Loop-and-allied QG bibliography

[atyy]

http://arxiv.org/abs/1004.0476
Perturbative Quantum Gravity from Gauge Theory
Zvi Bern, John Joseph M. Carrasco, Henrik Johansson
(Submitted on 4 Apr 2010)
In a previous paper we observed that (classical) tree-level gauge theory amplitudes can be rearranged to display a duality between color and kinematics. Once this is imposed, gravity amplitudes are obtained using two copies of gauge-theory diagram numerators. Here we suggest that this duality persists to all quantum loop orders and can thus be used to obtain multi-loop gravity amplitudes easily from gauge-theory ones. As a non-trivial test, we show that the three-loop four-point amplitude of N=4 super-Yang-Mills theory can be arranged into a form satisfying the duality, and by taking double copies of the diagram numerators we obtain the corresponding amplitude of N=8 supergravity. We also remark on a non-supersymmetric two-loop test based on pure Yang-Mills theory resulting in gravity coupled to an anti-symmetric tensor and dilaton.

http://arxiv.org/abs/1004.0576
Galactic rotation curves in modified gravity with non-minimal coupling between matter and geometry
T. Harko
(Submitted on 5 Apr 2010)
We investigate the possibility that the behavior of the rotational velocities of test particles gravitating around galaxies can be explained in the framework of modified gravity models with non-minimal matter-geometry coupling. Generally, the dynamics of test particles around galaxies, as well as the corresponding mass deficit, is explained by postulating the existence of dark matter. The extra-terms in the gravitational field equations with geometry-matter coupling modify the equations of motion of test particles, and induce a supplementary gravitational interaction. Starting from the variational principle describing the particle motion in the presence of the non-minimal coupling, the expression of the tangential velocity of a test particle, moving in the vacuum on a stable circular orbit in a spherically symmetric geometry, is derived. The tangential velocity depends on the metric tensor components, as well as of the coupling function between matter and geometry. The Doppler velocity shifts are also obtained in terms of the coupling function. If the tangential velocity profile is known, the coupling term between matter and geometry can be obtained explicitly in an analytical form. The functional form of this function is obtained in two cases, for a constant tangential velocity, and for an empirical velocity profile obtained from astronomical observations, respectively. Therefore, these results open the possibility of directly testing the modified gravity models with non-minimal coupling between matter and geometry by using direct astronomical and astrophysical observations at the galactic or extra-galactic scale.

http://arxiv.org/abs/1004.0597
The Superfluid Universe
G.E. Volovik
(Submitted on 5 Apr 2010)
We discuss phenomenology of quantum vacuum. Phenomenology of macroscopic systems has three sources: thermodynamics, topology and symmetry. Thermodynamics of the self-sustained vacuum allows us to treat the problems related to the vacuum energy: the cosmological constant problems. The natural value of the energy density of the equilibrium the self-sustained vacuum is zero. Cosmology is discussed as the process of relaxation of vacuum towards the equilibrium state. The present value of the cosmological constant is very small compared to the Planck scale, because the present Universe is very old and thus is close to equilibrium. Momentum space topology determines the universality classes of fermionic vacua. The Standard Model vacuum both in its massless and massive states is topological medium. The vacuum in its massless state shares the properties of superfluid 3He-A, which is topological superfluid. It belongs to the Fermi-point universality class, which has topologically protected fermionic quasiparticles. At low energy they behave as relativistic massless Weyl fermions. Gauge fields and gravity emerge together with Weyl fermions at low energy. This allows us to treat the hierarchy problem in Standard Model: the masses of elementary particles are very small compared to the Planck scale because the natural value of the quark and lepton masses is zero. The small nonzero masses appear in the infrared region, where the quantum vacuum acquires the properties of another topological superfluid, 3He-B, and 3+1 topological insulators. The other topological media in dimensions 2+1 and 3+1 are also discussed. In most cases, topology is supported by discrete symmetry of the underlying microscopic system, which indicates the important role of discrete symmetry in Standard Model.

 

[MTd2]

http://arxiv.org/abs/1004.0628

Fractional Dynamics from Einstein Gravity, General Solutions, and Black Holes

Sergiu I. Vacaru
(Submitted on 5 Apr 2010)
We study the fractional gravity for spacetimes with non-integer dimensions. Our constructions are based on a geometric formalism with the fractional Caputo derivative and integral calculus adapted to nonolonomic distributions. This allows us to define a fractional spacetime geometry with fundamental geometric/physical objects and a generalized tensor calculus all being similar to respective integer dimension constructions. Such models of fractional gravity mimic the Einstein gravity theory and various Lagrange-Finsler and Hamilton-Cartan generalizations in nonholonomic variables. The approach suggests a number of new implications for gravity and matter field theories with singular, stochastic, kinetic, fractal, memory etc processes. We prove that the fractional gravitational field equations can be integrated in very general forms following the anholonomic deformation method for constructing exact solutions. Finally, we study some examples of fractional black hole solutions, fractional ellipsoid gravitational configurations and imbedding of such objects in fractional solitonic backgrounds.

 

[MTd2]

http://arxiv.org/abs/1004.0718

Causal random geometry from stochastic quantization

J. Ambjorn, R. Loll, W. Westra, S. Zohren
(Submitted on 5 Apr 2010)
In this short note we review a recently found formulation of two-dimensional causal quantum gravity defined through Causal Dynamical Triangulations and stochastic quantization. This procedure enables one to extract the nonperturbative quantum Hamiltonian of the random surface model including the sum over topologies. Interestingly, the generally fictitious stochastic time corresponds to proper time on the geometries.

http://arxiv.org/abs/1004.0877

A No-go Theorem Prohibiting Inflation in the Entropic Force Scenario

Miao Li, Yi Pang
(Submitted on 6 Apr 2010)
We show that to accommodate inflation in the entropic force scenario of Verlinde, it is necessary to introduce a negative temperature on a holographic screen, this will introduce several puzzles such as energy non-conservation. If one tries to modify the derivation of the Einstein equations to avoid a negative temperature, we prove that it is impossible to find a proper new definition of temperature to derive the Einstein equations.

http://arxiv.org/abs/1004.0693

Gravity as the Square of Gauge Theory

Zvi Bern, Tristan Dennen, Yu-tin Huang, Michael Kiermaier
(Submitted on 5 Apr 2010)
We explore consequences of the recently discovered duality between color and kinematics, which states that kinematic numerators in a diagrammatic expansion of gauge-theory amplitudes can be arranged to satisfy Jacobi-like identities in one-to-one correspondence to the associated color factors. Using on-shell recursion relations, we give a field-theory proof showing that the duality implies that diagrammatic numerators in gravity are just the product of two corresponding gauge-theory numerators, as previously conjectured. These squaring relations express gravity amplitudes in terms of gauge-theory ingredients, and are a recasting of the Kawai, Lewellen and Tye relations. Assuming that numerators of loop amplitudes can be arranged to satisfy the duality, our tree-level proof immediately carries over to loop level via the unitarity method. We then present a Yang-Mills Lagrangian whose diagrams through five points manifestly satisfy the duality between color and kinematics. The existence of such Lagrangians suggests that the duality also extends to loop amplitudes, as confirmed at two and three loops in a concurrent paper. By "squaring" the novel Yang-Mills Lagrangian we immediately obtain its gravity counterpart. We outline the general structure of these Lagrangians for higher points. We also write down various new representations of gauge-theory and gravity amplitudes that follow from the duality between color and kinematics.

 

[marcus]

http://arxiv.org/abs/1004.1063
New length operator for loop quantum gravity
Yongge Ma, Chopin Soo, Jinsong Yang
10 pages, 2 figures
(Submitted on 7 Apr 2010)
"An alternative expression for the length operator in loop quantum gravity is presented. The operator is background-independent, symmetric, positive semi-definite, and well-defined on the kinematical Hilbert space. The expression for the regularized length operator can moreover be understood both from a simple geometrical perspective as the average of a formula relating the length to area, volume and flux operators, and also consistently as the result of direct substitution of the densitized triad operator with the functional derivative operator into the regularized expression of the length. Both these derivations are discussed, and the origin of an undetermined overall factor in each case is also elucidated."

http://arxiv.org/abs/1004.1110
Interplay between curvature and Planck-scale effects in astrophysics and cosmology
Antonino Marciano, Giovanni Amelino-Camelia, Nicola Rossano Bruno, Giulia Gubitosi, Gianluca Mandanici, Alessandro Melchiorri
26 pages.
(Submitted on 7 Apr 2010)
"Several recent studies have considered the implications for astrophysics and cosmology of some possible nonclassical properties of spacetime at the Planck scale. The new effects, such as a Planck-scale-modified energy-momentum (dispersion) relation, are often inferred from the analysis of some quantum versions of Minkowski spacetime, and therefore the relevant estimates depend heavily on the assumption that there could not be significant interplay between Planck-scale and curvature effects. We here scrutinize this assumption, using as guidance a quantum version of de Sitter spacetime with known Inonu-Wigner contraction to a quantum Minkowski spacetime. And we show that, contrary to common (but unsupported) beliefs, the interplay between Planck-scale and curvature effects can be significant. Within our illustrative example, in the Minkowski limit the quantum-geometry deformation parameter is indeed given by the Planck scale, while in the de Sitter picture the parameter of quantization of geometry depends both on the Planck scale and the curvature scalar. For the much-studied case of Planck-scale effects that intervene in the observation of gamma-ray bursts we can estimate the implications of "quantum spacetime curvature" within robust simplifying assumptions. For cosmology at the present stage of the development of the relevant mathematics one cannot go beyond semiheuristic reasoning, and we here propose a candidate approximate description of a quantum FRW geometry, obtained by patching together pieces (with different spacetime curvature) of our quantum de Sitter. This semiheuristic picture, in spite of its limitations, provides rather robust evidence that in the early Universe the interplay between Planck-scale and curvature effects could have been particularly significant."

 

[MTd2]

http://arxiv.org/abs/1004.1268

Principle of Relativity, 24 possiblw kinematical algebras and new geometries with Poincaré symmetry

Chao-Guang Huang
(Submitted on 8 Apr 2010)
From the principle of relativity with two universal invariant parameters $c$ and $l$, 24 possible kinematical (including geometrical and static) algebras can be obtained. Each algebra is of 10 dimensional, generating the symmetry of a 4 dimensional homogeneous space-time or a pure space. In addition to the ordinary Poincar\'e algebra, there is another Poincar\'e algebra among the 24 algebras. New 4d geometries with the new Poincar\'e symmetry are presented. The motion of free particles on one of the new space-times is discussed.

http://arxiv.org/abs/1004.1226

Logic is to the quantum as geometry is to gravity

Rafael D. Sorkin (Perimeter Institute and Syracuse University)
(Submitted on 8 Apr 2010)
I will propose that the reality to which the quantum formalism implicitly refers is a kind of generalized history, the word history having here the same meaning as in the phrase sum-over-histories. This proposal confers a certain independence on the concept of event, and it modifies the rules of inference concerning events in order to resolve a contradiction between the idea of reality as a single history and the principle that events of zero measure cannot happen (the Kochen-Specker paradox being a classic expression of this contradiction). The so-called measurement problem is then solved if macroscopic events satisfy classical rules of inference, and this can in principle be decided by a calculation. The resulting conception of reality involves neither multiple worlds nor external observers. It is therefore suitable for quantum gravity in general and causal sets in particular.

 

[marcus]

http://arxiv.org/abs/1004.1780
A new look at loop quantum gravity
Carlo Rovelli
15 pages, 5 figures
(Submitted on 11 Apr 2010)
"I describe a possible perspective on the current state of loop quantum gravity, at the light of the developments of the last years. I point out that a theory is now available, having a well-defined background-independent kinematics and a dynamics allowing transition amplitudes to be computed explicitly in different regimes. I underline the fact that the dynamics can be given in terms of a simple vertex function, largely determined by locality, diffeomorphism invariance and local Lorentz invariance. I emphasize the importance of approximations. I list open problems."

http://arxiv.org/abs/1004.1856
The Hubble Constant
Wendy L. Freedman, Barry F. Madore
To be published in Annual Reviews of Astronomy and Astrophysics, Vol. 48, 2010, consisting of 79 pages, 13 figures, 2 tables
(Submitted on 11 Apr 2010)
"Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, and focus on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy."

 

[marcus]

http://arxiv.org/abs/1004.2504
Primordial Gravitational Waves and Cosmology
Lawrence Krauss (1), Scott Dodelson (2,3) Stephan Meyer (3), ((1) Arizona State University, (2)Fermi National Laboratory, (3) University of Chicago)
12 pages. 4 figures
(Submitted on 14 Apr 2010)
"The observation of primordial gravitational waves could provide a new and unique window on the earliest moments in the history of the universe, and on possible new physics at energies many orders of magnitude beyond those accessible at particle accelerators. Such waves might be detectable soon in current or planned satellite experiments that will probe for characteristic imprints in the polarization of the cosmic microwave background (CMB), or later with direct space-based interferometers. A positive detection could provide definitive evidence for Inflation in the early universe, and would constrain new physics from the Grand Unification scale to the Planck scale."

http://arxiv.org/abs/1004.2586
Emergent Horizons in the Laboratory
Ralf Schützhold
7 pages, 4 figures
(Submitted on 15 Apr 2010)
"The concept of a horizon known from general relativity describes the loss of causal connection and can be applied to non-gravitational scenarios such as out-of-equilibrium condensed-matter systems in the laboratory. This analogy facilitates the identification and theoretical study (e.g., regarding the trans-Planckian problem) and possibly the experimental verification of 'exotic' effects known from gravity and cosmology, such as Hawking radiation. Furthermore, it yields a unified description and better understanding of non-equilibrium phenomena in condensed matter systems and their universal features. By means of several examples including general fluid flows, expanding Bose-Einstein condensates, and dynamical quantum phase transitions, the concepts of event, particle, and apparent horizons will be discussed together with the resulting quantum effects."

http://pirsa.org/10040067/
Neil Turok video lecture on Cosmology Basics

 

[MTd2]

http://arxiv.org/abs/1004.2552

Massive graviton propagation of the deformed Hořava-Lifgarbagez gravity without projectability condition

Yun Soo Myung
(Submitted on 15 Apr 2010)
We study graviton propagations of scalar, vector, and tensor modes in the deformed Ho\v{r}ava-Lifgarbagez gravity ($\lambda R$-model) without projectability condition. The quadratic Lagrangian is invariant under diffeomorphism only for $\lambda=1$ case, which contradicts to the fact that $\lambda$ is irrelevant to a consistent Hamiltonian approach to the $\lambda R$ model. In this case, as far as scalar propagations are concerned, there is no essential difference between deformed Ho\v{r}ava-Lifgarbagez gravity ($\lambda R$-model) and general relativity. This implies that there are two degrees of freedom for a massless graviton without Ho\v{r}ava scalar, and five degrees of freedom appear for a massive graviton when introducing Lorentz-violating and Fierz-Pauli mass terms. Finally, it is shown that for $\lambda=1$, the vDVZ discontinuity is absent in the massless limit of Lorentz-violating mass terms by considering external source terms.

http://arxiv.org/abs/1004.2632

Gravity and a Geometrization of Turbulence

Christopher Eling, Itzhak Fouxon, Yaron Oz
(Submitted on 15 Apr 2010)
The dynamics of fluids is a long standing challenge that remained as an unsolved problem for centuries. Understanding its main features, chaos and turbulence, is likely to provide an understanding of the principles and non-linear dynamics of a large class of systems far from equilibrium. We consider a conceptually new viewpoint to study these features using black hole dynamics. Since the gravitational field is characterized by a curved geometry, the gravity variables provide a geometrical framework for studying the dynamics of fluids: A geometrization of turbulence.

http://arxiv.org/abs/1004.2260

The new vertices and canonical quantization

Sergei Alexandrov
(Submitted on 13 Apr 2010)
We present two results on the recently proposed new spin foam models. First, we show how a (slightly modified) restriction on representations in the EPRL model leads to the appearance of the Ashtekar-Barbero connection, thus bringing this model even closer to LQG. As our second result, we however demonstrate that the quantization leading to the new models is completely inconsistent since it relies on the symplectic structure of the unconstraint BF theory.

 

[MTd2]

http://motls.blogspot.com/2010/04/d...sReferenceFrame+(Lubos+Motl's+reference+frame)

Dark energy, the holographic principle, and IPMUBy Paul Frampton, Professor of Physics and Astronomy at Chapel Hill, NC

It is a high honor to be invited, by Lubos, to write about dark energy. I first met Lubos, when he was an assistant professor, at Harvard University, and found him extremely intelligent and charming in person, quite different from the aggressively strident blogger. I confess to joking that Lubos is afflicted by "blog-rage", when he sits at his keyboard, by analogy with the road-rage suffered by drivers in California when they shoot dead the other driver, after a minor fender-bender.

 

[MTd2]

http://arxiv.org/abs/1004.2879

Quantizing Geometry or Geometrizing the Quantum?

Bejamin Koch
(Submitted on 16 Apr 2010)
The unsatisfactory status of the search for a consistent and predictive quantization of gravity is taken as motivation to study the question whether geometrical laws could be more fundamental than quantization procedures. In such an approach the quantum mechanical laws should emerge from the geometrical theory. A toy model that incorporates the idea is presented and its necessary formulation in configuration space is emphasized.

 

[marcus]

http://arxiv.org/abs/1004.2834
Abelian Chern-Simons theory, Stokes' Theorem, and generalized connections
Hanno Sahlmann, Thomas Thiemann
13 pages, 4 figures
(Submitted on 16 Apr 2010)
"Generalized connections and their calculus have been developed in the context of quantum gravity. Here we apply them to abelian Chern-Simons theory. We derive the expectation values of holonomies in U(1) Chern-Simons theory using Stokes' Theorem, flux operators and generalized connections. A framing of the holonomy loops arises in our construction, and we show how, by choosing natural framings, the resulting expectation values nevertheless define a functional over gauge invariant cylindrical functions.
The abelian theory considered in the present article is test case for our method. It can also be applied to the non-abelian theory. Results for that case will be reported elsewhere."

brief mention:
http://arxiv.org/abs/1004.2794
Revisiting the Entropic corrections to Newton's law
M. R. Setare, D. Momeni
6 pages
(Submitted on 16 Apr 2010)
"In this short letter we calculate separately the GUP and self gravitational corrections to the Newton's gravitational formula. We show that for a complete description of the GUP and self-gravity effects both temperature and the entropy must be modified."

http://arxiv.org/abs/1004.2901
Reviving Gravity's Aether in Einstein's Universe
Niayesh Afshordi (Perimeter Institute/ University of Waterloo)
4 pages, 1 figure, to appear in Physics in Canada
(Submitted on 16 Apr 2010)
"Einstein's theory of general relativity describes gravity as the interaction of particles with space-time geometry, as opposed to interacting with a physical fluid, as in the old gravitational aether theories. Moreover, any theoretical physicist would tell you that, despite its counter-intuitive structure, general relativity is one of the simplest, most beautiful, and successful theories in physics, that has withstood a diverse battery of precision tests over the past century. So, is there any motivation to relax its fundamental principle, and re-introduce a gravitational aether? Here, I give a short and non-technical account of why quantum gravity and cosmological constant problems provide this motivation."

 

[marcus]

http://arxiv.org/abs/1004.2952
Lectures on LQG/LQC
Ghanashyam Date
72 pages
(Submitted on 17 Apr 2010)
"A School on Loop Quantum Gravity was held at the IMSc during Sept 8 -- 18, 2009. In the first week a basic introduction to LQG was provided while in the second week the focus was on the two main application, to cosmology (LQC) and to the black hole entropy. These notes are an expanded written account of the lectures that I gave. These are primarily meant for beginning researchers."

 

[MTd2]

http://arxiv.org/abs/1004.2954

Influence on the entropic force by the virtual degree of freedom on the holographic screen

Qiyuan Pan, Bin Wang
(Submitted on 17 Apr 2010)
We generalize the study of entropic force to a general static spherical spacetime and examine the acceleration, temperature, equation of gravity and the energy associated with the holographic screen in this general background. We show that the virtual degree of freedom on the holographic screen plays a crucial role in interpreting field equations of gravity based on thermodynamical perspective.

 

[marcus]

http://arxiv.org/abs/1004.3450
Non-commutative flux representation for loop quantum gravity
Aristide Baratin, Bianca Dittrich, Daniele Oriti, Johannes Tambornino
21 pages, 1 figure
(Submitted on 20 Apr 2010)
"The Hilbert space of loop quantum gravity is usually described in terms of cylindrical functionals of the gauge connection, the electric fluxes acting as non-commuting derivation operators. Here we introduce a dual description of this space, by means of a Fourier transform mapping the usual loop gravity states to non-commutative functions on Lie algebras. We show that the Fourier transform defines a unitary equivalence of representations for loop quantum gravity. In the dual representation, flux operators act by star-multiplication and holonomy operators act by translation. We describe the gauge invariant dual states and discuss their geometrical meaning. Finally, we apply the construction to the simpler case of a U(1) gauge group and compare the resulting flux representation with the triad representation used in loop quantum cosmology."

 

[John86]

http://arxiv.org/abs/1004.3564

Topos Methods in the Foundations of Physics
Authors: Chris J. Isham
(Submitted on 20 Apr 2010)

Abstract: This article gives a conceptual introduction to the topos approach to the formulation of physical theories.

Comments: 24 pages; to appear in "Deep Beauty", ed. Hans Halvorson, Cambridge University Press (2010)
Subjects: Quantum Physics (quant-ph); General Relativity and Quantum Cosmology (gr-qc); Mathematical Physics (math-ph)
Cite as: arXiv:1004.3564v1 [quant-ph]

http://arxiv.org/abs/1004.3573

The physical interpretation of daseinisation
Authors: Andreas Doering
(Submitted on 20 Apr 2010)

Abstract: We provide a conceptual discussion and physical interpretation of some of the quite abstract constructions in the topos approach to physics. In particular, the daseinisation process for projection operators and for self-adjoint operators is motivated and explained from a physical point of view. Daseinisation provides the bridge between the standard Hilbert space formalism of quantum theory and the new topos-based approach to quantum theory. As an illustration, we will show all constructions explicitly for a three-dimensional Hilbert space and the spin-z operator of a spin-1 particle. This article is a companion to the article by Isham in the same volume.

http://arxiv.org/abs/1004.3561

Topos Quantum Logic and Mixed States
Authors: Andreas Doering
(Submitted on 20 Apr 2010)

Abstract: The topos approach to the formulation of physical theories includes a new form of quantum logic. We present this topos quantum logic, including some new results, and compare it to standard quantum logic, all with an eye to conceptual issues. In particular, we show that topos quantum logic is distributive, multi-valued, contextual and intuitionistic. It incorporates superposition without being based on linear structures, has a built-in form of coarse-graining which automatically avoids interpretational problems usually associated with the conjunction of propositions about incompatible physical quantities, and provides a material implication that is lacking from standard quantum logic. Importantly, topos quantum logic comes with a clear geometrical underpinning. The representation of pure states and truth-value assignments are discussed. It is briefly shown how mixed states fit into this approach.

 

[marcus]

http://arxiv.org/abs/1004.3979
The high-density regime of kinetic-dominated loop quantum cosmology
Martin Bojowald, David Mulryne, William Nelson, Reza Tavakol
26 pages, 7 figures
(Submitted on 22 Apr 2010)
"We study the dynamics of states perturbatively expanded about a harmonic system of loop quantum cosmology, exhibiting a bounce. In particular, the evolution equations for the first and second order moments of the system are analyzed. These moments back-react on the trajectories of the expectation values of the state and hence alter the energy density at the bounce. This analysis is performed for isotropic loop quantum cosmology coupled to a scalar field with a small but non-zero constant potential, hence in a regime in which the kinetic energy of matter dominates. Analytic restrictions on the existence of dynamical coherent states and the meaning of semi-classicality within these systems are discussed. A numerical investigation of the trajectories of states that remain semi-classical across the bounce demonstrates that, at least for such states, the bounce persists and that its properties are similar to the standard case, in which the moments of the states are entirely neglected. However the bounce density does change, implying that a quantum bounce may not be guaranteed to happen when the potential is no longer negligible."

 

[MTd2]

http://arxiv.org/abs/1004.4550

Spinfoams in the holomorphic representation

Eugenio Bianchi, Elena Magliaro, Claudio Perini
(Submitted on 26 Apr 2010)
We study a holomorphic representation for spinfoams. The representation is obtained via the Ashtekar-Lewandowski-Marolf-Mour\~ao-Thiemann coherent state transform. We derive the expression of the 4d spinfoam vertex for Euclidean and for Lorentzian gravity in the holomorphic representation. The advantage of this representation rests on the fact that the variables used have a clear interpretation in terms of a classical intrinsic and extrinsic geometry of space. We show how the peakedness on the extrinsic geometry selects a single exponential of the Regge action in the semiclassical large-scale asymptotics of the spinfoam vertex.

 

[marcus]

http://arxiv.org/abs/1004.4866
Unification of gravity, gauge fields, and Higgs bosons
A. Garrett Lisi, Lee Smolin, Simone Speziale
12 pages
(Submitted on 27 Apr 2010)
"We consider a diffeomorphism invariant theory of a gauge field valued in a Lie algebra that breaks spontaneously to the direct sum of the spacetime Lorentz algebra, a Yang-Mills algebra, and their complement. Beginning with a fully gauge invariant action -- an extension of the Plebanski action for general relativity -- we recover the action for gravity, Yang-Mills, and Higgs fields. The low-energy coupling constants, obtained after symmetry breaking, are all functions of the single parameter present in the initial action and the vacuum expectation value of the Higgs."

http://arxiv.org/abs/1004.4681
Entropy in Spin Foam Models: The Statistical Calculation
J.Manuel Garcia-Islas
11 pages, 1 figure
(Submitted on 26 Apr 2010)
"Recently an idea for computing the entropy of black holes in the spin foam formalism has been introduced. Particularly complete calculations for the three dimensional euclidean BTZ black hole were done. The whole calculation is based on observables living at the horizon of the black hole universe. Departing from this idea of observables living at the horizon, we now go further and compute the entropy of BTZ black hole in the spirit of statistical mechanics. We compare both calculations and show that they are very interrelated and equally valid. This latter behaviour is certainly due to the importance of the observables."

 

[marcus]

http://arxiv.org/abs/1004.5196
Bubble divergences from cellular homology
Valentin Bonzom (CPT), Matteo Smerlak (CPT)
5 pages
(Submitted on 29 Apr 2010)
"We consider a class of lattice topological field theories, among which are the weak-coupling limit of 2d Yang-Mills theory, the Ponzano-Regge model of 3d quantum gravity and discrete BF theory, whose dynamical variables are flat discrete connections with compact structure group on a cell 2-complex. In these models, it is known that the path integral measure is ill-defined in general, because of a phenomenon called `bubble divergences'. A common expectation is that the degree of these divergences is given by the number of `bubbles' of the 2-complex. In this note, we show that this expectation, although not realistic in general, is met in some special cases: when the 2-complex is simply connected, or when the structure group is Abelian -- in both cases, the divergence degree is given by the second Betti number of the 2-complex."


http://arxiv.org/abs/1004.5371
Classical general relativity as BF-Plebanski theory with linear constraints
Steffen Gielen, Daniele Oriti
15 pages, revtex, to be submitted to Class. Quant. Grav.
(Submitted on 29 Apr 2010)
"We investigate a formulation of continuum 4d gravity in terms of a constrained BF theory, in the spirit of the Plebanski formulation, but involving only linear constraints, of the type used recently in the spin foam approach to quantum gravity. We identify both the continuum version of the linear simplicity constraints used in the quantum discrete context and a linear version of the quadratic volume constraints that are necessary to complete the reduction from the topological theory to gravity. We illustrate and discuss also the discrete counterpart of the same continuum linear constraints. Moreover, we show under which additional conditions the discrete volume constraints follow from the simplicity constraints, thus playing the role of secondary constraints."

 

[MTd2]

http://arxiv.org/abs/1004.4016

On Dark Matter, Spiral Galaxies, and the Axioms of General Relativity

Hubert L. Bray
(Submitted on 22 Apr 2010)
Beginning with a geometric motivation for dark matter going back to the axioms of general relativity, we show how scalar field dark matter, which naturally forms dark matter density waves due to its wave nature, may cause the observed barred spiral pattern density waves in many disk galaxies and triaxial shapes with plausible brightness profiles in many elliptical galaxies. If correct, this would provide a unified explanation for spirals and bars in spiral galaxies and for the brightness profiles of elliptical galaxies. We compare the results of preliminary computer simulations with photos of actual galaxies.

******

There are things that might prove fundamental to quantum gravity here.

 

[marcus]

http://arxiv.org/abs/1005.0535
Comments on Nonlocality in Deformed Special Relativity, in reply to arXiv:1004.0664 by Lee Smolin and arXiv:1004.0575 by Jacob et al
Sabine Hossenfelder
(Submitted on 4 May 2010)
"It was previously shown that models with deformations of special relativity that have an energy-dependent yet observer-independent speed of light suffer from nonlocal effects that are in conflict with observation to very high precision. In a recent paper it has been proposed that these paradoxa arise only in the classical limit and can be prevented by an ad-hoc introduction of a quantum uncertainty that would serve to hide the nonlocality. We will show here that the proposed ansatz for this resolution is inconsistent with observer-independence and, when corrected, is in agreement with the earlier argument that revealed the troublesome nonlocality. We further offer an alternative derivation for the energy-dependent speed of light in the model used."

 

[MTd2]

http://arxiv.org/abs/1005.0475

The universal viscosity to entropy density ratio from entanglement

Goffredo Chirco, Christopher Eling, Stefano Liberati
(Submitted on 4 May 2010)
We present evidence that the universal Kovtun-Son-Starinets shear viscosity to entropy density ratio of 1/4\pi can be associated with a Rindler causal horizon in flat spacetime. Since there is no known holographic (gauge/gravity) duality for this spacetime, a natural microscopic explanation for this viscosity is in the peculiar properties of quantum entanglement. In particular, it is well-known that the Minkowski vacuum state is a thermal state and carries an area entanglement entropy density in the Rindler spacetime. Based on the fluctuation-dissipation theorem, we expect a similar notion of viscosity arising from vacuum fluctuations. Therefore, we propose a holographic Kubo formula in terms of a two-point function of the stress tensor of matter fields in the bulk. We calculate this viscosity assuming a minimally coupled scalar field theory and find that the ratio with respect to the entanglement entropy density is exactly 1/4\pi in four dimensions. The issues that arise in extending this result to non-minimally coupled scalar fields, higher spins, and higher dimensions provide interesting hints about the relationship between entanglement entropy and black hole entropy.

 

[MTd2]

http://arxiv.org/abs/1005.0619

Holography in Action

Sanved Kolekar, T. Padmanabhan
(Submitted on 4 May 2010)
Einstein-Hilbert action and its natural generalizations to higher dimensions (like the Lanczos-Lovelock action) have certain peculiar features. All of them can be separated into a bulk and a surface term, with a specific ("holographic") relationship between the two, so that either term can be used to extract information about the other. Further, the surface term leads to entropy of the horizons on-shell. It has been argued in the past that these features are impossible to understand in the conventional approach but find a natural explanation if we consider gravity as an emergent phenomenon. We provide further support for this point of view in this paper. We describe an alternative decomposition of the Einstein-Hilbert action and Lanczos-Lovelock action into a new pair of surface and bulk terms, such that the surface term becomes Wald entropy on a horizon and the bulk term is the energy density (which is the ADM Hamiltonian density for Einstein gravity). We show that this new pair also obeys a holographic relationship and give a thermodynamic interpretation to this relation in this context. Since the bulk and surface terms, in this decomposition, are related to energy and entropy, the holographic condition can be thought of as analogous to inverting the expression for entropy given as a function of energy S = S(E,V) to obtain the energy E = E(S,V) in terms of the entropy in a normal thermodynamic system. Thus the holographic nature of the action allows us to relate the descriptions of the same system in terms of two different thermodynamic potentials. Some further possible generalizations and implications are discussed.

http://arxiv.org/abs/1005.0764

Face amplitude of spinfoam quantum gravity

Eugenio Bianchi, Daniele Regoli, Carlo Rovelli
(Submitted on 5 May 2010)
The structure of the boundary Hilbert-space and the condition that amplitudes behave appropriately under compositions determine the face amplitude of a spinfoam theory. In quantum gravity the face amplitude turns out to be simpler than originally thought.

 

[marcus]

http://arxiv.org/abs/1005.0814
Observational hints on the Big Bounce
Jakub Mielczarek, Michal Kamionka, Aleksandra Kurek, Marek Szydlowski
25 pages, 8 figures
(Submitted on 5 May 2010)
"In this paper we study possible observational consequences of the bouncing cosmology. We consider a model where a phase of inflation is preceded by a cosmic bounce. While we consider in this paper only that the bounce is due to loop quantum gravity, most of the results presented here can be applied for different bouncing cosmologies. We concentrate on the scenario where the scalar field, as the result of contraction of the universe, is driven from the bottom of the potential well. The field is amplified, and finally the phase of the standard slow-roll inflation is realized. Such an evolution modifies the standard inflationary spectrum of perturbations by the additional oscillations and damping on the large scales. We extract the parameters of the model from the observations of the cosmic microwave background radiation. In particular, the value of inflaton mass is equal to m=(2.6 \pm 0.6) \cdot 10^{13} GeV. In our considerations we base on the seven years of observations made by the WMAP satellite. We propose the new observational consistency check for the phase of slow-roll inflation. We investigate the conditions which have to be fulfilled to make the observations of the Big Bounce effects possible. We translate them to the requirements on the parameters of the model and then put the observational constraints on the model. Based on assumption usually made in loop quantum cosmology, the Barbero-Immirzi parameter was shown to be constrained by \gamma<1100 from the cosmological observations. We have compared the Big Bounce model with the standard Big Bang scenario and showed that the present observational data is not informative enough to distinguish these models."

http://arxiv.org/abs/1005.0817
A regularization of the hamiltonian constraint compatible with the spinfoam dynamics
Emanuele Alesci, Carlo Rovelli
24 pages
(Submitted on 5 May 2010)
"We introduce a new regularization for Thiemann's Hamiltonian constraint. The resulting constraint can generate the 1-4 Pachner moves and is therefore more compatible with the dynamics defined by the spinfoam formalism. We calculate its matrix elements and observe the appearence of the 15j Wigner symbol in these."

 

[John86]

http://arxiv.org/abs/1005.0790

CMB acoustic scale in the entropic accelerating universe
Authors: Roberto Casadio, Alessandro Gruppuso
(Submitted on 5 May 2010)
Abstract: We show that the entropic accelerating universe recently proposed by Easson et al [4,5] is equivalent to a model with a dark energy component with constant parameter of state w_X = -1 + 2gamma/3, where gamma is related to the coefficients of the new terms in the Friedman equations. After discussing all the Friedman equations for an arbitrary gamma, we show how to recover the standard scalings for dust and radiation. The acoustic scale l_A, related to the peak positions in the pattern of the angular power spectrum of the Cosmic Microwave Background anisotropies, is also computed and yields the stringent bound gamma<<1, which implies that the correction proportional to dH/dt must be negligible with respect to that proportional to H^2. We then argue that future data might be able to distinguish this model from pure LambdaCDM (corresponding to gamma=0).

http://arxiv.org/abs/1005.0499

Schwarzschild Geometry Emerging from Matrix Models
Authors: Daniel N. Blaschke, Harold Steinacker
(Submitted on 4 May 2010)
Abstract: We demonstrate how various geometries can emerge from Yang-Mills type matrix models with branes, and consider the examples of Schwarzschild and Reissner-Nordstroem geometry. We provide an explicit embedding of these branes in R^{2,5} and R^{4,6}, as well as an appropriate Poisson resp. symplectic structure which determines the non-commutativity of space-time. The embedding is asymptotically flat with asymptotically constant \theta^{\mu\nu} for large r, and therefore suitable for a generalization to many-body configurations. This is an illustration of our previous work arXiv:1003.4132, where we have shown how the Einstein-Hilbert action can be realized within such matrix models.

http://arxiv.org/abs/0911.3397

Surprising phenomena in a rich new class of inflationary models
Authors: Pascal M. Vaudrevange, Dmitry I. Podolsky, Glenn D. Starkman
(Submitted on 18 Nov 2009 (v1), last revised 29 Apr 2010 (this version, v2))
Abstract: We report on a new class of fast-roll inflationary models. In a huge part of its parameter space, inflationary perturbations exhibit quite unusual phenomena such as scalar and tensor modes freezing out at widely different times, as well as scalar modes reentering the horizon during inflation. In another, narrower range of parameters, this class of models agrees with observations. One specific point in parameter space is characterized by extraordinary behavior of the scalar perturbations. Freeze-out of scalar perturbations as well as particle production at horizon crossing are absent. Also the behavior of the perturbations around this quasi-de Sitter background is dual to a quantum field theory in flat space-time. Finally, the form of the primordial power spectrum is determined by the interaction between different modes of scalar perturbations.

 

[ensabah6]

arXiv:1005.0380
http://arxiv.org/abs/1005.0838
First Dark Matter Results from the XENON100 Experimen
Authors: E. Aprile, K. Arisaka, F. Arneodo, A. Askin, L. Baudis, A. Behrens, E. Brown, J. M. R. Cardoso, B. Choi, D. B. Cline, S. Fattori, A. D. Ferella, K.-L. Giboni, K. Hugenberg, A. Kish, C. W. Lam, J. Lamblin, R. F. Lang, K. E. Lim, J. A. M. Lopes, T. Marrodán Undagoitia, Y. Mei, A. J. Melgarejo Fernandez, K. Ni, U. Oberlack, S. E. A. Orrigo, E. Pantic, G. Plante, A. C. C. Ribeiro, R. Santorelli, J. M. F. dos Santos, M. Schumann, P. Shagin, A. Teymourian, D. Thers, E. Tziaferi, H. Wang, C. Weinheimer (XENON100 Collaboration)
(Submitted on 3 May 2010)

Abstract: The XENON100 experiment, in operation at the Laboratori Nazionali del Gran Sasso in Italy, is designed to search for dark matter WIMPs scattering off 62 kg of liquid xenon in an ultra-low background dual-phase time projection chamber. In this letter, we present first dark matter results from the analysis of 11.17 live days of non-blind data, acquired in October and November 2009. In the selected fiducial target of 40 kg, and within the pre-defined signal region, we observe no events and hence exclude spin-independent WIMP-nucleon elastic scattering cross-sections above 3 x 10^-44 cm^2 for 50 GeV/c^2 WIMPs at 90% confidence level. Below 20 GeV/c^2, this result challenges the interpretation of the CoGeNT or DAMA signals as being due to spin-independent, elastic, light mass WIMP interactions.

 

[marcus]

http://arxiv.org/abs/1005.1057
Spin Foams and Noncommutative Geometry
Domenic Denicola (Caltech), Matilde Marcolli (Caltech), Ahmad Zainy al-Yasry (ICTP)
48 pages, 30 figures
(Submitted on 6 May 2010)
"We extend the formalism of embedded spin networks and spin foams to include topological data that encode the underlying three-manifold or four-manifold as a branched cover. These data are expressed as monodromies, in a way similar to the encoding of the gravitational field via holonomies. We then describe convolution algebras of spin networks and spin foams, based on the different ways in which the same topology can be realized as a branched covering via covering moves, and on possible composition operations on spin foams. We illustrate the case of the groupoid algebra of the equivalence relation determined by covering moves and a 2-semigroupoid algebra arising from a 2-category of spin foams with composition operations corresponding to a fibered product of the branched coverings and the gluing of cobordisms. The spin foam amplitudes then give rise to dynamical flows on these algebras, and the existence of low temperature equilibrium states of Gibbs form is related to questions on the existence of topological invariants of embedded graphs and embedded two-complexes with given properties. We end by sketching a possible approach to combining the spin network and spin foam formalism with matter within the framework of spectral triples in noncommutative geometry."

http://pirsa.org/10050002/
Deformations of General Relativity
Kirill Krasnov
2010-05-05
"I will describe a very special (infinite-parameter) family of gravity
theories that all describe, exactly like General Relativity, just two
propagating degrees of freedom. The theories are obtained by generalizing
Plebanski's self-dual (chiral) formulation of GR. I will argue that this
class of gravity theories provides a potentially powerful new framework for
testing the asymptotic safety conjecture in quantum gravity."

 

[John86]

http://arxiv.org/abs/1003.5179

Gauge fields in graphene
Authors: M. A. H. Vozmediano, M. I. Katsnelson, F. Guinea
(Submitted on 26 Mar 2010)
Abstract: The physics of graphene is acting as a bridge between quantum field theory and condensed matter physics due to the special quality of the graphene quasiparticles behaving as massless two dimensional Dirac fermions. Moreover, the particular structure of the 2D crystal lattice sets the arena to study and unify concepts from elasticity, topology and cosmology. In this paper we analyze these connections combining a pedagogical, intuitive approach with a more rigorous formalism when required.

 

[MTd2]

http://arxiv.org/abs/1005.1132

Tolman mass, generalized surface gravity, and entropy bounds

Gabriel Abreu (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 7 May 2010)
In any static spacetime the quasi-local Tolman mass contained within a volume can be reduced to a Gauss-like surface integral involving the flux of a suitably defined generalized surface gravity. By introducing some basic thermodynamics and invoking the Unruh effect one can then develop elementary bounds on the quasi-local entropy that are very similar in spirit to the holographic bound, and closely related to entanglement entropy.

http://arxiv.org/abs/1005.1169

Debye entropic force and modified Newtonian dynamics

Xin Li, Zhe Chang
(Submitted on 7 May 2010)
Verlinde has suggested that the gravity has an entropic origin, and a gravitational system could be regarded as a thermodynamical system. It is well-known that the equipartition law of energy is invalid at very low temperature. Therefore, entropic force should be modified while the temperature of the holographic screen is very low. It is shown that the modified entropic force is proportional to the square of the acceleration, while the temperature of the holographic screen is much lower than the Debye temperature $T_D$. The modified entropic force returns to the Newton's law of gravitation while the temperature of the holographic screen is much higher than the Debye temperature. The modified entropic force is connected with modified Newtonian dynamics (MOND). The constant $a_0$ involved in MOND is linear in the Debye frequency $\omega_D$, which can be regarded as the largest frequency of the bits in screen. We find that there do have a strong connection between MOND and cosmology in the framework of Verlinde's entropic force, if the holographic screen is taken to be bound of the Universe. The Debye frequency is linear in the Hubble constant $H_0$.http://arxiv.org/abs/1005.1174

Notes on "quantum gravity" and non-commutative geometry

Jose M. Gracia-Bondia
(Submitted on 7 May 2010)
I hesitated for a long time before giving shape to these notes, originally intended for preliminary reading by the attendees to the Summer School "New paths towards quantum gravity" (Holbaek Bay, Denmark, May 2008). At the end, I decide against just selling my mathematical wares, and for a survey, necessarily very selective, but taking a global phenomenological approach to its subject matter. After all, non-commutative geometry does not purport yet to solve the riddle of quantum gravity; it is more of an insurance policy against the probable failure of the other approaches. The plan is as follows: the introduction invites students to the fruitful doubts and conundrums besetting the application of even classical gravity. Next, the first experiments detecting quantum gravitational states inoculate us a healthy dose of skepticism on some of the current ideologies. In Section 3 we look at the action for general relativity as a consequence of gauge theory for quantum tensor fields. Section 4 briefly deals with the unimodular variants. Section 5 arrives at non-commutative geometry. I am convinced that, if this is to play a role in quantum gravity, commutative and non-commutative manifolds must be treated on the same footing; which justifies the place granted to the reconstruction theorem. Together with Section 3, this part constitutes the main body of the notes. Only very summarily at the end of this section we point to some approaches to gravity within the non-commutative realm. The last section delivers a last dose of skepticism. My efforts will have been rewarded if someone from the young generation learns to mistrust current mindsets.

 

[marcus]

http://arxiv.org/abs/1005.1291
The Immirzi Parameter as an Instanton Angle
Simone Mercuri, Andrew Randono
24 pages
(Submitted on 7 May 2010)
"The Barbero-Immirzi parameter is a one parameter quantization ambiguity underpinning the loop approach to quantum gravity that bears tantalizing similarities to the theta parameter of gauge theories such as Yang-Mills and QCD. Despite the apparent semblance, the Barbero-Immirzi field has resisted a direct topological interpretation along the same lines as the theta-parameter. Here we offer such an interpretation. Our approach begins from the perspective of Einstein-Cartan gravity as the symmetry broken phase of a de Sitter gauge theory. From this angle, just as in ordinary gauge theories, a theta-term emerges from the requirement that the vacuum is stable against quantum mechanical tunneling. The Immirzi parameter is then identified as a combination of Newton's constant, the cosmological constant, and the theta-parameter."

http://arxiv.org/abs/1005.1294
Gravity from a fermionic condensate of a gauge theory
Andrew Randono
16 pages
(Submitted on 7 May 2010)
"The most prominent realization of gravity as a gauge theory similar to the gauge theories of the standard model comes from enlarging the gauge group from the Lorentz group to the de Sitter group. To regain ordinary Einstein-Cartan gravity the symmetry must be broken, which can be accomplished by known quasi-dynamic mechanisms. Motivated by symmetry breaking models in particle physics and condensed matter systems, we propose that the symmetry can naturally be broken by a homogenous and isotropic fermionic condensate of ordinary spinors. We demonstrate that the condensate is compatible with the Einstein-Cartan equations and can be imposed in a fully de Sitter invariant manner. This lends support, and provides a physically realistic mechanism for understanding gravity as a gauge theory with a spontaneously broken local de Sitter symmetry."

http://arxiv.org/abs/1005.1488
Non-perturbative QEG Corrections to the Yang-Mills Beta Function
J.-E. Daum, U. Harst, M. Reuter
To appear in the proceedings of CORFU 2009
(Submitted on 10 May 2010)
"We discuss the non-perturbative renormalization group evolution of the gauge coupling constant by using a truncated form of the functional flow equation for the effective average action of the Yang-Mills-gravity system. Our result is consistent with the conjecture that Quantum Einstein Gravity (QEG) is asymptotically safe and has a vanishing gauge coupling constant at the non-trivial fixed point."