Loop-and-allied QG bibliography

[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