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Introduction To Loop Quantum Gravityby marlon
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#289
Nov311, 09:17 PM

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PF Gold
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First introductory LQG textbook went on sale 1 November 2011.
I checked today to see how it's doing. Earier books have covered related topics: canonical GR, application to cosmology etc. this is first wholly devoted to intro Loop. A First Course in Loop Quantum Gravity Rodolfo Gambini, Jorge Pullin Oxford University Press. http://www.amazon.com/FirstCourseL.../dp/0199590753 I found several thingsone is that you get to browse online. You can look inside the book. Another is that at least at 7PM Pacific time 3 November it is doing rather well for a textbook: Amazon Best Sellers Rank: #62,638 in Books #26 in Books > Professional & Technical > Professional Science > Physics > Mathematical Physics So it is #26 on the Mathematical Physics bestseller list. Here's the Oxford Press description: This book provides an accessible introduction to loop quantum gravity and some of its applications, at a level suitable for undergraduate students and others with only a minimal knowledge of college level physics. In particular it is not assumed that the reader is familiar with general relativity and only minimally familiar with quantum mechanics and Hamiltonian mechanics. Most chapters end with problems that elaborate on the text, and aid learning. Applications such as loop quantum cosmology, black hole entropy and spin foams are briefly covered. The text is ideally suited for an undergraduate course in the senior year of a physics major. It can also be used to introduce undergraduates to general relativity and quantum field theory as part of a 'special topics' type of course.Masters program at Nottingham, including Loop and other QG research http://pgstudy.nottingham.ac.uk/post...msc_1163.aspx Some papers: Loop quantum gravity: the first twenty five years Survey for nonspecialists, historical development and indications the theory recovers classical GR. http://arxiv.org/abs/1012.4707 Lectures on Loop Gravity Definitive presentation of Loop gravity, with some math prerequisites: http://arxiv.org/abs/1102.3660 The Big Bang and the Quantum Loop cosmology, as of mid2010: http://arxiv.org/abs/1005.5491. Testing: Loop cosmology derives from LQG and this provides the most accessible route to testing. Here are articles mostly by early universe phenomenologists about ways of testing LQC against CMB data, http://wwwlibrary.desy.de/cgibin/s...tecount%28d%29 These articles appeared 2009 or later. As of 3 November there were 45 of them. Loops 2011 the biannual Loops conference was held in Madrid. http://www.iem.csic.es/loops11/ Click on "scientific program" in the menu on the left to see the list of talks. Abstracts and slides pdf of most talks are now available, just click on the title. Videos of the plenary session morning talks are online. There were about 100 parallel session talks. ========================= Research trends, as of 3 November: http://howlonguntil.net/ day 307 162*365/307 = 193 816*365/307 = 970 LOOP RESEARCH BY YEAR (loop quantum gravity, loop quantum cosmology, spin foam) 2005 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (42 found) 2006 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (77 found) 2007 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (120 found) 2008 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (142 found) 2009 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (145 found) 2010 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (152 found) 2011 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (162 found, annualized 193) STRING,MEMBRANE,AdS/CFT RESEARCH BY YEAR (search terms "string model", "membrane model" and "AdS/CFT correspondence") 2005 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (988 found) 2006 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1029 found) 2007 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1050 found) 2008 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1128 found) 2009 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1132 found) 2010 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1046 found) 2011 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (816 found, annualized 970) 


#290
Nov1311, 10:25 PM

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There is currently a broken link in the Nottingham webpage given in the previous post:
Masters program at Nottingham, including Loop and other QG research http://pgstudy.nottingham.ac.uk/post...msc_1163.aspx The link to "the Quantum Gravity group" near the bottom of the page should be to: http://www.nottingham.ac.uk/mathemat...mgravity.aspx ======================================= Francesca has created a world map of places where LQG research is being done or where you can do a PhD in Loop. http://maps.google.com/maps/ms?ie=UT...e6c50e&t=h&z=0 It does not have UC Davis but on the whole it is remarkably complete. It's hard to keep such a map up to date. As confirmation that Carlip is advising LQG research as well as other QG, see: http://arxiv.org/pdf/1111.2107v1 


#291
Nov2111, 08:22 PM

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I want to assemble some information and links about the Quantum Geometry/Gravity program at Erlangen. It is getting to be quite a strong program. They have TWO QG groups, one in the Physics department led by Thiemann and now a second group in the Math department led by Meusburger. The intent is to have several postdocs in each, plus PhD students.
Here is about Meusburger: http://www.algeo.math.unierlangen.d...rchgroup.html A postdoc in Meusburger's group is Winston Fairbairn whom we know of as a Rovelli PhD and coauthor. There is another postdoc opening. Prof Meusburger recently sent out this email announcement: ==excerpt== A postdoc position will be available in the quantum gravity group within the algebra and geometry group at the Department of Mathematics, University of ErlangenNürnberg in Erlangen, Germany... ==endquote== for more information: http://www.algeo.math.unierlangen.d...positions.html Here is about the quantum gravity group in the Physics Department led by Thiemann: http://theorie3.physik.unierlangen.de/people.html This group has grown by the addition of some strong people who have had experience with various "new models" approaches. Maďté Dupuis comes there from Lyon, for example. She has coauthored a lot with Etera Livine, who was her PhD advisor, and also one paper with Freidel. Enrique Borja, who has coauthored with Etera Livine (several) and Freidel (one) Emanuele Alesci, a Rovelli PhD and coauthor. It's also interesting that John Baez' student Derek Wise is there. αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔~≈≠≡ ≤≥˝∞(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅) 


#292
Nov2211, 07:19 PM

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Here are results of an InSpire search is designed to turn up observational testrelated Loop gravity/cosmology papers. In conjunction with LQG/LQC, the DESY search categories are gravitational radiation, inflation, power spectrum, cosmic background radiation, primordial.
Loop pheno papers (increasingly observational testrelated)
http://inspirehep.net/search?ln=en&l...100&sc=0&of=hb If we look at the earlier 4year stretch (20042007) using the same categories, we get 27 papers. http://inspirehep.net/search?ln=en&l...100&sc=0&of=hb Here's the same search for the previous 4 years (20002003 inclusive), 6 papers found. http://inspirehep.net/search?ln=en&l...100&sc=0&of=hb There is a small amount of overlap in the time segments because InSpire counts a paper as having appeared in a certain year, say 2006, if either it was published in professional journal in 2006 or posted as a preprint on arxiv in 2006. αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔ ~≈≠≡ ≤≥˝∞(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅) 


#293
Nov2511, 06:34 PM

#294
Jan2912, 01:08 AM

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One way to keep track of current developments in LQG is to watch the ILQGS online seminar list. I've shortened some titles for brevity.
ILQGS SEMINAR SCHEDULE SPRING 2012 (All seminars will held at 9:00 AM Central Time)
Audio files and slides PDF are made availble online at the ILQGS website. http://relativity.phys.lsu.edu/ilqgs/ The site's blog has commentary by other LQG researchers and background notes. Note that the 2/28 topic is the Freidel Geiller Ziprick paper, currently leading in our MIP poll. The 4/10 talk is on the topic of the current MIP runnerup. Two of the talks are concerned with triangulation independence, an important issue, and one deals with the classical limit of spinfoam LQG. Pretty much all are topics of current interest. ================== For convenience I'll gather some potentially useful links: Here is Rovelli's group at Marseille http://www.cpt.univmrs.fr/~quantumgravity/ Here's the QG bunch at Perimeter http://www.perimeterinstitute.ca/Sci...antum_Gravity/ Here is Thiemann's group at Erlangen http://theorie3.physik.unierlangen.de/people.html Here is Meusburger's group at Erlangen http://www.algeo.math.unierlangen.d...rchgroup.html Here is Dittrich's group at AEI http://www.aei.mpg.de/english/resear...ics/index.html http://www.aei.mpg.de/english/resear...bers/index.php Here is Oriti's group at AEI http://www.aei.mpg.de/english/resear...bers/index.php Livine's QG group at Lyon http://www.enslyon.fr/PHYSIQUE/inde...uspage=gravite Pullin's QG group at LSU is part of overall gravity physics, no separate page http://relativity.phys.lsu.edu/ Ashtekar's Institute for Gravitation and the Cosmos http://gravity.psu.edu/ see the QG part By now anyone who follows QG will be familiar with the strong research group at Marseille, so I won't list names there. Perimeter has a lot of wellknown people, among the younger researchers: Eugenio Bianchi, Valentin Bonzom, Razvan Gurau, Tim Koslowski, Cecelia Flori, Joseph Ben Geloun. Thiemann's group currently includes Maďté Dupuis, Derek Wise, Emanuele Alesci, Enrique Borja among others. Meusburger's group includes Winston Fairbairn. Dittrich's AEI group incudes Dario Benedetti, Song He, James Ryan, Sebastian Steinhaus (giving the ILQGS presentation this semester), Wojciech Kaminski,... Oriti's group includes Fotini Markopoulou (as visitor), Matteo Smerlak, Isabeau PremontSchwarz, Gianluca Calcagni, Lorenzo Sindoni,... Livine's group at Lyon includes Johannes Tambornino QG at LSU incudes Jorge Pullin, Parampreet Singh, and Jacobo DiazPolo I should also ennumerate the QG bunch at Penn State. There is a lot to keep track of. Maybe in a later post. αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔ ~≈≠≡ ≤≥˝∞⇐⇑⇒⇓⇔∴∃⋅ℝℤℕℂ 


#295
Jan2912, 01:34 PM

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To continue the above list:
In the above I should have noted that Kristina Giesel has joined the faculty of Thiemann's group at Erlangen. Also in the above Perimeter QG bunch I should have included Steffen Gielen. This month he moved from AEI (Dittrich group) to Perimeter. http://www.aei.mpg.de/~gielen/ Barrett's QG group at Nottingham http://www.nottingham.ac.uk/mathemat...mgravity.aspx (faculty: John Barrett, Kirill Krasnov, Jorma Louko) More detail on QG at Penn State http://cft.igc.psu.edu/ (besides many wellknown faculty, younger members include Elena Magliaro, Claudio Perini, William Nelson, Adam Henderson, Ivan AgulloDavid Sloan until recently of this group is now at Utrecht, Ed WilsonEwing has moved on to Marseille, Jon Engle is now faculty at FloridaAtlantic, Param Singh now faculty at LSU) I shouldn't forget to mention two divisions of the University of Paris: Orsay (Vincent Rivasseau is there) and ParisDiderot (Marc Geiller). Geiller is giving the February 28 ILQGS talk mentioned in the preceding post, on the Freidel Geiller Ziprick paper. Hanno Sahlmann has started a QG group at the AsiaPacific CPT (Pohang University of Science and Technology) Some resource links: Loops 2011 (many videos and slides pdf of the talks) http://www.iem.csic.es/loops11/ Undergrad LQG textbook http://www.amazon.com/FirstCourseL.../dp/0199590753 A First Course in Loop Quantum Gravity Rodolfo Gambini, Jorge Pullin Oxford University Press. Master's program at Nottingham http://pgstudy.nottingham.ac.uk/post...msc_1163.aspx Latest review papers http://arxiv.org/abs/1201.4598 Introduction to Loop Quantum Gravity (Abhay Ashtekar) and http://arxiv.org/abs/1108.0893 Loop Quantum Cosmology: A Status Report (Abhay Ashtekar, Parampreet Singh) Searches for Loop and String/M papers by year (results for 2011 now appear complete): LOOP RESEARCH BY YEAR (loop quantum gravity, loop quantum cosmology, spin foam) 2006 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (77 found) 2007 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (120 found) 2008 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (142 found) 2009 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (145 found) 2010 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (152 found) 2011 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb (203 found) 2012 http://inspirebeta.net/search?ln=en&...=25&sc=0&of=hb STRING,MEMBRANE,AdS/CFT RESEARCH BY YEAR (search terms "string model", "membrane model" and "AdS/CFT correspondence") 2006 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1029 found) 2007 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1050 found) 2008 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1128 found) 2009 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1133 found) 2010 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (1044 found) 2011 http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb (928 found) 2012 http://inspirehep.net/search?ln=en&a...=10&sc=0&of=hb A great series of Perimeter video lectures going back to 2003 PIRSA QUANTUM GRAVITY SERIES http://pirsa.org/S005 select "semester"=all and "year"=all. Among the more recent are talks by Eugenio Bianchi, Elena Magliaro, Edward WilsonEwing, Aristide Baratin, Philipp Höhn, Maďté Dupuis, Muxin Han, Matteo Smerlak This recent talk by Joseph Ben Geloun was for some reason not catalogued in the QG series http://pirsa.org/12010132/ so it does not show up in the above search. (topic GFT, tensor field theory) ινξςυφΓΘΛΞΠΣΦΨ⋅ℤℕ 


#296
Feb212, 04:23 PM

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Phenomenology issues affecting various models, which may already be ruled out, are part of the general QG phenomenology picture http://backreaction.blogspot.com/201...timefoam.html that we need to keep track of.
Although the observations discussed here seem not relevant to LQG specifically. In LQG space is not in a simple or naive sense "grainy". Its network states are quantum states of geometry (i.e. refer to relations among geometric measurements)they're not imagined to be "what space is made of". Lorentz violation such as energydependent speed of light or the blurring of images one might get with a grainy substance are not predictions. One way to say it is QG is not about what space IS but rather it is about HOW IT RESPONDS to measurement. Analogous to basic quantum theory in other areas. ==quote Bee, above link== ...the authors have presented an analysis of the images of 157 highredshift (z > 4) quasistellar objects. They found no blurring. With that, also the holographic foam model is ruled out. Or, to be precise, the parameter α is constrained into a range that is implausible for quantum gravitational effects. As it is often the case in the phenomenology of quantum gravity, the plausible models are difficult, if not impossible, to constrain by data. And the implausible ones nobody misses when they are ruled out. This is a case of the latter. ==endquote== 


#297
Feb212, 09:03 PM

P: 62

I read alot of Three Roads to Quantum Gravity last night. I found a free, legal, online pdf source if anyone wants to check it out too. Kinda for laymen, but it filled in the blanks for me :)



#298
Feb212, 11:17 PM

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PF Gold
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Three Roads is certainly one way to get an introduction to QG. Right now I want to focus on the TESTING issue which has surfaced fairly recently. Bee Hossenfelder is a QG phenomenologist and she has just posted a paper written with Leonardo Modesto that discusses a completely new mode of QG testing.
It sounds weird to me and dubious but I respect Bee a lot and maybe I have to get used to it. It is based on Modesto's 2008 paper that CLAIMS that in LQG a black hole will not have a singularity but will open up out the bottom to make a new spacetime region. Maybe this could be tested and proved wrong without it showing the whole theory to be wrong. Maybe it is not an airtight case, maybe he made some special assumption or did not derive it correctly from the full LQG theory. Anyway it gives an opportunity to test SOMETHING, and Bee is involved. So here is Modesto s original 2008 paper: ==quote== http://arxiv.org/abs/0811.2196 SpaceTime Structure of Loop Quantum Black Hole Leonardo Modesto In this paper we have improved the semiclassical analysis of loop quantum black hole (LQBH) in the conservative approach of constant polymeric parameter. In particular we have focused our attention on the spacetime structure. We have introduced a very simple modification of the spherically symmetric Hamiltonian constraint in its holonomic version... =endquote== NOW HERE IS THE NEW PAPER: http://arxiv.org/abs/1202.0412 Emission spectra of selfdual black holes Sabine Hossenfelder, Leonardo Modesto, Isabeau PrémontSchwarz (Submitted on 2 Feb 2012) We calculate the particle spectra of evaporating selfdual black holes that are potential dark matter candidates. We first estimate the relevant mass and temperature range and find that the masses are below the Planck mass, and the temperature of the black holes is small compared to their mass. In this limit, we then derive the numberdensity of the primary emission particles, and, by studying the waveequation of a scalar field in the background metric of the black hole, show that we can use the low energy approximation for the greybody factors. We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of selfdual black holes. 15 pages, 6 figures I LIKE THIS BECAUSE it is a way to DISPROVE the existence of these weird doublemouthed black holes by looking for, and not finding, their distinctive radiation. It's empirical. And if they actually found some matching radiation it might be quite surprising. So good either way. 


#299
Feb412, 11:41 PM

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A good introduction to the topic of LQBH (loop quantum black holes) Modestostyle is
http://arxiv.org/abs/0905.3170 Selfdual Black Holes in LQG: Theory and Phenomenology Leonardo Modesto, Isabeau PrémontSchwarz (Submitted on 20 May 2009) In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular ReissnerNordstromlike metric is selfdual in the sense of Tduality: the form of the metric obtained in Loop quantum Gravity (LQG) is invariant under the exchange "r ↔ a_{0}/r" where "a_{0}" is proportional to the minimum area in LQG and "r" is the standard Schwarzschild radial coordinate at asymptotic infinity. Of particular interest, the symmetry imposes that if an observer at "r" close to infinity sees a black hole of mass "m" an observer in the other asymptotic infinity beyond the horizon (at "r" close to "0") sees a dual mass "m_{p}/m" ("m_{p}" is the Planck mass). We then show that small LQBH are stable and could be a component of dark matter. Ultralight LQBHs created shortly after the Big Bang would now have a mass of approximately "10^{5} m_{p}" and emit radiation with a typical energy of about 10^{13}  10^{14} eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultrahighenergycosmicrays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector. 18 pages, 32 figures. Extra Plot, Improved Numerical Results I think the more recent paper I'll call HMP http://arxiv.org/abs/1202.0412 Emission spectra of selfdual black holes refines and corrects some of these estimates. Here is an excerpt ==quote Hossenfelder Modesto PrémontSchwarz (HMP)== Integrating the inverse of dM/dt to obtain the lifetime, one finds that the time it takes for the black hole to completely evaporate exceeds the lifetime of the universe for m >~ 10^{−5}m_{p}. The primordially produced black holes with masses of about 10^{−3}m_{p} thus would still not have entirely decayed today. Moreover, they would have an average temperature of T ≈ 10^{−9}m_{p} ≈ 10^{9} TeV, which is about in the energy range of the ultra high energetic cosmic rays (UHECRs) whose origin is still unclear. We thus see why the selfdual black holes can make for an interesting phenomenology. However, to arrive at observational consequences we have to make this rough estimate more precise. For this, we take ... ==endquote== 


#300
Mar312, 04:56 PM

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PF Gold
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One way to see where a field is going is to check the conferences, what speakers and themes emerge as important. We've done that in the past in this thread. 2011 was a big year for Loop, in terms of research output and conferences, so 2012 is likely to be comparatively quiet. One upcoming event is this June Prague Relativity and Gravitation conference:
http://ae100prg.mff.cuni.cz/program Here are excerpts from the invited speaker list. ... Abhay Ashtekar (Institute for Gravitation and the Cosmos, Penn State University, University Park ) T.B.A.[my guess is Loop cosmology: inflation, initial conditions at the bounce, power spectrum] ... Julian Barbour (Department of Physics, University of Oxford, Oxford) Prague and the conception of general relativity: Kepler, Mach and Einstein ... Karel Kuchař (Department of Physics, University of Utah, Salt Lake City ) Canonical quantum gravity: Einstein's posthumous anathema Jerzy Lewandowski (Department of Physics, University of Warsaw, Warsaw) Loop quantum gravity: The status report ... Hermann Nicolai (AlbertEinsteinInstitut Golm, Potsdam) Quantum gravity: the view from particle physics ... Misao Sasaki (Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto) Inflation and birth of cosmological perturbations ... Bernard Schutz (AlbertEinsteinInstitute Golm, Potsdam) Gravity talks: observing the Universe with gravitational waves Alexei Starobinsky (Landau Institute for Theoretical Physics, Moscow) f(R) gravitythe most straightforward generalization of the Einstein gravity Robert Wald (Enrico Fermi Institute, University of Chicago, Chicago) T.B.A. Clifford Will (Department of Physics, Washington University, St. Louis) Testing general relativity: Centenary highlights and future prospects ================ Ashtekar has new quantum cosmology results with coauthors Agullo and Nelson, that have not been posted yet but were presented in seminar by Nelson http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf http://relativity.phys.lsu.edu/ilqgs/nelson101811.wav http://relativity.phys.lsu.edu/ilqgs/nelson101811.aif and which will be the subject of an invited presentation by Agullo at the April American Physical Society meeting. They are important results so it seems possible that Ashtekar will feature the new paper in his Prague talk this June. ================ This year's Marcel Grossmann meeting (July, in Stockholm) has these relevant parallel sessions: http://www.icra.it/mg/mg13/parallel_sessions.htm L. Quantum Gravity QG1 Loop Quantum Gravity, Quantum Geometry, Spin Foams (Jerzy Lewandowski) QG2 Quantum Gravity Phenomenology (Giovanni AmelinoCamelia) QG3 Asymptotic Safeness and Symmetry Breaking in Quantum Gravity (Eckehard W. Mielke) QG4 Loop quantum gravity: cosmology and black holes (Jorge Pullin, Parampreet Singh) for more information: http://www.icra.it/mg/mg13/ ================ 


#301
Jun312, 01:52 PM

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In the previous post I mentioned the Relativity and Gravitation conference scheduled to take place in Prague this month.
http://ae100prg.mff.cuni.cz/program at that point the title of Ashtekar's talk was TBA. Now it is listed on the program as: Abhay Ashtekar (Institute for Gravitation and the Cosmos, Penn State University, University Park ) Loop quantum gravity and the very early universe I certainly hope that the conference organizers will post PDF slide sets for the talks, online video would be even better. Two other talks could prove especially helpful: Jerzy Lewandowski (Department of Physics, University of Warsaw, Warsaw) Loop quantum gravity: The status report ... Hermann Nicolai (AlbertEinsteinInstitut Golm, Potsdam) Quantum gravity: the view from particle physics ... Back in May a PF member named Neutrino98 asked a basic question. How does Loop gravity work? This post is one possible very basic answer http://physicsforums.com/showthread....96#post3930796 This occasioned a positive reaction from Tom Stoer, who knows quite a lot about LQG, so I take that as a good sign. Maybe the explanation is worth keeping track of, so I will copy it here and maybe we can fill out some more detailsof the simplest possible entrylevel account of the theory's basics. ==quote== Loop gravity works first of all by focusing attention on geometric MEASUREMENT. You know that the Einstein 1915 theory of gravity is actually a theory of geometry. How geometry evolves and interacts with matter. How a concentration of matter will cause geometry to curve around it. So gravity = geometry. Any modern theory of gravity must be a theory of geometry. What is geometry? It's about measuring things like areas, volumes, angles. And about relations among these measurements, which can change depending on when and what order you measure them. And about predictions concerning future measurements. On a cosmic scale geometry can be about the pattern of expanding distances people call "expanding universe" and all the stuff that goes along with that. So Loop Gravity works first of all by setting up NETWORKS where each node corresponds to a volume measurement something might make, and the connections between nodes are labeled to indicate the AREA where those chunks of volume might touch. Another word for network is graph: something made up of a bunch of points (nodes) interconnected by lines (links). In this case it's a network of possible measurements and the nodes and links are labeled with volume and area numbers. A network could have millions of nodes but to picture one of the simplest examples it might just be a square: four nodes at the corners, connected by 4 links which are the sides of the square. So a network is an idealized STATE of the world's geometry representing the results of a bunch of interrelated measurements which some thing or somebody could be imagined making. (If matter measurements like particle detections are included, that means more labels on the network.) Then, as a theory of dynamically evolving geometry, LOOP HAS TO SAY HOW THESE NETWORKS, these states of geometry, EVOLVE. In a quantum theory the PROBABILITY of going from state A to state B is given by a complex number called an AMPLITUDE. So Loop has to say what the amplitude is of going from network state A to network state B. At the current stage of development of the theory, the tool used to arrive at amplitudes is a kind of path leading from one network to the other. the path looks like a FOAM. To take a simple example, suppose both A and B are squares as described earlier. A path between the two squares could be pictured as a CUBE with A on the bottom or "start" and B on the top or "finish". A hollow cube is all you need to make continuous connection between the two squares. It can even be open on the top and bottom. Such a cube might be one part of a much more elaborate foam. Loop has a rule for calculating amplitudes from foams. Given a foam that runs from network A (say at the bottom) to network B (say at the top) there is a way to calculate the amplitude of that foamthe probability amplitude of that path of evolution being taken from A to B. That's the theory. That's how it works. It gives results like replacing the BigBang "singularity" (where the old theory broke down) by a bounce. The present theory applied to cosmology predicts that a collapsing universe will rebound and undergo a brief very rapid phase of reexpansion. It lets us extrapolate backwards in time to before the start of our own universe's expansion and suggests things to look for as traces of the bounce (as a way of testing.) ==endquote== The special issue of the journal SIGMA devoted to LQG and LQC now has 19 articles that have gone thru peer review and whose final versions are online. The special issue is still listed as "in progress" so there may be more articles in the pipeline, but I do not know of any. So this may turn out to be the nearfinal version of the special issue. http://www.emis.de/journals/SIGMA/LQGC.html A useful crosssection sampling of current research in the Loop Gravity&Cosmology community. 


#302
Jun1312, 01:48 PM

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P: 23,124

More complete information is now available about the lineup of talks at the Prague June conference Relativity and Gravitation http://ae100prg.mff.cuni.cz/program
It's a large conference covering a wide range of topics (observation, numerical work, theory, testing) with talks by major people from many fields. So to get a good idea of the scope you need to look over the whole program. I'll just give some selected excerpts where there is relevance to Loop gravity/cosmology or to provide context. Here's a sample of the program. MONDAY June 25 ... ... Parallel Session 16:00 Jorge Pullin A local Hamiltonian for spherically symmetric gravity coupled to a scalar field 16:15 Maite Dupuis Loop Quantum gravity in terms of spinors and harmonic oscillators 16:30 Seth Major On the Observability of Granularity of Spatial Geometry 16:45 Yuri Bonder Quantum Gravity Phenomenology without Lorentz Invariance Violations 17:00 Break 17:15 Deborah Konkowski Quantum singularities in static and conformally static spacetimes 17:30 Vladimír Balek From 'nothing' to inflation and back again 17:45 Jakub Mielczarek Signature change in loop quantum cosmology 18:00 Andrzej Gorlich A transfer matrix model of volume fluctuations in 4D Causal Dynamical Triangulations 18:15 Igor Khavkine Time delay observable in classical and quantum geometries TUESDAY June 26 Plenary Talks in Blue Lecture Hall 8:40 Gary Gibbons Links between general relativity and other parts of physics 9:20 Abhay Ashtekar Loop quantum gravity and the very early universe 10:00 Coffee break 10:30 Misao Sasaki Inflation and birth of cosmological perturbations 11:10 Alexei Starobinsky f(R) gravity  the most straightforward generalization of the Einstein gravity WEDNESDAY June 27 Plenary Talks in Blue Lecture Hall 8:40 Bernard Schutz Gravity talks: observing the Universe with gravitational waves 9:20 Daniel Sudarsky The quantum gravity interface and the origin of the seeds of cosmic structure during inflation 10:00 Coffee break 10:30 Helmut Friedrich The large scale Einstein evolution problem 11:10 Hermann Nicolai Quantum Gravity: the view from particle physics ... ... Parallel Session in Yellow Lecture Hall 16:00 Alejandro Corichi Effective Dynamics of Anisotropic Cosmologies in Loop Quantum Cosmology 16:15 Steffen Gielen Spontaneous breaking of Lorentz symmetry for canonical gravity 16:30 Thomas Roman Probability Distributions of Quantum Stress Tensors in Two and Four Dimensions 16:45 Florian Girelli Geometric operators in loop quantum gravity with a cosmological constant FRIDAY June 29 Plenary Talks in Blue Lecture Hall ... ... 11:10 Jerzy Lewandowski Loop Quantum Gravity  where are we? ========================= In early July there will be another large conference, the MG13 (13th Marcel Grossmann) in Stockholm. For Loop and related QG there are several parallel sessions to check out. The chair of each session is supposed to list the lineup of talks in their brief descriptions, but this hasn't happened in all cases yet. I'll give the links to where the listing should appear. http://www.icra.it/mg/mg13/par_sessi...tm#lewandowski Parallel Session: QG2  Loop Quantum Gravity, Quantum Geometry, Spin Foams http://www.icra.it/mg/mg13/par_sessi...ils.htm#mielke Parallel Session: QG3  Asymptotic Safeness and Symmetry Breaking in Quantum Gravity http://www.icra.it/mg/mg13/par_sessi...ails.htm#moniz Parallel Session: CM4  Quantum Cosmology and Quantum Effects in the Early Universe http://www.icra.it/mg/mg13/par_sessi...ils.htm#pullin Parallel Session: QG4  Loop quantum gravity: cosmology and black holes http://www.icra.it/mg/mg13/par_sessi...ls.htm#novello Parallel Session: CM3  Nonsingular Cosmology http://www.icra.it/mg/mg13/par_sessi...amelinocamelia Parallel Session: QG2  Quantum Gravity Phenomenology 


#303
Jun1912, 08:55 PM

Astronomy
Sci Advisor
PF Gold
P: 23,124

The Prague "Relativity and Gravitation" conference starts in about 5 days and the abstracts of all the talks are posted. http://ae100prg.mff.cuni.cz/abstracts Here is a sample of the talks:
http://ae100prg.mff.cuni.cz/img/abst...539cd744e6.pdf Jerzy Lewandowski Loop Quantum Gravity  where are we? Abstract: For several models of gravity coupled to other fields, the algorithm of the canonical quantization has been completed and performed to an end. It gave rise to well defined, exact quantum theories. The Dirac observables are provided by the relational and the deparametrization frameworks. The quantum states, Hilbert spaces and concrete quantum operators are furnished by the canonical Loop Quantum Gravity framework. The models are not confirmed experimentally and admit ambiguities, but they are there, available for further study and applications. http://ae100prg.mff.cuni.cz/img/abst...3b4e153470.pdf Abhay Ashtekar Loop quantum gravity and the very early universe Abstract: Since the standard cosmological perturbation theory is based on QFT on curved spacetimes, it is not applicable in the Planck era. Using techniques from loop quantum gravity, the theory is extended to overcome this limitation. The new framework sharpens conceptual issues by distinguishing between true and apparent transPlanckian difficulties and shows that the true difficulties can be generically overcome in the standard inflationary scenario, with interesting lessons for both theory and observations. The talk will be based largely on some recently completed joint work with Ivan Agullo and William Nelson. http://ae100prg.mff.cuni.cz/img/abst...e3abe1f296.pdf Hermann Nicolai Quantum Gravity: the view from particle physics Abstract: In this talk I will review some facts and lessons that particle physics can offer to help in the search for a fully consistent theory of quantum gravity, including a brief discussion of recent LHC data from this perspective. Jorge Pullin A local Hamiltonian for spherically symmetric gravity coupled to a scalar field Abstract: We present a gauge fixing for gravity coupled to a scalar field in spherical symmetry that leads to a true Hamiltonian that is the integral over space of a local density. We discuss its potential use to study black hole evaporation. Maite Dupuis Loop Quantum gravity in terms of spinors and harmonic oscillators Abstract: Loop Quantum Gravity is an attempt to quantize general relativity. Its kinematical aspects are well understood and yield a description of space in terms of quanta. Spinorial tools provide a really nice geometrical picture of the classical phase space of Loop Gravity. Moving to the quantum level, spinors are simply quantized as harmonic oscillators. They are then the building blocks to define coherent states for Loop Quantum Gravity and to build spinfoam models which is a regularized path integral for general relativity. I will recall the main results of the spinorial formalism in the context of Loop Quantum Gravity and Spinfoam models and explain how it can be generalized to introduce a cosmological constant into the game. Yuri Bonder Quantum Gravity Phenomenology without Lorentz Invariance Violations Abstract: In the last years the phenomenology of quantum gravity has been dominated by the search of violations of Lorentz symmetry. However, there are very serious arguments that led us to assume that Lorentz invariance is a real symmetry in Nature. This motivated us to construct a phenomenological model describing how a Lorentz invariant discrete structure of spacetime could become manifest. The proposal is fully observer covariant, it involves nontrivial couplings of curvature to matter fields and leads to a well defined phenomenology. In fact, an experiment specially designed to test the model has been performed by the EötWash group allowing to put bounds on some of the model's free parameters. Jakub Mielczarek Signature change in loop quantum cosmology Abstract: The Wick rotation is commonly considered only as an useful computational trick. However, as it was suggested by Hartle and Hawking already in early eighties, Wick rotation may gain physical meaning at the Planck epoch. While such possibility is conceptually interesting, leading to noboundary proposal, mechanism behind the signature change remains mysterious. In this talk we show that the signature change anticipated by Hartle and Hawking may occur in result of the loop quantum gravity effects. Theory of cosmological perturbations with the effects of quantum holonomies is constructed. It is shown that such theory can be uniquely formulated in the anomalyfree manner. The algebra of quantum constraints turns out to be modified such that the signature is changing from Lorentzian in low curvature regime to Euclidean in high curvature regime. Implications of this phenomenon on propagation of cosmological perturbations are discussed. Possible relations with other approaches to quantum gravity are also outlined. Alejandro Corichi Effective Dynamics of Anisotropic Cosmologies in Loop Quantum Cosmology Abstract: We present results of numerical evolutions of effective equations for anisotropic cosmologies with spatial curvature in loop quantum cosmology. We address the issue of singularity resolution for different types of initial conditions and study the behavior of geometrical scalar quantities. Steffen Gielen Spontaneous breaking of Lorentz symmetry for canonical gravity Abstract: In Hamiltonian formulations of general relativity, in particular Ashtekar variables which serve as the classical starting point for loop quantum gravity, Lorentz covariance is a subtle issue which has been the focus of some debate, while at the same time being crucial with regard to possible experimental tests. After reviewing the sources of difficulty, we present a Lorentz covariant formulation in which we generalise the notion of a foliation of spacetime usually used in the Hamiltonian formalism to a field of ”local observers” which specify a time direction only locally. This field spontaneously breaks the local SO(3,1) symmetry down to a subgroup SO(3), in a way similar to systems in condensed matter and particle physics. The formalism is analogous to that in MacDowellMansouri gravity, where SO(4,1) is spontaneously broken to SO(3,1). We show that the apparent breaking of SO(3,1) to SO(3) is not in conflict with Lorentz covariance. We close by outlining other possible applications of the formalism of local observer, especially with regard to phenomenology of quantum gravity. Florian Girelli Geometric operators in loop quantum gravity with a cosmological constant Abstract: Loop quantum gravity is a candidate to describe the quantum gravity regime with zero cosmological constant. One of its key results is that geometric operators, such as area, angle, volume, are quantized. Not much is known when the cosmological constant is not zero. It is usually believed that to introduce this parameter in the game, we need to use quantum groups. However due to the complicated algebraic structure inherent to quantum groups not much is known in this case. Apart from the area operator, the geometric operators are not yet defined. I will discuss how the use of tensor operators can circumvent the difficulties and allow to construct a natural set of observables. In particular, I will construct the natural geometric observables such as angle or volume and discuss some of their properties. Franz Hinterleitner Quantization of plane gravitational waves Abstract: A longstanding problem in Loop Qauntum Gravity (LQG) is the semiclassical limit and the question of Lorentz invariance violation due to the "granularity" of quantum spacetime. In full 3+1 LQG there are strong indications for such violations, but no definitve answer to this issue has been given so far. Unidirectional plane gravitational waves are 1+1 dimensional fully generalrelativistic systems, which are convenient for an investigation of possible dispersion of gravitational radiation, quantum fluctuations of flat space, and the speed of light in a quantum spacetime environmant. In a recent paper a classical canonical approach to plane waves was found, where the reduction from arbitrarily forth and back running waves to unidirectional ones is formulated in terms of firstclass constraints. This means that this step of symmetry reduction can be carried out after quantization. The presently ongoing work deals with the formulation of the corresponding quantum constraint operators and the construction of solutions. More information about the schedule and program of the conference is here: http://ae100prg.mff.cuni.cz/program 


#304
Jun2912, 03:22 PM

Astronomy
Sci Advisor
PF Gold
P: 23,124

The triennial Marcel Grossmann meeting "MG13" starts next week in Stockholm.
To see the QGrelated talks to be given go to: http://www.icra.it/mg/mg13/parallel_sessions.htm Scroll down to the QG block Clicking on QG4 A gives, for example, the first of two sessions chaired by Jorge Pullin, taking place Thursday 5 July and Friday 6 July. CLICKING ON THE TITLE OF THE TALK links to a brief abstract. http://ntsrvg95.icra.it/mg13/FMPro?...&Max=50&Find 1) 14:00  14:30 Rovelli, Carlo Recent developments in Black Hole physics 2) 14:30  15:00 Perez, Alejandro Black holes in LQG, a local perspective. 3) 15:00  15:30 Bianchi, Eugenio Entropy of NonExtremal Black Holes from Loop Gravity 4) 15:30  16:00 Barbero, Fernando The thermodynamic limit for black holes in loop quantum gravity. COFFEE BREAK 5) 16:30  17:00 Diaz Polo, Jacobo Testable predictions from loop quantum gravity in evaporating microscopic black holes 6) 17:00  17:30 Gambini, Rodolfo Quantum scalar field in spherical quantum gravity: Standard and nonstandard techniques. 7) 17:30  18:00 Pranzetti, Daniele Dynamical evaporation of quantum horizons 8) 18:00  18:30 Frodden, Ernesto Toward the LQG quantization of rotating black holes 


#305
Jun2912, 03:27 PM

Astronomy
Sci Advisor
PF Gold
P: 23,124

See also the previous post #304
The triennial Marcel Grossmann meeting "MG13" starts next week in Stockholm. To see the QGrelated talks to be given go to: http://www.icra.it/mg/mg13/parallel_sessions.htm Scroll down to the QG block and click on, for example, QG1A. This will give the Tuesday 3 July session, the first of two Loopandallied sessions chaired by Jerzy Lewandowski. CLICKING ON THE TITLE OF THE TALK links to a brief abstract. 1) 14:00  14:30 Rovelli, Carlo Covariant dynamics 2) 14:30  15:00 Lewandowski, Jerzy Quantizable canonical LQG 3) 15:00  15:30 Ma, Yongge Loop Quantum ScalarTensor Gravity and Cosmology 4) 15:30  16:00 Bianchi, Eugenio Horizons in spin foam gravity COFFEE BREAK (I've added some possibly incomplete place tags to help keep track of the next batch of speakers.) 5) 16:30  16:50 Vidotto, Francesca (Mars. Utr.) Spinfoam Cosmology 6) 16:50  17:10 Puchta, Jacek (Wars. Mars.) Operator Spinnetwork Diagrams in calculations of higher order amplitudes in dipole cosmology 7) 17:10  17:30 Zipfel, Antonia (Erl.) Solving the Euclidean scalar constraint with Spin Foam methods 8) 17:30  17:50 Pawlowski, Tomasz (Wars. Mad. UNB Penn.) Computable framework of Loop Quantum Gravity 9) 17:50  18:10 Alesci, Emanuele (Mars. Erl.) A new perspective on early cosmology 10) 18:10  18:30 Duston, Christopher (FSUMarcolli) Topspin Networks and Loop Quantum Gravity 11) 18:30  18:50 Smerlak, Matteo (Mars. Perim.) The shift symmetry in spin foam models of BF theory 12) 18:50  19:10 Ryan, James (AEI, Unam) Tensor models and discrete quantum gravity 


#306
Jun2912, 03:49 PM

Astronomy
Sci Advisor
PF Gold
P: 23,124

See also post #304 on the previous page.
I should note that session QG2 A, the first of two chaired by Giovanni AmelinoCamelia, has a talk by Bee Hossenfelder discussing the possibility of FTL exchange of information within the context of quantum gravity. Try clicking on QG2A and check out her abstract. On the other hand, still in the QG block, clicking on QG4 B will give Friday 6 July the second of two sessions chaired by Jorge Pullin http://ntsrvg95.icra.it/mg13/FMPro?...&Max=50&Find Here as on the previous page, the abstract shows up when you click on the talk's title. 1) 14:00  14:30 Agullo, Ivan A quantum gravity extension of the inflationary paradigm 2) 14:30  15:00 WilsonEwing, Edward Lattice LQC 3) 15:00  15:20 Cailleteau, Thomas Observational consequences of Loop Quantum Cosmology 4) 15:20  15:40 Vidotto, Francesca Spinfoam for cosmologists 5) 15:40  16:00 Dapor, Andrea Loop Quantum Cosmology for nonminimally coupled Scalar Field COFFEE BREAK 6) 16:30  17:00 Pawlowski, Tomasz Geometric time in quantum cosmology 7) 17:00  17:30 Velhinho, Jose Uniqueness of the Fock quantization of scalar fields with time dependent mass 8) 17:30  17:50 FernándezMéndez, Mikel Hybrid quantization of an inhomogeneous inflationary scenario 9) 17:50  18:10 Martínde Blas, Daniel Approximated quantum solutions in inhomogeneous loop quantum cosmology 10) 18:10  18:50 Dapor, Andrea Quantum Field Theory on LQC Bianchi Spacetimes 11) 18:30  18:50 Cianfrani, Francesco Inhomogeneous Universe in Loop Quantum Gravity 


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