Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1311.1798
Topological lattice field theories from intertwiner dynamics
Bianca Dittrich, Wojciech Kaminski
(Submitted on 7 Nov 2013)
We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.
35+9 pages

possibly of general interest:
http://arxiv.org/abs/1311.1608
On the Reality of Unruh Temperature
Manfred Requardt
(Submitted on 7 Nov 2013)
In contrast to recent criticism we undertake to show that the notion of Unruh temperature describes a real thermal property of the vacuum if viewed from an accelerated reference frame. We embed our investigation in a more general analysis of general relativistic temperature (Tolman-Ehrenfest effect) with the entropy-maximum principle being our guiding principle. We show that the Unruh effect neatly fits into this more general framework. Our criterion of reality is, first, the possibility to transfer a quantum of acceleration radiation to the inertial laboratory where it can be studied in principle under ordinary thrmodynamical conditions. Second, we emphasize as another criterion the importance of the coincidence of the accelerated and inertial observer description as far as the final objective result is concerned.
21 pages

 

[marcus]

http://arxiv.org/abs/1311.2530
Spectral dimension in causal set quantum gravity
Astrid Eichhorn, Sebastian Mizera
(Submitted on 11 Nov 2013)
We evaluate the spectral dimension in causal set quantum gravity by simulating random walks on causal sets. In contrast to other approaches to quantum gravity, we find an increasing spectral dimension at small scales. This observation can be connected to the non-locality of causal set theory that is deeply rooted in its fundamentally Lorentzian nature. Based on its large-scale behaviour, we conjecture that the spectral dimension can serve as a tool to distinguish causal sets that approximate manifolds from those that do not. As a new tool to probe quantum spacetime in different quantum gravity approaches, we introduce a novel dimensional estimator, the causal spectral dimension, based on the meeting probability of two random walkers, which respect the causal structure of the quantum spacetime. We discuss a causal-set example, where the spectral dimension and the causal spectral dimension differ, due to the existence of a preferred foliation.
16 pages, 11 figures

http://arxiv.org/abs/1311.2148
On the Axioms of Causal Set Theory
Benjamin F. Dribus
(Submitted on 9 Nov 2013)
This paper offers suggested improvements to the causal sets program in discrete gravity, which treats spacetime geometry as an emergent manifestation of causal structure at the fundamental scale. This viewpoint, which I refer to as the causal metric hypothesis, is summarized by Rafael Sorkin's phrase, "order plus number equals geometry." Proposed improvements include recognition of a generally nontransitive causal relation more fundamental than the causal order, an improved local picture of causal structure, development and use of relation space methods, and a new background-independent version of the histories approach to quantum theory. Besides causal set theory, à la Bombelli, Lee, Meyer, and Sorkin, this effort draws on Isham's topos-theoretic framework for physics, Sorkin's quantum measure theory, Finkelstein's causal nets, and Grothendieck's structural principles. This approach circumvents undesirable structural features in causal set theory, such as the permeability of maximal antichains, studied by Major, Rideout, and Surya, and the configuration space pathology arising from the asymptotic enumeration of Kleitman and Rothschild. The paper culminates in the theory of co-relative histories and kinematic schemes, combining the causal metric hypothesis, the histories approach to quantum theory, and Grothendieck's relative viewpoint. This leads to the derivation of causal Schrödinger-type equations as dynamical laws for discrete quantum spacetime.
147 pages, 32 figures

 

[marcus]

http://arxiv.org/abs/1311.2898
Matter matters in asymptotically safe quantum gravity
Pietro Donà, Astrid Eichhorn, Roberto Percacci
(Submitted on 12 Nov 2013)
We investigate the compatibility of minimally coupled scalar, fermion and gauge fields with asymptotically safe quantum gravity, using nonperturbative functional Renormalization Group methods. We study d=4,5 and 6 dimensions and within certain approximations find that for a given number of gauge fields there is a maximal number of scalar and fermion degrees of freedom compatible with an interacting fixed point at positive Newton coupling. The bounds impose severe constraints on grand unification with fundamental Higgs scalars. Supersymmetry and universal extra dimensions are also generally disfavored. The standard model and its extensions accommodating right-handed neutrinos, the axion and dark-matter models with a single scalar are compatible with a fixed point.
22 pages, 18 figures, 4 tables

general interest:
http://arxiv.org/abs/1311.2619
The New Quantum Logic
Robert B. Griffiths
(Submitted on 11 Nov 2013)
It is shown how all the major conceptual difficulties of standard (textbook) quantum mechanics, including the two measurement problems and the (supposed) nonlocality that conflicts with special relativity, are resolved in the consistent or decoherent histories interpretation of quantum mechanics by using a modified form of quantum logic to discuss quantum properties (subspaces of the quantum Hilbert space), and treating quantum time development as a stochastic process. The histories approach in turn gives rise to some conceptual difficulties, in particular the correct choice of a framework (probabilistic sample space) or family of histories, and these are discussed. The central issue is that the principle of unicity, the idea that there is a unique single true description of the world, is incompatible with our current understanding of quantum mechanics.
23 pages
[Robert Griffiths was the originator of the "consistent histories" way of understanding quantum mechanics. At his Carnegie Mellon webpage he terms it Consistent Quantum Theory. The approach has been taken up and developed in work by Hartle, Gell-Mann among others.]

 

[John86]

http://lanl.arxiv.org/abs/1311.3135
Planck-scale dimensional reduction without a preferred frame
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 13 Nov 2013)
Several approaches to quantum gravity suggest that the standard description of spacetime as probed at low-energy, with four dimensions, is replaced in the Planckian regime by a spacetime with a spectral dimension of two. The implications for relativistic symmetries can be momentous, and indeed the most tangible picture for "running" of the spectral dimension, found within Horava-Lifschitz gravity, requires the breakdown of relativity of inertial frames. In this Letter we incorporate running spectral dimensions in a scenario that does not require the emergence of a preferred frame. We consider the best studied mechanism for deforming relativistic symmetries whilst preserving the relativity of inertial frames, based on a momentum space with curvature at the Planck scale. We show explicitly how running of the spectral dimension can be derived from these models.

 

[marcus]

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
We define and investigate a quantisation of null hypersurfaces in the context of loop quantum gravity on a fixed graph. The main tool we use is the parametrisation of the theory in terms of twistors, which has already proved useful in discussing the interpretation of spin networks as the quantization of twisted geometries. The classical formalism can be extended in a natural way to null hypersurfaces, with the Euclidean polyhedra replaced by null polyhedra with space-like faces, and SU(2) by the little group ISO(2). The main difference is that the simplicity constraints present in the formalims are all first class, and the symplectic reduction selects only the helicity subgroup of the little group. As a consequence, information on the shapes of the polyhedra is lost, and the result is a much simpler, abelian geometric picture. It can be described by an Euclidean singular structure on the 2-dimensional space-like surface defined by a foliation of space-time by null hypersurfaces. This geometric structure is naturally decomposed into a conformal metric and scale factors, forming locally conjugate pairs. Proper action-angle variables on the gauge-invariant phase space are described by the eigenvectors of the Laplacian of the dual graph. We also identify the variables of the phase space amenable to characterize the extrinsic geometry of the foliation. Finally, we quantise the phase space and its algebra using Dirac's algorithm, obtaining a notion of spin networks for null hypersurfaces. Such spin networks are labelled by SO(2) quantum numbers, and are embedded non-trivially in the unitary, infinite-dimensional irreducible representations of the Lorentz group.
22 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1311.3340
Spectral dimension of quantum geometries
Gianluca Calcagni, Daniele Oriti, Johannes Thürigen
(Submitted on 13 Nov 2013)
The spectral dimension is an indicator of geometry and topology of spacetime and a tool to compare the description of quantum geometry in various approaches to quantum gravity. This is possible because it can be defined not only on smooth geometries but also on discrete (e.g., simplicial) ones. In this paper, we consider the spectral dimension of quantum states of spatial geometry defined on combinatorial complexes endowed with additional algebraic data: the kinematical quantum states of loop quantum gravity (LQG). Preliminarily, the effects of topology and discreteness of classical discrete geometries are studied in a systematic manner. We look for states reproducing the spectral dimension of a classical space in the appropriate regime. We also test the hypothesis that in LQG, as in other approaches, there is a scale dependence of the spectral dimension, which runs from the topological dimension at large scales to a smaller one at short distances. While our results do not give any strong support to this hypothesis, we can however pinpoint when the topological dimension is reproduced by LQG quantum states. Overall, by exploring the interplay of combinatorial, topological and geometrical effects, and by considering various kinds of quantum states such as coherent states and their superpositions, we find that the spectral dimension of discrete quantum geometries is more sensitive to the underlying combinatorial structures than to the details of the additional data associated with them.
38 pages, 18 figures

 

[John86]

http://arxiv.org/abs/1311.3912
A covariant causal set approach to discrete quantum gravity
Stan Gudder
(Submitted on 15 Nov 2013)
A covariant causal set (c-causet) is a causal set that is invariant under labeling. Such causets are well-behaved and have a rigid geometry that is determined by a sequence of positive integers called the shell sequence. We first consider the microscopic picture. In this picture, the vertices of a c-causet have integer labels that are unique up to a label isomorphism. This labeling enables us to define a natural metric d(a,b) between time-like separated vertices a and b. The time metric d(a,b) results in a natural definition of a geodesic from a to b. It turns out that there can be n≥1 such geodesics. Letting a be the origin (the big bang), we define the curvature K(b) of b to be n−1. Assuming that particles tend to move along geodesics, K(b) gives the tendency that vertex b is occupied. In this way, the mass distribution is determined by the geometry of the c-causet. We next consider the macroscopic picture which describes the growth process of c-causets. We propose that this process is governed by a quantum dynamics given by complex amplitudes. At present, these amplitudes are unknown. But if they can be found, they will determine the (approximate) geometry of the c-causet describing our particular universe. As an illustration, we present a simple example of an amplitude process that may have physical relevance. We also give a discrete analogue of Einstein's field equations.

http://arxiv.org/abs/1311.3787
Cosmic structure, averaging and dark energy
David L. Wiltshire
(Submitted on 15 Nov 2013)
These lecture notes review the theoretical problems associated with coarse-graining the observed inhomogeneous structure of the universe at late epochs, of describing average cosmic evolution in the presence of growing inhomogeneity, and of relating average quantities to physical observables. In particular, a detailed discussion of the timescape scenario is presented. In this scenario, dark energy is realized as a misidentification of gravitational energy gradients which result from gradients in the kinetic energy of expansion of space, in the presence of density and spatial curvature gradients that grow large with the growth of structure. The phenomenology and observational tests of the timescape model are discussed in detail, with updated constraints from Planck satellite data. In addition, recent results on the variation of the Hubble expansion on < 100/h Mpc scales are discussed. The spherically averaged Hubble law is significantly more uniform in the rest frame of the Local Group of galaxies than in the conventional rest frame assumed for the Cosmic Microwave Background. This unexpected result supports a fundamental revision of the notion of the cosmic rest frame, consistent with the expectations of the timescape scenario.

 

[marcus]

http://arxiv.org/abs/1311.4481
Cosmological signature change in Cartan Gravity with dynamical symmetry breaking
Joao Magueijo, Matias Rodriguez-Vazquez, Hans Westman, T.G. Zlosnik
(Submitted on 18 Nov 2013)
We investigate the possibility for classical metric signature change in a straightforward generalization of the first order formulation of gravity, dubbed "Cartan gravity". The mathematical structure of this theory mimics the electroweak theory in that the basic ingredients are an SO(1,4) Yang-Mills gauge field A^ab_μ and a symmetry breaking Higgs field V^a, with no metric or affine structure of spacetime presupposed. However, these structures can be recovered, with the predictions of General Relativity exactly reproduced, whenever the Higgs field breaking the symmetry to SO(1,3) is forced to have a constant (positive) norm V^aV_a. This restriction is usually imposed "by hand", but in analogy with the electroweak theory we promote the gravitational Higgs field Va to a genuine dynamical field, subject to non-trivial equations of motion. Even though we limit ourselves to actions polynomial in these variables, we discover a rich phenomenology. Most notably we derive classical cosmological solutions exhibiting a smooth transition between Euclidean and Lorentzian signature in the four-metric. These solutions are non-singular and arise whenever the SO(1,4) norm of the Higgs field changes sign i.e. the signature of the metric of spacetime is determined dynamically by the gravitational Higgs field. It is possible to find a plethora of such solutions and in some of them this dramatic behaviour is confined to the early universe, with the theory asymptotically tending to Einstein gravity at late times. Curiously the theory can also naturally embody a well-known dark energy model: Peebles-Ratra quintessence.
21 pages, 5 figures

 

[MTd2]

Power Point Ppaer!

It cites Garrett Lisi!

http://arxiv.org/abs/1311.4413

Gravi-Weak Unification and Multiple Point Principle

C.D. Froggatt, C.R. Das, L.V. Laperashvili, H.B. Nielsen, A. Tureanu
(Submitted on 18 Nov 2013)
We construct a model unifying gravity with weak SU(2) gauge and "Higgs" scalar fields. We assume the existence of a visible and an invisible (hidden) sector of the Universe. We used the extension of Plebanski's 4-dimensional gravitational theory, in which the fundamental fields are two-forms containing tetrads, spin connections and additional auxiliary fields. Considering a Spin(4,4) invariant extended Plebanski action, we recover the actions in both (visible and invisible) sectors of the Universe. After symmetry breaking of the graviweak (GW) unification, its physical constants (Newton's constants, cosmological constants, YM-couplings, etc.), are determined by a parameter guni of the GW unification. It is discussed that if this "Higgs" field coming in the GW unification could be the Higgs of the Standard Model, then the idea that its vacuum value could be, according to the Multiple Point Principle, a second minimum of the Higgs field effective potential, turns out not to be viable. Then other scalar "Higgs" field, giving the inflation and axion fields, has a Planck scale expectation value, and could have a better chance of being the scalar field unified with gravity.

 

[MTd2]

http://arxiv.org/abs/1311.4712

Euclidean Dynamical Triangulation revisited: is the phase transition really first order?

Tobias Rindlisbacher, Philippe de Forcrand
(Submitted on 19 Nov 2013)
The transition between the two phases of 4D Euclidean Dynamical Triangulation [1] was long believed to be of second order until in 1996 first order behavior was found for sufficiently large systems [3,4]. However, one may wonder if this finding was affected by the numerical methods used: to control volume fluctuations, in both studies [3,4] an artificial harmonic potential was added to the action; in [4] measurements were taken after a fixed number of accepted instead of attempted moves which introduces an additional error. Finally the simulations suffer from strong critical slowing down which may have been underestimated.
In the present work, we address the above weaknesses: we allow the volume to fluctuate freely within a fixed interval; we take measurements after a fixed number of attempted moves; and we overcome critical slowing down by using an optimized parallel tempering algorithm [6]. With these improved methods, on systems of size up to 64k 4-simplices, we confirm that the phase transition is first order.

 

[marcus]

http://arxiv.org/abs/1311.4979
Cosmology: theory
Mikhail Shaposhnikov
(Submitted on 20 Nov 2013)
The discovery of 126 GeV Higgs boson and observations of no signs of new physics at the LHC implies that the Standard Model of elementary particles is a self-consistent weakly-coupled effective field theory all the way up to the Planck scale without the addition of any new particles. I will discuss possible consequences of these findings for cosmology.
11 pages, 5 figures. Plenary talk delivered at the European Physical Society Conference on High Energy Physics, 18-24 July, 2013, Stockholm, Sweden

 

[marcus]

http://arxiv.org/abs/1311.5325
Note on the super inflation in loop quantum cosmology
Kui Xiao, Xiao-Kai He, Jian-Yang Zhu
(Submitted on 21 Nov 2013)
Phenomenological effect of the super-inflation in loop quantum cosmology (LQC) is discussed. We investigate the case that the Universe is filled with the interacting field between massive scalar field and radiation. Considering the damping coefficient Γ as a constant, the changes of the scale factor during super-inflation with four different initial conditions are discussed, and we find that the changes of the scale factor depend on the initial values of energy density of the scalar field and radiation at the bounce point. But no matter which initial condition is chosen, the radiation always dominated at the late time. Moreover, we investigate whether the super-inflation can provide enough e-folding number. For the super-inflation starts from the quantum bounce point, the initial value of Hubble parameter H(t_i)∼0, then it is possible to solve the flatness problem and horizon problem. As an example, following the method of Amoros to calculate particle horizon on the condition that the radiation dominated at bounce point, we find that the Universe has had enough time to be homogeneous and isotopic.
9 pages, 4 figures; published Physics Letters B (2013)

 

[marcus]

http://inspirehep.net/record/1266031?ln=en
http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
The discrete spatial geometry underlying Loop Quantum Gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalised the LQG representation so as to describe states labelled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods. Motivated by the desire to model asymptotically flat quantum geometry by states with triad labels which are non- degenerate at infinity but not necessarily so in the interior, we initiate a generalisation of Sahlmann's considerations to triads of varying degeneracy. In doing so, we include delicate phase contributions to the averaging procedure which are crucial for the correct implementation of the gauge and diffeomorphism constraints, and whose existence can be traced to the background exponential functions recently constructed by one of us. Our treatment emphasizes the role of symmetries of quantum states in the averaging procedure. Semianalyticity, influential in the proofs of the beautiful uniqueness results for LQG, plays a key role in our considerations. As a by product, we re-derive the group averaging map for standard LQG, highlighting the role of state symmetries and explicitly exhibiting the essential uniqueness of its specification.
45 pages.

[comment:see also
TK http://arxiv.org/0709.3465
HS http://arxiv.org/1006.0388
MV http://arxiv.org/1306.6126
MC and MV in progress, refs 24 and 25 on pages 35 and 36.]

 

[marcus]

http://arxiv.org/abs/1311.6841
Observables in Loop Quantum Gravity with a cosmological constant
Maïté Dupuis, Florian Girelli
(Submitted on 26 Nov 2013)
An open issue in loop quantum gravity (LQG) is the introduction of a non-vanishing cosmological constant Λ. In 3d, Chern-Simons theory provides some guiding lines: Λ appears in the quantum deformation of the gauge group. The Turaev-Viro model, which is an example of spin foam model is also defined in terms of a quantum group. By extension, it is believed that in 4d, a quantum group structure could encode the presence of Λ≠0. In this article, we introduce by hand the quantum group U_q(su(2)) into the LQG framework, that is we deal with U_q(su(2))-spin networks. We explore some of the consequences, focusing in particular on the structure of the observables. Our fundamental tools are tensor operators for U_q(su(2)). We review their properties and give an explicit realization of the spinorial and vectorial ones. We construct the generalization of the U(n) formalism in this deformed case, which is given by the quantum group U_q(u(n)). We are then able to build geometrical observables, such as the length, area or angle operators ... We show that these operators characterize a quantum discrete hyperbolic geometry in the 3d LQG case. Our results confirm that the use of quantum group in LQG can be a tool to introduce a non-zero cosmological constant into the theory.
29 pages, 2 figures

http://arxiv.org/abs/1311.6942
A note on the spinor construction of Spin Foam amplitudes
Giorgio Immirzi
(Submitted on 27 Nov 2013)
I discuss the use of spinors in the construction of spin-foam models, in particular the form of the closure and simplicity constraints for triangles that are space-like,
i.e. with (area)² = 1/2 S^IJ S_IJ > 0,
regardless of whether they belong the tetrahedra with a space-like or time-like normal, emphasizing the role of the light-like 4-vector u_tσ^I u ̄_t. In the quantization of the model, with the representations of SL(2,C) acting on spaces of functions of light-like vectors, one may use the canonical basis of SU(2) representations, or the pseudobasis limited to the discrete representations of SU(1,1); in alternative it is proposed to use instead a basis of eigenstates of (L₃,K₃), which might give matrix elements and vertex functions with the same classical limit. A detailed example of a small triangulation is presented, which among other things indicates, on the basis of a classical calculation, that it would be impractical to limit oneself to tetrahedra with time-like normals.
20 pages, 1 figure.

 

[marcus]

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 29 Nov 2013)
We argue that refining, coarse graining and entangling operators can be obtained from time evolution operators. This applies in particular to geometric theories, such as spin foams. We point out that this provides a construction principle for the physical vacuum in quantum gravity theories and more generally allows to construct a (cylindrically) consistent continuum limit of the theory.
33 pages, 9 figures

http://arxiv.org/abs/1311.7498
Noncommutative Space-time from Quantized Twistors
Jerzy Lukierski, Mariusz Woronowicz
(Submitted on 29 Nov 2013)
We consider the relativistic phase space coordinates (x_μ,p_μ) as composite, described by functions of the primary pair of twistor coordinates. It appears that if twistor coordinates are canonicaly quantized the composite space-time coordinates are becoming noncommutative. We obtain deformed Heisenberg algebra which in order to be closed should be enlarged by the Pauli-Lubanski four-vector components. We further comment on star-product quantization of derived algebraic structures which permit to introduce spin-extended deformed Heisenberg algebra.
Comments: 7 pages; talk given at the Conference in Honour of 90-th Birthday of Freeman Dyson at Nanyang Technical University, Singapore,26-29 August 2013; to be published in Int.Journ.Mod.Phys.A

 

[John86]

http://arxiv.org/abs/1311.7619
Casimir forces on atoms in optical cavities
Alvaro M. Alhambra, Achim Kempf, Eduardo Martin-Martinez
(Submitted on 29 Nov 2013)
Casimir-type forces, such as those between two neutral conducting plates, or a sphere, atom or molecule and a plate have been widely studied and are becoming of increasing significance, for example, in nanotechnology. A key challenge is to better understand, from a fundamental microscopic approach, why the Casimir force is in some circumstances attractive and in others repulsive. Here, we study the Casimir-Polder forces experienced by small quantum systems such as atoms or molecules in an optical cavity. In a 1+1 dimensional setting, we model the small quantum systems as 2-level 'atoms', i.e., as qubits, and we model the electromagnetic field components as scalar fields with Dirichlet or Neumann boundary conditions. In this setting we determine the conditions for the Casimir force being attractive or repulsive for individual atoms. We also study the microscopic-macroscopic transition, finding that as the number of atoms in the cavity is increased, the atoms start to affect the Casimir force exerted on the cavity walls similarly to a dielectric medium.

http://arxiv.org/abs/1311.7146
Spacetime Entanglement Entropy in 1+1 Dimensions
Mehdi Saravani, Rafael D. Sorkin, Yasaman K. Yazdi
(Submitted on 27 Nov 2013)
arXiv:1205.2953 defines an entropy for a gaussian scalar field ϕ in an arbitrary region of either a causal set or a continuous spacetime, given only the correlator ⟨ϕ(x)ϕ(y)⟩ within the region. As a first application, we compute numerically the entanglement entropy in two cases where the asymptotic form is known or suspected from conformal field theory, finding excellent agreement when the required ultraviolet cutoff is implemented as a truncation on spacetime mode-sums. We also show how the symmetry of entanglement entropy reflects the fact that RS and SR share the same eigenvalues, R and S being arbitrary matrices.

 

[John86]

Not related but interesting.

http://arxiv.org/abs/1312.0580
Quantization of Emergent Gravity
Hyun Seok Yang
(Submitted on 2 Dec 2013)
Emergent gravity is based on a novel form of the equivalence principle known as the Darboux theorem or the Moser lemma in symplectic geometry stating that the electromagnetic force can always be eliminated by a local coordinate transformation as far as spacetime admits a symplectic structure, in other words, a microscopic spacetime becomes noncommutative (NC). If gravity emerges from U(1) gauge theory on NC spacetime, this picture of emergent gravity suggests a completely new quantization scheme where quantum gravity is defined by quantizing spacetime itself, leading to a dynamical NC spacetime. Therefore the quantization of emergent gravity is radically different from the conventional approach trying to quantize a phase space of metric fields. This approach for quantum gravity allows a background independent formulation where spacetime as well as matter fields is equally emergent from a universal vacuum of quantum gravity.

http://arxiv.org/abs/1312.0405
Ray tracing Einstein-Æther black holes: Universal versus Killing horizons
Bethan Cropp, Stefano Liberati, Arif Mohd, Matt Visser
(Submitted on 2 Dec 2013)
Violating Lorentz-invariance, and so implicitly permitting some form of superluminal communication, necessarily alters the notion of a black hole. Nevertheless, in both Einstein-{\AE}ther gravity, and Ho\v{r}ava-Lifshitz gravity, there is still a causally disconnected region in black-hole solutions; now being bounded by a "Universal horizon", which traps excitations of arbitrarily high velocities. To better understand the nature of these black holes, and their Universal horizons, we study ray trajectories in these spacetimes. We find evidence that Hawking radiation is associated with the Universal horizon, while the "lingering" of ray trajectories near the Killing horizon hints at reprocessing there. In doing this we solve an apparent discrepancy between the surface gravity of the Universal horizon and the associated temperature derived by tunneling method. These results advance the understanding of these exotic horizons, and provide hints for a full understanding of black-hole thermodynamics in Lorentz-violating theories.

 

[marcus]

http://arxiv.org/abs/arXiv:1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus
(Submitted on 3 Dec 2013)
So far spin foam models are hardly understood beyond a few of their basic building blocks. To make progress on this question, we define analogue spin foam models, so called spin nets, for quantum groups SU(2)_k and examine their effective continuum dynamics via tensor network renormalization. In the refinement limit of this coarse graining procedure, we find a vast non-trivial fixed point structure beyond the degenerate and the BF phase. In comparison to previous work, we use fixed point intertwiners, inspired by Reisenberger's construction principle [1] and the recent work [2], as the initial parametrization. In this new parametrization fine tuning is not required in order to flow to these new fixed points. Encouragingly, each fixed point has an associated extended phase, which allows for the study of phase transitions in the future. Finally we also present an interpretation of spin nets in terms of melonic spin foams. The coarse graining flow of spin nets can thus be interpreted as describing the effective coupling between two spin foam vertices or space time atoms.
30+5 pages, many figures

general interest/prominent authors:
http://arxiv.org/abs/1312.0613
The Habitable Epoch of the Early Universe
Abraham Loeb (Harvard)
(Submitted on 2 Dec 2013)
In the redshift range 100<(1+z)<110, the cosmic microwave background (CMB) had a temperature of 273-300K (0-30 degrees Celsius), allowing early rocky planets (if any existed) to have liquid water chemistry on their surface and be habitable, irrespective of their distance from a star. In the standard LCDM cosmology, the first star-forming halos within our Hubble volume started collapsing at these redshifts, allowing the chemistry of life to possibly begin when the Universe was merely 15 million years old. The possibility of life starting when the average matter density was a million times bigger than it is today argues against the anthropic explanation for the low value of the cosmological constant.
6 pages, submitted to Astrobiology

http://arxiv.org/abs/1312.0612
Holographic thermal field theory on curved spacetimes
Donald Marolf, Mukund Rangamani, Toby Wiseman
(Submitted on 2 Dec 2013)
The AdS/CFT correspondence relates certain strongly coupled CFTs with large effective central charge ceff to semi-classical gravitational theories with AdS asymptotics. We describe recent progress in understanding gravity duals for CFTs on non-trivial spacetimes at finite temperature, both in and out of equilibrium. Such gravity methods provide powerful new tools to access the physics of these strongly coupled theories, which often differs qualitatively from that found at weak coupling.
Our discussion begins with basic aspects of AdS/CFT and progresses through thermal CFTs on the Einstein Static Universe and on periodically identified Minkowski spacetime. In the latter context we focus on states describing so-called plasma balls, which become stable at large ceff. We then proceed to out-of-equilibrium situations associated with dynamical bulk black holes. In particular, the non-compact nature of these bulk black holes allows stationary solutions with non-Killing horizons that describe time-independent flows of CFT plasma. As final a topic we consider CFTs on black hole spacetimes. This discussion provides insight into how the CFT transports heat between general heat sources and sinks of finite size. In certain phases the coupling to small sources can be strongly suppressed, resulting in negligible heat transport despite the presence of a deconfined plasma with sizeable thermal conductivity. We also present a new result, explaining how this so-called droplet behaviour is related to confinement via a change of conformal frame.
77 pages. 9 figures

http://arxiv.org/abs/1312.0739
Sailing through the big crunch-big bang transition
Itzhak Bars, Paul Steinhardt, Neil Turok
(Submitted on 3 Dec 2013)
In a recent series of papers, we have shown that theories with scalar fields coupled to gravity (e.g., the standard model) can be lifted to a Weyl-invariant equivalent theory in which it is possible to unambiguously trace the classical cosmological evolution through the transition from big crunch to big bang. The key was identifying a sufficient number of finite, Weyl-invariant conserved quantities to uniquely match the fundamental cosmological degrees of freedom across the transition. In so doing we had to account for the well-known fact that many Weyl-invariant quantities diverge at the crunch and bang. Recently, some authors rediscovered a few of these divergences and concluded based on their existence alone that the theories cannot be geodesically complete. In this note, we show that this conclusion is invalid. Using conserved quantities we explicitly construct the complete set of geodesics and show that they pass continuously through the big crunch-big bang transition.
3 pages, 1 figure

 

[John86]

Nice article at Sabine Hossenfelder's "Backreaction" blog it's about the CDT program.

http://backreaction.blogspot.nl/

 

[John86]

http://arxiv.org/abs/1312.1229
Hamiltonian formalism for integer-valued variables and integer time steps and a possible application in quantum physics
Gerard 't Hooft
(Submitted on 4 Dec 2013)
Most classical mechanical systems are based on dynamical variables whose values are real numbers. Energy conservation is then guaranteed if the dynamical equations are phrased in terms of a Hamiltonian function, which then leads to differential equations in the time variable. If these real dynamical variables are instead replaced by integers, and also the time variable is restricted to integers, it appears to be hard to enforce energy conservation unless one can also derive a Hamiltonian formalism for that case. We here show how the Hamiltonian formalism works here, and how it may yield the usual Hamilton equations in the continuum limit. The question was motivated by the author's investigations of special quantum systems that allow for a deterministic interpretation. The 'discrete Hamiltonian formalism' appears to shed new light on these approaches.

 

[marcus]

http://arxiv.org/abs/1312.1538
Gravitational Energy, Local Holography and Non-equilibrium Thermodynamics
Laurent Freidel
(Submitted on 5 Dec 2013)
We study the properties of gravitational system in finite regions bounded by gravitational screens. We present the detail construction of the total energy of such regions and of the energy and momentum balance equations due to the flow of matter and gravitational radiation through the screen. We establish that the gravitational screen possesses analogs of surface tension, internal energy and viscous stress tensor, while the conservations are analogs of non-equilibrium balance equations for a viscous system. This gives a precise correspondence between gravity in finite regions and non-equilibrium thermodynamics.
41 pages, 3 figures

brief mention---possible interest:
I keep seeing more papers about so called "f(T)" gravity aka "teleparallel". The torsion T plays an important role. My hunch is that I should start including at least some of the f(T) teleparallel research in our bibliography.
http://arxiv.org/abs/1303.3144
On the Gravitational Energy-Momentum Vector in f(T) Theories
S. C. Ulhoa, E. P. Spaniol
(Submitted on 13 Mar 2013 (v1), last revised 4 Dec 2013)
This work is devoted to analyze the energy of the Universe in the context of f(T) theories. Such theories are the analogous counterpart of the well known f(R) theories that, however, uses torsion instead of curvature. We obtain a general expression for the gravitational energy-momentum vector in this framework. Using the hypothesis of the isotropy of spacetime, we find the energy of the Universe and compare it with the energy obtained in the realm of teleparallelism equivalent to general relativity (TEGR).
9 pages
http://arxiv.org/abs/1312.1367
On Gravitational Entropy of de Sitter Universe
S. C. Ulhoa, E. P. Spaniol
(Submitted on 4 Dec 2013)
The paper deals with the calculation of the gravitational entropy in the context of teleparallel gravity for de Sitter space-time. In such a theory it is possible to define gravitational energy and pressure, thus we use those expressions to construct the gravitational entropy. We interpret the cosmological constant as the temperature and write the first law of thermodynamics. In the limit Λ≪1 we find that the entropy is proportional to volume and ΔS≥0.
11 pages

 

[marcus]

http://arxiv.org/abs/1312.1696
BTZ Black Hole Entropy and the Turaev-Viro model
Marc Geiller, Karim Noui
(Submitted on 5 Dec 2013)
We show the explicit agreement between the derivation of the Bekenstein-Hawking entropy of a Euclidean BTZ black hole from the point of view of spin foam models and canonical quantization. This is done by considering a graph observable (corresponding to the black hole horizon) in the Turaev-Viro state sum model, and then analytically continuing the resulting partition function to negative values of the cosmological constant.
22+4 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1312.2385
Magnetic Bianchi type II string cosmological model in loop quantum cosmology
Victor Rikhvitsky, Bijan Saha, Mihai Visinescu
(Submitted on 9 Dec 2013)
The loop quantum cosmology of the Bianchi type II string cosmological model in the presence of a homogeneous magnetic field is studied. We present the effective equations which provide modifications to the classical equations of motion due to quantum effects. The numerical simulations confirm that the big bang singularity is resolved by quantum gravity effects.
11 pages, 9 figures

==quote from introduction==
More recently, it has been shown that the big bang singularity is also resolved for anisotropic
Bianchi type I (BI) [1], II (BII) [2] and IX [3]. Usually the matter source that was considered is
a massless scalar field that plays the role of internal time. The investigations have been extended
to more complicated models including a perfect fluid, magnetic fields [4], cosmological strings
[5]. It is remarkable the fact that in all these studies it was observed that the bounce prediction is
robust.
The purpose of this paper is to investigate the dynamics of a BII string cosmological model
in the presence of a magnetic field in the framework of LQC. We show that a bounce occurs in a collapsing magnetized BII string cosmological model, thus extending the known cases of singularity resolution.
==endquote==
At first the LQC bounce was shown in the simple homogeneous isotropic case with a massless scalar field serving as matter. To test the robustness of this prediction, researchers have been examining different anisotropic versions (e.g. Bianchi types) with different sorts of matter. The authors of this paper had an earlier article which I accidentally overlooked when it came out. It was published in Astrophysics and Space Science 339, 371-377 (2012) and investigated the LQC bounce (with magnetized cosmic string as matter) in the Bianchi type I case.

http://inspirehep.net/record/945923?ln=en
Bianchi type-I string cosmological model in the presence of a magnetic field: classical versus loop quantum cosmology approaches
Victor Rikhvitsky, Bijan Saha, Mihai Visinescu
(Submitted on 15 Nov 2011)
A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature. Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw a parallel between the classical and quantum approaches.
14 pages, 8 figures

 

[MTd2]

http://arxiv.org/abs/1312.2831

A gauge theoretic approach to Einstein 4-manifolds

Joel Fine, Kirill Krasnov, Dmitri Panov
(Submitted on 10 Dec 2013)
This article investigates a new gauge theoretic approach to Einstein's equations in dimension 4. Whilst aspects of the formalism are already explained in various places in the mathematics and physics literature, our first goal is to give a single coherent account of the theory in purely mathematical language. We then explain why the new approach may have important mathematical applications: the possibility of using the calculus of variations to find Einstein 4-manifolds, as well as links to symplectic topology. We also carry out some of the technical groundwork to attack these problems.

 

[marcus]

http://arxiv.org/abs/1312.3253
General Relativity from a Thermodynamic Perspective
T. Padmanabhan
(Submitted on 11 Dec 2013)
Several recent results suggest that gravity is an emergent phenomenon with its field equations having the same status as, say, the equations of fluid dynamics. I describe several additional results, supporting this paradigm and connecting the gravitational dynamics in a bulk region of space with a thermodynamic description in the boundary of that region:

(1) The Noether charge contained in a bulk region, associated with a specific time evolution vector field, has a direct thermodynamic interpretation as the gravitational heat content of the boundary surface.

(2) This result, in turn, shows that all static spacetimes maintain holographic equipartition; in these spacetimes, the number of degrees of freedom in the boundary is equal to the number of degrees of freedom in the bulk.

(3) In a general, evolving spacetime, the rate of change of gravitational momentum is related to the difference between the number of bulk and boundary degrees of freedom. It is this departure from the holographic equipartition which drives the time evolution of the spacetime.

(4) When the equations of motion hold, the (naturally defined) total energy of the gravity plus matter within a bulk region, will be equal to the boundary heat content.

(5) After motivating the need for an alternate description of gravity (if we have to solve the cosmological constant problem), I describe a thermodynamic variational principle based on null surfaces to achieve this goal. The concept of gravitational heat density of the null surfaces arises naturally from the Noether charge associated with the null congruence. The null surface variational principle, in fact, extremises the total heat content of the matter plus gravity system. Several variations on this theme and implications are described. [Abridged]
53 pages

The more complete, detailed, and elaborate form of the abstract is on page 1 of the PDF:
http://arxiv.org/pdf/1312.3253v1.pdf
What appears here is an abridged form. It could be advisable to read BOTH versions of the abstract.

http://arxiv.org/abs/1312.3220
Multisymplectic effective General Boundary Field Theory
Mona Arjang, José A. Zapata
(Submitted on 11 Dec 2013)
The transfer matrix in lattice field theory connects the covariant and the initial data frameworks; in spin foam models, it can be written as a composition of elementary cellular amplitudes/propagators. We present a framework for discrete spacetime classical field theory in which solutions to the field equations over elementary spacetime cells may be amalgamated if they satisfy simple gluing conditions matching the composition rules of cellular amplitudes in spin foam models. Furthermore, the formalism is endowed with a multisymplectic structure responsible for local conservation laws.
Some models within our framework are effective theories modeling a system at a given scale. Our framework allows us to study coarse graining and the continuum limit.

http://arxiv.org/abs/1312.3313
Planck Data Reconsidered
David Spergel, Raphael Flauger, Renee Hlozek
(Submitted on 11 Dec 2013)
The tension between the best fit parameters derived by the Planck team and a number of other astronomical measurements suggests either systematics in the astronomical measurements, systematics in the Planck data, the need for new physics, or a combination thereof. We re-analyze the Planck data and find that the 217GHz×217GHz detector set spectrum used in the Planck analysis is responsible for some of this tension. We use a map-based foreground cleaning procedure, relying on a combination of 353 GHz and 545 GHz maps to reduce residual foregrounds in the intermediate frequency maps used for cosmological inference. For our baseline data analysis, which uses 47% of the sky and makes use of both 353 and 545 GHz data for foreground cleaning, we find the ΛCDM cosmological parameters Ω_ch²=0.1169±0.0025, n_s=0.9671±0.0069, H₀=68.0±1.1kms⁻¹Mpc⁻¹, Ω_bh²=0.02197±0.00027, ln10¹⁰A_s=3.080±0.025, and τ=0.089±0.013. While in broad agreement with the results reported by the Planck team, these revised parameters imply a universe with a lower matter density of Ω_m=0.302±0.015, and parameter values generally more consistent with pre-Planck CMB analyses and astronomical observations. We compare our cleaning procedure with the foreground modeling used by the Planck team and find good agreement. The difference in parameters between our analysis and that of the Planck team is mostly due to our use of cross-spectra from the publicly available survey maps instead of their use of the detector set cross-spectra which include pixels only observed in one of the surveys. We show evidence suggesting residual systematics in the detector set spectra used in the Planck likelihood code, which is substantially reduced for our spectra.
Comments: 15 pages

 

[marcus]

http://arxiv.org/abs/1312.3595
Hawking radiation from a spherical loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 12 Dec 2013)
We introduce quantum field theory on quantum space-times techniques to characterize the quantum vacua as a first step towards studying black hole evaporation in spherical symmetry in loop quantum gravity and compute the Hawking radiation. We use as quantum space time the recently introduced exact solution of the quantum Einstein equations in vacuum with spherical symmetry and consider a spherically symmetric test scalar field propagating on it. The use of loop quantum gravity techniques in the background space-time naturally regularizes the matter content, solving one of the main obstacles to back reaction calculations in more traditional treatments. The discreteness of area leads to modifications of the quantum vacua, eliminating the trans-Planckian modes close to the horizon, which in turn eliminates all singularities from physical quantities, like the expectation value of the stress energy tensor. Apart from this, the Boulware, Hartle--Hawking and Unruh vacua differ little from the treatment on a classical space-time. The asymptotic modes near scri are reproduced very well. We show that the Hawking radiation can be computed, leading to an expression similar to the conventional one but with a high frequency cutoff. Since many of the conclusions concern asymptotic behavior, where the spherical mode of the field behaves in a similar way as higher multipole modes do, the results can be readily generalized to non spherically symmetric fields.
13 pages
==quote Conclusions==
We have studied the quantization of a scalar field on a quantum space time that approximates well the geometry of a Schwarzschild black hole. The treatment reproduces the results of quantum field theory on a classical space-time well, with some interesting differences. The presence of a discrete structure for the space-time eliminates the divergences associated with the Boulware and Unruh vacua arising from the trans-Planckian modes and only slight modifications for the Hartle–Hawking vacuum. All the different vacua’s modes change considerably on the horizons where all the singularities present in the usual analysis disappear.
We have carried out the analysis for a given spin network, but it is valid and can be extended without significant changes (except the one we will mention next) for generic refinements of the given spin network that include more vertices such that k_n grows monotonically with n. …
...The cutoff the type of discreteness here considered introduces is similar in nature to the one considered by [12] and leads to a similar calculation of the Hawking radiation, which does not suffer significant modifications with respect to the continuum, at least for large black holes and typical frequencies.
Summarizing, we have shown that the midisuperspace formulation of loop quantum gravity with spherical symmetry is able to reproduce many features of standard analysis of quantum vacua in black hole space-times in the limit in which one considers a quantum test field living on a quantum space time. The discreteness of the quantum space-time has implications for some of the vacua even in regions of low curvature, in particular eliminating singularities. ...We have only taken the first steps towards computing Hawking radiation in loop quantum gravity. A more complete treatment, including superpositions of the quantum spin network states and a more complete discussion of the properties of the Green’s functions of the theory, in particular their Lorentz invariance will be pursued in further publications.
==endquote==

 

[marcus]

http://arxiv.org/abs/1312.3657
Structural aspects of loop quantum gravity and loop quantum cosmology from an algebraic perspective
Alexander Stottmeister, Thomas Thiemann
(Submitted on 12 Dec 2013)
We comment on structural properties of the algebras A_LQG/LQC underlying loop quantum gravity and loop quantum cosmology, especially the representation theory, relating the appearance of the (dynamically induced) superselection structure (θ-sectors) in loop quantum cosmology to recently proposed representations with non-degenerate background geometries in loop quantum gravity with Abelian structure group. To this end, we review and employ the concept of extending a given (observable) algebra with possibly non-trivial centre to a (charged) field algebra with (global) gauge group.We also interpret the results in terms of the geometry of the structure group G. Furthermore, we analyze the Koslowski-Sahlmann representations with non-degenerate background in the case of a non-Abelian structure group. We find that these representations can be interpreted from two different, though related, points view: Either, the standard algebras of loop quantum gravity need to be extended by a (possibly) central term, or the elementary flux vector fields need to acquire a shift related to the (classical) background to make these representations well-defined. Both perspectives are linked by the fact that the background shift is not an automorphism of the algebras, but rather an affine transformation. Finally, we show how similar algebraic mechanisms, which are used to explain the breaking of chiral symmetry and the occurrence of θ-vacua in quantum field theory, extend to loop quantum gravity. Thus, opening a path for the discussion of these questions in loop quantum gravity.
45 pages

http://arxiv.org/abs/1312.3674
Scalar Field Theory in Curved Momentum Space
Laurent Freidel, Trevor Rempel
(Submitted on 12 Dec 2013)
We derive an action for scalar quantum field theory with cubic interaction in the context of relative locality. Beginning with the generating functional for standard φ³--theory and the corresponding Feynman rules we modify them to account for the non--trivial geometry of momentum space. These modified rules are then used to reconstruct the generating functional and extract the action for the theory. A method for performing a covariant Fourier transform is then developed and applied to the action. We find that the transformed fields depend implicitly on a fixed point in momentum space with fields based at different points being related by a non-local transformation. The interaction term in the action is also non--local, but the kinetic term can be made local by choosing the base point to be the origin of momentum space.
26 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1312.4173
The microlocal spectrum condition, initial value formulations and background independence
Alexander Stottmeister, Thomas Thiemann
(Submitted on 15 Dec 2013)
We analyze the implications of the microlocal spectrum/Hadamard condition for states in a (linear) quantum field theory on a globally hyperbolic spacetime M in the context of a (distributional) initial value formulation. More specifically, we work in a 3+1-split M≅ℝ×Σ and give a bound, independent of the spacetime metric, on the wave front sets of the initial data for a quasi-free Hadamard state in the quantum field theory defined by a normally hyperbolic differential operator P acting in a vector bundle π: E→M. This aims at a possible way to apply the concept of Hadamard states within approaches to quantum field theory/gravity relying on a Hamiltonian formulation, potentially without a (classical) background metric g.
24 pages

general interest:
http://arxiv.org/abs/1312.4100
The Planck and LHC results and particle physics
Fedor Bezrukov
(Submitted on 15 Dec 2013)
I will discuss the recent LHC and Planck results, which are completely compatible with the Standard Model of particle physics, and the standard cosmological model (ΛCDM), respectively. It turns out that the extension of the Standard Model is, of course, required, but can be very minimal. I will discuss also what future measurements may be important to test this approach.
7 pages, talk on the EPS-HEP 2013 prepared for conference proceedings

http://arxiv.org/abs/1312.4057
Aristotle's physics
Carlo Rovelli
(Submitted on 14 Dec 2013)
I show that Aristotelian physics is a correct approximation of Newtonian physics in its appropriate domain, in the same precise sense in which Newton theory is an approximation of Einstein's theory. Aristotelian physics lasted long not because it became dogma, but because it is a very good theory.
9 pages

 

[marcus]

http://arxiv.org/abs/1312.4591
Motion of a mirror under infinitely fluctuating quantum vacuum stress
Qingdi Wang, William G. Unruh
(Submitted on 16 Dec 2013)
The actual value of the quantum vacuum energy density is generally regarded as irrelevant in non-gravitational physics. However, this paper gives a non-gravitational system where this value does have physical significance. The system is a mirror with an internal degree of freedom which interacts with a scalar field. We find that the force exerted on the mirror by the field vacuum undergoes wild fluctuations with a magnitude proportional to the value of the vacuum energy density, which is mathematically infinite. This infinite fluctuating force gives infinite instantaneous acceleration of the mirror. We show that this infinite fluctuating force and infinite instantaneous acceleration make sense because they will not result in infinite fluctuation of the mirror's position. On the contrary, the mirror's fluctuating motion will be confined in a small region due to two special properties of the quantum vacuum: (1) the vacuum friction which resists the mirror's motion and (2) the strong anti-correlation of vacuum fluctuations which constantly changes the direction of the mirror's infinite instantaneous acceleration and thus cancels the effect of infinities to make the fluctuation of the mirror's position finite.
15 pages, 6 figures

 

[marcus]

not QG but possibly of general interest:
http://arxiv.org/abs/1312.4947
Tight bonds between sterile neutrinos and dark matter
Torsten Bringmann, Jasper Hasenkamp, Joern Kersten
(Submitted on 17 Dec 2013)
Despite the astonishing success of standard ΛCDM cosmology, there is mounting evidence for a tension with observations at small and intermediate scales. We introduce a simple model where both cold dark matter (DM) and sterile neutrinos are charged under a new U(1)X gauge interaction. The resulting DM self-interactions resolve the tension with the observed abundances and internal density structures of dwarf galaxies. At the same time, the sterile neutrinos can account for both the small hot DM component favored by cosmological observations and the neutrino anomalies found in short-baseline experiments.
7 pages, 3 figures

 

[atyy]

http://arxiv.org/abs/1312.5396
Action and Vertices in the Worldine Formalism
Laurent Freidel, Trevor Rempel
(Submitted on 19 Dec 2013)
Utilizing the worldline formalism we study the effects of demanding local interactions on the corresponding vertex factor. We begin by reviewing the familiar case of a relativistic particle in Minkowksi space, showing that localization gives rise to the standard conservation of momentum at each vertex. A generalization to curved geometry is then studied and a notion of covariant Fourier transform is introduced to aid in the analysis. The vertex factor is found to coincide with the one derived for flat spacetime. Next, we apply this formalism to a loop immersed in a gravitational field, demonstrating that the loop momenta is determined entirely by the external momenta. Finally, we postulate that the semi--classical effects of quantum gravity on the Feynman path integral can be accounted for by a modification to the vertex factor which de-localizes the vertex. We study one particular Lorentz invariant de-localization which, remarkably, has no effect on conservation of vertex momenta.

http://arxiv.org/abs/1312.5512
An introduction to spherically symmetric loop quantum gravity black holes
Rodolfo Gambini, Jorge Pullin
(Submitted on 19 Dec 2013)
We review recent developments in the treatment of spherically symmetric black holes in loop quantum gravity. In particular, we discuss an exact solution to the quantum constraints that represents a black hole and is free of singularities. We show that new observables that are not present in the classical theory arise in the quantum theory. We also discuss Hawking radiation by considering the quantization of a scalar field on the quantum spacetime.

http://arxiv.org/abs/1312.5646
Ising Model from Intertwiners
Bianca Dittrich, Jeff Hnybida
(Submitted on 19 Dec 2013)
Spin networks appear in a number of areas, for instance in lattice gauge theories and in quantum gravity. They describe the contraction of intertwiners according to the underlying network. We show that a certain generating function of intertwiner contractions leads to the partition function of the 2d Ising model. This implies that the intertwiner model possesses a second order phase transition, thus leading to a continuum limit with propagating degrees of freedom.

 

[John86]

http://arxiv.org/abs/1312.5373
Amplification, Redundancy, and the Quantum Chernoff Information
Michael Zwolak, C. Jess Riedel, Wojciech H. Zurek
(Submitted on 18 Dec 2013)
Amplification was regarded, since the early days of quantum theory, as a mysterious ingredient that endows quantum microstates with macroscopic consequences, key to the "collapse of the wavepacket", and a way to avoid embarrassing problems exemplified by Schr\"odinger's cat. Such a bridge between the quantum microworld and the classical world of our experience was postulated ad hoc in the Copenhagen Interpretation. Quantum Darwinism views amplification as replication, in many copies, of the information about quantum states. We show that such amplification is a natural consequence of a broad class of models of decoherence, including the photon environment we use to obtain most of our information. This leads to objective reality via the presence of robust and widely accessible records of selected quantum states. The resulting redundancy (the number of copies deposited in the environment) follows from the Quantum Chernoff Information that quantifies the information transmitted by a typical elementary subsystem of the environment

 

[marcus]

http://arxiv.org/abs/1312.6425
The Last 50 Years of General Relativity and Gravitation: From GR3 to GR20 Warsaw Conferences
Abhay Ashtekar
(Submitted on 22 Dec 2013)
This article has a dual purpose: i) to provide a flavor of the scientific highlights of the landmark conference, GR3, held in July 1962 at Jablonna, near Warsaw; and, ii) to present a bird's eye view of the tremendous advances that have occurred over the half century that separates GR3 and GR20, which was again held in Warsaw in July 2013.
15 pages, one figure

http://arxiv.org/abs/1312.6322
Time in Fundamental Physics
Abhay Ashtekar
(Submitted on 21 Dec 2013)
The first three sections of this article contain a broad brush summary of the profound changes in the notion of time in fundamental physics that were brought about by three revolutions: the foundations of mechanics distilled by Newton in his Principia, the discovery of special relativity by Einstein and its reformulation by Minkowski, and, finally, the fusion of geometry and gravity in Einstein's general relativity. The fourth section discusses two aspects of yet another deep revision that waits in the wings as we attempt to unify general relativity with quantum physics.
10 pages

possible side interest:
http://arxiv.org/abs/1312.6449
Quantum mechanics, matter waves, and moving clocks
Holger Mueller
(Submitted on 23 Dec 2013)
This paper is divided into three parts. In the first (section 1), we demonstrate that all of quantum mechanics can be derived from the fundamental property that the propagation of a matter wave packet is described by the same gravitational and kinematic time dilation that applies to a clock. We will do so in several steps, first deriving the Schroedinger equation for a nonrelativistic particle without spin in a weak gravitational potential, and eventually the Dirac equation in curved space-time describing the propagation of a relativistic particle with spin in strong gravity.
In the second part (sections 2-4), we present interesting consequences of the above quantum mechanics: that it is possible to use wave packets as a reference for a clock, to test general relativity, and to realize a mass standard based on a proposed redefinition of the international system of units, wherein the Planck constant would be assigned a fixed value. The clock achieved an absolute accuracy of 4 parts per billion (ppb). The experiment yields the fine structure constant α=7.297352589(15)×10⁻³ with 2.0 ppb accuracy. We present improvements that have reduced the leading systematic error about 8-fold and improved the statistical uncertainty to 0.33 ppb in 6 hours of integration time, referred to α.
In the third part (sections 5-7), we present possible future experiments with atom interferometry: A gravitational Aharonov-Bohm experiment and its application as a measurement of Newton's gravitational constant, antimatter interferometry, interferometry with charged particles, and interferometry in space.
We will give a review of previously published material when appropriate, but will focus on new aspects that haven't been published before.
74 pages. To be published in the proceedings of the International School of Physics "Enrico Fermi" 2013, Course 188 - Atom Interferometry

http://arxiv.org/abs/1312.6268
Quantum speed limits and optimal Hamiltonians for driven systems in mixed states
Ole Andersson, Hoshang Heydari
(Submitted on 21 Dec 2013)
Inequalities of Mandelstam-Tamm and Margolus-Levitin type provide lower bounds on the time it takes for a quantum system to evolve between two states. Knowledge of such bounds, called quantum speed limits, is of utmost importance in virtually all areas of physics, where determination of the minimum time required for a quantum process is of interest...
14 pages, 2 figures

 

[MTd2]

Well, this is very unrelated, but given the overall interest in numerology on this subforum, I think many people will enjoy this paper:

http://arxiv.org/abs/1312.6523

A magic pyramid of supergravities

A. Anastasiou, L. Borsten, M. J. Duff, L. J. Hughes, S. Nagy
(Submitted on 23 Dec 2013)
By formulating N = 1, 2, 4, 8, D = 3, Yang-Mills with a single Lagrangian and single set of transformation rules, but with fields valued respectively in R,C,H,O, it was recently shown that tensoring left and right multiplets yields a Freudenthal-Rosenfeld-Tits magic square of D = 3 supergravities. This was subsequently tied in with the more familiar R,C,H,O description of spacetime to give a unified division-algebraic description of extended super Yang-Mills in D = 3, 4, 6, 10. Here, these constructions are brought together resulting in a magic pyramid of supergravities. The base of the pyramid in D = 3 is the known 4x4 magic square, while the higher levels are comprised of a 3x3 square in D = 4, a 2x2 square in D = 6 and Type II supergravity at the apex in D = 10. The corresponding U-duality groups are given by a new algebraic structure, the magic pyramid formula, which may be regarded as being defined over three division algebras, one for spacetime and each of the left/right Yang-Mills multiplets. We also construct a conformal magic pyramid by tensoring conformal supermultiplets in D = 3, 4, 6. The missing entry in D = 10 is suggestive of an exotic theory with G/H duality structure F4(4)/Sp(3) x Sp(1).

 

[marcus]

http://arxiv.org/abs/1312.6835
Thermodynamics of the polymeric quantized Schwarzschild black hole
M. A. Gorji, K. Nozari, B. Vakili
(Submitted on 24 Dec 2013)
Polymer representation of quantum mechanics is an effective approach to loop quantum gravity. In this paper we develop statistical mechanics in the polymer framework. While to obtain energies of microstates one needs usually to solve the polymer-modified Schrödinger equation, we have not adopted this strategy here since it is not an easy task due to the complicated form of the Schrödinger equation in the polymer picture. Instead, we formulate the ensemble theory in polymer framework in a semi-classical regime through deformed density of states. We show that our results are in good agreement with those arising from quantum mechanical considerations. Applying this method to thermodynamics of quantum Schwarzschild black hole, we obtain corrections to the Bekenstein-Hawking entropy due to loop quantum gravity effects.
12 pages
Subjects: