Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1108.3269
An introduction to quantum gravity
Giampiero Esposito
(Submitted on 16 Aug 2011)
Quantum gravity was born as that branch of modern theoretical physics that tries to unify its guiding principles, i.e., quantum mechanics and general relativity. Nowadays it is providing new insight into the unification of all fundamental interactions, while giving rise to new developments in mathematics. The various competing theories, e.g. string theory and loop quantum gravity, have still to be checked against observations. We review the classical and quantum foundations necessary to study field-theory approaches to quantum gravity, the passage from old to new unification in quantum field theory, canonical quantum gravity, the use of functional integrals, the properties of gravitational instantons, the use of spectral zeta-functions in the quantum theory of the universe, Hawking radiation, some theoretical achievements and some key experimental issues.
58 pages, invited contribution to an encyclopedia sponsored by UNESCO (EOLSS)

brief mention:
http://arxiv.org/abs/1108.3080
How unitary cosmology generalizes thermodynamics and solves the inflationary entropy problem
Max Tegmark (MIT)
(Submitted on 15 Aug 2011)
We analyze cosmology assuming unitary quantum mechanics, using a tripartite partition into system, observer and environment degrees of freedom. This generalizes the second law of thermodynamics to "The system's entropy can't decrease unless it interacts with the observer, and it can't increase unless it interacts with the environment." We show that because of the long-range entanglement created by cosmological inflation, the cosmic entropy decreases exponentially rather than linearly with the number of bits of information observed, so that a given observer can reduce entropy by much more than the amount of information her brain can store...
...
...
18 pages, 5 figs

 

[marcus]

http://arxiv.org/abs/1108.3932
A second-order phase transition in CDT
J. Ambjorn, S. Jordan, J. Jurkiewicz, R. Loll
(Submitted on 19 Aug 2011)
Causal Dynamical Triangulations (CDT) are a concrete attempt to define a nonperturbative path integral for quantum gravity. We present strong evidence that the lattice theory has a second-order phase transition line, which can potentially be used to define a continuum limit in the conventional sense of nongravitational lattice theories.
10 pages, 3 figures

 

[MTd2]

http://vixra.org/abs/1108.0026

How to Learn to Ask Good Questions in Physics

Authors: Johan Noldus

[This paper deals with] how to learn to ask good questions in physics.

*****

Very interesting philosophical paper!

 

[marcus]

http://arxiv.org/abs/1108.4577
Algebras of Quantum Variables for Loop Quantum Gravity, I. Overview
Diana Kaminski
(Submitted on 19 Aug 2011)
The operator algebraic framework plays an important role in mathematical physics. Many different operator algebras exist for example for a theory of quantum mechanics. In Loop Quantum Gravity only two algebras have been introduced until now. In the project about 'Algebras of Quantum Variables (AQV) for LQG' the known holonomy-flux *-algebra and the Weyl C*-algebra will be modified and a set of new algebras will be proposed and studied. The idea of the construction of these algebras is to establish a finite set of operators, which generates (in the sense of Woronowicz, Schm\"udgen and Inoue) the different O*- or C*-algebras of quantum gravity and to use inductive limits of these algebras. In the Loop Quantum Gravity approach usually the basic classical variables are connections and fluxes. Studying the three constraints appearing in the canonical quantisation of classical general relativity in the ADM-formalism some other variables like curvature appear. Consequently the main difficulty of a quantisation of gravity is to find a suitable replacement of the set of elementary classical variables and constraints. The algebra of quantum gravity is supposed to be generated by a set of the operators associated to holonomies, fluxes and in some cases even the curvature. There are two reasonable choices for this algebra: The set of constraints of Quantum Gravity are contained in or at least the constraints are affilliated with this algebra. Secondly, the algebra of quantum variables is said to be physical if it contains complete observables. In the project of 'Algebras of Quantum Variables for LQG' different algebras will be studied with respect to the property of being a physical algebra. Furthermore the existence of KMS-states on these algebras will be argued.
45 pages

http://arxiv.org/abs/1108.4578
AQV II. A new formulation of the Weyl C*-algebra
Diana Kaminski
(Submitted on 19 Aug 2011)
In this article a new formulation of the Weyl C*-algebra, which has been invented by Fleischhack, in terms of C*-dynamical systems is presented. The quantum configuration variables are given by the holonomies along paths in a graph. Functions depending on these quantum variables form the analytic holonomy C*-algebra. Each classical flux variable is quantised as an element of a flux group associated to a certain surface set and a graph. The quantised spatial diffeomorphisms are elements of the group of bisections of a finite graph system. Then different actions of the flux group associated to surfaces and the group of bisections on the analytic holonomy C*-algebra are studied. The Weyl C*-algebra for surfaces is generated by unitary operators, which implements the group-valued quantum flux operators, and certain functions depending on holonomies along paths that satisfy canonical commutation relations. Furthermore there is a unique pure state on the commutative Weyl C*-algebra for surfaces, which is a path- or graph-diffeomorphism invariant.
71 pages, 10 figures

http://arxiv.org/abs/1108.4579
AQV III. The holonomy-flux cross-product C*-algebra
Diana Kaminski
(Submitted on 19 Aug 2011)
In this article a new C*-algebra derived from the basic quantum variables: holonomies along paths and group-valued quantum flux operators in the framework of Loop Quantum Gravity is constructed. This development is based on the theory of cross-products and C*-dynamical systems. The author has presented a set of actions of the flux group associated to a surface set on the analytic holonomy C*-algebra, which define C*-dynamical systems. These objects are used to define the holonomy-flux cross-product C*-algebra associated to a surface set. Furthermore surface-preserving path- and graph-diffeomorphism-invariant states of the new C*-algebra are analysed. Finally the holonomy-flux cross-product C*-algebra is extended such that the graph-diffeomorphisms generate among other operators the holonomy-flux-graph-diffeomorphism cross-product C*-algebra associated to a surface set.
49 pages, 4 figures

http://arxiv.org/abs/1108.4580
AQV IV. A new formulation of the holonomy-flux *-algebra
Diana Kaminski
(Submitted on 19 Aug 2011)
In this article the holonomy-flux *-algebra, which has been introduced by Lewandowski, Okolow, Sahlmann and Thiemann, is modificated. The new *-algebra is called the holonomy-flux cross-product *-algebra. This algebra is an abstract cross-product *-algebra. It is given by the universal algebra of the algebra of continuous and differentiable functions on the configuration space of generalised connections and the universal enveloping flux algebra associated to a surface set, and some canonical commutator relations. There is a uniqueness result for a certain path- and graph-diffeomorphism invariant state of the holonomy-flux cross-product *-algebra. This new *-algebra is not the only *-algebra, which is generated by the algebra of certain continuous and differentiable functions on the configuration space of generalised connections and the universal enveloping flux algebra associated to a surface set. The theory of abstract cross-product algebras allows to define different new *-algebras. Some of these algebras are presented in this article.
46 pages, 7 figures

http://arxiv.org/abs/1108.4581
AQV V. The localised holonomy-flux cross-product *-algebra
Diana Kaminski
(Submitted on 19 Aug 2011)
In the project AQV the issue of quantum constraints, KMS-states and algebras of quantum configuration and momentum variables in Loop Quantum Gravity has been argued. There a physical algebra has been required to contain complete observables and the quantum constraints, or at least the quantum constraints are affilliated with this algebra. In this context a first conjecture for a physical algebra is presented in this article. A new *-algebra for LGQ, which is called the localised holonomy-flux cross-product *-algebra, is studied. A suggestion for a physical *-algebra, which contains the localised holonomy-flux cross-product *-algebra, a modified quantum Hamilton constraint, a localised quantum diffeomorphism constraint and even a modified quantum Master constraint, is given.
44 pages, 6 figures

http://arxiv.org/abs/1108.4582
AQV VI. A holonomy groupoid formulation
Diana Kaminski
(Submitted on 19 Aug 2011)
The philosophy of the Loop Quantum Gravity approach is to construct the canonical variables by using the duality of infinitesimal connections and holonomies along loops. Based on this fundamental property for example the holonomy-flux *-algebra has been formulated. A generalisation of the one-to-one correspondence between infinitesimal objects: connections and curvature and path based objects: holonomy maps and parallel transports is used to replace the configuration space of the theory. This generalised duality is related to the concept of path connections and holonomy groupoids, which originally has been invented by Mackenzie and which is presented shortly in this article. Finally these objects are used to propose some new algebras of quantum variables for Loop Quantum Gravity.
27 pages, 1 figure

http://arxiv.org/abs/1108.4670
Charged Quantum Black Holes : Thermal Stability Criterion
Abhishek Majhi, Parthasarathi Majumdar
(Submitted on 23 Aug 2011)
A criterion of thermal stability is derived for electrically charged quantum black holes having large horizon area (compared to Planck area), as an inequality between the mass of the black hole and its microcanonical entropy. The derivation is based on key results of Loop Quantum Gravity and the grand canonical ensemble of equilibrium statistical mechanics, with Gaussian fluctuations around an equilibrium thermal configuration assumed here to be a quantum isolated horizon. No aspect of classical black hole geometry is used in deducing the stability criterion, even though it is tested against known charged black hole solutions as a fiducial check. The equilibrium Hawking temperature is shown to have an additional quantum correction over the semiclassical value. We also discuss the validity of the saddle point approximation used to incorporate thermal fluctuations.
14 pages

 

[marcus]

http://arxiv.org/abs/1108.4686
Lower bound on the spectral dimension near a black hole
S. Carlip, D. Grumiller
(Submitted on 23 Aug 2011)
We consider an evaporating Schwarzschild black hole in a framework in which the spectral dimension of spacetime varies continuously from four at large distances to a number smaller than three at small distances, as suggested by various approaches to quantum gravity. We demonstrate that the evaporation stops when the horizon radius reaches a scale at which spacetime becomes effectively 3-dimensional, and argue that an observer remaining outside the horizon cannot probe the properties of the black hole at smaller scales. This result is universal in the sense that it does not depend on the details of the effective dimension as a function of the diffusion time. Observers falling into the black hole can resolve smaller scales, as can external observers in the presence of a cosmological constant. Even in these cases, though, we obtain an absolute bound D>2 on the effective dimension that can be seen in any such attempt to measure the properties of the black hole.
6 pp, 2 figs

brief mention:
http://arxiv.org/abs/1108.4837
The Dynamics of Shape
Henrique Gomes
(Submitted on 24 Aug 2011)
This thesis consists of two parts, connected by one central theme: the dynamics of the "shape of space". The first part of the thesis concerns the construction of a theory of gravity dynamically equivalent to general relativity (GR) in 3+1 form (ADM). What is special about this theory is that it does not possesses foliation invariance, as does ADM. It replaces that "symmetry" by another: local conformal invariance...
...The second part of the thesis will develop a gauge theory for "shape of space"--theories. To be more precise, if one admits that the physically relevant observables are given by shape, our descriptions of Nature carry a lot of redundancy, namely absolute local size and absolute spatial position. This redundancy is related to the action of the infinite-dimensional conformal and diffeomorphism groups on the geometry of space. ...
137 pages, 7 figures. PhD thesis

http://arxiv.org/abs/1108.4731
Quantisation, Representation and Reduction; How Should We Interpret the Quantum Hamiltonian Constraints of Canonical Gravity?
Karim P. Y. Thebault
(Submitted on 24 Aug 2011)
...These questions will be refined and explored in the context of modern approaches to the quantisation of canonical general relativity.
18 Pages

 

[marcus]

http://arxiv.org/abs/1108.5224
Shape Dynamics
Tim Koslowski
(Submitted on 26 Aug 2011)
General Relativity can be reformulated as a geometrodynamical theory, called Shape Dynamics, that is not based on spacetime (in particular refoliation) symmetry but on spatial diffeomorphism and local spatial conformal symmetry. This leads to a constraint algebra that is (unlike General Relativity) a Lie algebra, where all local constraints are linear in momenta and may thus be quantized as vector fields on the geometrodynamic configuration space. The Hamiltonian of Shape Dynamics is complicated but admits simple expressions whenever spatial derivatives are negligible.
4 pages

http://arxiv.org/abs/1108.5261
On the limits of quantum theory: contextuality and the quantum-classical cut
George F R Ellis
(Submitted on 26 Aug 2011)
This paper is based on four assumptions: 1. Physical reality is made of linearly behaving components combined in non-linear ways. 2. Higher level behaviour emerges from this lower level structure. 3. The way the lower level elements behaves depends on the context in which they are imbedded. 4. Quantum theory applies to the lower level entities. An implication is that higher level effective laws, based in the outcomes of non-linear combinations of lower level linear interactions, will generically not be unitary. This leads to the view that both state vector preparation and the quantum measurement process are crucially based in top-down causal effects, supports the contention that the flow of time is real, and helps provide criteria for the Heisenberg cut that challenge some views on Schroedinger's cat and the existence of the wave function of the universe.
55 pages, 10 diagrams

http://arxiv.org/abs/1108.5240
Conservative entropic forces
Matt Visser (Victoria University of Wellington)
(Submitted on 26 Aug 2011)
Entropic forces have recently attracted considerable attention as ways to reformulate, retrodict, and perhaps even "explain'" classical Newtonian gravity from a rather specific thermodynamic perspective. In this article I point out that if one wishes to reformulate classical Newtonian gravity in terms of an entropic force, then the fact that Newtonian gravity is described by a conservative force places significant constraints on the form of the entropy and temperature functions. (These constraints also apply to entropic reinterpretations of electromagnetism, and indeed to any conservative force derivable from a potential.)
The constraints I will establish are sufficient to present real and significant problems for any reasonable variant of Verlinde's entropic gravity proposal, though for technical reasons the constraints established herein do not directly impact on either Jacobson's or Padmanabhan's versions of entropic gravity. In an attempt to resolve these issues, I will extend the usual notion of entropic force to multiple heat baths with multiple "temperatures'" and multiple "entropies".
21 pages;

 

[marcus]

http://arxiv.org/abs/1108.5389
Phase Transition in Dually Weighted Colored Tensor Models
Dario Benedetti, Razvan Gurau
(Submitted on 26 Aug 2011)
Tensor models are a generalization of matrix models (their graphs being dual to higher-dimensional triangulations) and, in their colored version, admit a 1/N expansion and a continuum limit. We introduce a new class of colored tensor models with a modified propagator which allows us to associate weight factors to the faces of the graphs, i.e. to the bones (or hinges) of the triangulation, where curvature is concentrated. They correspond to dynamical triangulations in three and higher dimensions with generalized amplitudes. We solve analytically the leading order in 1/N of the most general model in arbitrary dimensions. We then show that a particular model, corresponding to dynamical triangulations with a non-trivial measure factor, undergoes a third-order phase transition in the continuum characterized by a jump in the susceptibility exponent.
17 pages, 4 figures

 

[MTd2]

http://arxiv.org/abs/1108.5886

The Possibility of Inflation in Asymptotically Safe Gravity

Sungwook E. Hong, Young Jae Lee, Heeseung Zoe
(Submitted on 30 Aug 2011)
We explore possible inflationary trajectories in the phase space of the Hubble parameter arising from the cubic curvature theories in the context of asymptotically safe gravity, without introducing the inflaton field. We find the slow roll points in the phase space, where most of the e-foldings are attained, and then analyze the asymptotic behaviors around them. The coupling constants and the relative energy scale of inflation to the cutoff should be incorporated to give a successful inflation with more than 60 e-foldings.

http://arxiv.org/abs/1108.6005

No quantum gravity signature from the farthest quasars

Fabrizio Tamburini (1), Carmine Cuofano (2), Massimo Della Valle (3,4), Roberto Gilmozzi (5) ((1) Dept. of Astronomy, University of Padova, Italy, (2) Dept. of Physics, University of Ferrara, Italy, (3) INAF - Osservatorio Astronomico di Capodimonte, Naples, Italy, (4) International Center for Relativistic Astrophysics Network, Pescara, Italy, (5) European Southern Observatory, Garching bei Muenchen, Germany)
(Submitted on 30 Aug 2011)
Context: Strings and other alternative theories describing the quantum properties of space-time suggest that space-time could present a foamy structure and also that, in certain cases, quantum gravity (QG) may manifest at energies much below the Planck scale. One of the observable effects could be the degradation of the diffraction images of distant sources.
Aims: We searched for this degradation effect, caused by QG fluctuations, in the light of the farthest quasars (QSOs) observed by the Hubble Space Telescope with the aim of setting new limits on the fluctuations of the space-time foam and QG models.
Methods: We developed a software that estimates and compares the phase variation in the interference patterns of the high-redshift QSOs, taken from the snapshot survey of HST-SDSS, with those of stars that are expected to not be affected by QG effects. We used a two-parameter function to determine, for each test star and QSO, the maximum of the diffraction pattern and to calculate the Strehl ratio.
Results: Our results go far beyond those already present in the literature. By adopting the most conservative approach where the correction terms, that describe the possibility for space-time fluctuations cumulating across long distances and partially compensate for the effects of the phase variations, are taken into account. We exclude the random walk model and most of the holographic models of the space-time foam. Without considering these correction terms, all the main QG scenarios are excluded. Finally, our results show the absence of any directional dependence of QG effects and the validity of the cosmological principle with an independent method; that is, viewed on a large scale, the properties of the Universe are the same for all observers, including the effects of space-time fluctuations.

 

[marcus]

This seems an intriguing result and may be of use to loop researchers as well as others. The CFT is on a 3D boundary of a 4D spacetime. Due to observed accelerated expansion, the real universe becomes increasingly dS-like, and is not at all AdS. We are already in an approximately dS universe and the approximation gets better all the time. So a result about standard 4D deSitter spacetime immediately sounds a lot more realistic/relevant than the earlier conjectured duality. Mitchell called attention to this one:
http://arxiv.org/abs/1108.5735
Higher Spin Realization of the dS/CFT Correspondence
Dionysios Anninos, Thomas Hartman, Andrew Strominger
(Submitted 29 August 2011)
We conjecture that Vasiliev's theory of higher spin gravity in four-dimensional de Sitter space (dS) is holographically dual to a three-dimensional conformal field theory (CFT) living on the spacelike boundary of dS at future timelike infinity. The CFT is the Euclidean Sp(N) vector model with anticommuting scalars. The free CFT flows under a double-trace deformation to an interacting CFT in the IR. We argue that both CFTs are dual to Vasiliev dS gravity but with different future boundary conditions on the bulk scalar field. Our analysis rests heavily on analytic continuations of bulk and boundary correlators in the proposed duality relating the O(N) model with Vasiliev gravity in AdS.
18 pages

 

[marcus]

http://arxiv.org/abs/1108.6269
The Ising Model on Random Lattices in Arbitrary Dimensions
Valentin Bonzom, Razvan Gurau, Vincent Rivasseau
(Submitted on 31 Aug 2011)
We study analytically the Ising model coupled to random lattices in dimension three and higher. The family of random lattices we use is generated by the large N limit of a colored tensor model generalizing the two-matrix model for Ising spins on random surfaces. We show that, in the continuum limit, the spin system does not exhibit a phase transition at finite temperature, in agreement with numerical investigations. Furthermore we outline a general method to study critical behavior in colored tensor models.
13 pages

 

[MTd2]

http://arxiv.org/abs/1109.0080

Emergent Braided Matter of Quantum Geometry

Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman, Yidun Wan
(Submitted on 1 Sep 2011)
We review and present a few new results of the program of emergent matter as braid excitations of quantum geometry that is represented by braided ribbon networks, which are a generalisation of the spin networks proposed by Penrose and those in models of background independent quantum gravity theories, such as Loop Quantum Gravity and Spin Foam models. This program has been developed in two parallel but complimentary schemes, namely the trivalent and tetravalent schemes. The former studies the trivalent braids on trivalent braided ribbon networks, while the latter investigate the tetravalent braids on tetravalent braided ribbon networks. Both schemes have been fruitful. The trivalent scheme has been quite successful at establishing a correspondence between the trivalent braids and Standard Model particles, whereas the tetravalent scheme has naturally substantiated a rich, dynamical theory of interactions and propagation of tetravalent braids, which is ruled by topological conservation laws. Some recent advances in the program indicate that the two schemes may converge to yield a fundamental theory of matter in quantum spacetime.

http://arxiv.org/abs/1109.0248

Quantum gravity in the very early universe

Martin Bojowald
Comments: 10 pages, plenary talk at "6th International Conference on Physics and Astrophysics of Quark Gluon Plasma" (ICPAQGP 2010), Goa, India
Journal-ref: Nuclear Physics A 862-863 (2011) 98-103
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
General relativity describes the gravitational field geometrically and in a self-interacting way because it couples to all forms of energy, including its own. Both features make finding a quantum theory difficult, yet it is important in the high-energy regime of the very early universe. This review article introduces some of the results for the quantum nature of space-time which indicate that there is a discrete, atomic picture not just for matter but also for space and time. At high energy scales, such deviations from the continuum affect the propagation of matter, the expansion of the universe, and perhaps even the form of symmetries such as Lorentz or CP transformations. All these effects may leave traces detectable by sensitive measurements, as pointed out here by examples.

 

[MTd2]

http://arxiv.org/abs/1109.0118

Emergence of Space-Time from Topologically Homogeneous Causal Networks

Giacomo Mauro D'Ariano, Alessandro Tosini
(Submitted on 1 Sep 2011)
In this paper we study the emergence of Minkowski space-time from a causal network. Differently from previous approaches, we require the network to be topologically homogeneous, so that the metric is derived from pure event-counting. Emergence from events has an operational motivation in requiring that every physical quantity---including space-time---be defined through precise measurement procedures. Topological homogeneity is a requirement for having space-time metric emergent from the pure topology of causal connections, whereas physically corresponds to the universality of the physical law. We analyze in detail the case of 1+1 dimension. Coordinate systems are established via an Einsteinian protocol, and lead to a digital version of the Lorentz transformations. In a computational analogy, the foliation construction can also be regarded as the synchronization with a global clock of the calls to independent subroutines (corresponding to the causally independent events) in a parallel distributed computation, and the Lorentz time-dilation emerges as an increased density of leaves within a single tic-tac of a clock, whereas space-contraction results from the corresponding decrease of density of events per leaf. The operational procedure of building up the coordinate system introduces an in-principle indistinguishability between neighboring events, resulting in a network that is coarse-grained, the thickness of the event being a function of the observer clock. The present simple cinematical construction does not extend straightforwardly to space dimension greater than one, due to anisotropy of the maximal speed: this issue is cured by a superposition of causal paths, specializing the causal network to a quantum computational one.

 

[atyy]

http://arxiv.org/abs/1109.0016
de Sitter gauge theories and induced gravities
R. F. Sobreiro, A. A. Tomaz, V. J. Vasquez Otoya
(Submitted on 31 Aug 2011)
Pure de Sitter and anti de Sitter gauge theories in four-dimensional Euclidean spacetime are studied. It is shown that, if the theory is asymptotic free and a dynamical mass is generated, then an effective geometry can be induced and a gravity theory emerges. The asymptotic freedom and running of the mass might account for an In\"on\"u-Wigner contraction which induces a breaking of the gauge group to the Lorentz one while the mass itself is responsible for the coset sector of the gauge field to be identified with the effective vierbein. Further, the resulting local isometries are Lorentzian for the anti de Sitter group and Euclidean for the de Sitter one.

http://arxiv.org/abs/1109.0036
Decomposition of entanglement entropy in lattice gauge theory
William Donnelly
(Submitted on 31 Aug 2011)
We consider entanglement entropy between regions of space in lattice gauge theory. The Hilbert space corresponding to a region of space includes "edge states" that transform nontrivially under gauge transformations. By decomposing the edge states in irreducible representations of the gauge group, the entropy of an arbitrary state is expressed as the sum of three positive terms: a term associated with the classical Shannon entropy of the distribution of boundary representations, a term that appears only for non-abelian gauge theories and depends on the dimension of the boundary representations, and a term representing non-local correlations. The first two terms are the entropy of the edge states, and depend only on observables measurable at the boundary. These results are applied to several examples of lattice gauge theory states, including the ground state in the strong coupling expansion of Kogut and Susskind. In all these examples we find that the entropy of the edge states is the dominant contribution to the entanglement entropy.

 

[marcus]

http://arxiv.org/abs/1109.0499
Asymptotics of Spinfoam Amplitude on Simplicial Manifold: Lorentzian Theory
Muxin Han, Mingyi Zhang
(Submitted on 2 Sep 2011)
The present paper studies the large-j asymptotics of the Lorentzian EPRL spinfoam amplitude on a 4d simplicial complex with an arbitrary number of simplices. The asymptotics of the spinfoam amplitude is determined by the critical configurations. Here we show that, given a critical configuration in general, there exists a partition of the simplicial complex into three type of regions R_Nondeg, R_Deg-A, R_Deg-B, where the three regions are simplicial sub-complexes with boundaries. The critical configuration implies different types of geometries in different types of regions, i.e. (1) the critical configuration restricted into R_Nondeg implies a nondegenerate discrete Lorentzian geometry, (2) the critical configuration restricted into R_Deg-A is degenerate of type-A in our definition of degeneracy, but implies a nondegenerate discrete Euclidean geometry on R_Deg-A, (3) the critical configuration restricted into R_Deg-B is degenerate of type-B, and implies a vector geometry on R_Deg-B. With the critical configuration, we further make a subdivision of the regions R_Nondeg and R_Deg-A into sub-complexes (with boundary) according to their Lorentzian/Euclidean oriented 4-simplex volume V₄(v), such that sgn(V₄(v)) is a constant sign on each sub-complex. Then in the each sub-complex, the spinfoam amplitude at the critical configuration gives the Regge action in Lorentzian or Euclidean signature respectively on R_Nondeg or R_Deg-A. The Regge action reproduced here contains a sign factor sgn(V₄(v)) of the oriented 4-simplex volume. Therefore the Regge action reproduced here can be viewed a discretized Palatini action with on-shell connection. Finally the asymptotic formula of the spinfoam amplitude is given by a sum of the amplitudes evaluated at all possible critical configurations, which are the products of the amplitudes associated to different type of geometries.
54 pages, 2 figures

http://arxiv.org/abs/1109.0500
Asymptotics of Spinfoam Amplitude on Simplicial Manifold: Euclidean Theory
Muxin Han, Mingyi Zhang
(Submitted on 2 Sep 2011)
We study the large-j asymptotics of the Euclidean EPRL/FK spin foam amplitude on a 4d simplicial complex with arbitrary number of simplices. We show that for a critical configuration (j_f, g_ve, n_ef) in general, there exists a partition of the simplicial complex into three regions: Non-degenerate region, Type-A degenerate region and Type-B degenerate region. On both the non-degenerate and Type-A degenerate regions, the critical configuration implies a non-degenerate Euclidean geometry, while on the Type-B degenerate region, the critical configuration implies a vector geometry. Furthermore we can split the Non-degenerate and Type-A regions into sub-complexes according to the sign of Euclidean oriented 4-simplex volume. On each sub-complex, the spin foam amplitude at critical configuration gives a Regge action that contains a sign factor sgn(V₄(v)) of the oriented 4-simplices volume. Therefore the Regge action reproduced here can be viewed as a discretized Palatini action with on-shell connection. The asymptotic formula of the spin foam amplitude is given by a sum of the amplitudes evaluated at all possible critical configurations, which are the products of the amplitudes associated to different type of geometries.
27 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1109.0740
Observables in gravity: a review
Johannes Tambornino
(Submitted on 4 Sep 2011)
We present an overview on observables in gravity mainly from a loop quantum gravity perspective. The gauge group of general relativity is the diffeomorphism group of the underlying manifold. Consequently, general relativity is a totally constrained theory with vanishing canonical Hamiltonian. This fact, often referred to as the problem of time, provides the main conceptual difficulty towards the construction of gauge-invariant local observables. Nevertheless, within the framework of complete observables, that encode relations between dynamical fields, remarkable progress has been made during the last 20 years. Although analytic control over observables for full gravity is still lacking, perturbative calculations have been performed and within de-parameterizable toy models it was possible for the first time to construct a full set of gauge invariant observables for a background independent field theory. We review these developments and comment on their implications for quantum gravity.
31 pages. contribution for a special issue of SIGMA on Loop Quantum Gravity and Cosmology

 

[marcus]

http://arxiv.org/abs/1109.1085
Non-Commutative Worlds - Classical Constraints, Relativity and the Bianchi Identity
Louis H. Kauffman
(Submitted on 6 Sep 2011)
This paper shows how discrete measurement leads to commutators and how discrete derivatives are naturally represented by commutators in a non-commutative extension of the calculus in which they originally occurred. We show how the square root of minus one (i) arises naturally as a time-sensitive observable for an elementary oscillator. In this sense the square root of minus one is a clock and/or a clock/observer. This sheds new light on Wick rotation, which replaces t (temporal quantity) by it. In this view, the Wick rotation replaces numerical time with elementary temporal observation. The relationship of this remark with the Heisenberg commutator [P,Q]=ihbar is explained in the Introduction. After a review of previous work, the paper begins with a section of iterants - a generalization of the complex numbers as described above. This generalization includes all of matrix algebra in a temporal interpretation. We then give a generalization of the Feynman-Dyson derivation of electromagnetism in the context of non-commutative worlds. This generalization depends upon the definitions of derivatives via commutators and upon the way the non-commutative calculus mimics standard calculus. We then begin a project of examining constraints that link standard and non-commutative calculus, summarizing work Anthony Deakin and formulating problems related to the algebra of constraints. The paper ends with a discussion of the Bianchi identity in non-commutative worlds and with an appendix about the constraint algebra.
34 pages, 7 figures

Brief mention:
http://arxiv.org/abs/1109.1209
Entropy and the uncertainty principle
Rupert L. Frank, Elliott H. Lieb
(Submitted on 6 Sep 2011)
We generalize, improve and unify theorems of Rumin, and Maassen--Uffink about classical entropies associated to quantum density matrices. These theorems refer to the classical entropies of the diagonals of a density matrix in two different bases. Thus they provide a kind of uncertainty principle. Our inequalities are sharp because they are exact in the high-temperature or semi-classical limit.
6 pages

 

[marcus]

http://arxiv.org/abs/1109.1290
Linking covariant and canonical LQG: new solutions to the Euclidean Scalar Constraint
Emanuele Alesci, Thomas Thiemann, Antonia Zipfel
(Submitted on 6 Sep 2011)
It is often emphasized that spin-foam models could realize a projection on the physical Hilbert space of canonical Loop Quantum Gravity (LQG). As a first test we analyze the one-vertex expansion of a simple Euclidean spin-foam. We find that for fixed Barbero-Immirzi parameter gamma=1 the one vertex-amplitude in the KKL prescription annihilates the Euclidean Hamiltonian constraint of LQG. Since for gamma=1 the Lorentzian part of the Hamiltonian constraint does not contribute this gives rise to new solutions of the Euclidean theory. Furthermore, we find that the new states only depend on the diagonal matrix elements of the volume. This seems to be a generic property when applying the spin-foam projector.
33 pages, figures

Brief mention:
http://arxiv.org/abs/1109.1437
Scale-Dependent Newton's Constant G in the Conformal Newtonian Gauge
Herbert W. Hamber, Reiko Toriumi
(Submitted on 7 Sep 2011)
In classical gravity deviations from the predictions of the Einstein theory are often discussed within the framework of the conformal Newtonian gauge, where scalar perturbations are described by two potentials phi and psi. In this paper we use the above gauge to explore possible cosmological consequences of a running Newton's constant G, as suggested by the nontrivial ultraviolet fixed point scenario arising from the quantum field-theoretic treatment of Einstein gravity with a cosmological constant term...

 

[marcus]

http://arxiv.org/abs/1109.2530
Discrete Wheeler-DeWitt Equation
Herbert W. Hamber, Ruth M. Williams
(Submitted on 12 Sep 2011)
We present a discrete form of the Wheeler-DeWitt equation for quantum gravitation, based on the lattice formulation due to Regge. In this setup the infinite-dimensional manifold of 3-geometries is replaced by a space of three-dimensional piecewise linear spaces, with the solutions to the lattice equations providing a suitable approximation to the continuum wave functional. The equations incorporate a set of constraints on the quantum wavefunctional, arising from the triangle inequalities and their higher dimensional analogs. The character of the solutions is discussed in the strong coupling (large G) limit, where it is shown that the wavefunctional only depends on geometric quantities, such as areas and volumes. An explicit form, determined from the discrete wave equation supplemented by suitable regularity conditions, shows peaks corresponding to integer multiples of a fundamental unit of volume. An application of the variational method using correlated product wavefunctions suggests a relationship between quantum gravity in n+1 dimensions, and averages computed in the Euclidean path integral formulation in n dimensions. The proposed discrete equations could provide a useful, and complementary, computational alternative to the Euclidean lattice path integral approach to quantum gravity.
45 pages, 8 figures

 

[MTd2]

One more string theorist with an "alternative theory" of QG, now based on holography, similar to Verlinde's. He even cites his paper:

http://arxiv.org/abs/1109.2435

Holographic Space-Time: The Takeaway

T. Banks
(Submitted on 12 Sep 2011)
The theory of holographic space-time (HST) generalizes both string theory and quantum field theory. It provides a geometric rationale for supersymmetry (SUSY) and a formalism in which super-Poincare invariance follows from Poincare invariance. HST unifies particles and black holes, realizing both as excitations of non-commutative geometrical variables on a holographic screen. Compact extra dimensions are interpreted as finite dimensional unitary representations of super-algebras, and have no moduli. Full field theoretic Fock spaces, and continuous moduli are both emergent phenomena of super-Poincare invariant limits in which the number of holographic degrees of freedom goes to infinity. Finite radius de Sitter (dS) spaces have no moduli, and break SUSY with a gravitino mass scaling like $\Lambda^{1/4}$. We present a holographic theory of inflation and fluctuations. The inflaton field is an emergent concept, describing the geometry of an underlying HST model, rather than "a field associated with a microscopic string theory". We argue that the phrase in quotes is meaningless in the HST formalism.

 

[MTd2]

Debunks Kobakhidze regarding the failure of the neutron interference experiments to be explained by Verlinde's gravity. There was a recent paper of his, but I didn't post it because I didn't want to feed two trolls.

http://arxiv.org/abs/1109.2794

On entropic gravity: the entropy postulate, entropy content of screens and relation to quantum mechanics

Masud Chaichian, Markku Oksanen, Anca Tureanu
(Submitted on 13 Sep 2011)
We consider the controversial hypothesis that gravity is an entropic force that has its origin in the thermodynamics of holographic screens. Several key aspects of entropic gravity are discussed. In particular, we revisit and elaborate on our criticism of the recent claim that entropic gravity fails to explain observations involving gravitationally-bound quantum states of neutrons in the GRANIT experiment and gravitationally induced quantum interference. We argue that the analysis leading to this claim is troubled by a misinterpretation concerning the relation between the microstates of a holographic screen and the state of a particle in the emergent space, engendering inconsistencies. A point of view that could resolve the inconsistencies is presented. We expound the general idea of the aforementioned critical analysis of entropic gravity in such a consistent setting. This enables us to clarify the problem and to identify a premise whose validity will decide the faith of the criticism against entropic gravity. It is argued that in order to reach a sensible conclusion we need more detailed knowledge on entropic gravity. These arguments are relevant to any theory of emergent space, where the entropy of the microscopic system depends on the distribution of matter in the emergent space.

 

[MTd2]

http://arxiv.org/abs/1109.2789v1

Conformal and Affine Hamiltonian Dynamics of General Relativity

V.N. Pervushin, A.B. Arbuzov, B.M. Barbashov, R.G. Nazmitdinov, A. Borowiec, K.N. Pichugin, A.F. Zakharov
(Submitted on 13 Sep 2011)
The Hamiltonian approach to the General Relativity is formulated as a joint nonlinear realization of conformal and affine symmetries via the Dirac scalar dilaton and spin-connection coefficients. The dominance of the Casimir vacuum energy of physical fields provides a good description of the type Ia supernova luminosity distance--redshift relation. The uncertainty principle at Planck's epoch yields the hierarchy of the Universe energy scales which is supported by the observational data. The solution of Hamiltonian constraints yields new Jeans-like oscillations of Schwarzschild-type potentials.
We found that due to the affine symmetry condition for spin-connection coefficients there is only one degree of freedom of strong gravitational waves. Hamiltonian dynamics of the model gives the effect of an intensive vacuum creation of gravitons and Higgs bosons in the Early Universe.

 

[MTd2]

http://arxiv.org/abs/1109.2609

General relativity limit of Horava-Lifgarbagez gravity with a scalar field in gradient expansion

A.Emir Gumrukcuoglu, Shinji Mukohyama, Anzhong Wang
(Submitted on 12 Sep 2011)
We present a fully nonlinear study of long wavelength cosmological perturbations within the framework of the projectable Horava-Lifgarbagez gravity, coupled to a single scalar field. Adopting the gradient expansion technique, we explicitly integrate the dynamical equations up to any order of the expansion, then restrict the integration constants by imposing the momentum constraint. While the gradient expansion relies on the long wavelength approximation, amplitudes of perturbations do not have to be small. When the $\lambda\to 1$ limit is taken, the obtained nonlinear solutions exhibit a continuous behavior at any order of the gradient expansion, recovering general relativity in the presence of a scalar field and the "dark matter as an integration constant". This is in sharp contrast to the results in the literature based on the "standard" (and naive) perturbative approach where in the same limit, the perturbative expansion of the action breaks down and the scalar graviton mode appears to be strongly coupled. We carry out a detailed analysis on the source of these apparent pathologies and determine that they originate from an improper application of the perturbative approximation in the momentum constraint. We also show that there is a new branch of solutions, valid in the regime where $|\lambda-1|$ is smaller than the order of perturbations. In the limit $\lambda\to1$, this new branch allows the theory to be continuously connected to general relativity (plus "dark matter").

 

[MTd2]

http://arxiv.org/abs/1109.3062

Scale-dependent Planck mass and Higgs VEV from holography and functional renormalization

Daniel Litim, Roberto Percacci, Leslaw Rachwal
(Submitted on 14 Sep 2011)
We compute the scale-dependence of the Planck mass and of the vacuum expectation value of the Higgs field using two very different renormalization group methods: a "holographic" procedure based on Einstein's equations in five dimensions with matter confined to a 3-brane, and a "functional" procedure in four dimensions based on a Wilsonian momentum cutoff. Both calculations lead to very similar results, suggesting that the coupled theory approaches a non-trivial fixed point in the ultraviolet.

 

[marcus]

http://arxiv.org/abs/1109.3572
Loop gravity in terms of spinors
Etera R. Livine, Johannes Tambornino
(Submitted on 16 Sep 2011)
We show that loop gravity can equally well be formulated in in terms of spinorial variables (instead of the group variables which are commonly used), which have recently been shown to provide a direct link between spin network states and discrete geometries. This results in a new, unitarily equivalent formulation of the theory on a generalized Bargmann space. Since integrals over the group are exchanged for straightforward integrals over the complex plane we expect this formalism to be useful to efficiently organize practical calculations.
4 pages, based on a talk given at Loops '11, Madrid, to appear in Journal of Physics: Conference Series

brief mention
http://arxiv.org/abs/1109.3448
An Entropic Story
Jeremy Bernstein
(Submitted on 15 Sep 2011)
A pedagogical account of entropy and its history.

 

[marcus]

http://arxiv.org/abs/1109.3946
Parity and reality properties of the EPRL spinfoam
Yasha Neiman
(Submitted on 19 Sep 2011)
We study the parity behavior of the Lorentzian EPRL spinfoam model. We demonstrate that the vertex amplitude does not depend on the sign of the Immirzi parameter. We present numerical results for the transition amplitude and the graviton propagator in the large-spin 4-simplex approximation. The results suggest a simple relation between the contributions of the two parity-related critical points. Finally, we observe that the graviton propagator is not invariant under parity-odd permutations of equivalent nodes. Thus, the Lorentzian model has the same chirality problem as the Euclidean.
31 pages, 1 figure

brief mention:
(Would seem to be work at level of Master's thesis, which could serve a useful purpose)
http://arxiv.org/abs/1109.3879
Introductory Causal Dynamical Triangulation
Alex Forcier
(Submitted on 18 Sep 2011)
This report aims to present the main ideas of Regge calculus necessary to understand the basic premise of CDT. Next, the main strategy of the CDT approach is introduced in general terms. The main focus of this report is the 2-D model of CDT...independent work in attempting to find and prove how the results presented were obtained by CDT authors. Because these derivations were made explicit by the author, this paper can act as a guide to those who are new to CDT.

 

[marcus]

http://arxiv.org/abs/1109.4239
Probing Loop Quantum Gravity with Evaporating Black Holes
Aurelien Barrau, Xiangyu Cao, Jacobo Diaz-Polo, Julien Grain, Thomas Cailleteau
(Submitted on 20 Sep 2011)
This letter aims at showing that the observation of evaporating black holes should allow distinguishing between the usual Hawking behavior and Loop Quantum Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.
5 pages, 3 figures

brief mention:
http://arxiv.org/abs/1109.4187
Inflationary Scalars Don't Affect Gravitons at One Loop
Sohyun Park, R. P. Woodard (University of Florida)
(Submitted on 19 Sep 2011)
Primordial inflation results in the production of a vast ensemble of highly infrared, massless, minimally coupled scalars. We use a recent fully renormalized computation of the one loop contribution to the graviton self-energy from these scalars to show that they have no effect on the propagation of dynamical gravitons. Our computation motivates a conjecture for the first correction to the vacuum state wave functional of gravitons...
21 pages

 

[marcus]

http://arxiv.org/abs/1109.4688
Loop quantum gravity vacuum with nondegenerate geometry
Tim Koslowski, Hanno Sahlmann
(Submitted on 22 Sep 2011)
In loop quantum gravity, states of the gravitational field turn out to be excitations over a vacuum state that is sharply peaked on a degenerate spatial geometry. While this vacuum is singled out as fundamental due to its invariance properties, it is also important to consider states that describe non-degenerate geometries. Such states have features of Bose condensate ground states. We discuss their construction for the Lie-algebra as well as the Weyl-algebra setting, and point out possible applications in effective field theory, Loop Quantum Cosmology, as well as further generalizations.
15 pages; prepared for special issue "Loop Quantum Gravity and Cosmology" of the journal SIGMA

http://arxiv.org/abs/1109.4927
Coarse graining methods for spin net and spin foam models
Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito
(Submitted on 22 Sep 2011)
We undertake first steps in making a class of discrete models of quantum gravity, spin foams, accessible to a large scale analysis by numerical and computational methods. In particular, we apply Migdal-Kadanoff and Tensor Network Renormalization schemes to spin net and spin foam models based on finite Abelian groups and introduce `cutoff models' to probe the fate of gauge symmetries under various such approximated renormalization group flows. For the Tensor Network Renormalization analysis, a new Gauss constraint preserving algorithm is introduced to improve numerical stability and aid physical interpretation. We also describe the fixed point structure and establish an equivalence of certain models.
39 pages, 13 figures, 1 table

http://arxiv.org/abs/1109.4812
Colored Tensor Models - a review
Razvan Gurau, James P. Ryan
(Submitted on 22 Sep 2011)
Colored tensor models have recently burst onto the scene as a promising conceptual and computational tool in the investigation of problems of random geometry in dimension three and higher. We present a snapshot of the cutting edge in this rapidly expanding research field. Colored tensor models have been shown to share many of the properties of their direct ancestor, matrix models, which encode a theory of fluctuating two-dimensional surfaces. These features include the possession of Feynman graphs encoding topological spaces, a 1/N expansion of graph amplitudes, embedded matrix models inside the tensor structure, a resumable leading order with critical behavior and a continuum large volume limit, Schwinger-Dyson equations satisfying a Lie algebra (akin to the Virasoro algebra in two dimensions), non-trivial classical solutions and so on. In this review, we give a detailed introduction of colored tensor models and pointers to current and future research directions.
80 pages

brief mention:
http://arxiv.org/abs/1109.4632
A new cosmological distance measure using AGN
D. Watson (1), K. D. Denney (1), M. Vestergaard (1), T. M. Davis (2) ((1) Dark Cosmology Centre, U. Copenhagen, (2) U. Queensland)
(Submitted on 21 Sep 2011)
Accurate distances to celestial objects are key to establishing the age and energy density of the Universe and the nature of dark energy. A distance measure using active galactic nuclei (AGN) has been sought for more than forty years, as they are extremely luminous and can be observed at very large distances. We report here the discovery of an accurate luminosity distance measure using AGN. We use the tight relationship between the luminosity of an AGN and the radius of its broad line region established via reverberation mapping to determine the luminosity distances to a sample of 38 AGN. All reliable distance measures up to now have been limited to moderate redshift -- AGN will, for the first time, allow distances to be estimated to z~4, where variations of dark energy and alternate gravity theories can be probed.
Comments: Apj in press; 5 pages, 3 figures

 

[MTd2]

http://arxiv.org/abs/1109.4897

Measurement of the neutrino velocity with the OPERA detector in the CNGS beam

OPERA
(Submitted on 22 Sep 2011)
The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km with much higher accuracy than previous studies conducted with accelerator neutrinos. The measurement is based on high-statistics data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies. An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (60.7 \pm 6.9 (stat.) \pm 7.4 (sys.)) ns was measured. This anomaly corresponds to a relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.48 \pm 0.28 (stat.) \pm 0.30 (sys.)) \times 10-5.

 

[marcus]

http://arxiv.org/abs/1109.5023
The kernel and the injectivity of the EPRL map
Wojciech Kaminski, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 23 Sep 2011)
In this paper we prove injectivity of the EPRL map for |gamma|<1, filling the gap of our previous paper.
17 pages, 3 figures
========
my comment: this augments the results in
[3] Kamiński W,Kisielowski M, Lewandowski J (2010), Spin-Foams for All Loop Quantum Gravity, Class. Quantum Grav. 27 095006 (Preprint arXiv:0909.0939v2)
[4] Kamiński W,Kisielowski M, Lewandowski J (2010), The EPRL intertwiners and corrected partition function, Class. Quantum Grav. 27 165020 (Preprint arXiv:0912.0540v1)
========

 

[marcus]

http://arxiv.org/abs/1109.5793
Chern-Simons expectation values and quantum horizons from LQG and the Duflo map
Hanno Sahlmann, Thomas Thiemann
(Submitted on 27 Sep 2011)
We report on a new approach to the calculation of Chern-Simons theory expectation values, using the mathematical underpinnings of loop quantum gravity, as well as the Duflo map, a quantization map for functions on Lie-algebras. These new developments can be used in the quantum theory for certain types of black hole horizons, and they may offer new insights for loop quantum gravity, Chern-Simons theory and the theory of quantum groups.
5 pages, 2 figures

 

[John86]

http://arxiv.org/abs/1109.5353
Action principle for the Fluid-Gravity correspondence and emergent gravity
Authors: Sanved Kolekar, T. Padmanabhan
(Submitted on 25 Sep 2011)
Abstract: It has been known for a long time that Einstein's field equations when projected onto a black hole horizon looks very similar to a Navier-Stokes equation in suitable variables. More recently, it was shown that the projection of Einstein's equation on to any null surface in any spacetime reduces exactly to the Navier-Stokes form when viewed in the freely falling frame. We develop an action principle, the extremization of which leads to the above result, in an arbitrary spacetime. The degrees of freedom varied in the action principle are the null vectors in the spacetime and not the metric tensor. The same action principle was introduced earlier in the context of emergent gravity paradigm wherein it was shown that the corresponding Lagrangian can be interpreted as the entropy density of spacetime. The current analysis strengthens this interpretation and reinforces the idea that field equations in gravity can be thought of as emergent. We also find that the degrees of freedom on the null surface are equivalent to a fluid with equation of state PA = TS. We demonstrate that the same relation arises in the context of a spherical shell collapsing to form a horizon.

http://arxiv.org/abs/1109.5383
Can Electroweak Theory Without A Higgs Particle Be Renormalizable?
Authors: J. W. Moffat
(Submitted on 25 Sep 2011)
Abstract: Whether there exists a massive electroweak (EW) theory, without a Higgs spontaneous symmetry breaking mechanism, that is gauge invariant and renormalizable is investigated. A Stueckelberg formalism for massive $W$ and $Z$ bosons is used to derive a gauge invariant EW theory. Negative energy scalar fields that emerge from the gauge invariance of the Lagrangian are removed by invoking an indefinite metric in Hilbert space. A unitary S-matrix and a positive energy spectrum can be obtained by using the PT symmetric formulation of the pseudo-Hermitian Hamiltonian. The theory predicts that if for a system of particles the scalar boson energy $E_s < \lambda^{1/2}M_W$, where $\lambda$ is a gauge parameter and $M_W$ is the $W$ boson mass, then as $\lambda\rightarrow\infty$ the scalar boson mass $\mu=\lambda^{1/2}M_W$ tends to infinity. The theory is perturbatively renormalizable and does not violate longitudinally polarized $W_L W_L\rightarrow W_L W_L$ scattering in the energy range $E < \lambda^{1/2}M_W$ for which the scalar bosons have an undetected mass. This means that with this scenario the EW theory can only be treated as an effective renomalizable theory and not as a UV complete theory.

http://arxiv.org/abs/1109.5521
Non-commutative geometry and matrix models
Authors: Harold Steinacker
(Submitted on 26 Sep 2011)
Abstract: These notes provide an introduction to the noncommutative matrix geometry which arises within matrix models of Yang-Mills type. Starting from basic examples of compact fuzzy spaces, a general notion of embedded noncommutative spaces (branes) is formulated, and their effective Riemannian geometry is elaborated. This class of configurations is preserved under small deformations, and is therefore appropriate for matrix models. A realization of generic 4-dimensional geometries is sketched, and the relation with spectral geometry and with noncommutative gauge theory is explained. In a second part, dynamical aspects of these matrix geometries are discussed. The one-loop effective action for the maximally supersymmetric IKKT or IIB matrix model is discussed, which is well-behaved on 4-dimensional branes.

 

[John86]

Maybe not immediatly related, but interesting


http://arxiv.org/abs/1109.6190
Quantum anomalies and Newtonian gravity on quantum spacetime
Authors: Shahn Majid
(Submitted on 28 Sep 2011)
Abstract: It is known that sufficiently noncommutative quantum spacetimes do not admit covariant differential calculi of classical dimensions. The bicrossproduct model $\lambda$-Minkowski (or `$\kappa$-Minkowski') quantum spacetime introduced by the author and H. Ruegg has such an anomaly for the action of the Poincare quantum group which was resolved by an extra cotangent direction $\theta'$ not visible classically. We show that gauging a coefficient of $\theta'$ introduces gravity into the model. Effects include an induced constant term in the potential energy and a weakening of gravity as the test particle mass increases.

 

[MTd2]

http://arxiv.org/abs/1109.6538

Lorentzian spinfoam propagator

Eugenio Bianchi, You Ding
(Submitted on 29 Sep 2011)
The two-point correlation function is calculated in the Lorentzian EPRL spinfoam model, and shown to match with the one in Regge calculus in a proper limit: large boundary spins, and small Barbero-Immirzi parameter, keeping the size of the quantum geometry finite and fixed. Compared to the Euclidean case, the definition of a Lorentzian boundary state involves a new feature: the notion of past- and future-pointing intertwiners. The semiclassical correlation function is obtained for a time-oriented semiclassical boundary state.

 

[marcus]

http://arxiv.org/abs/1109.6636
Contrasting features of anisotropic loop quantum cosmologies: the role of spatial curvature
Brajesh Gupt, Parampreet Singh
(Submitted on 29 Sep 2011)
A characteristic feature of loop quantization of the isotropic and Bianchi-I spacetimes is the existence of universal bounds on the energy density and the expansion and shear scalars, independent of the matter content. In this work, we investigate the properties of these physical quantities in the Bianchi-II and Bianchi-IX spacetimes which have been recently loop quantized using the connection operator approach. Using the effective Hamiltonian approach, we show that energy density and the expansion and shear scalars turn out to be bounded, albeit not by universal values. In contrast to the isotropic and Bianchi-I models, there are important differences in the way underlying quantum geometry effects lead to these bounds. In particular, the role of energy conditions and the inverse triad modifications are shown to be significant. These results bring out subtle physical distinctions between the quantization using holonomies over closed loops performed for isotropic and Bianchi-I models and the connection operator approach. We find that qualitative differences in physics exist for these quantization methods even for the isotropic models in the presence of spatial curvature. As an illustration, we show that, unlike the holonomy based quantization, the expansion scalar in the effective spacetime description corresponding to the connection operator approach is unbounded for isotropic spatially closed and open models.
26 pages, 3 figures

brief mention:
http://arxiv.org/abs/1109.6462
Collapse of the State Vector
Steven Weinberg
(Submitted on 29 Sep 2011)
Modifications of quantum mechanics are considered, in which the state vector of any system, large or small, undergoes a stochastic evolution. The general class of theories is described, in which the probability distribution of the state vector collapses to a sum of delta functions, one for each possible final state, with coefficients given by the Born rule.
11 pages

 

[marcus]

http://arxiv.org/abs/1109.6801
Introduction to loop quantum cosmology
Kinjal Banerjee, Gianluca Calcagni, Mercedes Martín-Benito
(Submitted on 30 Sep 2011)
This is an introduction to loop quantum cosmology (LQC) reviewing mini- and midisuperspace models as well as homogeneous and inhomogeneous effective dynamics.
94 pages, 1 figure. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

 

[marcus]

http://arxiv.org/abs/1110.0291
Inflationary spectra and observations in loop quantum cosmology
Gianluca Calcagni
(Submitted on 3 Oct 2011)
We review some recent progress in the extraction of inflationary observables in loop quantum cosmology. Inverse-volume quantum corrections induce a growth of power in the large-scale cosmological spectra and are constrained by observations.
4 pages, 2 figures. To appear in the Proceedings of Loops 11, Madrid, 23-28 May 2011 (J. Phys.: Conf. Ser.)

 

[marcus]

http://arxiv.org/abs/1110.0646
Numerical techniques in loop quantum cosmology
D. Brizuela, D. Cartin, G. Khanna
(Submitted on 4 Oct 2011)
In this article, we review the use of numerical techniques to obtain solutions for the quantum Hamiltonian constraint in loop quantum cosmology (LQC). First, we summarize the basic features of LQC, and describe features of the constraint equations to solve -- generically, these are difference (rather than differential) equations. Important issues such as differing quantization methods, stability of the solutions, the semi-classical limit, and the relevance of lattice refinement in the difference equations are discussed. Finally, the cosmological models already considered in the literature are listed, along with typical features in these models and open issues.
26 pages, 1 figure; prepared for SIGMA's special issue "Loop Quantum Gravity and Cosmology"

http://arxiv.org/abs/1110.0686
Emergent models for gravity: an overview
L. Sindoni
(Submitted on 4 Oct 2011)
We give a critical overview of various attempts to describe gravity as an emergent phenomenon, starting from examples of condensed matter physics, to arrive to more sophisticated pregeometric models. The common line of thought is to view the graviton as a composite particle/collective mode. However, we will describe many different ways in which this idea is realized in practice.
Comments: 54 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

brief mention
http://arxiv.org/abs/1110.0549
On the origin of probability in quantum mechanics
Stephen D.H. Hsu

http://arxiv.org/abs/1110.0496
Eternal Symmetree
Daniel Harlow, Stephen Shenker, Douglas Stanford, Leonard Susskind

 

[marcus]

brief mention:
http://arxiv.org/abs/1110.1575
Some Considerations on Discrete Quantum Gravity
Gabriele Gionti, S.J.
(Submitted on 7 Oct 2011)
Recent results in Local Regge Calculus are confronted with Spin Foam Formalism. Introducing Barrett-Crane Quantization in Local Regge Calculus makes it possible to associate a unique Spin j_h with an hinge h, fulfilling one of the requirements of Spin Foam definition. It is shown that inter-twiner terms of Spin Foam can follow from the closure constraint in Local Regge Calculus.
Dedicated to Beppe Marmo for his 65th Birthday
9 pages, FunInGeo Conference proccedings, Ischia-Italy, 08-12 June 2011

http://arxiv.org/abs/1110.1539
Lattice diffeomorphism invariance
C.Wetterich
(Submitted on 7 Oct 2011)
We propose a lattice counterpart of diffeomorphism symmetry in the continuum. A functional integral for quantum gravity is regularized on a discrete set of space-time points. When the space-time points are positioned as discrete points of a continuous manifold, the lattice action can be defined in terms of average degrees of freedom within local cells and lattice derivatives. Lattice diffeomorphism invariance is realized if the action is independent of the positioning of the space-time points. Regular as well as rather irregular lattices are then described by the same action. The continuum limit of an action with lattice diffeomorphism invariance is invariant under general coordinate transformations - the basic ingredient for general relativity. Lattice diffeomorphism invariant actions are formulated without introducing a metric as fundamental degree of freedom. The latter rather arises as the expectation value of a suitable collective field. As examples, we present lattice diffeomorphism invariant actions for lattice spinor gravity and a bosonic non-linear sigma-model.
8 pages
[Strange that Wetterich does not cite Bianca Dittrich's work on this exact topic.]

http://arxiv.org/abs/1110.1492
A no-go for no-go theorems prohibiting cosmic acceleration in extra dimensional models
Rik Koster, Marieke Postma
(Submitted on 7 Oct 2011)
A four-dimensional effective theory that arises as the low-energy limit of some extra-dimensional model is constrained by the higher dimensional Einstein equations. Steinhardt & Wesley use this to show that accelerated expansion in our four large dimensions can only be transient in a large class of Kaluza-Klein models that satisfy the (higher dimensional) null energy condition [1]. We point out that these no-go theorems are based on a rather ad-hoc assumption on the metric, without which no strong statements can be made.
20 pages
[We will have to see how Steinhardt and Wesley respond to this.]

 

[marcus]

http://arxiv.org/abs/1110.1941
Loop quantum cosmology of the Bianchi I model: complete quantization
M Martín-Benito, L J Garay, G A Mena Marugán, E. Wilson-Ewing
(Submitted on 10 Oct 2011)
We complete the canonical quantization of the vacuum Bianchi I model within the improved dynamics scheme of loop quantum cosmology, characterizing the Hilbert structure of the physical states and providing a complete set of observables acting on them. In order to achieve this task, it has been essential to determine the structure of the separable superselection sectors that arise owing to the polymeric quantization, and to prove that the initial value problem obtained when regarding the Hamiltonian constraint as an evolution equation, interpreting the volume as the evolution parameter, is well-posed.
4 pages, Proceedings of Loops'11, Madrid, to appear in Journal of Physics: Conference Series

http://arxiv.org/abs/1110.2157
Lessons from toy-models for the dynamics of loop quantum gravity
Valentin Bonzom, Alok Laddha
(Submitted on 10 Oct 2011)
We review some approaches to the Hamiltonian dynamics of (loop) quantum gravity, the main issues being the regularization of the Hamiltonian and the continuum limit. First, Thiemann's definition of the quantum Hamiltonian is presented, and then more recent approaches. They are based on toy models which provide new insights into the difficulties and ambiguities faced in Thiemann's construction. The models we use are parametrized field theories, the topological BF model of which a special case is three-dimensional gravity which describes quantum flat space, and Regge lattice gravity.
41 pages

 

[marcus]

http://arxiv.org/abs/1110.2429
The role of time in relational quantum theories
Sean Gryb, Karim Thebault
(Submitted on 11 Oct 2011)
We propose a solution to the problem of time for systems with a single global Hamiltonian constraint. Our solution stems from the observation that, for these theories, conventional gauge theory methods fail to capture the classical dynamics of the full system. We propose a new strategy for consistently quantizing systems with a relational notion of time that captures the full classical dynamics of the system and allows for evolution parametrized by an equitable internal clock. This proposal contains the minimal temporal structure necessary to retain the ordering of events required to describe classical evolution. In the context of shape dynamics, an equivalent formulation of general relativity that is locally scale invariant and free of the local problem of time, our proposal constitutes a natural methodology for describing dynamical evolution in quantum gravity.
19 pages, 2 figures

 

[John86]

http://arxiv.org/abs/1110.2768
Rapidly-rotating non-Kerr black holes: an open window on the quantum gravity region
Authors: Cosimo Bambi, Francesco Caravelli, Leonardo Modesto
(Submitted on 12 Oct 2011)
Abstract: Recently, two of us have argued that non-Kerr black holes in gravity theories different from General Relativity may have a topologically non-trivial event horizon. More precisely, the spatial topology of the horizon of non-rotating and slow-rotating objects would be a 2-sphere, like in Kerr space-time, while it would change above a critical value of the spin parameter. When the topology of the horizon changes, the black hole central singularity shows up. The accretion process from a thin disk can potentially overspin these black holes and induce the topology transition, violating the Weak Cosmic Censorship Conjecture. If the astrophysical black hole candidates are not the black holes predicted by General Relativity, we might have the quite unique opportunity to see their central region, where classical physics breaks down and quantum gravity effects must appear. Even if the quantum gravity region turned out to be extremely small, at the level of the Planck scale, the size of its apparent image would be finite and potentially observable with future facilities

http://arxiv.org/abs/1110.2199
False loss of coherence
Authors: William G. Unruh
(Submitted on 10 Oct 2011)
Abstract: The loss of coherence of a quantum system coupled to a heat bath as expressed by the reduced density matrix is shown to lead to the miss-characterization of some systems as being incoherent when they are not. The spin boson problem and the harmonic oscillator with massive scalar field heat baths are given as examples of reduced incoherent density matrices which nevertheless still represent perfectly coherent systems.

http://arxiv.org/abs/1110.2217
Temperature of a Decoherent Oscillator with Strong Coupling
Authors: William G. Unruh
(Submitted on 10 Oct 2011)
Abstract: The temperature of an oscillator coupled to the vacuum state of a heat bath via ohmic coupling is non-zero, as measured by the reduced density matrix of the oscillator. This paper shows that the actual temperature, as measured by a thermometer is still zero (or in the thermal state of the bath, the temperature of the bath). The decoherence temperature is due to "false-decoherence", with the heat bath state being dragged along with the oscillator.

 

[marcus]

http://arxiv.org/abs/1110.2905
General covariant transition amplitudes in quantum cosmology
Francesca Vidotto
(Submitted on 13 Oct 2011)
The path-integral approach to cosmology consists in the computation of transition amplitudes between states of the quantum geometry of the universe. In the past, the concrete computation of these transitions amplitudes has been performed in a perturbative regime, breaking the full general covariance of the theory. Here I present how it is possible to define a general covariant path integral in quantum cosmology, by relying on the most recent results of the canonical and covariant formulations of Loop Quantum Gravity. I present two strategies that have been implemented. The first starts from the full Spinfoam theory, i.e. the path-integral framework for Loop Quantum Gravity, and defines a cosmological system. This is not obtained from symmetry reduced variables that are successively quantized, but directly considering the approximations that are characteristic of the full theory. The Spinfoam Cosmology obtained in this way includes quantum fluctuations beyond standard perturbation theory. The second strategy exploits the Hamiltonian constraint of Loop Quantum Cosmology, that is exponentiated in the formal expression of the usual path integral. The result is a general covariant path integral, that reproduces the form of the amplitude in the full Spinfoam theory. Therefore, this procedure connects the canonical and the covariant formalisms.
5 pages. Review for Scientifica Acta

http://arxiv.org/abs/1110.3020
Learning about quantum gravity with a couple of nodes
Enrique F. Borja, Iñaki Garay, Francesca Vidotto
(Submitted on 13 Oct 2011)
Loop Quantum Gravity provides a natural truncation of the infinite degrees of freedom of gravity, obtained by studying the theory on a given finite graph. We review this procedure and we present the construction of the canonical theory on a simple graph, formed by only two nodes. We review the U(N) framework, which provides a powerful tool for the canonical study of this model, and a formulation of the system based on spinors. We consider also the covariant theory, which permits to derive the model from a more complex formulation, paying special attention to the cosmological interpretation of the theory.
43 pages, 8 figures. Prepared for SIGMA's special issue on Loop Quantum Gravity and Cosmology

 

[marcus]

http://arxiv.org/abs/1110.3272
A new Hamiltonian for the Topological BF phase with spinor networks
Valentin Bonzom, Etera R. Livine
(Submitted on 14 Oct 2011)
We describe fundamental equations which define the topological ground states in the lattice realization of the SU(2) BF phase. We introduce a new scalar Hamiltonian, based on recent works in quantum gravity and topological models, which is different from the plaquette operator. Its gauge-theoretical content at the classical level is formulated in terms of spinors. The quantization is performed with Schwinger's bosonic operators on the links of the lattice. In the spin network basis, the quantum Hamiltonian yields a difference equation based on the spin 1/2. In the simplest case, it is identified as a recursion on Wigner 6j-symbols. We also study it in different coherent states representations, and compare with other equations which capture some aspects of this topological phase.
40 pages

http://arxiv.org/abs/1110.3278
Curved geometry and Graphs
Francesco Caravelli
(Submitted on 14 Oct 2011)
Quantum Graphity is an approach to quantum gravity based on a background independent formulation of condensed matter systems on graphs. We summarize recent results obtained on the notion of emergent geometry from the point of view of a particle hopping on the graph. We discuss the role of connectivity in emergent Lorentzian perturbations in a curved background and the BoseHubbard (BH) model defined on graphs with particular symmetries.
Comments: welcome. 4pp, 1 fig. Proceedings of Loops'11 Conference, Madrid

 

[marcus]

http://arxiv.org/abs/1110.3416
Revisiting canonical gravity with fermions
Ghanashyam Date
(Submitted on 15 Oct 2011)
Fermions constitute an important component of matter and their quantization in presence of dynamical gravity is essential for any theory of quantum gravity. We revisit the classical formulation adapted for a background free quantization. The analysis is carried out with the Hilbert-Palatini form for gravity together with the Nieh-Yan topological term which keeps the nature of Barbero-Immirzi parameter independent of inclusion of arbitrary matter with arbitrary couplings. With dynamical gravity, a priori, there are two distinct notions of `parity' - orientation reversing diffeomorphisms and improper Lorentz rotations. The invariance properties of the action and the canonical framework are different with respect to these and gravitational origin of parity violation seems ambiguous.
29 pages

 

[marcus]

http://arxiv.org/abs/1110.4055
A local first law for isolated horizons
Ernesto Frodden, Amit Ghosh, Alejandro Perez
(Submitted on 18 Oct 2011)
We show that isolated horizons satisfy a unique form of first law (in analogy to the first law of stationary black holes) if a local physical input is introduced. Our considerations single out a unique notion of isolated horizon (local) energy E_{IH}=A/(8\pi \ell) and (local) surface gravity \kappa_{IH}=1/\ell where A is the isolated horizon area and \ell is a proper length characterizing the distance to the horizon of a certain family of local observers (suitable for thermodynamics). Even though this result might be of limited relevance for classical considerations, it provides a clear-cut framework for the thermodynamical and statistical mechanical study of quantum isolated horizon models in loop quantum gravity.
[if the LaTex is not at first generated, it may help to click on "reload"]

http://arxiv.org/abs/1110.3874
Matter in Loop Quantum Gravity
Ghanashyam Date, Golam Mortuza Hossain
(Submitted on 18 Oct 2011)
Loop quantum Gravity, a non-perturbative and manifestly background free, quantum theory of gravity implies that at the kinematical level the spatial geometry is discrete in a specific sense. The spirit of background independence also requires a non-standard quantum representation of matter. While loop quantization of standard model fields has been proposed, detail study of its implications is not yet available. This review aims to survey the various efforts and results.
39 pages, invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

http://arxiv.org/abs/1110.3837
Coupling Shape Dynamics to Matter Gives Spacetime
Henrique Gomes, Tim Koslowski
(Submitted on 17 Oct 2011)
Shape Dynamics is a metric theory of pure gravity, equivalent to General Relativity, but formulated as a gauge theory of spatial diffeomporphisms and local spatial conformal transformations. In this paper we extend the construction of Shape Dynamics from pure gravity to gravity-matter systems and find that there is no obstruction for the coupling of gravity to standard matter. We use the matter gravity system to construct a clock and rod model for Shape Dynamics which allows us to recover a spacetime interpretation of Shape Dynamics trajectories.
10 pages

http://arxiv.org/abs/1110.3947
Canonical formalism for simplicial gravity
Philipp A. Hoehn
(Submitted on 18 Oct 2011)
We summarise a recently introduced general canonical formulation of discrete systems which is fully equivalent to the covariant formalism. This framework can handle varying phase space dimensions and is applied to simplicial gravity in particular.
4 pages, 5 figures, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.4051
Perfect discretization of path integrals
Sebastian Steinhaus
(Submitted on 18 Oct 2011)
In order to obtain a well-defined path integral one often employs discretizations. In the case of General Relativity these generically break diffeomorphism symmetry, which has severe consequences since these symmetries determine the dynamics of the corresponding system.
In this article we consider the path integral of reparametrization invariant systems as a toy example and present an improvement procedure for the discretized propagator. Fixed points and convergence of the procedure are discussed. Furthermore we show that a reparametrization invariant path integral implies discretization independence and acts as a projector onto physical states.
4 pages, 1 figure, based on a talk given at Loops '11, Madrid, to appear in Journal of Physics: Conference Series (JPCS)

brief mention:
The Frodden Ghosh Perez paper above cites a paper by Jacobson et al which was not noticed here at the time it appeared (back in 2008). It now seems clear that the Jacobson et al paper on horizon surface area is intriguing and should be retroactively added to bibliography.
http://arxiv.org/abs/0806.1677
Horizon surface gravity as 2d geodesic expansion
Ted Jacobson, Renaud Parentani
(Submitted on 10 Jun 2008 (v1), last revised 13 Aug 2008 (this version, v2))
The surface gravity of any Killing horizon, in any spacetime dimension, can be interpreted as a local, two-dimensional expansion rate seen by freely falling observers when they cross the horizon. Any two-dimensional congruence of geodesics invariant under the Killing flow can be used to define this expansion, provided that the observers have unit Killing energy.
8 pages,

 

[marcus]

http://arxiv.org/abs/1110.4578
U(N) and holomorphic methods for LQG and Spin Foams
Enrique F. Borja, Jacobo Diaz-Polo, Iñaki Garay
(Submitted on 20 Oct 2011)
The U(N) framework and the spinor representation for loop quantum gravity are two new points of view that can help us deal with the most fundamental problems of the theory. Here, we review the detailed construction of the U(N) framework explaining how one can endow the Hilbert space of N-leg intertwiners with a Fock structure. We then give a description of the classical phase space corresponding to this system in terms of the spinors, and we will study its quantization using holomorphic techniques. We take special care in constructing the usual holonomy operators of LQG in terms of spinors, and in the description of the Hilbert space of LQG with the different polarization given by these spinors.
16 pages. Proceedings for the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

brief mention:
http://arxiv.org/abs/1110.4492
Information dynamics and new geometric foundations of quantum theory
Ryszard Paweł Kostecki
(Submitted on 20 Oct 2011)
We discuss new approach to mathematical foundations of quantum theory, which is completely independent of Hilbert spaces and measure spaces. New kinematics is defined by non-linear geometry of spaces of integrals on abstract non-commutative algebras. New dynamics is defined by constrained maximisation of quantum relative entropy. We recover Hilbert space based approach (including unitary evolution and the von Neumann-Lüders rule) and measure theoretic approach to probability theory (including Bayes' rule) as special cases of our approach.
Submitted to Proceedings of the Foundations of Probability and Physics 6 conference, Linneuniversitetet, Vaxjo, June 13-16, 2011

 

[marcus]

http://arxiv.org/abs/1110.4833
Continuous formulation of the Loop Quantum Gravity phase space
Laurent Freidel, Marc Geiller, Jonathan Ziprick
(Submitted on 21 Oct 2011)
In this paper, we study the discrete classical phase space of loop gravity, which is expressed in terms of the holonomy-flux variables, and show how it is related to the continuous phase space of general relativity. In particular, we prove an isomorphism between the loop gravity discrete phase space and the symplectic reduction of the continuous phase space with respect to a flatness constraint. This gives for the first time a precise relationship between the continuum and holonomy-flux variables. Our construction shows that the fluxes depend on the three-geometry, but also explicitly on the connection, explaining their non commutativity. It also clearly shows that the flux variables do not label a unique geometry, but rather a class of gauge-equivalent geometries. This allows us to resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewise flat geometry, since we establish that both geometries belong to the same equivalence class. This finally gives us a clear understanding of the relationship between the piecewise flat spin foam geometries and Regge geometries, which are only piecewise-linear flat: While Regge geometry corresponds to metrics whose curvature is concentrated around straight edges, the loop gravity geometry correspond to metrics whose curvature is concentrated around not necessarily straight edges.
27 pages

http://arxiv.org/abs/1110.4694
Poincaré 2-group and quantum gravity
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 21 Oct 2011)
We show that General Relativity can be formulated as a constrained topological theory for flat 2-connections associated to the Poincaré 2-group. Matter can be consistently coupled to gravity in this formulation. We also show that the edge lengths of the spacetime manifold triangulation arise as the basic variables in the path-integral quantization, while the state-sum amplitude is an evaluation of a colored 3-complex, in agreement with the category theory results.
12 pages

 

[John86]

http://arxiv.org/abs/1110.5224
Fractal space-times under the microscope: A Renormalization Group view on Monte Carlo data
Martin Reuter, Frank Saueressig
(Submitted on 24 Oct 2011)
The emergence of fractal features in the microscopic structure of space-time is a common theme in many approaches to quantum gravity. In this work we carry out a detailed renormalization group study of the spectral dimension $d_s$ and walk dimension $d_w$ associated with the effective space-times of asymptotically safe Quantum Einstein Gravity (QEG). We discover three scaling regimes where these generalized dimensions are approximately constant for an extended range of length scales: a classical regime where $d_s = d, d_w = 2$, a semi-classical regime where $d_s = 2d/(2+d), d_w = 2+d$, and the UV-fixed point regime where $d_s = d/2, d_w = 4$. On the length scales covered by three-dimensional Monte Carlo simulations, the resulting spectral dimension is shown to be in very good agreement with the data. This comparison also provides a natural explanation for the apparent puzzle between the short distance behavior of the spectral dimension reported from Causal Dynamical Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic Safety.

http://arxiv.org/abs/1110.4649
Roaming moduli space using dynamical triangulations
J. Ambjorn, J. Barkley, T. Budd
(Submitted on 20 Oct 2011)
In critical as well as in non-critical string theory the partition function reduces to an integral over moduli space after integration over matter fields. For non-critical string theory this moduli integrand is known for genus one surfaces. The formalism of dynamical triangulations provides us with a regularization of non-critical string theory. We show how to assign in a simple and geometrical way a moduli parameter to each triangulation. After integrating over possible matter fields we can thus construct the moduli integrand. We show numerically for $c=0$ and $c=-2$ non-critical strings that the moduli integrand converges to the known continuum expression when the number of triangles goes to infinity.

http://arxiv.org/abs/0906.2016
UV and IR quantum-spacetime effects for the Chandrasekhar model
Giovanni Amelino-Camelia, Niccolo' Loret, Gianluca Mandanici, Flavio Mercati
(Submitted on 11 Jun 2009 (v1), last revised 24 Oct 2011 (this version, v2))
We modify the Chandrasekhar model of white dwarfs by introducing novel momentum-space features that characterize the analysis of some quantum-spacetime scenarios. We find that the rather standard ultraviolet effects of spacetime quantization can only be significant in a regime where the Chandrasekhar model anyway lacks any contact with observations. But a new class of quantum-spacetime effects inspired by the mechanism of "ultraviolet/infrared mixing" could be relevant for white dwarfs whose mass is roughly half the mass of the Sun, some of which are described in the literature as "strange white dwarfs". We also offer a preliminary argument suggesting that Planck-scale (ultraviolet) effects could be significant in cases where ultra-high densities are present, even when the relevant star is still gigantic in Planck-length units.

http://arxiv.org/abs/1110.5081
On the fate of Lorentz symmetry in relative-locality momentum spaces
Giovanni Amelino-Camelia
(Submitted on 23 Oct 2011)
The most studied doubly-special-relativity scenarios, theories with both the speed-of-light scale and a length/inverse-momentum scale as non-trivial relativistic invariants, have concerned the possibility of enforcing relativistically some nonlinear laws on momentum space. For the "relative-locality framework" recently proposed in arXiv:1101.0931 a central role is played by nonlinear laws on momentum space, with the guiding principle that they should provide a characterization of the geometry of momentum space. Building on previous doubly-special-relativity results I here identify a criterion for establishing whether or not a given geometry of the relative-locality momentum space is "DSR compatible", i.e. compatible with an observer-independent formulation of theories on that momentum space. I find that given some chosen parametrization of momentum-space geometry the criterion takes the form of an elementary algorithm. I show that relative-locality momentum spaces that fail my criterion definitely "break" Lorentz invariance, i.e. theories on such momentum spaces necessarily are observer-dependent "ether" theories. By working out a few examples I provide evidence that when the criterion is instead satisfied one does manage to produce a relativistic formulation. The examples I use to illustrate the applicability of my criterion also have some intrinsic interest, including two particularly noteworthy cases of $\kappa$-Poincar\'e-inspired momentum spaces.

 

[MTd2]

http://arxiv.org/abs/1110.5088

Simplicial Gravity and Strings

John Swain
(Submitted on 23 Oct 2011)
String theory, as a theory containing quantum gravity, is usually thought to require more dimensions of spacetime than the usual 3+1. Here I argue on physical grounds that needing extra dimensions for strings may well be an artefact of forcing a fixed flat background space. I also show that discrete simplicial approaches to gravity in 3+1 dimensions have natural string-like degrees of freedom which are inextricably tied to the dynamical space in which they evolve. In other words, if simplicial approaches to 3+1 dimensional quantum gravity do indeed give consistent theories, they may essentially contain consistent background-independent string theories.

 

[marcus]

http://arxiv.org/abs/1110.5631
Effective relational dynamics
Philipp A. Hoehn
(Submitted on 25 Oct 2011)
We provide a synopsis of an effective approach to the problem of time in the semiclassical regime. The essential features of this new approach to evaluating relational quantum dynamics in constrained systems are illustrated by means of a simple toy model.
4 pages, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.5480
Coherent State Functional Integrals in Quantum Cosmology
Li Qin, Yongge Ma
(Submitted on 25 Oct 2011)
Coherent state functional integrals for the minisuperspace models of quantum cosmology are studied. By the well-established canonical theories, the transition amplitudes in the path-integral representations of Wheeler-DeWitt quantum cosmology and loop quantum cosmology can be formulated through group averaging. The effective action and Hamiltonian with higher-order quantum corrections are thus obtained in both models. It turns out that for a non-symmetric Hamiltonian constraint operator, the Moyal (star)-product emerges naturally in the effective Hamiltonian. This reveals the intrinsic relation among coherent state functional integral, effective theory and star-product. Moreover, both the resulted effective theories imply a possible quantum cosmological effect in large scale limit.
15 pages

http://arxiv.org/abs/1110.5606
The microscopic dynamics of quantum space as a group field theory
Daniele Oriti
(Submitted on 25 Oct 2011)
We provide a rather extended introduction to the group field theory approach to quantum gravity, and the main ideas behind it. We present in some detail the GFT quantization of 3d Riemannian gravity, and discuss briefly the current status of the 4-dimensional extensions of this construction. We also briefly report on recent results obtained in this approach and related open issues, concerning both the mathematical definition of GFT models, and possible avenues towards extracting interesting physics from them.
60 pages. Extensively revised version of the contribution to "Foundations of Space and Time: Reflections on Quantum Gravity", edited by G. Ellis, J. Murugan, A. Weltman, published by Cambridge University Press