Loop-and-allied QG bibliography

[qsa]

http://arxiv.org/PS_cache/gr-qc/pdf/0006/0006061v3.pdf

Carlo Rovelli

I sketch the main lines of development of the research in quantum gravity,
from the first explorations in the early thirties to nowadays.

 

[jfy4]

http://arxiv.org/abs/1006.2369

Torsten and Helge have discussed their idea some with us here at PF Beyond. It is a radical and high-risk idea.
http://arxiv.org/abs/1006.2230
On the geometrization of matter by exotic smoothness
Torsten Asselmeyer-Maluga, Helge Rose
17 pages
(Submitted on 11 Jun 2010)
"Clifford's hypothesis is investigated: A particle is made up of nothing but a distinct type of a space manifold, differing from the surrounding manifold of empty space. It is shown that this distinct space manifold representing matter differs from the surrounding vacuum by the exotic smoothness of its spacetime. The smoothness structure of spacetime can be described by a tree-like subset -- the Casson handle -- consisting of immersed discs and connecting tubes between them. The Weierstrass representation shows that the immersed discs are represented by spinors fulfilling the Dirac equation and leading to a mass-less Dirac term in the Einstein-Hilbert action. The connecting tubes between the discs realize an action term of a gauge field. Both terms are purly geometrical and characterized by the mean curvature of the components of the Casson handle. This gives a good support to Clifford's conjecture that matter is nothing more but an exotic kind of space."

interesting conjecture quoted from right at the very end of the paper, on page 15:
"At the end we want to give another interpretation of the Casson handle. Connes [37] showed that by means of the non-commutative geometry the action of the standard model can be reproduced. His model is based on the space M×F where the additional space F is ad hoc and has no relation to the spacetime M.
In our model the space F could be interpreted as an expression of the Casson handle and so of the smoothness of spacetime establishing a deep relation between quantum matter and space."

Ahh! recently I have been considering to myself whether it would be possible to construct matter from space using a transformation of sorts. in fact it was 2 days ago that this idea hit me, and then yesterday I come to PF only to find a Paper on that very idea, and a long history of interest in the possibility forming matter out of geometry. My intuition told me that using a discrete set of space-time quanta, one could assemble a wave in much the same way a discrete transform takes quanta inputs and out-puts a continuous wave. A very alluring idea.

 

[marcus]

jfy4,
If we discussed and commented on papers in the bibliography thread it would get overcrowded. The practice here is to just list papers in this thread, with the paper's own abstract summary or a brief sample quote indicating what it is about. If you want to comment or discuss a particular paper please start a different thread for that purpose.
Since you are new and didn't know the custom, I will start a thread for you to comment on the Helge Torsten paper. I don't know but other people might want to comment, so a longer discussion could develop.

 

[marcus]

http://arxiv.org/abs/1006.4908
An Explicit Embedding of Gravity and the Standard Model in E8
A. Garrett Lisi
14 pages. For peer review and publication in the "Proceedings of the Conference on Representation Theory and Mathematical Physics."
(Submitted on 25 Jun 2010)
"The algebraic elements of gravitational and Standard Model gauge fields acting on a generation of fermions may be represented using real matrices. These elements match a subalgebra of spin(11,3) acting on a Majorana-Weyl spinor, consistent with GraviGUT unification. This entire structure embeds in the quaternionic real form of the largest exceptional Lie algebra, E8. These embeddings are presented explicitly and their implications discussed."

 

[marcus]

http://arxiv.org/abs/1006.5145
Functional Renormalization of Noncommutative Scalar Field Theory
Alessandro Sfondrini, Tim A. Koslowski
37 pages
(Submitted on 26 Jun 2010)
"In this paper we apply the Functional Renormalization Group Equation of Wetterich to the non-commutative scalar field theory proposed by Grosse and Wulkenhaar. We derive the flow equation in the matrix representation, and construct the theory space for the self-dual model. The features introduced by the external dimensionful scale provided by the non-commutativity parameter, originally pointed out in [1], are discussed in this new context. Using a technical assumption, but without resorting to a truncation, it is then shown that the theory is asymptotically safe for a suitable small value of the couplings, recovering the result of [2]. Finally, we show how the FRGE can be easily used to compute the one loop beta-functions of the duality covariant model."

http://arxiv.org/abs/1006.5666
Revisiting the Simplicity Constraints and Coherent Intertwiners
Maité Dupuis, Etera R. Livine
28 pages
(Submitted on 29 Jun 2010)
"In the context of loop quantum gravity and spinfoam models, the simplicity constraints are essential in that they allow to write general relativity as a constrained topological BF theory. In this work, we apply the recently developed U(N) framework for SU(2) intertwiners to the issue of imposing the simplicity constraints to spin network states. More particularly, we focus on solving them on individual intertwiners in the 4d Euclidean theory. We review the standard way of solving the simplicity constraints using coherent intertwiners and we explain how these fit within the U(N) framework. Then we show how these constraints can be written as a closed u(N) algebra and we propose a set of U(N) coherent states that solves all the simplicity constraints weakly for an arbitrary Immirzi parameter."

 

[marcus]

http://arxiv.org/abs/1007.0718
On limitations of the extent of inertial frames in non-commutative relativistic spacetimes
Lee Smolin
21 pages
(Submitted on 5 Jul 2010)
"We study the interplay of non-locality and Lorentz invariance in a version of deformed or doubly special relativity (DSR) based on kappa-Minkowski spacetime. We find that Einstein's procedure for an inertial observer to assign coordinates to distant events becomes ambiguous for sufficiently distant events. The accuracy to which two clocks can be synchronized turns out to depend on the distance between them. These are consequences of the non-commutativity of space and time coordinates or a dependence of the speed of light on energy in relativistic theories. These ambiguities grow with distance and only become relevant for real observations for the description of cosmologically distant events. They do not afflict the interpretation of the detection of gamma rays in stationary or moving frames near the detector. Consequently there is no disagreement between the principles of DSR and the observation that interactions in nature are local down to currently observable scales."

http://arxiv.org/abs/1007.0735
Unimodular Loop Quantum Cosmology
Dah-Wei Chiou, Marc Geiller
26 pages
(Submitted on 5 Jul 2010)
"Unimodular gravity is based on a modification of the usual Einstein-Hilbert action that allows one to recover general relativity with a dynamical cosmological constant. It also has the interesting property of providing, as the momentum conjugate to the cosmological constant, an emergent clock variable. In this paper we investigate the cosmological reduction of unimodular gravity, and its quantization within the framework of flat homogeneous and isotropic loop quantum cosmology. It is shown that the unimodular clock can be used to construct the physical state space, and that the fundamental features of the previous models featuring scalar field clocks are reproduced. In particular, the classical singularity is replaced by a quantum bounce, which takes place in the same condition as obtained previously. We also find that requirement of semi-classicality demands the expectation value of the cosmological constant to be small (in Planck units). The relation to spin foam models is also studied, and we show that the use of the unimodular time variable leads to a unique vertex expansion."

 

[marcus]

http://arXiv.org/abs/1007.0402
Introductory lectures to loop quantum gravity
Pietro Doná, Simone Speziale
Based on lectures given at the 3eme Ecole de Physique Theorique de Jijel, Algeria, 26 Sep -- 3 Oct, 2009. 52 pages, many figures. To be published in the proceedings
(Submitted on 2 Jul 2010)
"We give a standard introduction to loop quantum gravity, from the ADM variables to spin network states. We include a discussion on quantum geometry on a fixed graph and its relation to a discrete approximation of general relativity."

 

[marcus]

http://arxiv.org/abs/1007.1317
Black holes in an asymptotically safe gravity theory with higher derivatives
Yi-Fu Cai, Damien A. Easson
22 pages, 3 figures
(Submitted on 8 Jul 2010)
"We present a class of spherically symmetric vacuum solutions to an asymptotically safe theory of gravity containing high-derivative terms. We find quantum corrected Schwarzschild-(anti)-de Sitter solutions with running gravitational coupling parameters. The evolution of the couplings is determined by their corresponding renormalization group flow equations. These black holes exhibit properties of a classical Schwarzschild solution at large length scales. At the center, the metric factor remains smooth but the curvature singularity, while softened by the quantum corrections, persists. The solutions have an outer event horizon and an inner Cauchy horizon which equate when the physical mass decreases to a critical value. Super-extremal solutions with masses below the critical value correspond to naked singularities. The Hawking temperature of the black hole vanishes when the physical mass reaches the critical value. Hence, the black holes in the asymptotically safe gravitational theory never completely evaporate. For appropriate values of the parameters such stable black hole remnants make excellent dark matter candidates."

In case anyone missed it, Easson co-authored earlier this year with N-laureate George Smoot. There were a couple of Easson Frampton Smoot cosmology papers.

 

[MTd2]

http://arxiv.org/abs/1007.1437

Time Evolution of Horizons

Arundhati Dasgupta
(Submitted on 8 Jul 2010)
A density matrix is defined using coherent states for space-times with apparent horizons. Evolving the density matrix in time gives the origin of Hawking radiation.

 

[marcus]

http://arxiv.org/abs/1007.1563
New formulation of Horava-Lifgarbagez quantum gravity as a master constraint theory
Chopin Soo, Jinsong Yang, Hoi-Lai Yu
4 pages
(Submitted on 9 Jul 2010)
"Horava-Lifgarbagez theory of quantum gravity attempts to preserve unitarity by relinquishing space-time covariance, and improve renormalizability by including higher order (spatial) derivatives. For theories without full space-time covariance, departures of the constraint algebra from the Dirac algebra are to be expected. In the non-projectable version of Horava-Lifgarbagez gravity, the commutator of two local Hamiltonian constraints leads to severely restrictive secondary constraints and perplexing 'troubles'. On the other hand, the projectable version has an integrated Hamiltonian constraint and consistent constraint algebra. But an extra graviton mode which can be problematic is then allowed, whereas in Einstein's theory the spurious mode is eliminated precisely by the local Hamiltonian constraint. A new formulation of Horava-Lifgarbagez gravity, naturally realized as a representation of the master constraint algebra studied by loop quantum gravity researchers, is presented in this work. This reformulation yields a consistent canonical theory with 1st class constraints. It captures the essence of Horava-Lifgarbagez gravity in retaining only spatial diffeomorphisms (instead of full space-time covariance) as the physically relevant non-trivial gauge symmetry; at the same time the local Hamiltonian constraint which is needed to remove the spurious mode is equivalently enforced by the master constraint."

"Master constraint" is a version of canonical LQG developed by Thiemann.

 

[atyy]

http://arxiv.org/abs/1007.1724
Gravity, Two Times, Tractors, Weyl Invariance and Six Dimensional Quantum Mechanics
Roberto Bonezzi, Emanuele Latini, Andrew Waldron
(Submitted on 10 Jul 2010)
Fefferman and Graham showed some time ago that four dimensional conformal geometries could be analyzed in terms of six dimensional, ambient, Riemannian geometries admitting a closed homothety. Recently it was shown how conformal geometry provides a description of physics manifestly invariant under local choices of unit systems. Strikingly, Einstein's equations are then equivalent to the existence of a parallel scale tractor (a six component vector subject to a certain first order covariant constancy condition at every point in four dimensional spacetime). These results suggest a six dimensional description of four dimensional physics, a viewpoint promulgated by the two times physics program of Bars. The Fefferman--Graham construction relies on a triplet of operators corresponding, respectively to a curved six dimensional light cone, the dilation generator and the Laplacian. These form an sp(2) algebra which Bars employs as a first class algebra of constraints in a six-dimensional gauge theory. In this article four dimensional gravity is recast in terms of six dimensional quantum mechanics by melding the two times and tractor approaches. This "parent" formulation of gravity is built from an infinite set of six dimensional fields. Successively integrating out these fields yields various novel descriptions of gravity including a new four dimensional one built from a scalar doublet, a tractor vector multiplet and a conformal class of metrics.

http://arxiv.org/abs/1007.1795
Emergent Geometry and Quantum Gravity
Hyun Seok Yang
(Submitted on 11 Jul 2010)
We explain how quantum gravity can be defined by quantizing spacetime itself. A pinpoint is that the gravitational constant G = L_P^2 whose physical dimension is of (length)^2 in natural unit introduces a symplectic structure of spacetime which causes a noncommutative spacetime at the Planck scale L_P. The symplectic structure of spacetime $M$ leads to an isomorphism between symplectic geometry (M, \omega) and Riemannian geometry (M, g) where the deformations of symplectic structure \omega in terms of electromagnetic fields F=dA are transformed into those of Riemannian metric g. This approach for quantum gravity allows a background independent formulation where spacetime as well as matter fields is equally emergent from a universal vacuum of quantum gravity which is thus dubbed as the quantum equivalence principle.

http://arxiv.org/abs/1007.1813
Entanglement entropy for the n-sphere
H. Casini, M. Huerta
(Submitted on 12 Jul 2010)
We calculate the entanglement entropy for a sphere and a massless scalar field in any dimensions. The reduced density matrix is expressed in terms of the infinitesimal generator of conformal transformations keeping the sphere fixed. The problem is mapped to the one of a thermal gas in a hyperbolic space and solved by the heat kernel approach. The coefficient of the logarithmic term in the entropy for 2 and 4 spacetime dimensions are in accordance with previous numerical and analytical results. In particular, the four dimensional result, together with the one reported by Solodukhin, gives support to the Ryu-Takayanagi holographic anzats. We also find there is no logarithmic contribution to the entropy for odd space time dimensions.

 

[John86]

http://arxiv.org/abs/1006.1769
Eddington's theory of gravity and its progeny
Authors: Maximo Banados (PUC, Chile and Oxford), Pedro G. Ferreira (Oxford)
(Submitted on 9 Jun 2010)
Abstract: We resurrect Eddington's proposal for the gravitational action in the presence of a cosmological constant and extend it to include matter fields. We show that the Newton-Poisson equation is modified in the presence of sources and that charged black holes show great similarities with those arising in Born-Infeld electrodynamics coupled to gravity. When we consider homogeneous and isotropic space-times we find that there is a minimum length (and maximum density) at early times, clearly pointing to an alternative theory of the Big Bang. We thus argue that the modern formulation of Eddington's theory, Born-Infeld gravity, presents us with a novel, non-singular description of the Universe.

 

[marcus]

http://arxiv.org/abs/1007.2594
Coupling the inflaton to an expanding aether
William Donnelly, Ted Jacobson
13 pages
(Submitted on 15 Jul 2010)
"We consider a Lorentz-violating theory of inflation consisting of Einstein-aether theory with a scalar inflaton coupled bilinearly to the expansion of the aether. We determine the conditions for linearized stability, positive energy and vanishing of preferred-frame post-Newtonian parameters, and find that all these conditions can be met. In homogeneous and isotropic cosmology, the inflaton-aether expansion coupling leads to a driving force on the inflaton that is proportional to the Hubble parameter. This force affects the slow-roll dynamics, but still allows for a natural end to inflation."

http://arxiv.org/abs/1007.2560
CDT---an Entropic Theory of Quantum Gravity
J. Ambjorn, A. Goerlich, J.Jurkiewicz, R. Loll
49 pages, many figures. Lectures presented at the "School on Non-Perturbative Methods in Quantum Field Theory" and the "Workshop on Continuum and Lattice Approaches to Quantum Gravity", Sussex, September 15th-19th 2008 . To appear as part of a volume in the Springer Lecture Notes in Physics series.
(Submitted on 15 Jul 2010)
"In these lectures we describe how a theory of quantum gravity may be constructed in terms of a lattice formulation based on so-called causal dynamical triangulations (CDT). We discuss how the continuum limit can be obtained and how to define and measure diffeomorphism-invariant correlators. In four dimensions, which has our main interest, the lattice theory has an infrared limit which can be identified with de Sitter spacetime. We explain why this infrared property of the quantum spacetime is nontrivial and due to 'entropic' effects encoded in the nonperturbative path integral measure. This makes the appearance of the de Sitter universe an example of true emergence of classicality from microscopic quantum laws. We also discuss nontrivial aspects of the UV behaviour, and show how to investigate quantum fluctuations around the emergent background geometry. Finally, we consider the connection to the asymptotic safety scenario, and derive from it a new, conjectured scaling relation in CDT quantum gravity."

http://arxiv.org/abs/1007.2410
General Covariance in Quantum Gravity at a Lifgarbagez Point
Petr Horava, Charles M. Melby-Thompson
40 pages
(Submitted on 14 Jul 2010)
"In the minimal formulation of gravity with Lifgarbagez-type anisotropic scaling, the gauge symmetries of the system are foliation-preserving diffeomorphisms of spacetime. Consequently, compared to general relativity, the spectrum contains an extra scalar graviton polarization. Here we investigate the possibility of extending the gauge group by a local U(1) symmetry to 'nonrelativistic general covariance.' This extended gauge symmetry eliminates the scalar graviton, and forces the coupling constant lambda in the kinetic term of the minimal formulation to take its relativistic value, lambda=1. The resulting theory exhibits anisotropic scaling at short distances, and reproduces many features of general relativity at long distances."

 

[marcus]

http://arxiv.org/abs/1007.2768
Generic isolated horizons in loop quantum gravity
Christopher Beetle, Jonathan Engle
13 pages
(Submitted on 16 Jul 2010)
"Isolated horizons model equilibrium states of classical black holes. A detailed quantization, starting from a classical phase space restricted to spherically symmetric horizons, exists in the literature and has since been extended to axisymmetry. This paper extends the quantum theory to horizons of arbitrary shape. Surprisingly, the Hilbert space obtained by quantizing the full phase space of all generic horizons with a fixed area is identical to that originally found in spherical symmetry. The entropy of a large horizon remains one quarter its area, with the Barbero-Immirzi parameter retaining its value from symmetric analyses. These results suggest a reinterpretation of the intrinsic quantum geometry of the horizon surface."

 

[marcus]

http://arxiv.org/abs/1007.3150
Quantum Corrections in the Group Field Theory Formulation of the EPRL/FK Models
Thomas Krajewski, Jacques Magnen, Vincent Rivasseau, Adrian Tanasa, Patrizia Vitale
35 pages, 5 figures
(Submitted on 19 Jul 2010)
"We investigate the group field theory formulation of the EPRL/FK spin foam models. These models aim at a dynamical, i.e. non-topological formulation of 4D quantum gravity. We introduce a saddle point method for general group field theory amplitudes and compare it with existing results, in particular for a second order correction to the EPRL/FK propagator."

http://arxiv.org/abs/1007.3086
A New Solution of The Cosmological Constant Problems
John D. Barrow, Douglas J. Shaw
5 pages
(Submitted on 19 Jul 2010)
"We extend the usual gravitational action principle by promoting the bare cosmological constant (CC) from a parameter to a field which can take many possible values. Variation leads to a new integral constraint equation which determines the classical value of the effective CC that dominates the wave function of the universe. In a Friedmann background cosmology with observed matter and radiation content the expected value of the effective CC, is calculated from measurable quantities to be O(t_U^-2)~ 10^-122 (in natural units), as observed, where t_U is the present age of the universe. Any application of our model produces a falsifiable prediction for Lambda in terms of other measurable quantities. This leads to a specific prediction for the observed spatial curvature parameter of Omega_k0 = 5.2 x 10^-5, which is of the magnitude expected if inhomogeneities have an inflationary origin. This explanation of the CC requires no fine tunings, extra dark energy fields, or Bayesian selection in a multiverse."


Brief mention:
http://arxiv.org/abs/1007.2875
Unstable growth of curvature perturbation in non-singular bouncing cosmologies
BingKan Xue, Paul J. Steinhardt
4 pages, 2 figures
(Submitted on 16 Jul 2010)
"We consider non-singular bouncing cosmologies, such as the new ekpyrotic model,..."

http://arxiv.org/abs/1007.2971
Reasoning by analogy: attempts to solve the cosmological constant paradox
Rafael A. Porto, A. Zee
4 pages. To appear in Proceedings of the Conference in Honor of Murray Gell-Mann's 80th Birthday
(Submitted on 18 Jul 2010)

http://arxiv.org/abs/1007.2873
Is There Unification in the 21st Century?
Yuan K. Ha
Lecture delivered in Conference in Honor of Murray Gell-Mann's 80th Birthday, February 24 - 26, 2010.
(Submitted on 16 Jul 2010)

 

[MTd2]

http://arxiv.org/abs/1007.2976

Holograph in noncommutative geometry: Part 1

Jingbo Wang
(Submitted on 18 Jul 2010)
In this paper, we consider the holograph principle emergent from noncommutative geometry, based on the spectral action principle. We show that under some appropriate conditions, the gravity theory on a manifold with boundary could be equivalent to a gauge theory $SU(N)$ on the boundary. Then an expression for $N$ with the geometrical quantities of the manifold is given. Based on this result, we find that the volume of the manifold and the boundary have some discrete structure. Applying the result to the black hole, we get that the radium of the Schwarzschild black hole is quantized. We also find an explanation why the extremal RN-black hole has zero temperature but with finite entropy.

http://arxiv.org/abs/1007.2941

The modified wave function of test particle approaching holographic screen from entropy force-> This paper discusses the nature of Verlinde's entropy.

Bin Liu, Yun-Chuan Dai, Xian-Ru Hu, Jian-Bo Deng
(Submitted on 17 Jul 2010)
In this note we generalize entropy based on the quantum mechanical probability density distribution. Motivated by J. Munkhammar and the uncertainty of entropy we modified the origin wave function of the test particle. The corrected one acting on the quantum particle is subject to the uncertainty principle. Considering the uncertain relation, the corrected probability of the particle for measurement on holographic screen has been proposed. We also derivate the speed of information transfer described by wave function. Our quantum approach to entropy stress the information in a physical system is directly associated with its quantum

 

[marcus]

http://arxiv.org/abs/1007.3368
The Definition of Mach's Principle
Julian Barbour
30 pages. To be published in Foundations of Physics as invited contribution to Peter Mittelstaedt's 80th Birthday Festschrift.
(Submitted on 20 Jul 2010)
"Two definitions of Mach's principle are proposed. Both are related to gauge theory, are universal in scope and amount to formulations of causality that take into account the relational nature of position, time, and size. One of them leads directly to general relativity and may have relevance to the problem of creating a quantum theory of gravity."

http://arxiv.org/abs/1007.3723
Vertex Expansion for the Bianchi I model
Miguel Campiglia, Adam Henderson, William Nelson
15 pages, 2 figures
(Submitted on 21 Jul 2010)
"A perturbative expansion of Loop Quantum Cosmological transitions amplitudes of Bianchi I models is performed. Following the procedure outlined in [1,2] for isotropic models, it is shown that the resulting expansion can be written in the form of a series of amplitudes each with a fixed number of transitions mimicking a spin foam expansion. This analogy is more complete than in the isotropic case, since there are now the additional anisotropic degrees of freedom which play the role of 'colouring' of the spin foams. Furthermore, the isotropic expansion is recovered by integrating out the anisotropies."

http://arxiv.org/pdf/1007.3732v1
Chiral Effects in Quantum Gravity as Consequence of Instantonic Transitions
Simone Mercuri
8 pages
(Submitted on 21 Jul 2010)
"Instantonic solutions of the Holst modified action for General Relativity indicate that gravity becomes chiral through quantum effects. The resulting violation of parity reflects in a different Newton's constant for right and left modes: a measurement of the TB correlation on CMB can reveal the existence of such an effect."
==sample excerpt from Mercuri's paper==
" In this paper, by using a WKB approximation, we demonstrate that in fact the Holst action can produce a parity violation in the quantum regime. This provides a clear theoretical support to the assumption made in [1] that left and right gravitons are characterized by a different gravitational constant.
Furthermore, this fact is particularly interesting in consideration of the fact that the Holst action is the Lagrangian counterpart of the Ashtekar–Barbero reformulation of canonical General Relativity [5, 6], which is the basis of the non-perturbative quantization of GR, i.e. Loop Quantum Gravity [7–10].
Therefore a result indicating that in fact a parity violation can be generated by the quantization of the Holst action, even in the WKB approximation, would suggest that the same effect could characterize the full LQG theory.
==endquote==
Simone Mercuri is one of Ashtekar's group at Penn State IGC.

 

[atyy]

http://arxiv.org/abs/1007.3970
The Jarzynski Identity and the AdS/CFT Duality
Djordje Minic, Michel Pleimling
We point out a remarkable analogy between the Jarzynski identity from non-equilibrium statistical physics and the AdS/CFT duality. We apply the logic that leads to the Jarzynski identity to renormalization group (RG) flows of quantum field theories and then argue for the natural connection with the AdS/CFT duality formula. This application can be in principle checked in Monte Carlo simulations of RG flows. Given the existing generalizations of the Jarzynski identity in non-equilibrium statistical physics, and the analogy between the Jarzynski identity and the AdS/CFT duality, we are led to suggest natural but novel generalizations of the AdS/CFT dictionary.

 

[marcus]

This has a lot about LQG even though the main topic is a different QG approach:

http://arxiv.org/abs/1007.4094
Modular Theory, Non-commutative Geometry and Quantum Gravity
Paolo Bertozzini, Roberto Conti, Wicharn Lewkeeratiyutkul
48 pages, contribution to SIGMA special issue "Noncommutative Spaces and Fields"
(Submitted on 23 Jul 2010)
"This paper contains the first written exposition of some ideas (announced in a previous survey) on an approach to quantum gravity based on Tomita-Takesaki modular theory and A.Connes non-commutative geometry aiming at the reconstruction of spectral geometries from an operational formalism of states and categories of observables in a covariant theory. Care has been taken to provide a coverage of the relevant background on modular theory, its applications in non-commutative geometry and physics and to the detailed discussion of the main foundational issues raised by the proposal."

 

[atyy]

http://arxiv.org/abs/1007.4234
Topological Insulators and Superconductors from String Theory
Shinsei Ryu, Tadashi Takayanagi
Relevant in a non-string bibliography because "our string theory realization includes the honeycomb lattice Kitaev model in two spatial dimensions, and its higher-dimensional extensions".

 

[atyy]

http://arxiv.org/abs/1007.4829
New Phenomena in NC Field Theory and Emergent Spacetime Geometry
Badis Ydri
(Submitted on 27 Jul 2010)
Abstract: We give a brief review of two nonperturbative phenomena typical of noncommutative field theory which are known to lead to the perturbative instability known as the UV-IR mixing. The first phenomena concerns the emergence/evaporation of spacetime geometry in matrix models which describe perturbative noncommutative gauge theory on fuzzy backgrounds. In particular we show that the transition from a geometrical background to a matrix phase makes the description of noncommutative gauge theory in terms of fields via the Weyl map only valid below a critical value g_*. The second phenomena concerns the appearance of a nonuniform ordered phase in noncommutative scalar \phi^4 field theory and the spontaneous symmetry breaking of translational/rotational invariance which happens even in two dimensions. We argue that this phenomena also originates in the underlying matrix degrees of freedom of the noncommutative field theory. Furthermore it is conjectured that in addition to the usual WF fixed point at $\theta=0$ there must exist a novel fixed point at \theta=\infty corresponding to the quartic hermitian matrix model.

http://arxiv.org/abs/1007.5049
Gauge non-invariance as tests of emergent gauge symmetry
John F. Donoghue, Mohamed Anber, Ufuk Aydemir
(Submitted on 28 Jul 2010)
Abstract: We motivate the concept of emergent gauge symmetry and discuss ways that this concept can be tested. The key idea is that if a symmetry is emergent, one should look for small violations of this symmetry because the underlying fundamental theory does not contain the symmetry. We describe our recent work implementing this idea in the gravity sector. We also describe the reasons why violations of gauge symmetry may well be linked to violations of Lorentz invariance.

 

[MTd2]

http://arxiv.org/abs/1007.5066

Finite entanglement entropy from the zero-point-area of spacetime
Authors: T. Padmanabhan
(Submitted on 28 Jul 2010)

Abstract: The calculation of entanglement entropy S of quantum fields in spacetimes with horizon shows that, quite generically, S (a) is proportional to the area A of the horizon and (b) is divergent. I argue that this divergence, which arises even in the case of Rindler horizon in flat spacetime, is yet another indication of a deep connection between horizon thermodynamics and gravitational dynamics. In an emergent perspective of gravity, which accommodates this connection, the fluctuations around the equipartition value in the area elements will lead to a minimal quantum of area, of the order of L_P^2, which will act as a regulator for this divergence. In a particular prescription for incorporating L_P^2 as zero-point-area of spacetime, this does happen and the divergence in entanglement entropy is regularized, leading to S proportional to (A/L_P^2) in Einstein gravity. In more general models of gravity, the surface density of microscopic degrees of freedom is different which leads to a modified regularisation procedure and the possibility that the entanglement entropy - when appropriately regularised - matches the Wald entropy.

 

[marcus]

http://arxiv.org/abs/1007.5066
Finite entanglement entropy from the zero-point-area of spacetime
Thanu Padmanabhan
9 pages
(Submitted on 28 Jul 2010)
"The calculation of entanglement entropy S of quantum fields in spacetimes with horizon shows that, quite generically, S (a) is proportional to the area A of the horizon and (b) is divergent. I argue that this divergence, which arises even in the case of Rindler horizon in flat spacetime, is yet another indication of a deep connection between horizon thermodynamics and gravitational dynamics. In an emergent perspective of gravity, which accommodates this connection, the fluctuations around the equipartition value in the area elements will lead to a minimal quantum of area, of the order of L_P², which will act as a regulator for this divergence. In a particular prescription for incorporating L_P² as zero-point-area of spacetime, this does happen and the divergence in entanglement entropy is regularized, leading to S proportional to (A/L_P²) in Einstein gravity. In more general models of gravity, the surface density of microscopic degrees of freedom is different which leads to a modified regularisation procedure and the possibility that the entanglement entropy - when appropriately regularised - matches the Wald entropy."

 

[marcus]

http://arxiv.org/abs/1007.5500
The propagator in polymer quantum field theory
Golam Mortuza Hossain, Viqar Husain, Sanjeev S. Seahra
4 pages, 1 figure
(Submitted on 30 Jul 2010)
"We study free scalar field theory on flat spacetime using a background independent (polymer) quantization procedure. Specifically we compute the propagator using a method that takes the energy spectrum and position matrix elements of the harmonic oscillator as inputs. We obtain closed form results in the infrared and ultraviolet regimes that give Lorentz invariance violating dispersion relations, and show suppression of propagation at sufficiently high energy."
==sample excerpt==
In this paper we explore a background independent (“polymer”) quantization method that arose in loop quantum gravity (LQG) [1] and apply it to scalar field theory. In this approach the Hilbert space used for quantization is different from the one employed in usual quantum theory. This Hilbert space is such that its inner product does not make use of a spacetime metric, even if one is available, as in QFT on a fixed background. Rather the inner product comes from an underlying group structure...
==endquote==

 

[MTd2]

http://arxiv.org/abs/1008.0354

EPRL/FK Group Field Theory

Joseph Ben Geloun, Razvan Gurau, Vincent Rivasseau
(Submitted on 2 Aug 2010)
The purpose of this short note is to clarify the Group Field Theory vertex and propagators corresponding to the EPRL/FK spin foam models and to detail the subtraction of leading divergences of the model.

 

[John86]

http://arxiv.org/abs/1008.0283
Unknown system boundaries cannot be determined within quantum Darwinism
Authors: Chris Fields
(Submitted on 2 Aug 2010)
Abstract: Observers restricted to interactions with environmental degrees of freedom that nondestructively encode pointer states of a system of interest S cannot determine from such interactions which degrees of freedom of S interact directly or indirectly with the environment E. Without a specification of the S-E boundary, such observers cannot use einselection and quantum Darwinism to calculate the pointer states of S or their environmental encodings. Quantum Darwinism requires S-E boundary specifications assumed or stipulated on the basis of classical-scale observations, and therefore cannot be regarded as providing a predictive, purely quantum-mechanical explanation of the "emergence" of classicality.

http://arxiv.org/abs/1008.0373
Little Boxes: The Simplest Demonstration of the Failure of Einstein's Attempt to Show the Incompleteness of Quantum Theory
Authors: John D. Norton
(Submitted on 2 Aug 2010)
Abstract: The failure of Einstein's co-authored "EPR" attempt to show the incompleteness of quantum theory is demonstrated directly for spatial degrees of freedom using only elementary notions. A GHZ construction is realized in the position properties of three particles whose quantum waves are distributed over three two-chambered boxes. The same system is modeled more realistically using three spatially separated, singly ionized hydrogen molecules.

http://arxiv.org/abs/1008.0033
Emergent Photons and Gravitons:The Problem of Vacuum Structure
Authors: J. D. Bjorken
(Submitted on 30 Jul 2010)
Abstract: We discuss vacuum condensates associated with emergent QED and with torsion, as well as the possible role of the Kodama wave function in quantum cosmology

http://arxiv.org/abs/1008.0154
Gauge Theories under Incorporation of a Generalized Uncertainty Principle
Authors: Martin Kober
(Submitted on 1 Aug 2010)
Abstract: It is considered an extension of gauge theories according to the assumption of a generalized uncertainty principle which implies a minimal length scale. A modification of the usual uncertainty principle implies an extended shape of matter field equations like the Dirac equation. If it is postulated invariance of such a generalized field equation under local gauge transformations, the usual covariant derivative containing the gauge potential has to be replaced by a generalized covariant derivative. This leads to a generalized interaction between the matter field and the gauge field as well as to an additional self interaction of the gauge field. Since the existence of a minimal length scale seems to be a necessary assumption of any consistent quantum theory of gravity, the gauge principle is a constitutive ingredient of the standard model and even gravity can be described as gauge theory of local translations or Lorentz transformations, the presented extension of gauge theories appears as a very important consideration.

 

[John86]

http://arxiv.org/abs/1006.3480
Six-dimensional Methods for Four-dimensional Conformal Field Theories
Authors: Steven Weinberg
(Submitted on 17 Jun 2010 (v1), last revised 2 Aug 2010 (this version, v2))
Abstract: The calculation of both spinor and tensor Green's functions in four-dimensional conformally invariant field theories can be greatly simplified by six-dimensional methods. For this purpose, four-dimensional fields are constructed as projections of fields on the hypercone in six-dimensional projective space, satisfying certain transversality conditions. In this way some Green's functions in conformal field theories are shown to have structures more general than those commonly found by use of the inversion operator. These methods fit in well with the assumption of AdS/CFT duality. In particular, it is transparent that if fields on AdS$_5$ approach finite limits on the boundary of AdS$_5$, then in the conformal field theory on this boundary these limits transform with conformal dimensionality zero if they are tensors (of any rank), but with conformal dimension 1/2 if they are spinors or spinor-tensors.

 

[marcus]

Someone at Beyond forum was asking about BFL Ward's work on resummed QG. Here's a new paper.
http://arxiv.org/abs/1008.1046
An Estimate of Λ in Resummed Quantum Gravity in the Context of Asymptotic Safety
B.F.L. Ward (1) ((1) Department of Physics, Baylor University, Waco, TX, USA)
(Submitted on 5 Aug 2010)
"We show that, by using recently developed exact resummation techniques based on the extension of the methods of Yennie, Frautschi and Suura to Feynman's formulation of Einstein's theory, we get quantum field theoretic descriptions for the UV fixed-point behaviors of the dimensionless gravitational and cosmological constants postulated by Weinberg. Connecting our work to the attendant phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter, we predict the value of the cosmological constant Λ. We find the encouraging estimate \rho_\Lambda\equiv \frac{\Lambda}{8\pi G_N} \simeq (2.379\times 10^{-3}eV)^4 "

I remember now! It was Murray92 who was asking about Ward's work. https://www.physicsforums.com/showthread.php?p=2813182#post2813182
Here is another Ward paper that just appeared on arxiv.

http://arxiv.org/abs/1008.1052
"Low'' Energy GUTs
B.F.L. Ward (1) ((1) Department of Physics, Baylor University, Waco, TX, USA)
9 pages
(Submitted on 5 Aug 2010)
"We introduce a new approach to the subject of grand unification which allows the GUT scale to be small, \lesssim 200 TeV, so that it is within the reach of {\em conceivable} laboratory accelerated colliding beam devices. Central to the approach is a novel abstraction of the heterotic string symmetry group physics ideas to render baryon number violating effects small enough to have escaped detection to date."

This Chamseddine Connes paper is a good explanation of their NCG+standard model for a wide audience. It communicates the basics without showing a lot of calculation.
http://arxiv.org/abs/1008.0985
Space-Time from the spectral point of view
Ali H. Chamseddine, Alain Connes
19 pages. To appear in the Proceedings of the 12th Marcel Grossmann meeting
(Submitted on 5 Aug 2010)
"We develop the spectral point of view on geometry based on the formalism of quantum physics. We start from the simple physical question of specifying our position in space and explain how the spectral geometric point of view provides a new paradigm to model space-time whose fine structure can be encoded by a finite geometry. The classification of the irreducible finite geometries of KO-dimension 6 singles out a "symplectic--unitary" candidate F, which when used as the fine texture of space-time delivers from pure gravity on M x F the Standard Model coupled to gravity and, once extrapolated to unification scale, gives testable predictions."

 

[MTd2]

http://arxiv.org/abs/1008.1045

Quantum Gravity via Manifold Positivity

Michael Freedman
(Submitted on 5 Aug 2010)
The macroscopic dimensions of space should not be input but rather output of a general model for physics. Here, dimensionality arises from a recently discovered mathematical bifurcation: positive versus indefinite manifold pairings. It is used to build an action on a formal chain of combinatorial space-times of arbitrary dimension. The context for such actions is 2-field theory where Feynman integrals are not over classical, but previously quantized configurations. A topologically enforced singularity of the action terminates the dimension at four and, in fact, the final fourth dimension is Lorentzian due to light-like vectors in the four dimensional manifold pairing. Our starting point is the action of causal dynamical triangulations but in a dimension-agnostic setting. It is encouraging that some hint of extra small dimensions emerges from our action.

 

[atyy]

http://focus.aps.org/story/v26/st6
http://arxiv.org/abs/1007.1130
Metric Signature Transitions in Optical Metamaterials
Igor I. Smolyaninov, Evgenii E. Narimanov
(Submitted on 7 Jul 2010)
We demonstrate that the extraordinary waves in indefinite metamaterials experience (- - + +) effective metric signature. During a metric signature change transition in such a metamaterial, a Minkowski space-time is "created" together with large number of particles populating this space-time. Such metamaterial models provide a table top realization of metric signature change events suggested to occur in Bose-Einstein condensates and quantum gravity theories.