Loop-and-allied QG bibliography

[MTd2]

http://arxiv.org/abs/1106.2544

Relationships Between Exact RGs and some Comments on Asymptotic Safety

Oliver J. Rosten
(Submitted on 13 Jun 2011)
The standard flow equation for the effective average action can be derived from a Legendre transform of Polchinski's exact renormalization group equation. However, the latter is not well adapted for finding fixed-points with non-zero anomalous dimension. Instead, it is more convenient to use a modified version which ensures that the redundant coupling associated with the normalization of the field never appears in the action. Taking this as the starting point, a Legendre transform is constructed allowing a direct derivation of the corresponding flow equation for the effective average action. This equation is then used to exactly construct some illuminating though essentially trivial) asymptotically safe trajectories emanating from various non-unitary fixed-points. Finally, in the context of asympotically safe quantum gravity, it is pointed out that the standard argument that the anomalous dimension of Newton's constant is necessarily 2-d at a non-trivial fixed-point is incomplete. The implications of this are discussed.

 

[marcus]

Delightful imaginary encounter with Isaac Newton
of general interest (and also related to the topic of quantum gravity)
http://arxiv.org/abs/1106.2541
Is Reality Digital or Analog?
Jarmo Mäkelä
(Submitted on 13 Jun 2011)
A report of a discussion with Isaac Newton.
This essay, written in a form of a fictitious dialogue with Isaac Newton, received the First Prize in the Foundational Questions Institute (FQXi) essay contest "Is Reality Digital or Analog?"
===========
Maybe Jarmo M. is an unfamiliar name so here is his FQXi Bio:

"I received my PhD in theoretial physics from the University of Jyväskylä, Finland, in 1994, and did a post-doc in the Department of Applied Mathematics and Theoretical Physics of the University of Cambridge during the years 1995-1996. Since the year 2000 I have worked as a Senior Lecturer of mathematics and physics in the Vaasa University of Applied Sciences located in Vaasa, Finland."

 

[marcus]

http://arxiv.org/abs/1106.3179
The thermodynamic limit and black hole entropy in the area ensemble
J. Fernando Barbero G., Eduardo J. S. Villaseñor
(Submitted on 16 Jun 2011)
We discuss the thermodynamic limit in the canonical area ensemble used in loop quantum gravity to model quantum black holes. The computation of the thermodynamic limit is the rigorous way to obtain a smooth entropy from the counting entropy given by a direct determination of the number of microstates compatible with macroscopic quantities (the energy in standard statistical mechanics or the area in the framework presented here). As we will show in specific examples the leading behavior of the smoothed entropy for large horizon areas is the same as the counting entropy but the subleading contributions differ. This is important because these corrections determine the concavity or convexity of the entropy as a function of the area.
23 pages, 3 figures

 

[atyy]

http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.241602
Two is good, four is better
Abhishek Agarwal

http://arxiv.org/abs/1104.3850
The 4d Superconformal Index from q-deformed 2d Yang-Mills
Abhijit Gadde, Leonardo Rastelli, Shlomo S. Razamat, Wenbin Yan
(Submitted on 19 Apr 2011)
We identify the 2d topological theory underlying the N=2 4d superconformal index with an explicit model: q-deformed 2d Yang-Mills. By this route we are able to evaluate the index of some strongly-coupled 4d SCFTs, such as Gaiotto's T_N theories.

 

[atyy]

http://arxiv.org/abs/1106.3375
Geometrical Description of the Fractional Quantum Hall Effect
F. D. M. Haldane
(Submitted on 17 Jun 2011)
The fundamental collective degree of freedom of fractional quantum Hall states is identified as a unimodular two-dimensional spatial metric that characterizes the local shape of the correlations of the incompressible fluid. Its quantum fluctuations are controlled by a topologically-quantized "guiding-center spin". Charge fluctuations are proportional to its Gaussian curvature.

 

[marcus]

http://arxiv.org/abs/1106.3744
Anomaly-free vector perturbations with holonomy corrections in loop quantum cosmology
Jakub Mielczarek, Thomas Cailleteau, Aurelien Barrau, Julien Grain
(Submitted on 19 Jun 2011)
We investigate vector perturbations with holonomy corrections in the framework of loop quantum cosmology. Conditions to achieve anomaly freedom for these perturbations are found at all orders. This requires the introduction of counter-terms in the hamiltonian constraint. We also show that anomaly freedom requires the diffeomorphism constraint to hold its classical form when matter is added. The gauge-invariant variable and the corresponding equation of motion are derived. The propagation of vector modes through the bounce is finally discussed.
8 pages, 1 figure

http://arxiv.org/abs/1106.3793
Effective dynamics of the hybrid quantization of the Gowdy T³ universe
David Brizuela, Guillermo A. Mena Marugan, Tomasz Pawlowski
(Submitted on 20 Jun 2011)
The quantum dynamics of the linearly polarized Gowdy T³ model (compact inhomogeneous universes admitting linearly polarized gravitational waves) is analyzed within Loop Quantum Cosmology by means of an effective dynamics. The analysis, performed via analytical and numerical methods, proves that the behavior found in the evolution of vacuum (homogeneous) Bianchi I universes is preserved qualitatively also in the presence of inhomogeneities. More precisely, the initial singularity is replaced by a big bounce which joins deterministically two large classical universes. In addition, we show that the size of the universe at the bounce is at least of the same order of magnitude (roughly speaking) as the size of the corresponding homogeneous universe obtained in the absence of gravitational waves. In particular, a precise lower bound for the ratio of these two sizes is found. Finally, the comparison of the amplitudes of the gravitational wave modes in the distant future and past shows that, statistically (i.e., for large samples of universes), the difference in amplitude is enhanced for nearly homogeneous universes, whereas this difference vanishes in inhomogeneity dominated cases. The presented analysis constitutes the first systematic effective study of an inhomogeneous system within Loop Quantum Cosmology, and it proves the robustness of the results obtained for homogeneous cosmologies in this context.
21 pages, 11 figures,

 

[MTd2]

This is the 3rd version of the paper, posted today, and the authors changed their minds concerning many of their negative attitudes towards spin foam.

http://arxiv.org/abs/1103.1428

Graviton propagator asymptotics and the classical limit of ELPR/FK spin foam models

Aleksandar Mikovic, Marko Vojinovic
(Submitted on 8 Mar 2011 (v1), last revised 20 Jun 2011 (this version, v3))
We study the classical limit of the ELPR/FK spin foam models by analyzing the large-distance asymptotics of the corresponding graviton propagators. This is done by examining the large-spin asymptotics of the Hartle-Hawking wavefunction which is peaked around a classical flat spatial geometry. By using the stationary phase method we determine the wavefunction asymptotics. The obtained asymptotics does not give the desired large-distance asymptotics for the corresponding graviton propagator. However, we show that the ELPR/FK vertex amplitude can be redefined such that the corresponding Hartle-Hawking wavefunction gives the desired asymptotics for the graviton propagator.

The second is related to Verlinde's gravity and the last to Volovik's analog model.

http://arxiv.org/abs/1103.1428

Particles statistics of the holographic screen lead to Modified Newtonian Dynamics(MOND)

E. Pazy
(Submitted on 21 Jun 2011)
Employing a thermodynamical interpretation of gravity based on the holographic principle and assuming underlaying particle statistics, fermionic or bosonic, for excitations of the holographic screen leads to Modified Newtonian Dynamics (MOND) in the low energy limit. A connection between the acceleration scale a_0 appearing in MOND and the Fermi energy of the holographic fermionic degrees of freedom is obtained. In this formulation the physics of MOND results from the fermionic specific heat. Due to the to dimensionality of the screen the formalism is general and applies to two dimensional bosonic excitations as well. Based on the statistical mechanical interpretation of MOND, its cosmological implications are understood: the connection between a_0 and the Hubble constant is described as a quantum uncertainty relation; and the relationship between a_0 and the http://arxiv.org/abs/1002.4651

Quantum Non-Gravity and Stellar Collapse

C. Barcelo, L.J. Garay, G. Jannes
(Submitted on 24 Feb 2010 (v1), last revised 21 Jun 2011 (this version, v2))
Observational indications combined with analyses of analogue and emergent gravity in condensed matter systems support the possibility that there might be two distinct energy scales related to quantum gravity: the scale that sets the onset of quantum gravitational effects $E_B$ (related to the Planck scale) and the much higher scale $E_L$ signalling the breaking of Lorentz symmetry. We suggest a natural interpretation for these two scales: $E_L$ is the energy scale below which a special relativistic spacetime emerges, $E_B$ is the scale below which this spacetime geometry becomes curved. This implies that the first `quantum' gravitational effect around $E_B$ could simply be that gravity is progressively switched off, leaving an effective Minkowski quantum field theory up to much higher energies of the order of $E_L$. This scenario may have important consequences for gravitational collapse, inasmuch as it opens up new possibilities for the final state of stellar collapse other than an evaporating black hole.

 

[marcus]

brief mention (not Loop-and-allied QG, but may be of general interest):

http://arxiv.org/abs/1106.4789
Knot Invariants from Four-Dimensional Gauge Theory
Davide Gaiotto, Edward Witten
(Submitted on 23 Jun 2011)
It has been argued based on electric-magnetic duality and other ingredients that the Jones polynomial of a knot in three dimensions can be computed by counting the solutions of certain gauge theory equations in four dimensions. Here, we attempt to verify this directly by analyzing the equations and counting their solutions, without reference to any quantum dualities. After suitably perturbing the equations to make their behavior more generic, we are able to get a fairly clear understanding of how the Jones polynomial emerges...
117 pages

 

[marcus]

http://arxiv.org/abs/1106.5473
Coupling of gravity to matter, spectral action and cosmic topology
Branimir Cacic, Matilde Marcolli, Kevin Teh
(Submitted on 27 Jun 2011)
We consider a model of modified gravity based on the spectral action functional, for a cosmic topology given by a spherical space form, and the associated slow-roll inflation scenario. We consider then the coupling of gravity to matter determined by an almost commutative geometry over the spherical space form. We show that this produces a multiplicative shift of the amplitude of the power spectra for the density fluctuations and the gravitational waves, by a multiplicative factor equal to the total number of fermions in the matter sector of the model. We obtain the result by an explicit nonperturbative computation, based on the Poisson summation formula and the spectra of twisted Dirac operators on spherical space forms, as well as by a heat-kernel computation.
28 pages

http://arxiv.org/abs/1106.5095
Invariants of Spin Networks from Braided Ribbon Networks
Jonathan Hackett
(Submitted on 25 Jun 2011)
We connect Braided Ribbon Networks to the states of loop quantum gravity. Using this connection we present the reduced link as an invariant which captures information from the embedding of the spin-networks. We also present a means of understanding higher valent nodes in the context of braided ribbon networks and an interpretation of the dual of these nodes as polygons or polyhedra.
11 pages, 21 figures

http://arxiv.org/abs/1106.5096
Invariants of Braided Ribbon Networks
Jonathan Hackett
(Submitted on 25 Jun 2011)
We present a consistent definition for braided ribbon networks in 3-dimensional manifolds, unifying both three and four valent networks in a single framework. We present evolution moves for these networks which are dual to the Pachner moves on simplices and present an invariant of this evolution. Finally we relate these results back to previous work in the subject.
11 pages, 17 figures

http://arxiv.org/abs/1106.5397
Five-Dimensional Mechanics as the Starting Point for the Magueijo-Smolin Doubly Special Relativity
B. F. Rizzuti, A. A. Deriglazov
(Submitted on 27 Jun 2011)
We discuss a way to obtain the doubly special relativity kinematical rules (the deformed energy-momentum relation and the nonlinear Lorentz transformations of momenta) starting from a singular Lagrangian action of a particle with linearly realized SO(1,4) symmetry group. The deformed energy-momentum relation appears in a special gauge of the model. The nonlinear transformations of momenta arise from the requirement of covariance of the chosen gauge.
5 pages, to be published in Physics Letters B

 

[marcus]

http://arxiv.org/abs/1106.5710
Relative Locality in kappa-Poincaré
Giulia Gubitosi, Flavio Mercati
(Submitted on 28 Jun 2011)
We show that the kappa-Poincaré Hopf algebra can be interpreted in the framework of curved momentum space leading to the relativity of locality [reference AFKS]. We study the geometric properties of the momentum space described by kappa-Poincaré, and derive the consequences for particles propagation and energy-momentum conservation laws in interaction vertices, obtaining for the first time a coherent and fully workable model of the deformed relativistic kinematics implied by kappa-Poincaré. We describe the action of boost transformations on multi-particles systems, showing that in order to keep covariant the composed momenta it is necessary to introduce a dependence of the rapidity parameter on the particles momenta themselves. Finally, we show that this particular form of the boost transformations keeps the validity of the relativity principle, demonstrating the invariance of the equations of motion under boost transformations.
20 pages, 3 figures, 1 table

 

[marcus]

http://arxiv.org/abs/1106.6016
Holonomy observables in Ponzano-Regge type state sum models
John W. Barrett, Frank Hellmann
(Submitted on 29 Jun 2011)
We study observables on group elements in the Ponzano-Regge model. We show that these observables have a natural interpretation in terms of Feynman diagrams on a sphere and contrast them to the well studied observables on the spin labels. We elucidate this interpretation by showing how they arise from the no-gravity limit of the Turaev-Viro model and Chern-Simons theory.
5 pages, 2 figures

http://arxiv.org/abs/1106.5787
Geometry of fractional spaces
Gianluca Calcagni
(Submitted on 28 Jun 2011)
We introduce fractional flat space, described by a continuous geometry with constant non-integer Hausdorff and spectral dimensions. This is the analogue of Euclidean space, but with anomalous scaling and diffusion properties. The basic tool is fractional calculus, which is cast in a way convenient for the definition of the differential structure, distances, volumes, and symmetries. By an extensive use of concepts and techniques of fractal geometry, we clarify the relation between fractional calculus and fractals, showing that fractional spaces can be regarded as fractals when the ratio of their Hausdorff and spectral dimension is greater than one. All the results are analytic and constitute the foundation for field theories living on multi-fractal spacetimes, which will be presented in a companion paper.
1+77 pages, 6 figures, 4 tables

Brief mention (not directly connected with Loop-and-allied gravity but possibly of interest):

http://arxiv.org/abs/1106.5868
Massive Gravity Simplified: A Quadratic Action
Ali H. Chamseddine, Viatcheslav Mukhanov
(Submitted on 29 Jun 2011)
We present a simplified formulation of massive gravity where the Higgs fields have quadratic kinetic term. This new formulation allows us to prove in a very explicit way that all massive gravity theories considered so far inevitably have Boulware-Deser ghost in non-trivial fluctuations of background metric.
7 pages

http://arxiv.org/abs/1106.5894
Black Hole Masses are Quantized
Gia Dvali, Cesar Gomez, Slava Mukhanov
(Submitted on 29 Jun 2011)
We give a simple argument showing that in any sensible quantum field theory the masses of black holes cannot assume continuous values and must be quantized... In particular, this implies, that black holes cannot emit/absorb arbitrarily soft quanta. The effect has phenomenological model-independent implications for black holes and other classicalons that may be created at LHC. We predict, that contrary to naive intuition, the black holes and/or classicalons, will be produced in form of fully-fledged quantum resonances of discrete masses, with the level-spacing controlled by the inverse square-root of cross-section.
23 pages

 

[marcus]

brief mention:
http://arxiv.org/abs/1106.6073
The gauge/gravity duality
Juan Maldacena
(Submitted on 29 Jun 2011)
Short introduction...
Chapter of the book "Black Holes in Higher Dimensions" to be published by Cambridge University Press, editor: G. Horowitz. 23 pages

 

[MTd2]

http://arxiv.org/abs/1106.6346

Comment on [arXiv:1106.1417] "Small Lorentz violations in quantum gravity: do they lead to unacceptably large effects?"

Joseph Polchinski
(Submitted on 30 Jun 2011)
A recent paper by Gambini, Rastgoo and Pullin [arXiv:1106.1417] investigates the important issue of constraints from Lorentz invariance on Planck scale physics, arguing that the classic analysis of Collins, Perez, Sudarsky, Urrutia and Vucetich is not valid. We argue that the new work is based on models that do not capture the relevant physics, and that the constraints are in fact strong.

 

[atyy]

http://arxiv.org/abs/1106.6153
Split noncommutativity and compactified brane solutions in matrix models
Harold Steinacker
(Submitted on 30 Jun 2011)
Solutions of the undeformed IKKT matrix model with structure R^{3,1} x K are presented, where the noncommutativity relates the compact with the non-compact space. The extra dimensions are stabilized by angular momentum, and the scales of K are generic moduli of the solutions. Explicit solutions are given for K= T^2, K= T^4, K = S^2 x T^2 and K = S^2 x S^2. Infinite towers of Kaluza-Klein modes may arise in some directions, along with an effective UV cutoff on the non-compact space. Deformations of these solutions carry NC gauge theory coupled to (emergent) gravity. Analogous solutions of the BFSS model are also given.

http://arxiv.org/abs/1106.6166
Propagators and Matrix Basis on Noncommutative Minkowski Space
Andre Fischer, Richard J. Szabo
(Submitted on 30 Jun 2011)
We describe an analytic continuation of the Euclidean Grosse-Wulkenhaar and LSZ models which defines a one-parameter family of duality covariant noncommutative field theories interpolating between Euclidean and Minkowski space versions of these models, and provides an alternative regularization to the usual Feynman prescription. This regularization allows for a matrix model representation of the field theories in terms of a complex generalization of the usual basis of Landau wavefunctions. The corresponding propagators are calculated and identified with the Feynman propagators of the field theories. The regulated quantum field theories are shown to be UV/IR-duality covariant. We study the asymptotics of the regularized propagators in position and matrix space representations, and confirm that they generically possesses a comparably good decay behaviour as in the Euclidean case.

 

[atyy]

http://arxiv.org/abs/1107.0265
Intersecting branes and a standard model realization in matrix models
Athanasios Chatzistavrakidis, Harold Steinacker, George Zoupanos
(Submitted on 1 Jul 2011)
We consider intersecting brane solutions of the type IIB matrix model. It is shown that ermionic zero-modes arise on such backgrounds, localized at the brane intersections. They lead to chiral fermions in four dimensions under certain conditions. Such configurations reproduce many of the welcome features in similar string-theoretic constructions. Therefore they can be used to construct semi-realistic particle physics models in the framework of Yang-Mills matrix models. In particular, we present a brane configuration which realizes the correct chiral spectrum of the standard model in the matrix model. Furthermore, the stability of intersecting branes is discussed by analyzing the 1-loop effective action. It is shown that intersecting branes may form a bound state for certain flux configurations. The four-dimensional geometry of the branes is generic, and determined by the (emergent) gravity sector of the matrix model.

 

[marcus]

http://arxiv.org/abs/1107.0709
The Plebanski sectors of the EPRL vertex
Jonathan Engle
(Submitted on 4 Jul 2011)
Modern spin-foam models of four dimensional gravity are based on a discrete version of the Spin(4) Plebanski formulation. Beyond what is already in the literature, we clarify the meaning of different Plebanski sectors in this classical discrete model. We show that the linearized simplicity constraints used in the EPRL and FK models are not sufficient to impose a restriction to a single Plebanski sector, but rather, three Plebanski sectors are mixed. We propose this as the reason for certain extra 'undesired' terms in the asymptotics of the EPRL vertex analyzed by Barrett et al. This explanation for the extra terms is new and different from that sometimes offered in the spin-foam literature thus far.
17 pages

http://arxiv.org/abs/1107.0318
Chern-Simons Inflation and Baryogenesis
Stephon Alexander, Antonino Marciano, David Spergel
(Submitted on 1 Jul 2011)
We propose a model of inflation where the Chern-Simons interaction and vector fields play a central role in generating an inflationary epoch. As a result, in accord with the APS mechanism, the Sakharov conditions for baryogenesis are self-consistently satisfied, and we calculate the net baryon asymmetry index in terms of the gauge configuration necessary for inflation, based on the chiral anomaly. Inflation begins with a large plasma density of interacting gauge fields and fermions, which interact through gravity and the Chern Simons term. The Chern-Simons term drives power from an initial white-noise spectrum of gauge fields into a narrow-band of superhorizon wave vectors. At the same time, the fermionic current and metric coupling amplifies the gauge field on superhorizon scales. This phase-correlation and amplification of the gauge field produces the correct conditions to maintain more than 60 e-folds of inflation. Eventually the gauge field dissipates by producing the observed baryon asymmetry n_b/s ~ 10^-10, through the chiral anomaly and inflation ends.
7 pages

http://arxiv.org/abs/1107.0708
Generalized Uncertainty Principle and Self-dual Black Holes
Bernard Carr, Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 4 Jul 2011)
The Generalized Uncertainty Principle suggests corrections to the Uncertainty Principle as the energy increases towards the Planck value. It provides a natural transition between the expressions for the Compton wavelength below the Planck mass and the black hole event horizon size above this mass. It also suggests corrections to the the event horizon size as the black hole mass falls towards the Planck value, leading to the concept of a Generalized Event Horizon. Extrapolating below the Planck mass suggests the existence of a new class of black holes, whose size is of order the Compton wavelength for their mass. Such sub-Planckian black holes have recently been discovered in the context of loop quantum gravity and it is possible that this applies more generally. This suggests an intriguing connection between black holes, the Uncertainty Principle and quantum gravity.
13 Pages, 6 figures

http://arxiv.org/abs/1107.0698
Scalar Field Theory on a Causal Set in Histories Form
Rafael D. Sorkin
(Submitted on 4 Jul 2011)
We recast into histories-based form a quantum field theory defined earlier in operator language for a free scalar field on a background causal set. The resulting decoherence functional resembles that of the continuum theory. The counterpart of the d'Alembertian operator is nonlocal and is a generalized inverse of the discrete retarded Green function. We comment on the significance of this and we also suggest how to include interactions.
25 pages.

http://arxiv.org/abs/1107.0650
Pure 4-geometry of quantum magnetic spin matter from Kondo effect
T. Asselmeyer-Maluga, J. Krol
(Submitted on 4 Jul 2011)
We determine a smooth Euclidean 4-geometry on R⁴ from quantum interacting spin matter like in the multichannel Kondo effect. The CFT description of both: the k-channel Kondo effect of spin magnetic impurities quantum interacting with spins of conducting electrons and exotic smooth R⁴, by the level k WZW model on SU(2), indicates the relation between smooth R⁴'s and the quantum matter. We propose a model which shows: exotic smooth R⁴_k generates fermionic fields via the topological structure of Casson handles and when this handle is attached to some subspace A of R⁴ these fermions represent electrons bounded by the magnetic impurity. Thus the Kondo bound state of k conducting electrons with magnetic impurity of spin s is created like in the low temperature Kondo effect. Then the quantum character of the interactions is encoded in 4-exoticness. The complexity as well the number of Casson handles correspond to the number of channels in the Kondo effect. When the smoothness structure is the standard one, no quantum interactions are carried on by standard R⁴.
7 pages

 

[MTd2]

http://arxiv.org/abs/1107.0870

Classical Dimensional Transmutation and Confinement

Gia Dvali, Cesar Gomez, Slava Mukhanov
(Submitted on 5 Jul 2011)
We observe that probing certain classical field theories by external sources uncovers the underlying renormalization group structure, including the phenomenon of dimensional transmutation, at purely-classical level. We perform this study on an example of $\lambda\phi^{4}$ theory and unravel asymptotic freedom and triviality for negative and positives signs of $\lambda$ respectively. We derive exact classical $\beta$ function equation. Solving this equation we find that an isolated source has an infinite energy and therefore cannot exist as an asymptotic state. On the other hand a dipole, built out of two opposite charges, has finite positive energy. At large separation the interaction potential between these two charges grows indefinitely as a distance in power one third.

**** They don't cite Reuter! Why?!?

 

[marcus]

http://arxiv.org/abs/1107.1320
Black hole entropy and isolated horizons thermodynamics
Amit Ghosh, Alejandro Perez
(Submitted on 7 Jul 2011)
We present a statistical mechanical calculation of the thermodynamical properties of (non rotating) isolated horizons. The introduction of Planck scale allows for the definition of an universal horizon temperature (independent of the mass of the black hole) and a well-defined notion of energy (as measured by suitable local observers) proportional to the horizon area in Planck units. The microcanonical and canonical ensembles associated with the system are introduced. Black hole entropy and other thermodynamical quantities can be consistently computed in both ensembles and results are in agreement with Hawking's semiclassical analysis for all values of the Immirzi parameter.
5 pages

brief mention:
http://arxiv.org/abs/1107.1421
Penrose's circles in the CMB and a test of inflation
Paul Tod
(Submitted on 7 Jul 2011)
We present a calculation of the angular size of the circles in the CMB predicted by Penrose on the basis of his Conformal Cyclic Cosmology. If these circles are detected, the existence of an upper limit on their angular radius would provide a challenge for inflation.
6 pages

 

[marcus]

http://arxiv.org/abs/1107.1540
Observational test of inflation in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa
(Submitted on 8 Jul 2011)
We study in detail the power spectra of scalar and tensor perturbations generated during inflation in loop quantum cosmology (LQC). After clarifying in a novel quantitative way how inverse-volume corrections arise in inhomogeneous settings, we show that they can generate large running spectral indices, which generally lead to an enhancement of power at large scales. We provide explicit formulas for the scalar/tensor power spectra under the slow-roll approximation, by taking into account corrections of order higher than the runnings. We place observational bounds on the inverse-volume quantum correction δ ~ a^-σ (σ >0, a is the scale factor) and the slow-roll parameter ε_V for power-law potentials as well as exponential potentials by using the data of WMAP 7yr combined with other observations. We derive the constraints on δ for two pivot wavenumbers k₀ for several values of δ. The quadratic potential can be compatible with the data even in the presence of the LQC corrections, but the quartic potential is in tension with observations. We also find that the upper bounds on δ (k₀) for given σ and k₀ are insensitive to the choice of the inflaton potentials.
1+36 pages, 6 figures, 1 table

 

[marcus]

http://arxiv.org/abs/1107.1724
Relative-locality distant observers and the phenomenology of momentum-space geometry
Giovanni Amelino-Camelia, Michele Arzano, Jerzy Kowalski-Glikman, Giacomo Rosati, Gabriele Trevisan
(Submitted on 8 Jul 2011)
We study the translational invariance of the relative-locality framework proposed in arXiv:1101.0931, which had been previously established only for the case of a single interaction. We provide an explicit example of boundary conditions at endpoints of worldlines, which indeed ensures the desired translational invariance for processes involving several interactions, even when some of the interactions are causally connected (particle exchange). We illustrate the properties of the associated relativistic description of distant observers within the example of a kappa-Poincaré-inspired momentum-space geometry, with de Sitter metric and parallel transport governed by a non-metric and torsionful connection. We find that in such a theory simultaneously-emitted massless particles do not reach simultaneously a distant detector, as expected in light of the findings of arXiv:1103.5626 on the implications of non-metric connections. We also show that the theory admits a free-particle limit, where the relative-locality results of arXiv:1102.4637 are reproduced. We establish that the torsion of the kappa-Poincaré connection introduces a small (but observably-large) dependence of the time of detection, for simultaneously-emitted particles, on some properties of the interactions producing the particles at the source.
45 pages, 10 figures

 

[marcus]

http://arxiv.org/abs/1107.2310
Discretizing parametrized systems: the magic of Ditt-invariance
Carlo Rovelli
(Submitted on 12 Jul 2011)
Peculiar phenomena appear in the discretization of a system invariant under reparametrization. The structure of the continuum limit is markedly different from the usual one, as in lattice QCD. First, the continuum limit does not require tuning a parameter in the action to a critical value. Rather, there is a regime where the system approaches a sort of asymptotic topological invariance ("Ditt-invariance"). Second, in this regime the expansion in the number of discretization points provides a good approximation to the transition amplitudes. These phenomena are relevant for understanding the continuum limit of quantum gravity. I illustrate them here in the context of a simple system.
9 pages, 4 figures

Brief mention (not quantum gravity but of potential interest to QG research):
http://arxiv.org/abs/1107.0727
Testing model independent modified gravity with future large scale surveys
Daniel B. Thomas, Carlo R. Contaldi
(Submitted on 4 Jul 2011)
Model-independent parametrisations of modified gravity have attracted a lot of attention over the past few years and numerous combinations of experiments and observables have been suggested to constrain the parameters used in these models. Galaxy clusters have been mentioned, but not looked at as extensively in the literature as some other probes. Here we look at adding galaxy clusters into the mix of observables and examine how they could improve the constraints on the modified gravity parameters. In particular, we forecast the constraints from combining Planck satellite Cosmic Microwave Background (CMB) measurements and Sunyaev-Zeldovich (SZ) cluster catalogue with a DES-like weak lensing survey. We find that cluster counts significantly improve the constraints over those derived using CMB and WL. We then look at surveys further into the future, to see how much better it may be feasible to make the constraints.
14 pages, 8 figures

 

[MTd2]

http://arxiv.org/abs/1107.2403

Super-renormalizable Quantum Gravity

Leonardo Modesto
(Submitted on 12 Jul 2011)
In this paper we study perturbatively an extension of the Stelle higher derivative gravity involving an infinite number of derivative terms. We know that the usual quadratic action is renormalizable but suffers of the unitarity problem because of the presence of a ghost (state of negative norm) in the theory. The new theory is instead ghost-free since the introduction of (in general) two entire functions in the model with the property do not introduce new poles in the propagator. The local high derivative theory is recovered expanding the entire functions to the lowest order in the mass scale of the theory. Any truncation of the entire functions gives rise to the unitarity violation but if we keep all the infinite series we do not fall into these troubles. The theory is renormalizable at one loop and finite from two loops on. Since only a finite number of graphs are divergent then the theory is super-renormalizable. We analyze the fractal properties of the theory at high energy showing a reduction of the spacetime dimension at short scales. Black hole spherical symmetric solutions are also studied omitting the high curvature corrections in the equation of motions. The solutions are regular and the classical singularity is replaced by a "de Sitter-like core" in r=0. Black holes may show a "multi-horizon" structure depending on the value of the mass.

http://arxiv.org/abs/1107.2633

Many-nodes/many-links spinfoam: the homogeneous and isotropic case

Francesca Vidotto
(Submitted on 13 Jul 2011)
I compute the Lorentzian EPRL/FK/KKL spinfoam vertex amplitude for regular graphs, with an arbitrary number of links and nodes, and coherent states peaked on a homogeneous and isotropic geometry. This form of the amplitude can be applied for example to a dipole with an arbitrary number of links or to the 4-simplex given by the compete graph on 5 nodes. All the resulting amplitudes have the same support, independently of the graph used, in the large j (large volume) limit. This implies that they all yield the Friedmann equation: I show this in the presence of the cosmological constant. This result indicates that in the semiclassical limit quantum corrections in spinfoam cosmology do not come from just refining the graph, but rather from relaxing the large j limit.

 

[marcus]

http://arxiv.org/abs/1107.2678
Effective Conformal Descriptions of Black Hole Entropy
Steven Carlip
(Submitted on 13 Jul 2011)
It is no longer considered surprising that black holes have temperatures and entropies. What remains surprising, though, is the universality of these thermodynamic properties: their exceptionally simple and general form, and the fact that they can be derived from many very different descriptions of the underlying microscopic degrees of freedom. I review the proposal that this universality arises from an approximate conformal symmetry, which permits an effective "conformal dual" description that is largely independent of the microscopic details.
27 pages; solicited review article, to appear in Entropy

http://arxiv.org/abs/1107.2712
de Sitter invariance of the dS graviton vacuum
Atsushi Higuchi, Donald Marolf, Ian A. Morrison
(Submitted on 14 Jul 2011)
The two-point function of linearized gravitons on de Sitter space is infrared divergent in the standard transverse traceless synchronous gauge defined by k=0 cosmological coordinates (also called conformal or Poincare coordinates). We show that this divergence can be removed by adding a linearized diffeomorphism to each mode function; i.e., by an explicit change of gauge. It follows that the graviton vacuum state is well-defined and de Sitter invariant in agreement with various earlier arguments.
13 pages, 1 figure

http://arxiv.org/abs/1107.2669
Quantum Noise in Amplifiers and Hawking/Dumb-Hole Radiation as Amplifier Noise
W. G. Unruh
(Submitted on 13 Jul 2011)
The quantum noise in a linear amplifier is shown to be thermal noise. The theory of linear amplifiers is applied first to the simplest, single or double oscillator model of an amplifier, and then to linear model of an amplifier with continuous fields and input and outputs. Finally it is shown that the thermal noise emitted by black holes first demonstrated by Hawking, and of dumb holes (sonic and other analogs to black holes), arises from the same analysis as for linear amplifiers. The amplifier noise of black holes acting as amplifiers on the quantum fields living in the spacetime surrounding the black hole is the radiation discovered by Hawking. For any amplifier, that quantum noise is completely characterized by the attributes of the system regarded as a classical amplifier, and arises out of those classical amplification factors and the commutation relations of quantum mechanics.
13 pages, 4 figures

brief mention (possibly of general interest):
There is a short non-technical article by Gennady Gorelik in the SciAm recalling the 1936 roots of Quantum Gravity in the work of Matvei Bronstein (1906-1938):
http://www.scientificamerican.com/blog/post.cfm?id=why-is-quantum-gravity-so-hard-and-2011-07-15
==quote Gorelik's July 2011 SciAm article==

...In Bronstein's full-dress research on quantum gravity, published in 1936, the most fascinating part was the essential difference between quantum electrodynamics and the quantum theory of gravity. Bronstein showed that the quantum limit of measurability would become apparent for particles with a certain characteristic mass, which is now known as the Planck mass (which Max Planck had introduced in 1900 with no reference to quantum gravity). Since gravity in general relativity is described by geometry, Bronstein concluded that the difficulty of quantizing gravity brought the entire nature of space and time into question: "The elimination of the logical inconsistencies ... requires a radical reconstruction of the theory, and in particular, the rejection of a Riemannian geometry dealing, as we have seen here, with quantities which are unobservable in principle, and perhaps also the rejection of our ordinary concepts of space and time, replacing them by some much deeper and nonevident concepts. Wer's nicht glaubt, bezahlt einen Thaler."
==endquote==
For a related PF thread, to which Gennady Gorelik himself contributed photos and further details, see:
https://www.physicsforums.com/showthread.php?t=375799

 

[atyy]

http://arxiv.org/abs/1107.2897
What we don't know about time
Vijay Balasubramanian
(Submitted on 14 Jul 2011)
String theory has transformed our understanding of geometry, topology and spacetime. Thus, for this special issue of Foundations of Physics commemorating "Forty Years of String Theory", it seems appropriate to step back and ask what we do not understand. As I will discuss, time remains the least understood concept in physical theory. While we have made significant progress in understanding space, our understanding of time has not progressed much beyond the level of a century ago when Einstein introduced the idea of space-time as a combined entity. Thus, I will raise a series of open questions about time, and will review some of the progress that has been made as a roadmap for the future.

http://arxiv.org/abs/1107.2610
The Purely Geometric Part of "Dark Matter" -- A Fresh Playground for "String Theory"
Hagen Kleinert
(Submitted on 13 Jul 2011)
We argue that part of "dark matter" is not made of matter, but of the singular world-surfaces in the solutions of Einstein's vacuum field equation G_{\mu\nu}=0. Their Einstein-Hilbert action governs also their quantum fluctuation. It coincides with the action of closed bosonic "strings" in four spacetime dimensions, which appear here in a new physical context. Thus, part of dark matter is of a purely geometric nature, and its quantum physics is governed by the same string theory, whose massless spin-2 particles interact like the quanta of Einstein's theory.

http://arxiv.org/abs/1107.2839
Discrete gravity from statistical mechanics
Antonio Enea Romano
(Submitted on 14 Jul 2011)
We show how to construct space time lattices with a Regge action proportional to the energy of a given Ising or Potts model macrostate. This allows to take advantage of the existence of exact solutions for these models to calculate the quantum wave function of the universe using the sum over the histories approach to quantum gravity. Motivated by this isomorphism we show how the Regge equations, i.e. the discrete equivalent of the vacuum Einstein equations, can be derived using statistical mechanics under the assumption that the energy of a given space time geometry is proportional to the Regge action.

 

[marcus]

http://arxiv.org/abs/1107.3122
Classical Group Field Theory
Joseph Ben Geloun
(Submitted on 15 Jul 2011)
The ordinary formalism for classical field theory is applied to dynamical group field theories. Focusing first on a local group field theory over one copy of SU(2) and, then, on more involved nonlocal theories (colored and non colored) defined over a tensor product of the same group, we address the issue of translation and dilatation symmetries and the corresponding Noether theorem. The energy momentum tensor and dilatation current are derived and their properties identified for each case.

http://arxiv.org/abs/1107.3110
Asymptotic safety goes on shell
Dario Benedetti
(Submitted on 15 Jul 2011)
It is well known in quantum field theory that the off-shell effective action depends on the gauge choice and field parametrization used in calculating it. Nevertheless, the typical scheme in which the scenario of asymptotically safe gravity is investigated is an off-shell version of the functional renormalization group equation. Working with the Einstein-Hilbert truncation as a test bed, we develop a new scheme for the analysis of asymptotically safe gravity in which the on-shell part of the effective action is singled out and we show that the beta function for the essential coupling has no explicit gauge-dependence. In order to reach our goal, we introduce several technical novelties, including a different decomposition of the metric fluctuations, a new implementation of the ghost sector, and a new cut-off scheme. We find a non-trivial fixed point, with a value of the cosmological constant which is independent of the gauge-fixing parameters.
20 pages

 

[marcus]

http://arxiv.org/abs/1107.3164
A proposal for testing Quantum Gravity in the lab
Ahmed Farag Ali, Saurya Das, Elias C. Vagenas
(Submitted on 15 Jul 2011)
Attempts to formulate a quantum theory of gravitation are collectively known as {\it Quantum Gravity}. Various approaches to Quantum Gravity such as String Theory and Loop Quantum Gravity, as well as Black Hole Physics and Doubly Special Relativity Theories predict a minimum measurable length, or a maximum observable momentum, and related modifications of the Heisenberg Uncertainty Principle to a so-called Generalized Uncertainty Principle (GUP). We have proposed a GUP consistent with String Theory, Black Hole Physics and Doubly Special Relativity Theories, and have showed that this modifies all quantum mechanical Hamiltonians. When applied to an elementary particle, it suggests that the space which confines it must be quantized, and in fact that all measurable lengths are quantized in units of a fundamental length (which can be the Planck length). On the one hand, this may signal the breakdown of the spacetime continuum picture near that scale, and on the other hand, it can predict an upper bound on the quantum gravity parameter in the GUP, from current observations. Furthermore, such fundamental discreteness of space may have observable consequences at length scales much larger than the Planck scale. Because this influences all the quantum Hamiltonians in an universal way, it predicts quantum gravity corrections to various quantum phenomena. Therefore, in the present work we compute these corrections to the Lamb Shift, simple harmonic oscillator, Landau levels, and the tunneling current in a scanning tunneling microscope.
10 pages, accepted for publication in Phys. Rev. D., has substantial overlap with arXiv:0906.5396 , published in a different journal.

http://arxiv.org/abs/1107.3334
Modeling transverse relative locality
Giovanni Amelino-Camelia, Leonardo Barcaroli, Niccoló Loret
(Submitted on 17 Jul 2011)
We investigate some aspects of relativistic classical theories with "relative locality", in which pairs of events established to be coincident by nearby observers may be described as non-coincident by distant observers. While previous studies focused mainly on the case of longitudinal relative locality, where the effect occurs along the direction connecting the distant observer to the events, we here focus on transverse relative locality, in which instead the effect is found in a direction orthogonal to the one connecting the distant observer to the events. Our findings suggest that, at least for theories of free particles such as the one in arXiv:1006.2126, transverse relative locality is as significant as longitudinal relative locality both conceptually and quantitatively. And we observe that "dual gravity lensing", first discussed in arXiv:1103.5626, can be viewed as one of two components of transverse relative locality. We also speculate about a type of spacetime noncommutativity for which transverse relative locality could be particularly significant.
13 pages

 

[MTd2]

http://arxiv.org/abs/1107.3144

Earthy Lagrangian for Arbitrary Spin

Benjamin Koch, Nicolas Rojas
(Submitted on 15 Jul 2011)
A simple Lagrangian is proposed that by the choice of the representation of SU(2), gives rise to field equations for arbitrary spin. In explicit examples it is shown, how the Klein-Gordon, the Dirac, and the Proca equation can be obtained from this Lagrangian. On the same footing field equations for arbitrary spin are given. Finally, symmetries are discussed, the fields are quantized, and Feynman rules are derived.

http://arxiv.org/abs/1107.3448

On the origin of the holographic principle

Jae-Weon Lee
(Submitted on 18 Jul 2011)
Recently, it has been suggested that quantum mechanics and gravity are not fundamental but emerge from information loss at causal horizons. Based on the formalism it is shown that the holographic principle also aries naturally from the information loss of bulk fields observed by an outside observer. As an application, Witten's prescription for the Rindler spacetime is derived.

http://arxiv.org/abs/1107.3458

Constructing a quantum field theory from spacetime

Torsten Asselmeyer-Maluga, Jerzy Krol
(Submitted on 18 Jul 2011)
The paper shows deep connections between exotic smoothings of a small R^4 (the spacetime), the leaf space of codimension-1 foliations (related to noncommutative algebras) and quantization. At first we relate a small exotic R^4 to codimension-1 foliations of the 3-sphere unique up to foliated cobordisms and characterized by the real-valued Godbillon-Vey invariant. Special care is taken for the integer case which is related to flat PSL(2,R)-$bundles. Then we discuss the leaf space of the foliation using noncommutative geometry. This leaf space contains the hyperfinite III_1 factor of Araki and Woods important for quantum field theory (QFT) and the I_{\infty} factor. Using Tomitas modular theory, one obtains a relation to a factor II_{\infty} algebra given by the horocycle foliation of the unit tangent bundle of a surface S of genus g>1. The relation to the exotic R^4 is used to construct the (classical) observable algebra as Poisson algebra of functions over the character variety of representations of the fundamental group \pi_{1}(S) into the SL(2,C). The Turaev-Drinfeld quantization (as deformation quantization) of this Poisson algebra is a (complex) skein algebra which is isomorphic to the hyperfinite factor II_{1} algebra determining the factor II_{\infty}=II_{1}\otimes I_{\infty} algebra of the horocycle foliation. Therefore our geometrically motivated hyperfinite III_1 factor algebra comes from the quantization of a Poisson algebra. Finally we discuss the states and operators to be knots and knot concordances, respectively.

 

[marcus]

brief mention (a curious result--not QG but may be of general interest)
http://arxiv.org/abs/1107.4337
Can an astrophysical black hole have a topologically non-trivial event horizon?
Cosimo Bambi, Leonardo Modesto
(Submitted on 21 Jul 2011)
In 4-dimensional General Relativity, there are several theorems restricting the topology of the event horizon of a black hole. In the stationary case, black holes must have a spherical horizon, while a toroidal spatial topology is allowed only for a short time. In this paper, we consider spinning black holes inspired by Loop Quantum Gravity and by alternative theories of gravity. We show that the spatial topology of the event horizon of these objects changes when the spin parameter exceeds a critical value...
7 pages, 2 figures

 

[marcus]

brief mention:
(A non-technical speculative essay by GFT expert Daniele Oriti.)
http://arxiv.org/abs/1107.4534
On the depth of quantum space
Daniele Oriti
(Submitted on 22 Jul 2011)
We focus on the question: "Is space fundamentally discrete or continuous?" in the context of current quantum gravity research. In particular, we paint a scenario based on the idea that 'quantum space' is a sort of peculiar condensed matter system, and on the speculation that its microscopic dynamics is described by a 'group field theory' formalism...
...We also speculate on further physical implications of the suggested scenario.
18 pages, 4 figures; submitted for the third FQXi Essay Contest: "Is Reality digital or analog?"

 

[MTd2]

http://arxiv.org/abs/1107.5002

Twistorial phase space for complex Ashtekar variables

Wolfgang M. Wieland
(Submitted on 25 Jul 2011)
We generalise the SU(2) spinor framework of twisted geometries developed by Dupuis, Freidel, Livine, Speziale and Tambornino to the Lorentzian case, that is the group SL(2,C). We show that the phase space for complex valued Ashtekar variables on a spinnetwork graph can be decomposed in terms of twistorial variables. To every link there are two twistors---one to each boundary point---attached. The formalism provides a new derivation of the solution space of the simplicity constraints of loop quantum gravity. Key properties of the EPRL spinfoam model are perfectly recovered.

http://arxiv.org/abs/1107.4605

Area law for black hole entropy in the SU(2) quantum geometry approach

P. Mitra
(Submitted on 22 Jul 2011)
SU(2) loop quantum gravity can be considered with the level k of the Chern-Simons theory held fixed. Then the black hole entropy is proportional to the area without any logarithmic correction. However, if k is made large, there is the -(3/2)log A correction.

http://arxiv.org/abs/1107.4921

Loop quantum $f(R)$ theories

Xiangdong Zhang, Yongge Ma
(Submitted on 25 Jul 2011)
As modified gravity theories, the 4-dimensional metric $f(R)$ theories are cast into connection dynamical formalism with real $su(2)$-connections as configuration variables. This formalism enables us to extend the non-perturbative loop quantization scheme of general relativity to any metric $f(R)$ theories. The quantum kinematical framework of $f(R)$ gravity is rigorously constructed, where the quantum dynamics can be launched. Both Hamiltonian constraint operator and master constraint operator for $f(R)$ theories are well defined. Our results show that the non-perturbative quantization procedure of loop quantum gravity are valid not only for general relativity but also for a rather general class of 4-dimensional metric theories of gravity.