Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1101.1615
A taste of Hamiltonian constraint in spin foam models
Valentin Bonzom
16 pages
(Submitted on 8 Jan 2011)
"The asymptotics of some spin foam amplitudes for a quantum 4-simplex is known to display rapid oscillations whose frequency is the Regge action. In this note, we reformulate this result through a difference equation, asymptotically satisfied by these models, and whose semi-classical solutions are precisely the sine and the cosine of the Regge action. This equation is then interpreted as coming from the canonical quantization of a simple constraint in Regge calculus. This suggests to lift and generalize this constraint to the phase space of loop quantum gravity parametrized by twisted geometries. The result is a reformulation of the flat model for topological BF theory from the Hamiltonian perspective. The Wheeler-de-Witt equation in the spin network basis gives difference equations which are exactly recursion relations on the 15j-symbol. Moreover, the semi-classical limit is investigated using coherent states, and produces the expected results. It mimics the classical constraint with quantized areas, and for Regge geometries it reduces to the semi-classical equation which has been introduced in the beginning."

http://arxiv.org/abs/1101.1690
Chern-Simons theory, Stokes' Theorem, and the Duflo map
Hanno Sahlmann, Thomas Thiemann
26 pages, 8 figures
(Submitted on 10 Jan 2011)
"We consider a novel derivation of the expectation values of holonomies in Chern-Simons theory, based on Stokes' Theorem and the functional properties of the Chern-Simons action. It involves replacing the connection by certain functional derivatives under the path integral integral. It turns out that ordering choices have to be made in the process, and we demonstrate that, quite surprisingly, the Duflo isomorphism gives the right ordering, at least in the simple cases that we consider. In this way, we determine the expectation values of unknotted, but possibly linked, holonomy loops for SU(2) and SU(3), and sketch how the method may be applied to more complicated cases. Our manipulations of the path integral are formal but well motivated by a rigorous calculus of integration on spaces of generalized connections which has been developed in the context of loop quantum gravity."

http://arxiv.org/abs/1101.1738
Loop Quantum Cosmology: A cosmological theory with a view
Guillermo A. Mena Marugan
17 pages, to appear in Proceedings of Spanish Relativity Meeting 2010 (ERE 2010) held in Granada, Spain
(Submitted on 10 Jan 2011)
"Loop Quantum Gravity is a background independent, nonperturbative approach to the quantization of General Relativity. Its application to models of interest in cosmology and astrophysics, known as Loop Quantum Cosmology, has led to new and exciting views of the gravitational phenomena that took place in the early universe, or that occur in spacetime regions where Einstein's theory predicts singularities. We provide a brief introduction to the bases of Loop Quantum Cosmology and summarize the most important results obtained in homogeneous scenarios. These results include a mechanism to avoid the cosmological Big Bang singularity and replace it with a Big Bounce, as well as the existence of processes which favor inflation. We also discuss the extension of the frame of Loop Quantum Cosmology to inhomogeneous settings."

http://arxiv.org/abs/1101.1752
Extension of loop quantum gravity to f(R) theories
Xiangdong Zhang, Yongge Ma
4 pages
(Submitted on 10 Jan 2011)
"The 4-dimensional metric f(R) theories of gravity are cast into connection-dynamical formalism with real SU(2)-connections as configuration variables. Through this formalism, the classical metric f(R) theories are quantized by extending the loop quantization scheme of general relativity. Our results imply that the non-perturbative quantization procedure of loop quantum gravity is valid not only for general relativity but also for a rather general class of 4-dimensional metric theories of gravity."

 

[marcus]

http://arxiv.org/abs/1101.2174
Cosmology within Noncommutative Spectral Geometry
Mairi Sakellariadou
14 pages. Invited talk in the Corfu Summer Institute on Elementary Particles and Physics - Workshop on Non Commutative Field Theory and Gravity, September 8-12, 2010 Corfu Greece
(Submitted on 11 Jan 2011)
"Close to the Planck energy scale, the quantum nature of space-time reveals itself and all forces, including gravity, should be unified so that all interactions correspond to just one underlying symmetry. In the absence of a full quantum gravity theory, one may follow an effective approach and consider space-time as the product of a four-dimensional continuum compact Riemanian manifold by a tiny discrete finite noncommutative space. Since all available data are of a spectral nature, one may argue that it is more appropriate to apply the spectral action principle in this almost commutative space. Following this procedure one obtains an elegant geometric explanation for the most successful particle physics model, namely the standard model (and supersymmetric extensions) of electroweak and strong interactions in all its details, as determined by experimental data. Moreover, since this gravitational theory lives by construction at very high energy scales, it offers a perfect framework to address some of the early universe cosmological questions still awaiting for an answer.
After introducing some of the main mathematical elements of noncommutative spectral geometry, I will discuss various cosmological and phenomenological consequences of this theory, focusing in particular on constraints imposed on the gravitational sector of the theory."

Brief mention:
http://arxiv.org/abs/1101.2186
Making the Case for Conformal Gravity
Philip D. Mannheim
30 pages, 20 figures. Presentation at the International Conference on Two Cosmological Models, Universidad Iberoamericana, Mexico City, November 17-19, 2010.
(Submitted on 11 Jan 2011)
"We review some recent developments in the conformal gravity theory that has been advanced as a candidate alternative to standard Einstein gravity. As a quantum theory the conformal theory is both renormalizable and unitary, with unitarity being obtained because the theory is a PT symmetric rather than a Hermitian theory. We show that in the theory there can be no a priori classical curvature, with all curvature having to result from quantization. In the conformal theory gravity requires no independent quantization of its own, with it being quantized solely by virtue of its being coupled to a quantized matter source...
...We show that the macroscopic classical theory that results from the quantum conformal theory incorporates global physics effects that provide for a detailed accounting of a comprehensive set of 110 galactic rotation curves with no adjustable parameters other than the galactic mass to light ratios, and with the need for no dark matter whatsoever...
...Finally, we review some recent work by 't Hooft in which a connection between conformal gravity and Einstein gravity has been found."

 

[marcus]

For connection to LQG and Loop cosmology, see page 10 and following.
http://arxiv.org/abs/1101.2397
Uniqueness of the Fock quantization of fields with unitary dynamics in nonstationary spacetimes
Jeronimo Cortez, Guillermo A. Mena Marugan, Javier Olmedo, Jose M. Velhinho
(Submitted on 12 Jan 2011)
"The Fock quantization of fields propagating in cosmological spacetimes is not uniquely determined because of several reasons. Apart from the ambiguity in the choice of the quantum representation of the canonical commutation relations, there also exists certain freedom in the choice of field: one can scale it arbitrarily absorbing background functions, which are spatially homogeneous but depend on time. Each nontrivial scaling turns out into a different dynamics and, in general, into an inequivalent quantum field theory. In this work we analyze this freedom at the quantum level for a scalar field in a nonstationary, homogeneous spacetime whose spatial sections have S³ topology. A scaling of the configuration variable is introduced as part of a linear, time dependent canonical transformation in phase space. In this context, we prove in full detail a uniqueness result about the Fock quantization requiring that the dynamics be unitary and the spatial symmetries of the field equations have a natural unitary implementation. The main conclusion is that, with those requirements, only one particular canonical transformation is allowed, and thus only one choice of field-momentum pair (up to irrelevant constant scalings). This complements another previous uniqueness result for scalar fields with a time varying mass on S³, which selects a specific equivalence class of Fock representations of the canonical commutation relations under the conditions of a unitary evolution and the invariance of the vacuum under the background symmetries. In total, the combination of these two different statements of uniqueness picks up a unique Fock quantization for the system. We also extend our proof of uniqueness to other compact topologies and spacetime dimensions."

 

[atyy]

http://arxiv.org/abs/1101.1958
What Really Sets the Upper Bound on Quantum Correlations?Joy Christian (Oxford)
(Submitted on 10 Jan 2011)
The discipline of parallelization in the manifold of all possible measurement results is shown to be responsible for the existence of all quantum correlations, with the upper bound on their strength stemming from the maximum of possible torsion within all norm-composing parallelizable manifolds. A profound interplay is thus uncovered between the existence and strength of quantum correlations and the parallelizability of the spheres S^0, S^1, S^3, and S^7 necessitated by the four real division algebras. In particular, parallelization within a unit 3-sphere is shown to be responsible for the existence of EPR and Hardy type correlations, whereas that within a unit 7-sphere is shown to be responsible for the existence of all GHZ type correlations. Moreover, parallelizability in general is shown to be equivalent to the completeness criterion of EPR, in addition to necessitating the locality condition of Bell. It is therefore shown to predetermine both the local outcomes as well as the quantum correlations among the remote outcomes, dictated by the infinite factorizability of points within the spheres S^3 and S^7. The twin illusions of quantum entanglement and non-locality are thus shown to stem from the topologically incomplete accountings of the measurement results.

 

[PhilKravitz]

in classical mechanics things move along trajectories---curved paths parametrized by time---and when you quantize the trajectories go away.

the curved paths things travel along don't exist any more, you have to erase the trajectories (or in Feynman sum over histories you "integrate" all possible ways of getting from here to there---in any case the clear picture of a path loses reality and dissipates)

It seems to me more like Feynmann's sum over paths changes not at all with a network of discrete paths. It is still the sum over all possible paths. The classical trajectory is now more like diffusion along the discrete edges of the mesh.

 

[marcus]

Phil,
please do start a separate thread for discussion of the topic you have in mind. This thread began back in 2003 with some discussion, but quickly evolved into a bibliography. It is no longer used for discussion. It's sole function now is to list current QG research articles: Loop-and-allied quantum gravity.
Mixing in discussion of various topics would dilute the thread and make it less usable as a bibliography.

 

[atyy]

http://arxiv.org/abs/1101.2451
From Navier-Stokes To Einstein
Irene Bredberg, Cynthia Keeler, Vyacheslav Lysov, Andrew Strominger
(Submitted on 12 Jan 2011)
We show by explicit construction that for every solution of the incompressible Navier-Stokes equation in p+1 dimensions, there is a uniquely associated "dual" solution of the vacuum Einstein equations in p+2 dimensions. The dual geometry has an intrinsically flat timelike boundary segment $\Sigma_c$ whose extrinsic curvature is given by the stress tensor of the Navier-Stokes fluid. We consider a "near-horizon" limit in which $\Sigma_c$ becomes highly accelerated. The near-horizon expansion in gravity is shown to be mathematically equivalent to the hydrodynamic expansion in fluid dynamics, and the Einstein equation reduces to the incompressible Navier-Stokes equation. For p=2, we show that the full dual geometry is algebraically special Petrov type II. The construction is a mathematically precise realization of suggestions of a holographic duality relating fluids and horizons which began with the membrane paradigm in the 70's and resurfaced recently in studies of the AdS/CFT correspondence.

 

[marcus]

http://arxiv.org/abs/1101.2744
Time in quantum gravity and black-hole information paradox
Harvey Nikolic
(Submitted on 14 Jan 2011)
"The fact that canonical quantum gravity does not possesses a fundamental notion of time implies that the theory is unitary in a trivial sense. At the fundamental level, this trivial unitarity leaves no room for a black-hole information loss. Yet, a phenomenological loss of information may appear when some matter degrees of freedom are reinterpreted as a clock-time. This explains how both fundamental unitarity and phenomenological information loss may peacefully coexist, which offers a resolution of the black-hole information paradox."

http://arxiv.org/abs/1101.2841
Palatini Actions and Quantum Gravity Phenomenology
Gonzalo J. Olmo
7 pages, 1 figure
(Submitted on 14 Jan 2011)
"We show that a quadratic gravitational Lagrangian in the Palatini formulation is able to capture different aspects of quantum gravity phenomenology in a single framework. In particular, we show that in this theory field excitations propagating with different energy-densities perceive different background metrics, which is a fundamental characteristic of the DSR and Rainbow Gravity approaches. Also, the resulting isotropic and anisotropic cosmologies are free from the big bang singularity. This singularity avoidance occurs non-perturbatively and shares some similitudes with the effective dynamics of loop quantum cosmology."

 

[marcus]

http://arxiv.org/abs/1101.3264
Spinfoam Fermions: PCT Symmetry, Dirac Determinant, and Correlation Functions
Muxin Han, Carlo Rovelli
26 pages, 9 figures
(Submitted on 17 Jan 2011)
"We discuss fermion coupling in the framework of spinfoam quantum gravity. We analyze the gravity-fermion spinfoam model and its fermion correlation functions. We show that there is a spinfoam analog of PCT symmetry for the fermion fields on spinfoam model, where a PCT theorem is proved for spinfoam fermion correlation functions. We compute the determinant of the Dirac operator for the fermions, where two presentations of the Dirac determinant are given in terms of diagram expansions. We compute the fermion correlation functions and show that they can be given by Feynman diagrams on the spinfoams, where the Feynman propagators can be represented by a discretized path integral of a world-line action along the edges of the underlying 2-complex."

http://arxiv.org/abs/1101.3294
Finiteness of the EPRL/FK spin foam model
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 17 Jan 2011)
"We show that the EPRL/FK spin foam model of quantum gravity can be made finite by dividing the vertex amplitude with an appropriate power p of the product of dimensions of the vertex spins and intertwiners. This power is independent of the spin foam and we find a lower bound for p which makes the state sum absolutely convergent."

http://arxiv.org/abs/1101.3151
Quantum buoyancy, generalized second law, and higher-dimensional entropy bounds
Shahar Hod
6 pages
(Submitted on 17 Jan 2011)
Bekenstein has presented evidence for the existence of a universal upper bound of magnitude 2 pi R/hbarc to the entropy-to-energy ratio S/E of an arbitrary three dimensional system of proper radius R and negligible self-gravity. In this paper we derive a generalized upper bound on the entropy-to-energy ratio of a (D+1)-dimensional system. We consider a box full of entropy lowered towards and then dropped into a (D+1)-dimensional black hole in equilibrium with thermal radiation. In the canonical case of three spatial dimensions, it was previously established that due to quantum buoyancy effects the box floats at some neutral point very close to the horizon. We find here that the significance of quantum buoyancy increases dramatically with the number D of spatial dimensions. In particular, we find that the neutral (floating) point of the box lies near the horizon only if its length b is large enough such that b/b_C>F(D), where b_C is the Compton length of the body and F(D) ~ D^D/2 >> 1 for D >> 1. A consequence is that quantum buoyancy severely restricts our ability to deduce the universal entropy bound from the generalized second law of thermodynamics in higher-dimensional spacetimes with D >> 1. Nevertheless, we find that the universal entropy bound is always a sufficient condition for operation of the generalized second law in this type of gedanken experiments."

http://arxiv.org/abs/1101.3168
Gravitational sources induced by exotic smoothness
T. Asselmeyer-Maluga, C.H. Brans
17 pages, 8 figures, 5 appendices
(Submitted on 17 Jan 2011)
"In this paper we construct a coordinate atlas in an exotic R⁴ using Bizaca's construction. The main source for such an atlas is the handle body decomposition of a Casson handle, which of course is an infinite, but periodic, process. The immersion of the end-periodic manifold into R⁴ is directly related to the exoticness of the R⁴ and also gives rise naturally to a spinor field. Thus we obtain the interesting result that the simplest exotic R⁴ generates an extra spinor field by exoticness."

αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔~≈≠≡ ≤≥½∞(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅)
:

 

[MTd2]

http://arxiv.org/abs/1101.3524

The Hamiltonian constraint in 3d Riemannian loop quantum gravity

Valentin Bonzom, Laurent Freidel
(Submitted on 18 Jan 2011)
We discretize the Hamiltonian scalar constraint of three-dimensional Riemannian gravity on a graph of the loop quantum gravity phase space. This Hamiltonian has a clear interpretation in terms of discrete geometries: it computes the extrinsic curvature from dihedral angles. The Wheeler-DeWitt equation takes the form of difference equations, which are actually recursion relations satisfied by Wigner symbols. On the boundary of a tetrahedron, the Hamiltonian generates the exact recursion relation on the 6j-symbol which comes from the Biedenharn-Elliott (pentagon) identity. This fills the gap between the canonical quantization and the symmetries of the Ponzano-Regge state-sum model for 3d gravity.

 

[marcus]

http://arxiv.org/abs/1101.3660
Detailed black hole state counting in loop quantum gravity
Ivan Agullo, J. Fernando Barbero G., Enrique F. Borja, Jacobo Diaz-Polo, Eduardo J. S. Villaseñor
Phys.Rev.D82:084029,2010
(Submitted on 19 Jan 2011)
"We give a complete and detailed description of the computation of black hole entropy in loop quantum gravity by employing the most recently introduced number-theoretic and combinatorial methods. The use of these techniques allows us to perform a detailed analysis of the precise structure of the entropy spectrum for small black holes, showing some relevant features that were not discernible in previous computations. The ability to manipulate and understand the spectrum up to the level of detail that we describe in the paper is a crucial step towards obtaining the behavior of entropy in the asymptotic (large horizon area) regime."

http://arxiv.org/abs/1101.3662
Statistical description of the black hole degeneracy spectrum
J. Fernando Barbero G., Eduardo J. S. Villaseñor
41 pages, 12 figures
(Submitted on 19 Jan 2011)
"We use mathematical methods based on generating functions to study the statistical properties of the black hole degeneracy spectrum in loop quantum gravity. In particular we will study the persistence of the observed effective quantization of the entropy as a function of the horizon area. We will show that this quantization disappears as the area increases despite the existence of black hole configurations with a large degeneracy. The methods that we describe here can be adapted to the study of the statistical properties of the black hole degeneracy spectrum for all the existing proposals to define black hole entropy in loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/1101.4049
Cosmological constant in spinfoam cosmology
Eugenio Bianchi, Thomas Krajewski, Carlo Rovelli, Francesca Vidotto
4 pages, 2 figures
(Submitted on 20 Jan 2011)
"We consider a simple modification of the amplitude defining the dynamics of loop quantum gravity, corresponding to the introduction of the cosmological constant, and possibly related to the SL(2,C)q extension of the theory recently considered by Fairbairn-Meusburger and Han. We show that in the context of spinfoam cosmology, this modification yields the de Sitter cosmological solution."

http://arxiv.org/abs/1101.4182
The 1/N expansion of colored tensor models in arbitrary dimension
Razvan Gurau, Vincent Rivasseau
4 pages, 3 figures
(Submitted on 21 Jan 2011)
"In this paper we extend the 1/N expansion introduced in [1] to group field theories in arbitrary dimension and prove that only graphs corresponding to spheres S^D contribute to the leading order in the large N limit."

 

[marcus]

http://arxiv.org/abs/1101.4294
Radiative corrections in the Boulatov-Ooguri tensor model: The 2-point function
Joseph Ben Geloun, Valentin Bonzom
24 pages, 3 figures
(Submitted on 22 Jan 2011)
"The Boulatov-Ooguri tensor model generates a sum over spacetime topologies for the D-dimensional BF theory. We study here the quantum corrections to the propagator of the theory. In particular, we find that the radiative corrections at the second order in the coupling constant yield a mass renormalization. They also exhibit a divergence which cannot be balanced with a counter-term in the initial action, and which usually corresponds to the wave-function renormalization."

 

[Kevin_Axion]

http://www.perimeterscholars.org/303.html
Renate Loll is beginning her course at PI for the PSI program. The first lecture was posted today which will be followed by 14 more. If anyone is interested in learning Quantum Gravity (including LQG, CDT etc.) this is a very good resource, make sure you know your GR and Differential Geometry!

Move if necessary

 

[marcus]

http://arxiv.org/abs/1101.4788
Gravity as a diffeomorphism invariant gauge theory
Kirill Krasnov
24 pages
(Submitted on 25 Jan 2011)
"A general diffeomorphism invariant SU(2) gauge theory is a gravity theory with two propagating polarisations of the graviton. We develop this description of gravity, in particular for future applications to the perturbative quantisation. Thus, the linearised theory, gauge symmetries, gauge fixing are discussed in detail, and the propagator is obtained. The propagator takes a simple form of that of Yang-Mills theory with an additional projector on diffeomorphism equivalence classes of connections inserted. In our approach the gravitational perturbation theory takes a rather unusual form in that the Planck length determined from the self-coupling of the graviton is no longer fundamental but becomes a derived quantity."

http://arxiv.org/abs/1101.4775
Perfect discretization of reparametrization invariant path integrals
Benjamin Bahr, Bianca Dittrich, Sebastian Steinhaus
22+8 pages
(Submitted on 25 Jan 2011)
"To obtain a well defined path integral one often employs discretizations. In the case of gravity and reparametrization invariant systems, the latter of which we consider here as a toy example, discretizations generically break diffeomorphism and reparametrization symmetry, respectively. This has severe implications, as these symmetries determine the dynamics of the corresponding system. Indeed we will show that a discretized path integral with reparametrization invariance is necessarily also discretization independent and therefore uniquely determined by the corresponding continuum quantum mechanical propagator. We use this insight to develop an iterative method for constructing such a discretized path integral, akin to a Wilsonian RG flow. This allows us to address the problem of discretization ambiguities and of an anomaly--free path integral measure for such systems. The latter is needed to obtain a path integral, that can act as a projector onto the physical states, satisfying the quantum constraints. We will comment on implications for discrete quantum gravity models, such as spin foams."

brief mention:
http://arxiv.org/abs/1101.4916
On the Semiclassical Approach to Quantum Cosmology
Edward Anderson

 

[MTd2]

http://arxiv.org/abs/1101.4783

NC GUTs: A Status Report

C.P.Martin
(Submitted on 25 Jan 2011)
I review the main results that have been obtained so far on the construction of noncommutative GUTs

***

One of these days I asked something about this...

 

[marcus]

http://arxiv.org/abs/1101.5061
A Short and Subjective Introduction to the Spinfoam Framework for Quantum Gravity
Etera R. Livine
90 pages
(Submitted on 26 Jan 2011)
"This is my Thèse d'Habilitation (HDR) on the topic of spinfoam models for quantum gravity, which I presented in l'Ecole Normale Supérieure de Lyon on december 16 2010. The spinfoam framework is a proposal for a regularized path integral for quantum gravity, inspired from Topological Quantum Field Theory (TQFT) and state-sum models. It can also be seen as defining transition amplitudes for the quantum states of geometry for Loop Quantum Gravity (LQG)."

http://arxiv.org/abs/1101.5136
Extremal and nonextremal Kerr/CFT correspondences
S. Carlip
16 pages
(Submitted on 26 Jan 2011)
"I rederive the Kerr/CFT correspondence without first taking the near-horizon extremal Kerr limit. This method extends easily to nonextremal black holes, for which the temperature and central charge behave poorly at the horizon but the entropy remains finite. A computation yields one-half of the standard Bekenstein-Hawking entropy, with hints that the other half may be related to a conformal field theory at the inner horizon. I then present an alternative approach, based on a stretched Killing horizon, in which the full entropy is obtained and the temperature and central charge remain well-behaved even in the nonextremal case."

 

[marcus]

http://arxiv.org/abs/1101.5391
Observational constraints on loop quantum cosmology
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa
4 pages, 3 figures
(Submitted on 27 Jan 2011)
"In the inflationary scenario of loop quantum cosmology (LQC) in the presence of inverse-volume corrections, we give analytic formulas for the power spectra of scalar and tensor perturbations convenient to confront with observations. Since inverse-volume corrections can provide strong contributions to the running spectral indices, inclusion of terms higher than the second-order runnings in the power spectra is crucially important. Using the recent data of cosmic microwave background (CMB) and other cosmological experiments, we place bounds on the quantum corrections for a quadratic inflaton potential."

Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Report number: IGC-11/1-1, AEI-2011-004
Cite as: arXiv:1101.5391v1 [astro-ph.CO]

I bolded some key language. Interesting that this was posted in the Astrophysics section of Arxiv, in astro-ph, not in hep-th or gr-qc. Tsujikawa is astrophysics-phenomenology---not a QG-ist. It's a shift towards the observational side.

 

[atyy]

http://arxiv.org/abs/1101.5185
Einstein Manifolds As Yang-Mills Instantons
John J. Oh, Hyun Seok Yang
(Submitted on 27 Jan 2011)
We show that every Einstein manifolds are Yang-Mills instantons in O(4) = SU(2)L × SU(2)R
gauge theory. It is shown that any Einstein manifold with or without a cosmological constant always arises as the sum of SU(2)L instantons and SU(2)R anti-instantons. It explains why an Einstein manifold is stable because two kinds of instantons belong to different gauge groups, one in SU(2)L and the other in SU(2)R, and so they cannot decay into a vacuum. We discuss the topological aspects of Einstein manifolds.

http://arxiv.org/abs/1101.5315
Special Geometries Emerging from Yang-Mills Type Matrix Models
Daniel N. Blaschke
(Submitted on 27 Jan 2011)
Abstract: I review some recent results which demonstrate how various geometries, such as Schwarzschild and Reissner-Nordstroem, can emerge from Yang-Mills type matrix models with branes. Furthermore, explicit embeddings of these branes as well as appropriate Poisson structures and star-products which determine the non-commutativity of space-time are provided. These structures are motivated by higher order terms in the effective matrix model action which semi-classically lead to an Einstein-Hilbert type action.

 

[MTd2]

http://arxiv.org/abs/1101.5552

Asymptotic freedom of Yang-Mills theory with gravity

Sarah Folkerts, Daniel F. Litim, Jan M. Pawlowski
(Submitted on 28 Jan 2011)
We study the high energy behaviour of Yang-Mills theory under the inclusion of gravity. In the weak-gravity limit, the running gauge coupling receives no contribution from the gravitational sector, if all symmetries are preserved. This holds true with and without cosmological constant. We also show that asymptotic freedom persists in general field-theory-based gravity scenarios including gravitational shielding as well as asymptotically safe gravity.

http://arxiv.org/abs/1101.5401

Asymptotically Safe Cosmology

Mark Hindmarsh, Daniel Litim, Christoph Rahmede
(Submitted on 27 Jan 2011)
We study quantum modifications to cosmology in a Friedmann-Robertson-Walker universe with and without scalar fields by taking the renormalisation group running of gravitational and matter couplings into account. We exploit the Bianchi identity to relate the renormalisation group scale with scale factor and derive the improved cosmological evolution equations. We find two types of cosmological fixed points where the renormalisation group scale either freezes in, or continues to evolve with scale factor. We discuss the implications of each of these, and classify the different cosmological fixed points with and without gravity displaying an asymptotically safe renormalisation group fixed point. We state conditions of existence for an inflating ultraviolet cosmological fixed point for Einstein gravity coupled to a scalar field. We also discuss other fixed point solutions such as "scaling" solutions, or fixed points with equipartition between kinetic and potential energies.

http://arxiv.org/abs/1101.5585

2+1 gravity with positive cosmological constant in LQG: a proposal for the physical state

Daniele Pranzetti
(Submitted on 28 Jan 2011)
In this paper, I investigate the possible quantization, in the context of LQG, of three dimensional gravity in the case of positive cosmological constant {\Lambda} and try to make contact with alternative quantization approaches already existing in the literature. Due to the appearance of an anomaly in the constraints algebra, previously studied as a first step of the analysis, alternative techniques developed for the quantization of systems with constraints algebras not associated with a structure Lie group need to be adopted. Therefore, I introduce an ansatz for a physical state which gives some transition amplitudes in agreement with what one would expect from the Turaev-Viro model. Moreover, in order to check that this state implements the right dynamicss, I show that it annihilates the master constraint for the theory up to the first order in {\Lambda}.

http://arxiv.org/abs/1101.5592

Loop Quantum Gravity and Cosmology: A dynamical introduction

Martin Bojowald
(Submitted on 28 Jan 2011)
Loop quantum gravity and cosmology are reviewed with an emphasis on evaluating the dynamics, rather than constructing it. The three crucial parts of such an analysis are (i) deriving effective equations, (ii) controlling the theory's microscopic degrees of freedom that lead to its spatial discreteness and refinement, and (iii) ensuring consistency and anomaly-freedom. All three issues are crucial for making the theory testable by conceptual and observational means, and they remain challenging. Throughout this review, the Hamiltonian nature of the theory will play a large role for properties of space-time structure within the framework discussed.

 

[marcus]

http://arxiv.org/abs/1101.5792
Particle Creation by Loop Black Holes
Emanuele Alesci, Leonardo Modesto
14 pages, 6 figures
(Submitted on 30 Jan 2011)
"We study the black hole particle production in a regular spacetime metric obtained in a minisuperspace approach to loop quantum gravity. In different previous papers the static solution was obtained and shown to be singularity-free and self-dual. In this paper expanding a previous study of the black hole dynamics we repeat the Hawking analysis which leads to a thermal flux of particles at the future infinity. The evaporation time is infinite and the unitarity is recovered due to the regularity of the spacetime and to the characteristic behavior of the surface gravity."

http://arxiv.org/abs/1101.6078
Induced standard model and unification
John W. Barrett
(Submitted on 31 Jan 2011)
"A proposal that the bosonic action of the standard model is induced from the fermionic action is investigated."

http://arxiv.org/abs/1101.6007
QED coupled to QEG
Ulrich Harst, Martin Reuter
25 pages, 3 figures
(Submitted on 31 Jan 2011)
"We discuss the non-perturbative renormalization group flow of Quantum Electrodynamics (QED) coupled to Quantum Einstein Gravity (QEG) and explore the possibilities for defining its continuum limit at a fixed point that would lead to a non-trivial, i.e. interacting field theory. We find two fixed points suitable for the Asymptotic Safety construction. In the first case, the fine-structure constant vanishes at the fixed point and its infrared ("renormalized") value is a free parameter not determined by the theory itself. In the second case, the fixed point value of the fine-structure constant is non-zero, and its infrared value is a computable prediction of the theory."

A notable paper included despite no direct explicit connection with QG
http://arxiv.org/abs/1101.5690
Division Algebras and Quantum Theory
John C. Baez
30 pages, 3 figures
(Submitted on 29 Jan 2011)
"Quantum theory may be formulated using Hilbert spaces over any of the three associative normed division algebras: the real numbers, the complex numbers and the quaternions. Indeed, these three choices appear naturally in a number of axiomatic approaches. However, there are internal problems with real or quaternionic quantum theory. Here we argue that these problems can be resolved if we treat real, complex and quaternionic quantum theory as part of a unified structure. Dyson called this structure the 'three-fold way'. It is perhaps easiest to see it in the study of irreducible unitary representations of groups on complex Hilbert spaces. These representations come in three kinds: those that are not isomorphic to their own dual (the truly 'complex' representations), those that are self-dual thanks to a symmetric bilinear pairing (which are 'real', in that they are the complexifications of representations on real Hilbert spaces), and those that are self-dual thanks to an antisymmetric bilinear pairing (which are 'quaternionic', in that they are the underlying complex representations of representations on quaternionic Hilbert spaces). This three-fold classification sheds light on the physics of time reversal symmetry, and it already plays an important role in particle physics. More generally, Hilbert spaces of anyone of the three kinds - real, complex and quaternionic - can be seen as Hilbert spaces of the other kinds, equipped with extra structure."

 

[atyy]

http://arxiv.org/abs/1102.0270
Topological Phases: An Expedition off Lattice
Michael H. Freedman, Lukas Gamper, Charlotte Gils, Sergei V. Isakov, Simon Trebst, Matthias Troyer
(Submitted on 1 Feb 2011)
Motivated by the goal to give the simplest possible microscopic foundation for a broad class of topological phases, we study quantum mechanical lattice models where the topology of the lattice is one of the dynamical variables. However, a fluctuating geometry can remove the separation between the system size and the range of local interactions, which is important for topological protection and ultimately the stability of a topological phase. In particular, it can open the door to a pathology, which has been studied in the context of quantum gravity and goes by the name of `baby universe', Here we discuss three distinct approaches to suppressing these pathological fluctuations. We complement this discussion by applying Cheeger's theory relating the geometry of manifolds to their vibrational modes to study the spectra of Hamiltonians. In particular, we present a detailed study of the statistical properties of loop gas and string net models on fluctuating lattices, both analytically and numerically.

 

[marcus]

http://arxiv.org/abs/1102.0954
The Holst Action by the Spectral Action Principle
Frank Pfaeffle, Christoph A. Stephan
11 pages
(Submitted on 4 Feb 2011)
"We investigate the Holst action for closed Riemannian 4-manifolds with orthogonal connections. For connections whose torsion has zero Cartan type component we show that the Holst action can be recovered from the heat asymptotics for the natural Dirac operator acting on left-handed spinor fields."

(If anyone is coming in new to this, Holst action is the action used in Spinfoam LQG and Spectral Action Principle means they are talking Connes NC geometry realization of standard model matter. The title indicates they want to put Connes-style standard model matter together with LQG. Read the paper to get more detail.)

In post #1410 I logged a paper by John Barrett, "Induced standard model and unification". In the course of revision this has been given a new title:
http://arxiv.org/abs/1101.6078
State sum models, induced gravity and the spectral action
John W. Barrett
13 pages
(Submitted on 31 Jan 2011)
"A proposal that the bosonic action of the standard model is induced from the fermionic action is investigated. It is suggested that this might occur naturally in state sum models."

http://arxiv.org/abs/1102.0789
The emergence of a universal limiting speed
Mohamed M. Anber, John F. Donoghue
13 pages, 3 figures
(Submitted on 3 Feb 2011)
"We display several examples of how fields with different limiting velocities (the "speed of light") at a high energy scale can nevertheless have a common limiting velocity at low energies due to the effects of interactions. We evaluate the interplay of the velocities through the self-energy diagrams and use the renormalization group to evolve the system to low energy. The differences normally vanish only logarithmically, so that an exponentially large energy trajectory is required in order to satisfy experimental constraints. However, we also display a model in which the running is power-law, which could be more phenomenologically useful. The largest velocity difference should be in system with the weakest interaction, which suggests that the study of the speed of gravitational waves would be the most stringent test of this phenomenon."

 

[marcus]

http://arxiv.org/abs/1102.1688
State Sums and Geometry
Frank Hellmann
PhD Thesis, 106 pages
(Submitted on 8 Feb 2011)
"In this thesis I review the definition of topological quantum field theories through state sums on triangulated manifolds. I describe the construction of state sum invariants of 3-manifolds from a graphical calculus and show how to evaluate the invariants as boundary amplitudes. I review how to define such a graphical calculus through SU(2) representation theory. I then review various geometricity results for the representation theory of SU(2), Spin(4) and SL(2,C), and define coherent boundary manifolds for state sums based on these representations. I derive the asymptotic geometry of the SU(2) based Ponzano-Regge invariant in three dimensions, and the SU(2) based Ooguri models amplitude in four dimensions. As a corollary to the latter results I derive the asymptotic behaviour of various recently proposed spin foam models motivated from the Plebanski formulation of general relativity. Finally the asymptotic geometry of the SL(2,C) based model is derived."

 

[marcus]

http://arxiv.org/abs/1102.1844
Comment on `Lost in Translation: Topological Singularities in Group Field Theory'
Matteo Smerlak
(Submitted on 9 Feb 2011)
"Gurau argued in [arXiv:1006.0714] that the gluing spaces arising as Feynman diagrams of three-dimensional group field theory are not all pseudo-manifolds. I dispute this conclusion: albeit not properly triangulated, these spaces are genuine pseudo-manifolds, viz. their singular locus is of codimension at most two."

 

[atyy]

http://arxiv.org/abs/1102.1592
Emergent of non-gravitational fields in dimensional reduction of 4d spin foam models
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 9 Feb 2011 (this version, v2))
We consider a Kaluza-Klein like approach for a 4d spin foam model. We apply this approach to a 4d TOCY model based on group field theory; and using the Peter-Weyl expansion of the gravitational field we find a mechanism for generation of matter and new dimensions from pure gravity.

 

[MTd2]

http://arxiv.org/abs/1102.2226

Discrete and continuum third quantization of Gravity

Steffen Gielen, Daniele Oriti
(Submitted on 10 Feb 2011)
We give a brief introduction to matrix models and the group field theory (GFT) formalism as realizations of the idea of a third quantization of gravity, and present in some more detail the idea and basic features of a continuum third quantization formalism in terms of a field theory on the space of connections, building up on the results of loop quantum gravity that allow to make the idea slightly more concrete. We explore to what extent one can rigorously define such a field theory. Concrete examples are given for the simple case of Riemannian GR in 3 spacetime dimensions. We discuss the relation between GFT and this formal continuum third quantized gravity, and what it can teach us about the continuum limit of GFTs.

 

[marcus]

Not directly about QG, but Baez is "family" and it could apply to Jacobson, Verlinde gravity conceivably

http://arxiv.org/abs/1102.2098
Renyi Entropy and Free Energy
John C. Baez
(Submitted on 10 Feb 2011)
"The Renyi entropy is a generalization of the usual concept of entropy which depends on a parameter beta. Any probability distribution can be seen as the Gibbs state for some Hamiltonian at temperature 1. Starting with this Hamiltonian, we can then compute the free energy at temperature T, and up to a certain factor this turns out to be the Renyi entropy where beta = 1/T. More precisely, the free energy is (1 - T) times the Renyi entropy. This is true not only classically but also quantum-mechanically."

 

[physikdan]

http://arxiv.org/abs/1102.1592
Emergent of non-gravitational fields in dimensional reduction of 4d spin foam models
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 9 Feb 2011 (this version, v2))
We consider a Kaluza-Klein like approach for a 4d spin foam model. We apply this approach to a 4d TOCY model based on group field theory; and using the Peter-Weyl expansion of the gravitational field we find a mechanism for generation of matter and new dimensions from pure gravity.

It is inersting that they do Kaluza-Klein strategy to a very different subject ! and ern some how similar results.

 

[marcus]

Please start a separate discussion thread about the paper. We don't discuss papers in this thread. This is a bibliography thread to help us keep track of current Loop-and-allied research. It overloads and makes it less useful to have discussion crowding in.
I would welcome a discussion thread about this paper by the two guys at Teheran (have already asked Atyy what he thinks of it.)