# Loop-and-allied QG bibliography

1. Jun 23, 2013

### marcus

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli
(Submitted on 21 Jun 2013)
We show that Oeckl's boundary formalism incorporates quantum statistical mechanics naturally, and we formulate general-covariant quantum statistical mechanics in this language. We illustrate the formalism by showing how it accounts for the Unruh effect. We observe that the distinction between pure and mixed states weakens in the general covariant context, and surmise that local gravitational processes are indivisibly statistical with no possible quantal versus probabilistic distinction.
8 pages, 2 figures

http://arxiv.org/abs/1306.4995
Dust time in quantum cosmology
Viqar Husain, Tomasz Pawlowski
(Submitted on 20 Jun 2013)
We give a formulation of quantum cosmology with a pressureless dust and arbitrary additional matter fields. The dust provides a natural time gauge corresponding to a cosmic time, yielding a physical time independent Hamiltonian. The approach simplifies the analysis of both Wheeler-deWitt and loop quantum cosmology models, broadening the applicability of the latter.
3 pages. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012

brief mention:
http://arxiv.org/abs/1306.5063
Entropy from near-horizon geometries of Killing horizons
Olaf Dreyer, Amit Ghosh, Avirup Ghosh
(Submitted on 21 Jun 2013)
We derive black hole entropy based on the near-horizon symmetries of black hole space-times. To derive these symmetries we make use of an (R,T)-plane close to a Killing horizon. We identify a set of vector fields that preserves this plane and forms a Witt algebra. The corresponding algebra of Hamiltonians is shown to have a non-trivial central extension. Using the Cardy formula and the central charge we obtain the Bekenstein-Hawking entropy.
8 pages.

Last edited: Jun 23, 2013
2. Jun 25, 2013

### John86

http://www.fqxi.org/community/forum/topic/1816
Essay Abstract
Relative information at the foundation of physics by Carlo Rovelli
I observe that Shannon's notion of relative information between two physical systems can effectively function as a foundation for statistical mechanics and quantum mechanics, without referring to any subjectivism or idealism. It can also represent the key missing element in the foundation of the naturalistic picture of the world, providing the conceptual tool for dealing with its apparent limitations. I comment on the relation between these ideas and Democritus.

http://www.fqxi.org/community/forum/topic/1831
Information-Based Physics and the Influence Network by Kevin H Knuth
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.

3. Jun 25, 2013

### John86

http://arxiv.org/abs/1306.5332
Super-accelerating bouncing cosmology in asymptotically-free non-local gravity
Gianluca Calcagni, Leonardo Modesto, Piero Nicolini
(Submitted on 22 Jun 2013)
Recently, evidence has been collected that a class of gravitational theories with certain non-local operators is renormalizable. We consider one such model which, at the linear perturbative level, reproduces the effective non-local action for the light modes of bosonic closed string field theory. Using the property of asymptotic freedom in the ultraviolet and fixing the classical behaviour of the scale factor at late times, an algorithm is proposed to find general homogeneous cosmological solutions valid both at early and late times. Imposing a power-law classical limit, these solutions (including anisotropic ones) display a bounce instead of a big-bang singularity, and super-accelerate near the bounce even in the absence of an inflaton or phantom field.

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah
(Submitted on 24 Jun 2013)
Since black holes can be formed through widely varying processes, the horizon structure is highly complicated in the dynamical phase. Nonetheless, as numerical simulations show, the final state appears to be universal, well described by the Kerr geometry. How are all these large and widely varying deviations from the Kerr horizon washed out? To investigate this issue, we introduce a well-suited notion of horizon multipole moments and equations governing their dynamics, thereby providing a coordinate and slicing independent framework to investigate the approach to equilibrium. In particular, our flux formulas for multipoles can be used as analytical checks on numerical simulations and, in turn, the simulations could be used to fathom possible universalities in the way black holes approach their final equilibrium.

http://arxiv.org/abs/1306.5470
Bimetric Gravity, Variable Speed of Light Cosmology and Planck2013
J. W. Moffat
(Submitted on 23 Jun 2013)
A bimetric gravity model with a variable speed of light is shown to be in agreement with the results reported from the Planck satellite in 2013. The predicted scalar mode spectral index is $n_s\approx 0.96$ and its running is $\alpha_s\approx 8\times 10^{-4}$ when the fundamental length scale $\ell_0$ in the model is fixed to be $\ell_0\approx 10^5\ell_P$, where $\ell_P$ is the Planck length $\ell_P=1.62\times 10^{-33}\,{\rm cm}$, giving the observed CMB fluctuations: $\delta_H\approx 10^{-5}$. The enlarged lightcone ensures that horizon and flatness problems are solved. The model is free from many of the fine-tuning problems of the inflationary models and the fluctuations that form the seeds of structure formation do not lead to a chaotic inhomogeneous universe and the need for a multiverse.

http://arxiv.org/abs/1306.5471
Quantum Structures: A View of the Quantum World
Jasmina Jeknic-Dugic, Momir Arsenijevic, Miroljub Dugic
(Submitted on 23 Jun 2013)
We oer a systematic account of decomposition of quantum systems into parts. Dierent decompositions (structures) are mutually linked via the proper linear canonical transformations. Dierent kinds of structures, as well as their relations, are considered. Emphasis is placed on mutually global and irreducible structures. Is there a privileged structure of the closed system? Is there a preferred (bipartite) structure of an open system? Are there any practical advantages of certain alternative structures of the system? Is there a simple dynamical relation for a pair of structures? The [necessarily partial] answers are rather intriguing. Relativity of quantum correlations (that include entanglement as well as the "one-way" and "two-way" discord) is carefully presented. Emphasis is placed on the "parallel occurrence of decoherence" in the quantum Brownian motion. The environment-selected preferred structure of an open composite system is presented for a pair of harmonic oscillators (or the ?eld modes). A limitation of the Nakajima-Zwanzig projection method appears as a consequence of quantum correlations relativity. Hence, describing dynamics of an alternate open-system is a delicate task. Certain interpretational issues, which include "quantum reference frames" issue, are discussed. Some recent experiments are also discussed. To this end, the option that "there are no particles" on the most fundamental physical level naturally follows. Certain open questions and prospects for further research are highlighted.

4. Jun 26, 2013

### marcus

http://arxiv.org/abs/1306.6126
The generator of spatial diffeomorphisms in the Koslowski- Sahlmann representation
(Submitted on 26 Jun 2013)
A generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity, to accomodate states labelled by smooth spatial geometries, was discovered by Koslowski and further studied by Sahlmann. We show how to construct the diffeomorphism constraint operator in this Koslowski- Sahlmann (KS) representation from suitable connection and triad dependent operators. We show that the KS representation supports the action of hitherto unnoticed "background exponential" operators which, in contrast to the holonomy-flux operators, change the smooth spatial geometry label of the states. These operators are shown to be quantizations of certain connection dependent functions and play a key role in the construction of the diffeomorphism constraint operator.
8 pages

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh
(Submitted on 26 Jun 2013)
A fundamental issue for any quantum cosmological theory is to specify how probabilities can be assigned to various quantum events or sequences of events such as the occurrence of singularities or bounces. In previous work, we have demonstrated how this issue can be successfully addressed within the consistent histories approach to quantum theory for Wheeler-DeWitt-quantized cosmological models. In this work, we generalize that analysis to the exactly solvable loop quantization of a spatially flat, homogeneous and isotropic cosmology sourced with a massless, minimally coupled scalar field known as sLQC. We provide an explicit, rigorous and complete decoherent histories formulation for this model and compute the probabilities for the occurrence of a quantum bounce vs. a singularity. Using the scalar field as an emergent internal time, we show for generic states that the probability for a singularity to occur in this model is zero, and that of a bounce is unity, complementing earlier studies of the expectation values of the volume and matter density in this theory. We also show from the consistent histories point of view that all states in this model, whether quantum or classical, achieve arbitrarily large volume in the limit of infinite 'past' or 'future' scalar 'time', in the sense that the wave function evaluated at any arbitrary fixed value of the volume vanishes in that limit. Finally, we briefly discuss certain misconceptions concerning the utility of the consistent histories approach in these models.
22 pages, 3 figures

Last edited: Jun 26, 2013
5. Jun 27, 2013

### John86

http://arxiv.org/abs/1306.5805
Is the Pusey-Barrett-Rudolph Theorem Compatible with Quantum Nonseparability?
Maximilian Schlosshauer, Arthur Fine
(Submitted on 24 Jun 2013)
The Pusey-Barrett-Rudolph (PBR) no-go theorem targets a class of "epistemic" hidden-variables models in which hidden variables associated with distinct quantum states overlap. We show that the PBR strategy leads to a no-go result that would also rule out nonepistemic ("ontic") models. Moreover, it would rule out a vast class of deterministic hidden-variables theories, even those known to be consistent. The strength of this result calls into question a central assumption of the PBR strategy about how hidden variables of composite systems relate to hidden variables of components.

http://www.fqxi.org/community/forum/topic/1842
Essay Abstract
It from Qubit by Giacomo Mauro D'Ariano
In this essay I will embark on the venture of changing the realist reader’s mind about the informational viewpoint for physics: “It from Bit”. I will try to convince him of the amazing theoretical power of such paradigm. Contrary to the common belief, the whole history of physics is indeed a winding road making the notion of “physical object”–the “It”–fade away. Such primary concept, on which the structure of contemporary theoretical physics is still grounded, is no longer logically tenable. The thesis I advocate here is that the “It” is emergent from pure information, an information of special kind: quantum. The paradigm then becomes: “It from Qubit”. Quantum fields, particles, space-time and relativity simply emerge from countably infinitely many quantum systems in interaction. Don’t think that, however, I can cheat by suitably programming a “simulation” of what we see. On the contrary: the quantum software is constrained by very strict rules of topological nature, which minimize the algorithmic complexity. These are locality, unitariety, homogeneity, and isotropy of the processing, with minimal quantum dimension. What is amazing is that from just such simple rules, and without using relativity, we obtain the Dirac field dynamics as emergent.

6. Jun 27, 2013

### Blackforest

I have read both, the rules for that forum and the article. I find that it is an interesting and very clear exposé of a controversial and difficult problem (The inflation of our universe and the equation of state for the vacuum remain two puzzling item, so far I know).

Staying exclusively at the mathematical and physical level of the discussion, I get some confusion because of the fact that the conversation introduces (see p. 6) three different mass densities: the effective, the gravitational and the inertial one. Furthermore, equation (21) page 6 indicates an obvious difference between the gravitational and the inertial mass density. Isn't it in contradiction with some fundamental principle stating the equivalence between inerty and gravitation? What did I certainly miss? Thanks for explaining better, if possible.

7. Jun 27, 2013

### marcus

Blackforest, our custom here is to reserve this thread for bibliography and start a separate discussion thread for any paper we want to discuss. I have quoted your post, with your question, in order to start a discussion thread.
The idea is not to overload this thread and interfere with bibliography function.

Last edited: Jun 27, 2013
8. Jul 2, 2013

### MTd2

http://arxiv.org/abs/1307.0096

Elementary Particles as Gates for Universal Quantum Computation
Deepak Vaid
(Submitted on 29 Jun 2013)
It is shown that there exists a mapping between the fermions of the Standard Model (SM) represented as braids in the Bilson-Thompson model, and a set of gates which can perform Universal Quantum Computation (UQC). This leads us to conjecture that the "Computational Universe Hypothesis" (CUH) can be given a concrete implementation in a new physical framework where elementary particles and the gauge bosons (which intermediate interactions between fermions) are interpreted as the components of a quantum computational network, with the particles serving as quantum computational gates and the gauge fields as the information carrying entities.

9. Jul 2, 2013

### MTd2

http://arxiv.org/abs/1101.5185

Einstein Manifolds As Yang-Mills Instantons

John J. Oh, Hyun Seok Yang
(Submitted on 27 Jan 2011 (v1), last revised 2 Jul 2013 (this version, v4))
It is well-known that Einstein gravity can be formulated as a gauge theory of Lorentz group where spin connections play a role of gauge fields and Riemann curvature tensors correspond to their field strengths. One can then pose an interesting question: What is the Einstein equations from the gauge theory point of view? Or equivalently, what is the gauge theory object corresponding to Einstein manifolds? We show that the Einstein equations in four dimensions are precisely self-duality equations in Yang-Mills gauge theory and so Einstein manifolds correspond to Yang-Mills instantons in SO(4) = SU(2)_L x SU(2)_R gauge theory. Specifically, we prove that any Einstein manifold with or without a cosmological constant always arises as the sum of SU(2)_L instantons and SU(2)_R anti-instantons. This result explains why an Einstein manifold must be stable because two kinds of instantons belong to different gauge groups, instantons in SU(2)_L and anti-instantons in SU(2)_R, and so they cannot decay into a vacuum. We further illuminate the stability of Einstein manifolds by showing that they carry nontrivial topological invariants.

10. Jul 3, 2013

### John86

http://arxiv.org/abs/1307.0256
Quantum astrometric observables II: time delay in linearized quantum gravity
Béatrice Bonga, Igor Khavkine
(Submitted on 1 Jul 2013)
A clock synchronization thought experiment is modeled by a diffeomorphism invariant "time delay" observable. In a sense, this observable probes the causal structure of the ambient Lorentzian spacetime. Thus, upon quantization, it is sensitive to the long expected smearing of the light cone by vacuum fluctuations in quantum gravity. After perturbative linearization, its mean and variance are computed in the Minkowski Fock vacuum of linearized gravity. The na\"ive divergence of the variance is meaningfully regularized by a length scale $\mu$, the physical detector resolution. This is the first time vacuum fluctuations have been fully taken into account in a similar calculation. Despite some drawbacks this calculation provides a useful template for the study of a large class of similar observables in quantum gravity. Due to their large volume, intermediate calculations were performed using computer algebra software. The resulting variance scales like $(s \ell_p/\mu)^2$, where $\ell_p$ is the Planck length and $s$ is the distance scale separating the ("lab" and "probe") clocks. Additionally, the variance depends on the relative velocity of the lab and the probe, diverging for low velocities. This puzzling behavior may be due to an oversimplified detector resolution model or a neglected second order term in the time delay.

http://arxiv.org/abs/1307.0745
Rainbow gravity and scale-invariant fluctuations
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 2 Jul 2013)
We re-examine a recently proposed scenario where the deformed dispersion relations associated with a flow of the spectral dimension to a UV value of 2 leads to a scale-invariant spectrum of cosmological fluctuations, without the need for inflation. In that scenario Einstein gravity was assumed. The theory displays a wavelength-dependent speed of light but by transforming to a suitable "rainbow frame" this feature can be removed, at the expense of modifying gravity. We find that the ensuing rainbow gravity theory is such that gravity switches off at high energy (or at least leads to a universal conformal coupling). This explains why the fluctuations are scale-invariant on all scales: there is no horizon scale as such. For dispersion relations that do not lead to exact scale invariance we find instead esoteric inflation in the rainbow frame. We argue that these results shed light on the behaviour of gravity under the phenomenon of dimensional reduction

http://arxiv.org/abs/1307.0722
Linear lattice gauge theory
C.Wetterich
(Submitted on 2 Jul 2013)
Linear lattice gauge theory is based on link variables that are arbitrary complex or real $N\times N$ matrices. This contrasts with the usual (non-linear) formulation with unitary or orthogonal matrices. The additional degrees of freedom correspond to massive particles. We discuss a limit in parameter space where linear lattice gauge theory becomes equivalent to the standard formulation. We argue that the continuum limit of linear lattice gauge theory may be a useful setting for an analytic description of confinement. The running gauge coupling corresponds to the flow of the minimum of a "link potential". This minimum occurs for nonzero values $l_0$ in the perturbative regime, while $l_0$ vanishes in the confinement regime.

http://arxiv.org/abs/1307.0246
Noisy soccer balls
Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin
(Submitted on 30 Jun 2013)
In her Comment arXiv:1202.4066 [hep-th] Hossenfelder proposes a generalization of the results we reported in Phys. Rev. D84 (2011) 087702 and argues that thermal fluctuations introduce incurable pathologies for the description of macroscopic bodies in the relative-locality framework. We here show that Hossenfelder's analysis, while raising a very interesting point, is incomplete and leads to incorrect conclusions. Her estimate for the fluctuations did not take into account some contributions from the geometry of momentum space which must be included at the relevant order of approximation. Using the full expression here derived one finds that thermal fluctuations are not in general large for macroscopic bodies in the relative-locality framework. We find that such corrections can be unexpectedly large only for some choices of momentum-space geometry, and we comment on the possibility of developing a phenomenology suitable for possibly ruling out such geometries of momentum space.

11. Jul 4, 2013

### MTd2

Relative Locality in $κ$-Poincaré

http://arxiv.org/abs/1106.5710

Giulia Gubitosi, Flavio Mercati
(Submitted on 28 Jun 2011 (v1), last revised 3 Jul 2013 (this version, v2))
We show that the $\kappa$-Poincar\'e Hopf algebra can be interpreted in the framework of curved momentum space leading to the relativity of locality \cite{AFKS}. We study the geometric properties of the momentum space described by $\kappa$-Poincar\'e, and derive the consequences for particles propagation and energy-momentum conservation laws in interaction vertices, obtaining for the first time a coherent and fully workable model of the deformed relativistic kinematics implied by $\kappa$-Poincar\'e. We describe the action of boost transformations on multi-particles systems, showing that in order to keep covariant the composed momenta it is necessary to introduce a dependence of the rapidity parameter on the particles momenta themselves. Finally, we show that this particular form of the boost transformations keeps the validity of the relativity principle, demonstrating the invariance of the equations of motion under boost transformations.

12. Jul 4, 2013

### marcus

http://arxiv.org/abs/1307.1420
Approximation methods in Loop Quantum Cosmology: From Gowdy cosmologies to inhomogeneous models in Friedmann-Robertson-Walker geometries
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
(Submitted on 4 Jul 2013)
We develop approximation methods in the hybrid quantization of the Gowdy model with linear polarization and a massless scalar field, for the case of three-torus spatial topology. The loop quantization of the homogeneous gravitational sector of the Gowdy model (according to the improved dynamics prescription) and the presence of inhomogeneities lead to a very complicated Hamiltonian constraint. Therefore, the extraction of physical results calls for the introduction of well justified approximations. We first show how to approximate the homogeneous part of the Hamiltonian constraint, corresponding to Bianchi I geometries, as if it described a Friedmann-Robertson-Walker (FRW) model corrected with anisotropies. This approximation is valid in the high-energy sector of the FRW geometry (concerning its contribution to the constraint) and for anisotropy profiles that are sufficiently smooth. In addition, for certain families of states associated to regimes of physical interest, with negligible effects of the anisotropies and small inhomogeneities, one can approximate the Hamiltonian constraint of the inhomogeneous system by that of an FRW geometry with a relatively simple matter content, and then obtain its solutions.
20 pages, 3 figures

not QG, however possibly of general interest:
http://arxiv.org/abs/1307.1331
A new perspective on CP and T violation
Abhay Ashtekar
(Submitted on 4 Jul 2013)
It is shown that the results of CP and T violation experiments can be interpreted using a very general framework that does not require a Hilbert space of states or linear operators to represent the symmetries or dynamics. Analysis within this general framework brings out the aspects of quantum mechanics that are essential for this interpretation. More importantly, should quantum mechanics be eventually replaced by a new paradigm, this framework could still be used to establish violation of CP and T invariance from the already known experimental results.
This work arose as a 'formal response' to a talk "Three Merry Roads to T-Violation" by Dr. Bryan Roberts [1]. Both talks were given at a "Workshop on Cosmology and Time" held at Penn State in April 2013, which brought together physicists and philosophers of science.
10 pages; 'Formal Response' to the talk "Three Merry Roads to T-Violation" by Bryan Roberts at the Workshop on Cosmology and Time, held at the Institute for Gravitation and the Cosmos, Penn State, in April 2013

Last edited: Jul 4, 2013
13. Jul 7, 2013

### marcus

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski
(Submitted on 5 Jul 2013)
We study the asymptotic geometry of the spin foam partition function for a large class of models, including the models of Barrett and Crane, Engle, Pereira, Rovelli and Livine, and, Freidel and Krasnov.
The asymptotics is taken with respect to the boundary spins only, no assumption of large spins is made in the interior. We give a sufficient criterion for the existence of the partition function. We find that geometric boundary data is suppressed unless its interior continuation satisfies certain accidental curvature constraints. This means in particular that most Regge manifolds are suppressed in the asymptotic regime. We discuss this explicitly for the case of the configurations arising in the 3-3 Pachner move. We identify the origin of these accidental curvature constraints as an incorrect twisting of the face amplitude upon introduction of the Immirzi parameter and propose a way to resolve this problem, albeit at the price of losing the connection to the SU(2) boundary Hilbert space.
The key methodological innovation that enables these results is the introduction of the notion of wave front sets, and the adaptation of tools for their study from micro local analysis to the case of spin foam partition functions.
63 pages, 5 figures
Note: Frank Hellmann is giving a plenary talk at Loops 2013 later this month, quite possibly the topic will be holonomy spin foams, but i haven't yet seen the title of his talk listed.

Last edited: Jul 7, 2013
14. Jul 8, 2013

### marcus

http://arxiv.org/abs/1307.1848
High Energy Physics - Theory
Local Conformal Symmetry in Physics and Cosmology
Itzhak Bars, Paul Steinhardt, Neil Turok
(Submitted on 7 Jul 2013)
We review some of the arguments for why scale symmetry may be a fundamental principle in nature and, if so, why it is likely to be manifest as a local conformal symmetry including gravity. We show how to lift a generic non-scale invariant theory in Einstein frame into a Weyl-invariant theory and present the general form for Lagrangians consistent with Weyl symmetry. Various applications to cosmology are then discussed: the construction of classically geodesically complete cosmologies, the determination of initial conditions after the big bang, inflation, the metastability of the Higgs and cyclic cosmology. As examples, we focus on the standard model and Higgs cosmology, exploring the notion that the Higgs alone could be sufficient to explain the stages of cosmic evolution after (and perhaps before) the big bang and the large-scale features of the universe.
28 pages, 1 figure

brief mention:
http://arxiv.org/abs/1307.2060
Pseudo Weyl invariance is still anomalous
Enrique Alvarez, Mario Herrero-Valea
(Submitted on 8 Jul 2013)
A Weyl invariant extension of Einstein gravity is studied. It simply consists in the group averaging of Einstein's action under Weyl transformations. Contradicting cherished beliefs, a conformal anomaly is found in the trace of the equations of motion ...It is however possible to keep Weyl invariance as a bona fide symmetry at the price of losing full diffeomorphism invariance. This is what happens in unimodular gravity, a closely related theory.
19 pages

Last edited: Jul 8, 2013
15. Jul 9, 2013

### marcus

http://arxiv.org/abs/1307.2227
The two faces of Hawking radiation
Matteo Smerlak
(Submitted on 5 Jul 2013)
What happens when Alice falls into a black hole? In spite of recent challenges by Almheiri et al. -- the ""firewall" hypothesis -- the consensus on this question tends to remain "nothing special". Here I argue that something rather special can happen near the horizon, already at the semiclassical level: besides the standard Hawking outgoing modes, Alice can record a quasi-thermal spectrum of ingoing modes, whose temperature and intensity diverges as Alice's Killing energy E goes to zero. I suggest that this effect can be thought of in terms a "horizon-infinity duality", which relates the perception of near-horizon and asymptotic geodesic observers -- the two faces of Hawking radiation.
7 pages Honorable Mention in the Gravity Research Foundation 2013 Essay Competition

Last edited: Jul 9, 2013
16. Jul 10, 2013

### marcus

http://arxiv.org/abs/1307.2719
Deformations of Polyhedra and Polygons by the Unitary Group
Etera R. Livine
(Submitted on 10 Jul 2013)
We introduce the set of framed (convex) polyhedra with N faces as the symplectic quotient C2N //SU(2). A framed polyhedron is then parametrized by N spinors living in C2 satisfying suitable closure constraints and defines a usual convex polyhedron plus extra U(1) phases attached to each face. We show that there is a natural action of the unitary group U(N) on this phase space, which changes the shape of faces and allows to map any (framed) polyhedron onto any other with the same total (boundary) area. This identifies the space of framed polyhedra to the Grassmannian space U(N )/ (SU(2)×U(N −2)). We show how to write averages of geometrical observables (polynomials in the faces’ area and the angles between them) over the ensemble of polyhedra (distributed uniformly with respect to the Haar measure on U(N)) as polynomial integrals over the unitary group and we provide a few methods to compute these integrals systematically. We also use the Itzykson-Zuber formula from matrix models as the generating function for these averages and correlations.
In the quantum case, a canonical quantization of the framed polyhedron phase space leads to the Hilbert space of SU(2) intertwiners (or, in other words, SU(2)-invariant states in tensor products of irreducible representations). The total boundary area as well as the individual face areas are quantized as half-integers (spins), and the Hilbert spaces for fixed total area form irreducible representations of U(N). We define semi-classical coherent intertwiner states peaked on classical framed polyhedra and transforming consistently under U(N) transformations. And we show how the U(N) character formula for unitary transformations is to be considered as an extension of the Itzykson-Zuber to the quantum level and generates the traces of all polynomial observables over the Hilbert space of intertwiners.
We finally apply the same formalism to two dimensions and show that classical (convex) polygons can be described in a similar fashion trading the unitary group for the orthogonal group. We conclude with a discussion of the possible (deformation) dynamics that one can define on the space of polygons or polyhedra. This work is a priori useful in the context of discrete geometry but it should hopefully also be relevant to (loop) quantum gravity in 2+1 and 3+1 dimensions when the quantum geometry is defined in terms of gluing of (quantized) polygons and polyhedra.
33 pages

Last edited: Jul 10, 2013
17. Jul 11, 2013

### marcus

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto
(Submitted on 11 Jul 2013)
A simple argument indicates that covariant loop gravity (spinfoam theory) predicts a maximal acceleration, and hence forbids the development of curvature singularities. This supports the results obtained for cosmology and black holes using canonical methods.
4 pages, 1 figure

18. Jul 15, 2013

### MTd2

http://arxiv.org/abs/1102.1592

From Dimensional Reduction of 4d Spin Foam Model to Adding Non-Gravitational Fields to 3d Spin Foam Model
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 14 Jul 2013 (this version, v5))
A Kaluza-Klein like approach for a 4d spin foam model is considered. By applying this approach to a model based on group field theory in 4d (TOCY model), and using the Peter-Weyl expansion of the gravitational field, reconstruction of new non gravitational fields and interactions in the action are found. The perturbative expansion of the partition function produces graphs colored with su(2) algebraic data, from which one can reconstruct a 3d simplicial complex representing space-time and its geometry; (like in the Ponzano-Regge formulation of pure 3d quantum gravity), as well as the Feynman graph for typical matter fields. Thus a mechanism for generation of matter and construction of new dimensions are found from pure gravity.

19. Jul 17, 2013

### marcus

http://arxiv.org/abs/1307.4747
Asymptotic of Lorentzian Polyhedra Propagator
Jacek Puchta
(Submitted on 17 Jul 2013)
A certain operator $$T=\int_{SL(2,C)}dgY^{\dagger}gY$$ can be found in various Lorentzian EPRL calculations. The properties of this operator has been studied here in large j limit. The leading order of T is proportional to the identity operator.
Knowing the operator T one can renormalize spin-foam's edge self-energy by computing the amplitude of sum of a series of edges with increasing number of vertices and bubbles. This amplitude is calculated and is shown to be convergent.
Moreover some technical tools useful in Lorentzian Spin-Foam calculation have been developed.
28 pages, 1 figure

http://arxiv.org/abs/1307.4687
Comment on "Time and a Physical Hamiltonian for Quantum Gravity"
Jedrzej Świeżewski
(Submitted on 17 Jul 2013)
The aim of this comment is to present a simple argument showing that in the irrotational dust model the Hamilotnian constraint is indeed free of the square root if the time-gauge is chosen. This feature requires no additional assumptions, namely no choices of signs.
2 pages (comment on Husain Pawlowski http://arxiv.org/abs/1108.1145 )

brief mention:
http://arxiv.org/abs/1307.4706
Gauge/Gravity Duality and the Black Hole Interior
Donald Marolf, Joseph Polchinski
(Submitted on 17 Jul 2013)
We present a further argument that typical black holes with field theory duals have firewalls at the horizon...
... We also address the ER=EPR conjecture of Maldacena and Susskind, arguing that the correlations in generic highly entangled states cannot be geometrized as a smooth wormhole.
5 pages
[my comment: Ashtekar pointed out at GR20 that LQG BH eliminates singularity and necessity of "firewall"]

http://gr20-amaldi10.edu.pl/userfiles/book_07_07_2013.pdf [Broken]
Quantum space-times and unitarity of black hole evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.
page 216 of the book of abstracts of the July 2013 GR20 conference.

There is also a paper by Don Marolf presented at the same joint session

Ads/cft, unitary black hole evaporation, and firewalls
Marolf D
We review arguments that black hole evaporation is unitary in AdS/CFT. As a result, the physics expe- rienced by infalling observers at the horizon of at least sufficiently old black holes described by AdS/CFT must be dramatically different from that described by familiar field theory in a smooth spacetime.
page 217 of the GR20 abstracts.

Last edited by a moderator: May 6, 2017
20. Jul 17, 2013

### MTd2

http://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379

http://arxiv.org/abs/1303.6878

A Universe without expansion
C. Wetterich
(Submitted on 27 Mar 2013 (v1), last revised 5 Jul 2013 (this version, v2))
We discuss a cosmological model where the universe shrinks rather than expands during the radiation and matter dominated periods. Instead, the Planck mass and all particle masses grow exponentially. Together with a preceding inflationary phase and a late dark energy dominated epoch this model is compatible with all observations. The curvature is almost constant during all epochs. Cosmology has no big bang singularity. There exist other, equivalent choices of field variables for which the universe shows the usual expansion or is static during the radiation or matter dominated epochs. For those field coordinates the big bang is singular. Thus the big bang singularity turns out to be related to a singular choice of field coordinates.