Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/1012.0535
Physics as Information Processing
Authors: Giacomo Mauro D'Ariano
(Submitted on 2 Dec 2010)
Abstract: I review some recent advances in foundational research at Pavia QUIT group. The general idea is that there is only Quantum Theory without quantization rules, and the whole Physics---including space-time and relativity--is emergent from the quantum-information processing. And since Quantum Theory itself is axiomatized solely on informational principles, the whole Physics must be reformulated in information-theoretical terms: this is the "It from Bit of J. A. Wheeler. The review is divided into four parts: a) the informational axiomatization of Quantum Theory; b) how space-time and relativistic covariance emerge from quantum computation; c) what is the information-theoretical meaning of inertial mass and of $\hbar$, and how the quantum field emerges; d) an observational consequence of the new quantum field theory: a mass-dependent refraction index of vacuum. I will conclude with the research lines that will follow in the immediate future.

http://arxiv.org/abs/1011.6451
nformational derivation of Quantum Theory
Authors: G. Chiribella, G. M. D'Ariano, P. Perinotti
(Submitted on 30 Nov 2010)
Abstract: Quantum theory can be derived from purely informational principles. Five elementary axioms-causality, perfect distinguishability, ideal compression, local distinguishability, and pure conditioning-define a broad class of theories of information-processing that can be regarded as a standard. One postulate-purification-singles out quantum theory within this class. The main structures of quantum theory, such as the representation of mixed states as convex combinations of perfectly distinguishable pure states, are derived directly from the principles without using the Hilbert space framework

 

[marcus]

Matteo Smerlak just gave an interesting seminar talk (30 November) based on this paper, which we add to bibliography now since it was not listed here at the time it was posted on arxiv.
http://arxiv.org/abs/1008.1476
Bubble divergences from twisted cohomology
Valentin Bonzom, Matteo Smerlak
23 pages
(Submitted on 9 Aug 2010)
"We consider a class of lattice topological field theories, among which are the weak-coupling limit of 2d Yang-Mills theory and 3d Riemannian quantum gravity, whose dynamical variables are flat discrete connections with compact structure group on a cell 2-complex. In these models, it is known that the path integral measure is ill-defined because of a phenomenon known as 'bubble divergences'. In this paper, we extend recent results of the authors to the cases where these divergences cannot be understood in terms of cellular cohomology. We introduce in its place the relevant twisted cohomology, and use it to compute the divergence degree of the partition function. We also relate its dominant part to the Reidemeister torsion of the complex, thereby generalizing previous results of Barrett and Naish-Guzman. The main limitation to our approach is the presence of singularities in the representation variety of the fundamental group of the complex; we illustrate this issue in the well-known case of two-dimensional manifolds."

The slides of Smerlak's talk Bubble divergences in state-sum models:
http://relativity.phys.lsu.edu/ilqgs/smerlak113010.pdf
Two versions of the audio are here, "wav" sounds slightly better to me:
http://relativity.phys.lsu.edu/ilqgs/
In the talk, considerable attention is paid to the 4D case, I think relatively more so than in the August paper.

 

[marcus]

http://arxiv.org/abs/1012.0713
Quantum Gravity coupled to Matter via Noncommutative Geometry
Johannes Aastrup, Jesper M. Grimstrup, Mario Paschke
15 pages, 1 figure
(Submitted on 3 Dec 2010)
"We show that the principal part of the Dirac Hamiltonian in 3+1 dimensions emerges in a semi-classical approximation from a construction which encodes the kinematics of quantum gravity. The construction is a spectral triple over a configuration space of connections. It involves an algebra of holonomy loops represented as bounded operators on a separable Hilbert space and a Dirac type operator. Semi-classical states, which involve an averaging over points at which the product between loops is defined, are constructed and it is shown that the Dirac Hamiltonian emerges as the expectation value of the Dirac type operator on these states in a semi-classical approximation."

brief mention--NCC not quantum cosmology but connects gravity with SM matter:
http://arxiv.org/abs/1012.0780
The coupling of topology and inflation in Noncommutative Cosmology
Matilde Marcolli, Elena Pierpaoli, Kevin Teh
30 pages, 11 figures
(Submitted on 3 Dec 2010)
"We show that, in a model of modified gravity based on the spectral action functional, there is a nontrivial coupling between cosmic topology and inflation, in the sense that the shape of the possible slow-roll inflation potentials obtained in the model from the nonperturbative form of the spectral action are sensitive not only to the geometry (flat or positively curved) of the universe, but also to the different possible non-simply connected topologies..."

 

[John86]

http://arxiv.org/abs/1012.0756
The Quantum Field as a Quantum Computer
Authors: Giacomo Mauro D'Ariano
(Submitted on 2 Dec 2010)
Abstract: It is supposed that at very small scales a quantum field is an infinite homogeneous quantum computer. On a quantum computer the information cannot propagate faster than $c=a/\tau$, $a$ and $\tau$ being the minimum space and time distances between gates, respectively. It is shown that the information flow satisfies a Dirac equation, with speed $v=\zeta c$ and $\zeta=\zeta(m)$ mass-dependent. For $a/\tau=c$ the speed of light $\zeta^{-1}$ is a vacuum refraction index increasing monotonically from $\zeta^{-1}(0)=1$ to $\zeta^{-1}(M)=\infty$, $M$ being the Planck mass for $2a$ the Planck length.


Now quite something different

http://arxiv.org/abs/1011.3440
Are There Quantum Effects Coming from Outside Space-time? Nonlocality, free will and "no many-worlds"
Authors: Nicolas Gisin
(Submitted on 15 Nov 2010)
Abstract: Observing the violation of Bell's inequality tells us something about all possible future theories: they must all predict nonlocal correlations. Hence Nature is nonlocal. After an elementary introduction to nonlocality and a brief review of some recent experiments, I argue that Nature's nonlocality together with the existence of free will is incompatible with the many-worlds view of quantum physics.

 

[marcus]

http://arxiv.org/abs/1012.1307
Exotic singularities and spatially curved Loop Quantum Cosmology
Parampreet Singh, Francesca Vidotto
12 pages, 9 figures
(Submitted on 6 Dec 2010)
"We investigate the occurrence of various exotic spacelike singularities in the past and the future evolution of k = ± 1 Friedmann-Robertson-Walker model and loop quantum cosmology using a sufficiently general phenomenological model for the equation of state. We highlight the non-trivial role played by the intrinsic curvature for these singularities and the new physics which emerges at the Planck scale. We show that quantum gravity effects generically resolve all strong curvature singularities including big rip and big freeze singularities. The weak singularities, which include sudden and big brake singularities are ignored by quantum gravity when spatial curvature is negative, as was previously found for the spatially flat model. Interestingly, for the spatially closed model there exist cases where weak singularities may be resolved when they occur in the past evolution. The spatially closed model exhibits another novel feature. For a particular class of equation of state, this model also exhibits an additional physical branch in loop quantum cosmology, a baby universe separated from the parent branch. Our analysis generalizes previous results obtained on the resolution of strong curvature singularities in flat models to isotropic spacetimes with non-zero spatial curvature."

 

[atyy]

http://arxiv.org/abs/1012.1244
Gravity on a multifractal
Gianluca Calcagni
(Submitted on 6 Dec 2010)
Despite their diversity, many of the most prominent candidate theories of quantum gravity share the property to be effectively lower-dimensional at small scales. In particular, dimension two plays a fundamental role in the finiteness of these models of Nature. Thus motivated, we entertain the idea that spacetime is a multifractal with integer dimension 4 at large scales, while it is two-dimensional in the ultraviolet. Consequences for particle physics, gravity and cosmology are discussed.

 

[John86]

http://arxiv.org/abs/1011.0061
The Conformal Constraint in Canonical Quantum Gravity
Authors: Gerard 't Hooft
(Submitted on 30 Oct 2010)
Abstract: Perturbative canonical quantum gravity is considered, when coupled to a renormalizable model for matter fields. It is proposed that the functional integral over the dilaton field should be disentangled from the other integrations over the metric fields. This should generate a conformally invariant theory as an intermediate result, where the conformal anomalies must be constrained to cancel out. When the residual metric is treated as a background, and if this background is taken to be flat, this leads to a novel constraint: in combination with the dilaton contributions, the matter lagrangian should have a vanishing beta function. The zeros of this beta function are isolated points in the landscape of quantum field theories, and so we arrive at a denumerable, or perhaps even finite, set of quantum theories for matter, where not only the coupling constants, but also the masses and the cosmological constant are all fixed, and computable, in terms of the Planck units.

http://arxiv.org/abs/1009.0669
Probing the small distance structure of canonical quantum gravity using the conformal group
Authors: Gerard 't Hooft
(Submitted on 3 Sep 2010 (v1), last revised 2 Oct 2010 (this version, v2))
Abstract: In canonical quantum gravity, the formal functional integral includes an integration over the local conformal factor, and we propose to perform the functional integral over this factor before doing any of the other functional integrals. By construction, the resulting effective theory would be expected to be conformally invariant and therefore finite. However, also the conformal integral itself diverges, and the effects of a renormalization counter term are considered. It generates problems such as unitarity violation, due to a Landau-like ghost, and conformal anomalies. Adding (massive or massless) matter fields does not change the picture. Various alternative ideas are offered, including a more daring speculation, which is that no counter term should be allowed for at all. This has far-reaching and important consequences, which we discuss. A surprising picture emerges of quantized elementary particles interacting with a gravitational field, in particular gravitons, which are "partly classical". This approach was inspired by a search towards the reconciliation of Hawking radiation with unitarity and locality, and it offers basic new insights there.

 

[marcus]

John the two items in your post were listed here earlier.

http://arxiv.org/abs/1009.0669
Probing the small distance structure of canonical quantum gravity using the conformal group
Gerard 't Hooft
...

http://arxiv.org/abs/1011.0061
The Conformal Constraint in Canonical Quantum Gravity
Authors: Gerard 't Hooft
...

If you would like to delete your post, I will delete this---it will reduce clutter.

 

[John86]

That's ok Marcus

John

 

[marcus]

http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
6 pages, 1 figure
(Submitted on 8 Dec 2010)
"The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covariance is manifest. K can be described in terms of a certain subset of the 'projected' spin networks studied by Livine, Alexandrov and Dupuis. It is formed by SL(2,C) functions completely determined by their restriction on SU(2). These are square-integrable in the SU(2) scalar product, but not in the SL(2,C) one. Thus, SU(2)-spin-network states can be represented by Lorentz-covariant SL(2,C) functions, as two-component photons can be described in the Lorentz-covariant Gupta-Bleuler formalism. As shown by Wolfgang Wieland in a related paper, this manifestly Lorentz-covariant formulation can also be directly obtained from canonical quantization. We show that the spinfoam dynamics of loop quantum gravity is locally SL(2,C)-invariant in the bulk, and yields states that are preciseley in K on the boundary. This clarifies how the SL(2,C) spinfoam formalism yields an SU(2) theory on the boundary. These structures define a tidy Lorentz-covariant formalism for loop gravity."

http://arxiv.org/abs/1012.1738
Complex Ashtekar variables and reality conditions for Holst's action
Wolfgang Wieland
19 pages, 2 pictures
(Submitted on 8 Dec 2010)
"From the Holst action in terms of complex valued Ashtekar variables additional reality conditions mimicking the linear simplicity constraints of spin foam gravity are found. In quantum theory with the results of You and Rovelli we are able to implement these constraints weakly, that is in the sense of Gupta and Bleuler. The resulting kinematical Hilbert space matches the original one of loop quantum gravity, that is for real valued Ashtekar connection. Our result perfectly fit with recent developments of Rovelli and Speziale concerning Lorentz covariance within spin-form gravity."

 

[marcus]

http://arxiv.org/abs/1012.1982
The kinematical Hilbert space of Loop Quantum Gravity from BF theories
Francesco Cianfrani
5 pages
(Submitted on 9 Dec 2010)
"In this work, it is demonstrated how the kinematical Hilbert space of BF theories can be reduced to the one of Loop Quantum Gravity via the imposition of the Hamiltonian constraints. In particular, it is outlined how the projection to the representations associated with Ashtekar-Barbero connections provides the correct procedure to implement second-class constraints. Then, the reduction to SU(2) intertwiners at vertices takes place by virtue of the vanishing behavior of the momenta associated to the boost parameters."

 

[marcus]

http://arxiv.org/abs/1012.2324
Matter in inhomogeneous loop quantum cosmology: the Gowdy T³ model
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
12 pages, submitted to PRD
(Submitted on 10 Dec 2010)
"We apply a hybrid approach which combines loop and Fock quantizations to fully quantize the linearly polarized Gowdy T³ model in the presence of a massless scalar field with the same symmetries as the metric. Like in the absence of matter content, the application of loop techniques leads to a quantum resolution of the classical cosmological singularity. Most importantly, thanks to the inclusion of matter, the homogeneous sector of the model contains flat Friedmann-Robertson-Walker (FRW) solutions, which are not allowed in vacuo. Therefore, this model provides a simple setting to study at the quantum level interesting physical phenomena such as the effect of the anisotropies and inhomogeneities on flat FRW cosmologies."

 

[marcus]

http://arxiv.org/abs/1012.2776
Vector fields and Loop Quantum Cosmology
Michal Artymowski, Zygmunt Lalak
15 pages, 6 figures
(Submitted on 13 Dec 2010)
"In the context of the Loop Quantum Cosmology we have analysed the holonomy correction to the classical evolution of the simplified Bianchi I model in the presence of vector fields. For the Universe dominated by a massive vector field or by a combination of a scalar field and a vector field a smooth transition between Kasner-like and Kasner-unlike solutions for a Bianchi I model has been demonstrated. In this case a lack of initial singularity and a finite maximal energy density appear already at the level of General Relativity, which simulates a classical Big Bounce."

http://arxiv.org/abs/1012.2419
Multiple quantum collapse of the inflaton field and its implications on the birth of cosmic structure
Gabriel León, Adolfo De Unánue, Daniel Sudarsky
17 pages
(Submitted on 11 Dec 2010)
"The standard inflationary account for the origin of cosmic structure is, without a doubt, extremely successful. However, it is not fully satisfactory as has been argued in [A. Perez, H. Sahlmann, and D. Sudarsky, Class. Quantum Grav., 23, 2317, (2006) arXiv:gr-qc/0508100]. The central point is that, in the standard accounts, the inhomogeneity and anisotropy of our universe seems to emerge, unexplained, from an exactly homogeneous and isotropic initial state through processes that do not break those symmetries. The proposal made there to address this shortcoming calls for a dynamical and self- induced quantum collapse of the original homogeneous and isotropic state of the inflaton. In this article, we consider the possibility of a multiplicity of collapses in each one of the modes of the Quantum Field. As we will see, the results are sensitive to a more detailed characterization of the collapse than those studied in the previous works, and in this regard two simple options will be studied. We find important constraints on the model, most remarkably on the number of possible collapses for each mode."

http://arxiv.org/abs/1012.2680
Planck Scale Cosmology and Asymptotic Safety in Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX)
5 pages; presented by B.F.L. Ward at ICHEP 2010
(Submitted on 13 Dec 2010)
"In Weinberg's asymptotic safety approach, a finite dimensional critical surface for a UV stable fixed point generates a theory of quantum gravity with a finite number of physical parameters. We argue that, in an extension of Feynman's original formulation of the theory, we recover this fixed-point UV behavior from an exact re-arrangement of the respective perturbative series. Our results are consistent with the exact field space Wilsonian renormalization group results of Reuter et al. and with recent Hopf-algebraic Dyson-Schwinger renormalization theory results of Kreimer. We obtain the first "first principles" predictions of the dimensionless gravitational and cosmological constants and our results support the Planck scale cosmology of Bonanno and Reuter. We conclude with an estimate for the currently observed value of the cosmological constant."

 

[marcus]

http://arXiv.org/abs/1012.1798
Generalization of the Bollobás-Riordan polynomial for tensor graphs
Adrian Tanasa
17 pages, 12 figures
(Submitted on 8 Dec 2010)
"Tensor models are used nowadays for implementing a fundamental theory of quantum gravity. We define here a polynomial T encoding the supplementary topological information. This polynomial is a natural generalization of the Bollobás-Riordan polynomial (used to characterize matrix graphs) and is different of the Gurau polynomial, (R. Gurau, "Topological Graph Polynomials in Colored Group Field Theory", Annales Henri Poincaré 11, 565-584 (2010)) defined for a particular class of tensor graphs, the colorable ones. The polynomial T is defined for both colorable and non-colorable graphs and it is proved to satisfy the contraction/deletion relation. A non-trivial example of a non-colorable graphs is analyzed."

This comes to qg by way of group field theory. A large proportion of the cited references are to Loop-and-allied papers. Despite this, I am not sure how it fits in with LQG--just guessing that it does.

 

[marcus]

http://arxiv.org/abs/1012.3081
The Universal RG Machine
Dario Benedetti, Kai Groh, Pedro F. Machado, Frank Saueressig
38 pages
(Submitted on 14 Dec 2010)
"Functional Renormalization Group Equations constitute a powerful tool to encode the perturbative and non-perturbative properties of a physical system. We present an algorithm to systematically compute the expansion of such flow equations in a given background quantity specified by the approximation scheme. The method is based on off-diagonal heat-kernel techniques and can be implemented on a computer algebra system, opening access to complex computations in, e.g., Gravity or Yang-Mills theory. In a first illustrative example, we re-derive the gravitational beta-functions of the Einstein-Hilbert truncation, demonstrating their background-independence. As an additional result, the heat-kernel coefficients for transverse vectors and transverse-traceless symmetric matrices are computed to second order in the curvature."

 

[marcus]

http://arxiv.org/abs/1012.3713
Quantum Gowdy model within the new loop quantum cosmology improved dynamics
M Martín-Benito, L J Garay, G A Mena Marugán
4 pages, jpconf.cls, to appear in Proceedings of Spanish Relativity Meeting 2010 (ERE 2010) held in Granada, Spain
(Submitted on 16 Dec 2010)
The linearly polarized Gowdy T³ model can be regarded as compact Bianchi I cosmologies with inhomogeneous modes allowed to travel in one direction. We study a hybrid quantization of this model that combines the loop quantization of the Bianchi I background, adopting the improved dynamics scheme put forward by Ashtekar and Wilson-Ewing, with a Fock quantization for the inhomogeneities. The Hamiltonian constraint operator provides a resolution of the cosmological singularity and superselects separable sectors. We analyze the complicated structure of these sectors. In any of them the Hamiltonian constraint provides an evolution equation with respect to the volume of the associated Bianchi I universe, with a well posed initial value problem. This fact allows us to construct the Hilbert space of physical states and to show that we recover the standard quantum field theory for the inhomogeneities."

Westra is a former PhD student of Renate Loll at Utrecht, with research in CDT
http://arxiv.org/abs/1012.3472
Localization of particles in quantum field theory
W. Westra
23 pages
(Submitted on 15 Dec 2010)
"We put forward an interpretation of scalar quantum field theory as relativistic quantum mechanics by curing well known problems related to locality. A probabilistic interpretation of quantum field theory similar to quantum mechanics is difficult if particle localization is defined using the Newton-Wigner position operator as it is non-local and non-covariant. An alternative bilinear covariant position operator is discussed which incorporates a time operator that can be exponentiated to a unitary operator. Moreover, it satisfies an algebra that unifies special relativity and quantum mechanics and has the same form for particles with spin. Higher power position operators are derived which yield Heisenberg's uncertainty relations. Our ideas are illustrated with a relativistic wave function whose probability density can be perfectly localized."

http://arxiv.org/abs/1012.3473
A Causal Alternative to Feynman's Propagator
Jurjen F. Koksma, W. Westra
31 pages, 3 figures
(Submitted on 15 Dec 2010)
"The Feynman propagator used in the conventional in-out formalism in quantum field theory is not a causal propagator as wave packets are propagated virtually instantaneously outside the causal region of the initial state. We formulate a causal in-out formalism in quantum field theory by making use of the Wheeler propagator, the time ordered commutator propagator, which is manifestly causal. Only free scalar field theories and their first quantization are considered. We identify the real Klein Gordon field itself as the wave function of a neutral spinless relativistic particle. Furthermore, we derive a probability density for our relativistic wave packet using the inner product between states that live on a suitably defined Hilbert space of real quantum fields. We show that the time evolution of our probability density is governed by the Wheeler propagator, such that it behaves causally too."

http://arxiv.org/abs/1012.3629
From Quantum Deformations of Relativistic Symmetries to Modified Kinematics and Dynamics
Jerzy Lukierski
29 pages, 1 fig. Invited talk at 50-th Cracow School of Theoretical Physics "Particle Physics at the Dawn of the LHC'', Zakopane, Poland (June 9-19, 2010). To be published in Acta Physica Polonica-B.
(Submitted on 16 Dec 2010)
"We present a short review describing the use of noncommutative space-time in quantum-deformed dynamical theories: classical and quantum mechanics as well as classical and quantum field theory. We expose the role of Hopf algebras and their realizations (noncommutative modules) as important mathematical tool describing quantum-deformed symmetries: quantum Lie groups and quantum Lie algebras. We consider in some detail the most studied examples of noncommutative space-time geometry: the canonical and kappa-deformed cases. Finally we briefly describe the modifications of Einstein gravity obtained by introduction of noncommutative space-time coordinates."


Brief mention:
http://arxiv.org/abs/1012.3744
Testing the Gaussianity and Statistical Isotropy of the Universe
Dragan Huterer, Sarah Shandera, Eiichiro Komatsu
(Submitted on 16 Dec 2010)
"The last few years have seen a surge in excitement about measurements of statistics of the primordial fluctuations beyond the power spectrum. New ideas for precision tests of Gaussianity and statistical isotropy in the data are developing simultaneously with proposals for a wide range of new theoretical possibilities. From both the observations and theory, it has become clear that there is a huge discovery potential from upcoming measurements. In this Special Issue of Advances in Astronomy we have collected articles that summarize the theoretical predictions for departures from Gaussianity or statistical isotropy from a variety of potential sources, together with the observational approaches to test these properties using the CMB or large-scale structure..."

 

[John86]

http://arxiv.org/abs/1012.3701
Decoherence in Quantum Mechanics
Authors: Jurjen F. Koksma, Tomislav Prokopec, Michael G. Schmidt
(Submitted on 16 Dec 2010)
Abstract: We study decoherence in a simple quantum mechanical model using two approaches. Firstly, we follow the conventional approach to decoherence where one is interested in solving the reduced density matrix from the perturbative master equation. Secondly, we consider our novel correlator approach to decoherence where entropy is generated by neglecting observationally inaccessible correlators. We show that both methods can accurately predict decoherence time scales. However, the perturbative master equation generically suffers from instabilities which prevents us to reliably calculate the system's total entropy increase. We also discuss the relevance of the results in our quantum mechanical model for interacting field theories.

 

[Kevin_Axion]

Sorry about that, it must have slipped my eye.

 

[marcus]

Kevin, thanks for deleting the duplicates!

http://arxiv.org/abs/1012.3832
U(N) invariant dynamics for a simplified Loop Quantum Gravity model
Enrique F. Borja, Jacobo Díaz-Polo, Iñaki Garay, Etera R. Livine
4 pages, to appear in Proceedings of Spanish Relativity Meeting 2010 (ERE 2010) held in Granada
(Submitted on 17 Dec 2010)
"The implementation of the dynamics in Loop Quantum Gravity (LQG) is still an open problem. Here, we discuss a tentative dynamics for the simplest class of graphs in LQG: Two vertices linked with an arbitrary number of edges. We use the recently introduced U(N) framework in order to construct SU(2) invariant operators and define a global U(N) symmetry that will select the homogeneous/isotropic states. Finally, we propose a Hamiltonian operator invariant under area-preserving deformations of the boundary surface and we identify possible connections of this model with Loop Quantum Cosmology."

 

[marcus]

http://arxiv.org/abs/1012.4216
4-dimensional Spin-foam Model with Quantum Lorentz Group
Muxin Han
22 pages, 3 figures
(Submitted on 19 Dec 2010)
"We study the quantum group deformation of the Lorentzian EPRL spin-foam model. The construction uses the harmonic analysis on the quantum Lorentz group. We show that the quantum group spin-foam model so defined is free of the infrared divergence, thus gives a finite partition function on a fixed triangulation. We expect this quantum group spin-foam model is a spin-foam quantization of discrete gravity with a cosmological constant."

http://arxiv.org/abs/1012.4280
Renormalization Group Flow of the Holst Action
J.-E. Daum, M.Reuter
11 pages, 3 figures
(Submitted on 20 Dec 2010)
"The renormalization group (RG) properties of quantum gravity are explored, using the vielbein and the spin connection as the fundamental field variables. The scale dependent effective action is required to be invariant both under space time diffeomorphisms and local frame rotations. The nonperturbative RG equation is solved explicitly on the truncated theory space defined by a three parameter family of Holst-type actions which involve a running Immirzi parameter. We find evidence for the existence of an asymptotically safe fundamental theory, probably inequivalent to metric quantum gravity constructed in the same way."

 

[marcus]

http://arxiv.org/abs/1012.4707
Loop quantum gravity: the first twenty five years
Carlo Rovelli
24 pages, 3 figures
(Submitted on 21 Dec 2010)
"This is a review paper invited by the journal 'Classical ad Quantum Gravity' for a 'Cluster Issue' on approaches to quantum gravity. I give a synthetic presentation of loop gravity. I spell-out the aims of the theory and compare the results obtained with the initial hopes that motivated the early interest in this research direction. I give my own perspective on the status of the program and attempt of a critical evaluation of its successes and limits."

http://arxiv.org/abs/1012.4719
Spinfoam fermions
Eugenio Bianchi, Muxin Han, Elena Magliaro, Claudio Perini, Carlo Rovelli, Wolfgang Wieland
8 pages
(Submitted on 21 Dec 2010)
"We describe a minimal coupling of fermions and Yang Mills fields to the loop quantum gravity dynamics. The coupling takes a very simple form."

http://arxiv.org/abs/1012.4476
Lessons from Classical Gravity about the Quantum Structure of Spacetime
T. Padmanabhan
Expanded version of the lectures given on several ocassions including (i) Plenary talk at ERE 2010, Granada, September 2010; (ii) Keynote address at DICE 2010, Castiglioncello, September 2010; (iii) Special Lecture at the Indian Academy of Sciences, Bangalore, July 2010
(Submitted on 20 Dec 2010)
"I present the theoretical evidence which suggests that gravity is an emergent phenomenon like gas dynamics or elasticity with the gravitational field equations having the same status as, say, the equations of fluid dynamics/elasticity. This paradigm views a wide class of gravitational theories - including Einstein's theory - as describing the thermodynamic limit of the statistical mechanics of 'atoms of spacetime'. The evidence for this paradigm is hidden in several classical features of the gravitational theories and depends on just one quantum mechanical input, viz. the existence of Davies-Unruh temperature of horizons. I discuss several conceptual ingredients of this approach."

http://arxiv.org/abs/1012.4784
Quantum deformation of two four-dimensional spin foam models
Winston J. Fairbairn, Catherine Meusburger
38 pages, 3 figures
(Submitted on 21 Dec 2010)
"We construct the q-deformed version of two four-dimensional spin foam models, the Euclidean and Lorentzian EPRL model. The q-deformed models are based on the representation theory of two copies of U_q(su(2)) at a root of unity and on the quantum Lorentz group with a real deformation parameter. For both models we give a definition of the quantum EPRL intertwiners, study their convergence and braiding properties and construct an amplitude for the four-simplexes. We find that both of the resulting models are convergent."

Brief mention (Classical, not QG):
http://arxiv.org/abs/1012.4467
The causal structure of spacetime is a parameterized Randers geometry
Jozef Skakala (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
8 pages
(Submitted on 20 Dec 2010)
"There is a by now well-established isomorphism between stationary 4-dimensional spacetimes and 3-dimensional purely spatial Randers geometries - these Randers geometries being a particular case of the more general class of 3-dimensional Finsler geometries. We point out that in stably causal spacetimes, by using the (time-dependent) ADM decomposition, this result can be extended to general non-stationary spacetimes - the causal structure (conformal structure) of the full spacetime is completely encoded in a parameterized (time-dependent) class of Randers spaces, which can then be used to define a Fermat principle, and also to reconstruct the null cones and causal structure."

 

[marcus]

Some loop researchers experiment here with applying LQG method (among other things) to string.
http://arxiv.org/abs/1012.5073
Equivalence of the self-dual and Nambu-Goto strings
Winston J. Fairbairn, Karim Noui, Francesco Sardelli
20 pages
(Submitted on 22 Dec 2010)
"We establish explicitly the relation between the algebraic and Nambu-Goto strings when the target space is a four dimensional flat space. We find that the two theories are exactly equivalent only when the algebraic string is restricted to the self-dual or anti self-dual sectors. In its Hamiltonian formulation, the algebraic string defines a constrained system with first and second class constraints. In the self-dual case, we exhibit the appropriate set of second class constraints such that the resulting physical phase space is formulated in the same way as it is in the standard Nambu-Goto string. We conclude with a discussion on alternative quantisation schemes."

Thiemann tried some Lqg-string investigation back around 2004. They cite him and also a bunch of regular Lqg papers (Ashtekar Baez Perez Oriti and others...). They treat a case with no extra dimensions. Could just be a curiosity.

 

[marcus]

http://arxiv.org/abs/1012.5421
Ideal Gas in a strong Gravitational field: Area dependence of Entropy
Sanved Kolekar, T. Padmanabhan
18 pages
(Submitted on 24 Dec 2010)
"We study the thermodynamic parameters like entropy, energy etc. of a box of gas made up of indistinguishable particles when the box is kept in various static background spacetimes having a horizon. We compute the thermodynamic variables using both statistical mechanics as well as by solving the hydrodynamical equations for the system. When the box is far away from the horizon, the entropy of the gas depends on the volume of the box except for small corrections due to background geometry. As the box is moved closer to the horizon with one (leading) edge of the box at about Planck length (L_p) away from the horizon, the entropy shows an area dependence rather than a volume dependence. More precisely, it depends on a small volume A*L_p/2 of the box, upto an order O(L_p/K)² where A is the transverse area of the box and K is the (proper) longitudinal size of the box related to the distance between leading and trailing edge in the vertical direction (i.e in the direction of the gravitational field). Thus the contribution to the entropy comes from only a fraction O(L_p/K) of the matter degrees of freedom and the rest are suppressed when the box approaches the horizon. Near the horizon all the thermodynamical quantities behave as though the box of gas has a volume A*L_p/2 and is kept in a Minkowski spacetime. These effects are: (i) purely kinematic in their origin and are independent of the spacetime curvature (in the sense that Rindler approximation of the metric near the horizon can reproduce the results) and (ii) observer dependent. When the equilibrium temperature of the gas is taken to be equal to the the horizon temperature, we get the familiar A/L_p² dependence in the expression for entropy. All these results hold in a D+1 dimensional spherically symmetric spacetime."

 

[marcus]

I vaguely remember this guy posting at PF Beyond forum some time ago.
The paper is not directly related to QG but want to give brief mention anyway. We feature Asymptotic Safety, don't we?

http://arxiv.org/abs/1012.5529
Asymptotically safe weak interactions
Xavier Calmet
(Submitted on 26 Dec 2010)
"We emphasize that the electroweak interactions without a Higgs boson are very similar to quantum general relativity. The Higgs field could just be a dressing field and might not exist as a propagating particle. In that interpretation, the electroweak interactions without a Higgs boson could be renormalizable at the non-perturbative level because of a non-trivial fixed point. Tree-level unitarity in electroweak bosons scattering is restored by the running of the weak scale."

Happy New Year All!

 

[marcus]

http://arxiv.org/abs/1101.0590
Diffeomorphisms in group field theories
Aristide Baratin, Florian Girelli, Daniele Oriti
31 pages, many figures
(Submitted on 3 Jan 2011)
"We study the issue of diffeomorphism symmetry in group field theories (GFT), using the recently introduced noncommutative metric representation. In the colored Boulatov model for 3d gravity, we identify a field (quantum) symmetry which ties together the vertex translation invariance of discrete gravity, the flatness constraint of canonical quantum gravity, and the topological (coarse-graining) identities for the 6j-symbols. We also show how, for the GFT graphs dual to manifolds, the invariance of the Feynman amplitudes encodes the discrete residual action of diffeomorphisms in simplicial gravity path integrals. We extend the results to GFT models for higher dimensional BF theories and discuss various insights that they provide on the GFT formalism itself."

http://arxiv.org/abs/1101.0367
Observables in the General Boundary Formulation
Robert Oeckl (UNAM)
20 pages, contribution to the proceedings of the conference "Quantum Field Theory and Gravity" (Regensburg, 2010)
(Submitted on 1 Jan 2011)
"We develop a notion of quantum observable for the general boundary formulation of quantum theory. This notion is adapted to spacetime regions rather than to hypersurfaces and naturally fits into the topological quantum field theory like axiomatic structure of the general boundary formulation. We also provide a proposal for a generalized concept of expectation value adapted to this type of observable. We show how the standard notion of quantum observable arises as a special case together with the usual expectation values. We proceed to introduce various quantization schemes to obtain such quantum observables including path integral quantization (yielding the time-ordered product), Berezin-Toeplitz (antinormal ordered) quantization and normal ordered quantization and discuss some of their properties."
20 pages, contribution to the proceedings of the conference "Quantum Field Theory and Gravity" (Regensburg, 2010)


Brief mention:
http://arxiv.org/abs/1101.0389
Cosmological Big Bounce Transition
Wlodzimierz Piechocki
(Submitted on 2 Jan 2011)
9 pages, talk presented at the 11th international symposium `Frontiers of Fundamental Physics [FFP11]', 6-9 July, 2010, Paris, France; to be published in the AIP Conference Proceedings Series
"We analyze the big bounce transition of the quantum FRW model in the setting of the nonstandard loop quantum cosmology (LQC). Elementary observables are used to quantize compound observables. The spectrum of the energy density operator is bounded and continuous. The spectrum of the volume operator is bounded from below and discrete. It has equally distant levels defining a quantum of the volume. The discreteness may imply a foamy structure of spacetime at semiclassical level which may be detected in astro-cosmo observations. The nonstandard LQC method has a free parameter that should be fixed in some way to specify the big bounce transition."

 

[MTd2]

I think it was posted before, but this version, including the results, are completely changed! ENJOY!

http://arxiv.org/abs/1006.2230v2

On the geometrization of matter by exotic smoothness

Torsten Asselmeyer-Maluga, Helge Rose
(Submitted on 11 Jun 2010 (v1), last revised 4 Jan 2011 (this version, v2))
In this paper we will discuss the question how matter emerges from space. For that purpose we consider the smoothness structure as underlying structure of the spacetime manifold. The smoothness structure depends on an infinite structure -- the Casson handle -- representing the failure to smoothly embed a disc without self-intersections (immersed disc). By using the Weierstrass representation, we are able to show 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. Between the immersed discs, there are "connecting tubes" which are realized by an action term of a gauge field. Both terms are genuinely geometrical and characterized by the mean curvature of the components of the Casson handle. We also discuss the gauge group of the theory.Comments: 22 pages, no figures, elsevier style, many changes according to the referee's, thanks to Ashtekar for the argumentation about the boundary terms, argumentation about gauge group added

 

[marcus]

Brief mention. In spite of the title, this paper is not solely about string theory. It compares and contrasts a number of different approaches to QG---discussing general problems faced by all, and the state of present knowledge. The author is at the University of Giessen, at the Institute for Philosophy of Science:
http://arxiv.org/abs/1101.0690
String Theory - Nomological Unification and the Epicycles of the Quantum Field Theory Paradigm
Reiner Hedrich
23 pages

 

[marcus]

http://arxiv.org/abs/1101.0931
The principle of relative locality
Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin
12 pages, 3 figures
(Submitted on 5 Jan 2011)
"We propose a deepening of the relativity principle according to which the invariant arena for non-quantum physics is a phase space rather than spacetime. Descriptions of particles propagating and interacting in spacetimes are constructed by observers, but different observers, separated from each other by translations, construct different spacetime projections from the invariant phase space. Nonetheless, all observers agree that interactions are local in the spacetime coordinates constructed by observers local to them.
This framework, in which absolute locality is replaced by relative locality, results from deforming momentum space, just as the passage from absolute to relative simultaneity results from deforming the linear addition of velocities. Different aspects of momentum space geometry, such as its curvature, torsion and non-metricity, are reflected in different kinds of deformations of the energy-momentum conservation laws. These are in principle all measurable by appropriate experiments. We also discuss a natural set of physical hypotheses which singles out the cases of momentum space with a metric compatible connection and constant curvature."

 

[MTd2]

http://arxiv.org/abs/1101.1081

General Covariance in Gravity at a Lifgarbagez Point

Petr Horava
(Submitted on 5 Jan 2011)
This paper is based on the invited talks delivered by the author at GR 19: the 19th International Conference on General Relativity and Gravitation, Ciudad de M\'exico, M\'exico, July 2010. In Part 1, we briefly review some of the main features of quantum gravity with anisotropic scaling, and comment on its possible relation to the causal dynamical triangulations (CDT) approach to lattice quantum gravity. Part 2 explains the construction of gravity with anisotropic scaling with an extended gauge symmetry -- essentially a nonrelativistic version of general covariance. This extra symmetry eliminates the scalar graviton polarization, and thus brings the theory closer to general relativity at long distances.

 

[MTd2]

http://arxiv.org/abs/1101.1424

On Gravity, Torsion and the Spectral Action Principle

Frank Pfaeffle, Christoph A. Stephan
(Submitted on 7 Jan 2011)
We consider closed Riemannian spin manifolds with orthogonal connections. We regard the induced Dirac operators and the associated commutative spectral triples. In case of dimension four we compute the Chamseddine-Connes spectral action, deduce the equations of motions and discuss critical points.

Marcus deleted, but I will post!

http://arxiv.org/abs/1101.1417
Canonical Relational Quantum Mechanics from Information Theory

Joakim Munkhammar
(Submitted on 7 Jan 2011)
In this paper we construct a theory of quantum mechanics based on Shannon information theory. We define a few principles regarding information-based frames of reference, including explicitly the concept of information covariance, and show how an ensemble of all possible physical states can be setup on the basis of the accessible information in the local frame of reference. In the next step the Bayesian principle of maximum entropy is utilized in order to constrain the dynamics. We then show, with the aid of Lisi's universal action reservoir approach, that the dynamics is equivalent to that of quantum mechanics. Thereby we show that quantum mechanics emerges when classical physics is subject to incomplete information. We also show that the proposed theory is relational and that it in fact is a path integral version of Rovelli's relational quantum mechanics. Furthermore we give a discussion on the relation between the proposed theory and quantum mechanics, in particular the role of observation and correspondence to classical physics is addressed. In addition to this we derive a general form of entropy associated with the information covariance of the local reference frame. Finally we give a discussion and some open problems.