Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/gr-qc/0602119
Matters of Gravity, The Newsletter of The Topical Group in Gravitation of the American Physical Society, Volume 27, Spring 2006
Jorge Pullin (Editor)
30 pages

"GGR News:
GGR program at the APS April meeting in Dallas
We hear that..., 100 years ago, by Jorge Pullin
Research Briefs:
What's new in LIGO, by David Shoemaker
LISA Pathfinder, by Paul McNamara
Recent progress in binary black hole simulations, by Thomas Baumgarte
Conference reports:
Workshop on Emergence of Spacetime, by Olaf Dreyer
Quantum gravity subprogram at the Isaac Newton Institute, by Jorma Louko
Global problems in Math Relativity at the Newton Institute, by Jim Isenberg
Loops '05, by Thomas Thiemann
Numrel 2005, by Scott Hawley and Richard Matzner
Apples With Apples Workshop in Argentina, by Sascha Husa"
====================

http://arxiv.org/abs/gr-qc/0602120
Categorical Geometry and the Mathematical Foundations of Quantum General Relativity
Louis Crane
Contribution to the Oxford University Press volume on Quantum Gravity

"We explore the possibility of replacing point set topology by higher category theory and topos theory as the foundation for quantum general relativity. We discuss the BC model and problems of its interpretation, and connect with the construction of causal sites."

 

[marcus]

http://arxiv.org/abs/hep-th/0603022
Quantum Gravity and the Standard Model
Sundance O. Bilson-Thompson, Fotini Markopoulou, Lee Smolin
12 pages, 21 figures

"We show that a class of background independent models of quantum spacetime have local excitations that can be mapped to the first generation fermions of the standard model of particle physics. These states propagate coherently as they can be shown to be noiseless subsystems of the microscopic quantum dynamics. These are identified in terms of certain patterns of braiding of graphs, thus giving a quantum gravitational foundation for the topological preon model proposed by one of us.
These results apply to a large class of theories in which the Hilbert space has a basis of states given by ribbon graphs embedded in a three-dimensional manifold up to diffeomorphisms, and the dynamics is given by local moves on the graphs, such as arise in the representation theory of quantum groups. For such models, matter appears to be already included in the microscopic kinematics and dynamics."

==================
Sundance did a seminar talk at Perimeter on this back in November 2005. The video is online in the PI streaming media center. It is impressive to see him responding to questions and working stuff out at the blackboard. At that time Sundance was collaborating with Smolin and Markopoulou on this and they were putting it together----merging Sundance preon model of matter with Spin-network spacetime.

I will get the link to that video, or a link to the link

Here is a link to a PF thread about the Preons of Sundance Bilson-Thompson that we had back in November 2005
https://www.physicsforums.com/showthread.php?t=100183

that will have a link to the seminar talk

also try this:
http://streamer.perimeterinstitute....fa7-485f-8d5d-3b62fb7d3e4c&shouldResize=False

and flip to page 5 of the seminar series. the date is 16 November and that is currently on page 5------they list latest first

the title is "Braid New World"
==================

Etera Livine must always be kept track of and he posted today as well

http://arxiv.org/abs/gr-qc/0603008
Reconstructing Quantum Geometry from Quantum Information: Area Renormalisation, Coarse-Graining and Entanglement on Spin Networks
Etera R. Livine, Daniel R. Terno
27 pages, 12 figures

"After a brief review of spin networks and their interpretation as wave functions for the (space) geometry, we discuss the renormalisation of the area operator in loop quantum gravity. In such a background independent framework, we propose to probe the structure of a surface through the analysis of the coarse-graining and renormalisation flow(s) of its area. We further introduce a procedure to coarse-grain spin network states and we quantitatively study the decrease in the number of degrees of freedom during this process. Finally, we use these coarse-graining tools to define the correlation and entanglement between parts of a spin network and discuss their potential interpretation as a natural measure of distance in such a state of quantum geometry."

============
briefly noted
Sabine H.
http://arxiv.org/hep-th/0603032
Thanu P.
http://arxiv.org/astro-ph/0603114

 

[marcus]

DSR chapter of Oriti

http://arxiv.org/abs/gr-qc/0603022

Doubly Special Relativity: facts and prospects
Jerzy Kowalski-Glikman
Submitted to the volume "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", D. Oriti ed

"In this short review of Doubly Special Relativity I describe first the relations between DSR and (quantum) gravity. Then I show how, in the case of a field theory with curved momentum space, the Hopf algebra of symmetries naturally emerges. I conclude with some remarks concerning DSR phenomenology and description of open problems."

 

[marcus]

CDT paper from Paris Einstein conference last summer

http://arxiv.org/abs/hep-th/0603079
Nonperturbative sum over topologies in 2D Lorentzian quantum gravity
R. Loll (U. Utrecht), W. Westra (U. Utrecht), S. Zohren (U. Utrecht, RWTH Aachen)
10 pages, 4 figures. Talk given by S. Zohren at the Albert Einstein Century International Conference (Paris, July 18-22 2005)

"The recent progress in the Causal Dynamical Triangulations (CDT) approach to quantum gravity indicates that gravitation is nonperturbatively renormalizable. We review some of the latest results in 1+1 and 3+1 dimensions with special emphasis on the 1+1 model. In particular we discuss a nonperturbative implementation of the sum over topologies in the gravitational path integral in 1+1 dimensions. The dynamics of this model shows that the presence of infinitesimal wormholes leads to a decrease in the effective cosmological constant. Similar ideas have been considered in the past by Coleman and others in the formal setting of 4D Euclidean path integrals. A remarkable property of the model is that in the continuum limit we obtain a finite space-time density of microscopic wormholes without assuming fundamental discreteness. This shows that one can in principle make sense out of a gravitational path integral including a sum over topologies, provided one imposes suitable kinematical restrictions on the state-space that preserve large scale causality."

this is listed for completeness although I think there is nothing new here that was not already in
http://arxiv.org/abs/hep-th/0507012
and its referencesbriefly noted:
http://arxiv.org/abs/gr-qc/0603029
Counting of Black Hole Microstates
A. Ghosh, P. Mitra

 

[marcus]

Recent contributions from Baez, Smolin, Rovelli

a couple of these are discussed in https://www.physicsforums.com/showthread.php?t=114140

http://math.ucr.edu/home/baez/where_we_stand/
"Fundamental Physics: Where We Stand Today"
John Baez

===============

http://www.nyas.org/publications/UpdateUnbound.asp?UpdateID=41
"A Crisis in Fundamental Physics"
Lee Smolin
published in the New York Academy of Sciences magazine.

The Houghton-Mifflin publishing house plans to release a new book by Lee Smolin this year---what the book is about is suggested by:

http://www.lse.ac.uk/collections/CPNSS/events/Conferences/AnnualPublicLecture.htm
Abstract for something called "The Annual Lecture" to be given by Smolin at The London School of Economics CPNSS, in June 2006."...I will begin by proposing an answer to the question of what science is and why it works. Part of the answer is that scientists form a community that is defined by adherence to a set of ethics which encourages honest reporting of observations and results as well as an awareness that future generations will know far more than us. As such science is based on a tragic understanding of how easily we fool ourselves and others and its success is due to the discovery of techniques whose use helps us discover error. In this and other ways I will discuss science is tied to democracy in that both require members of a community to adhere to ethics designed to allow us to achieve as a community far more than would be possible each on our own..."

[my comment: a healthy scientific community requires ethical behavior and some "checks and balances" because even prestigeous elites can succumb to self-delusion and error. it is not simply an egalitarian democracy, nor is it simply an amoral turf-based aristocratic hierarchy---a key ingredient is the empirical ethic]

=============

Recent contributions from Rovelli are mentioned here:

https://www.physicsforums.com/showthread.php?t=114024

A 59 slide lecture given at Lyon, Introduction to Loop Quantum Gravity
http://www.cpt.univ-mrs.fr/~rovelli/Lyon2006II.pdf

Course outline for a History of Science course that Rovelli is currently teaching is here
http://www.cpt.univ-mrs.fr/~rovelli/

=============================
We recently had a thread at PF about George Ellis finding misinformation in Susskind's book
https://www.physicsforums.com/showthread.php?t=114068

http://arxiv.org/abs/astro-ph/0603266
On horizons and the cosmic landscape
George F R Ellis

"Susskind claims in his recent book The Cosmic Landscape that evidence for the existence and nature of 'pocket universes' in a multiverse would be available via detailed study of the Cosmic Blackbody Background Radiation. I point out that apart from any other queries one might have about the chain of argument involved, this claim is invalid because it rests on a confusion between the nature of a particle horizon and an event horizon in cosmology."

Ellis is co-author, with Stephen Hawking, of a well-known reference "The Large-Scale Structure of Spacetime". He gives this link to an erroneous passage of Susskind's book.
http://www.twbookmark.com/books/28/0316155799/chapter_excerpt22014.html

 

[marcus]

new Loll and Benedetti, plus others of interest

http://arxiv.org/abs/hep-lat/0603013
Unexpected Spin-Off from Quantum Gravity
D. Benedetti (U. Utrecht), R. Loll (U. Utrecht)
10 pages, 4 figures

"We propose a novel way of investigating the universal properties of spin systems by coupling them to an ensemble of causal dynamically triangulated lattices, instead of studying them on a fixed regular or random lattice. Somewhat surprisingly, graph-counting methods to extract high- or low-temperature series expansions can be adapted to this case. For the two-dimensional Ising model, we present evidence that this ameliorates the singularity structure of thermodynamic functions in the complex plane, and improves the convergence of the power series."

title seems to involve a pun. Benedetti got his Masters from Rome IIRC and joined Loll's group as a PhD student sometime in the past two years---I have the feeling it was about a year ago

http://arxiv.org/abs/gr-qc/0603062
Conformal decomposition in canonical general relativity
Charles H.-T. Wang
6 pages
"A new canonical transformation is found that enables the direct canonical treatment of the conformal factor in general relativity. The resulting formulation significantly simplifies the previously presented conformal geometrodynamics. It provides a further theoretical basis for the conformal approach to loop quantum gravity and offers a generic framework for the conformal analysis of spacetime dynamics."

Charles Wang is physics prof IIRC at Aberdeen, UK. He does LQG his own independent way and is someone to watch out of the corner of one's eye. He gave a paper at last September QG conference on Sardinia. My fear is that if I ignore mavericks that seem disconnected from the rest of the researchers one of them may sneak up and surprise me. So without recommendation, I keep a link on Wang.

===============
Cosmology is a principle feed and motivation for QG. This book by Loeb (Harvard) will be a valuable reference, I suspect. Around 160 pages.
It is being published by Springer in Germany.

http://arxiv.org/abs/astro-ph/0603360
First Light
Abraham Loeb (Harvard)
158 pages, lecture notes for the 36th Swiss (SAAS-Fee) Winter School, April 2006 (to be published by Springer Verlag); http://obswww.unige.ch/saas-fee2006/preannouncement/course_pres/overview_f.html

"The first dwarf galaxies, which constitute the building blocks of the collapsed objects we find today in the Universe, had formed hundreds of millions of years after the big bang. This pedagogical review describes the early growth of their small-amplitude seed fluctuations from the epoch of inflation through dark matter decoupling and matter-radiation equality, to the final collapse and fragmentation of the dark matter on all mass scales above \~10^{-4} solar masses. The condensation of baryons into halos in the mass range of ~10^5-10^{10} solar masses led to the formation of the first stars and the re-ionization of the cold hydrogen gas, left over from the big bang. The production of heavy elements by the first stars started the metal enrichment process that eventually led to the formation of rocky planets and life. A wide variety of instruments currently under design [including large-aperture infrared telescopes on the ground or in space (JWST), and low-frequency arrays for the detection of redshifted 21cm radiation], will establish better understanding of the first sources of light during an epoch in cosmic history that was largely unexplored so far. Numerical simulations of reionization are computationally challenging, as they require radiative transfer across large cosmological volumes as well as sufficently high resolution to identify the sources of the ionizing radiation. The technological challenges for observations and the computational challenges for numerical simulations, will motivate intense work in this field over the coming decade."

 

[arivero]

Duff on three generations?

Just noticed this one:
http://arxiv.org/abs/hep-th/0602160
Hidden symmetries of the Nambu-Goto action

It seems a sort of follow-up of

http://arxiv.org/abs/hep-th/0601134
String triality, black hole entropy and Cayley's hyperdeterminant

 

[marcus]

thanks! good to have your contributions.

 

[arivero]

hmmm and sorry I put it using "new reply" instead of "new thread"

 

[marcus]

hmmm and sorry I put it using "new reply" instead of "new thread"

well actually it might be simpler to keep this thread specialized for non-string QG papers, as a general rule. and there is a sticky for string literature.

but I hope you will keep watch for interesting Loop-and-allied QG papers and contribute links here---I think you have an eye for what research could be important

 

[marcus]

baez "loop braid group" paper

no-brainer, this has to go on the link list

http://arxiv.org/abs/gr-qc/0603085
Exotic Statistics for Loops in 4d BF Theory
John C. Baez, Derek K. Wise, Alissa S. Crans
40 pages, many figures

"After a review of exotic statistics for point particles in 3d BF theory, and especially 3d quantum gravity, we show that loop-like defects in 4d BF theory obey exotic statistics governed by the 'loop braid group'. This group has a set of generators that switch two loops just as one would normally switch point particles, but also a set of generators that switch two loops by passing one through the other. The first set generates a copy of the symmetric group, while the second generates a copy of the braid group. Thanks to recent work of Xiao-Song Lin, we can give a presentation of the whole loop braid group, which turns out to be isomorphic to the 'braid permutation group' of Fenn, Rimanyi and Rourke. In the context 4d BF theory this group naturally acts on the moduli space of flat G-bundles on the complement of a collection of unlinked unknotted circles in R^3. When G is unimodular, this gives a unitary representation of the loop braid group. We also discuss 'quandle field theory', in which the gauge group G is replaced by a quandle."here's a discussion thread, with exerpts

https://www.physicsforums.com/showthread.php?t=115082

=============================

f-h called attention to this new paper, also about the interesting (4D) case of BF theory:
http://arxiv.org/abs/gr-qc/0603076
Alternative symplectic structures for SO(3,1) and SO(4) four-dimensional BF theories
Merced Montesinos
13 pages
Class. Quantum Grav. 23 (2006) 2267-2278
"The most general action, quadratic in the B fields as well as in the curvature F, having SO(3,1) or SO(4) as the internal gauge group for a four-dimensional BF theory is presented and its symplectic geometry is displayed. It is shown that the space of solutions to the equations of motion for the BF theory can be endowed with symplectic structures alternative to the usual one. The analysis also includes topological terms and cosmological constant. The implications of this fact for gravity are briefly discussed."

========briefly noted========
http://arxiv.org/abs/gr-qc/0603090
Fundamental decoherence from quantum gravity: a pedagogical review
Rodolfo Gambini, Rafael Porto, Jorge Pullin
9 pages, dedicated to Octavio Obregon on his 60th birthday

I have no reason to recommend the next one. Just want to stay aware of other (particle) approaches to the c.c. problem. here is an example.
http://arxiv.org/abs/gr-qc/0603088
A solution to the cosmological constant problem
Tomislav Prokopec (Utrecht University)
22 pages, 4 figures
"We argue that, when coupled to Einstein's theory of gravity, the Yukawa theory may solve the cosmological constant problem in the following sense: The radiative corrections of fermions generate an effective potential for the scalar field, such that the effective cosmological term Lambda_eff is dynamically driven to zero. Thence, for any initial positive cosmological constant Lambda_0, Lambda_eff = 0 is an attractor of the semiclassical Einstein theory coupled to fermionic and scalar matter fields. When the initial cosmological term is negative, Lambda_eff=Lambda_0 does not change. Next we argue that the dark energy of the Universe may be explained by a GUT scale fermion with a mass, m = 4.3 * 10^15 (Lambda_0/10^13GeV)^(1/2) GeV.
Finally, we comment on how the inflationary paradigm, BEH mechanism and phase transitions in the early Universe get modified in the light of our findings."

No special reason to recommend this either. philosophical/historical paper suggesting that there's something wrong with the ideas of b.i. and diffeo-invariance. If true, a serious critique of LQG and allied research.
http://arxiv.org/abs/gr-qc/0603087
Some remarks on the notions of general covariance and background independence
Domenico Giulini
26 pages, 3 figures. Contribution to "An assessment of current paradigms in the physics of fundamental interactions'', edited by I.O. Stamatescu (Springer Verlag, to appear)

 

[marcus]

John Swain about a way of seeing Higgs

John Swain is a experimental particle physicist/phenomenologist who works part at CERN and part in the physics department at Northeastern.

He interests me in part because he also writes LQG papers. He is a working nutsbolts high energy physicist who has enough style to also think creatively in Quantum Gravity.

So when he has new-sounding ideas about anything, I try to pay attention. This has been sitting on my desktop since last September.
Maybe someone here at PF could be interested. It is a way of seeing Higgs field by mass shifts, not by actually detecting the particle quantum of the field.

I suspect that the higgs field will ultimately be found built into the quantum geometry of some QG----intrinsic to the quantum state of spacetime. I would bet this thought has crossed Swain's mind too.

http://arxiv.org/hep-ph/0509151
Probing the Higgs Field Using Massive Particles as Sources and Detectors
S. Reucroft, Y.N. Srivastava, J. Swain, A. Widom
6 pages, no figures; Version 2 corrects some typographical errors of factors of 2 in equations 14, 17, 18 and 19 (all of the same origin) and mentions a linear collider as an interesting place to test the results of this paper

"In the Standard Model, all massive elementary particles acquire their masses by coupling to a background Higgs field with a non-zero vacuum expectation value. What is often overlooked is that each massive particle is also a source of the Higgs field. A given particle can in principle shift the mass of a neighboring particle. The mass shift effect goes beyond the usual perturbative Feynman diagram calculations which implicitly assume that the mass of each particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs physics since they do not require the production of on-shell Higgs bosons. We provide theoretical estimates showing that the mass shift effect can be large and measurable, especially near pair threshold, at both the Tevatron and the LHC."

The general idea (which has some historical validity I believe) is to look for the field instead of for the particle which represents the field in certain situations.

======================
TIMBUQTU AND F-H thanks for your help with this biblio-links thread! About CD's blog as time goes on it is getting more and more active and useful isn't it, contributions from f-h and also from John Baez among others. It is an odd coincidence that we should post a link to Timb.'s professor's paper at the same time as he assigns homework from it.

 

[Timbuqtu]

http://arxiv.org/abs/gr-qc/0603088
A solution to the cosmological constant problem
Tomislav Prokopec (Utrecht University)
22 pages, 4 figures
"We argue that, when coupled to Einstein's theory of gravity, the Yukawa theory may solve the cosmological constant problem in the following sense: The radiative corrections of fermions generate an effective potential for the scalar field, such that the effective cosmological term Lambda_eff is dynamically driven to zero. Thence, for any initial positive cosmological constant Lambda_0, Lambda_eff = 0 is an attractor of the semiclassical Einstein theory coupled to fermionic and scalar matter fields. When the initial cosmological term is negative, Lambda_eff=Lambda_0 does not change. Next we argue that the dark energy of the Universe may be explained by a GUT scale fermion with a mass, m = 4.3 * 10^15 (Lambda_0/10^13GeV)^(1/2) GeV.
Finally, we comment on how the inflationary paradigm, BEH mechanism and phase transitions in the early Universe get modified in the light of our findings."

That's funny. Last week, one day before this paper was posted, Prokopec gave us a homework exercise to derive this result (the mass calculation). Still have to do it. But apparently there is a squareroot missing in the mass-formula in this abstract, because it must be m = 4.3 * 10^15 ((Lambda_0)^(1/2)/10^13GeV)^(1/2) GeV (according to his own paper).

 

[f-h]

"f-h called attention to this new paper, also about the interesting (4D) case of BF theory"

Actuall I first saw it on CDs blog.

 

[marcus]

http://arxiv.org/abs/gr-qc/0603110
Quantum Cosmology
Martin Bojowald
10 pages, published in Encyclopedia of Mathematical Physics, eds. J.-P. Franccoise, G. L. Naber and Tsou S. T., Oxford: Elsevier, 2006 (ISBN 978-0-1251-2666-3), volume 4, page 153

"Quantum cosmology in general denotes the application of quantum physics to the whole universe and thus gives rise to many realizations and examples, covering problems at different mathematical and conceptual levels. It is related to quantum gravity and more specifically describes the application to cosmological situations rather than the construction and analysis of quantum field equations. As there are several different approaches to quantum gravity, equations for quantum cosmology are not unique. Most investigations have been performed in the context of canonical quantization, where Wheeler--DeWitt like equations are the prime object. Applications are mostly conceptual, ranging from possible resolutions of classical singularities and explanations of the uniqueness of the universe to the origin of seeds for a classical world and its initial conditions."

this is in rather dry handbook style and covers the general subject instead of focusing on Loop cosmology. For a more focused and detailed discussion see Bojowald's Living Reviews article
http://arxiv.org/abs/gr-qc/0601085
or this short survey for non-specialists, also by Bojowald:
http://arxiv.org/abs/astro-ph/0511557

while on the subject, a recent cosmology overview by Ned Wright just came out
http://arxiv.org/abs/astro-ph/0603750
A Century of Cosmology
E. L. Wright (UCLA Astronomy)GLAST mission reference
http://arxiv.org/abs/astro-ph/0603762
The GLAST mission, LAT and GRBs
Nicola Omodei, for the GLAST/LAT GRB Science Group
6 pages, 3 figures. Proceedings for the 16th Annual October Astrophysics Conference in Maryland: Gamma Ray Bursts in the Swift Era

 

[marcus]

Mohammad Ansari is at Perimeter and has been sitting in on the Smolin lectures
Several times Smolin has referred to Ansari's work, and they've talked back and forth, but Smolin has the mike and I can't distinguish what Mohammad says.

Here is some recent work by him

http://arxiv.org/abs/gr-qc/0603121
Entanglement entropy in loop quantum geometry
Mohammad H. Ansari
4 pages, 3 figures
"The notion of entanglement entropy of a non-rotating black hole in the context of loop quantum geometry is introduced. By defining the notion of degeneracy of spin network states and determining the Barbero-Immirzi parameter from the highly damping quasinormal modes, we show that the entropy of SO(3) representations coincides with the Bekenstein-Hawking entropy."
=====================
http://arxiv.org/abs/gr-qc/0601013
Fermions in Ashtekar-Barbero Connections Formalism for Arbitrary Values of the Immirzi Parameter
Simone Mercuri
15 pp., no figures, title changed, version accepted for publication on Phys. Rev. D"The Ashtekar-Barbero-Immirzi formulation of General Relativity is extended to include spinor matter fields. Our formulation applies to generic values of the Immirzi parameter and reduces to the Ashtekar-Romano-Tate approach when the Immirzi parameter is taken equal to the imaginary unit. The dynamics of the gravity-fermions coupled system is described by the Holst plus Dirac action with a non-minimal coupling term. The non-minimal interaction together with the Holst modification to the Hilbert-Palatini action reconstruct the Nieh-Yan invariant, so that the effective action coming out is the one of Einstein-Cartan theory with a typical Fermi-like interaction term: in spite of the presence of spinor matter fields, the Immirzi parameter plays no role in the classical effective dynamics and results to be only a multiplicative factor in front of a total divergence.
We reduce the total action of the theory to the sum of dynamically independent Ashtekar-Romano-Tate actions for self and anti-self dual connections, with different weights depending on the Immirzi parameter. This allows to calculate the constraints of the complete theory in a simple way, it is only necessary to realize that the Barbero-Immirzi connection is a weighted sum of the self and anti-self dual Ashtekar connections. Finally the obtained constraints for the separated action result to be polynomial in terms of the self and anti-self dual connections, this could have implications in the inclusion of spinor matter in the framework of non-perturbative quantum gravity."From page 5, discussion of equation (22):

"This result shows that the Immirzi parameter, which appears in the action for the gravitational field used as starting point in the construction of LQG, not only appears in non-perturbative quantum effects but also in the classical equations of motion, when fermions are present, leading to (independently from the quantum theory) possible observable effects. It plays the role of coupling constant in front of the four fermions interacting term..."

 

[marcus]

new Ashtekar paper, quantum nature of big bang

http://arxiv.org/abs/gr-qc/0604013
Quantum Nature of the Big Bang: An Analytical and Numerical Investigation I
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
59 pages, 19 figures

"Analytical and numerical methods are developed to analyze the quantum nature of the big bang in the setting of loop quantum cosmology. They enable one to explore the effects of quantum geometry both on the gravitational and matter sectors and significantly extend the known results on the resolution of the big bang singularity. Specifically, the following results are established for the homogeneous isotropic model with a massless scalar field: i) the scalar field is shown to serve as an internal clock, thereby providing a detailed realization of the `emergent time' idea; ii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously; iii) the Hamiltonian constraint is solved numerically to show that the big bang is replaced by a big bounce. Thanks to the non-perturbative, background independent methods, unlike in other approaches the quantum evolution is deterministic across the deep Planck regime. Our constructions also provide a conceptual framework and technical tools which can be used in more general models. In this sense, they provide foundations for analyzing physical issues associated with the Planck regime of loop quantum cosmology as a whole."
===============
not sure about this. any discussion of making QM deterministic is bound to be over my head. but there was discussion of this paper in a separate thread---some folks might be interested in checking it out

http://arxiv.org/abs/quant-ph/0604008
The mathematical basis for deterministic quantum mechanics
Gerard 't Hooft
15 pages, 3 figures

"If there exists a classical, i.e. deterministic theory underlying quantum mechanics, an explanation must be found of the fact that the Hamiltonian, which is defined to be the operator that generates evolution in time, is bounded from below. The mechanism that can produce exactly such a constraint is identified in this paper. It is the fact that not all classical data are registered in the quantum description. Large sets of values of these data are assumed to be indistinguishable, forming equivalence classes. It is argued that this should be attributed to information loss, such as what one might suspect to happen during the formation and annihilation of virtual black holes.
The nature of the equivalence classes is further elucidated, as it follows from the positivity of the Hamiltonian. Our world is assumed to consist of a very large number of subsystems that may be regarded as approximately independent, or weakly interacting with one another. As long as two (or more) sectors of our world are treated as being independent, they all must be demanded to be restricted to positive energy states only. What follows from these considerations is a unique definition of energy in the quantum system in terms of the periodicity of the limit cycles of the deterministic model."

 

[marcus]

new Freidel paper

http://arxiv.org/abs/gr-qc/0604016
Hidden Quantum Gravity in 3d Feynman diagrams
Aristide Baratin, Laurent Freidel
35 pages, 4 figures
"In this work we show that 3d Feynman amplitudes of standard QFT in flat and homogeneous space can be naturally expressed as expectation values of a specific topological spin foam model. The main interest of the paper is to set up a framework which gives a background independent perspective on usual field theories and can also be applied in higher dimensions. We also show that this Feynman graph spin foam model, which encodes the geometry of flat space-time, can be purely expressed in terms of algebraic data associated with the Poincare group. This spin foam model turns out to be the spin foam quantization of a BF theory based on the Poincare group, and as such is related to a quantization of 3d gravity in the limit where the Newton constant G_N goes to 0. We investigate the 4d case in a companion paper where the strategy proposed here leads to similar results."

 

[marcus]

Rovelli, graviton propagator

http://arxiv.org/abs/gr-qc/0604044
Graviton propagator in loop quantum gravity
Eugenio Bianchi, Leonardo Modesto, Carlo Rovelli, Simone Speziale
41 pages, 6 figures
"We compute some components of the graviton propagator in loop quantum gravity, using the spinfoam formalism, up to some second order terms in the expansion parameter."

===========================
Chapter for Oriti's book "Towards Quantum Gravity"

http://arxiv.org/abs/gr-qc/0604045
Unfinished revolution
Carlo Rovelli
8 pages

"Introductive chapter of a book on Quantum Gravity, edited by Daniele Oriti, that will appear with Cambridge University Press."

=================

http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
7 pages
"We argue that EPR-type correlations do not entail any form of "non-locality", when viewed in the context of a relational interpretation of quantum mechanics. The abandonment of strict Einstein realism advocated by this interpretation permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."

=================

http://arxiv.org/abs/gr-qc/0604050
Tachyon Matter in Loop-Inspired Cosmology
A. A. Sen
5 pages

"An analytical approach for studying the cosmological scenario with a homogeneous tachyon field within the framework of loop quantum gravity is developed. Our study is based on the semi-classical regime where space time can be approximated as a continuous manifold, but matter Hamiltonian gets non-perturbative quantum corrections. A formal correspondence between classical and loop quantum cosmology is also established. The Hamilton-Jacobi method for getting exact solutions is constructed and some exact power-law as well as bouncing solutions are presented."

http://arxiv.org/abs/gr-qc/0604040
Macroscopic observables and Lorentz violation in discrete quantum gravity
Joe Henson
15 pages, 1 figure

"This article concerns the fate of local Lorentz invariance in quantum gravity, particularly for approaches in which a discrete structure replaces continuum spacetime. Some features of standard quantum mechanics, presented in a sum-over-histories formulation, are reviewed, and their consequences for such theories are discussed. It is argued that, if the individual histories of a theory give bad approximations to macroscopic continuum properties in some frames, then it is inevitable that the theory violates Lorentz symmetry.

 

[marcus]

I don't think discussion of anthropics is so important because I expect nonperturbative approaches to Quantum Gravity (the new understanding of spacetime and matter that is emerging, see Oriti's book when it comes out will explain the CC---what its role is and why it is what it is.

And then people will lose interest in the "Anthropic Lack of Principles" as it has been called. But meanwhile, here's something to check out!

http://arxiv.org/abs/astro-ph/0604242
An Observational Test for the Anthropic Origin of the Cosmological Constant
Abraham Loeb (Harvard)
5 pages, submitted to JCAP

"The existence of multiple regions of space beyond the observable Universe (within the so-called "multiverse") where the vacuum energy density takes different values, has been postulated as an explanation for the low non-zero value observed for it in our Universe. It is often argued that our existence pre-selects regions where the cosmological constant is sufficiently small to allow galaxies like the Milky Way to form and intelligent life to emerge. Here we propose a simple empirical test for this anthropic argument within the boundaries of the observable Universe. We make use of the fact that dwarf galaxies formed in our Universe at redshifts as high as z~10 when the mean matter density was larger by a factor of ~10^3 than today. Existing technology enables to check whether planets form in nearby dwarf galaxies and globular clusters by searching for microlensing or transit events of background stars. The oldest of these nearby systems may have formed at z~10. If planets are as common per stellar mass in these descendents as they are in the Milky Way galaxy, then the anthropic argument would be weakened considerably since planets could have formed in our Universe even if the cosmological constant was three orders of magnitude larger than observed. For a flat probability distribution, this would imply that the probability for us to reside in a region where the cosmological constant obtains its observed value is lower than approx. 10^{-3}. A precise version of the anthropic argument could then be ruled-out at a confidence level of ~99.9%, which constitutes a satisfactory measure of a good experimental test."http://arxiv.org/abs/gr-qc/0604053
Violation of Strong Energy Condition in Effective Loop Quantum Cosmology
Hua-Hui Xiong, Jian-Yang Zhu
7 pages

must keep tabs on cousins at Beijing Normal----getting quite strong in LQG and LQC.
several presentations from Beijing Normal at Loops '05 last October.

 

[marcus]

Joao Magueijo is a hero of mine, ever since I watched an online video seminar talk of him from February 2006 at Perimeter.

I really liked a MOND paper he wrote with Bekenstein posted this year.
Now he has a paper with Rafael Sorkin----familiar to everybody too I guess.
I personally could not get much out of this Magueijo Sorkin paper. But because of who they are, and because AXIS OF EVIL is topical in cosmology and because of WMAP3, I figure there might be someone to whom this is interesting. Even though I don't someone else might.

http://arxiv.org/abs/astro-ph/0604410
Occam's razor meets WMAP
Joao Magueijo, Rafael D. Sorkin

"Using a variety of quantitative implementations of Occam's razor we examine the low quadrupole, the ``axis of evil'' effect and other detections recently made appealing to the excellent WMAP data. We find that some razors fully demolish the much lauded claims for departures from scale-invariance. They all reduce to pathetic levels the evidence for a low quadrupole (or any other low ell cut-off), both in the first and third year WMAP releases. The 'axis of evil' effect is the only anomaly examined here that survives the humiliations of Occam's razor, and even then in the category of 'strong' rather than 'decisive' evidence. Statistical considerations aside, differences between the various renditions of the datasets remain worrying."Hey look again, this shows ago that Magueijo is a lean-mean phenomenologist. Lots of other folks are talking about the axis of evil and believing departure from scale-invariance at large angles and he says NO! the confidence levels for most of that are pitiful. At least he is frank and willing to take contrary stands. It might be a good paper.

Magueijo has QG importance because he co-authors with Lee Smolin and is into QG phenomenology.

=================================

Couple of Shahn Majid QG-related papers came out today:
OTE=marcus]http://arxiv.org/abs/hep-th/0604130
Algebraic approach to quantum gravity II: noncommutative spacetime
S. Majid
26 pages, 2 figures; book chapter to appear in D. Oriti, ed., Cambridge Univ. Press
"We provide a self-contained introduction to the quantum group approach to noncommutative geometry as the next-to-classical effective geometry that might be expected from any successful quantum gravity theory. We focus particularly on a thorough account of the bicrossproduct model noncommutative spacetimes of the form [t,x_i]=i lambda x_i and the correct formulation of predictions for it including a variable speed of light. We also study global issues in the Poincaré group in the model with the 2D case as illustration. We show that any off-shell momentum can be boosted to infinite negative energy by a finite Lorentz transformaton."

http://arxiv.org/abs/hep-th/0604132
Algebraic approach to quantum gravity III: noncommmutative Riemannian geometry
S. Majid
25 pages, 1 figure; to appear in collection B. Fauser and J. Tolksdorf, eds., Birkhauser

"This is a self-contained introduction to quantum Riemannian geometry based on quantum groups as frame groups, and its proposed role in quantum gravity. Much of the article is about the generalisation of classical Riemannian geometry that arises naturally as the classical limit; a theory with nonsymmetric metric and a skew version of metric compatibilty. Meanwhile, in quantum gravity a key ingredient of our approach is the proposal that the differential structure of spacetime is something that itself must be summed over or 'quantise' as a physical degree of freedom. We illustrate such a scheme for quantum gravity on small finite sets."

 

[marcus]

http://arxiv.org/abs/gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
"Non-minimally coupling a scalar field to gravity introduces an additional curvature term into the action which can change the general behavior in strong curvature regimes, in particular close to classical singularities. While one can conformally transform any non-minimal model to a minimally coupled one, that transformation can itself become singular. It is thus not guaranteed that all qualitative properties are shared by minimal and non-minimal models. This paper addresses the classical singularity issue in isotropic models and extends singularity removal in quantum gravity to non-minimal models."

http://arxiv.org/abs/hep-th/0604181
6J Symbols Duality Relations
L. Freidel, K. Noui, P. Roche
28 pages, 2 figures
"It is known that the Fourier transformation of the square of (6j) symbols has a simple expression in the case of su(2) and U_q(su(2)) when q is a root of unity. The aim of the present work is to unravel the algebraic structure behind these identities. We show that the double crossproduct construction $$H_1\bowtie H_2$$ of two Hopf algebras and the bicrossproduct construction
$$H_2^{*}\bowtie H_1$$
are the Hopf algebras structures behind these identities by analysing different examples. We study the case where
$$D= H_1\bowtie H_2$$
is equal to the group algebra of ISU(2), SL(2,C) and where D is a quantum double of a finite group, of SU(2) and of U_q(su(2)) when q is real."

the year 2006 is turning out to be the wonderyear of Laurent Freidel

http://arxiv.org/abs/hep-th/0604184
Towards a solution of pure Yang-Mills theory in 3+1 dimensions
Laurent Freidel, Robert G. Leigh, Djordje Minic
12 pages
We discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime. The approach is based on the use of local gauge invariant variables in the Schrödinger representation and the large N, planar limit. In particular, within this approach we point out unexpected parallels between pure Yang-Mills theory in 2+1 and 3+1 dimensions. The most important parallel shows up in the analysis of the ground state wave-functional especially in view of the numerical similarity of the existing large N lattice simulations of the spectra of 2+1 and 3+1 Yang Mills theories.

http://arxiv.org/abs/hep-th/0604185
On pure Yang-Mills theory in 3+1 dimensions: Hamiltonian, vacuum and gauge invariant variables
Laurent Freidel
35 pages
In this work we discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime which strongly suggests that the recent strategy already applied to pure Yang-Mills theory in 2+1 can be extended to 3+1 dimensions. We show that the local gauge invariant variables introduced by Bars gives a natural generalisation to any dimension of the formalism of Karabali and Nair which recently led to a new understanding of the physics of QCD in dimension 2+1. After discussing the kinematics of these variables, we compute the jacobian between the Yang-Mills and Bars variables and propose a regularization procedure which preserves a generalisation of holomorphic invariance. We discuss the construction of the QCD hamiltonian properly regularized and compute the behavior of the vacuum wave functional both at weak and strong coupling. We argue that this formalism allows the developpement of a strong coupling expansion in the continuum by computing the first local eigenstate of the kinetic part of Yang-Mills hamiltonian.
-------------------

the next paper is discussed in Bee's thread about it here
https://www.physicsforums.com/showthread.php?t=118641

http://arxiv.org/abs/gr-qc/0508013
Anti-Gravitation
S. Hossenfelder
Phys. Lett. B 636 (2006) 119-125

 

[marcus]

http://arxiv.org/abs/gr-qc/0604112
Background independent quantizations: the scalar field II
W. Kaminski, J. Lewandowski, A. Okolow
51 pages
"We are concerned with the issue of quantization of a scalar field in a diffeomorphism invariant manner. We apply the method used in Loop Quantum Gravity. It relies on the specific choice of scalar field variables referred to as the polymer variables. The quantization, in our formulation, amounts to introducing the `quantum' polymer *-star algebra and looking for positive linear functionals, called states. Assumed in our paper homeomorphism invariance allows to derive the complete class of the states. They are determined by the homeomorphism invariant states defined on the CW-complex *-algebra. The corresponding GNS representations of the polymer *-algebra and their self-adjoint extensions are derived, the equivalence classes are found and invariant subspaces characterized. In the preceding letter (the part I) we outlined those results. Here, we present the technical details."

can't evaluate the next one. list it in part to keep tabs on the Karpacz Winterschool, which several times has chosen a topic related to DSR or quantum gravity

http://arxiv.org/abs/gr-qc/0604120
The Chrono-geometrical Structure of Special and General Relativity: a Re-Visitation of Canonical Geometrodynamics
Luca Lusanna (INFN, Firenze)
33 pages, Lectures given at the 42nd Karpacz Winter School of Theoretical Physics, "Current Mathematical Topics in Gravitation and Cosmology", Ladek, Poland, 6-11 February 2006
A modern re-visitation of the consequences of the lack of an intrinsic notion of instantaneous 3-space in relativistic theories leads to a reformulation of their kinematical basis emphasizing the role of non-inertial frames centered on an arbitrary accelerated observer. In special relativity the exigence of predictability implies the adoption of the 3+1 point of view, which leads to a well posed initial value problem for field equations in a framework where the change of the convention of synchronization of distant clocks is realized by means of a gauge transformation. This point of view is also at the heart of the canonical approach to metric and tetrad gravity in globally hyperbolic asymptotically flat space-times, where the use of Shanmugadhasan canonical transformations allows the separation of the physical degrees of freedom of the gravitational field (the tidal effects) from the arbitrary gauge variables. Since a global vision of the equivalence principle implies that only global non-inertial frames can exist in general relativity, the gauge variables are naturally interpreted as generalized relativistic inertial effects, which have to be fixed to get a deterministic evolution in a given non-inertial frame. As a consequence, in each Einstein's space-time in this class the whole chrono-geometrical structure, including also the clock synchronization convention, is dynamically determined and a new approach to the Hole Argument leads to the conclusion that "gravitational field" and "space-time" are two faces of the same entity. This view allows to get a classical scenario for the unification of the four interactions in a scheme suited to the description of the solar system or our galaxy with a deperametrization to special relativity and the subsequent possibility to take the non-relativistic limit."

several former string theorists are publishing mainly in pure Mathematics now,
I don't know about Michael Douglas but he has posted some papers in the math part of arxiv.
the most recent is today
http://arxiv.org/abs/math.AG/0604597

Algebraic Geometry no less! classy!

 

[marcus]

http://arxiv.org/abs/hep-th/0604212
Quantum Gravity, or The Art of Building Spacetime
J. Ambjorn, J. Jurkiewicz, R. Loll
22 pages, 6 figures. Contribution to the book "Approaches to Quantum Gravity", ed. D. Oriti, Cambridge University Press

"The method of four-dimensional Causal Dynamical Triangulations provides a background-independent definition of the sum over geometries in quantum gravity, in the presence of a positive cosmological constant. We present the evidence accumulated to date that a macroscopic four-dimensional world can emerge from this theory dynamically. Using computer simulations we observe in the Euclidean sector a universe whose scale factor exhibits the same dynamics as that of the simplest mini-superspace models in quantum cosmology, with the distinction that in the case of causal dynamical triangulations the effective action for the scale factor is not put in by hand but obtained by integrating out in the quantum theory the full set of dynamical degrees of freedom except for the scale factor itself."



http://arxiv.org/abs/gr-qc/0604124
On Recovering Continuum Topology from a Causal Set
Seth Major, David Rideout, Sumati Surya
31 pages, 5 figs. Dedicated to our friend and teacher Rafael Sorkin, to celebrate his 60th year

"An important question that discrete approaches to quantum gravity must address is how continuum features of spacetime can be recovered from the discrete substructure. Here, we examine this question within the causal set approach to quantum gravity, where the substructure replacing the spacetime continuum is a locally finite partial order. A new topology on causal sets using 'thickened antichains' is constructed. This topology is then used to recover the homology of a globally hyperbolic spacetime from a causal set which faithfully embeds into it at sufficiently high sprinkling density. This implies a discrete-continuum correspondence which lends support to the fundamental conjecture or 'Hauptvermutung' of causal set theory."

http://arxiv.org/abs/gr-qc/0605006
Discreteness without symmetry breaking: a theorem
Luca Bombelli, Joe Henson, Rafael D. Sorkin
7 pages
"This paper concerns sprinklings into Minkowski space (Poisson processes). It proves that there exists no equivariant measurable map from sprinklings to spacetime directions (even locally). Therefore, if a discrete structure is associated to a sprinkling in an intrinsic manner, then the structure will not pick out a preferred frame, locally or globally. This implies that the discreteness of a sprinkled causal set will not give rise to 'Lorentz breaking'' effects like modified dispersion relations. Another consequence is that there is no way to associate a finite-valency graph to a sprinkling consistently with Lorentz invariance."

 

[marcus]

http://arxiv.org/abs/gr-qc/0605011
Gravity, Geometry and the Quantum
Abhay Ashtekar
16 pages, 2 figures. To appear in the Proceedings of the `Einstein Century' Conference, 15-22 July, Paris, edited by J-M Alimi et al (American Institute of Physics)

"After a brief introduction, basic ideas of the quantum Riemannian geometry underlying loop quantum gravity are summarized. To illustrate physical ramifications of quantum geometry, the framework is then applied to homogeneous isotropic cosmology. Quantum geometry effects are shown to replace the big bang by a big bounce. Thus, quantum physics does not stop at the big-bang singularity. Rather there is a pre-big-bang branch joined to the current post-big-bang branch by a 'quantum bridge'. Furthermore, thanks to the background independence of loop quantum gravity, evolution is deterministic across the bridge."

http://arxiv.org/abs/gr-qc/0605014
Entropy counting for microscopic black holes in LQG
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 6 figures

"Quantum black holes within the loop quantum gravity (LQG) framework are considered. The number of microscopic states that are consistent with a black hole of a given horizon area A_0 are computed and the statistical entropy, as a function of the area, is obtained for A_0 up to 550 l^2_P. The results are consistent with an asymptotic linear relation and a logarithmic correction with a coefficient equal to -1/2. The Barbero-Immirzi parameter that yields the asymptotic linear relation compatible with the Bekenstein-Hawking entropy is shown to coincide with a value close to \gamma=0.274, which has been previously obtained analytically. However, a new and unexpected functional form for the entropy is found for small, Planck size, black holes that calls for a physical interpretation."

=============
a lot depends on the GLAST mission, I am hoping very much that it flies in 2007 as scheduled
this technical article is a sign that the mission is still going ahead
http://arxiv.org/abs/astro-ph/0605050
GLAST Tracker
Hiroyasu Tajima, for GLAST Tracker Team (Stanford Linear Accelerator Center)
5 pages, 5 figues, Invited talk at Vertex 2005, Chuzenji Lake, Nikko, Japan, November 7-November 11, 2005, To be published in Nuclear Instruments and Methods A
SLAC-PUB-11833
"The Large Area Telescope (LAT) on board the Gamma-ray Large-Area Space Telescope (GLAST) is a pair-conversion gamma-ray detector designed to explore the gamma-ray universe in the 20 MeV-300 GeV energy band. The Tracker subsystem of the LAT will perform tracking of electron and positrons to determine the origin of the gamma-ray. The design and performance of the GLAST LAT Tracker are described in this paper."

 

[marcus]

http://arxiv.org/abs/hep-th/0605052
Generic predictions of quantum theories of gravity
Lee Smolin
For inclusion in "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

"I discuss generic consequences (sometimes called "soft predictions") of a class of background independent quantum theories of spacetime called causal spin network theories. These are theories whose kinematics and dynamics is based on the evolution of labeled graphs, by local moves, such as in loop quantum gravity and spin foam models. Some generic consequences are well known, including the discreteness of quantum geometry, the elimination of spacetime singularities, the entropy of black hole and cosmological horizons and the fact that positive cosmological constant spacetimes are hot. Within the last few years three possible generic consequences have come to light. These are:

1) Deformed special relativity as the symmetry of the ground state,

2) Elementary particles as coherent excitations of quantum geometry,

3) Locality is disordered.
I discuss some possible experimental consequences of each."

 

[marcus]

Larry Krauss is a prominent cosmophysicist and neutrinos get into everything including ultimately QG and how we will observe the very early universe. Neutrinos-in-cosmology deserves a wide-audience survey and Krauss has the writing style to give a good one.

http://arxiv.org/abs/astro-ph/0605378
What is the Role of Neutrinos in Shaping the Universe?
Lawrence M. Krauss (Case Western Reserve University)
10 pages. Invited review lecture, International Workshop on NO-VE, Venice, 2006

"I review various aspects of the role neutrinos have played in shaping various cosmological observables: the nature of large scale structure, observed fluctuations in the CMB, the nature of matter, and the shape of things to come. (Invited review lecture III International Workshop on NO-VE, Venice, 2006)"

==========

Back in post 471, in March, I mentioned this paper of Mohammad Ansari, which Danny recently brought up in a post:
"Recently, I read a beautiful paper in which it is proven that ANY surface in LQG contains degeneracy, no matter it being a boundary horizon or whatever else. (http://uk.arxiv.org/abs/gr-qc/0603121) This degeneracy is such that the surface degeneracy is A/4. This is a critial discovery in LQG and can follow up Padmanabhan's idea of associating the lack of information in (http://uk.arxiv.org/abs/gr-qc/0405072) into quantum gravity."

post 471 is here
https://www.physicsforums.com/showpost.php?p=951694&postcount=471
https://www.physicsforums.com/showthread.php?p=951694#post951694
http://arxiv.org/abs/gr-qc/0603121
Entanglement entropy in loop quantum geometry
Mohammad H. Ansari
4 pages, 3 figures
"The notion of entanglement entropy of a non-rotating black hole in the context of loop quantum geometry is introduced. By defining the notion of degeneracy of spin network states and determining the Barbero-Immirzi parameter from the highly damping quasinormal modes, we show that the entropy of SO(3) representations coincides with the Bekenstein-Hawking entropy."

that post also had an article about the Immirzi parameter by Simone Mercuri which was accepted for
publication by Physical Review D

Danny's thread about the Ansari paper is here
https://www.physicsforums.com/showthread.php?t=120887
called "Ansari's degeneracy in LQG"
=============================
this appeared yesterday

http://arxiv.org/abs/gr-qc/0605087
Quantization of strings and branes coupled to BF theory
John C. Baez, Alejandro Perez
"BF theory is a topological theory that can be seen as a natural generalization of 3-dimensional gravity to arbitrary dimensions. Here we show that the coupling to point particles that is natural in three dimensions generalizes in a direct way to BF theory in d dimensions coupled to (d-3)-branes. In the resulting model, the connection is flat except along the membrane world-sheet, where it has a conical singularity whose strength is proportional to the membrane tension. As a step towards canonically quantizing these models, we show that a basis of kinematical states is given by 'membrane spin networks', which are spin networks equipped with extra data where their edges end on a brane."

it is a followup on the Baez Wise Crans article mentioned earlier in this thread
Exotic statistics for loops in 4d BF theory
http://arxiv.org/gr-qc/0603085 =====================

Ignacio Navarro and Karel Van Acoleyen recently posted two articles on a variant of MOND
http://arxiv.org/abs/astro-ph/0605322
Dark energy, MOND and sub-millimeter tests of gravity
I. Navarro, K. Van Acoleyen
6 pages, to appear in proceedings of the XLIrst Rencontres de Moriond

"We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds."http://arxiv.org/abs/gr-qc/0605091
Long distance modifications of gravity in four dimensions
I. Navarro, K. Van Acoleyen
4 pages, contribution to the proceedings of the Rencontres de Moriond: Contents and Structures of the Universe, March 18-25, 2006, La Thuile

"We discuss some general characteristics of modifications of the 4D Einstein-Hilbert action that become important for low space-time curvatures. In particular we focus on the chameleon-like behaviour of the massive gravitational degrees of freedom. Generically there is at least one extra scalar that is light on cosmic scales, but for certain models it becomes heavy close to any mass source."

an earlier paper from last year by Navarro and Van Acoleyen
http://arxiv.org/abs/gr-qc/0512109
Modified gravity, Dark Energy and MOND
Ignacio Navarro, Karel Van Acoleyen
24 pages, 2 figures.

"We propose a class of actions for the spacetime metric that introduce corrections to the Einstein-Hilbert Lagrangian depending on the logarithm of some curvature scalars. We show that for some choices of these invariants the models are ghost free and modify Newtonian gravity below a characteristic acceleration scale given by a_0 = c mu, where c is the speed of light and mu is a parameter of the model that also determines the late-time Hubble constant: H_0 ~mu.

In these models, besides the massless spin two graviton, there is a scalar excitation of the spacetime metric whose mass depends on the background curvature. This dependence is such that this scalar, although almost massless in vacuum, becomes massive and effectively decouples when one gets close to any source and we recover an acceptable weak field limit at short distances. There is also a (classical) 'running' of Newton's constant with the distance to the sources and gravity is easily enhanced at large distances by a large ratio. We comment on the possibility of building a model with a MOND-like Newtonian limit that could explain the rotation curves of galaxies without introducing Dark Matter using this kind of actions. We also explore briefly the characteristic gravitational phenomenology that these models imply: besides a long distance modification of gravity they also predict deviations from Newton's law at short distances. This short distance scale depends on the local background curvature of spacetime, and we find that for experiments on the Earth surface it is of order ~ 0.1mm, while this distance would be bigger in space where the local curvature is significantly lower."

==========
more work on black hole state counting and determining the Immirzi parameter
http://arxiv.org/abs/hep-th/0605125
Counting of isolated horizon states
A. Ghosh, P. Mitra
4 pages
"The entropy of an isolated horizon has been obtained by counting states in loop quantum gravity. We revisit the calculation of the horizon states using statistical methods and find the possibility of additional states, leading to an increase in the entropy. Apart from this, an isolated horizon temperature is introduced in this framework."

the paper of Ghosh and Mitra cites this recent one of Corichi et al
http://arxiv.org/abs/gr-qc/0605014
Entropy counting for microscopic black holes in LQG
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 6 figures

"Quantum black holes within the loop quantum gravity (LQG) framework are considered. The number of microscopic states that are consistent with a black hole of a given horizon area A₀ are computed and the statistical entropy, as a function of the area, is obtained for A₀ up to 550 L²_P The results are consistent with an asymptotic linear relation and a logarithmic correction with a coefficient equal to -1/2. The Barbero-Immirzi parameter that yields the asymptotic linear relation compatible with the Bekenstein-Hawking entropy is shown to coincide with a value close to gamma = 0.274, which has been previously obtained analytically. However, a new and unexpected functional form for the entropy is found for small, Planck size, black holes that calls for a physical interpretation."

The recent paper of Ghosh Mitra also cites one of theirs that was published in Physical Review Letters B, in 2005. It gives the same value of the Immirzi parameter, namely about 0.274.

http://arxiv.org/abs/gr-qc/0411035
An improved estimate of black hole entropy in the quantum geometry approach
A. Ghosh, P. Mitra
5 pages, LaTeX
Journal-ref: Phys.Lett. B616 (2005) 114-117

"A proper counting of states for black holes in the quantum geometry approach shows that the dominant configuration for spins are distributions that include spins exceeding one-half at the punctures. This raises the value of the Immirzi parameter and the black hole entropy. However, the coefficient of the logarithmic correction remains -1/2 as before."

=====================
not sure about this: an odd thing, don't remember ever seeing anything like it
http://arxiv.org/abs/gr-qc/0605098
Area and Entropy: A New Perspective
Jarmo Mäkelä
4 pages. This paper received an honorary mention in the annual Awards for Essays in Gravitation contest

"We consider a spacelike two-plane originally at rest with respect to electromagnetic radiation in equilibrium. We find that if the plane is moved with respect to the radiation, the plane shrinks such that the maximum amount of entropy carried by radiation through the plane is, in natural units, exactly one-half of the decrease in the area of the plane. This result suggests that the equivalence between area and entropy may not be limited in black holes, nor even in the spacetime horizons only, but the equivalence between horizon area and entropy may be a special case of some general and simple, still undiscovered fundamental principle of nature."

 

[marcus]

http://arxiv.org/abs/gr-qc/0605113
Avoidance of future singularities in loop quantum cosmology
M. Sami, Parampreet Singh, Shinji Tsujikawa
6 pages, 2 figures

"We consider the fate of future singularities in the effective dynamics of loop quantum cosmology. Non-perturbative quantum geometric effects which lead to $\rho^2$ modification of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density $\rho$ diverges at future singularities of Friedmann dynamics. Such quantum effects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a finite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory."

I list the following since is to appear in Oriti's book
http://arxiv.org/abs/hep-th/0605202
String Field Theory
Washington Taylor (MIT, Stanford)
To appear in "Towards Quantum Gravity", ed. Daniele Oriti, Cambridge University Press; 22 pages
"This elementary introduction to string field theory highlights the features and the limitations of this approach to quantum gravity as it is currently understood. String field theory is a formulation of string theory as a field theory in space-time with an infinite number of massive fields. Although existing constructions of string field theory require expanding around a fixed choice of space-time background, the theory is in principle background-independent, in the sense that different backgrounds can be realized as different field configurations in the theory. String field theory is the only string formalism developed so far which, in principle, has the potential to systematically address questions involving multiple asymptotically distinct string backgrounds. Thus, although it is not yet well defined as a quantum theory, string field theory may eventually be helpful for understanding questions related to cosmology in string theory."

the following really are raw lecture notes, very sketchy, with some spelling mistakes and awkward wording, by someone at Ben-Gurion university who wants to cover all the QM you ever heard of, at an undergraduate level, in 200-some pages. It is an exercise in writing down one or two sentences and half dozen equations, and moving on. Like a stand-up comedian or serial exhibitionist. Frankly I thought it was a good idea---it had more EFFRONTERY than the usual textbook, I thought, so I wanted to include it:

http://arxiv.org/abs/quant-ph/0605180
Lecture Notes in Quantum Mechanics
Doron Cohen
239 pages
"These lecture notes cover undergraduate textbook topics (e.g. as in Sakurai), and also additional advanced topics at the same level of presentation. In particular: EPR and Bell; Basic postulates; The probability matrix; Measurement theory; Entanglement; Quantum computation; Wigner-Weyl formalism; The adiabatic picture; Berry phase; Linear response theory; Kubo formula; Modern approach to scattering theory with mesoscopic orientation; Theory of the resolvent and the Green function; Gauge and Galilei Symmetries; Motion in magnetic field; Quantum Hall effect; Quantization of the electromagnetic field; Fock space formalism."

Let me know if you check it out and think I should not have included it.

 

[marcus]

http://arxiv.org/abs/gr-qc/0605123
Towards the graviton from spinfoams: higher order corrections in the 3d toy model
Etera R. Livine, Simone Speziale, Joshua L. Willis
24 pages, many figures
"We consider the recent calculation gr-qc/0508124 of the graviton propagator in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102, we test how the spinfoam formalism can be used to construct the perturbative expansion of graviton amplitudes. We compute explicitly the next to leading and next to next to leading orders, corresponding to one-loop and two-loop corrections. We show that while the first arises entirely from the expansion of the Regge action around the flat background, the latter receives contributions from the microscopic, non Regge-like, quantum geometry. Surprisingly, this new contribution reduces the magnitude of the next to next to leading order. It thus appears that the spinfoam formalism is likely to substantially modify the conventional perturbative expansion at higher orders.
This result supports the interest in this approach. We then address a number of open issues in the rest of the paper. First, we discuss the boundary state ansatz, which is a key ingredient in the whole construction. We propose a way to enhance the ansatz in order to make the edge lengths and dihedral angles conjugate variables in a mathematically well-defined way. Second, we show that the leading order is stable against different choices of the face weights of the spinfoam model; the next to leading order, on the other hand, is changed in a simple way, and we show that the topological face weight minimizes it. Finally, we extend the leading order result to the case of a regular, but not equilateral, tetrahedron."

 

[marcus]

just keeping the link-basket up to date:
===========
http://arxiv.org/abs/gr-qc/0605141
Time Delay Predictions in a Modified Gravity Theory
J. W. Moffat
5 pages
"The time delay effect for planets and spacecraft is obtained from a fully relativistic modified gravity theory including a fifth force skew symmetric field by fitting to the Pioneer 10/11 anomalous acceleration data. A possible detection of the predicted time delay corrections to general relativity for the outer planets and future spacecraft missions is considered. The time delay correction to GR predicted by the modified gravity is consistent with the observational limit of the Doppler tracking measurement reported by the Cassini spacecraft on its way to Saturn, and the correction increases to a value that could be measured for a spacecraft approaching Neptune and Pluto."

?
========
http://arxiv.org/abs/gr-qc/0605138
Quantization of diffeomorphism invariant theories of connections with a non-compact structure group - an example
Andrzej Okolow
76 pages,
"A simple diffeomorphism invariant theory of connections with the non-compact structure group R of real numbers is quantized. The theory is defined on a four-dimensional 'space-time' by an action resembling closely the self-dual Plebanski action for general relativity. The space of quantum states is constructed by means of projective techniques by Kijowski. Except this point the applied quantization procedure is based on Loop Quantum Gravity methods."

Okolow is the "O" in the L.O.S.T theorem

 

[marcus]

http://arxiv.org/abs/gr-qc/0606032
Non-Singular Bouncing Universes in Loop Quantum Cosmology
Parampreet Singh, Kevin Vandersloot, G. V. Vereshchagin
12 pages, 9 figures

"Non-perturbative quantum geometric effects in Loop Quantum Cosmology predict a rho^2 modification to the Friedmann equation at high energies. The quadratic term is negative definite and can lead to generic bounces when the matter energy density becomes equal to a critical value of the order of the Planck density. The non-singular bounce is achieved for arbitrary matter without violation of positive energy conditions. By performing a qualitative analysis we explore the nature of the bounce for inflationary and Cyclic model potentials. For the former we show that inflationary trajectories are attractors of the dynamics after the bounce implying that inflation can be harmoniously embedded in LQC. For the latter difficulties associated with singularities in cyclic models can be overcome. We show that non-singular cyclic models can be constructed with a small variation in the original Cyclic model potential by making it slightly positive in the regime where scalar field is negative."

http://arxiv.org/abs/gr-qc/0604124
On Recovering Continuum Topology from a Causal Set
Seth Major, David Rideout, Sumati Surya
31 pages, 5 figs. Dedicated to our friend and teacher Rafael Sorkin, to celebrate his 60th year.

"An important question that discrete approaches to quantum gravity must address is how continuum features of spacetime can be recovered from the discrete substructure. Here, we examine this question within the causal set approach to quantum gravity, where the substructure replacing the spacetime continuum is a locally finite partial order. A new topology on causal sets using 'thickened antichains' is constructed. This topology is then used to recover the homology of a globally hyperbolic spacetime from a causal set which faithfully embeds into it at sufficiently high sprinkling density. This implies a discrete-continuum correspondence which lends support to the fundamental conjecture or 'Hauptvermutung' of causal set theory."

http://arxiv.org/abs/gr-qc/0606026
The black hole horizon as a dynamical system
Gerard 't Hooft
16 pages. Presented at "Einstein's Legacy in the New Millennium", Puri, India, 15-22 Dec. 2005. An extension of gr-qc/0504120 with chapter added and small sign changes

"Interactions between outgoing Hawking particles and ingoing matter are determined by gravitational forces and Standard Model interactions. In particular the gravitational interactions are responsible for the unitarity of the scattering against the horizon, as dictated by the holographic principle, but the Standard Model interactions also contribute, and understanding their effects is an important first step towards a complete understanding of the horizon's dynamics. The relation between in- and outgoing states is described in terms of an operator algebra. In this contribution, in which earlier results are rederived and elaborated upon, we first describe the algebra induced on the horizon by U(1) vector fields and scalar fields, including the case of an Englert-Brout-Higgs mechanism, and a more careful consideration of the transverse vector field components. We demonstrate that, unlike classical black holes, the quantized black hole has on its horizon an imprint of its (recent) past history, i.e., quantum hair. The relation between in- and outgoing states depends on this imprint. As a first step towards the inclusion of non-Abelian interactions, we then compute the effects of magnetic monopoles both in the in-states and in the out-states. They completely modify, and indeed simplify, our algebra."John Baez has an online video of an interesting talk given 31 May at Perimeter

Higher-Dimensional Algebra: A Langauge for Quantum Spacetime

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
select "seminar series" from the left margin menu

the slides/lecturenotes for this talk are available at Baez site

http://math.ucr.edu/home/baez/quantum_spacetime/

http://math.ucr.edu/home/baez/quantum_spacetime/qs.pdf

 

[marcus]

Online videos of 3 talks at last week's "Theory Canada" conference

Gravitational collapse in quantum gravity
Viqar Husain

"I will describe a framework for studying the dynamical collapse of a scalar field in a Hamiltonian formulation of quantum gravity. The approach makes use of operators that test states for trapped surface formation as a state is evolves, and allows the extraction of invariant information about the scalar field and geometry."

==============

Nonlocal remnants of microscopic discreteness?
Rafael Sorkin

"If 'quantum spacetime' is discrete, then any propagation equation compatible with the Lorentz transformations is necessarily nonlocal. I will present evidence that this sort of nonlocality survives on intermediate length scales much greater than that of the Planck length, yielding for example a nonlocal equation of motion for a scalar field."

================Protected information and emergent locality in background independent quantum gravity
Fotini Markopoulou

"A successful quantum theory of gravity needs to explain spacetime as an effective, approximate description of a more fundamental theory. I describe an approach to quantum gravity in which the pre-spacetime structure is a directed graph of operator algebras. They may also be thought of as a network of quantum information flow. Methods from quantum information theory can be used to extract degrees of freedom protected from the microscopic evolution and which define the causal structure of the emergent spacetime. As a result of the background independence we find that emergent spacetime means emergent locality and hence locality corrections at all scales. This is a new direction in quantum gravity phenomenology currently under investigation."

==========
There are plenty of others to choose from as well. I didnt sample all by any means, and mention only these.
To get the videos, go here
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
and scroll down to "Theory Canada 2" in the menu on the left

The format is split screen so you get to see both the lecturer and the projected slide.
Several of the talks are brief (20-25 minutes). You may find other talks of particular interest to you.
Among the short talks, I was especially impressed by those given by Sorkin and by Husain. Sorkin's abstract does not indicate the full scope of what he presented.

==============
the talk by Robert Leigh is not yet put online, but it may appear later.
The Spectrum of 2+1 Dimensional Yang-Mills Theory (so far this talk is not available on line)
Robert Leigh
"I review our recent work on an analytic solution of Yang-Mills theory in 2+1 dimensions."

It was about http://arxiv.org/abs/hep-th/0604060
and there was a follow-up
http://arxiv.org/abs/hep-th/0604184
of which Laurent Freidel was co-author

======================
now for something completely different

http://arxiv.org/abs/gr-qc/0606061
Gravity: A New Holographic Perspective
T. Padmanabhan
Plenary talk at the International Conference on Einstein's Legacy in the New Millennium, December 15 - 22, 2005, Puri, India; to appear in the Proceedings to be published in IJMPD; 16 pages

"A general paradigm for describing classical (and semiclassical) gravity is presented. This approach brings to the centre-stage a holographic relationship between the bulk and surface terms in a general class of action functionals and provides a deeper insight into several aspects of classical gravity which have no explanation in the conventional approach. After highlighting a series of unresolved issues in the conventional approach to gravity, I show that (i) principle of equivalence, (ii) general covariance and (iii)a reasonable condition on the variation of the action functional, suggest a generic Lagrangian for semiclassical gravity of the form $L=Q_a^{bcd}R^a_{bcd}$ with $\nabla_b Q_a^{bcd}=0$ . The expansion of $$Q_a^{bcd}$$ in terms of the derivatives of the metric tensor determines the structure of the theory uniquely. The zeroth order term gives the Einstein-Hilbert action and the first order correction is given by the Gauss-Bonnet action. Any such Lagrangian can be decomposed into a surface and bulk terms which are related holographically. The equations of motion can be obtained purely from a surface term in the gravity sector. Hence the field equations are invariant under the transformation $T_{ab} \to T_{ab} + \lambda g_{ab}$ and gravity does not respond to the changes in the bulk vacuum energy density. The cosmological constant arises as an integration constant in this approach. The implications are discussed."

 

[marcus]

http://arxiv.org/abs/hep-th/0606135
Fixed points of quantum gravity in higher dimensions
Peter Fischer, Daniel F. Litim
Talk presented at Einstein Century Meeting, Paris, 15-22 July 2005
SHEP-06-14, CERN-PH-TH-2006/066, PITHA 06/06
"We study quantum gravity in more than four dimensions by means of an exact functional flow. A non-trivial ultraviolet fixed point is found in the Einstein-Hilbert theory. It is shown that our results for the fixed point and universal scaling exponents are stable. If the fixed point persists in extended truncations, quantum gravity in the metric field is asymptotically safe. We indicate physical consequences of this scenario in phenomenological models with low-scale quantum gravity and large extra dimensions."

cites several papers of Martin Reuter and by Ambjorn, Jurkiewicz, Loll.
suggests ways to make higher dimensional models testable, by showing that einstein metric gravity is assympt. safe, so manageable, in higher dimensions.

http://arxiv.org/abs/gr-qc/0606065
Evidence for an entropy bound from fundamentally discrete gravity
D. Rideout, S. Zohren
25 pages, 9 figures
IMPERIAL/TP/06/SZ/01
"The various entropy bounds that exist in the literature suggest that spacetime is fundamentally discrete, and hint at an underlying relationship between geometry and 'information'. The foundation of this relationship is yet to be uncovered, but should manifest itself in a theory of quantum gravity. We present a measure for the maximal entropy of spherically symmetric spacelike regions within the causal set approach to quantum gravity. In terms of the proposal, a bound for the entropy contained in this region can be derived from a counting of potential 'degrees of freedom' associated to the Cauchy horizon of its future domain of dependence. For different spherically symmetric spacelike regions in Minkowski spacetime of arbitrary dimension, we show that this proposal leads, in the continuum approximation, to Susskind's well-known spherical entropy bound."

Stefan Zohren got his masters at Utrecht, in Loll's group, and then went to London Imperial for PhD. London is strong in Causal Sets. Rideout is experienced researcher in that approach. Did his PhD at Syracuse with Rafael Sorkin in 2002. (Dynamics of Causal Sets gr-qc/0212064)

http://arxiv.org/abs/gr-qc/0606067
How often does the Unruh-DeWitt detector click? Regularisation by a spatial profile
Jorma Louko, Alejandro Satz
28 pages, 1 figure
"We analyse within first-order perturbation theory the instantaneous transition rate of an accelerated Unruh-DeWitt particle detector whose coupling to a massless scalar field on four-dimensional Minkowski space is regularised by a spatial profile. For the Lorentzian profile introduced by Schlicht, the zero size limit is computed explicitly and expressed as manifestly finite integral formula that no longer involves regulators or limits. The same transition rate is obtained for an arbitrary profile of compact support under a modified definition of spatial smearing. Consequences for the asymptotic behaviour of the transition rate are discussed. A number of stationary and nonstationary trajectories are analysed, recovering in particular the Planckian spectrum for uniform acceleration."

Alejandro Satz is an Argentine studying for PhD in Nottingham. He has a blog called REALITY CONDITIONS
Jorma Louko has co-authored with Carlo Rovelli, Don Marolf, Rafael Sorkin, Kirill Krasnov. 35 of his papers are here:
http://arxiv.org/find/gr-qc/1/au:+Louko_J/0/1/0/all/0/1

 

[marcus]

http://arxiv.org/abs/gr-qc/0606074
A semiclassical tetrahedron
Carlo Rovelli, Simone Speziale
10 pages
"We construct a macroscopic semiclassical state state for a quantum tetrahedron. The expectation values of the geometrical operators representing the volume, areas and dihedral angles are peaked around assigned classical values, with vanishing relative uncertainties."

 

[marcus]

http://arxiv.org/abs/gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
"Non-minimal actions with matter represented by a scalar field coupled to gravity are considered in the context of a homogeneous and isotropic universe. The coupling is of the form $$-\xi/2 \phi^2 R$$. The possibility of successful inflation is investigated taking into account features of loop cosmology. For that end a conformal transformation is performed. That brings the theory into the standard minimally coupled form (Einstein frame) with some effective field and its potential. Both analytical and numerical estimates show that a negative coupling constant is preferable for successful inflation. Moreover, provided fixed initial conditions, larger$$|\xi|$$ leads to a greater number of {\em e}-folds. The latter is obtained for a reasonable range of initial conditions and the coupling parameter and indicates a possibility for successful inflation."http://arxiv.org/abs/gr-qc/0606078
On obtaining classical mechanics from quantum mechanics
Ghanashyam Date
22 pages

"Constructing a classical mechanical system associated with a given quantum mechanical one, entails construction of a classical phase space and a corresponding Hamiltonian function from the available quantum structures and a notion of coarser observations. The Hilbert space of any quantum mechanical system naturally has the structure of an infinite dimensional symplectic manifold (`quantum phase space'). There is also a systematic, quotienting procedure which imparts a bundle structure to the quantum phase space and extracts a classical phase space as the base space. This works straight forwardly when the Hilbert space carries weakly continuous representation of the Heisenberg group and recovers the linear classical phase space $$\mathbb{R}^{\mathrm{2N}}$$. We report on how the procedure also allows extraction of non-linear classical phase spaces and illustrate it for Hilbert spaces being finite dimensional (spin-j systems), infinite dimensional but separable (particle on a circle) and infinite dimensional but non-separable (Bohr quantization). To construct a corresponding classical dynamics, one needs to choose a suitable section and identify an effective Hamiltonian. The effective dynamics mirrors the quantum dynamics provided the section satisfies conditions of semiclassicality and tangentiality. There is also an alternative and a priori distinct notion of effective Hamiltonian used in the context of a WKB approximation. These two notions are compared for the example of loop quantum cosmology."