Loop-and-allied QG bibliography

[selfAdjoint]

the guy is at Austen Texas
I don't know whether this paper by itself is so weighty but it impresses me that stuff keeps piling up about Kodama state

I don't know who Andrew Randono is either, but if the paper holds up, it's path-breaking. He has embedded the existing, problematical Kodama state (proposed as the ground state of quantum gravity, the QG vacuum if you will) in a continuum of such states parametrized by the Immirzi parameter. Since the Baez et. al. path integral paper strongly suggests the I.P. as a little bigger than 1 and real, while existing Kodama has Immirzi = -i, he wants to, well, sort of continue through his new continuum to get to the real value. And when he does this he gets a new kind of Kodama state which is nice. I hope he's right, because it could really simplify things for QG, and might even convert some more string theorists!

 

[marcus]

... I hope he's right, because it could really simplify things for QG, and might even convert some more string theorists!

I so much hope he is right too, or that something along those lines can be rigorously established. Bringing in a few more string theorists is a nice idea too

I should post something about this newly arxived paper by Sorkin:

http://arxiv.org/abs/hep-th/0504037
Ten Theses on Black Hole Entropy
Rafael D. Sorkin (Perimeter Institute and Syracuse University)

To appear in the Proceedings of the European Science Foundation Conference on Philosophical and Foundational Issues in Statistical Physics, held Utrecht, the Netherlands, 28-30 November 2003. The proceedings are to be published in a special issue of Studies in History and Philosophy of Modern Physics (2005)

"I present a viewpoint on black hole thermodynamics according to which the entropy: derives from horizon "degrees of freedom''; is finite because the deep structure of spacetime is discrete; is "objective'' thanks to the distinguished coarse graining provided by the horizon; and obeys the second law of thermodynamics precisely because the effective dynamics of the exterior region is not unitary."

I wonder what our long-time PF-member whose handle is "nonunitary" thinks of Sorkin's paper. Also PF-member Edgar1813, whose friends Gambini and Pullin have argued that time-evolution (told with a realistic quantum clock) must be nonunitary. The idea presses in from several directions.

 

[marcus]

http://arxiv.org/abs/gr-qc/0504029

Black hole evaporation: A paradigm
Abhay Ashtekar, Martin Bojowald
18 pages, 4 figures
IGPG04/8-4, AEI-2004-072

"A paradigm describing black hole evaporation in non-perturbative quantum gravity is developed by combining two sets of detailed results: i) resolution of the Schwarzschild singularity in loop quantum gravity; and ii) time-evolution of black holes in the dynamical horizon framework. Quantum geometry effects introduce a major modification in the traditional space-time diagram of black hole evaporation, providing a possible mechanism for recovery of information that is classically lost in the process of black hole formation. The paradigm is developed directly in the Lorentzian regime and necessary conditions for its viability are discussed. If these conditions are met, much of the tension between expectations based on space-time geometry and structure of quantum theory would be resolve."

this is a long-awaited paper. Ashtekar gave a talk at Penn State last year about this (audio and slides are available). the paper has already been cited in some others.

 

[marcus]

Ashtekar and Bojowald have another BH paper in preparation called
Non-singular quantum geometry of the Schwarzschild black hole interior

that is reference [1] in their paper that just came out.

in the present paper they discuss the results in [1] somewhat, as in the abstract, and on pages 5 and 7:

abstract: " i) resolution of the Schwarzschild singularity in loop quantum gravity[1]; "

page 5:"Since the key issues involve the final black hole singularity and since we expect this singularity to be generically space-like, one can first focus just on the interior of the Schwarzschild horizon. This region is naturally foliated by 3-manifolds ...Using quantum geometry, we can go to the exact quantum theory [1]. The situation is similar but technically more complicated than that encountered in the rigorous treatment of spatially homogeneous and isotropic cosmologies [16]. (See also [15] where the same kind of representation is used, based on ADM variables.)... "

page 7: "... Thus, as in quantum cosmology, one finds that the quantum evolution does not stop at the singularity; one can evolve right through it [1]. The state remains pure. However, in the deep Planck regime around the singularity, the notion of a classical space-time geometry fails to make even an approximate sense. Nonetheless, since there is no longer a final boundary in the interior, the full quantum evolution is quite different from the classical one..."

page 7: "...This calculation was done [1] in the Kantowski-Sachs mini-superspace and |Psi> represents the state of the Schwarzschild black hole interior in loop quantum gravity. This black hole can not evaporate: there is no matter and because of the restriction to spherical symmetry there can not be Hawking radiation of gravitons either. However, since the generic singularity is expected to be space-like, one may hope that the general intuition about the resolution of the Schwarzschild singularity it provides can be taken over to models in which gravity is coupled to scalar fields, where the evaporation does occur. We will assume that the overall, qualitative features of our singularity resolution will continue to be valid in these models."

 

[marcus]

http://arxiv.org/abs/gr-qc/0504043
Quantum Gravitational Collapse
Leonardo Modesto
13 pages

"We apply the recent results in Loop Quantum Cosmology and in the resolution of Black Hole singularity to the gravitational collapse of a star. We study the dynamic of the space time in the interior of the Schwarzschild radius. In particular in our simple model we obtain the evolution of the matter inside the star and of the gravity outside the region where the matter is present. The boundary condition identify an unique time inside and outside the region where the matter is present. We consider a star during the collapse in the particular case in which inside the collapsing star we take null pressure, homogeneity and isotropy. The space-time outside the matter is homogeneous and anisotropic. We show that the space time is singularity free and that we can extend dynamically the space-time beyond the classical singularity."

 

[ohwilleke]

If Modesto's paper really new? I thought that LQG implies no singularities had been established for some time. Is the new contribution the description of what is going on inside the black hole?

 

[marcus]

here is post #302 on this thread

just out
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures

"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."

Bojowald removed the cosmological singularity in 2001, assuming isotropy. The result has since been extended to more general cases---post #301 has a link to a recent review.

Removing the black hole singularity is just happening this year, for the first time.

Just because the cosmological (BB) singularity was cured does not mean that the gravitational collapse (BH) singularity was cured.
In any given case the LQG analysis has to be done to see if the theory breaks down (and makes a singularity) or not. Including matter makes for some additional technical complications
========================
a new paper by Ashtekar was posted Tuesday 12 April
http://arxiv.org/abs/gr-qc/0504052
Semiclassical States for Constrained Systems
Abhay Ashtekar, Luca Bombelli, Alejandro Corichi
25 pages, 3 figures

 

[marcus]

short reading list for "LQG explains the constants" thread:

1.these two papers, and the references therein, discuss the removal of the classical BB ex-singularity in a range of cases that has gradually extended the generality of Bojowald's initial 2001 result.

http://arxiv.org/abs/gr-qc/0402053
Loop Quantum Cosmology: Recent Progress
Martin Bojowald
17 pages, 2 figures, Plenary talk at ICGC 2004

"Aspects of the full theory of loop quantum gravity can be studied in a simpler context by reducing to symmetric models like cosmological ones. This leads to several applications where loop effects play a significant role when one is sensitive to the quantum regime. As a consequence, the structure of and the approach to classical singularities are very different from general relativity: The quantum theory is free of singularities, and there are new phenomenological scenarios for the evolution of the very early universe including inflation. We give an overview of the main effects, focussing on recent results obtained by several different groups."

http://arxiv.org/abs/gr-qc/0503020
The Early Universe in Loop Quantum Cosmology
Martin Bojowald
10 pages, 3 figures, plenary talk at VI Mexican School on Gravitation and Mathematical Physics, Nov 21-27, 2004

"Loop quantum cosmology applies techniques derived for a background independent quantization of general relativity to cosmological situations and draws conclusions for the very early universe. Direct implications for the singularity problem as well as phenomenology in the context of inflation or bouncing universes result, which will be reviewed here. The discussion focuses on recent new results for structure formation and generalizations of the methods."

2. these papers, and references therein, go towards understanding the BH ex-singularity and showing that it evolves into the BB ex-singularity:

http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures

"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."

http://arxiv.org/abs/gr-qc/0504043
Quantum Gravitational Collapse
Leonardo Modesto
13 pages

We apply the recent results in Loop Quantum Cosmology and in the resolution of Black Hole singularity to the gravitational collapse of a star. We study the dynamic of the space time in the interior of the Schwarzschild radius. In particular in our simple model we obtain the evolution of the matter inside the star and of the gravity outside the region where the matter is present. The boundary condition identify an unique time inside and outside the region where the matter is present. We consider a star during the collapse in the particular case in which inside the collapsing star we take null pressure, homogeneity and isotropy. The space-time outside the matter is homogeneous and anisotropic. We show that the space time is singularity free and that we can extend dynamically the space-time beyond the classical singularity."

3. if the models of BH collapse and BB expansion can be joined in theory then one can take seriously the CNS (cosm. nat. selection) hypothesis as a way of understanding why the basic constants in the Standard Models are what they are. The CNS theory can be tested empirically by observation and experiment. This paper discusses CNS:

http://arxiv.org/abs/hep-th/0407213
Scientific alternatives to the anthropic principle
Lee Smolin
for "Universe or Multiverse", ed. by Bernard Carr et. al., to be published by Cambridge University Press.

It is explained in detail why the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science. Cases which have been claimed as successful predictions from the AP are shown to be not that. Either they are uncontroversial applications of selection principles in one universe (as in Dicke's argument), or the predictions made do not actually logically depend on any assumption about life or intelligence, but instead depend only on arguments from observed facts (as in the case of arguments by Hoyle and Weinberg). The Principle of Mediocrity is also examined and shown to be unreliable, as arguments for factually true conclusions can easily be modified to lead to false conclusions by reasonable changes in the specification of the ensemble in which we are assumed to be typical.
We show however that it is still possible to make falsifiable predictions from theories of multiverses, if the ensemble predicted has certain properties specified here. An example of such a falsifiable multiverse theory is cosmological natural selection. It is reviewed here and it is argued that the theory remains unfalsified. But it is very vulnerable to falsification by current observations, which shows that it is a scientific theory.
The consequences for recent discussions of the AP in the context of string theory are discussed."

 

[marcus]

wolram found this paper and contributed the link:

http://arxiv.org/gr-qc/0504059
Geometric spin foams, Yang-Mills theory and background-independent models
Florian Conrady (CPT, Marseille & Potsdam, Max Planck Inst.)
28 pages, 27 diagrams
AEI-2005-090

"We review the dual transformation from pure lattice gauge theory to spin foam models with an emphasis on a geometric viewpoint. This allows us to give a simple dual formulation of SU(N) Yang-Mills theory, where spin foam surfaces are weighted with the exponentiated area. In the case of gravity, we introduce a symmetry condition which demands that the amplitude of an individual spin foam depends only on its geometric properties and not on the lattice on which it is defined. For models that have this property, we define a new sum over abstract spin foams that is independent of any choice of lattice or triangulation. We show that a version of the Barrett-Crane model satisfies our symmetry requirement."

a thread for discussing the paper:
https://www.physicsforums.com/showthread.php?p=530764#post530764

 

[marcus]

http://arxiv.org/gr-qc/9404011
The fate of black hole singularities and the parameters of the standard models of particle physics and cosmology
Lee Smolin
27 pages
CGPG-94/3-5

"A cosmological scenario which explains the values of the parameters of the standard models of elementary particle physics and cosmology is discussed. In this scenario these parameters are set by a process analogous to natural selection which follows naturally from the assumption that the singularities in black holes are removed by quantum effects leading to the creation of new expanding regions of the universe. The suggestion of J. A. Wheeler that the parameters change randomly at such events leads naturally to the conjecture that the parameters have been selected for values that extremize the production of black holes. This leads directly to a prediction, which is that small changes in any of the parameters should lead to a decrease in the number of black holes produced by the universe. On plausible astrophysical assumptions it is found that changes in many of the parameters do lead to a decrease in the number of black holes produced by spiral galaxies. These include the masses of the proton,neutron, electron and neutrino and the weak, strong and electromagnetic coupling constants. Finally,this scenario predicts a natural time scale for cosmology equal to the time over which spiral galaxies maintain appreciable rates of star formation, which is compatible with current observations that Omega = .1-.2."

 

[marcus]

A recent review article about BH vibration modes (relevant to LQG)
http://arxiv.org/abs/gr-qc/0411025

A critique of Smolin CNS by Rudy Vaas (in his capacity as Philosophy of Science expert) with a good bibliography of other people's reactions to CNS

http://arxiv.org/gr-qc/0205119
Is there a Darwinian Evolution of the Cosmos? - Some Comments on Lee Smolin's Theory of the Origin of Universes by Means of Natural Selection
Ruediger Vaas
Comments: 20 pages; extended version of a contribution to the MicroCosmos - MacroCosmos conference in Aachen, Germany, September 2-5 1998; finished in late 1998 and published in the conference proceedings

"For Lee Smolin, our universe is only one in a much larger cosmos (the Multiverse) - a member of a growing community of universes, each one being born in a bounce following the formation of a black hole. In the course of this, the values of the free parameters of the physical laws are reprocessed and slightly changed. This leads to an evolutionary picture of the Multiverse, where universes with more black holes have more descendants. Smolin concludes, that due to this kind of Cosmological Natural Selection our own universe is the way it is. The hospitality for life of our universe is seen as an offshoot of this self-organized process. - This paper outlines Smolin's hypothesis, its strength, weakness and limits, its relationship to the anthropic principle and evolutionary biology, and comments on the hypothesis from different points of view: physics, biology, philosophy of science, philosophy of nature, and metaphysics..."

 

[marcus]

LQC paper by G. M. Hossain

http://arxiv.org/abs/gr-qc/0504125
Large volume quantum correction in loop quantum cosmology: Graviton illusion?
Golam Mortuza Hossain
4 pages
IMSc/2005/04/10

"The leading quantum correction to Einstein-Hilbert Hamiltonian coming from large volume vacuum isotropic loop quantum cosmology, is independent of quantization ambiguity parameters. It is shown here that this correction can be viewed as finite volume gravitational Casimir energy due to one-loop 'graviton' contributions. In vacuum case sub-leading quantum corrections and in non-vacuum case even leading quantum correction depend on ambiguity parameters. It may be recalled that these are in fact analogous features of perturbative quantum gravity where it is well-known that pure gravity (on-shell) is one-loop finite whereas higher-loops contributions are not even renormalizable. These features of the quantum corrections coming from non-perturbative quantization, sheds a new light on a major open issue; how to communicate between non-perturbative and perturbative approaches of quantum gravity."

 

[marcus]

The Creation of General Relativity

http://arxiv.org/abs/physics/0504179
Einstein and Hilbert: The Creation of General Relativity
Ivan T. Todorov (Institut fuer Theoretische Physik, Universitaet Goettingen, Germany, and Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria)
Colloquium talk; 15 pages
Subj-class: History of Physics
"It took eight years after Einstein announced the basic physical ideas behind the relativistic gravity theory before the proper mathematical formulation of general relativity was mastered. The efforts of the greatest physicist and of the greatest mathematician of the time were involved and reached a breathtaking concentration during the last month of the work.
Recent controversy, raised by a much publicized 1997 reading of Hilbert's proof-sheets of his article of November 1915, is also discussed."

knowing something about the history of how Einstein (and David Hilbert who was working along similar lines) arrived at General Relativity, in several ways a completely new kind of physical theory, can be a help in understanding it and even today's efforts to quantize this geometrical theory of gravity. this account draws on a lot of contemporary stuff, letters, quotes, to make an interesting story.

Rovelli's book Quantum Gravity also has much of this history in chapter 2. Rovelli obviously considered it essential to LQG that one get some perspective on GR and the challenge of finding a common ground with QM.

I don't happen to know other online accounts of Einstein's "Long March" to general relativity----what he went through over about 8 years to get there and the help he got from others. If anyone knows of another worthwhile online history of the years 1907-1915 or thereabout, please let us know.

 

[marcus]

a long-awaited paper by Lewandowski, Okolow, Sahlmann, Thiemann

http://arxiv.org/abs/gr-qc/0504147
Uniqueness of diffeomorphism invariant states on holonomy-flux algebras
Jerzy Lewandowski, Andrzej Okolow, Hanno Sahlmann, Thomas Thiemann
38 pages, one figure

"Loop quantum gravity is an approach to quantum gravity that starts from the Hamiltonian formulation in terms of a connection and its canonical conjugate. Quantization proceeds in the spirit of Dirac: First one defines an algebra of basic kinematical observables and represents it through operators on a suitable Hilbert space. In a second step, one implements the constraints. The main result of the paper concerns the representation theory of the kinematical algebra: We show that there is only one cyclic representation invariant under spatial diffeomorphisms.
While this result is particularly important for loop quantum gravity, we are rather general: The precise definition of the abstract *-algebra of the basic kinematical observables we give could be used for any theory in which the configuration variable is a connection with a compact structure group. The variables are constructed from the holonomy map and from the fluxes of the momentum conjugate to the connection. The uniqueness result is relevant for any such theory invariant under spatial diffeomorphisms or being a part of a diffeomorphism invariant theory."

 

[marcus]

http://arxiv.org/abs/gr-qc/0505002
Absence of the Kasner singularity in the effective dynamics from loop quantum cosmology
Ghanashyam Date
4 pages
IMSc/2005/4/11
"In classical general relativity, the generic approach to the initial singularity is usually understood in terms of the BKL scenario. In this scenario, along with the Bianchi IX model, the exact, singular, Kasner solution of vacuum Bianchi I model also plays a pivotal role. Using an effective classical Hamiltonian obtained from loop quantization of vacuum Bianchi I model, exact solution is obtained which is non-singular due to a discreteness parameter. The solution is parameterized in exactly the same manner as the usual Kasner solution and reduces to the Kasner solution as discreteness parameter is taken to zero. At the effective Hamiltonian level, the avoidance of Kasner singularity uses a mechanism distinct from the `inverse volume' modifications characteristic of loop quantum cosmology."

Ghanashyam Date is a senior relativist at Chennai Institute, who is an LQC expert. he has co-authored with Bojowald. Golam Hossain, several of whose LQC papers we have seen, is the student of Ghanashyam Date. some of Date's previous papers are Genericity of Inflation in LQC and
another one about the absence of cosmological singularity which showed that a bounce was generic in LQC.

 

[marcus]

Causal Dynamical Triangulations at Perimeter

http://arxiv.org/abs/hep-th/0505004
Foliations and 2+1 Causal Dynamical Triangulation Models
Tomasz Konopka
9 pages, 3 figures
"Most models of causal dynamical triangulations construct space-time by arranging a set of simplices in layers separated by a fixed time-like distance. The importance of the foliation structure in the 2+1 dimensional model is studied by considering variations of the model in which this property is relaxed. It turns out that the fixed-lapse condition can be equivalently replaced by a set of global constraints that have geometrical interpretation. On the other hand, the introduction of new types of simplices that puncture the foliating sheets in general leads to different low-energy behavior compared to the original model."

A year ago there was the Marseille conference and Renate Loll delivered the DT paper "Emergence of a 4D World..." which raised a lot of interest in Causal DT. So Smolin and Markopoulou have done a little with DT in less than 4 dimensions, since then, and I guess that Tom Konopka is a grad student or postdoc maybe of Fotini Markopoulou, she said she had someone at perimeter/waterloo who was working on Dynamical Triangulations.

 

[marcus]

a new paper by Laurent Freidel

http://arxiv.org/abs/hep-th/0505016
Group Field Theory: An overview
Laurent Freidel (PI, ENS-Lyon)
10 pages
"We give a brief overview of the properties of a higher dimensional generalization of matrix model which arises naturally in the context of a background independent approach to quantum gravity, the so called group field theory. We show that this theory leads to a natural proposal for the physical scalar product of quantum gravity. We also show in which sense this theory provides a third quantization point of view on quantum gravity."

 

[marcus]

new paper by Gambini and Pullin

Gambini and Pullin have a unique approach to quantum gravity, not LQG but able to make contact with LQG in some of its results. There is no Hamiltonian constraint in their approach unlike in LQG, and no "problem of time"

Ashtekar considers G&P "consistent discretization" one of 3 or 4 promising approaches to quantizing General Relativity (particularly as to the dynamics) and evidently he is going to include G&P in the landmark book he is putting together called "100 Years of Relativity".

http://arxiv.org/abs/gr-qc/0505023
Discrete space-time
Rodolfo Gambini, Jorge Pullin
16 pages, submitted to the volume "100 Years of Relativity - Space-time Structure: Einstein and Beyond", A. Ashtekar, ed., to be published by World Scientific.

"We review recent efforts to construct gravitational theories on discrete space-times, usually referred to as the "consistent discretization'' approach. The resulting theories are free of constraints at the canonical level and therefore allow to tackle many problems that cannot be currently addressed in continuum quantum gravity. In particular the theories imply a natural method for resolving the big bang (and other types) of singularities and predict a fundamental mechanism for decoherence of quantum states that might be relevant to the black hole information paradox. At a classical level, the theories may provide an attractive new path for the exploration of issues in numerical relativity. Finally, the theories can make direct contact with several kinematical results of continuum loop quantum gravity. We review in broad terms several of these results and present in detail as an illustration the classical treatment with this technique of the simple yet conceptually challenging model of two oscillators with constant energy sum."

 

[marcus]

A growing number of researchers are involved with improved models of BH that don't have the singularity problem. I want to keep the links to their papers handy. here are some of the people:

Abhay Ashtekar, Viqar Husain, Oliver Winkler, Leonardo Modesto, Martin Bojowald, Roy Maartens, Rituparno Goswami, Parampreet Singh,

and here are some of their recent papers:

http://arxiv.org/abs/gr-qc/0504029
http://arxiv.org/abs/gr-qc/0503041
http://arxiv.org/abs/gr-qc/0504043
http://arxiv.org/abs/gr-qc/0411032
http://arxiv.org/abs/gr-qc/0407097
http://arxiv.org/abs/gr-qc/0412039
http://arxiv.org/abs/gr-qc/0410125

 

[marcus]

http://arxiv.org/abs/gr-qc/0505002
Absence of the Kasner singularity in the effective dynamics from loop quantum cosmology
Ghanashyam Date
4 pages
IMSc/2005/4/11
... the absence of cosmological singularity which showed that a bounce was generic in LQC.

I just reported a paper by Ghanashyam Date, here is another:
http://arxiv.org/abs/gr-qc/0505030
Pre-classical solutions of the vacuum Bianchi I loop quantum cosmology
Ghanashyam Date
Comments: 4 pages, revtex4, no figures
IMSc/2005/4/12
"Loop quantization of diagonalized Bianchi class A models, leads to a partial difference equation as the Hamiltonian constraint at the quantum level. In the absence of an adequate candidate for a physical inner product and/or physical observables, a criterion for testing a viable semiclassical limit has been formulated in terms of existence of the so-called pre-classical solutions. We demonstrate the existence of pre-classical solutions of the quantum equation for the vacuum Bianchi I model. All these solutions avoid the classical singularity at vanishing volume."
=========================
Here is one in Loop Quantum Cosmology by Thiemann, a new field for him.

http://arxiv.org/abs/gr-qc/0505032
On (Cosmological) Singularity Avoidance in Loop Quantum Gravity
Johannes Brunnemann, Thomas Thiemann
34 pages, 16 figures
AEI-2005-098

"Loop Quantum Cosmology (LQC), mainly due to Bojowald, is not the cosmological sector of Loop Quantum Gravity (LQG). Rather, LQC consists of a truncation of the phase space of classical General Relativity to spatially homogeneous situations which is then quantized by the methods of LQG. Thus, LQC is a quantum mechanical toy model (finite number of degrees of freedom) for LQG(a genuine QFT with an infinite number of degrees of freedom) which provides important consistency checks. However, it is a non trivial question whether the predictions of LQC are robust after switching on the inhomogeneous fluctuations present in full LQG. Two of the most spectacular findings of LQC are that 1. the inverse scale factor is bounded from above on zero volume eigenstates which hints at the avoidance of the local curvature singularity and 2. that the Quantum Einstein Equations are non -- singular which hints at the avoidance of the global initial singularity. We display the result of a calculation for LQG which proves that the (analogon of the) inverse scale factor, while densely defined, is {\it not} bounded from above on zero volume eigenstates. Thus, in full LQG, if curvature singularity avoidance is realized, then not in this simple way. In fact, it turns out that the boundedness of the inverse scale factor is neither necessary nor sufficient for curvature singularity avoidance and that non -- singular evolution equations are neither necessary nor sufficient for initial singularity avoidance because none of these criteria are formulated in terms of observable quantities.After outlining what would be required, we present the results of a calculation for LQG which could be a first indication that our criteria at least for curvature singularity avoidance are satisfied in LQG."

this is a companion, or auxilliary paper by the same authors:

http://arxiv.org/abs/gr-qc/0505033
Unboundedness of Triad -- Like Operators in Loop Quantum Gravity
Johannes Brunnemann, Thomas Thiemann
57 pages, 19 figures
AEI-2005-099
"In this paper we deliver the proofs for the claims, made in a companion paper, concerning the avoidance of cosmological curvature singularities in in full Loop Quantum Gravity (LQG)."

 

[marcus]

http://arxiv.org/abs/hep-th/0505069
Knot theory and a physical state of quantum gravity
Tomas Liko, Louis H. Kauffman
37 pages, 4 figures; review paper; comments/suggestions welcome

"We discuss the theory of knots, and describe how knot invariants arise naturally in theoretical physics. The focus of this review is to delineate the relationship between topological field theory and conformal field theory at both the classical and quantum levels, and to describe in detail the loop representation of non-perturbative canonical quantum general relativity (loop quantum gravity). This leads naturally to a discussion of the Kodama wavefunction, and a framing of the loop observables. The latter may be important for a background-independent formulation of perturbative string theory. This review can serve as a self-contained introduction to loop quantum gravity and related areas."

 

[marcus]

http://arxiv.org/abs/gr-qc/0505043

Consistent discretizations: the Gowdy spacetimes
Rodolfo Gambini, Marcelo Ponce, Jorge Pullin
10 pages, 8 figures

"We apply the consistent discretization scheme to general relativity particularized to the Gowdy space-times. This is the first time the framework has been applied in detail in a non-linear generally-covariant gravitational situation with local degrees of freedom. We show that the scheme can be correctly used to numerically evolve the space-times. We show that the resulting numerical schemes are convergent and preserve approximately the constraints as expected."

We now seem to have several distinct but related quantum gravity approaches to cosmology.

1. full LQG
2. LQC (as Bojowald and others do it)
3. the older pre-Loop quantum gravity (various people: Husain, Modesto, Reuter)
using the Wheeler-DeWitt eqn and quantizing the metric.
4. Gambini-Pullin consistent discretizations approach

and more (don't forget Renate Loll, and the semiclassical analysis of Ganashyam Date)

it will be interesting to see who confirms whose results.

 

[marcus]

Gambini and Pullin have been posting a lot lately. Three papers just this month (May 2005). Here is one from earlier this month.

Gambini and Pullin have a unique approach to quantum gravity, not LQG but able to make contact with LQG in some of its results. There is no Hamiltonian constraint in their approach unlike in LQG, and no "problem of time"

Ashtekar considers G&P "consistent discretization" one of 3 or 4 promising approaches to quantizing General Relativity (particularly as to the dynamics) and evidently he is going to include G&P in the landmark book he is putting together called "100 Years of Relativity".

http://arxiv.org/abs/gr-qc/0505023
Discrete space-time
Rodolfo Gambini, Jorge Pullin
16 pages, submitted to the volume "100 Years of Relativity - Space-time Structure: Einstein and Beyond", A. Ashtekar, ed., to be published by World Scientific.

"We review recent efforts to construct gravitational theories on discrete space-times, usually referred to as the "consistent discretization'' approach. The resulting theories are free of constraints at the canonical level and therefore allow to tackle many problems that cannot be currently addressed in continuum quantum gravity. In particular the theories imply a natural method for resolving the big bang (and other types) of singularities and predict a fundamental mechanism for decoherence of quantum states that might be relevant to the black hole information paradox. At a classical level, the theories may provide an attractive new path for the exploration of issues in numerical relativity. Finally, the theories can make direct contact with several kinematical results of continuum loop quantum gravity. We review in broad terms several of these results and present in detail as an illustration the classical treatment with this technique of the simple yet conceptually challenging model of two oscillators with constant energy sum."

Here is one they posted today:
http://arxiv.org/abs/gr-qc/0505052
Classical and quantum general relativity: a new paradigm
Rodolfo Gambini, Jorge Pullin
Comments: 8 pages, one figure

"We argue that recent developments in discretizations of classical and quantum gravity imply a new paradigm for doing research in these areas. The paradigm consists in discretizing the theory in such a way that the resulting discrete theory has no constraints. This solves many of the hard conceptual problems of quantum gravity. It also appears as a useful tool in some numerical simulations of interest in classical relativity. We outline some of the salient aspects and results of this new framework."

 

[marcus]

Here is Martin Bojowald's latest survey of LQC

http://arxiv.org/abs/gr-qc/0505057
Elements of Loop Quantum Cosmology
Martin Bojowald
30 pages, 4 figures, Chapter contributed to "100 Years of Relativity - Space-time Structure: Einstein and Beyond", Ed. A. Ashtekar (World Scientific)
Report-no: AEI-2005-025

"The expansion of our universe, when followed backward in time, implies that it emerged from a phase of huge density, the big bang. These stages are so extreme that classical general relativity combined with matter theories is not able to describe them properly, and one has to refer to quantum gravity. A complete quantization of gravity has not yet been developed, but there are many results about key properties to be expected. When applied to cosmology, a consistent picture of the early universe arises which is free of the classical pathologies and has implications for the generation of structure which are potentially observable in the near future."

the style of this survey is very un-headline grabbing.
according to LQC a gravitational collapse preceded the current expansion and that the turnaround from contraction to expansion, sometimes called the bounce, involved a flip in the orientation of the spatial triad or volume element.
also according to LQC inflation is generic, it happens after a bounce automatically without fine tuning or much extra paraphernalia. some Bojo short papers headline these results.

however you can either think of LQC as a testable theory IN ITS OWN RIGHT or you can think of it as a simplified stripped-down version of the LQG FULL THEORY and if you think of it as a simplification of the full LQG theory then all these conclusions have to be checked by more elaborate calculation in the full theory. this is now in progress.

So now I guess that LQC people are being very modest and are politely waiting for the elaborate LQG ceremony of consulting the entrails of the full theory to see in what sense they confirm the LQC results.

I may put together a reading list of a few SHORT Bojowald papers that are more exciting reading than this careful survey.

 

[marcus]

Here is a recent Ambjorn Jurkiewicz Loll (AJL) paper. They say their long article called Reconstructing the Universe dated May 2005 from the University of Utrecht, is to appear. But it is not yet out on archiv.


http://arxiv.org/abs/hep-th/0505113
Spectral Dimension of the Universe
J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
10 pages, 1 figure
SPIN-05/05, ITP-UU-05/07

"We measure the spectral dimension of universes emerging from nonperturbative quantum gravity, defined through state sums of causal triangulated geometries. While four-dimensional on large scales, the quantum universe appears two-dimensional at short distances. We conclude that quantum gravity may be "self-renormalizing" at the Planck scale, by virtue of a mechanism of dynamical dimensional reduction."

=========
this thread is a substitute for a sticky thread devolted to Loop-and-allied LINKS: a kind of library to store useful online LQG-and-related stuff.
Periodically I try to sort it out and organize the links selectively and by topic. this has not been done for a while.

There are other Renate Loll or AJL links back further in this thread
==========

this present paper, about the spacetime dimension being less than 4D at very small scale, has its own thread for discussion
https://www.physicsforums.com/showthread.php?t=75472

 

[marcus]

http://arxiv.org/abs/gr-qc/0505081
Physical effects of the Immirzi parameter
Alejandro Perez, Carlo Rovelli
3 pages

"The Immirzi parameter is a constant appearing in the version of the general relativity action utilized as a starting point for the loop quantization of gravity.The parameter is commonly believed not to show up in the equations of motion, because it appears in front of a term in the action that vanishes on shell. We show that in the presence of fermions, instead, the Immirzi term of the action does not vanish on shell, and the Immirzi parameter appears in the equations of motion. It is the coupling constant of a parity violating four-fermion interaction. Therefore the nontriviality of the Immirzi parameter leads to effects that are observables in principle, even independently from nonperturbative quantum gravity."

 

[marcus]

http://arxiv.org/abs/hep-th/0505154
Reconstructing the Universe
J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
52 pages, 20 postscript figures

"We provide detailed evidence for the claim that nonperturbative quantum gravity, defined through state sums of causal triangulated geometries, possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale, and at the same time provides a nontrivial consistency check of the method of causal dynamical triangulations. A closer look at the quantum geometry reveals a number of highly nonclassical aspects, including a dynamical reduction of spacetime to two dimensions on short scales and a fractal structure of slices of constant time."

 

[Spin_Network]

http://arxiv.org/abs/gr-qc/0505081
Physical effects of the Immirzi parameter
Alejandro Perez, Carlo Rovelli
3 pages

"The Immirzi parameter is a constant appearing in the version of the general relativity action utilized as a starting point for the loop quantization of gravity.The parameter is commonly believed not to show up in the equations of motion, because it appears in front of a term in the action that vanishes on shell. We show that in the presence of fermions, instead, the Immirzi term of the action does not vanish on shell, and the Immirzi parameter appears in the equations of motion. It is the coupling constant of a parity violating four-fermion interaction. Therefore the nontriviality of the Immirzi parameter leads to effects that are observables in principle, even independently from nonperturbative quantum gravity."

Marcus, this is a bu**er!

I spent some time on arxiv, looking for a paper on The Immirzi parameter !..hoping to link it to some recent postings, I came offline and spent three hours going through pre-prints I have myself, this is amazing!

Great, and thanks for this link.

P.S I did not see this paper when browsing arxiv so I guess you have a time advantage over the UK

 

[ohwilleke]

Can anyone connect the dots and explain what sort of context one could observe the Immirzi parameter effects in?

 

[marcus]

Spin_Network, I'm glad you found the Perez Rovelli paper useful.

Ohwilleke, I started a separate thread about the paper in hopes of some comment or clarification.

Can anyone connect the dots and explain what sort of context one could observe the Immirzi parameter effects in?

thanks to selfAdjoint for noticing another new CDT paper. the rate of posting CDT papers seems to be up this year. here is the one that sA flagged:

http://arxiv.org/hep-th/0505165
A statistical formalism of Causal Dynamical Triangulations
Mohammad H. Ansari, Fotini Markopoulou
20 pages, 19 pictures, 1 graph

"We rewrite the 1+1 Causal Dynamical Triangulations model as a spin system and thus provide a new method of solution of the model."

here's another Loop related paper, by Kirill Krasnov, who has co-authored with Laurent Freidel IIRC

http://arxiv.org/abs/hep-th/0505174
Quantum Gravity with Matter via Group Field Theory
Kirill Krasnov
43 pages, many figures
A generalization of the matrix model idea to quantum gravity in three and higher dimensions is known as group field theory (GFT). In this paper we show how GFT can be used to describe 3D quantum gravity coupled to point particles. This is achieved by a generalization similar to the one used in 2D where multi-matrix models or matrix quantum mechanics are considered. Thus, we replace the group that leads to pure quantum gravity by the twisted product of the group with its dual -the so-called Drinfeld double of the group. The Drinfeld double is a quantum group in that it is an algebra that is both non-commutative and non-cocommutative, and special care is needed to define group field theory for it. We show how this is done, and consider the resulting GFT models. Of special interest is a new topological model that is the "Ponzano-Regge'' model for the Drinfeld double. We also consider a more general class of models that are defined using not GFT, but the so-called chain mail techniques. A general model of this class does not produce 3-manifold invariants, but has an interpretation in terms of point particle Feynman diagrams."

I have not checked this one out, but Krasnov is an old hand and it is probably worth keeping the link within easy reach