiv.org/abs/0907.4325
    Regge calculus from a new angle
     
    Last edited: Feb 8, 2015
  2. jcsd
  3. Jan 10, 2015 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

  4. Jan 10, 2015 #3

    atyy

    User Avatar
    Science Advisor

    I didn't know whether to laugh or cry when I read what you had named the approach of Chamseddine, Connes and colleagues. :):L
     
  5. Jan 10, 2015 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    heh heh, I guess it is a bit crude. We could try for a more polite name: "Spectral geometry and the Standard Model"
    I'm personally not partial to something with a lot of syllables like "Noncommutative geometry and the standard model" (15 syllables).

    If you just laugh I'm happy but I don't want anyone to be aggrieved or grossed out.
     
  6. Jan 10, 2015 #5

    atyy

    User Avatar
    Science Advisor

    Oh I mean cry :L in a good way :). In my country we like corny jokes.
     
  7. Jan 10, 2015 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    As in this country one might groan and look to the Heavens in response to a really awful pun. It is a form of ironical appreciation or at least acknowledgment.
    Algebraic geomatter sounds a bit like algebraic geometry. But let's pretend we don't notice that. The main thing is they have somehow merged dynamical geometry and matter
    so that quanta of geometry and quanta of matter blend into a common quantum geomatter
    and they accomplished the merger in a highly algebraic way. So it is algebraic geomatter :w
    Please clue me in anytime you see better terminology or decide some of these terms sound too dumb. We can go with them on a provisional basis.

    Atyy I am kind of interested in which of these 6 themes will be in evidence at the February EFI Winter Conference at the ski resort of Tux in Austrian Alps.
    http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
    For some reason I do not see Lanery and Thiemann on the list of participants.
    But I see Ed Wilson-Ewing (so maybe LambdaCDM bounce will be discussed)
    and I see Wolfgang Wieland (his family is Austrian so how could he miss this one) so maybe Causal Spinfoams might be discussed
    and Starbounce for sure.
    So that is 3 out of 6.
    I'll check to see if Dittrich or Geiller will be there, if so they might touch on the "flux formulation" theme as well.

    Yes Marc Geiller will be there, so there is a chance. Dittrich is not listed though.
    So that means possibly four out of the six.
    The program for the EFI Winter Conference should appear on line sometime in the next two weeks or so, I imagine.
     
    Last edited: Jan 10, 2015
  8. Jan 10, 2015 #7

    atyy

    User Avatar
    Science Advisor

    An older theme, but one which I'm hoping to see more of is the Chirco-Haggard-Riello-Rovelli (CHRR) approach to entanglement, gravity and thermodynamics. The Bianchi-Myers paper indicated this was a common interest for loops and strings. The stringy side is represented by the ongoing efforts to understand AdS/CFT, while CHRR seems to be the main loopy development of Bianchi-Myers. I think you pointed this out in the other thread, and I owe you a reply, but it needed an intelligence far beyond mine, so I'm just hoping some of those guys will add connections.

    Also, what is the status of EPRL?
     
  9. Jan 11, 2015 #8

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Let me paste in a couple of abstracts as reminders of the CHRR paper and the more recent collaboration of Haggard and Riello (with two others). I need a little time to think about where CHRR might lead. As far as what's happening with EPRL some things that occur to me (as a not too well-informed onlooker) are:
    1. Wieland's new action is a spin foam action and he argues it is an improvement
    2. Dittrich's flux formulation is argued to be an improvement. I'm not sure why these researchers think that EPRL should evolve.
    3. Haggard and Riello (with Han and Kaminski) have a new improved EPRL with Lambda (constant curvature in the bulk interior of the simplexes, not flat ones) and they think that is better. They call it "LambdaEPRL" sometimes, I think. I need to refresh my memory. Let me put up those abstracts

    http://arxiv.org/abs/1412.7546
    SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
    Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
    (Submitted on 23 Dec 2014)
    We study the expectation value of a nonplanar Wilson graph operator in SL(2,C) Chern-Simons theory on S3. In particular we analyze its asymptotic behaviour in the double-scaling limit in which both the representation labels and the Chern-Simons coupling are taken to be large, but with fixed ratio. When the Wilson graph operator has a specific form, motivated by loop quantum gravity, the critical point equations obtained in this double-scaling limit describe a very specific class of flat connection on the graph complement manifold. We find that flat connections in this class are in correspondence with the geometries of constant curvature 4-simplices. The result is fully non-perturbative from the perspective of the reconstructed geometry. We also show that the asymptotic behavior of the amplitude contains at the leading order an oscillatory part proportional to the Regge action for the single 4-simplex in the presence of a cosmological constant. In particular, the cosmological term contains the full-fledged curved volume of the 4-simplex. Interestingly, the volume term stems from the asymptotics of the Chern-Simons action. This can be understood as arising from the relation between Chern-Simons theory on the boundary of a region, and a theory defined by an F2 action in the bulk. Another peculiarity of our approach is that the sign of the curvature of the reconstructed geometry, and hence of the cosmological constant in the Regge action, is not fixed a priori, but rather emerges semiclassically and dynamically from the solution of the equations of motion. In other words, this work suggests a relation between 4-dimensional loop quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in 3-dimensions with knotted graph defects.
    54+11 pages, 9 figures

    The earlier Haggard Riello was the CHRR one that you mentioned, Atyy. Let me get that. Sometimes following people could help follow ideas, I think. As I recall that was not SPECIFICALLY about EPRL, it was more general. Any QG degrees of freedom that satisfied certain conditions could be the basis of a Jacobson-like analysis. So you could be starting either from EPRL or an improved version of EPRL or some other simplicial QG? I'm not sure but I think it was not tied to a particular spinfoam model. Let's see.
    http://arxiv.org/abs/1401.5262
    Spacetime thermodynamics without hidden degrees of freedom
    Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
    (Submitted on 21 Jan 2014)
    A celebrated result by Jacobson is the derivation of Einstein's equations from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius relation. This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric. We show that a different interpretation of Jacobson result is possible, which does not imply the existence of additional degrees of freedom, and follows only from the quantum properties of gravity. We introduce the notion of quantum gravitational Hadamard states, which give rise to the full local thermodynamics of gravity.
    12 pages, 1 figure
     
    Last edited: Jan 11, 2015
  10. Jan 11, 2015 #9

    atyy

    User Avatar
    Science Advisor

    Yes, I dind't mean to associate EPRL with CHRR. That was a separate question. Is there a problem with EPRL that is motivating flux LQG or projective LQG?
     
  11. Jan 11, 2015 #10

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Atyy you jogged my memory when you mentioned the CHRR paper, and also current developments with EPRL, even if not directly associated.
    This made me realize that we couldn't leave out the OTHER Haggard&Riello paper: the one that appeared in December 2014 after their ILQGS online seminar talk in November. I copied the abstract a couple of posts back.

    It's an important development and actually chimes conceptually with what you were talking about because it involves a relation between one theory on a boundary and another in bulk. E.g. notice the last sentence of the HHKR abstract I just posted:
    "In other words, this work suggests a relation between 4-dimensional loop quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in 3-dimensions with knotted graph defects."
     
    Last edited: Jan 11, 2015
  12. Jan 11, 2015 #11

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I was wondering earlier which of these 6 themes (algebraic geomatter, chern-simons spinfoam, flux formulation, lambdaCDM bounce, projectiveLQG, and "starbounce") are likely to be represented at the 16-20 February 2015 EFI Winter Conference.
    Han will be there so they will hear about Chern-Simons spinfoam with cosmological constant. Putting positive Lambda into EPRL was one of the main motivations for hooking up with Chern-Simons. Han has been working on that several years and gave a talk at Perimeter in 2011.

    Marc Geiller will be there and he has been working with Dittrich on flux formulation of LQG (the authors say it is closer to EPRL than past formulations of LQG)

    Edward Wilson-Ewing will be at the conference---so LambdaCDM bounce cosmology may be discussed.

    Rovelli and Vidotto will be there so one way or another the Planck star idea will come up. So that's four out of the 6 topics. Planck star theme is important because related to possible observations (gamma ray bursts, unexplained brief radio-frequency bursts). But it is also possible that one or the other could surprise by giving a talk on something unexpected, say related to the Chamseddine, Connes, Mukhanov work. Rovelli co-authored a paper with Connes some years back about thermal time, he could have gotten interested in the CCM work---which appears to be unifying quantum GR with StdMdl matter--- and have something to say about it. Then 5 out of the 6 themes would be covered.

    It will be interesting to see the EFI Winter conference program when it is posted. Also there's Loops 2015 coming up, at Erlangen this year.
    http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
    http://www.gravity.physik.fau.de/events/loops15/loops15.shtml
     
    Last edited: Jan 24, 2015
  13. Jan 12, 2015 #12

    Demystifier

    User Avatar
    Science Advisor

    What is your country? :)
     
  14. Jan 12, 2015 #13

    atyy

    User Avatar
    Science Advisor

    I'm a Singaporean who is a postdoc in the US. My country only refers to the former, I don't intend to impugn my hosts! :)
     
  15. Jan 17, 2015 #14

    MTd2

    User Avatar
    Gold Member

    So, are we already getting TOE from "eigen deformations" of the geometry of space time, or like coupled fields to the geometry of it?
     
  16. Jan 23, 2015 #15

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    a nice extra resource, we have the Cai&Wilson-Ewing paper ( http://arxiv.org/abs/1412.2914 ) and now there is a video lecture at Perimeter, by Wilson-Ewing, to go with it:
    http://pirsa.org/15010074/
    A ΛCDM Bounce Scenario
    Speaker(s): Edward Wilson-Ewing
    Abstract:
    We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. Assuming that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, we calculate the spectrum of scalar and tensor perturbations after the bounce, assuming initial quantum vacuum fluctuations. We find that the modes that exit the (sound) Hubble radius during matter-domination when the effective equation of state is slightly negative due to the cosmological constant will be nearly scale-invariant with a slight red tilt, in agreement with observations. The tensor perturbations are also nearly scale-invariant, and the predicted tensor-to-scalar ratio is small. Finally, as this scenario predicts a positive running of the scalar index, it can be differentiated from inflationary models.
    21/01/2015 - 4:00 pm

    Also possibly of interest, a pirsa video talk by Karim Noui titled "Getting rid of the Barbero-Immirzi parameter in LQG"
    http://pirsa.org/15010130/
    http://pirsa.org/displayFlash.php?id=15010130
     
    Last edited: Jan 23, 2015
  17. Jan 23, 2015 #16

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The program for the Feb 2015 EFI winter school in Tirol ski country has been posted. I want to see if it reflects some of the themes mentioned, and if others stand out as well.
    Here is a list of talks to be given in the afternoons. Mornings are left free for ski. The time slots seem to be mostly 40 minutes.

    Mehdi Assanioussi: Construction of a hamiltonian operator in LQG

    Tomasz Pawlowski: Interfacing loop quantum gravity with cosmology

    Beatriz Elizaga: Effective homogeneous and isotropic scenarios emerging from states of the hybrid Gowdy model

    Maciej Dunajski: Non-relativistic twistor theory and Newton-Cartan geometry

    Giuseppe Sellaroli: Spinor operators in 3D Lorentzian gravity

    Muxin Han: Chern-Simons Theory, Flat Connections and 4d Quantum Geometry

    Marcin Kisielowski: First-order Dipole Cosmology

    Guillermo Mena Marugan: Mukhanov-Sasaki equations in Loop Quantum Cosmology

    John Schliemann: Coherent Quantum Dynamics: What Fluctuations Can Tell

    Ilkka Mäkinen: Coherent state operators in loop quantum gravity

    Ivan Agullo: Unitarity and ultraviolet-regularity in QFT in curved spaces

    Maximillian Hanusch: Symmetry Actions and Invariance Conditions in LQG

    Edward Wilson-Ewing: A Lambda-CDM Bounce Scenario

    Andrzej Dragan: Ideal clocks - convenient fiction

    Goffredo Chirco: Thermally correlated states in LQG

    Martin Ammon: Recent developments in AdS/CFT and higher spin gravity

    Jorge Pullin: Recent results in spherically symmetric LQG

    Mercedes Martin-Benito: More information about the early Universe than meets the eye

    Carlo Rovelli: Can we test quantum gravity with black hole explosions?

    Benjamin Bahr: Background-independent renormalization in Spin Foam models

    Maite Dupuis: Towards the Turaev-Viro amplitudes from a Hamiltonian constraint

    Jedrzej Swiezewski: Radial gauge - reduced phase space of General Relativity

    Xiangdong Zhang: Loop quantum cosmology in 2+1 dimensions

    Simone Speziale: First order gravity on the light front

    Wolfgang Wieland: Covariant Loop Quantum Gravity as the Scattering of Defects

    Florian Girelli: Turaev Viro GFT and its Fourier transform

    Norbert Bodendorfer: A quantum reduction to Bianchi I models in LQG

    Lacina Kamil: The problem of time in background independence

    Andrea Dapor: Rainbows from Quantum Gravity

    Francesca Vidotto: The compact phase space of Loop Quantum Gravity

    Marc Geiller: Flux formulation of loop quantum gravity

    http://www.gravity.physik.fau.de/events/tux3/program.pdf
    http://www.gravity.physik.fau.de/events/tux3/tux3.shtml


    So, four out of our 6 themes and theme #2 is treated in two papers :w
    I have five of the scheduled talks highlighted in blue. Another one that interests me is Wieland's. I do not know what it entails. His most recent paper is the July 2014 one with arXiv number
    14070025 in which an action for spinfoamQG is constructed as a kind of "scattering of tetrahedra", where a conserved fluid of tetrahedra flows timewards through a network. I have not seen a "scattering of defects" paper by him so I cannot see how to connect the Tux talk with the previous paper. I have highlighted the talk in green as exemplifying an unknown theme---maybe one to add to our list.
     
    Last edited: Feb 4, 2015
  18. Jan 24, 2015 #17

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    For me, the Wilson-Ewing recorded talk was actually an improvement over his paper with Cai ( http://arxiv.org/abs/1412.2914 ) that appeared last month.
    Since I did not get the video, I downloaded the slides PDF and then turned on the audio MP3. Highly satisfactory. Clear rapid-fire delivery. Emphasis on TESTABILITY.
    W-E mentioned some ways the scenario could definitely be FALSIFIED. It replaces inflation with the effects of the prior contraction in the presence of dark matter, radiation, and the Lambda constant. This leads to slightly different predictions, so the scenario is DISTINGUISHABLE by observations which are practical to make and some of which are already being made.

    The inflation paradigm, as the BICEP events showed, is overly flexible and seems to adapt to whatever new observations, so it was refreshing to hear clear prospects of testability emphasized, in a model which is alternative to inflation---achieves the general results which originally motivated cosmologists to dream up inflation and invent the never-observed exotic "inflaton" field, but does not need to invoke the fabulous. I liked the talk a lot---here's the link and abstract again:

    http://pirsa.org/15010074/
    A ΛCDM Bounce Scenario
    Speaker(s): Edward Wilson-Ewing
    Abstract:
    We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. Assuming that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, we calculate the spectrum of scalar and tensor perturbations after the bounce, assuming initial quantum vacuum fluctuations. We find that the modes that exit the (sound) Hubble radius during matter-domination when the effective equation of state is slightly negative due to the cosmological constant will be nearly scale-invariant with a slight red tilt, in agreement with observations. The tensor perturbations are also nearly scale-invariant, and the predicted tensor-to-scalar ratio is small. Finally, as this scenario predicts a positive running of the scalar index, it can be differentiated from inflationary models.
    21/01/2015 - 4:00 pm
     
    Last edited: Jan 24, 2015
  19. Jan 24, 2015 #18

    julian

    User Avatar
    Gold Member

    Thiemann has always been a constructionist of new mathematical machinery. He is indeed making progress in the construction of more satisfactory coherent states that allows for a more balanced treatment of the holonomy and flux variables. As I understand there are remain obstructions to the completion of this project.

    I wonder to what extent this construction will eventually address old outstanding problems with his previous attempts of the construction of coherent states which suffered the following issues:

    (i) The semi-classical machinery developed was only appropriate to non-graph-changing operators, however, Thiemann's Hamiltonian constraint is a graph-changing operator - the new graph it generates has degrees of freedom upon which the coherent state does not depend and so their quantum fluctuations are not suppressed.

    (i) There is also the restriction, as far as I know, that these coherent states are only defined at the Kimematic level, and now one has to lift them to the level of the Diff and Phys Hilbert space

    which prevented a proper evaluation of the semi-classical limit of canonical LQG. Will his/their new construction address or circumvention these issues in some sense? What then would constitute a successful establishment of LQG having the correct semi-classical limit in the context of their new construction? What physical applications could this new construction have? Have other researchers started looking into this question? Perhaps these reasons and the obstructions and because it is so recent is why it is not prominent there in the talks? Even though as we know Thiemann et al are always people to watch...
     
    Last edited: Jan 24, 2015
  20. Jan 24, 2015 #19

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It's a good question to be asking! Thiemann will deliver his message the week before:
    http://www.gravity.physik.fau.de/events/cosmo2015/cosmo2015.shtml
    there will be some partial overlap of participants and topics between this workshop at Erlangen 9-13 Feb and the conference at Tux 16-20 Feb
    http://www.gravity.physik.fau.de/events/tux3/tux3.shtml
    Invited speakers at the Cosmology and Quantum Gravity workshop include:
    Ivan Agullo (Louisiana State University)
    Abhay Ashtekar (Pennsylvania State University)
    Latham Boyle (Perimeter Institute)
    Edmund Copeland (University of Nottingham)
    Raphael Flauger (Princeton University)
    Steffen Gielen (Imperial College London)
    Jean-Luc Lehners (Albert Einstein Institute Golm)
    Thomas Thiemann (University of Erlangen-Nürnberg)
    Edward Wilson-Ewing (Albert Einstein Institute Golm)

    It seems like a good idea to put energy into a "Cosmology and Quantum Gravity" meeting because there is a strong trend (I sense) away from speculative untestable paradigms involving stuff we can't see and that may be made up. Cosmology is richly observational and it will be good for QG as a field to join in with Cosmology. the two lines of research have implications for each other. It's time for QG to be seen as empirical, as observation-based.

    There was that strongly worded article in Nature recently, by George Ellis and Joe Silk.
    Scientific method: Defend the integrity of physics. You probably read it and may have already commented, Julian. If anyone wants it
    google: "Ellis Silk integrity physics Nature". That should get it.

    Basically IMHO for Lanery and Thiemann's reformulation of LQG to succeed in a real sense, it should go all the way and get rid of the cosmological singularity---which most likely means predicting a bounce, as you go back in time. And when you put the cosmological constant and dark matter and radiation in, then it should make some predictions we can check about early universe things we can see.
     
    Last edited by a moderator: Jan 25, 2015
  21. Jan 24, 2015 #20

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Julian, I'm glad you referred to the Lanery&Thiemann Projective LQG work which is one of the six research directions I especially interested in watching.
    The program for the Cosmology&QG Workshop 9-13 February has not been posted yet. But to me it seems likely that TWO of our themes will surface there.
    Thiemann is giving a talk (which could very well be about Projective LQG) and also there is to be one by Latham Boyle.
    Boyle has had dealings with the Connes Chamseddine Spectral Geometry and Standard Model stuff
    http://arxiv.org/abs/1408.5367
    Rethinking Connes' approach to the standard model of particle physics via non-commutative geometry
    http://arxiv.org/abs/1401.5083
    Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
    http://arxiv.org/abs/1303.1782
    Non-Associative Geometry and the Spectral Action Principle
    So it's hardly a sure thing but he might touch on what I called "algebraic geomatter".

    Having seen the EFI Winter School program we know that 4 of the six are on the agenda at Tux 16-20 February
    and it's not impossible that the other 2 will be come up at the CosmologyQG Workshop at Erlangen 9-13 February.
    I'm optimistic :w
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Six themes for QG in 2015 (developments to watch for)
  1. A project on QG (Replies: 2)

  2. Toe = Qg? (Replies: 14)

  3. Experiments for QG (Replies: 2)

  4. Experimental QG (Replies: 1)

Loading...