Loop-and-allied QG bibliography

[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 :grumpy:

 

[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

 

[marcus]

http://arxiv.org/abs/gr-qc/0505111
Entropy and Area in Loop Quantum Gravity
John Swain
7 pages, this essay received an Honourable Mention in the Gravity Research Foundation Essay Competition 2005

"Black hole thermodynamics suggests that the maximum entropy that can be contained in a region of space is proportional to the area enclosing it rather than its volume. I argue that this follows naturally from loop quantum gravity and a result of Kolmogorov and Bardzin' on the the realizability of networks in three dimensions. This represents an alternative to other approaches in which some sort of correlation between field configurations helps limit the degrees of freedom within a region. It also provides an approach to thinking about black hole entropy in terms of states inside rather than on its surface. Intuitively, a spin network complicated enough to imbue a region with volume only let's that volume grow as quickly as the area bounding it."

 

[marcus]

David Poulin investigates relational time and gradual decoherence
I am not sure if his papers are relevant enough to QG for me to flag them, here is one
http://arxiv.org/abs/quant-ph/0505175
Relational time for systems of oscillators
G.J.Milburn, David Poulin
Contribution to the Int. J. of Quant. Info. issue dedicated to the memory of Asher Peres

"Using an elementary example based on two simple harmonic oscillators, we show how a relational time may be defined that leads to an approximate Schrodinger dynamics for subsystems, with corrections leading to an intrinsic decoherence in the energy eigenstates of the subsystem."

here is another
http://arxiv.org/abs/quant-ph/0505081
A Relational Formulation of Quantum Theory
David Poulin
14 pages, comments welcome

"We investigate, with the help of a simple model, how a relational quantum theory can emerge from the combination of the general framework of quantum mechanics with the requirement of background independence of general relativity. More precisely, we argue that any quantum mechanical experiment admits a purely relational description at a fundamental level, from which the original "non-relational" theory emerges in a semi-classical limit. According to this thesis, the non-relational theory is therefore an approximation of the fundamental relational theory. We propose four simple rules that can be used to translate an "orthodox" quantum mechanical description into a relational description, independent of an external spatial reference frame or clock. The techniques used to construct these relational theories are motivated by a Bayesian approach to quantum mechanics, and rely on the noiseless subsystem method of quantum information science used to protect quantum states against undesired noise. The relational theory naturally predicts a fundamental decoherence mechanism, so an arrow of time emerges from a time-symmetric theory. Moreover, there is no need for a "collapse of the wave packet" in this theory: the probability interpretation is only applied to diagonal density operators. Finally, the physical states of the relational theory can be described in terms of "spin networks" introduced by Penrose as a combinatorial description of geometry, and widely studied in the loop formulation of quantum gravity. Thus, our simple bottom-up approach (starting from the semi-classical limit to derive the quantum theory) may offer interesting insights on the low energy limit of quantum gravity."

i cannot evaluate this or vouch for it. just feel a nagging sense that we ought to keep tabs on research in relational time. Gambini and Pullin have some papers about it

 

[marcus]

http://arxiv.org/abs/gr-qc/0506035
Counting a black hole in Lorentzian product triangulations
B. Dittrich (AEI, Golm), R. Loll (U. Utrecht)
42 pages, 11 figures

"We take a step toward a nonperturbative gravitational path integral for black-hole geometries by deriving an expression for the expansion rate of null geodesic congruences in the approach of causal dynamical triangulations. We propose to use the integrated expansion rate in building a quantum horizon finder in the sum over spacetime geometries. It takes the form of a counting formula for various types of discrete building blocks which differ in how they focus and defocus light rays. In the course of the derivation, we introduce the concept of a Lorentzian dynamical triangulation of product type, whose applicability goes beyond that of describing black-hole configurations."

http://arxiv.org/abs/gr-qc/0506031
Hermann Nicolai's contribution to Abhay Ashtekar's new book

http://arxiv.org/abs/gr-qc/0506024
a new Loop Quantum Cosmology paper

http://arxiv.org/abs/gr-qc/0506021
new MOND paper by Moffat

There are now at least 7 chapters of Ashtekar's book (A Hundred Years of Relativity) available as arxiv preprint. Here is a post about the book giving links to the other chapters:
https://www.physicsforums.com/showpost.php?p=566800&postcount=56

 

[marcus]

http://arxiv.org/gr-qc/0506075
General Relativity in the Undergraduate Physics Curriculum
James B. Hartle
9 pages, 2 figures

"Einstein's general relativity is increasingly important in contemporary physics on the frontiers of both the very largest distance scales (astrophysics and cosmology) and the very smallest(elementary particle physics). This paper makes the case for a 'physics first' approach to introducing general relativity to undergraduate physics majors."




http://arxiv.org/gr-qc/0506067
A group field theory for 3d quantum gravity coupled to a scalar field
Laurent Freidel, Daniele Oriti, James Ryan
11 pages

"We present a new group field theory model, generalising the Boulatov model, which incorporates both 3-dimensional gravity and matter coupled to gravity. We show that the Feynman diagram amplitudes of this model are given by Riemannian quantum gravity spin foam amplitudes coupled to a scalar matter field. We briefly discuss the features of this model and its possible generalisations."

---------------------------
some comment: I've been watching Freidel's work with the greatest interest for the past couple of years. He made some waves earlier this year with two papers, Freidel/Starodubtsev (that Baez called to our attention) and Freidel/Livine (Ponzano-Regge revisited III).

Freidel is at Uni. Lyon in France (also part time Perimeter in Canada) and the other two authors are at Cambridge in the UK.
Here is an exerpt from the Introduction section of the new Freidel paper:

---quote gr-qc/0506067---
Spin foam models [1, 2] represent a purely combinatorial and algebraic implementation of the sum-over-histories approach to quantum gravity, in any signature and spacetime dimension, with an abstract 2-complex playing the role of a discrete spacetime, and algebraic data from the representation theory of the Lorentz group playing the role of geometric data assigned to it.

This approach has recently been developed to a great extent in the 3-dimensional case. It is now clear that it provides a full quantisation of pure gravity[3], whose relation with the one obtained by other approaches is well understood[4, 5].

Moreover, matter can be consistently included in the picture[3, 6], providing a link between spin foam models and effective field theory[7] living on a non-commutative geometry. This picture allows us to naturally address the semi-classical limit of spin foam models and shows that quantum gravity in dimension 3 effectively follows the principle of the so-called deformed (or doubly) special relativity[8].

The group field theory formalism[9] represents a generalisation of matrix models of 2-dimensional quantum gravity [10]. It is a universal structure lying behind any spin foam model for quantum gravity[11, 12], providing a third quantisation point of view on gravity[9] and allowing us to sum over pure quantum gravity amplitudes associated with different topologies[13].

In this picture, spin foams, and thus spacetime itself, appear as (higher-dimensional analogues of) Feynman diagrams of a field theory defined on a group manifold and spin foam amplitudes are simply the Feynman amplitudes weighting the different graphs in the perturbative expansion of the quantum field theory.

On the other hand, we can construct a noncommutative field theory whose Feynman diagram amplitudes reproduce the coupling of matter fields to 3d quantum gravity for a trivial topology of spacetime[7]. Remarkably, the momenta of the fields are labelled also by group elements.

Moreover, in three dimensions there is a duality between matter and geometry, and the insertion of matter can be understood as the insertion of a topological defect charged under the Poincaré group[3].

This suggests that one should be able to treat the third quantisation of gravity and the second quantisation of matter fields in one stroke (see[14] for an early attempt). The purpose of this paper is to study how the coupling of matter to quantum gravity is realized in the group field theory, and whether it is possible to write down a group field theory for gravity and particles that reproduces the amplitudes derived in [3] coupling quantum matter to quantum geometry. This is what we achieve in the present work.

The way the correct amplitudes are generated as Feynman amplitudes of the group field theory is highly non-trivial. It requires an extension of the usual group field theory (gft) formalism to a higher number of field variables, and produces an interesting intertwining of gravity and matter degrees of freedom, as we are going to discuss in the following...
---endquote---

back in post #339 of this thread there is a link to a related paper that also appeared recently:
http://arxiv.org/abs/hep-th/0505174
Quantum Gravity with Matter via Group Field Theory
Kirill Krasnov
43 pages, many figures

(as one would expect, the Krasnov paper is cited by Freidel et al)

 

[marcus]

new paper DSR paper by Kowalski-Glikman

http://arxiv.org/abs/gr-qc/0506082
Quantized Black Holes, Their Spectrum and Radiation
I.B. Khriplovich
----time only to note this to check out later----

http://arxiv.org/abs/gr-qc/0506084
Doubly Special Relativity as a Limit of Gravity
Katarzyna Imilkowska, Jerzy Kowalski-Glikman
26 pages, Submitted to Lecture Notes in Physics

"Doubly Special Relativity (DSR) is a theory with two observer-independent scales, of velocity and mass, which is expected to replace Special Relativity at ultra-high energies. In these notes we first discuss the postulates of DSR, and then turn to presenting arguments supporting the hypothesis that DSR can be regarded as a flat space, semiclassical limit of gravity. The notes are based on the talk presented at the conference Special Relativity -- Will it Survive the Next 100 Years?''

my comment: a significant development this year was the paper by Freidel and Starodubtsev hep-th/0501191 "Quantum Gravity in Terms of Topological Observables". this is some way a follow-up on that.
At PF we have discussed Kowalski-Glikman work on several occasions, he being one of the leading theorists involved in DSR. He organized the Polish Winterschool workshop on QG Phenomenology of February 2004. Often works with QG people.
Now see page 8 of the KG et al paper----they are taking off from the QG formalism of Freidel and Starodubtsev. this is the "BF" approach where it was found there is a possible way to get a BACKGROUND INDEPENDENT, but nevertheless PERTURBATIVE approach, with the cosmological constant and the Barbero-Immirzi parameter playing significant roles.

KG is arguing as generally as he can that the flat limit of QG should be not Minkowski space but the corresponding DSR space (very much like Minkowski but with a second invariant scale)

the exposition is pedagogical, the level is of lecture notes for graduate students, so it is easier reading than usual Kowalski-Glikman. and also kind of an update since it comes after the landmark Freidel-Starodubtsev.

 

[selfAdjoint]

http://arxiv.org/abs/gr-qc/0506082
Quantized Black Holes, Their Spectrum and Radiation
I.B. Khriplovich
----time only to note this to check out later----

I read the paper. Here is the abstract:

Under quite natural general assumptions, the following results are obtained. The maximum entropy of a quantized surface is demonstrated to be proportional to the surface area in the classical limit. The general structure of the horizon spectrum is found. The discrete spectrum of thermal radiation of a black hole Under quite natural general assumptions, the following results are obtained. The maximum entropy of a quantized surface is demonstrated to be proportional to the surface area in the classical limit. The general structure of the horizon spectrum is found. The discrete spectrum of thermal radiation of a black hole fits the Wien profile. The natural widths of the lines are much smaller than the distances between them. The total intensity of the thermal radiation is estimated.
In the special case of loop quantum gravity, the value of the Barbero -- Immirzi parameter is found. Different values for this parameter, obtained under additional assumption that the horizon is described by a U(1) Chern -- Simons theory, are demonstrated to be in conflict with the firmly established holographic bound.

His derivation of the "holographic bound", which he uses several times to show other people's calculations are wrong, is particularly intuitive. But the whole argument is really just baby statistical mechanics applied to a surface constructed of patches forming the event horizon of a black hole. Many of us here should be able to follow it.

 

[marcus]

new paper by Shahar Hod

more about the black hole radiation spectrum

http://arxiv.org/abs/hep-th/0506214
Selection Rules for Black-Hole Quantum Transitions
Shahar Hod, Uri Keshet
4 pages, 2 figures

"We suggest that quantum transitions of black holes comply with selection rules, analogous to those of atomic spectroscopy. In order to identify such rules, we apply Bohr's correspondence principle to the quasinormal ringing frequencies of black holes. In this context, classical ringing frequencies with an asymptotically vanishing real part
$$\omega_R$$
correspond to virtual quanta, and may thus be interpreted as forbidden quantum transitions. With this motivation, we calculate the quasinormal spectrum of neutrino fields in spherically symmetric black-hole spacetimes. It is shown that
$$\omega_R \rightarrow 0$$
for these resonances, suggesting that the corresponding fermionic transitions are quantum mechanically forbidden."


Shahar Hod was who started the uproar about quasinormal vibration modes of black holes in the first place. He cites his own 1998 paper

 

[marcus]

new black hole paper by Bojowald

http://arxiv.org/abs/gr-qc/0506128
Nonsingular Black Holes and Degrees of Freedom in Quantum Gravity

Martin Bojowald
4 pages

"Spherically symmetric space-times provide many examples for interesting black hole solutions, which classically are all singular. Following a general program, space-like singularities in spherically symmetric quantum geometry, as well as other inhomogeneous models, are shown to be absent. Moreover, one sees how the classical reduction from infinitely many kinematical degrees of freedom to only one physical one, the mass, can arise, where aspects of quantum cosmology such as the problem of initial conditions play a role."

 

[Spin_Network]

http://arxiv.org/abs/gr-qc/0506128
Nonsingular Black Holes and Degrees of Freedom in Quantum Gravity

Martin Bojowald
4 pages

"Spherically symmetric space-times provide many examples for interesting black hole solutions, which classically are all singular. Following a general program, space-like singularities in spherically symmetric quantum geometry, as well as other inhomogeneous models, are shown to be absent. Moreover, one sees how the classical reduction from infinitely many kinematical degrees of freedom to only one physical one, the mass, can arise, where aspects of quantum cosmology such as the problem of initial conditions play a role."

Marcus the last two papers are great, Hod's in perticular, very interesting!

 

[marcus]

...the last two papers are great, Hod's in perticular, very interesting!

Spin Network, I am so glad you found the papers readable and of interest to you! At first sight, I could not understand much of the Hod paper. but it was his intuition (more than 5 years ago now IIRC) that set off that long train of research into BH quasinormal modes (with considerable consequences for quantum gravity, especially Loop). so posting the Hod link was a no brainer.

this next link is about nothing in particular. I just need a place to stash it so as to have it handy.
http://adsabs.harvard.edu/physics_service.html
it is a good search engine, but the database is limited in some way
I'm not certain about

 

[marcus]

Here's an odd one!

http://arxiv.org/abs/gr-qc/0506129
Quantum evaporation of a naked singularity
Rituparno Goswami, Pankaj S. Joshi, Parampreet Singh
4 pages, 2 figures

I respect Parampreet Singh. He is a postdoc of Ashtekar at Penn State who has coauthored interesting papers with Bojowald and with Roy Maartens.
Several of his seminar talks at Penn State are online---talking about LQC phenomenology: observable signature of Loop gravity in CMB and so forth. He is very focused on observable quantum gravity effects.

I wasnt familiar with the other two authors, but now I see that e.g. Goswami has 15 papers and has co-authored with Bojowald on an interesting one that we discussed earlier at PF
http://arxiv.org/abs/gr-qc/0503041

This present paper talks about something very strange. Not sure what to make of it!

"We investigate here gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that non-perturbative semi-classical modifications near the singularity, based on loop quantum gravity, give rise to a strong outward flux of energy. This leads to the dissolution of the collapsing cloud before a naked singularity can form. Quantum gravitational effects can thus censor naked singularities by avoiding their formation. Further, quantum gravity induced mass flux has a distinct feature which can lead to a novel observable signature in astrophysical bursts."

it seems that the authors may have found a quantum reason for the absence of naked glitches ("cosm. censorship") and also they may have may have may have a prediction about gammaray bursts which could provide a way of empirically testing what they are saying.