# Loop-and-allied QG bibliography

1. Jul 18, 2013

### marcus

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer
(Submitted on 18 Jul 2013)
We propose a derivation for computing black hole entropy for spherical non-rotating isolated horizons from loop quantum gravity in four and higher dimensions. The state counting problem effectively reduces to the well studied 3+1-dimensional one based on an SU(2)-Chern-Simons theory, differing only in the precise form of the area spectrum and the restriction to integer spins.
5 pages

http://arxiv.org/abs/1307.5026
Melonic phase transition in group field theory
Aristide Baratin, Sylvain Carrozza, Daniele Oriti, James P. Ryan, Matteo Smerlak
(Submitted on 18 Jul 2013)
Group field theories have recently been shown to admit a 1/N expansion dominated by so-called melonic graphs', dual to triangulated spheres. In this note, we deepen the analysis of this melonic sector. We obtain a combinatorial formula for the melonic amplitudes in terms of a graph polynomial related to a higher dimensional generalization of the Kirchhoff tree-matrix theorem. Simple bounds on these amplitudes show the existence of a phase transition driven by melonic interaction processes. We restrict our study to the Boulatov-Ooguri models, which describe topological BF theories and are the basis for the construction of four dimensional models of quantum gravity.
8 pages, 4 figures

2. Jul 21, 2013

### marcus

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau
(Submitted on 19 Jul 2013)
This article addresses the issue of the closure of the algebra of constraints for generic (cosmological) perturbations when taking into account simultaneously the two main corrections of effective loop quantum cosmology, namely the holonomy and the inverse-volume terms. Previous works on either the holonomy or the inverse volume case are reviewed and generalized. In the inverse-volume case, we point out new possibilities. An anomaly-free solution including both corrections is found for perturbations, and the corresponding equations of motion are derived.
19 pages.

http://arxiv.org/abs/1307.5281
Double Scaling in Tensor Models with a Quartic Interaction
Stephane Dartois, Razvan Gurau, Vincent Rivasseau
(Submitted on 19 Jul 2013)
In this paper we identify and analyze in detail the subleading contributions in the 1/N expansion of random tensors, in the simple case of a quartically interacting model. The leading order for this 1/N expansion is made of graphs, called melons, which are dual to particular triangulations of the D-dimensional sphere, closely related to the "stacked" triangulations. For D<6 the subleading behavior is governed by a larger family of graphs, hereafter called cherry trees, which are also dual to the D-dimensional sphere. They can be resummed explicitly through a double scaling limit. In sharp contrast with random matrix models, this double scaling limit is stable. Apart from its unexpected upper critical dimension 6, it displays a singularity at fixed distance from the origin and is clearly the first step in a richer set of yet to be discovered multi-scaling limits.
40 pages.

brief mention:
http://arxiv.org/abs/1307.5303
Invariant Connections in Loop Quantum Gravity
Maximilian Hanusch
(Submitted on 19 Jul 2013)
38 pages

Last edited: Jul 21, 2013
3. Jul 22, 2013

### marcus

http://arxiv.org/abs/1307.5461
Quantum hyperbolic geometry in loop quantum gravity with cosmological constant
Maite Dupuis, Florian Girelli
(Submitted on 20 Jul 2013)
Loop Quantum Gravity (LQG) is an attempt to describe the quantum gravity regime. Introducing a non-zero cosmological constant Λ in this context has been a withstanding problem. Other approaches, such as Chern-Simons gravity, suggest that quantum groups can be used to introduce Λ in the game. Not much is known when defining LQG with a quantum group. Tensor operators can be used to construct observables in any type of discrete quantum gauge theory with a classical/quantum gauge group. We illustrate this by constructing explicitly geometric observables for LQG defined with a quantum group and show for the first time that they encode a quantized hyperbolic geometry. This is a novel argument pointing out the usefulness of quantum groups as encoding a non-zero cosmological constant. We conclude by discussing how tensor operators provide the right formalism to unlock the LQG formulation with a non-zero cosmological constant.
6 pages, 1 figure

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll
(Submitted on 20 Jul 2013)
We present a detailed analysis of a recently introduced version of Causal Dynamical Triangulations (CDT) that does not rely on a distinguished time slicing. Focussing on the case of 2+1 spacetime dimensions, we analyze its geometric and causal properties, present details of the numerical set-up and explain how to extract "volume profiles". Extensive Monte Carlo measurements of the system show the emergence of a de Sitter universe on large scales from the underlying quantum ensemble, similar to what was observed previously in standard CDT quantum gravity. This provides evidence that the distinguished time slicing of the latter is not an essential part of its kinematical set-up.
44 pages, 29 figures

http://arxiv.org/abs/1307.5527
Loop Quantum Cosmology and the Fine Structure Constant
David Sloan
(Submitted on 21 Jul 2013)
The cosmological implications of introducing a variation to the fine structure 'constant', α are examined within the context of Loop Quantum Cosmology. The evolution of α is described using the model introduced by Bekenstein, Sandvik, Barrow and Magueijo, in which a ghost scalar field produces the variation. The dynamics of the system are examined in flat and closed cosmological settings. Matter consisting of the scalar field and radiation are examined with a thermodynamically motivated coupling between the two, which can lead to a series of bounces induced by both the negative density effects of the ghost field and the loop effects.
10 pages, 5 figures

http://arxiv.org/abs/1307.5352
Cosine problem in EPRL/FK spinfoam model
Marko Vojinovic
(Submitted on 19 Jul 2013)
We calculate the classical limit effective action of the EPRL/FK spinfoam model of quantum gravity coupled to matter fields. By employing the standard QFT background field method adapted to the spinfoam setting, we find that the model has many different classical effective actions. Most notably, these include the ordinary Einstein-Hilbert action coupled to matter, but also an action which describes antigravity. All those multiple classical limits appear as a consequence of of the fact that the EPRL/FK vertex amplitude has cosine-like large spin asymptotics. We discuss some possible ways to eliminate the unwanted classical limits.
20 pages

possibly of general interest:
http://arxiv.org/abs/1307.5737
Structures in the Planck map of the CMB
Daniel An, Krzysztof A. Meissner, Pawel Nurowski
(Submitted on 22 Jul 2013)
We present the results of the quest for ring-type structures on the maps observed by the Planck satellite.

Last edited: Jul 22, 2013
4. Jul 23, 2013

### marcus

http://arxiv.org/abs/1307.6167
The Universe as a Process of Unique Events
Marina Cortês, Lee Smolin
(Submitted on 23 Jul 2013)
We describe a new class of models of quantum space-time based on energetic causal sets and show that under natural conditions space-time emerges from them. These are causal sets whose causal links are labelled by energy and momentum and conservation laws are applied at events. The models are motivated by principles we propose govern microscopic physics which posit a fundamental irreversibility of time. One consequence is that each event in the history of the universe has a distinct causal relationship to the rest; this requires a novel form of dynamics which an be applied to uniquely distinctive events. We hence introduce a new kind of deterministic dynamics for a causal set in which new events are generated from pairs of progenitor events by a rule which is based on extremizing the distinctions between causal past sets of events. This dynamics is asymmetric in time, but we find evidence from numerical simulations of a 1+1 dimensional model, that an effective dynamics emerges which restores approximate time reversal symmetry. Finally we also present a natural twistorial representation of energetic causal sets.
26 pages, 5 figures

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel
(Submitted on 22 Jul 2013)
In a seminal paper, Kaminski, Kisielowski an Lewandowski for the first time extended the definition of spin foam models to arbitrary boundary graphs. This is a prerequisite in order to make contact to the canonical formulation of Loop Quantum Gravity (LQG) and allows to investigate the question whether any of the presently considered spin foam models yield a rigging map for any of the presently defined Hamiltonian constraint operators. The KKL extension cannot be described in terms of Group Field Theory (GFT) since arbitrary foams are involved while GFT is tied to simplicial complexes. Therefore one has to define the sum over spin foams with given boundary spin networks in an independent fashion using natural axioms, most importantly a gluing property for 2-complexes. These axioms are motivated by the requirement that spin foam amplitudes should define a rigging map (physical inner product) induced by the Hamiltonian constraint. This is achieved by constructing a spin foam operator based on abstract 2-complexes that acts on the kinematical Hilbert space of Loop Quantum Gravity. In the analysis of the resulting object we are able to identify an elementary spin foam transfer matrix that allows to generate any finite foam as a finite power of the transfer matrix. It transpires that the sum over spin foams, as written, does not define a projector on the physical Hilbert space. This statement is independent of the concrete spin foam model and Hamiltonian constraint. However, the transfer matrix potentially contains the necessary ingredient in order to construct a proper rigging map in terms of a modified transfer matrix.
62 pages, 14 figures

http://arxiv.org/abs/1307.5979
The Large-Volume Limit of a Quantum Tetrahedron is a Quantum Harmonic Oscillator
John Schliemann
(Submitted on 23 Jul 2013)
It is shown that the volume operator of a quantum tetrahedron is, in the sector of large eigenvalues, accurately described by a quantum harmonic oscillator. This result relies on the fact that (i) the volume operator couples only neighboring states of its standard basis, and (ii) its matrix elements show a unique maximum as a function of internal angular momentum quantum numbers. These quantum numbers, considered as a continuous variable, are the coordinate of the oscillator describing its quadratic potential, while the corresponding derivative defines a momentum operator. We also analyze the scaling properties of the oscillator parameters as a function of the size of the tetrahedron, and the role of different angular momentum coupling schemes.
10 pages, 3 figures; comments welcome

not directly QG-related, but possibly of interest:
http://arxiv.org/abs/1307.6122
Black-hole entropy and minimal diffusion
Michele Arzano, Gianluca Calcagni
(Submitted on 23 Jul 2013)
The density of states reproducing the Bekenstein-Hawking entropy-area scaling can be modelled via a nonlocal field theory. We define a diffusion process based on the kinematics of this theory and find a spectral dimension whose flow exhibits surprising properties. While it asymptotes the infrared value of four from above, in the ultraviolet the spectral dimension diverges at a finite (Planckian) value of the diffusion length, signalling a breakdown of the notion of diffusion on a continuum spacetime below that scale. The correlation length remains finite throughout the flow. We comment on the implications of this minimal diffusion scale for the entropy bound in a holographic and field-theoretic context.
4 pages, 1 figure

http://arxiv.org/abs/1307.6169
The world is discrete
Olaf Dreyer
(Submitted on 23 Jul 2013)
We argue that the scale-free spectrum that is observed in the cosmic microwave background is the result of a phase transition in the early universe. The observed tilt of the spectrum, which has been measured to be 0.04, is shown to be equal to the anomalous scaling dimension of the correlation function. The phase transition replaces inflation as the mechanism that produces this spectrum. The tilt further indicates that there is a fundamental small length scale in nature that we have not yet observed in any other way.
12 pages, 1 figure

Last edited: Jul 23, 2013
5. Jul 29, 2013

### John86

Maybe Freidel is trying to close the gap between strings and loops ?

http://arxiv.org/abs/1307.7080
Born Reciprocity in String Theory and the Nature of Spacetime
Laurent Freidel, Robert G. Leigh, Djordje Minic
(Submitted on 26 Jul 2013)
After many years, the deep nature of spacetime in string theory remains an enigma. In this letter we incorporate the concept of Born reciprocity in order to provide a new point of view on string theory in which spacetime is a derived dynamical concept. This viewpoint may be thought of as a dynamical chiral phase space formulation of string theory, in which Born reciprocity is implemented as a choice of a Lagrangian submanifold of the phase space, and amounts to a generalization of T-duality. In this approach the fundamental symmetry of string theory contains phase space diffeomorphism invariance and the underlying string geometry should be understood in terms of dynamical bi-Lagrangian manifolds and an apparently new geometric structure, somewhat reminiscent of para-quaternionic geometry, which we call Born geometry.

6. Jul 30, 2013

### John86

http://arxiv.org/abs/1307.7376
Multiversality
Frank Wilczek
(Submitted on 28 Jul 2013)
Valid ideas that physical reality is vastly larger than human perception of it, and that the perceived part may not be representative of the whole, exist on many levels and have a long history. After a brief general inventory of those ideas and their implications, I consider the cosmological "multiverse" much discussed in recent scientific literature. I review its theoretical and (broadly) empirical motivations, and its disruptive implications for the traditional program of fundamental physics. I discuss the inflationary axion cosmology, which provides an example where firmly rooted, plausible ideas from microphysics lead to a well-characterized "mini-multiverse" scenario, with testable phenomenological consequences.

7. Jul 30, 2013

### marcus

not directly Loop-or-allied gravity but of general interest:
http://arxiv.org/abs/1307.8106
Cyclic Cosmology, Conformal Symmetry and the Metastability of the Higgs
Itzhak Bars, Paul J. Steinhardt, Neil Turok
(Submitted on 30 Jul 2013)
Recent measurements at the LHC suggest that the current Higgs vacuum could be metastable with a modest barrier (height 1010-12GeV4) separating it from a ground state with negative vacuum density of order the Planck scale. We note that metastability is problematic for big bang to end one cycle, bounce, and begin the next. In this paper, motivated by the approximate scaling symmetry of the standard model of particle physics and the primordial large-scale structure of the universe, we use our recent formulation of the Weyl-invariant version of the standard model coupled to gravity to track the evolution of the Higgs in a regularly bouncing cosmology. We find a band of solutions in which the Higgs field escapes from the metastable phase during each big crunch, passes through the bang into an expanding phase, and returns to the metastable vacuum, cycle after cycle after cycle. We show that, due to the effect of the Higgs, the infinitely cycling universe is geodesically complete, in contrast to inflation.
16 pages, 4 figures

http://arxiv.org/abs/1307.7988
Pathways to relativistic curved momentum spaces: de Sitter case study
Giovanni Amelino-Camelia, Giulia Gubitosi, Giovanni Palmisano
(Submitted on 30 Jul 2013)
Several arguments suggest that the Planck scale could be the characteristic scale of curvature of momentum space. As other recent studies we assume that the metric of momentum space determines the condition of on-shellness while the momentum-space affine connection governs the form of the law of composition of momenta. We show that the possible choices of laws of composition of momenta are more numerous than the possible choices of affine connection on a momentum space. This motivates us to propose a new prescription for associating an affine connection to momentum composition, which we compare to the one most used in the recent literature. We find that the two prescriptions lead to the same picture of the so-called κ-momentum space, with de Sitter metric and κ-Poincaré connection. We also examine in greater detail than ever before the DSR-relativistic properties of κ-momentum space, particularly in relation to its noncommutative law of composition of momenta. We then show that in the case of "proper de Sitter momentum space", with the de Sitter metric and its Levi-Civita connection, the two prescriptions are inequivalent. Our novel prescription leads to a picture of proper de Sitter momentum space which is DSR-relativistic and is characterized by a commutative law of composition of momenta, a possibility for which no explicit curved-momentum-space picture had been previously found. We argue that our construction provides a natural test case for the study of momentum spaces with commutative, and yet deformed, laws of composition of momenta. Moreover, it can serve as laboratory for the exploration of the properties of DSR-relativistic theories which are not connected to group-manifold momentum spaces and Hopf algebras.
36 pages, 1 figure

Footnote: one of the authors gives a 20 minute talk on this here (start at minute 49:00 of the recording):
http://pirsa.org/13070052/
Phenomenology - 1
Jonathan Granot, Julien Bolmont, Giulia Gubitosi, Giovanni Palmisano, Linqing Chen
21:00, 49:00, 75:00, 95:00
===========
For the complete list of parallel session talks, with minute-marks, go here:

Last edited: Jul 30, 2013
8. Aug 1, 2013

### atyy

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick
(Submitted on 31 Jul 2013)
It is well known that the $\SU(2)$-gauge invariant phase space of loop gravity can be represented in terms of twisted geometries. These are piecewise-linear-flat geometries obtained by gluing together polyhedra, but the resulting geometries are not continuous across the faces. Here we show that this phase space can also be represented by continuous, piecewise-flat three-geometries called spinning geometries. These are composed of metric-flat three-cells glued together consistently. The geometry of each cell and the manner in which they are glued is compatible with the choice of fluxes and holonomies.
We first remark that the fluxes provide each edge with an angular momentum. By studying the piecewise-flat geometries which minimize edge lengths, we show that these angular momenta can be literally interpreted as the spin of the edges: the geometries of all edges are necessarily helices. We also show that the compatibility of the gluing maps with the holonomy data results in the same conclusion. This shows that a spinning geometry represents a way to glue together the three-cells of a twisted geometry to form a continuous geometry which represents a point in the loop gravity phase space.

http://arxiv.org/abs/1308.0300
Snyder Momentum Space in Relative Locality
Andrzej Banburski, Laurent Freidel
(Submitted on 1 Aug 2013)
The standard approaches of phenomenology of Quantum Gravity have usually explicitly violated Lorentz invariance, either in the dispersion relation or in the addition rule for momenta. We investigate whether it is possible in 3+1 dimensions to have a non local deformation that preserves fully Lorentz invariance, as it is the case in 2+1D Quantum Gravity. We answer positively to this question and show for the first time how to construct a homogeneously curved momentum space preserving the full action of the Lorentz group in dimension 4 and higher, despite relaxing locality. We study the property of this relative locality deformation and show that this space leads to a noncommutativity related to Snyder spacetime.

http://arxiv.org/abs/1212.5233
Causal loop in the theory of Relative Locality
Lin-Qing Chen
(Submitted on 20 Dec 2012 (v1), last revised 3 Feb 2013 (this version, v2))
We find that Relative Locality, a recently proposed Planck-scale deformation of Special Relativity, suffers from the existence of causal loops. A simple and general construction of such on-shell loop processes is studied. We then show that even in one of the weakest deformations of the Poincar\'e group in Relative Locality, causality can be violated.

http://arxiv.org/abs/1308.0318
Orientability of loop processes in Relative Locality
Lin-Qing Chen
(Submitted on 1 Aug 2013)
We introduce a way to classify loop processes in relative locality in the case of Kappa-Poincare momentum space. We show that orientability is connected to a few essential properties in loop processes. Non-orientable loops have "effective curvature", which explicitly breaks translation symmetry, and can lead to breaking of causality and global momentum conservation. Orientable loops are "flat". Causality and global momentum conservation are all preserved in this kind of loops. We argue that the non-trivial classical loops in relative locality might be understood as dual effects from general relativity.

Last edited: Aug 1, 2013
9. Aug 5, 2013

### marcus

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 3 Aug 2013)
The dynamics of a homogeneous, anisotropic universe are investigated within the context of spinfoam cosmology. Transition amplitudes are calculated for a graph consisting of a single node and three links - the Daisy graph' - probing the behaviour a classical Bianchi I spacetime. It is shown further how the use of such single node graphs gives rise to a simplification of states such that all orders in the spin expansion can be calculated, indicating that it is the vertex expansion that contains information about quantum dynamics.
28 pages, 1 figure

http://arxiv.org/abs/1308.1012
A look at area Regge calculus
Yasha Neiman
(Submitted on 5 Aug 2013)
Area Regge calculus is a candidate theory of simplicial gravity, based on the Regge action with triangle areas as the dynamical variables. It is characterized by metric discontinuities and vanishing deficit angles. Area Regge calculus arises in the large-spin limit of the Barrett-Crane spinfoam model, but not in the newer EPRL/FK model. We address the viability of area Regge calculus as a discretization of General Relativity. We argue that when all triangles are spacelike and all tetrahedra have the same signature, non-trivial solutions of the area calculus are associated with a nonzero Ricci scalar. Our argument rests on a seemingly natural regularization of the metric discontinuities. It rules out the Euclidean area calculus, as well as the Lorentzian sector with all tetrahedra spacelike - the two setups usually considered in spinfoam models. On the other hand, we argue that the area calculus has attractive properties from the point of view of finite-region observables in quantum gravity.
15 pages, 2 figures

general interest (brief mention):
http://arxiv.org/abs/1308.1007
The Fate of the Quantum
Gerard 't Hooft
(Submitted on 5 Aug 2013)
Although the suspicion that quantum mechanics is emergent has been lingering for a long time, only now we begin to understand how a bridge between classical and quantum mechanics might be squared with Bell's inequalities and other conceptual obstacles...
14 pages, 1 figure, talk given at the "Time and Matter" conference, Venice March 2013.

Last edited: Aug 5, 2013
10. Aug 6, 2013

### MTd2

http://arxiv.org/abs/1308.1278

Second Order Standard Model
Johnny Espin, Kirill Krasnov
(Submitted on 6 Aug 2013)
We rewrite the Lagrangian of the fermionic sector of the Standard Model in a novel compact form. The new Lagrangian is second order in derivatives, and is obtained from the usual first order Lagrangian by integrating out all primed (or dotted) 2-component spinors. The Higgs field enters the new Lagrangian non-polynomially, very much like the metric enters the Einstein-Hilbert Lagrangian of General Relativity. We also discuss unification in the second order formalism, and describe a natural in this framework SU(2)xSU(4) unified theory.

11. Aug 11, 2013

### marcus

http://arxiv.org/abs/1308.1982
Polyhedra in spacetime from null vectors
Yasha Neiman
(Submitted on 8 Aug 2013)
We consider convex spacelike polyhedra oriented in Minkowski space. These are the classical analogues of spinfoam intertwiners. We point out a parametrization of these shapes using null face normals, with no constraints or redundancies. Our construction is dimension-independent. In 3+1d, it provides the spacetime picture behind a well-known property of the loop quantum gravity intertwiner space in spinor form, namely that the closure constraint is always satisfied after some SL(2,C) rotation. As a simple application of our variables, we incorporate them in a 4-simplex action that reproduces the large-spin behavior of the Barrett-Crane vertex amplitude.
12 pages, 1 figure

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin
(Submitted on 9 Aug 2013)
We propose an approach to quantum theory based on the energetic causal sets, introduced in Cortês and Smolin (2013). Fundamental processes are causal sets whose events carry momentum and energy, which are transmitted along causal links and conserved at each event. Fundamentally there are amplitudes for such causal processes, but no space-time. An embedding of the causal processes in an emergent space-time arises only at the semiclassical level. Hence, fundamentally there are no commutation relations, no uncertainty principle and, indeed, no hbar. All that remains of quantum theory is the relationship between the absolute value squared of complex amplitudes and probabilities. Consequently, we find that neither locality, nor non locality, are primary concepts, only causality exists at the fundamental level.
9 pages. Article companion to http://arxiv.org/abs/1307.6167

general interest:
http://arxiv.org/abs/1308.1977
Holography without strings?
Donald Marolf
(Submitted on 8 Aug 2013)
A defining feature of holographic dualities is that, along with the bulk equations of motion, boundary correlators at any given time t determine those of observables deep in the bulk. We argue that this property emerges from the bulk gravitational Gauss law together with bulk quantum entanglement as embodied in the Reeh-Schlieder theorem. Stringy bulk degrees of freedom are not required and play little role even when they exist. As an example we study a toy model whose matter sector is a free scalar field. The energy density (ρ) sources what we call a pseudo-Newtonian potential (Φ) through Poisson's equation on each constant time surface, but there is no back-reaction on the matter. We show the Hamiltonian to be essentially self-adjoint on the domain generated from the vacuum by acting with boundary observables localized in an arbitrarily small neighborhood of the chosen time t. Since the Gauss law represents the Hamiltonian as a boundary term, the model is holographic in the sense stated above.
13 pages

Last edited: Aug 11, 2013
12. Aug 14, 2013

### John86

http://arxiv.org/abs/1308.2746
On the renormalization of the Gibbons-Hawking boundary term
Ted Jacobson, Alejandro Satz
(Submitted on 13 Aug 2013)
The bulk (Einstein-Hilbert) and boundary (Gibbons-Hawking) terms in the gravitational action are generally renormalized differently when integrating out quantum fluctuations. The former is affected by nonminimal couplings, while the latter is affected by boundary conditions. We use the heat kernel method to analyze this behavior for a nonminimally coupled scalar field, the Maxwell field, and the graviton field. Allowing for Robin boundary conditions, we examine in which cases the renormalization preserves the ratio of boundary and bulk terms required for the effective action to possess a stationary point. The implications for field theory and black hole entropy computations are discussed.

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Chaharsough Shirazi, Jonathan Engle
(Submitted on 13 Aug 2013)
Spin-foams are a proposal for defining the dynamics of loop quantum gravity via path integral. In order for a path integral to be at least formally equivalent to the corresponding canonical quantization, at each point in the space of histories it is important that the integrand have not only the correct phase -- a topic of recent focus in spin-foams -- but also the correct modulus, usually referred to as the measure factor. The correct measure factor descends from the Liouville measure on the reduced phase space, and its calculation is a task of canonical analysis.
The covariant formulation of gravity from which spin-foams are derived is the Plebanski-Holst formulation, in which the basic variables are a Lorentz connection and a Lorentz-algebra valued two-form, called the Plebanski two-form. However, in the final spin-foam sum, one sums over only spins and intertwiners, which label eigenstates of the Plebanski two-form alone. The spin-foam sum is therefore a discretized version of a Plebanski-Holst path integral in which only the Plebanski two-form appears, and in which the connection degrees of freedom have been integrated out. We call this a purely geometric Plebanski-Holst path integral.
In prior work in which one of the authors was involved, the measure factor for the Plebanski-Holst path integral with both connection and two-form variables was calculated. Before one discretizes this measure and incorporates it into a spin-foam sum, however, one must integrate out the connection in order to obtain the purely geometric version of the path integral. To calculate this purely geometric path integral is the principal task of the present paper, and it is done in two independent ways. Gauge-fixing and the background independence of the resulting path integral are discussed in the appendices.

13. Aug 15, 2013

### John86

Distant related, but interesting

http://arxiv.org/abs/1308.3044
Nonperturbative analysis of the evolution of cosmological perturbations through a nonsingular bounce
BingKan Xue, David Garfinkle, Frans Pretorius, Paul J. Steinhardt
(Submitted on 14 Aug 2013)
In bouncing cosmology, the primordial fluctuations are generated in a cosmic contraction phase before the bounce into the current expansion phase. For a nonsingular bounce, curvature and anisotropy grow rapidly during the bouncing phase, raising questions about the reliability of perturbative analysis. In this paper, we study the evolution of adiabatic perturbations in a nonsingular bounce by nonperturbative methods including numerical simulations of the nonsingular bounce and the covariant formalism for calculating nonlinear perturbations. We show that the bounce is disrupted in regions of the universe with significant inhomogeneity and anisotropy over the background energy density, but is achieved in regions that are relatively homogeneous and isotropic. Sufficiently small perturbations, consistent with observational constraints, can pass through the nonsingular bounce with negligible alteration from nonlinearity. We follow scale invariant perturbations generated in a matter-like contraction phase through the bounce. Their amplitude in the expansion phase is determined by the growing mode in the contraction phase, and the scale invariance is well preserved across the bounce.

14. Aug 16, 2013

### John86

http://arxiv.org/abs/1308.3337
Information-Based Physics and the Influence Network
Kevin H. Knuth
(Submitted on 15 Aug 2013)
This essay considers a simple model of observers that are influenced by the world around them. Consistent quantification of information about such influences results in a great deal of familiar physics. The end result is a new perspective on relativistic quantum mechanics, which includes both a way of conceiving of spacetime as well as particle "properties" that may be amenable to a unification of quantum mechanics and gravity. Rather than thinking about the universe as a computer, perhaps it is more accurate to think about it as a network of influences where the laws of physics derive from both consistent descriptions and optimal information-based inferences made by embedded observers.

http://arxiv.org/abs/1308.3398
Higher Derivative Gravity and Asymptotic Safety in Diverse Dimensions
(Submitted on 15 Aug 2013)
We derive the one-loop beta functions for a theory of gravity with generic action containing up to four derivatives. The calculation is done in arbitrary dimension and on an arbitrary background. The special cases of three, four, near four, five and six dimensions are discussed in some detail. We find that the theories have nontrivial UV fixed points and are asymptotically safe in all dimensions we study. We also find an indication that Weyl-invariant fixed point exists in four dimensions. The new massive gravity in three dimensions does not correspond to any fixed point.

http://arxiv.org/abs/1308.3488
Statistical physics of black holes as quantum-mechanical systems
Steven B. Giddings
(Submitted on 15 Aug 2013)
Some basic features of black-hole statistical mechanics are investigated, assuming that black holes respect the principles of quantum mechanics. Care is needed in defining an entropy S_bh corresponding to the number of microstates of a black hole, given that the black hole interacts with its surroundings. An open question is then the relationship between this entropy and the Bekenstein-Hawking entropy S_BH. For a wide class of models with interactions needed to ensure unitary quantum evolution, these interactions produce extra energy flux beyond that predicted by Hawking. Arguments are then presented that this results in an entropy S_bh that is smaller than S_BH. Correspondingly, in such scenarios equilibrium properties of black holes are modified. We examine questions of consistency of such an inequality; if it is not consistent, that provides significant constraints on models for quantum-mechanical black hole evolution.

15. Aug 19, 2013

### marcus

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han
(Submitted on 19 Aug 2013)
A low-energy perturbation theory is developed from the nonperturbative framework of covariant Loop Quantum Gravity (LQG) by employing the background field method. The resulting perturbation theory is a 2-parameter expansion in the semiclassical and low-energy regime. The two expansion parameters are the large spin and small curvature. The leading order effective action coincides with the Einstein-Hilbert action. The subleading corrections organized by the two expansion parameters give the modifications of Einstein gravity in quantum and high-energy regime from LQG. The result of the paper may be viewed as the first step toward understanding the UV completeness of LQG.
4 pages, 1 figure

brief mention:
http://arxiv.org/abs/1308.4099
Null tests of the cosmological constant using supernovae
Sahba Yahya, Marina Seikel, Chris Clarkson, Roy Maartens, Mathew Smith
(Submitted on 19 Aug 2013)
7 pages, 5 figures.

Last edited: Aug 19, 2013
16. Aug 20, 2013

### marcus

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 20 Aug 2013)
We identify a signature of quantum gravitational effects that survives from the early universe to the current era: Fluctuations of quantum fields as seen by comoving observers are significantly influenced by the history of the early universe. In particular we will show how the existence (or not) of a quantum bounce leaves a trace in the background quantum noise that is not damped and would be non-negligible even nowadays. Furthermore, we will estimate an upper bound to the typical energy and length scales where quantum effects are relevant. We will discuss how this signature might be observed and therefore used to build falsifiability tests of quantum gravity theories.
5 pages, 3 figures

17. Aug 20, 2013

### MTd2

http://arxiv.org/abs/1308.4044

Notes on several phenomenological laws of quantum gravity

Jean-Philippe Bruneton
(Submitted on 19 Aug 2013)
Phenomenological approaches to quantum gravity try to infer model-independent laws by analyzing thought experiments and combining both quantum, relativistic, and gravitational ingredients. We first review these ingredients -three basic inequalities- and discuss their relationships with the nature of fundamental constants. In particular, we argue for a covariant mass bound conjecture: in a spacetime free of horizon, the mass inside a surface $A$ cannot exceed $16 \pi G^2 m^2< A$, while the reverse holds in a spacetime with horizons. This is given a precise definition using the formalism of light-sheets. We show that $\hbar/c$ may be also given a geometrical interpretation, namely $4 \pi \hbar^2/m^2< A$. We then combine these inequalities and find/review the following: (1) Any system must have a size greater than the Planck length, in the sense that there exists a minimal area (2) We comment on the Minimal Length Scenarios and the fate of Lorentz symmetry near the Planck scale (3) Quanta with transplanckian frequencies are allowed in a large enough boxes (4) There exists a mass-dependent maximal acceleration given by $m c^3/\hbar$ if $m<m_p$ and by $c^4/G m$ if $m>m_p$ (5) There exists a mass dependent maximal force and power (6) There exists a maximal energy density and pressure (7) Physical systems must obey the Holographic Principle (8) Holographic bounds can only be saturated by systems with $m>m_p$; systems lying on the Compton line'' $l \sim 1/m$ are fundamental objects without substructures (9) We speculate on a new bound from above for the action. In passing, we note that the maximal acceleration is of the order of Milgrom's acceleration $a_0$ for ultra-light particles ($m\sim H_0)$ that could be associated to the Dark Energy fluid. This suggests designing toy-models in which modified gravity in galaxies is driven by the DE field, via the maximal acceleration principle.

18. Aug 22, 2013

### John86

http://arxiv.org/abs/1308.4667
Conformal Symmetry, Rindler Space and The AdS/CFT Correspondence
(Submitted on 21 Aug 2013)
Field theories in black hole spacetimes undergo dimensional reduction near horizon (in the Rindler limit) to two dimensional conformal field theories. We investigate this enhancement of symmetries in the context of gauge/gravity duality by considering Rindler space as boundary of Anti-de Sitter space in three spacetime dimensions. We show that the loxodromy conjugacy class of the SO(2,2) isometry group is responsible for generating the special conformal transformations on the boundary under RG flow. We use this approach to present an alternative derivation of the two-point function in Rindler space using AdS/CFT correspondence.

19. Aug 26, 2013

### John86

http://arxiv.org/abs/1308.4976
Evolution of quantum field, particle content and classicality in the three stage universe
Suprit Singh, Sujoy Kumar Modak, T. Padmanabhan
(Submitted on 22 Aug 2013)
We study the evolution of a quantum scalar field in a toy universe which has three stages of evolution, viz., (i) an early (inflationary) de Sitter phase (ii) radiation dominated phase and (iii) late-time (cosmological constant dominated) de Sitter phase. Using Schr\"odinger picture, the scalar field equations are solved separately for the three stages and matched at the transition points. The boundary conditions are chosen so that field modes in the early de Sitter evolves from Bunch-Davies vacuum state. We determine the (time-dependent) particle content of this quantum state for the entire evolution of the universe and describe the various features both numerically and analytically. We also describe the quantum to classical transition in terms of a {\it classicality parameter} which tracks the particle creation and its effect on phase space correlation of the quantum field.

http://arxiv.org/abs/1308.5009
Quantum nonlocal correlations are not dominated
Adrian Kent (Centre for Quantum Information and Foundations, DAMTP, University of Cambridge and Perimeter Institute)
(Submitted on 22 Aug 2013)
We show that no probability distribution of spin measurement outcomes on pairs of spin 1/2 particles is unambiguously more nonlocal than the quantum correlations. That is, any distribution that produces a CHSH violation larger than the quantum violation for some axis choices also produces a smaller CHSH violation for some other axis choices. In this sense, it is not possible for nature to be strictly more nonlocal than quantum theory allows.

this is quite an interesting paper

http://arxiv.org/abs/1308.5097
Foundations of Quantum Gravity : The Role of Principles Grounded in Empirical Reality
M. Holman
(Submitted on 23 Aug 2013)
When attempting to assess the strengths and weaknesses of various principles in their potential role of guiding the formulation of a theory of quantum gravity, it is crucial to distinguish between principles which are strongly supported by empirical data - either directly or indirectly - and principles which instead (merely) rely heavily on theoretical arguments for their justification. These remarks are illustrated in terms of the current standard models of cosmology and particle physics, as well as their respective underlying theories, viz. general relativity and quantum (field) theory. It is argued that if history is to be of any guidance, the best chance to obtain the key structural features of a putative quantum gravity theory is by deducing them, in some form, from the appropriate empirical principles (analogous to the manner in which, say, the idea that gravitation is a curved spacetime phenomenon is arguably implied by the equivalence principle). It is subsequently argued that the appropriate empirical principles for quantum gravity should at least include (i) quantum nonlocality, (ii) irreducible indeterminacy, (iii) the thermodynamic arrow of time, (iv) homogeneity and isotropy of the observable universe on the largest scales. In each case, it is explained - when appropriate - how the principle in question could be implemented mathematically in a theory of quantum gravity, why it is considered to be of fundamental significance and also why contemporary accounts of it are insufficient.

Last edited: Aug 26, 2013
20. Aug 26, 2013

### marcus

http://arxiv.org/abs/1308.5599
Why Gauge?
Carlo Rovelli
(Submitted on 26 Aug 2013)
The world appears to be well described by gauge theories; why? I suggest that gauge is more than mathematical redundancy. Gauge variables describe handles though which systems couple. Gauge-dependent quantities can not be predicted, but there is a sense in which they can be measured. This observation leads to a physical interpretation for the ubiquity of gauge: it is a consequence of a relational structure of the physical quantities.
7 pages

http://arxiv.org/abs/1308.5648
Semiclassical states in quantum gravity: Curvature associated to a Voronoi graph
Jacobo Diaz-Polo, Iñaki Garay
(Submitted on 26 Aug 2013)
The building blocks of a quantum theory of general relativity are expected to be discrete structures. Loop quantum gravity is formulated using a basis of spin networks (wave functions over oriented graphs with coloured edges), thus realizing the aforementioned expectation. Semiclassical states should, however, reproduce the classical smooth geometry in the appropriate limits. The question of how to recover a continuous geometry from these discrete structures is, therefore, relevant in this context. Following previous works by Bombelli et al. we explore this problem from a rather general mathematical perspective using, in particular, properties of Voronoi graphs to search for their compatible continuous geometries. We test the previously proposed methods for computing the curvature associated to such graphs and analyse the framework in detail, in the light of the results obtained.
16 pages

possible interest, no time to evaluate:
http://arxiv.org/abs/1308.5290

Last edited: Aug 26, 2013