Loop-and-allied QG bibliography

[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.

 

[Spin_Network]

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 this paper may be of great interest:http://uk.arxiv.org/abs/quant-ph/0506228

he thanks Rovelli and Smolin to name but two!

and this paper may/will? be of interest to the Hod paper:http://uk.arxiv.org/abs/quant-ph/0506231

 

[selfAdjoint]

Marcus this paper may be of great interest:http://uk.arxiv.org/abs/quant-ph/0506228

he thanks Rovelli and Smolin to name but two!

and this paper may/will? be of interest to the Hod paper:http://uk.arxiv.org/abs/quant-ph/0506231

http://uk.arxiv.org/abs/quant-ph/0506228 is certainly a greatly interesting paper! I am going to nring it to the attention of the quantum physics subforum.

 

[marcus]

http://uk.arxiv.org/abs/quant-ph/0506228 is certainly a greatly interesting paper! I am going to bring it to the attention of the quantum physics subforum.

thanks for fielding that one!

 

[marcus]

another shoe drops re Pioneer anomaly

http://arxiv.org/abs/gr-qc/0506139

"...The existence of the Pioneer anomaly is no longer in doubt. Further, after much understandable hesitancy, a steadily growing part of the community has concluded that the anomaly should be subject to interpretation. Our program presents an ordered approach to doing this..."


"...This mission is designed to determine the origin of the discovered anomaly and to characterize its properties to an accuracy of at least three orders of magnitude below its measured value ..."

A MISSION TO EXPLORE THE PIONEER ANOMALY

the list of authors has some 39 names, they call themselves the Pioneer Collaboration.

it is an 8 page paper.

the noises keep getting louder that something about gravity needs adjustment

 

[marcus]

http://arxiv.org/hep-th/0507012
Taming the cosmological constant in 2D causal quantum gravity with topology change

R. Loll (U. Utrecht), W. Westra (U. Utrecht), S. Zohren (U. Utrecht, RWTH Aachen)
19 pages, 4 figures

"As shown in previous work, there is a well-defined nonperturbative gravitational path integral including an explicit sum over topologies in the setting of Causal Dynamical Triangulations in two dimensions. In this paper we derive a complete analytical solution of the quantum continuum dynamics of this model, obtained uniquely by means of a double-scaling limit. We show that the presence of infinitesimal wormholes leads to a decrease in the effective cosmological constant, reminiscent of the suppression mechanism considered by Coleman and others in the four-dimensional Euclidean path integral. Remarkably, in the continuum limit we obtain a finite spacetime density of microscopic wormholes without assuming fundamental discreteness. This shows that one can in principle make sense of a gravitational path integral which includes a sum over topologies, provided suitable causality restrictions are imposed on the path integral histories."

this is the paper they will present this month in Paris at the Einstein2005 conference

Willem Westra did his Masters at Utrecht working for Loll, on this problem, and they published a paper in 2003 about it, and now he is doing his PhD.
Including topology-change in the path integral is very interesting.
one takes a weighted average not only over all possible spacetime geometries, but also adds up all possible spacetime topologies and all possible geometries of each topology. it could get amusing

 

[marcus]

Abhay Ashtekar and Martin Bojowald have posted an updated version of this
http://arxiv.org/abs/gr-qc/0504029
Black hole evaporation: A paradigm
Abhay Ashtekar, Martin Bojowald
21 pages, 4 figures, v2: new references and discussion of relation to other ideas

"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 using quantum geometry methods; and ii) time-evolution of black holes in the trapping and dynamical horizon frameworks. 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 resolved."

Black hole evaporation seems to be a hot topic in quantum gravity now---thinking of the recent paper by Joshi, Goswami, and P.Singh. Also a couple recent papers by Bojowald solo.
BTW in this one Ashtekar and Bojo cite this highly readable and provocative paper by Sean Hayward
http://arxiv.org/abs/gr-qc/0504038
The disinformation problem for black holes (pop version)
Sean A. Hayward
6 pages
The supposed information paradox for black holes is based on the fundamental misunderstanding that black holes are usefully defined by event horizons. Understood in terms of locally defined trapping horizons, the paradox disappears: information will escape from an evaporating black hole. According to classical properties of trapping horizons, a general scenario is outlined whereby a black hole evaporates completely without singularity, event horizon or loss of energy or information.

as another BTW here are Ruth Williams' papers
http://arxiv.org/find/grp_physics/1/au:+Williams_Ruth/0/1/0/all/0/1
(she co-authored with Tullio Regge around 2000) and here is a new one
http://arxiv.org/abs/gr-qc/0506137
Nonlocal Effective Field Equations for Quantum Cosmology
Herbert W. Hamber, Ruth M. Williams
9 pages

"The possibility that the strength of gravitational interactions might slowly increase with distance, is explored by formulating a set of effective field equations, which incorporate the gravitational, vacuum-polarization induced, running of Newton's constant G. The resulting long distance (or large time) behaviour depends on only one adjustable parameter \xi, and the implications for the Robertson-Walker universe are calculated, predicting an accelerated power-law expansion at later times t \sim \xi \sim 1/H.

 

[marcus]

This week, Loll and Westra have posted an updated version of their 2003 paper
http://arxiv.org/hep-th/0306183
Sum over topologies and double-scaling limit in 2D Lorentzian quantum gravity
9 pages, 3 Postscript figures; added comments on strip versus bulk partition function

"We construct a combined non-perturbative path integral over geometries and topologies for two-dimensional Lorentzian quantum gravity. The Lorentzian structure is used in an essential way to exclude geometries with unacceptably large causality violations. The remaining sum can be performed analytically and possesses a unique and well-defined double-scaling limit, a property which has eluded similar models of Euclidean quantum gravity in the past."

this was a first. before, the moment you allowed wormholes the sum would blow up and you would get infinities. too many baby universes, too many possibilities. so you had to make a rule against topology-change at the outset.
I am oversimplifying. Anyway in earlier CDT the topology of spacetime had to be restricted to be simple, and then within that you could have all different shape geometries. But this little paper of Loll and Westra is a kind of landmark because at least in 2D they are allowing topology-change and it is not a complete disaster.

now there is a little more progress
http://arxiv.org/hep-th/0507012
Taming the cosmological constant in 2D causal quantum gravity with topology change
They are getting their stuff together for the Paris conference this month.
there is this curious result of a finite density of wormholes.
they are going back and polishing the 2003 paper a little, because it will be a footnote in the 2005 paper they give in Paris. what busy people

 

[marcus]

Just a week or so ago this one came out, Parmapreet Singh being one of the co-authors.

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

"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."

Today, another P. Singh paper:

http://arxiv.org/abs/gr-qc/0507029
Semi-classical States, Effective Dynamics and Classical Emergence in Loop Quantum Cosmology
Parampreet Singh, Kevin Vandersloot
8 pages, 4 figures
IGPG-05/7-1, AEI-2005-122

"We construct physical semi-classical states annihilated by the Hamiltonian constraint operator in the framework of loop quantum cosmology as a method of systematically determining the regime and validity of the semi-classical limit of the quantum theory. Our results indicate that the evolution can be effectively described using continuous classical equations of motion with non-perturbative corrections down to near the Planck scale below which the universe can only be described by the discrete quantum constraint. These results, for the first time, provide concrete evidence of the emergence of classicality in loop quantum cosmology and also clearly demarcate the domain of validity of different effective theories. We prove that discrete quantum geometry effects may become very significant and lead to various new phenomenological applications. Furthermore the understanding of semi-classical states allows for a framework for interpreting the quantum wavefunctions and understanding questions of a semi-classical nature within the quantum theory of loop quantum cosmology."

mounting evidence that in the cosmology sector LQG is consistent with classical cosmology----that it has the right largescale limit in other words.

 

[marcus]

Thiemann decides which volume operator is right

http://arxiv.org/abs/gr-qc/0507036
Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity I
Kristina Giesel, Thomas Thiemann
20 pages, 5 figures

"The volume operator plays a pivotal role for the quantum dynamics of Loop Quantum Gravity (LQG). It is essential in order to construct Triad operators that enter the Hamiltonian constraint and which become densely defined operators on the full Hilbert space even though in the classical theory the triad becomes singular when classical GR breaks down. The expression for the volume and triad operators derives from the quantisation of the fundamental electric flux operator of LQG by a complicated regularisation procedure. In fact, there are two inequivalent volume operators available in the literature and, moreover, both operators are unique only up to a finite, multiplicative constant which should be viewed as a regularisation ambiguity. Now on the one hand, classical volumes and triads can be expressed directly in terms of fluxes and this fact was used to construct the corresponding volume and triad operators. On the other hand, fluxes can be expressed in terms of triads and therefore one can also view the volume operator as fundamental and consider the flux operator as a derived operator. In this paper we examine whether the volume, triad and flux quantisations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show: 1. The regularisation constant can be uniquely fixed. 2. One of the volume operators can be ruled out as inconsistent. 3. Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized. In this paper we present ideas and results of the consistency check. In a companion paper we supply detailed proofs."

http://arxiv.org/abs/gr-qc/0507037
Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity II

Kristina Giesel, Thomas Thiemann
67 pages, 6 figures, 36 pages paper, 31 pages appendix

"In this paper we provide the techniques and proofs for the resuls presented in our companion paper concerning the consistency check on volume and triad operator quantisation in Loop Quantum Gravity."

 

[marcus]

A new LQG primer!

http://arxiv.org/abs/gr-qc/0507038
Loop Quantum Geometry: A primer
Alejandro Corichi
Comments: 25 pages. Contribution for the Proceedings of the VI Mexican School of Gravitation and Mathematical Physics

"This is the written version of a lecture given at the ``VI Mexican School of Gravitation and Mathematical Physics" (Nov 21-27, 2004, Playa del Carmen, Mexico), introducing the basics of Loop Quantum Geometry. The purpose of the written contribution is to provide a Primer version, that is, a first entry into Loop Quantum Gravity and to present at the same time a friendly guide to the existing pedagogical literature on the subject. This account is geared towards graduate students and non-experts interested in learning the basics of the subject."

This is by someone who used to visit here at PF fairly often and made some very useful posts (but under an internet "handle" name, not his own) according to my considered opinion.

I am very glad that there is a new Primer, introduction to the subject for grad students getting into it. for many years the most convenient LQG Primer was the 1998 one of Rovelli Upadhya and it is good to have another, so the beginner can have more choice.

 

[Chronos]

Thanks marcus. I'm a pedagogologist, as you probably know.

 

[marcus]

Thanks marcus. I'm a pedagogologist, as you probably know.

a connoisseur of the art of teaching?
no I didn't know.
but actually Alejandro Corichi is aiming at a narrowly defined level of grad student who has completed a graduate course in General Relativity.
And one or two other substantial prerequisites. He is talking to a definite audience and not going out of his way to reach others.

If he was here i would tell him that on page 2 paragraphs 3 and 4 he misspells heart "hearth" and thought "though".
This is the pitfall of the spell-checker, which doesn't know what word you are trying to spell so just gives you SOME correctly spelled word.

 

[Spin_Network]

Bing that you are HERE marcus, I convey that "hearth" was meant to be 'Hearted', as in :not for the faint hearted! ;)

I myself have 'boo**ied' in spelling many times, actually in this very POST!..so I will refrain from altering it :(

 

[selfAdjoint]

New papers by Kristina Giesel and Thomas Thiemann

http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0507/0507036.pdf has the proofs.


From the abstract of gr-qc/0507036:

In this paper we examine whether the volume, triad and flux quantisations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show: 1. The regularisation constant can be uniquely fixed. 2. One of the volume operators can be ruled out as inconsistent. 3. Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized. In this paper we present ideas and results of the consistency check. In a companion paper we supply detailed proofs.

From the introduction:

First of all, there are in fact two unitarily
inequivalent volume operators [5, 6] which come from two, a priori equally justified background independent regularisation techniques. We will denote them by Rovelli – Smolin (RS) and Ashtekar – Lewandowski (AL) volume respectively for the rest of this paper. Secondly, both volume operators are anyway only determined up to a multiplicative regularisation constant C_reg [12] which remains undetermined when taking the limit, quite similar to finite regularisation constants that appear in counterterms of standard renormaisation of ordinary QFT. The ambiguity is further enhanced by factor ordering ambiguities once we consider triad operators. These ambiguities are parameterized by a spin quantum number ℓ = 1/2, 1, 3/2, ...
In this paper we will be able to remove all those ambiguities by the following consistency check: As we mentioned above, the volume and triad can be considered as functions of the fluxes. But the converse is also true: The fluxes can be written in terms of triads and thus the volume. Is it then true that there exists a regularisation constant for the volume operator and a factor ordering of the flux operator considered as a function of the triad operator or volume operator such that the corresponding alternative flux operator agrees (at least in the correspondence limit of large eigenvalues of the volume operator) with the fundamental flux operator, independent of the choice of ℓ? This better be possible as otherwise the inescapable conclusion would be that the volume operator is inconsistently quantised

Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.

 

[marcus]

...

Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.

Hi selfAdj, concise and nicely crafted. It's a definite plus to have more than one person scouting and flagging papers. So it looks like Ashtekar's volume wins over Smolin and Rovelli volume.
Here's a snapshot of Kristina Giesel
http://math.ucr.edu/home/baez/marseille/giesel.jpg

 

[Spin_Network]

http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0507/0507036.pdf has the proofs.


From the abstract of gr-qc/0507036:



From the introduction:


Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.

This is actually going to be very..very interesting, I have not gone through both papers fully, I actually was reading the proof one first, which set alarm bells ringing, and I am indulged in a number of Rovelli's papers, I believe Rovelli has allready highlighted a specific relevant aspect, but then again I might just go straight to the Volume operator Einstein detailed in an much overlooked correspondance, I believe Rovelli's veiwpoint not be 'over-ruled' just yet!

Measure..measure..measure!

 

[marcus]

Hawking finally comes out with it!

http://arxiv.org/abs/hep-th/0507171


he gave the talk almost exactly one year ago, and everybody said
they'd have to wait to read the paper

this 5-pager may not settle all the questions.

In the paper Hawking says information is not lost down a black hole, however reconstructing the information that fell in from the hawking radiation that comes from the hole as it evaporates is (in some sense) like reconstructing an encyclopedia from the smoke and ashes it leaves when consumed by fire.

At the end Hawking reflects that when he paid off the bet to John Preskill and gave him the Encyclopedia
perhaps he should have burned the Encyclopedia first and given Preskill the ashes.

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

WARNING: this paper may leave you quite frustrated. that's how it left me anyway.

In the paper he says:
"I adopt the Euclidean [path integral] approach, the only sane way to do quantum gravity nonperturbatively."

Hawking Euclidean QG path integral is a 1980s and 1990s precursor to Loll Lorentzian QG path integral. Loll papers refer back to Hawking Euclidean sum over histories and related Hawking QG matters. I'm persuaded that path integral sum over geometries is a promising way to do QG and Hawking deserves credit for establishing this research direction. But this paper has not convinced me that Hawking's specific (Euclidean) approach to it is necessarily the right one.

 

[marcus]

new paper by Martin Reuter

http://arxiv.org/abs/hep-th/0507167

From Big Bang to Asymptotic de Sitter: Complete Cosmologies in a Quantum Gravity Framework

M. Reuter, F. Saueressig
47 pages, 17 figures
"Using the Einstein-Hilbert approximation of asymptotically safe quantum gravity we present a consistent renormalization group based framework for the inclusion of quantum gravitational effects into the cosmological field equations. Relating the renormalization group scale to cosmological time via a dynamical cutoff identification this framework applies to all stages of the cosmological evolution. The very early universe is found to contain a period of "oscillatory inflation'' with an infinite sequence of time intervals during which the expansion alternates between acceleration and deceleration. For asymptotically late times we identify a mechanism which prevents the universe from leaving the domain of validity of the Einstein-Hilbert approximation and obtain a classical de Sitter era."

Martin Reuter is one of the invited speakers at the Loops 05 conference in October. He has his own approach to QG, which is different from LQG but when it is applied to cosmology it gets some similar results. Here he gets some results similar to Martin Bojowald's Loop Quantum Cosmology. So there is an interesting convergence. Bojowald derived this "oscillatory inflation" business earlier. Now reuter is getting it by a different method.

 

[Spin_Network]

http://arxiv.org/abs/hep-th/0507171


he gave the talk almost exactly one year ago, and everybody said
they'd have to wait to read the paper

this 5-pager may not settle all the questions.

In the paper Hawking says information is not lost down a black hole, however reconstructing the information that fell in from the hawking radiation that comes from the hole as it evaporates is (in some sense) like reconstructing an encyclopedia from the smoke and ashes it leaves when consumed by fire.

At the end Hawking reflects that when he paid off the bet to John Preskill and gave him the Encyclopedia
perhaps he should have burned the Encyclopedia first and given Preskill the ashes.

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

WARNING: this paper may leave you quite frustrated. that's how it left me anyway.

In the paper he says:
"I adopt the Euclidean [path integral] approach, the only sane way to do quantum gravity nonperturbatively."

Hawking Euclidean QG path integral is a 1980s and 1990s precursor to Loll Lorentzian QG path integral. Loll papers refer back to Hawking Euclidean sum over histories and related Hawking QG matters. I'm persuaded that path integral sum over geometries is a promising way to do QG and Hawking deserves credit for establishing this research direction. But this paper has not convinced me that Hawking's specific (Euclidean) approach to it is necessarily the right one.

Thanks marcus, I have been waiting for this for some time. On another forum, and at PF under a different name, questions have I asked about the ILP (Information Loss Paradox) now you have yourself read the paper and it frustrates to an extent?

Lets read between the path-integral lines? straight away I see Hawking has confirmed my 'past' question of Blackhole 'Time-dependant' reasoning. Take the last sentence in the paper:I gave John an encyclopedia of baseball, but maybe I should just have given him the ashes.

Is this really cricket? ..or is this fact that one can derive an integral of information entering a Blackhole, but cannot derieve the same integral of the information that 'rebounds' , scatters back out?

What this really means is that if you throw an English Encyclopedia into a Blackhole, the only thing you can guarantee, is that an English Encyclopedia will never emerge, the particles that went into the construction of the pre-blackhole Encyclopedia, can never re-construct it , the particles that scatter from a certain black hole horizon, are 'Time-Stamped', and are thus 'younger', 'older' but never the same 'age' as those that entered the Blackhole.

Into the fire, Out of the ashes ?..you will never get the particles of the Encyclopedia back scattered, but you could theoretically Get the Particles of the Trees that went into 'before' the Paper was created, and thus only re-create an Encyclopedia that has 'no-written-words', a sort of Encyclopedia Template!..not an 'Historical' Documentation of Factual Writings!

"The information loss corresponds to the classical relaxation of black holes according to the no hair theorem. One can not ask when the information gets out of a black hole because that would require the use of a semi-classical metric which has already lost the information"

The Time-Dependant paths of Galactic Blackholes have no Branching off to 'other-universes' . "If you jump into a black hole, your mass energy will
be returned to our universe but in a mangled form which contains the information about what you were like but in a state where it can not be easily recognized. It is like burning an encyclopedia. Information is not lost, if one keeps the smoke and the ashes. But it is difficult to read."

This paper contravines one of the most rigourous time-evolution paramiters of Big-Bang theory, if one rewinds our Galaxy, with the theorized Blackhole at its Core, then our Galaxy has a Time-Stamp 'information' that is Unique to our Galaxy. It is evident that all Galaxies that have theorized Blackhole's at their core's, are thus themselves 'Unique'.

The only way in GR to travel to another 'Time-PAST/FUTURE', is to remain inside you Galaxy, wait for another Galaxy to head your way as a merging process, then to 'jump-ship' when the intertwined Galaxies exchange information during the collision process.

You cannot leave our Galaxy and travel to Andromeda, which observationally is within our 'information-time' locally, but yet according to Hawking, if one waits for the 'future' collision of Andromeda and Milkyway, then this collision harbours a good chance of 'Time-Travel'..infact the ONLY chance.