Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1202.6183
A Gauge Theoretic Approach to Gravity
Kirill Krasnov
(Submitted on 28 Feb 2012)
Einstein's General Relativity (GR) is a dynamical theory of the spacetime metric. We describe an approach in which GR becomes an SU(2) gauge theory. We start at the linearised level and show how a gauge theoretic Lagrangian for non-interacting massless spin two particles (gravitons) takes a much more simple and compact form than in the standard metric description. Moreover, in contrast to the GR situation, the gauge theory Lagrangian is convex. We then proceed with a formulation of the full non-linear theory. The equivalence to the metric-based GR holds only at the level of solutions of the field equations, that is, on-shell. The gauge-theoretic approach also makes it clear that GR is not the only interacting theory of massless spin two particles, in spite of the GR uniqueness theorems available in the metric description. Thus, there is an inifnite-parameter class of gravity theories all describing just two propagating polarisations of the graviton. We describe how matter can be coupled to gravity in this formulation and, in particular, how both the gravity and Yang-Mills arise as sectors of a general diffeomorphism invariant gauge theory. We finish by outlining a possible scenario of the UV completion of quantum gravity within this approach.
40 pages; invited review to appear in Proceedings of the Royal Society A
[my comment: see page 34, section 4.3, and the next section Conclusions. conjecture about RG flow of theory, tie-in with asym-safe approach]

http://arxiv.org/abs/1202.6322
Multigraph models for causal quantum gravity and scale dependent spectral dimension
Georgios Giasemidis, John F Wheater, Stefan Zohren
(Submitted on 28 Feb 2012)
We study random walks on ensembles of a specific class of random multigraph graphs associated with theories of causal quantum gravity. In particular, we investigate the spectral dimension of the graph ensemble for recurrent as well as transient walks. We investigate the circumstances in which the spectral dimension and Hausdorff dimension are equal and show that this occurs when rho, the exponent for anomalous behaviour of the resistance to infinity, is zero. The concept of scale dependent spectral dimension in these models is introduced. We apply this notion to a multigraph ensemble with a measure induced by a size biased critical Galton-Watson process which has a scale dependent spectral dimension of two at large scales and one at small scales. We conclude by discussing a specific model related to four dimensional quantum gravity which has a spectral dimension of four at large scales and two at small scales.
30 pages, 3 figures
[my comment: most references are to "causal dynamical triangulations" and related qg. they call it "causal QG" but it's a close relative of CDT, more about the interesting running of dimensionality with scale]

 

[marcus]

http://arxiv.org/abs/1203.0174
Quantum isolated horizons and black hole entropy
J. Fernando Barbero G., Jerzy Lewandowski, Eduardo J. S. Villaseñor
(Submitted on 1 Mar 2012)
We give a short introduction to the approaches currently used to describe black holes in loop quantum gravity. We will concentrate on the classical issues related to the modeling of black holes as isolated horizons, give a short discussion of their canonical quantization by using loop quantum gravity techniques, and a description of the combinatorial methods necessary to solve the counting problems involved in the computation of the entropy.
28 pages. Contribution to the Proceedings of the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

 

[marcus]

http://arxiv.org/abs/1203.1040
Cleaning up the cosmological constant
Ian Kimpton, Antonio Padilla
(Submitted on 5 Mar 2012)
We present a novel idea for screening the vacuum energy contribution to the overall value of the cosmological constant, thereby enabling us to choose the bare value of the vacuum curvature empirically, without any need to worry about the zero-point energy contributions of each particle. The trick is to couple matter to a metric that is really a composite of other fields, with the property that the square-root of its determinant is the integrand of a topological invariant, and/or a total derivative. This ensures that the vacuum energy contribution to the Lagrangian is non-dynamical. We then give an explicit example of a theory with this property that is free from Ostrogradski ghosts, and is consistent with solar system physics and cosmological tests.
4 pages
[my comment: depends on prior work http://arxiv.org/abs/1106.2000 published in Physical Review Letters in 2012]

http://arxiv.org/abs/1203.1173
Cosmological particle creation in the lab?
Ralf Schützhold, William G. Unruh
(Submitted on 6 Mar 2012)
One of the most striking examples for the production of particles out of the quantum vacuum due to external conditions is cosmological particle creation, which is caused by the expansion or contraction of the Universe. Already in 1939, Schrödinger understood that the cosmic evolution could lead to a mixing of positive and negative frequencies and that this "would mean production or annihilation of matter, merely by the expansion". Later this phenomenon was derived via more modern techniques of quantum field theory in curved space-times by Parker (who apparently was not aware of Schrödinger's work) and subsequently has been studied in numerous publications. Even though cosmological particle creation typically occurs on extremely large length scales, it is one of the very few examples for such fundamental effects where we actually may have observational evidence: According to the inflationary model of cosmology, the seeds for the anisotropies in the cosmic microwave background (CMB) and basically all large scale structures stem from this effect. In this Chapter, we shall provide a brief discussion of this phenomenon and sketch a possibility for an experimental realization via an analogue in the laboratory.
13 pages

 

[marcus]

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

 

[marcus]

http://arxiv.org/abs/1203.2161
Highlights of Noncommutative Spectral Geometry
Mairi Sakellariadou
(Submitted on 9 Mar 2012)
A summary of noncommutative spectral geometry as an approach to unification is presented. The role of the doubling of the algebra, the seeds of quantization and some cosmological implications are briefly discussed.
4 pages, to be published in the Journal of Physics Conference Series under the title "Vishwa Mimansa - An Interpretative Exposition of the Universe"; talk given at the 7th International Conference on Gravitation and Cosmology, 14-19 December 2011, Goa, India

http://arxiv.org/abs/1203.1934
Loop Quantization of the Supersymmetric Two-Dimensional BF Model
Clisthenis P. Constantinidis, Ruan Couto, Ivan Morales, Olivier Piguet
(Submitted on 8 Mar 2012)
In this paper we consider the quantization of the 2d BF model coupled to topological matter. Guided by the rigid supersymmetry this system can be viewed as a super-BF model, where the field content is expressed in terms of superfields. A canonical analysis is done and the constraints are then implemented at the quantum level in order to construct the Hilbert space of the theory under the perspective of Loop Quantum Gravity methods.
17 pages

http://arxiv.org/abs/1203.1962
An effective action for asymptotically safe gravity
Alfio Bonanno
(Submitted on 8 Mar 2012)
Asymptotically safe theories of gravitation have received great attention in recent times. In this framework an effective action embodying the basic features of the renormalized flow around the non-gaussian fixed point is derived and its implications for the early universe are discussed. In particular, a "landscape" of a countably infinite number of cosmological inflationary solutions characterized by an unstable de Sitter phase lasting for a large enough number of e-folds is found.
5 pages, to appear as a Rapid Communication in Physical Review D

http://arxiv.org/abs/1203.2158
The "tetrad only" theory space: Nonperturbative renormalization flow and Asymptotic Safety
Ulrich Harst, Martin Reuter
(Submitted on 9 Mar 2012)
We set up a nonperturbative gravitational coarse graining flow and the corresponding functional renormalization group equation on the as to yet unexplored "tetrad only" theory space. It comprises action functionals which depend on the tetrad field (along with the related background and ghost fields) and are invariant under the semi-direct product of spacetime diffeomorphisms and local Lorentz transformations. This theory space differs from that of Quantum Einstein Gravity (QEG) in that the tetrad rather than the metric constitutes the fundamental variable and because of the additional symmetry requirement of local Lorentz invariance. It also differs from "Quantum Einstein Cartan Gravity" (QECG) investigated recently since the spin connection is not an independent field variable now. We explicitly compute the renormalization group flow on this theory space within the tetrad version of the Einstein-Hilbert truncation. A detailed comparison with analog results in QEG and QECG is performed in order to assess the impact the choice of a fundamental field variable has on the renormalization behavior of the gravitational average action, and the possibility of an asymptotically safe infinite cutoff limit is investigated. Implications for nonperturbative studies of fermionic matter coupled to quantum gravity are also discussed. It turns out that, in the context of functional flow equations, the "hybrid calculations" proposed in the literature (using the tetrad for fermionic diagrams only, and the metric in all others) are unlikely to be quantitatively reliable. Moreover we find that, unlike in perturbation theory, the non-propagating Faddeev-Popov ghosts related to the local Lorentz transformations may not be discarded but rather contribute quite significantly to the beta functions of Newton's constant and the cosmological constant.
45 pages, 10 figures

brief mention (not QG but possibly of general interest):
http://arxiv.org/abs/1203.2035
A Noether Theorem for Markov Processes
John C. Baez, Brendan Fong
(Submitted on 9 Mar 2012)
Noether's theorem links the symmetries of a quantum system with its conserved quantities, and is a cornerstone of quantum mechanics. Here we prove a version of Noether's theorem for Markov processes. In quantum mechanics, an observable commutes with the Hamiltonian if and only if its expected value remains constant in time for every state. For Markov processes that no longer holds, but an observable commutes with the Hamiltonian if and only if both its expected value and standard deviation are constant in time for every state.
9 pages

 

[marcus]

http://arxiv.org/abs/1203.2733
From Classical To Quantum Gravity: Introduction to Loop Quantum Gravity
Kristina Giesel, Hanno Sahlmann
(Submitted on 13 Mar 2012)
We present an introduction to the canonical quantization of gravity performed in loop quantum gravity, based on lectures held at the 3rd quantum geometry and quantum gravity school in Zakopane in 2011. A special feature of this introduction is the inclusion of new proposals for coupling matter to gravity that can be used to deparametrize the theory, thus making its dynamics more tractable. The classical and quantum aspects of these new proposals are explained alongside the standard quantization of vacuum general relativity in loop quantum gravity.
55 pages. Contribution to the Proceedings of the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

brief mention (not directly Loop-related but possibly of general interest):
http://arxiv.org/abs/1203.2622
The Optimal Cosmic Epoch for Precision Cosmology
Abraham Loeb (Harvard)
(Submitted on 12 Mar 2012)
The statistical uncertainty in measuring the primordial density perturbations on a given comoving scale is dictated by the number of independent regions of that scale that are accessible to an observer. This number varies with cosmic time and diminishes per Hubble volume in the distant past or future of the standard cosmological model. We show that the best constraints on the initial power spectrum of linear density perturbations are accessible (e.g. through 21-cm intensity mapping) at redshifts z~10-50, and that the ability to constrain the cosmological initial conditions will deteriorate quickly in our cosmic future.
4 pages, 4 figures

http://arxiv.org/abs/1203.2642
Very special relativity as particle in a gauge field and two-time physics

Juan M. Romero, Eric Escobar, Etelberto Vazquez
(Submitted on 12 Mar 2012)
The action for a particle in very special relativity is studied. It is shown that this system is equivalent to a relativistic particle in a gauge field. A new symmetry for this system is found. A general action with restored Lorentz symmetry is proposed for this system. It is shown that this new action contain very special relativity and two-time physics.
12 pages

http://arxiv.org/abs/1203.2679
Noncommutative Mixmaster Cosmologies
Christopher Estrada, Matilde Marcolli
(Submitted on 13 Mar 2012)
In this paper we investigate a variant of the classical mixmaster universe model of anisotropic cosmology, where the spatial sections are noncommutative 3-tori. We consider ways in which the discrete dynamical system describing the mixmaster dynamics can be extended to act on the noncommutative torus moduli, and how the resulting dynamics differs from the classical one, for example, in the appearance of exotic smooth structures. We discuss properties of the spectral action, focussing on how the slow-roll inflation potential determined by the spectral action affects the mixmaster dynamics. We relate the model to other recent results on spectral action computation and we identify other physical contexts in which this model may be relevant.
24 pages

http://arxiv.org/abs/1203.2641
Internal Relativity
Olaf Dreyer
(Submitted on 12 Mar 2012)
General relativity differs from other forces in nature in that it can be made to disappear locally. This is the essence of the equivalence principle. In general relativity the equivalence principle is implemented using differential geometry. The connection that comes from a metric is used to glue together the different gravity-free Minkowski spaces. In this article we argue that there is another way to implement the equivalence principle. In this new way it is not different Minkowski spaces that are connected but different vacua of an underlying solid-state like model. One advantage of this approach to gravity is that one can start with a quantum mechanical model so that the question of how to arrive at a quantum theory of gravity does not arise. We show how the gravitational constant can be calculated in this setup.
9 pages

 

[marcus]

A new ILQGS talk (given 13 March) by Diaz-Polo
Black Hole Evaporation in Loop Quantum Gravity
Slides PDF:
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.pdf
Audio:
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.wav
or
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.aif
based on this paper:
http://arxiv.org/abs/1109.4239
Probing Loop Quantum Gravity with Evaporating Black Holes
Aurelien Barrau, Xiangyu Cao, Jacobo Diaz-Polo, Julien Grain, Thomas Cailleteau
(Submitted on 20 Sep 2011)
This letter aims at showing that the observation of evaporating black holes should allow distinguishing between the usual Hawking behavior and Loop Quantum Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.
5 pages, 3 figures. Published in Physical Review Letters 107, 251301 (2011)

 

[marcus]

http://arxiv.org/abs/1203.3449
Non-singular Power-law and Assisted inflation in Loop Quantum Cosmology
Evan Ranken, Parampreet Singh
(Submitted on 15 Mar 2012)
We investigate the dynamics of single and multiple scalar fields with exponential potentials, leading to power-law and assisted inflation, in loop quantum cosmology. Unlike in the classical theory, dynamical trajectories in loop quantum cosmology are generically non-singular, with a big bounce replacing classical big bang in the Planck regime. Post bounce, after a phase of super-inflation, dynamical trajectories evolve towards the classical attractor in the inflationary scenarios. Depending on the initial conditions, bounce is shown to occur in kinetic as well as potential dominated regimes. We analyze the number of e-foldings resulting from the phase of super-inflation, and find the dependence of the maximum possible number of e-foldings on the equation of state at the bounce and on the steepness of the potential. We find that if the potential is not steep, this phase can lead to large number of e-foldings in power-law inflation. For the assisted inflation scenario, an increase in the number of fields can yield a significant increase in the number of e-foldings during super-inflation.
21 pages, 15 figures

 

[marcus]

http://arxiv.org/abs/1203.3591
Nonperturbative Quantum Gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 15 Mar 2012)
Asymptotic safety describes a scenario in which general relativity can be quantized as a conventional field theory, despite being nonrenormalizable when expanding it around a fixed background geometry. It is formulated in the framework of the Wilsonian renormalization group and relies crucially on the existence of an ultraviolet fixed point, for which evidence has been found using renormalization group equations in the continuum.
"Causal Dynamical Triangulations" (CDT) is a concrete research program to obtain a nonperturbative quantum field theory of gravity via a lattice regularization, and represented as a sum over spacetime histories. In the Wilsonian spirit one can use this formulation to try to locate fixed points of the lattice theory and thereby provide independent, nonperturbative evidence for the existence of a UV fixed point.
We describe the formalism of CDT, its phase diagram, possible fixed points and the "quantum geometries" which emerge in the different phases. We also argue that the formalism may be able to describe a more general class of Horava-Lifgarbagez gravitational models.
146 pages, many figures

 

[marcus]

http://arxiv.org/abs/1203.4105
Lorentz breaking Effective Field Theory and observational tests
Stefano Liberati
(Submitted on 19 Mar 2012)
Analogue models of gravity have provided an experimentally realizable test field for our ideas on quantum field theory in curved spacetimes but they have also inspired the investigation of possible departures from exact Lorentz invariance at microscopic scales. In this role they have joined, and sometime anticipated, several quantum gravity models characterized by Lorentz breaking phenomenology. A crucial difference between these speculations and other ones associated to quantum gravity scenarios, is the possibility to carry out observational and experimental tests which have nowadays led to a broad range of constraints on departures from Lorentz invariance. We shall review here the effective field theory approach to Lorentz breaking in the matter sector, present the constraints provided by the available observations and finally discuss the implications of the persisting uncertainty on the composition of the ultra high energy cosmic rays for the constraints on the higher order, analogue gravity inspired, Lorentz violations.
47 pages, 4 figures. Lecture Notes for the IX SIGRAV School on "Analogue Gravity", Como (Italy), May 2011

http://arxiv.org/abs/1203.4207
The phase diagram of quantum gravity from diffeomorphism-invariant RG-flows
Ivan Donkin, Jan M. Pawlowski
(Submitted on 19 Mar 2012)
We evaluate the phase diagram of quantum gravity within a fully diffeomorphism-invariant renormalisation group approach. The construction is based on the geometrical or Vilkovisky-DeWitt effective action. We also resolve the difference between the fluctuation metric and the background metric. This allows for fully background-independent flows in gravity. The results provide further evidence for the ultraviolet fixed point scenario in quantum gravity with quantitative changes for the fixed point physics. We also find a stable infrared fixed point related to classical Einstein gravity. Implications and possible extensions are discussed.
23 pages, 13 figures

brief mention:
http://arxiv.org/abs/1203.4197
Is the Cosmological Coincidence a Problem?
Navin Sivanandam
(Submitted on 19 Mar 2012)
The matching of our epoch of existence with the approximate equality of dark energy and dark matter energy densities is an apparent further fine-tuning, beyond the already troubling 120 orders of magnitude that separate dark energy from the Planck scale. In this paper I will argue that the coincidence is not a fine-tuning problem, but instead an artifact of anthropic selection. Rather than assuming measurements are equally likely in all epochs, one should insist that measurements of a quantity be typical amongst all such measurements. As a consequence, particular observations will reflect the epoch in which they are most easily made. In the specific case of cosmology, most measurements of dark energy and dark matter will done during an epoch when large numbers of linear modes are available to observers, so we should not be surprised at living at such a time. This is made precise in a particular model for the probability distribution for r=min(Ω_m/Ω_L, Ω_L/Ω_m), where it is shown that if p(r) ~ [N(r)]^b (where N(r) is the number of linear modes, and b is some arbitrary positive power), the probability that r is greater than its observed value of 0.4, is close to 1. Thus the cosmological coincidence is no longer problematic.
10 pages, 5 figures

http://arxiv.org/abs/1203.3827
Where will Einstein fail? Lessons for gravity and cosmology
Niayesh Afshordi (U-Waterloo/Perimeter Institute)
(Submitted on 16 Mar 2012)
Einstein's theory of General Relativity is the benchmark example for empirical success and mathematical elegance in theoretical physics. However, in spite of being the most successfully tested theory in physics, there are strong theoretical and observational arguments for why General Relativity should fail. It is not a question of if, but rather a question of where and when! I start by recounting the tremendous success in observational cosmology over the past three decades, that has led to the era of precision cosmology. I will then summarize the pathologies in Einstein's theory of gravity, as the cornerstone of standard cosmological model. Attempts to address these pathologies are either inspired by mathematical elegance, or empirical falsifiability. Here, I provide different arguments for why a falsifiable solution should violate Lorentz symmetry, or revive "gravitational aether". Deviations from Einstein's gravity are then expected in: 1) cosmological matter-radiation transition, 2) neutron stars, 3) gravitomagnetic effect, 4) astrophysical black holes, and their potential connection to dark energy, and 5) early Universe, where the predictions are ranked by their degree of robustness and falsifiability.
20 pages, 3 figures, Based on the Professor M.K. Vainu Bappu gold medal award (2008) lecture given at IUCAA, Pune on 2011 October 15, To appear in the 2012 March issue of the Bulletin of the Astronomical Society of India

 

[marcus]

http://arxiv.org/abs/1203.5082
Bubbles and jackets: new scaling bounds in topological group field theories
Sylvain Carrozza, Daniele Oriti
(Submitted on 22 Mar 2012)
We use a reformulation of topological group field theories in 3 and 4 dimensions in terms of variables associated to vertices, in 3d, and edges, in 4d, to obtain new scaling bounds for their Feynman amplitudes. In both 3 and 4 dimensions, we obtain a bubble bound proving the suppression of singular topologies with respect to the first terms in the perturbative expansion (in the cut-off). We also prove a new, stronger jacket bound than the one currently available in the literature. We expect these results to be relevant for other tensorial field theories of this type, as well as for group field theory models for 4d quantum gravity.
32 pages

brief mention:
http://arxiv.org/abs/1203.4995
Emergence of time in quantum gravity: is time necessarily flowing ?
Pierre Martinetti
(Submitted on 22 Mar 2012)
We discuss the emergence of time in quantum gravity, and ask whether time is always "something that flows"'. We first recall that this is indeed the case in both relativity and quantum mechanics, although in very different manners: time flows geometrically in relativity (i.e. as a flow of proper time in the four dimensional space-time), time flows abstractly in quantum mechanics (i.e. as a flow in the space of observables of the system). We then ask the same question in quantum gravity, in the light of the thermal time hypothesis of Connes and Rovelli. The latter proposes to answer the question of time in quantum gravity (or at least one of its many aspects), by postulating that time is a state dependent notion. ...
...
12 pages. Contribution to the Workshop "Temps et Emergence", Ecole Normale Supérieure, Paris 14-15 october 2011.

 

[marcus]

http://arxiv.org/abs/1203.5214
Universality of geometry
C. Wetterich
(Submitted on 23 Mar 2012)
In models of emergent gravity the metric arises as the expectation value of some collective field. Usually, many different collective fields with appropriate tensor properties are candidates for a metric. Which collective field describes the "physical geometry"? We resolve this "metric ambiguity" by an investigation of the most general form of the quantum effective action for several metrics. In the long-distance limit the physical metric is universal and accounts for a massless graviton. Other degrees of freedom contained in the various metric candidates describe very massive scalars and symmetric second rank tensors. They only play a role at microscopic distances, typically around the Planck length. The universality of geometry at long distances extends to the vierbein and the connection. On the other hand, for distances and time intervals of Planck size geometry loses its universal meaning. Time is born with the big bang.
6 pages

brief mention (not QG but of possible interest):
http://arxiv.org/abs/1203.5153
Self-healing of unitarity in effective field theories and the onset of new physics
Ufuk Aydemir, Mohamed M. Anber, John F. Donoghue
(Submitted on 23 Mar 2012)
In effective field theories it is common to identify the onset of new physics with the violation of tree-level unitarity. However, we show that this is parametrically incorrect in the case of chiral perturbation theory, and is probably theoretically incorrect in general. In the chiral theory, we explore perturbative unitarity violation as a function of the number of colors and the number of flavors, holding the scale of the "new physics" (i.e. QCD) fixed. This demonstrates that the onset of new physics is parametrically uncorrelated with tree-unitarity violation. When the latter scale is lower than that of new physics, the effective theory must heal its unitarity violation itself, which is expected because the field theory satisfies the requirements of unitarity. In the chiral theory, the self-healing results in a resonant structure with scalar quantum numbers. In the electroweak variant of this argument, the structure must have the properties of the Higgs and must couple proportional to the mass in both gauge boson and fermion scattering. A similar example can be seen in the case of general relativity coupled to multiple matter fields, where iteration of the vacuum polarization diagram restores unitarity. We present arguments that suggest the correct identification should be connected to the onset of inelasticity rather than unitarity violation. We describe how the onset of inelasticity can occur in the effective theory, although it does not appear possible to predict the onset reliably.
11 pages, 5 figures

http://arxiv.org/abs/1203.5238
The small-scale structure of quantum spacetime
Christopher D. Burton
(Submitted on 23 Mar 2012)
Planck-scale quantum spacetime undergoes probabilistic local curvature fluctuations whose distributions cannot explicitly depend on position otherwise vacuum's small-scale quantum structure would fail to be statistically homogeneous. Since the collection of fluctuations is a many-body system, the natural explanation for their position-independent statistics is that they are in equilibrium with each other and distributed at maximum entropy. Consequently, their probability distributions obey the laws of statistical physics which enforces small-scale smoothness, prevents the homogeneity-violating diffusion found in any free quantum system, and maintains decoherence. Their entropy, calculated using the explicitly-constructed phase space of the Riemann whose statistics are derived using a background-independent graviton exchange ensemble, is proportional to the Einstein-Hilbert action evaluated on the macroscopic expected geometry and includes a small, positive cosmological constant. Entropy maximization yields quantum spacetime's Ehrenfest equations of motion which are identical to Einstein's expectation-valued field equations. This background-independent dynamical formulation reveals curvature fluctuation entropy as a source of expansion and raises the possibility that matter's zero-point energy problem, which is action-based and not energy shift invariant, may not be a problem after all.
16 pages

 

[marcus]

http://arxiv.org/abs/1203.5425
CODATA Recommended Values of the Fundamental Physical Constants: 2010
Peter J. Mohr, Barry N. Taylor, David B. Newell
(Submitted on 24 Mar 2012)
This paper gives the 2010 self-consistent set of values of the basic constants and conversion factors of physics and chemistry recommended by the Committee on Data for Science and Technology (CODATA) for international use. The 2010 adjustment takes into account the data considered in the 2006 adjustment as well as the data that became available from 1 January 2007, after the closing date of that adjustment, until 31 December 2010, the closing date of the new adjustment. Further, it describes in detail the adjustment of the values of the constants, including the selection of the final set of input data based on the results of least-squares analyses. The 2010 set replaces the previously recommended 2006 CODATA set and may also be found on the World Wide Web at physics.nist.gov/constants.
94 pages, 8 figures, 48 tables

strange:
http://arxiv.org/abs/1203.5557
Quantum Theory without Planck's Constant
John P. Ralston
(Submitted on 25 Mar 2012)
Planck's constant was introduced as a fundamental scale in the early history of quantum mechanics. We find a modern approach where Planck's constant is absent: it is unobservable except as a constant of human convention. Despite long reference to experiment, review shows that Planck's constant cannot be obtained from the data of Ryberg, Davisson and Germer, Compton, or that used by Planck himself. In the new approach Planck's constant is tied to macroscopic conventions of Newtonian origin, which are dispensable. The precision of other fundamental constants is substantially improved by eliminating Planck's constant...
42 pages, 9 figures

brief mention:
http://arxiv.org/abs/1203.5367
Domain structures in quantum graphity
James Q. Quach, Chun-Hsu Su, Andrew M. Martin, Andrew D. Greentree
(Submitted on 23 Mar 2012)
Quantum graphity offers the intriguing notion that space emerges in the low energy states of the spatial degrees of freedom of a dynamical lattice. Here we investigate metastable domain structures which are likely to exists in the low energy phase of lattice evolution. Through a simulated annealing process we explore the formation of metastable defects at domain boundaries and the effects of domain structures on the propagation of bosons. We show that these structures should have observable consequences including scattering, double imaging, and gravitational lensing-like effects.
10 pages, 11 figures

 

[marcus]

http://arxiv.org/abs/1203.5875
Big bounce from gravitational four-fermion interaction
I.B. Khriplovich
(Submitted on 27 Mar 2012)
The four-fermion gravitational interaction is induced by torsion, and gets dominating on the Planck scale. The regular, axial-axial part of this interaction by itself does not stop the gravitational compression. However, the anomalous, vector-vector interaction results in a natural way in big bounce.
4 pages.

http://arxiv.org/abs/1203.6191
Minimal Length Scale Scenarios for Quantum Gravity
Sabine Hossenfelder
(Submitted on 28 Mar 2012)
We review the question whether the fundamental laws of nature limit our ability to probe arbitrarily short distances. First, we examine what insights can be gained from thought experiments for probes of shortest distances, and summarize what can be learned from different approaches to a theory of quantum gravity. Then we discuss some models that have been developed to implement a minimal length scale into quantum mechanics and quantum field theory. These models have entered the literature under the names of generalized uncertainty principle or modified dispersion relation, and have allowed to study the effects of a minimal length scale in quantum mechanics, quantum electrodynamics, thermodynamics, black hole physics and cosmology. Finally, we touch upon the question if there are ways to circumvent the manifestation of a minimal length scale in short-distance physics.
86 pages, prepared for Living Reviews in Relativity

http://arxiv.org/abs/1203.6164
Intersecting Quantum Gravity with Noncommutative Geometry - a Review
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 28 Mar 2012)
We review applications of noncommutative geometry in canonical quantum gravity. First, we show that the framework of loop quantum gravity includes natural noncommutative structures which have, hitherto, not been explored. Next, we present the construction of a spectral triple over an algebra of holonomy loops. The spectral triple, which encodes the kinematics of quantum gravity, gives rise to a natural class of semiclassical states which entail emerging fermionic degrees of freedom. In the particular semiclassical approximation where all gravitational degrees of freedom are turned off, a free fermionic quantum field theory emerges. We end the paper with an extended outlook section.

 

[marcus]

http://arxiv.org/abs/1203.6525
Loop quantum gravity without the Hamiltonian constraint
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn
(Submitted on 29 Mar 2012)
We show that under certain technical assumptions, including a generalisation of CMC foliability and strict positivity of the scalar field, general relativity conformally coupled to a scalar field can be quantised on a partially reduced phase space, meaning reduced only with respect to the Hamiltonian constraint and a proper gauge fixing. More precisely, we introduce, in close analogy to shape dynamics, the generator of a local conformal transformation acting on both, the metric and the scalar field. A new metric, which is invariant under this transformation, is constructed and used to define connection variables which can be quantised by standard loop quantum gravity methods. Since this connection is invariant under the local conformal transformation, the generator of which is shown to be a good gauge fixing for the Hamiltonian constraint, the Dirac bracket associated with implementing these constraints coincides with the Poisson bracket for the connection. Thus, the well developed kinematical quantisation techniques for loop quantum gravity are available, while the Hamiltonian constraint has been solved (more precisely, gauge fixed) classically. The physical interpretation of this system is that of general relativity on a fixed spatial slice, the associated "time" of which is given by the value of the generator of local conformal transformations. While it is hard to address dynamical problems in this framework (due to the complicated "time" function), it seems, due to good accessibility properties of the gauge in certain situations, to be well suited for problems such as the computation of black hole entropy, where actual physical states can be counted and the dynamics is only of indirect importance. Also, the interpretation of the geometric operators gets an interesting twist, which exemplifies the deep relationship between observables and the choice of a time function.
5 pages

http://arxiv.org/abs/1203.6526
On a partially reduced phase space quantisation of general relativity conformally coupled to a scalar field
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn
(Submitted on 29 Mar 2012)
The purpose of this paper is twofold: On the one hand, after a thorough review of the matter free case, we supplement the derivations in our companion paper on "loop quantum gravity without the Hamiltonian constraint" with calculational details and extend the results to standard model matter, a cosmological constant, and non-compact spatial slices. On the other hand, we provide a discussion on the role of observables, focussed on the situation of a symmetry exchange, which is key to our derivation. Furthermore, we comment on the relation of our model to reduced phase space quantisations based on deparametrisation.
51 pages, 5 figures

 

[atyy]

http://arxiv.org/abs/1203.6564
Infrared fixed point in quantum Einstein gravity
S. Nagy, J. Krizsan, K. Sailer
(Submitted on 29 Mar 2012)
We performed the renormalization group analysis of the quantum Einstein gravity in the deep infrared regime for different types of extensions of the model. It is shown that an attractive infrared point exists in the broken symmetric phase of the model. It is also shown that due to the Gaussian fixed point the IR critical exponent $\nu$ of the correlation length is 1/2. However, there exists a certain extension of the model which gives finite correlation length in the broken symmetric phase. It typically appears in case of models possessing a first order phase transitions as is demonstrated on the example of the scalar field theory with a Coleman-Weinberg potential.

 

[marcus]

http://arxiv.org/abs/1203.6688
Observable Equivalence between General Relativity and Shape Dynamics
Tim Koslowski
(Submitted on 29 Mar 2012)
In this conceptual paper we construct a local version of Shape Dynamics that is equivalent to General Relativity in the sense that the algebras of Dirac observables weakly coincide. This allows us to identify Shape Dynamics observables with General Relativity observables, whose observables can now be interpreted as particular representative functions of observables of a conformal theory at fixed York time. An application of the observable equivalence of General Relativity and Shape Dynamics is to define the quantization of General Relativity through quantizing Shape Dynamics and using observable equivalence. We investigate this proposal explicitly for gravity in 2+1 dimensions.
16 pages,

 

[marcus]

http://arxiv.org/abs/1204.0211
Constraint algebra in LQG reloaded : Toy model of a U(1)³ Gauge Theory I
Adam Henderson, Alok Laddha, Casey Tomlin
(Submitted on 1 Apr 2012)
We analyze the issue of anomaly-free representations of the constraint algebra in Loop Quantum Gravity (LQG) in the context of a diffeomorphism-invariant gauge theory in three spacetime dimensions. We construct a Hamiltonian constraint operator whose commutator matches with a quantization of the classical Poisson bracket involving structure functions. Our quantization scheme is based on a geometric interpretation of the Hamiltonian constraint as a generator of phase space-dependent diffeomorphisms. The resulting Hamiltonian constraint at finite triangulation has a conceptual similarity with the "mu-bar"-scheme in loop quantum cosmology and highly intricate action on the spin-network states of the theory. We construct a subspace of non-normalizable states (distributions) on which the continuum Hamiltonian constraint is defined which leads to an anomaly-free representation of the Poisson bracket of two Hamiltonian constraints in loop quantized framework.
60 pages, 6 figures

brief mention:
http://arxiv.org/abs/1204.0492
Non-detection of the Tooth Fairy at Optical Wavelengths

 

[marcus]

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito
(Submitted on 2 Apr 2012)
We achieve a group theoretical quantization of the flat Friedmann-Robertson-Walker model coupled to a massless scalar field adopting the improved dynamics of loop quantum cosmology. Deparemeterizing the system using the scalar field as internal time, we first identify a complete set of phase space observables whose Poisson algebra is isomorphic to the su(1,1) Lie algebra. It is generated by the volume observable and the Hamiltonian. These observables describe faithfully the regularized phase space underlying the loop quantization: they account for the polymerization of the variable conjugate to the volume and for the existence of a kinematical non-vanishing minimum volume. Since the Hamiltonian is an element in the su(1,1) Lie algebra, the dynamics is now implemented as SU(1,1) transformations. At the quantum level, the system is quantized as a time-like irreducible representation of the group SU(1,1). These representations are labeled by a half-integer spin, which gives the minimal volume. They provide superselection sectors without quantization anomalies and no factor ordering ambiguity arises when representing the Hamiltonian. We then explicitly construct SU(1,1) coherent states to study the quantum evolution. They not only provide semiclassical states but truly dynamical coherent states. Their use further clarifies the nature of the bounce that resolves the big bang singularity.
33 pages

http://arxiv.org/abs/1204.0683
Shape dynamics and Mach's principles: Gravity from conformal geometrodynamics
Sean Gryb
(Submitted on 3 Apr 2012)
In this PhD thesis, we develop a new approach to classical gravity starting from Mach's principles and the idea that the local shape of spatial configurations is fundamental. This new theory, "shape dynamics", is equivalent to general relativity but differs in an important respect: shape dynamics is a theory of dynamic conformal 3-geometry, not a theory of spacetime. Equivalence is achieved by trading foliation invariance for local conformal invariance (up to a global scale). After the trading, what is left is a gauge theory invariant under 3d diffeomorphisms and conformal transformations that preserve the volume of space. The local canonical constraints are linear and the constraint algebra closes with structure constants. Shape dynamics, thus, provides a novel new starting point for quantum gravity.
The procedure for the trading of symmetries was inspired by a technique called "best matching". We explain best matching and its relation to Mach's principles. The key features of best matching are illustrated through finite dimensional toy models. A general picture is then established where relational theories are treated as gauge theories on configuration space. Shape dynamics is then constructed by applying best matching to conformal geometry. We then study shape dynamics in more detail by computing its Hamiltonian and Hamilton-Jacobi functional perturbatively.
This thesis is intended as a pedagogical but complete introduction to shape dynamics and the Machian ideas that led to its discovery. The reader is encouraged to start with the introduction, which gives a conceptual outline and links to the relevant sections in the text for a more rigorous exposition. When full rigor is lacking, references to the literature are given. It is hoped that this thesis may provide a starting point for anyone interested in learning about shape dynamics.
117 pages, 2 tables, 10 figures, PhD thesis

http://arxiv.org/abs/1204.0702
Radiation from quantum weakly dynamical horizons in LQG
Daniele Pranzetti
(Submitted on 3 Apr 2012)
Using the recent thermodynamical study of isolated horizons by Ghosh and Perez, we provide a statistical mechanical analysis of isolated horizons near equilibrium in the grand canonical ensemble. By matching the description of the dynamical phase in terms of weakly dynamical horizons with this local statistical framework, we introduce a notion of temperature in terms of the local surface gravity. This provides further support to the recovering of the semiclassical area law just by means of thermodynamical considerations. Moreover, it allows us to study the radiation process generated by the LQG dynamics near the horizon, providing a quantum gravity description of the horizon evaporation. For large black holes, the spectrum we derive presents a discrete structure which could be potentially observable and might be preserved even after the inclusion of all the relevant transition lines.
9 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 5 Apr 2012)
The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB.
4 pages, ICGC (2011) Goa Conference proceedings

http://arxiv.org/abs/1204.0965
Quantum-gravity-induced matter self-interactions in the asymptotic-safety scenario
Astrid Eichhorn
(Submitted on 4 Apr 2012)
We investigate the high-energy properties of matter theories coupled to quantum gravity. Specifically, we show that quantum gravity fluctuations generically induce matter self-interactions in a scalar theory. Our calculations apply within asymptotically safe quantum gravity, where our results indicate that the UV is dominated by an interacting fixed point, with non-vanishing gravitational as well as matter couplings. We show that the number of relevant directions of the fixed point depends on the inclusion of these quantum-gravity induced matter self-interactions. Furthermore we point out that terms of this type can have observable consequences in the context of scalar-field driven inflation, where they can induce potentially observable non-Gaussianities in the CMB.
15 pages, 9 figures

Rovelli's introductory QG series "Explorations in quantum gravity" http://pirsa.org/C12012
Lecture 1: http://pirsa.org/12040019/
Lecture 2: http://pirsa.org/12040020/
Lecture 3: http://pirsa.org/12040021/
Lecture 4: http://pirsa.org/12040022/

 

[marcus]

http://arxiv.org/abs/1204.2550
Diffusion in quantum gravity
Gianluca Calcagni
(Submitted on 11 Apr 2012)
The change of the effective dimension of spacetime with the probed scale is a universal phenomenon shared by independent models of quantum gravity. Using tools of probability theory and multifractal geometry, we show how dimensional flow is controlled by a multiscale fractional diffusion equation, and physically interpreted as a composite stochastic process. The simplest example is a fractional telegraph process, describing quantum spacetimes with a spectral dimension equal to 2 in the ultraviolet and monotonically rising to 4 towards the infrared.
4 pages, 1 figure

http://arxiv.org/abs/1204.1530
Hawking radiation from dynamical horizons
Ayan Chatterjee, Bhramar Chatterjee, Amit Ghosh
(Submitted on 6 Apr 2012)
In completely local settings, we establish that a dynamically evolving black hole horizon can be assigned a Hawking temperature. Moreover, we calculate the Hawking flux and show that the radius of the horizon shrinks.
5 Pages

brief mention:
http://arxiv.org/abs/1204.2520
Quantum Black Holes from Cosmic Rays
Xavier Calmet, Lauretiu Ioan Caramete, Octavian Micu
(Submitted on 11 Apr 2012)
We investigate the possibility for cosmic ray experiments to discover non-thermal small black holes with masses in the TeV range. Such black holes would result due to the impact between ultra high energy cosmic rays or neutrinos with nuclei from the upper atmosphere and decay instantaneously. They could be produced copiously if the Planck scale is in the few TeV region. As their masses are close to the Planck scale, these holes would typically decay into two particles emitted back-to-back. Depending on the angles between the emitted particles with respect to the center of mass direction of motion, it is possible for the simultaneous showers to be measured by the detectors.
4 pages, 1 figure

 

[atyy]

http://arxiv.org/abs/1112.0302
Spacetime as a topological insulator: Mechanism for the origin of the fermion generations
David B. Kaplan, Sichun Sun
(Submitted on 1 Dec 2011 (v1), last revised 13 Apr 2012 (this version, v3))
We suggest a mechanism whereby the three generations of quarks and leptons correspond to surface modes in a five-dimensional theory. These modes arise from a nonlinear fermion dispersion relation in the extra dimension, much in the same manner as fermion surface modes in a topological insulator or lattice implementation of domain wall fermions. We also show that the topological properties can persist in a deconstructed version of the model in four dimensions.

 

[marcus]

http://arxiv.org/abs/1204.3039
The analogue cosmological constant in Bose-Einstein condensates: a lesson for quantum gravity
Stefano Finazzi, Stefano Liberati, Lorenzo Sindoni
(Submitted on 13 Apr 2012)
For almost a century, the cosmological constant has been a mysterious object, in relation to both its origin and its very small value. By using a Bose-Einstein condensate analogue model for gravitational dynamics, we address here the cosmological constant issue from an analogue gravity standpoint. Starting from the fundamental equations describing a system of condensed bosons, we highlight the presence of a vacuum source term for the analogue gravitational field, playing the role of a cosmological constant. In this simple system it is possible to compute from scratch the value of this constant, to compare it with other characteristic energy scales and hence address the problem of its magnitude within this framework, suggesting a different path for the solution of this longstanding puzzle. We find that, even though this constant term is related with quantum vacuum effects, it is not immediately related to the ground state energy of the condensate. On the gravity side this result suggests that the interpretation and computation of the cosmological term as a form of renormalized vacuum energy might be misleading, its origin being related to the mechanism that instead produces spacetime from its pregeometric progenitor, shedding a different light on the subject and at the same time suggesting a potentially relevant role of analogue models in the understanding of quantum gravity.
24 pages, 1 figure, Proceedings of the II Amazonian Symposium on Physics

http://arxiv.org/abs/1204.3505
Conserved quantities in isotropic loop quantum cosmology
Daniel Cartin
(Submitted on 16 Apr 2012)
We develop an action principle for those models arising from isotropic loop quantum cosmology, and show that there is a natural conserved quantity Q for the discrete difference equation arising from the Hamiltonian constraint. This quantity Q relates the semi-classical limit of the wavefunction at large values of the spatial volume, but opposite triad orientations. Moreover, there is a similar quantity for generic difference equations of one parameter arising from a self-adjoint operator.
6 pages, to be published in Europhysics Letters

http://arxiv.org/abs/1204.3541
The local potential approximation in quantum gravity
Dario Benedetti, Francesco Caravelli
(Submitted on 16 Apr 2012)
Within the context of the functional renormalization group flow of gravity, we suggest that a generic f(R) ansatz (i.e. not truncated to any specific form, polynomial or not) for the effective action plays a role analogous to the local potential approximation (LPA) in scalar field theory. In the same spirit of the LPA, we derive and study an ordinary differential equation for f(R) to be satisfied by a fixed point of the renormalization group flow. As a first step in trying to assess the existence of global solutions (i.e. true fixed point) for such equation, we investigate here the properties of its solutions by a comparison of various series expansions and numerical integrations. In particular, we study the analyticity conditions required because of the presence of fixed singularities in the equation, and we develop an expansion of the solutions for large R up to order N=29. Studying the convergence of the fixed points of the truncated solutions with respect to N, we find a characteristic pattern for the location of the fixed points in the complex plane, with one point stemming out for its stability. Finally, we establish that if a non-Gaussian fixed point exists within the full f(R) approximation, it corresponds to an R² theory.
31 pages, 7 figures

http://arxiv.org/abs/1204.3531
Interacting Generalised Cosmic Chaplygin gas in Loop quantum cosmology: A singularity free universe
Ratul Chowdhury (Jadavpur U.), Prabir Rudra (Bengal Engin. Sci. U., Howrah)
(Submitted on 13 Apr 2012)
In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by Loop quantum cosmology. Dark energy in the form of Generalised Cosmic Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is taken into account in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence problem is taken. The statefinder parameters are also calculated to classify the dark energy model. Graphs and phase diagrams are drawn to study the variations of these parameters. It is seen that the background dynamics of Generalised Cosmic Chaplygin gas is completely consistent with the notion of an accelerated expansion in the late universe. From the graphs, generalised cosmic Chaplygin gas is identified as a dark fluid with a lesser negative pressure compared to Modified Chaplygin gas, thus supporting a 'No Big Rip' cosmology. It has also been shown that in this model the universe follows the power law form of expansion around the critical point, which is consistent with the known results. Future singularities that may be formed in this model as an ultimate fate of the universe has been studied in detail. It was found that the model is completely free from any types of future singularities.
10 pages, 10 figures

brief mention (of general interest though not directly LoopQG-related)
http://arxiv.org/abs/1204.3138
Quantization of area for event and Cauchy horizons of the Kerr-Newman black hole
Matt Visser (Victoria University of Wellington)
(Submitted on 14 Apr 2012)
Based on various string theoretic constructions, there have been repeated suggestions that the areas of black hole event horizons should be quantized in a quite specific manner, involving linear combinations of square roots of natural numbers. It is important to realize the significant physical limitations of such proposals when one attempts to extend them outside their original framework. Specifically, in their most natural and direct physical interpretations, these specific proposals for horizon areas fail for the ordinary Kerr-Newman black holes in (3+1) dimensions, essentially because the fine structure constant is not an integer. A more baroque interpretation involves asserting the fine structure constant is the square root of a rational number; but such a proposal has its own problems. Insofar as one takes (3+1) general relativity (plus the usual quantization of angular momentum and electric charge) as being paramount, the known explicitly calculable spectra of horizon areas for the physically compelling Kerr-Newman spacetimes do not resemble those of currently available string theoretic constructions.
15 pages

 

[marcus]

http://arxiv.org/abs/1204.4345
The spacetime in the neighborhood of a general isolated black hole
Badri Krishnan
(Submitted on 19 Apr 2012)
We construct the spacetime in the vicinity of a general isolated, rotating, charged black hole. The black hole is modeled as a weakly isolated horizon, and we use the characteristic initial value formulation of the Einstein equations with the horizon as an inner boundary. The spacetime metric and other geometric fields are expanded in a power series in a radial coordinate away from the horizon by solving the characteristic field equations in the Newman-Penrose formalism. This is the first in a series of papers which investigate the near horizon geometry and its physical applications using the isolated horizon framework.
23 pages, 1 figure

http://arxiv.org/abs/1204.4344
Reply to the comment on "Black hole entropy and isolated horizons thermodynamics"
Amit Ghosh, Alejandro Perez
(Submitted on 19 Apr 2012)
The comment http://arxiv.org/abs/1204.2729v1 is completely wrong. The author makes serious mistakes in calculations and judgement...

brief mention:
http://arxiv.org/abs/1204.4339
Teleparallel Gravity as a Higher Gauge Theory
John C. Baez, Derek K. Wise
(Submitted on 19 Apr 2012)
We show that general relativity can be viewed as a higher gauge theory involving a categorical group, or 2-group, called the teleparallel 2-group. On any semi-Riemannian manifold M, we first construct a principal 2-bundle with the Poincaré 2-group as its structure 2-group. Any flat metric-preserving connection on M gives a flat 2-connection on this 2-bundle, and the key ingredient of this 2-connection is the torsion. Conversely, every flat strict 2-connection on this 2-bundle arises in this way if M is simply connected and has vanishing 2nd deRham cohomology. Extending from the Poincaré 2-group to the teleparallel 2-group, a 2-connection includes an additional piece: a coframe field. Taking advantage of the teleparallel reformulation of general relativity, in which a coframe field, a flat connection and its torsion are the key ingredients, this let's us rewrite general relativity as a theory with a 2-connection for the teleparallel 2-group as its only field.
35 pages

 

[marcus]

Unfortunately not available online, a talk given today at Princeton Institute for Advanced Studies:
Monday, April 23, 2012
High Energy Theory Seminar
“Loop Quantum Gravity: Recent Results and Open Problems”
Location: Bloomberg Lecture Hall
Time: 2:30 PM
Speaker(s): Carlo Rovelli, Centre de Physique Théorique de Luminy, Aix-Marseille University, France
Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit.
http://www.princeton.edu/physics/events/viewevent.xml?id=347

http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi
(Submitted on 23 Apr 2012)
We compute the entropy of non-extremal black holes using the quantum dynamics of Loop Gravity. The horizon entropy is finite, scales linearly with the area A, and reproduces the Bekenstein-Hawking expression S = A/4 with the one-fourth coefficient for all values of the Immirzi parameter. The near-horizon geometry of a non-extremal black hole - as seen by a stationary observer - is described by a Rindler horizon. We introduce the notion of a quantum Rindler horizon in the framework of Loop Gravity. The system is described by a quantum surface and the dynamics is generated by the boost Hamiltonion of Lorentzian Spinfoams. We show that the expectation value of the boost Hamiltonian reproduces the local horizon energy of Frodden, Ghosh and Perez. We study the coupling of the geometry of the quantum horizon to a two-level system and show that it thermalizes to the local Unruh temperature. The derived values of the energy and the temperature allow one to compute the thermodynamic entropy of the quantum horizon. The relation with the Spinfoam partition function is discussed.
6 pages, 1 figure

brief mention--not Loop-and-allied QG, but possibly of general interest:

http://arxiv.org/abs/1204.4926
Relating the quantum mechanics of discrete systems to standard canonical quantum mechanics
Gerard 't Hooft
(Submitted on 22 Apr 2012)
Discrete quantum mechanics is here defined to be a quantum theory of wave functions defined on integers P_i and Q_i, while canonical quantum mechanics is assumed to be based on wave functions on the real numbers, R^n. We study reversible mappings from the position operators q_i and their quantum canonical operators p_i of a canonical theory, onto the discrete, commuting operators Q_i and P_i. In this paper we are particularly interested in harmonic oscillators. In the discrete system, these turn into deterministic models, which is our motivation for this study. We regard the procedure worked out here as a "canonical formalism" for discrete dynamics, and as a stepping stone to handling discrete deterministic systems in a quantum formalism.
20 pages (incl. title page), 2 figures

http://arxiv.org/abs/1204.4683
A Long View of Particle Physics
Frank Wilczek
(Submitted on 20 Apr 2012)
2011 marked the hundredth anniversary both of the famous Solvay conferences, and of the Geiger-Marsden experiment that launched the modern understanding of subatomic structure. I was asked to survey the status and prospects of particle physics for the anniversary Solvay conference, with appropriate perspective. This is my attempt.
8 pages, no figures. Rapporteur talk at the 25th Solvay Conference on Physics, "Theory of the Quantum World", October 2011. To be published in the Proceedings

 

[negru]

I was at the talk at IAS, everyone was pretty confused by what he was doing. One point brought up was that there exist other models, like YM in 5d i think, whose discrete version has the correct classical limit and is uv and ir finite, but does not make sense quantum mechanically. And there was no concrete argument for why lqg would be a better example. Some numerical checks are needed, and he said they are very hard to do but people are working on them.

 

[MTd2]

This thread is for bibliographical help. You might wish to open a thread to tell your experience.

 

[marcus]

http://arxiv.org/abs/1204.5394
Discrete gravity models and Loop Quantum Gravity: a short review
Maite Dupuis, James P. Ryan, Simone Speziale
(Submitted on 24 Apr 2012)
We review the relation between loop quantum gravity on a fixed graph and discrete models of gravity. We compare Regge and twisted geometries, and discuss discrete actions based on twisted geometries and on the discretization of the Plebanski action. We discuss the role of discrete geometries in the spin foam formalism, with particular attention to the definition of the simplicity constraints.
32 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

brief mention:
http://arxiv.org/abs/1204.5455
Quantization of Lorentzian 3d Gravity by Partial Gauge Fixing
Rodrigo M S Barbosa, Clisthenis P Constantinidis, Zui Oporto, Olivier Piguet
(Submitted on 24 Apr 2012)
D = 2+1 gravity with a cosmological constant has been shown by Bonzom and Livine to present a Barbero-Immirzi like ambiguity depending on a parameter. We make use of this fact to show that, for positive cosmological constant, the Lorentzian theory can be partially gauge fixed and reduced to an SU(2) Chern-Simons theory...
19 pages

 

[marcus]

http://arxiv.org/abs/1204.6349
Gravitation and vacuum entanglement entropy
Ted Jacobson
(Submitted on 28 Apr 2012)
The vacuum of quantum fields contains correlated fluctuations. When restricted to one side of a surface these have a huge entropy of entanglement that scales with the surface area. If UV physics renders this entropy finite, then a thermodynamic argument implies the existence of gravity. That is, the causal structure of spacetime must be dynamical and governed by the Einstein equation with Newton's constant inversely proportional to the entropy density. Conversely, the existence of gravity makes the entanglement entropy finite. This thermodynamic reasoning is powerful despite the lack of a detailed description of the dynamics at the cutoff scale, but it has its limitations. In particular, we should not expect to understand corrections to Einstein gravity in this way.
9 pages; Essay written for the Gravity Research Foundation 2012 Awards for Essays on Gravitation

 

[marcus]

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 3 May 2012)
We study time-reversal and parity ---on the physical manifold and in internal space--- in covariant loop gravity. We consider a minor modification of the Holst action which makes it transform coherently under such transformations. The classical theory is not affected but the quantum theory is slightly different. In particular, the simplicity constraints are slightly modified and this restricts orientation flips in a spinfoam to occur only across degenerate regions, thus reducing the sources of potential divergences.
8 pages

http://pirsa.org/12050050 (online talk)
Shape Dynamics and General Relativity
Speaker(s): Julian Barbour
Abstract: Shape Dynamics first arose as a theory of particle interactions formulated without any of Newton's absolute structures. Its fundamental arena is shape space, which is obtained by quotienting Newton's kinematic framework with respect to translations, rotations and dilatations. This leads to a universe defined purely intrinsically in relational terms. It is then postulated that a dynamical history is determined by the specification in shape space of an initial shape and an associated rate of change of shape. There is a very natural way to create a theory that meets such a requirement. It fully implements Mach's principle and shows how time and local inertial frames are determined by the universe as whole. If the same principles are applied to a spatially closed universe in which geometry is dynamical, they lead rather surprisingly to a theory that, modulo some caveats, is dynamically equivalent to general relativity but dual to it in that refoliation invariance is traded for three-dimensional conformal invariance. This shows that there is a hidden three-dimensional conformal symmetry within general relativity. It is in fact what underlies York's crucial method of solution of the initial-value problem in general relativity. It is also remarkable that, as in York's work, shape dynamics inescapably introduces a mathematically distinguished notion of absolute simultaneity, the desirability of which has been found in two currently popular approaches to quantum gravity: causal dynamical triangulations and Horava gravity. I aim to express the key ideas and techniques of shape dynamics as simply as possible.
Date: 09/05/2012 - 2:00 pm
Series: Colloquium

 

[atyy]

http://arxiv.org/abs/1205.0911
The new spin foam models and quantum gravity
Alejandro Perez
(Submitted on 4 May 2012)
In this article we give a systematic definition of the recently introduced spin foam models for four dimensional quantum gravity reviewing the main results on their semiclassical limit on fixed discretizations.

 

[tom.stoer]

Seems to be ay very interesting paper - especially as the author points out that the construction of the SF measure due to the presence of second-class constraints is still a matter of debate and by no means fully understood.

 

[tom.stoer]

Seems to be ay very interesting paper - especially as the author points out that the construction of the SF measure due to the presence of second-class constraints is still a matter of debate and by no means fully understood.

 

[marcus]

http://arxiv.org/abs/1205.1229
Second- and First-Order Phase Transitions in CDT
J. Ambjorn, S. Jordan, J. Jurkiewicz, R. Loll
(Submitted on 6 May 2012)
Causal Dynamical Triangulations (CDT) is a proposal for a theory of quantum gravity, which implements a path-integral quantization of gravity as the continuum limit of a sum over piecewise flat spacetime geometries. We use Monte Carlo simulations to analyse the phase transition lines bordering the physically interesting de Sitter phase of the four-dimensional CDT model. Using a range of numerical criteria, we present strong evidence that the so-called A-C transition is first order, while the B-C transition is second order. The presence of a second-order transition may be related to an ultraviolet fixed point of quantum gravity and thus provide the key to probing physics at and possibly beyond the Planck scale.
24 pages, 9 figures

http://arxiv.org/abs/1205.1304
Curved Momentum Space and Relative Locality
Jerzy Kowalski-Glikman
(Submitted on 7 May 2012)
I briefly discuss the construction of a theory of particles with curved momentum space and its consequence, the principle of relative locality.
10 pages; based on the talk given at 29-th Max Born Symposium, to appear in the proceedings

not Loop-or-allied QG, but possibly of general interest:
http://arxiv.org/abs/1205.1256
Relationalism
Edward Anderson
(Submitted on 6 May 2012)
This article contributes to the debate of the meaning of relationalism and background independence, which has remained of interest in theoretical physics from Newton versus Leibniz through to foundational issues for today's leading candidate theories of quantum gravity. I contrast and compose the substantially different Leibniz--Mach--Barbour (LMB) and Rovelli--Crane (RC) uses of the word 'relational'. Leibniz advocated primary timelessness and Mach that 'time is to be abstracted from change'. I consider 3 distinct viewpoints on Machian time: Barbour's, Rovelli's and my own. I provide four expansions on Barbour's taking configuration space to be primary: to (perhaps a weakened notion of) phase space, categorizing, perspecting and propositioning. Categorizing means considering not only object spaces but also the corresponding morphisms and then functors between such pairs. Perspecting means considering the set of subsystem perspectives; this is an arena in which the LMB and Rovelli approaches make contact. By propositioning, I mean considering the set of propositions about a physical (sub)system. I argue against categorization being more than a formal pre-requisite for quantization in general; however, perspecting is a categorical operation, and propositioning leads one to considering topoi, with Isham and Doering's work represents one possibility for a mathematically sharp implementation of propositioning. Further applications of this article are arguing for Ashtekar variables as being relational in LMB as well as just the usually-ascribed RC sense, relationalism versus supersymmetry, string theory and M-theory. The question of whether scale is relational is also considered, with quantum cosmology in mind.
39 pages, 4 figures.

 

[MTd2]

http://arxiv.org/abs/1205.1788

Clockwork Quantum Universe

Donatello Dolce
(Submitted on 7 May 2012)
Besides the purely digital or analog interpretation of reality there is a third possibility which incorporates important aspects of both. This is the cyclic formulation of elementary systems, in which elementary particles are represented as classical strings vibrating in compact space-time dimensions with periodic boundary conditions. We will address these cyclic solutions as "de Broglie internal clocks". They constitute the deterministic gears of a consistent semi-classical description of quantum relativistic physics, providing in addition an appealing formulation of the notion of time.
http://arxiv.org/abs/1205.1636

Born's prophecy leaves no space for quantum gravity

Giovanni Amelino-Camelia
(Submitted on 8 May 2012)
I stress that spacetime is a redundant abstraction, since describing the physical content of all so-called "space-time measurements" only requires timing (by a physical/material clock) of particle detections (at a physical/material detector). It is interesting then to establish which aspects of our current theories afford us the convenient abstraction of a spacetime. I emphasize the role played by the assumed triviality of the geometry of momentum space, which makes room for an observer-independent notion of locality. This is relevant for some recent studies of the quantum-gravity problem that stumbled upon hints of a nontrivial geometry of momentum space, something which had been strikingly envisaged for quantum gravity already in 1938 by Max Born. If indeed momentum space has nontrivial geometry then the abstraction of a spacetime becomes more evidently redundant and less convenient: one may still abstract a spacetime but only allowing for the possibility of a relativity of spacetime locality. I also provide some examples of how all this could affect our attitude toward the quantum-gravity problem, including some for the program of emergent gravity and emergent spacetime and an indication of triviality of the holographic description of black holes. And in order to give an illustrative example of possible logical path for the "disappearance of spacetime" I rely on formulas inspired by the $\kappa$-Poincar\'e framework.