Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1107.5274
Holomorphic Lorentzian Simplicity Constraints
Maité Dupuis, Laurent Freidel, Etera R. Livine, Simone Speziale
(Submitted on 26 Jul 2011)
We develop an Hamiltonian representation of the sl(2,C) algebra on a phase space consisting of N copies of twistors, or bi-spinors. We identify a complete set of global invariants, and show that they generate a closed algebra including gl(N,C) as a subalgebra. Then, we define the linear and quadratic simplicity constraints which reduce the spinor variables to (framed) 3d spacelike polyhedra embedded in Minkowski spacetime. Finally, we introduce a new version of the simplicity constraints which (i) are holomorphic and (ii) Poisson-commute with each other, and show their equivalence to the linear and quadratic constraints.
20 pages

http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)
"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.
36 pages, 23 figures

A nice clear Higgs FAQ by Prof. Matt Strassler:
http://profmattstrassler.com/articles-and-posts/the-higgs-particle/360-2/
An earlier post by Strassler on the same topic:
http://profmattstrassler.com/2011/07/24/the-first-version-of-the-higgs-faq/

http://arxiv.org/abs/1107.5157
Nonperturbative Loop Quantization of Scalar-Tensor Theories of Gravity
Xiangdong Zhang, Yongge Ma
(Submitted on 26 Jul 2011)
The Hamiltonian formulation of scalar-tensor theories of gravity (with coupling parameter $\omega(\phi)\neq-3/2$) is derived from their Lagrangian formulation by Hamiltonian analysis. The canonical structure and constraint algebra of the theories are similar to those of general relativity coupled with a scalar field. By canonical transformations, we further obtain the connection dynamical formalism of the scalar-tensor theories with real su(2)-connections as configuration variables. This formalism enable us to extend the scheme of non-perturbative loop quantum gravity to the scalar-tensor theories. The quantum kinematical framework for the scalar-tensor theories is rigorously constructed. Both the Hamiltonian constraint operator and master constraint operator are well defined and proposed to represent quantum dynamics. Thus loop quantum gravity method is also valid for the rather general scalar-tensor theories.
8 pages

This deals with several approaches to QG including CDT, LQGspinfoam, AsymSafe, DSR...:
http://arxiv.org/abs/1107.5041
Geometry and field theory in multi-fractional spacetime
Gianluca Calcagni
(Submitted on 25 Jul 2011)
We construct a theory of fields living on continuous geometries with fractional Hausdorff and spectral dimensions, focussing on a flat background analogous to Minkowski spacetime. After reviewing the properties of fractional spaces with fixed dimension, presented in a companion paper, we generalize to a multi-fractional scenario inspired by multi-fractal geometry, where the dimension changes with the scale. This is related to the renormalization group properties of fractional field theories, illustrated by the example of a scalar field. Depending on the symmetries of the Lagrangian, one can define two models. In one, the scalar has a continuum of massive modes, while in the other it only has a mass pole. If the effective dimension flows from 2 in the ultraviolet (UV), geometry constrains the infrared limit to be four-dimensional. At the UV critical value, the model is rendered power-counting renormalizable. However, this is not the most fundamental regime. Compelling arguments of fractal geometry require an extension of the fractional action measure to complex order. In doing so, we obtain a hierarchy of scales characterizing different geometric regimes. At very small scales, discrete symmetries emerge and the notion of a continuous spacetime begins to blur, until one reaches a fundamental scale and an ultra-microscopic fractal structure. This fine hierarchy of geometries has implications for non-commutative theories and discrete quantum gravity. In the latter case, the present model can be viewed as a top-down realization of a quantum-discrete to classical-continuum transition.
1+80 pages, 1 figure, 2 tables

 

[MTd2]

http://arxiv.org/abs/1107.5274

Holomorphic Lorentzian Simplicity Constraints

Maité Dupuis, Laurent Freidel, Etera R. Livine, Simone Speziale
(Submitted on 26 Jul 2011)
We develop an Hamiltonian representation of the sl(2,C) algebra on a phase space consisting of N copies of twistors, or bi-spinors. We identify a complete set of global invariants, and show that they generate a closed algebra including gl(N,C) as a subalgebra. Then, we define the linear and quadratic simplicity constraints which reduce the spinor variables to (framed) 3d spacelike polyhedra embedded in Minkowski spacetime. Finally, we introduce a new version of the simplicity constraints which (i) are holomorphic and (ii) Poisson-commute with each other, and show their equivalence to the linear and quadratic constraints.

http://arxiv.org/abs/1107.5185

Feynman diagrammatic approach to spin foams

Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)
"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.

http://arxiv.org/abs/1107.5041

Geometry and field theory in multi-fractional spacetime

Gianluca Calcagni
(Submitted on 25 Jul 2011)
We construct a theory of fields living on continuous geometries with fractional Hausdorff and spectral dimensions, focussing on a flat background analogous to Minkowski spacetime. After reviewing the properties of fractional spaces with fixed dimension, presented in a companion paper, we generalize to a multi-fractional scenario inspired by multi-fractal geometry, where the dimension changes with the scale. This is related to the renormalization group properties of fractional field theories, illustrated by the example of a scalar field. Depending on the symmetries of the Lagrangian, one can define two models. In one, the scalar has a continuum of massive modes, while in the other it only has a mass pole. If the effective dimension flows from 2 in the ultraviolet (UV), geometry constrains the infrared limit to be four-dimensional. At the UV critical value, the model is rendered power-counting renormalizable. However, this is not the most fundamental regime. Compelling arguments of fractal geometry require an extension of the fractional action measure to complex order. In doing so, we obtain a hierarchy of scales characterizing different geometric regimes. At very small scales, discrete symmetries emerge and the notion of a continuous spacetime begins to blur, until one reaches a fundamental scale and an ultra-microscopic fractal structure. This fine hierarchy of geometries has implications for non-commutative theories and discrete quantum gravity. In the latter case, the present model can be viewed as a top-down realization of a quantum-discrete to classical-continuum transition.

 

[MTd2]

http://arxiv.org/abs/1107.5308

Fractional and noncommutative spacetimes

Michele Arzano, Gianluca Calcagni, Daniele Oriti, Marco Scalisi
(Submitted on 26 Jul 2011)
We establish a mapping between fractional and noncommutative spacetimes in configuration space. Depending on the scale at which the relation is considered, there arise two possibilities. For a fractional spacetime with log-oscillatory measure, the effective measure near the fundamental scale determining the log-period coincides with the non-rotation-invariant but cyclicity-preserving measure of \kappa-Minkowski. At scales larger than the log-period, the fractional measure is averaged and becomes a power-law with real exponent. This can be also regarded as the cyclicity-inducing measure in a noncommutative spacetime defined by a certain nonlinear algebra of the coordinates, which interpolates between \kappa-Minkowski and canonical spacetime. These results are based upon a braiding formula valid for any nonlinear algebra which can be mapped onto the Heisenberg algebra.

 

[marcus]

http://arxiv.org/abs/1107.5693
Spinors and Voros star-product for Group Field Theory: First Contact
Maité Dupuis, Florian Girelli, Etera R. Livine
(Submitted on 28 Jul 2011)
In the context of non-commutative geometries, we develop a group Fourier transform for the Lie group SU(2). Our method is based on the Schwinger representation of the Lie algebra su(2) in terms of spinors. It allows us to prove that the non-commutative R³ space dual to the SU(2) group is in fact of the Moyal-type and endowed with the Voros star-product when expressed in the spinor variables. Finally, from the perspective of quantum gravity, we discuss the application of these new tools to group field theories for spinfoam models and their interpretation as non-commutative field theories with quantum-deformed symmetries.
23 pages

Maybe Dupuis and friends have found something here. Here's an earlier post of mine with a snapshot of Dupuis and brief comment on her PhD thesis.
https://www.physicsforums.com/showthread.php?p=3249152#post3249152

 

[MTd2]

http://arxiv.org/abs/1107.5815

Asymptotically safe gravity as a scalar-tensor theory and its cosmological implications

Yi-Fu Cai, Damien A. Easson
Comments: 8 pages, 2 figures
Subjects: High Energy Physics - Theory (hep-th); Cosmology and Extragalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)
We study asymptotically safe gravity with Einstein-Hilbert truncation taking into account the renormalization group running of both gravitational and cosmological constants. We show the classical behavior of the theory is equivalent to a specific class of Jordan-Brans-Dicke theories with vanishing Brans-Dicke parameter, and potential determined by the renormalization group equation. The theory may be reformulated as an $f(R)$ theory. In the simplest cosmological scenario, we find large--field inflationary solutions near the Planck scale where the effective field theory description breaks down. Finally, we discuss the implications of a running gravitational constant to background dynamics via cosmological perturbation theory. We show that compatibility with General Relativity requires contributions from the running gravitational constant to the stress energy tensor to be taken into account in the perturbation analysis.

 

[marcus]

http://arxiv.org/abs/1108.0320
Unruh effect without entanglement
Carlo Rovelli, Matteo Smerlak
(Submitted on 1 Aug 2011)
We estimate the transition rates of a uniformly accelerated Unruh-DeWitt detector coupled to a quantum field with reflecting conditions on a boundary plane (a "mirror"). We find that these are essentially indistinguishable from the usual Unruh rates, viz. that the Unruh effect persists in the presence of the mirror. This shows that the Unruh effect is not merely a consequence of the entanglement between left and right Rindler quanta in the Minkowski vacuum. Since in this setup the state of the field in the Rindler wedge is pure, we argue furthermore that the relevant entropy in the Unruh effect cannot be the von Neumann entanglement entropy. We suggest, in alternative, that it is the Shannon entropy associated with Heisenberg uncertainty.
5 pages

http://arxiv.org/abs/1108.0369
Twistor Networks and Covariant Twisted Geometries
Etera R. Livine, Simone Speziale, Johannes Tambornino
(Submitted on 1 Aug 2011)
We study the symplectic reduction of the phase space of two twistors to the cotangent bundle of the Lorentz group. We provide expressions for the Lorentz generators and group elements in terms of the spinors defining the twistors. We use this to define twistor networks as a graph carrying the phase space of two twistors on each edge. We also introduce simple twistor networks, which provide a classical version of the simple projected spin networks living on the boundary Hilbert space of EPRL/FK spin foam models. Finally, we give an expression for the Haar measure in terms of spinors.
18 pages

http://arxiv.org/abs/1108.0005
Quantum memory of the Universe
Jakub Mielczarek, Wlodzimierz Piechocki
(Submitted on 29 Jul 2011)
We present results concerning propagation of the Gaussian state across the cosmological quantum bounce. The reduced phase space quantization of loop quantum cosmology is applied to the Friedman-Robertson-Walker universe with a free massless scalar field. The dispersion of observables are studied in the context of the Heisenberg uncertainty principle. We show that the bounce transition is the least quantum part of the evolution. We give an interpretation of this surprising result in terms of entropy. We examine the conservation of semiclassicality across the quantum bounce. The cosmic amnesia may occur or not depending on parameters of the quantum state. We show that this issue can be studied observationally. The preliminary estimations based on astronomical data support the Universe without cosmic forgetfulness.
4 pages,4 figures

http://arxiv.org/abs/1108.0079
Gravitational wave generation in loop quantum cosmology
Paulo M. Sá, Alfredo B. Henriques
(Submitted on 30 Jul 2011)
We calculate the full spectrum, as observed today, of the cosmological gravitational waves generated within a model based on loop quantum cosmology. It is assumed that the universe, after the transition to the classical regime, undergoes a period of inflation driven by a scalar field with a chaotic-type potential. Our analysis shows that, for certain conditions, loop quantum effects leave a clear signature on the spectrum, namely, an over-production of low-frequency gravitational waves. One of the aims of our work is to show that loop quantum cosmology models can be tested and that, more generally, pre-inflationary physical processes, contrary to what is usually assumed, leave their imprint in those spectra and can also be tested.
7 pages, 8 figures,

http://arxiv.org/abs/1108.0116
Anisotropic Structures and Wormholes with Loop Quantum Gravity Holonomy Corrections
Andrew DeBenedictis
(Submitted on 30 Jul 2011)
Anisotropic spherically symmetric systems are studied in the connection and densitized triad variables used in loop quantum gravity. The material source is an anisotropic fluid, which is arguably the most commonly used source term in anisotropic studies within general relativity. The gravitational+anisotropic fluid constraints are derived and analyzed and then quantum gravity inspired holonomy replacements are performed. The quantum properties of the fluid are dictated by the modified constraint equations. Particular attention is paid to wormhole throats, as they provide a simplistic model of the structures thought to be ubiquitous in the quantum gravity space-time foam at high energy scales. In comparison to the purely classical theory, the quantum corrections act to increase the energy density of the fluid, which indicates that they may lessen the energy condition violation present in the classical theory. Related to this, in principle it would be possible to have scenarios where the classical solution yields everywhere negative (with a zero at the throat) fluid energy density but the corresponding quantum corrected theory possesses only small regions of negative energy density or even everywhere non-negative energy density.
19 pages, 6 figures

 

[MTd2]

http://arxiv.org/abs/1108.0227

Asymptotically safe phantom cosmology

Rong-Jia Yang
(Submitted on 1 Aug 2011)
We consider quantum modifications to phantom cosmology in a Friedmann-Robertson-Walker spacetime. The cosmological evolution equations improved by renormalization group are obtained. For exponential potential, we find two types of cosmological fixed points where the renormalization group scale either freezes in, or continues to evolve with scale factor. We discuss the implications of each of these points, and investigate especially whether the big rip can be avoided. If the fixed point of renormalization group flow coincides with the cosmological fixed point, the universe will be dominated by dark matter and free from the big rip.

 

[MTd2]

Hmm, we missed a very nice paper yesterday:

http://arxiv.org/abs/1107.5927

Quantum Gravity in Plebanski Formulation

L. V. Laperashvili
(Submitted on 29 Jul 2011)
We present a theory of the four-dimensional quantum gravity with massive gravitons which may be essentially renormalizable. In Plebanski formulation of general relativity in which the tetrads, the connection and the curvature are all independent variables (and the usual relations among these quantities are valid only on-shell), we consider the nonperturbative theory of gravity with a nonzero background connection. We predict a tiny value of the graviton mass: $m_g\approx 1.5\times 10^{-42}\,{GeV}$ and extremely small dimensionless coupling constant of the perturbative gravitational interaction: $g\sim 10^{-60}$. We put forward the idea by H. Isimori \ct{15} on renormalizability of quantum gravity having multi-gravitons with masses $m_0, m_1,..., m_{N-1}$.

 

[marcus]

brief mention:
http://arxiv.org/abs/1108.0422
Renormalisation group improvement of scalar field inflation
Adriano Contillo, Mark Hindmarsh, Christoph Rahmede
(Submitted on 1 Aug 2011)
We study quantum corrections to Friedmann-Robertson-Walker cosmology with a scalar field under the assumption that the dynamics are subject to renormalisation group improvement. We use the Bianchi identity to relate the renormalisation group scale to the scale factor and obtain the improved cosmological evolution equations. We study the solutions of these equations in the renormalisation group fixed point regime, obtaining the time-dependence of the scalar field strength and the Hubble parameter...
... and find classical dynamics as an attractor solution for late times. We show that the solution found in the renormalisation group fixed point regime is also a cosmological fixed point in the autonomous phase space. We derive the power spectrum of cosmological perturbations and find that the scalar power spectrum is exactly scale-invariant and bounded up to arbitrarily small times, while the tensor perturbations are tilted as appropriate for the background power-law inflation. We specify conditions for the renormalisation group fixed point values of the couplings under which the amplitudes of the cosmological perturbations remain small.
17 pages; 2 figures

http://arxiv.org/abs/1108.0592
Lorentzian approach to noncommutative geometry
Nicolas Franco
(Submitted on 2 Aug 2011)
This thesis concerns the research on a Lorentzian generalization of Alain Connes' noncommutative geometry. ... In the last chapter, we investigate the problem of the generalization to Lorentzian manifolds. We present a first step of generalization of the distance function with the use of a global timelike eikonal condition. Then we set the first axioms of a temporal Lorentzian spectral triple as a generalization of a pseudo-Riemannian spectral triple together with a notion of global time in noncommutative geometry.
PhD thesis, 200 pages, 9 figures

 

[marcus]

http://arxiv.org/abs/1108.0832
On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit
Carlo Rovelli
(Submitted on 3 Aug 2011)
The Hamilton function is a powerful tool for studying the classical limit of quantum systems, which remains meaningful in background-independent systems. In quantum gravity, it clarifies the physical interpretation of the transitions amplitudes and their truncations.
7 pages

http://arxiv.org/abs/1108.0868
A critical look at strings
Carlo Rovelli
(Submitted on 3 Aug 2011)
This is an invited contribution to the Special Issue of "Foundations of Physics" titled "Forty Years Of String Theory: Reflecting On the Foundations". I have been asked to assess string theory as an outsider, and to compare it with the theory, methods, and expectations in my own field.
7 pages

Gerard 't Hooft is the chief editor of that special issue "Forty Years of String Theory". Presumably he is the one who invited Rovelli to provide an outside assessment of the program, in comparison with the Loop program.

http://arxiv.org/abs/1108.0883
Clocks and Relationalism in the Thermal Time Hypothesis
Nicolas C. Menicucci, S. Jay Olson, Gerard J. Milburn
(Submitted on 3 Aug 2011)
The Thermal Time Hypotheis (TTH) has been proposed as a general method for identifying a time variable from within background-free theories which do not come equipped with a pre-defined clock variable. Here, we explore some implications of the TTH in an entirely relational context by constructing a protocol for the creation of "thermal clocks" from components of a large but finite quantum mechanical system. The protocol applies locally, in the sense that we do not attempt to construct a single clock describing the evolution of the entire system, but instead we construct clocks which describe the evolution of each subsystem of interest. We find that a consistency condition required for the evolution of our clocks is operationally equivalent to the general relativistic Tolman-Ehrenfest relation for thermal equilibrium in a static gravitational field but without the assumption of gravity or a metric field of any kind.
11 pages, 3 figures

http://arxiv.org/abs/1108.0893
Loop Quantum Cosmology: A Status Report
Abhay Ashtekar, Parampreet Singh
(Submitted on 3 Aug 2011)
The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without a loss of continuity.
136 pages, 15 figures

http://arxiv.org/abs/1108.0910
The black hole information paradox and relative locality
Lee Smolin
(Submitted on 3 Aug 2011)
We argue that the recently proposed principle of relative locality offers a new way to resolve the black hole information puzzle.
11 pages, one figure

http://arxiv.org/abs/1108.0816
Chirality of tensor perturbations for complex values of the Immirzi parameter
Laura Bethke, Joao Magueijo
(Submitted on 3 Aug 2011)
In this paper we generalise previous work on tensor perturbations in a de Sitter background in terms of Ashtekar variables to cover all complex values of the Immirzi parameter gamma (previous work was restricted to imaginary gamma). Particular attention is paid to the case of real gamma. Following the same approach as in the imaginary case, we can obtain physical graviton states by invoking reality and torsion free conditions. The Hamiltonian in terms of graviton states has the same form whether gamma has a real part or not; however changes occur for the vacuum energy and fluctuations. Specifically, we observe a gamma dependent chiral asymmetry in the vacuum fluctuations only if gamma has an imaginary part. Ordering prescriptions also change this asymmetry. We thus present a measurable result for CMB polarisation experiments that could shed light on the workings of quantum gravity.
6 pages, 1 figure

http://arxiv.org/abs/1108.0829
Prescriptions in Loop Quantum Cosmology: A comparative analysis
Guillermo A. Mena Marugan, Javier Olmedo, Tomasz Pawlowski
(Submitted on 3 Aug 2011)
Various prescriptions proposed in the literature to attain the polymeric quantization of a homogeneous and isotropic flat spacetime coupled to a massless scalar field are carefully analyzed in order to discuss their differences. A detailed numerical analysis confirms that, for states which are not deep in the quantum realm, the expectation values and dispersions of some natural observables of interest in cosmology are qualitatively the same for all the considered prescriptions. On the contrary, the amplitude of the wave functions of those states differs considerably at the bounce epoch for these prescriptions. This difference cannot be absorbed by a change of representation. Finally, the prescriptions with simpler superselection sectors are clearly more efficient from the numerical point of view.
18 pages, 6 figures

http://arxiv.org/abs/1108.0896
Thermodynamics of Ideal Gas in Doubly Special Relativity
Nitin Chandra, Sandeep Chatterjee
(Submitted on 30 Jul 2011)
We study thermodynamics of an ideal gas in Doubly Special Relativity. New type of special functions (which we call Incomplete Modified Bessel functions) emerge. We obtain a series solution for the partition function and derive thermodynamic quantities. We observe that DSR thermodynamics is non-perturbative in the SR and massless limits. A stiffer equation of state is found.
11 pages, 6 figures

Brief mention:
http://arxiv.org/abs/1108.0877
On the Role of Space-Time Foam in Breaking Supersymmetry via the Barbero-Immirzi Parameter
John Ellis, Nick E. Mavromatos
(Submitted on 3 Aug 2011)
We discuss how: (i) a dilaton/axion superfield can play the role of a Barbero-Immirzi field in four-dimensional conformal quantum supergravity theories, (ii) a fermionic component of such a dilaton/axion superfield may play the role of a Goldstino in the low-energy effective action obtained from a superstring theory with F-type global supersymmetry breaking, (iii) this global supersymmetry breaking is communicated to the gravitational sector via the supergravity coupling of the Goldstino, and (iv) such a scenario may be realized explicitly in a D-foam model with D-particle defects fluctuating stochastically.
:24 pages, 2 figures

 

[MTd2]

http://arxiv.org/abs/1108.1178v1

Quantum simplicial geometry in the group field theory formalism: reconsidering the Barrett-Crane model

Aristide Baratin, Daniele Oriti
(Submitted on 4 Aug 2011)
Using the non-commutative metric formulation of group field theories (GFT), we define a model of 4-dimensional quantum gravity as a constrained BF theory, without Immirzi parameter, encoding the quantum simplicial geometry of any triangulation used to define its quantum amplitudes. This involves a generalization of the usual GFT framework, where the usual field variables, associated to the four triangles of a tetrahedron, are supplemented by an S^3 vector playing the role of the normal to the tetrahedron. This leads naturally to projected spin network states. We give both a simplicial path integral and a spin foam formulation of the Feynman amplitudes, which correspond to a variant of the Barrett-Crane amplitudes. We then re-examin the arguments against the Barrett-Crane model(s), in light of our construction. We argue that it can still be considered a plausible quantization of 4d gravity, and that further work is needed to either confirm or refute its validity.

 

[marcus]

http://arxiv.org/abs/1108.1145
Time and a physical Hamiltonian for quantum gravity
Viqar Husain, Tomasz Pawlowski
(Submitted on 4 Aug 2011)
We present a non-perturbative quantization of general relativity coupled to dust and other matter fields. The dust provides a natural time variable, leading to a physical Hamiltonian with spatial diffeomorphism symmetry. The surprising feature is that the Hamiltonian is not a square root. This property, together with the kinematical structure of loop quantum gravity, provides a complete theory of quantum gravity, and puts in technical reach applications to cosmology, quantum gravitational collapse and Hawking radiation.
4 pages

http://arxiv.org/abs/1108.1147
Dust reference frame in quantum cosmology
Viqar Husain, Tomasz Pawlowski
(Submitted on 4 Aug 2011)
We give a formulation of quantum cosmology with a pressureless dust and arbitrary additional matter fields. The system has the property that its Hamiltonian constraint is linear in the dust momentum. This feature provides a natural time gauge, leading to a physical hamiltonian that is not a square root. Quantization leads to Schrödinger equation for which unitary evolution is directly linked to geodesic completeness. Our approach simplifies the analysis of both Wheeler-deWitt and loop quantum cosmology (LQC) models, and significantly broadens the applicability of the latter. This is demonstrated for arbitrary scalar field potential and cosmological constant in LQC.
8 pages,

 

[marcus]

http://arxiv.org/abs/1108.1507
Anomalous dimension in semiclassical gravity
Emanuele Alesci, Michele Arzano
(Submitted on 6 Aug 2011)
The description of the phase space of relativistic particles coupled to three-dimensional Einstein gravity requires momenta which are coordinates on a group manifold rather than on ordinary Minkowski space. The corresponding field theory turns out to be a non-commutative field theory on configuration space and a group field theory on momentum space. Using basic non-commutative Fourier transform tools we introduce the notion of non-commutative heat-kernel associated with the Laplacian on the non-commutative configuration space. We show that the spectral dimension associated to the non-commutative heat kernel varies with the scale reaching a non-integer value smaller than three for Planckian diffusion scales.
9 pages, 1 figure

The majority of the references are to LQG papers but a considerable number are to CDT and AsymSafe QG as well, some also to Horava gravity. So this paper seems to generalize over a range of approaches.

 

[MTd2]

http://arxiv.org/abs/1108.1495

Simplicial quantum dynamics

David Ritz Finkelstein
(Submitted on 6 Aug 2011)
Present-day quantum field theory can be regularized by a decomposition into quantum simplices. This replaces the infinite-dimensional Hilbert space by a high-dimensional spinor space and singular canonical Lie groups by regular spin groups. It radically changes the uncertainty principle for small distances. Gaugeons, including the gravitational, are represented as bound fermion-pairs, and space-time curvature as a singular organized limit of quantum non-commutativity.

http://arxiv.org/abs/1108.1540

Expanding (3+1)-dimensional universe from a Lorentzian matrix model for superstring theory in (9+1)-dimensions

Sang-Woo Kim, Jun Nishimura, Asato Tsuchiya
(Submitted on 7 Aug 2011)
We reconsider the matrix model formulation of type IIB superstring theory in (9+1)-dimensional space-time. Unlike the previous proposal in which the Wick rotation was used to make the model well-defined, we regularize the Lorentzian model by introducing infrared cutoffs in both the spatial and temporal directions. Monte Carlo studies reveal that the two cutoffs can be removed in the large-N limit and that the theory thus obtained has no parameters other than one scale parameter. Moreover, we find that three out of nine spatial directions start to expand at some "critical time", after which the space has SO(3) symmetry instead of SO(9).

 

[marcus]

http://arxiv.org/abs/1108.1974
Canonical simplicial gravity
Bianca Dittrich, Philipp A Hoehn
(Submitted on 9 Aug 2011)
A general canonical formalism for discrete systems is developed which can handle varying phase space dimensions and constraints. The central ingredient is Hamilton's principle function which generates canonical time evolution and ensures that the canonical formalism reproduces the dynamics of the covariant formulation following directly from the action. We apply this formalism to simplicial gravity and (Euclidean) Regge calculus, in particular. A discrete forward/backward evolution is realized by gluing/removing single simplices step by step to/from a bulk triangulation and amounts to Pachner moves in the triangulated hypersurfaces. As a result, the hypersurfaces evolve in a discrete `multi-fingered' time through the full Regge solution. Pachner moves are an elementary and ergodic class of homeomorphisms and generically change the number of variables, but can be implemented as canonical transformations on naturally extended phase spaces. Some moves introduce a priori free data which, however, may become fixed a posteriori by constraints arising in subsequent moves. The end result is a general and fully consistent formulation of canonical Regge calculus, thereby removing a longstanding obstacle in connecting covariant simplicial gravity models to canonical frameworks. The present scheme is, therefore, interesting in view of many approaches to quantum gravity, but may also prove useful for numerical implementations.
52 pages, 14 figures, 3 tables

brief mention:
http://arxiv.org/abs/1108.1908
Large N Quantum Gravity
A. Codello
(Submitted on 9 Aug 2011)
We obtain the effective action for four dimensional quantum gravity, induced by minimally coupled matter fields, by integrating the RG flow of the relative effective average action. We show how different aspects of quantum gravity, as asymptotic safety, quantum corrections to the Newtonian potential and the anomaly induced Riegert action, are all represented by different terms of the effective action when this is expanded in powers of the curvature. When the number N of matter fields grows large, the form of the effective action we present should become a consistent approximation to the full quantum gravitational effective action, at least when metric fluctuation are not too strong.
20 pages, 1 figure, contribution to appear in "New Journal of Physics Focus Issue on Quantum Einstein Gravity"

 

[MTd2]

http://arxiv.org/abs/1108.1835

A positive energy theorem for Einstein-aether and Hořava gravity

David Garfinkle, Ted Jacobson
(Submitted on 9 Aug 2011)
Energy positivity is established for a class of solutions to Einstein-aether theory and the IR limit of Ho\v{r}ava gravity within a certain range of coupling parameters. The class consists of solutions where the aether 4-vector is divergence free on a spacelike surface to which it is orthogonal (which implies that the surface is maximal). In particular, this result holds for spherically symmetric solutions at a moment of time symmetry.

 

[MTd2]

http://arxiv.org/abs/1108.2258

Emergence of gravity from spinfoams

Elena Magliaro, Claudio Perini
(Submitted on 10 Aug 2011)
We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma), large spins (j) with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.

 

[John86]

http://arxiv.org/PS_cache/arxiv/pdf/1108/1108.2013v1.pdf
Trapped surfaces and emergent curved space in the Bose-Hubbard model
Francesco Caravelli, Alioscia Hamma, Fotini Markopoulou, Arnau Riera
(Submitted on 9 Aug 2011)
A Bose-Hubbard model on a dynamical lattice was introduced in previous work as a spin system analogue of emergent geometry and gravity. Graphs with regions of high connectivity in the lattice were identified as candidate analogues of spacetime geometries that contain trapped surfaces. We carry out a detailed study of these systems and show explicitly that the highly connected subgraphs trap matter. We do this by solving the model in the limit of no back-reaction of the matter on the lattice, and for states with certain symmetries that are natural for our problem. We find that in this case the problem reduces to a one-dimensional Hubbard model on a lattice with variable vertex degree and multiple edges between the same two vertices. In addition, we obtain a (discrete) differential equation for the evolution of the probability density of particles which is closed in the classical regime. This is a wave equation in which the vertex degree is related to the local speed of propagation of probability. This allows an interpretation of the probability density of particles similar to that in analogue gravity systems: matter inside this analogue system sees a curved spacetime. We verify our analytic results by numerical simulations. Finally, we analyze the dependence of localization on a gradual, rather than abrupt, fall-off of the vertex degree on the boundary of the highly connected region and find that matter is localized in and around that region.

 

[John86]

"not really related papers but sureley interesting"

http://arxiv.org/abs/1108.2550
Entropic Dynamics and the Quantum Measurement Problem
David T. Johnson, Ariel Caticha
(Submitted on 12 Aug 2011)
We explore the measurement problem in the entropic dynamics approach to quantum theory. The dual modes of quantum evolution---either continuous unitary evolution or abrupt wave function collapse during measurement---are unified by virtue of both being special instances of entropic updating of probabilities. In entropic dynamics particles have definite but unknown positions; their values are not created by the act of measurement. Other types of observables are introduced as a convenient way to describe more complex position measurements; they are not attributes of the particles but of the probability distributions; their values are effectively created by the act of measurement. We discuss the Born statistical rule for position, which is trivially built into the formalism, and also for generic observables.http://arxiv.org/abs/1108.2629
Momentum and Uncertainty Relations in the Entropic Approach to Quantum Theory
Shahid Nawaz, Ariel Caticha
(Submitted on 10 Aug 2011)
In the Entropic Dynamics (ED) approach to quantum theory the particles have well-defined positions but since they follow non differentiable Brownian trajectories they cannot be assigned an instantaneous momentum. Nevertheless, four different notions of momentum can be usefully introduced. We derive relations among them and the corresponding uncertainty relations. The main conclusion is that momentum is a statistical concept: in ED the momenta are not properties of the particles; they are attributes of the probability distributions.

 

[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