View Poll Results: Which talk(s) offered most valuable input to future research?  
The Feynman diagramatics for the spin foam models  1  14.29%  
Loop Gravity as the Dynamics of Topological Defects  3  42.86%  
Inhomogeneous loop quantum cosmology  1  14.29%  
Shape dynamics  4  57.14%  
Plebanski sectors of the new spin foam models  0  0%  
Canonical Time Evolution in Simplicial Gravity  1  14.29%  
Group Field Theory and Simplicial Path Integrals  0  0%  
Scalar Perturbations in Loop Quantum Cosmology  0  0%  
Continuous Formulation of the Loop Quantum Gravity Phase Space  0  0%  
Classical limit of spin foams on arbitrary triangulations  0  0%  
Fractal Spacetimes Under the Microscope: a RG View on Monte Carlo Data  2  28.57%  
Continuous formulation of the LQG phase space  2  28.57%  
Spinor Quantisation for Complex Ashtekar Variables  1  14.29%  
Black hole evaporation in LQG  1  14.29%  
Black hole entropy in LQG: new insights from a local perspective  1  14.29%  
Multiple Choice Poll. Voters: 7. You may not vote on this poll 
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Our picks for first quarter 2012 MIRS (online QG seminar talks) 
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#1
Mar2812, 06:16 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

Which talks' input will contribute most to future research in Loopandallied quantum gravity?
Since the poll is multiple choice, it's possible to vote for several, if you choose. Abstract summaries follow in the next post. PIRSA talks are online video. The ILQGS links are to the slides PDF. To get the online audio, replace .pdf by .wav in the URL. You will then be able to listen to the talk while you scroll through the slides. ILQGS Sept. 20 http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf The Feynman diagramatics for the spin foam models Jacek Puchta, University of Warszaw PIRSA Sept. 21 http://pirsa.org/11090125/ Loop Gravity as the Dynamics of Topological Defects Eugenio Bianchi, Perimeter Institute ILQGS Oct. 18 http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf Inhomogeneous loop quantum cosmology William Nelson, PennState ILQGS Nov. 1 http://relativity.phys.lsu.edu/ilqgs...wski110111.pdf Shape dynamics Tim Koslowski, Perimeter Institute ILQGS Nov. 15 http://relativity.phys.lsu.edu/ilqgs/engle111511.pdf Plebanski sectors of the new spin foam models Jonathan Engle, Florida Atlantic Univesrity PIRSA Dec. 7 http://pirsa.org/11120050/ Canonical Time Evolution in Simplicial Gravity Philipp Hoehn, Utrecht to Perimeter Institute PIRSA Jan. 11 http://pirsa.org/12010131/ Group Field Theory and Simplicial Path Integrals Aristide Baratin, AEI PIRSA Jan. 18 http://pirsa.org/12010115/ Scalar Perturbations in Loop Quantum Cosmology Edward WilsonEwing, Marseille to LSU PIRSA Feb. 1 http://pirsa.org/12020096/ Continuous Formulation of the Loop Quantum Gravity Phase Space Jonathan Ziprick, Perimeter Institute ILQGS Feb. 14 http://relativity.phys.lsu.edu/ilqgs/perini021412.pdf Classical limit of spin foams on arbitrary triangulations Claudio Perini, PennState PIRSA Feb. 15 http://pirsa.org/12020088/ Fractal Spacetimes Under the Microscope: a RG View on Monte Carlo Data Frank Saueressig, Univ. Mainz ILQGS Feb. 28 http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf Continuous formulation of the LQG phase space Marc Geiller, Univ. Paris PIRSA Feb. 29 http://pirsa.org/12020129/ Spinor Quantisation for Complex Ashtekar Variables Wolfgang Wieland, Marseille ILQGS Mar. 13 http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.pdf Black hole evaporation in LQG Jacobo DiazPolo, LSU ILQGS Mar. 27 http://relativity.phys.lsu.edu/ilqgs/perez032712.pdf Black hole entropy in LQG: new insights from a local perspective Alejandro Perez, Marseille ============= For reference, here's a link to 2011's 4th qtr MIP poll (MIP="Most Important Paper(s)" in the indicated field of research.) http://physicsforums.com/showthread.php?t=563724 This quarter, for a change we're considering online Recorded Seminar talks, instead of papers. So instead of MIP it's a MIRS poll. For those unfamiliar with the online seminar series, PIRSA stands for "Perimeter Institute Recorded Seminar Archive" and ILQGS stands for "International Loop Quantum Gravity Seminar". The ILQGS is held at several locations simultaneously via teleconferencing connection. 


#2
Mar2812, 06:49 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

This is intended as a semiannual poll ( twice a year, covering recorded seminar talks from the previous 6 months or so.)
Not all the talks are listed with abstracts at their respective sites, so where possible I'll retrieve the relevant abstract from the main source paper the talk is based on. ILQGS Sept. 20 http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf The Feynman diagramatics for the spin foam models Jacek Puchta, University of Warszaw [paper's abstract: Feynman diagrammatic approach to spin foams"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.] PIRSA Sept. 21 http://pirsa.org/11090125/ Loop Gravity as the Dynamics of Topological Defects Eugenio Bianchi, Perimeter Institute A charged particle can detect the presence of a magnetic field confined into a solenoid. The strength of the effect depends only on the phase shift experienced by the particle's wave function, as dictated by the Wilson loop of the Maxwell connection around the solenoid. In this seminar I'll show that Loop Gravity has a structure analogous to the one relevant in the AharonovBohm effect described above: it is a quantum theory of connections with curvature vanishing everywhere, except on a 1d network of topological defects. Loop states measure the flux of the gravitational magnetic field through a defect line. A feature of this reformulation is that the space of states of Loop Gravity can be derived from an ordinary QFT quantization of a classical diffeomorphisminvariant theory defined on a manifold. I'll discuss the role quantum geometry operators play in this picture, and the perspective of formulating the Spin Foam dynamics as the local interaction of topological defects. ILQGS Oct. 18 http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf Inhomogeneous loop quantum cosmology William Nelson, PennState [abstract of related talk based on the same research (invited talk to be given by Nelson's coauthor Ivan Agullo at the April meeting of the APS): Beyond the standard inflationary paradigmThe inflationary paradigm provides a compelling argument to account for the origin of the cosmic inhomogeneities that we observe in the CMB and galaxy distribution. In this talk we introduce a completion of the inflationary paradigm from a (loop) quantum gravity point of view, by addressing gravitational issues that have been open both for the background geometry and perturbations. These include a quantum gravity treatment of the Planck regime from which inflation arises, and a clarification of what the transPlanckian problems are and what they are not. In addition, this approach provides examples of effects that may have observational implications, that may provide a window to test the basic quantum gravity principles employed here.] ILQGS Nov. 1 http://relativity.phys.lsu.edu/ilqgs...wski110111.pdf Shape dynamics Tim Koslowski, Perimeter Institute [based on several papers including: Coupling Shape Dynamics to Matter Gives SpacetimeShape Dynamics is a metric theory of pure gravity, equivalent to General Relativity, but formulated as a gauge theory of spatial diffeomorphisms and local spatial conformal transformations. In this paper we extend the construction of Shape Dynamics form pure gravity to gravitymatter systems and find that there is no obstruction for the coupling of gravity to standard matter. We use the matter gravity system to construct a clock and rod model for Shape Dynamics which allows us to recover a spacetime interpretation of Shape Dynamics trajectories.] ILQGS Nov. 15 http://relativity.phys.lsu.edu/ilqgs/engle111511.pdf Plebanski sectors of the new spin foam models Jonathan Engle, Florida Atlantic University [paper's abstract: A proposed proper EPRL vertex amplitudeAs established in a prior work of the author, the linear simplicity constraints used in the construction of the socalled 'new' spinfoam models mix three of the five sectors of Plebanski theory, only one of which is gravity in the usual sense, and this is the reason for certain 'unwanted' terms in the asymptotics of the EPRL vertex amplitude as calculated by Barrett et al. In the present paper, an explicit classical discrete condition is derived that isolates the desired gravitational sector, which we call (II+), following other authors. This condition is quantized and used to modify the vertex amplitude, yielding what we call the 'proper EPRL vertex amplitude.' This vertex still depends only on standard SU(2) spinnetwork data on the boundary, is SU(2) gauge invariant, and is linear in the boundary state, as required. In addition, the asymptotics now consist in the single desired term of the form e^{iSRegge}, and all degenerate configurations are exponentially suppressed.] PIRSA Dec. 7 http://pirsa.org/11120050/ Canonical Time Evolution in Simplicial Gravity Philipp Hoehn, Utrecht to Perimeter Institute [paper's abstract: Canonical simplicial gravityA general canonical formalism for discrete systems is developed which can handle varying phase space dimensions and constraints. The central ingredient is Hamilton's principle function which generates canonical time evolution and ensures that the canonical formalism reproduces the dynamics of the covariant formulation following directly from the action. We apply this formalism to simplicial gravity and (Euclidean) Regge calculus, in particular. A discrete forward/backward evolution is realized by gluing/removing single simplices step by step to/from a bulk triangulation and amounts to Pachner moves in the triangulated hypersurfaces. As a result, the hypersurfaces evolve in a discrete `multifingered' time through the full Regge solution. Pachner moves are an elementary and ergodic class of homeomorphisms and generically change the number of variables, but can be implemented as canonical transformations on naturally extended phase spaces. Some moves introduce a priori free data which, however, may become fixed a posteriori by constraints arising in subsequent moves. The end result is a general and fully consistent formulation of canonical Regge calculus, thereby removing a longstanding obstacle in connecting covariant simplicial gravity models to canonical frameworks. The present scheme is, therefore, interesting in view of many approaches to quantum gravity, but may also prove useful for numerical implementations.] PIRSA Jan. 11 http://pirsa.org/12010131/ Group Field Theory and Simplicial Path Integrals Aristide Baratin, AEI [paper's abstract: Group field theory and simplicial gravity path integralsIn a recent work, a dual formulation of group field theories as noncommutative quantum field theories has been proposed, providing an exact duality between spin foam models and noncommutative simplicial path integrals for constrained BF theories. In light of this new framework, we define a model for 4d gravity which includes the Immirzi parameter gamma. It reproduces the BarrettCrane amplitudes when gamma goes to infinity, but differs from existing models otherwise; in particular it does not require any rationality condition for gamma. We formulate the amplitudes both as BF simplicial path integrals with explicit noncommutative B variables, and in spin foam form in terms of Wigner 15jsymbols. Finally, we briefly discuss the correlation between neighboring simplices, often argued to be a problematic feature, for example, in the BarrettCrane model.] PIRSA Jan. 18 http://pirsa.org/12010115/ Scalar Perturbations in Loop Quantum Cosmology Edward WilsonEwing, Marseille to LSU We study the dynamics of the scalar modes of linear perturbations around a flat, homogeneous and isotropic background in loop quantum cosmology. PIRSA Feb. 1 http://pirsa.org/12020096/ Continuous Formulation of the Loop Quantum Gravity Phase Space Jonathan Ziprick, Perimeter Institute We relate the discrete classical phase space of loop gravity to the continuous phase space of general relativity. Our construction shows that the flux variables do not label a unique geometry, but rather a class of gaugeequivalent geometries. We resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewiseflat geometry, showing that both geometries belong to the same equivalence class. We also establish a clear relationship between Regge geometries and the piecewiseflat spin foam geometries. All of this is based on arXiv:1110.4833. ILQGS Feb. 14 http://relativity.phys.lsu.edu/ilqgs/perini021412.pdf Classical limit of spin foams on arbitrary triangulations Claudio Perini, PennState [earlier paper's abstract: Emergence of gravity from spinfoamsWe find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma) and large spins (j), with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.] PIRSA Feb. 15 http://pirsa.org/12020088/ Fractal Spacetimes Under the Microscope: a RG View on Monte Carlo Data Frank Saueressig, Univ. Mainz The emergence of fractal features in the microscopic structure of spacetime is a common theme in many approaches to quantum gravity. In particular the spectral dimension, which measures the return probability of a fictitious diffusion process on spacetime, provides a valuable probe which is easily accessible both in the continuum functional renormalization group and discrete Monte Carlo simulations of the gravitational action. In this talk, I will give a detailed exposition of the fractal properties associated with the effective spacetimes of asymptotically safe Quantum Einstein Gravity (QEG). Comparing these continuum results to threedimensional Monte Carlo simulations, we demonstrate that the resulting spectral dimensions are in very good agreement. This comparison also provides a natural explanation for the apparent conflicts between the short distance behavior of the spectral dimension reported from Causal Dynamical Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic Safety. ILQGS Feb. 28 http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf Continuous formulation of the LQG phase space Marc Geiller, Univ. Paris [paper's abstract: Continuous formulation of the Loop Quantum Gravity phase space In this paper, we study the discrete classical phase space of loop gravity, which is expressed in terms of the holonomyflux variables, and show how it is related to the continuous phase space of general relativity. In particular, we prove an isomorphism between the loop gravity discrete phase space and the symplectic reduction of the continuous phase space with respect to a flatness constraint. This gives for the first time a precise relationship between the continuum and holonomyflux variables. Our construction shows that the fluxes depend on the threegeometry, but also explicitly on the connection, explaining their non commutativity. It also clearly shows that the flux variables do not label a unique geometry, but rather a class of gaugeequivalent geometries. This allows us to resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewise flat geometry, since we establish that both geometries belong to the same equivalence class. This finally gives us a clear understanding of the relationship between the piecewise flat spin foam geometries and Regge geometries, which are only piecewiselinear flat: While Regge geometry corresponds to metrics whose curvature is concentrated around straight edges, the loop gravity geometry correspond to metrics whose curvature is concentrated around not necessarily straight edges.] PIRSA Feb. 29 http://pirsa.org/12020129/ Spinor Quantisation for Complex Ashtekar Variables Wolfgang Wieland, Marseille During the last couple of years Dupuis, Freidel, Livine, Speziale and Tambornino developed a twistorial formulation for loop quantum gravity. Constructed from AshtekarBarbero variables, the formalism is restricted to SU(2) gauge transformations. In this talk, I perform the generalisation to the full Lorentzian case, that is the group SL(2,C). The phase space of SL(2,C) (i.e. complex or selfdual) Ashtekar variables on a spinnetwork graph is decomposed in terms of twistorial variables. To every link there are two twistorsone to each boundary pointattached. The formalism provides a clean derivation of the solution space of the reality conditions of loop quantum gravity. Key features of the EPRL spinfoam model are perfectly recovered. If there is still time, I'll sketch my current project concerning a twistorial path integral for spinfoam gravity as well. ILQGS Mar. 13 http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.pdf Black hole evaporation in LQG Jacobo DiazPolo, LSU [paper's abstract:Probing Loop Quantum Gravity with Evaporating Black Holes This letter aims at showing that the observation of evaporating black holes should allow distinguishing between the usual Hawking behavior and Loop Quantum Gravity (LQG) expectations. We present a full MonteCarlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.] ILQGS Mar. 27 http://relativity.phys.lsu.edu/ilqgs/perez032712.pdf Black hole entropy in LQG: new insights from a local perspective Alejandro Perez, Marseille [paper's abstract: A local first law for black hole thermodynamicsWe first show that stationary black holes satisfy an extremely simple local form of the first law ∂ E=κ(l) ∂ A/(8 π) where the thermodynamical energy E=A/(8π l) and (local) surface gravity κ(l)=1/l, where A is the horizon area and l is a proper length characterizing the distance to the horizon of a preferred family of local observers suitable for thermodynamical considerations. Our construction is extended to the more general framework of isolated horizons. The local surface gravity is universal. This has important implications for semiclassical considerations of black hole physics as well as for the fundamental quantum description arising in the context of loop quantum gravity.] NOTE the first 6 minutes of the Perez audio are spoiled by noise so I would advise dragging the time button to around 6:00 or 6:30 when you start it. This corresponds to around the slide numbered (2). The first part of the talk is a review, so missing the first few minutes need not be critical. 


#3
Mar2812, 09:12 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

Already we have 3 respondents to the poll, and I just barely got through editing a final copy of the abstracts! Thanks for the response. I learn from seeing other's perspective and what others find the most promising lines of research. Hopefully before long we'll see a bunch of different viewpoints represented.



#4
Mar2812, 09:21 PM

PF Gold
P: 1,963

Our picks for first quarter 2012 MIRS (online QG seminar talks)
I voted for things that look like non mainstream and weird as possible. LQG and its friends are almost mainstream.
I mean, if nothing seems to be working, at least it feels good to be underground or counterculture! And shape dynamics has a preferential foliation, which gives me hope to use it as a way to build a FTL spacecraft someday. I mean, if you are bound to a foliation, you won't violate causality no matter how fast you go, you have just to find a warp mechanism. 


#5
Mar2912, 10:07 AM

PF Gold
P: 1,963

I am pretty sure marcus answered me and the post stayed long enough to not allow it being deleted. Is there a problem with PF forum's server?



#6
Mar2912, 10:40 AM

Astronomy
Sci Advisor
PF Gold
P: 23,274

BTW I was glad to see that we both picked Frank Saueressig's PIRSA talk on the Asymptotic Safe approach. For me Frank S. now seems a very effective spokesman for Safe QG, on par with Reuter. He is also making strong progress in research. He has the advantage of being young, in the normally best years. People should watch that video! He gives a fast introduction to what Safe QG is, in the first part, and why it may be true to natureit would be a good introduction for anyone who is unfamiliar and wants to find out: wellorganized and clear. Then later in the talk he discusses the fact that dimensionality decays at small scale (not only in Safe but also in CDT and possibly Loop as well). We know this about Safe and CDT since 2005 (both Reuter and Loll gave plenary talks describing it at the Loops 2005 conference!). Also as I think you know Steve Carlip has explained why there are even hints of this in *classical* GR. The classical theory, as he interprets it, goes some way towards supporting the decay that is found the nonstring 4d quantum theories of geometry. ================== Again I should stress, and I think you probably agree, that talk of Saueressig's was excellent! People should watch it if at all interested in Safe QG. Also I think it's excellent that a talk can appeal to one person because of edgy "farout" qualities and to someone else it can appeal because of solid mainstreamness. I think of Asym Safe QG as one of the most mainstream quantum extensions of GR. It may even be more widely accepted than Loop. (And the decay of dimensionality down from 4d to around 2d at small scale having been around since 2005 is no longer so shocking for me.) ===================== EDIT: Oooops! I would have voted for Saueressig's talk but somehow omitted it when I was voting. I thought I had. It was definitely one of the best talks. I had too much to do yesterday and was rushing from one thing to another. I was glad to see that you voted for the talk. Not only insightful results and strong innovative research technique, but excellent presentation. He should probably become a "standard bearer" for the RGflow approach to QG. 


#7
Mar2912, 01:28 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

Sabine Hossenfelder discusses Safe QG starting on page 40 of her monograph on the minimal length scale http://arxiv.org/abs/1203.6191 (Living Reviews).
This theme (the limit to the resolution of structure that arises in various ways in several theoretical approaches to understanding spacetime) allows her to treat several major approaches on the same footing: String, Loop, Safe, and Spectral ("noncommutative geometry"). It's an important document. Evenhanded uncommitted but insightful style of the good phenomenologist. She explains why you get a minimal scale of resolution in Safe QG too, as well as in other leading approaches. Some people apparently don't like Reuter's name for it and have tried referring to it in other ways in their titles. Cai Easson: "Higgs Boson in RG running Inflationary Cosmology" (1202.1285) Donkin Pawlowski: "The phase diagram of quantum gravity from diffeomorphisminvariant RGflows" (1203.4207) And for whatever reason, Saueressig himself does not headline the usual nomenclature in the title of his PIRSA talk: "Fractal Spacetimes Under the Microscope: a RG View on Monte Carlo Data" (12020088) It seems that if your title includes a hint like "cosmology", "spacetime" or "quantum gravity" so people know you are talking about geometry, then you do not need to use clunky language like *asymptotic safety* or *quantum Einstein gravity* (Weinbergly or Reuterian phrases). You can just put in a tag like "RG running", "RGflows" or "RG view", as these authors do, and people will know what you mean. 


#8
Mar2912, 02:00 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

DiazPolo's talk is another important one because identifies a new way that Loop gravity can be tested by astrophysical observations. Distinctive radiation from evaporating black holes.
The paper it's based on is http://arxiv.org/abs/1109.4239 They find characteristic differences from the radiation spectrum Hawking predicted. Anything that opens a new route to testing QG is potentially significant. ================ Anybody with an interest in Loop should watch Wieland's talk. It's pretty amazing. I was impressed (I guess you can tell ) The paper it's based on is: http://arxiv.org/abs/1107.5002 Twistorial phase space for complex Ashtekar variables Wolfgang M. Wieland (Submitted on 25 Jul 2011 (v1), last revised 24 Jan 2012 (this version, v2)) We generalise the SU(2) spinor framework of twisted geometries developed by Dupuis, Freidel, Livine, Speziale and Tambornino to the Lorentzian case, that is the group SL(2,C). We show that the phase space for complex valued Ashtekar variables on a spinnetwork graph can be decomposed in terms of twistorial variables. To every link there are two twistorsone to each boundary pointattached. The formalism provides a new derivation of the solution space of the simplicity constraints of loop quantum gravity. Key properties of the EPRL spinfoam model are perfectly recovered. 18 pages, to appear in Classical and Quantum Gravity 


#9
Mar3012, 09:10 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

So far Koslowski (Shape Dynamics) and Bianchi (Dynamics of Topological Defects) talks have taken the lead with Saueressig's talk on Asymptotic Safe QG running third.
6 people have voted, with a total of 16 votes cast. Thanks to all who have responded so far! Interestingly, Bianchi's talk (in second place) is about a new way to formulate Loop geometry (curvature concentrated on the boundaries between flat cells, the socalled "defects" where cells join, so the geometry "lives" on these defects, so to speak, and the dynamics of the defects captures the whole story.) And this new formulation is drawn on in the research that both Ziprick and Geiller describe in their talks. 


#10
Apr112, 12:59 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

So far the poll is interesting for me partly because it goes against my personal assessment of where QG presently stands.
Of course we can't know the future, especially of research, so we can't tell what gambits/initiatives will pay off. But I have a strong hunch that Puchta's talk is the most breakthrough landmark one: The Feynman diagrammatics for the spin foam models http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf http://relativity.phys.lsu.edu/ilqgs/puchta092011.wav Of course I voted for several others besides, which I see as promising but secondary. But Puchta's is my strong favorite. Maybe I will try to explain why that one stands out. (Actually it would be interesting if some of the other respondents would explain their choices. MTd2 already has, which is a plus.) The key thingyou may need to think about it some before you realize why it is so importantis that to each of a large class of spinfoams it associates a unique labeled graph. Each graph can be "contracted" to give the amplitude of that "path"the amplitude associated with the original spinfoam jumps out, you don't have to dig for it. Now in contrast to foams, graphs can be straightforwardly enumerated. I have not followed thru enough to be sure but it looks to me that this opens the door to a true path integral. In other words you can choose two quantum states of geometryInitial and Final geometrieseach represented by a network graph. And you can generate by computer algorithm all the spinfoams* that evolve the geometry from one state to the other. And the process for generating each of those spinfoam "paths", that geometry could take, gives you an unique labeled operator graph associated with itand the amplitude of the path jumps out at you (by a straightforward contraction of the labels along the graph.) So you can sum the amplitudes of all the paths going from one geometric state to the other** and have a "sum over histories". *of a large class which the Warsaw people think is big enough, this is a question that needs to be settled. ** at least up to some cutoff level of complexity, the ultimate convergence issue also needs to be settled. I guess the basic point here is that you can describe each 2complex by a graph. I think of that as nontrivial mathematically, something I did not expect. Graphs are combinatorially simpler than 2complexes, and easier for a blind automata to enumerate systematically. The Warsaw people did something quite unexpected, and this stands out prominently. 


#11
Apr112, 04:51 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

To be more specific, here is the key passage from the Warsaw group's paper, page 2:
B. Our goal  the spin network diagramsThe point is that from a given operator spin network, a labeled graph in other words, one can derive the (presumably unique) foam (i.e. labeled 2complex) associated with it. And each operator spin network comes with a readily accessible probability amplitude. So if all works out as they expect (it's new) they can enumerate all the graphs representing paths from Initial to Final geometry, and add up their amplitudes. Bingo, a path integral. If you want to see the paper that Puchta's talk is based on, and that quoted here, you can simply google "arxiv feynman puchta" and get: http://arxiv.org/abs/1107.5185 Feynman diagrammatic approach to spin foams Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta 


#12
Apr112, 09:55 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

I got a PM that raised a question about William Nelson's talk the significance of which I ventured to discuss a little bit in Cosmology forum:
http://physicsforums.com/showthread....24#post3842724 So I will respond to the general thrust of the question here in case anyone else is interested. The point is that based on some work by Ivan Agullo and Leonard Parker (Harvard Phd 1966?) some of the structure BEFORE inflation comes thru due to stimulated particle creation. So inflation does not wipe the slate clean. The inflationary period (if it occurred as is commonly supposed) is, in a sense, transparent. We can see (signs of) the density map of what came before. This is of considerable interest to Loop cosmology researchers because what came before, a bounce and a brief period of naturally fasterthanexponential expansion (called "superinflation"), is of considerable interest to them. So in the Loop case we may be able to see back before inflation. And Ivan Agullo was invited to give a talk about this at the April meeting of the American Physical Society. 


#13
Apr112, 10:00 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

I got a PM today that raised a question about William Nelson's talk the significance of which I ventured to discuss a little bit in Cosmology forum:
http://physicsforums.com/showthread....24#post3842724 So I will respond to the general thrust of the question here. The point is that based on some work by Ivan Agullo and Leonard Parker (Harvard Phd 1966?) some of the structure BEFORE inflation comes thru due to stimulated particle creation. So inflation does not wipe the slate clean. The inflationary period (if it occurred as is commonly supposed) is, in a sense, transparent. We can see (signs of) the density map of what came before. This is of considerable interest to Loop cosmology researchers because what came before, a bounce and a brief period of naturally fasterthanexponential expansion (called "superinflation"), is of considerable interest to them. So in the Loop case we may be able to see back before inflation. And Ivan Agullo was invited to give a talk about this at the April meeting of the American Physical Society. http://physicsforums.com/showthread....76#post3842676 


#14
Apr512, 09:32 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

A paper came out today by Agullo, Ashtekar, and Nelson that gives us something else to use in understanding William Nelson's talk. Because this paper had not come out yet, I used the abstract of a talk that Nelson's coauthor Ivan Agullo just gave a few days ago at the Atlanta meeting of the American Physical Society.
Now we have the paper itself and can do an update. This is what we had earlier: ILQGS Oct. 18 http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf Inhomogeneous loop quantum cosmology William Nelson, PennState [abstract of related talk based on the same research (invited talk to be given by Nelson's coauthor Ivan Agullo at the April meeting of the APS): Beyond the standard inflationary paradigmThe inflationary paradigm provides a compelling argument to account for the origin of the cosmic inhomogeneities that we observe in the CMB and galaxy distribution. In this talk we introduce a completion of the inflationary paradigm from a (loop) quantum gravity point of view, by addressing gravitational issues that have been open both for the background geometry and perturbations. These include a quantum gravity treatment of the Planck regime from which inflation arises, and a clarification of what the transPlanckian problems are and what they are not. In addition, this approach provides examples of effects that may have observational implications, that may provide a window to test the basic quantum gravity principles employed here.] Now in addition to that we have the actual A,A,and N paper: http://arxiv.org/abs/1204.1288 Perturbations in loop quantum cosmology Ivan Agullo, Abhay Ashtekar, William Nelson (Submitted on 5 Apr 2012) The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB. 4 pages, ICGC (2011) Goa Conference proceedings Nelson's talk had the same title as the paper Perturbations in loop quantum cosmology (which had not come out yet) and the talk was clearly based on it. But the talk is actually wide ranging and hits several different areas of investigation listed on one of his first slides: *INFLATION (focus of Agullo's talk at aPS. the fact that preinflation inhomogeneities can persist even thru inflation) *HAMILTONIAN PERTURBATIONS IN COSMOLOGY *QFT IN QUANTUM (COSMOLOGICAL) SPACETIMES *PERTURBATIONS IN LQC 


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