Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1103.1900
Towards Renormalizing Group Field Theory
Vincent Rivasseau
(Submitted on 9 Mar 2011)
"We review some aspects of non commutative quantum field theory and group field theory, in particular recent progress on the systematic study of the scaling and renormalization properties of group field theory. We thank G. Zoupanos and the organizers of the Corfu 2010 Workshop on Noncommutative Field Theory and Gravity for encouraging us to write this review."

http://arxiv.org/abs/1103.2098
Quantum Mechanics on SO(3) via Non-commutative Dual Variables
Daniele Oriti, Matti Raasakka
27 pages
(Submitted on 10 Mar 2011)
"We formulate quantum mechanics on SO(3) using a non-commutative dual space representation for the quantum states, inspired by recent work in quantum gravity. The new non-commutative variables have a clear connection to the corresponding classical variables, and our analysis confirms them as the natural phase space variables, both mathematically and physically. In particular, we derive the first order (Hamiltonian) path integral in terms of the non-commutative variables, as a formulation of the transition amplitudes alternative to that based on harmonic analysis. We find that the non-trivial phase space structure gives naturally rise to quantum corrections to the action for which we find a closed expression. We then study both the semi-classical approximation of the first order path integral and the example of a free particle on SO(3). On the basis of these results, we comment on the relevance of similar structures and methods for more complicated theories with group-based configuration spaces, such as Loop Quantum Gravity and Spin Foam models."


Brief mention (not Loop-and-allied QG but possibly of general interest):
http://arxiv.org/abs/1103.1879
Disproof of Bell's Theorem
Joy Christian (Oxford)
1 page; Forthcoming in a FQXi sponsored book on Bell's Theorem and Quantum Entanglement (2011)
(Submitted on 9 Mar 2011)
"We illustrate an explicit counterexample to Bell's theorem by constructing a pair of dichotomic variables that exactly reproduce the EPR-Bohm correlations in a manifestly local-realistic manner."

 

[marcus]

http://arxiv.org/abs/1103.2475
Probability of Inflation in Loop Quantum Cosmology
Abhay Ashtekar, David Sloan
34 pages, 3 figures
(Submitted on 12 Mar 2011)
"Inflationary models of the early universe provide a natural mechanism for the formation of large scale structure. This success brings to forefront the question of naturalness: Does a sufficiently long slow roll inflation occur generically or does it require a careful fine tuning of initial parameters? In recent years there has been considerable controversy on this issue. In particular, for a quadratic potential, Kofman, Linde and Mukhanov have argued that the probability of inflation with at least 65 e-foldings is close to one, while Gibbons and Turok have argued that this probability is suppressed by a factor of ~ 10^-85. We first clarify that such dramatically different predictions can arise because the required measure on the space of solutions is intrinsically ambiguous in general relativity. We then show that this ambiguity can be naturally resolved in loop quantum cosmology (LQC) because the big bang is replaced by a big bounce and the bounce surface can be used to introduce the structure necessary to specify a satisfactory measure.
The second goal of the paper is to present a detailed analysis of the inflationary dynamics of LQC using analytical and numerical methods. By combining this information with the measure on the space of solutions, we address a sharper question than those investigated in the literature: What is the probability of a sufficiently long slow roll inflation WHICH IS COMPATIBLE WITH THE SEVEN YEAR WMAP DATA? We show that the probability is very close to 1.
The material is so organized that cosmologists who may be more interested in the inflationary dynamics in LQC than in the subtleties associated with measures can skip that material without loss of continuity."

http://arxiv.org/abs/1103.2723
The SU(2) Black Hole entropy revisited
Jonathan Engle, Karim Noui, Alejandro Perez, Daniele Pranzetti
31 pages, 8 figures
(Submitted on 14 Mar 2011)
"We study the state-counting problem that arises in the SU(2) black hole entropy calculation in loop quantum gravity. More precisely, we compute the leading term and the logarithmic correction of both the spherically symmetric and the distorted SU(2) black holes. Contrary to what has been done in previous works, we have to take into account "quantum corrections" in our framework in the sense that the level k of the Chern-Simons theory which describes the black hole is finite and not sent to infinity. Therefore, the new results presented here allow for the computation of the entropy in models where the quantum group corrections are important."

 

[marcus]

http://arxiv.org/abs/1103.2971
Gravity as a constrained BF theory: Noether charges and Immirzi parameter
R. Durka, J. Kowalski-Glikman
5 pages
(Submitted on 15 Mar 2011)
"We derive and analyze Noether charges associated with the diffeomorphism invariance for the constrained SO(2,3) BF theory. This result generalizes the Wald approach to the case of the first order gravity with a negative cosmological constant, the Holst modification and topological terms (Nieh-Yan, Euler, and Pontryagin). We show that differentiability of the action is automatically implemented by the the structure of the constrained BF model. Finally, we calculate the AdS--Schwarzschild black hole entropy from the Noether charge and we find that, unexpectedly, it does not depend on the Immirzi parameter."

 

[marcus]

http://arxiv.org/abs/1103.3149
Inflation in asymptotically safe f(R) theory
Adriano Contillo
Presented at 14th Conference on Recent Developments in Gravity: Ioannina, Greece, 8-11 Jun 2010
(Submitted on 16 Mar 2011)
"We discuss the existence of inflationary solutions in a class of renormalization group improved polynomial f(R) theories, which have been studied recently in the context of the asymptotic safety scenario for quantum gravity. These theories seem to possesses a nontrivial ultraviolet fixed point, where the dimensionful couplings scale according to their canonical dimensionality. Assuming that the cutoff is proportional to the Hubble parameter, we obtain modified Friedmann equations which admit both power law and exponential solutions. We establish that for sufficiently high order polynomial the solutions are reliable, in the sense that considering still higher order polynomials is very unlikely to change the solution."

 

[marcus]

http://arxiv.org/abs/1103.3415
Yet Another Recursion Relation for the 6j-Symbol
Valentin Bonzom, Etera R. Livine
7 pages
(Submitted on 17 Mar 2011)
"In the context of spinfoam path integral models for quantum gravity, we provide a new way to derive recursion relations for the 6j-symbol. This basic object from the recoupling theory of SU(2) representations is the building block of the Ponzano-Regge amplitudes for 3d quantum gravity. From this perspective, such recursion relations are understood to be not only useful for numerical study of the spinfoam amplitudes but also have been shown to be deeply related to the symmetries and the dynamics of the theory. Here, we show how to use the expression of the square of the 6j-symbol as a integral over SU(2) to derive a recursion relation and we discuss how to generalize this method to derive more general recursion relations on spinfoam amplitudes."

 

[marcus]

http://arxiv.org/abs/1103.3679
de Sitter relativity: a natural scenario for an evolving Lambda
J. P. Beltran Almeida, C. S. O. Mayor, J. G. Pereira
11 pages
(Submitted on 18 Mar 2011)
"Both cosmology and quantum gravity seem to require the existence of an invariant length-parameter at their scales. The most simple and natural way to introduce such parameter without spoiling the Lorentz symmetry is arguably to replace ordinary (Poincare-based) special relativity by de Sitter special relativity. In fact, the de Sitter group has Lorentz as subgroup, and at the same time involves an invariant length-parameter related to the cosmological term, whose (kinematic) source turns out to be the conformal current of ordinary matter. When applied to the quantum gravity scale, the de Sitter special relativity naturally endows spacetime with a holographic structure. When applied to the whole universe, it provides a natural scenario for the existence of an evolving Lambda, as well as an explanation for its currently observed value."

 

[MTd2]

http://arxiv.org/abs/1103.4057

A Lorentz-Covariant Connection for Canonical Gravity

Marc Geiller, Marc Lachieze-Rey, Karim Noui, Francesco Sardelli
(Submitted on 21 Mar 2011)
We construct a Lorentz-covariant connection in the context of first order canonical gravity with non-vanishing Barbero-Immirzi parameter. To do so, we start with the phase space formulation derived from the canonical analysis of the Holst action in which the second class constraints have been solved explictely. This allows us to avoid the use of Dirac brackets. In this context, we show that there is a "unique" Lorentz-covariant connection which is commutative in the sense of the Poisson bracket, and which furthermore agrees with the connection found by Alexandrov using the Dirac bracket. This result opens a new way toward the understanding of Lorentz-covariant loop quantum gravity.

 

[marcus]

http://arxiv.org/abs/1103.3961
Bubble divergences: sorting out topology from cell structure
Valentin Bonzom, Matteo Smerlak
19 pages
(Submitted on 21 Mar 2011)
"We conclude our analysis of bubble divergences in the flat spinfoam model. In [arXiv:1008.1476] we showed that the divergence degree of an arbitrary two-complex Gamma can be evaluated exactly by means of twisted cohomology. Here, we specialize this result to the case where Gamma is the two-skeleton of the cell decomposition of a pseudomanifold, and sharpen it with a careful analysis of the cellular and topological structures involved. Moreover, we explain in detail how this approach reproduces all the previous powercounting results for the Boulatov-Ooguri (colored) tensor models, and sheds light on algebraic-topological aspects of Gurau's 1/N expansion."

 

[marcus]

http://arxiv.org/abs/1103.4172
A field-theoretic approach to Spin Foam models in Quantum Gravity
Patrizia Vitale
16 pages, 3 figures. Proceedings of the Workshop on Non Commutative Field Theory and Gravity, September 8-12, 2010 Corfu Greece
(Submitted on 21 Mar 2011)
"We present an introduction to Group Field Theory models, motivating them on the basis of their relationship with discretized BF models of gravity. We derive the Feynman rules and compute quantum corrections in the coherent states basis."

Brief mention:
http://arxiv.org/abs/1103.4192
On the mass of the Universe born in a black hole
Nikodem J. Poplawski

 

[marcus]

http://arxiv.org/abs/1103.4498
New Action Principle for General Relativity
Kirill Krasnov
4 pages
(Submitted on 23 Mar 2011)
"General Relativity can be reformulated as a diffeomorphism invariant SU(2) gauge theory. A new action principle for this 'pure connection' formulation of GR is described."

http://arxiv.org/abs/1103.4602
Curvature in spinfoams
Elena Magliaro, Claudio Perini
6 pages, 3 figures
(Submitted on 23 Mar 2011)
"We consider spinfoam quantum gravity. We show in a simple case that the amplitude projects over a nontrivial (curved) classical geometry. This suggests that, at least for spinfoams without bubbles and for large values of the boundary spins, the amplitude takes the form of a path integral over Regge metrics, thus enforcing discrete Einstein equations in the classical limit."

 

[atyy]

http://arxiv.org/abs/1103.4841
The cosmological constant: a lesson from Bose-Einstein condensates
Stefano Finazzi, Stefano Liberati, Lorenzo Sindoni
(Submitted on 24 Mar 2011)
The cosmological constant is one of the most pressing problems in modern physics. In this Letter, we address the issue of its nature and computation using an analogue gravity standpoint as a toy model for an emergent gravity scenario. Even if it is well known that phonons in some condense matter systems propagate like a quantum field on a curved spacetime, only recently it has been shown that the dynamics of the analogue metric in a Bose-Einstein condensate can be described by a Poisson-like equation with a vacuum source term reminiscent of a cosmological constant. Here we directly compute this term and confront it with the other energy scales of the system. On the gravity side of the analogy, this model suggests that in emergent gravity scenarios it is natural for the cosmological constant to be much smaller than its naif value computed as the zero-point energy of the emergent effective field theory. The striking outcome of our investigation is that the value of this constant cannot be easily predicted by just looking at the ground state energy of the microscopic system from which spacetime and its dynamics should emerge. A proper computation would require the knowledge of both the full microscopic quantum theory and a detailed understanding about how Einstein equations emerge from such a fundamental theory. In this light, the cosmological constant appears even more a decisive test bench for any quantum/emergent gravity scenario.

 

[marcus]

http://arxiv.org/abs/1103.4967
Quantum Gravitational Contributions to the CMB Anisotropy Spectrum
Claus Kiefer, Manuel Kraemer
4 pages, 1 figure
(Submitted on 25 Mar 2011)
"We derive the primordial power spectrum of density fluctuations in the framework of quantum cosmology. For this purpose, we perform a Born-Oppenheimer approximation to the Wheeler-DeWitt equation for an inflationary universe with a scalar field. In this way we first recover the scale-invariant power spectrum that is found as an approximation in the simplest inflationary models. We then obtain quantum gravitational corrections to this spectrum and discuss whether they lead to measurable signatures in the CMB anisotropy spectrum. The non-observation so far of such corrections translates into an upper bound on the energy scale of inflation."

 

[marcus]

http://arxiv.org/abs/1103.5134
A Gravitational Mechanism for Cosmological Screening
N. C. Tsamis, R. P. Woodard
(Submitted on 26 Mar 2011)
"Infrared gravitons are continually produced during inflation. Like all particles, their contribution to the vacuum energy comes not only from their bare kinetic energy but also from the interactions they have with other gravitons. These interactions can be substantial -- despite the particles being highly infrared -- because they occur over the enormous spatial volume of the universe. Furthermore, the interactions grow with time evolution because more and more such gravitons come into causal contact with one another. Since gravity is universally attractive, these interactions can act to slow and eventually stop accelerated expansion."

brief mention:
http://arxiv.org/abs/1103.5331
What do we really know about Dark Energy?
Ruth Durrer
14p 2 figs. Invited talk at the meeting "Cosmological Tests of General Relativity" at the Kavli Royal Society Center for the Advancement of Science.
(Submitted on 28 Mar 2011)
"In this paper I discuss what we truly know about dark energy. I shall argue that up to date our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, CMB anisotropies and observations of baryon acoustic oscillations, they all simply tell us that the observed distance to a given redshift is larger than the one expected from a Friedmann Lemaitre universe with matter only and the locally measured Hubble parameter."

 

[marcus]

http://relativity.phys.lsu.edu/ilqgs/fairbairn031511.pdf
http://relativity.phys.lsu.edu/ilqgs/fairbairn031511.wav
Quantum deformation of 4d spin foam models
This is an excellent clear introduction to the subject matter covered in the paper:
http://arxiv.org/abs/1012.4784
Quantum deformation of two four-dimensional spin foam models
Winston J. Fairbairn, Catherine Meusburger
45 pages, 4 figures
(Submitted on 21 Dec 2010)
"We construct the q-deformed version of two four-dimensional spin foam models, the Euclidean and Lorentzian versions of the EPRL model. The q-deformed models are based on the representation theory of two copies of U_q(su(2)) at a root of unity and on the quantum Lorentz group with a real deformation parameter. For both models we give a definition of the quantum EPRL intertwiners, study their convergence and braiding properties and construct an amplitude for the four-simplexes. We find that both of the resulting models are convergent."

http://arxiv.org/abs/1103.5626
Gamma ray burst delay times probe the geometry of momentum space
Laurent Freidel, Lee Smolin
21 pages, 13 figures
(Submitted on 29 Mar 2011)
"We study the application of the recently proposed framework of relative locality to the problem of energy dependent delays of arrival times of photons that are produced simultaneously in distant events such as gamma ray bursts. Within this framework, possible modifications of special relativity are coded in the geometry of momentum space. The metric of momentum space codes modifications in the energy momentum relation, while the connection on momentum space describes possible non-linear modifications in the laws of conservation of energy and momentum. In this paper, we study effects of first order in the inverse Planck scale, which are coded in the torsion and non-metricity of momentum space. We find that time delays of order Distance * Energies/m_p are coded in the non-metricity of momentum space. Current experimental bounds on such time delays hence bound the components of this tensor of order 1/m_p. We also find a new effect, whereby photons from distant sources can appear to arrive from angles slightly off the direction to the sources, which we call gravitational lensing. This is found to be coded into the torsion of momentum space."

brief mention:
http://arxiv.org/abs/1103.5587
Status of Horava gravity: A personal perspective
Matt Visser (Victoria University of Wellington)
11 pages. Based on a talk at the ERE2010 conference, Granada, Spain, September 2010
(Submitted on 29 Mar 2011)
"Horava gravity is a relatively recent (Jan 2009) idea in theoretical physics for trying to develop a quantum field theory of gravity. It is not a string theory, nor loop quantum gravity, but is instead a traditional quantum field theory that breaks Lorentz invariance at ultra-high (presumably trans-Planckian) energies, while retaining approximate Lorentz invariance at low and medium (sub-Planckian) energies. The challenge is to keep the Lorentz symmetry breaking controlled and small - small enough to be compatible with experiment. I will give a very general overview of what is going on in this field, paying particular attention to the disturbing role of the scalar graviton."

 

[marcus]

http://arxiv.org/abs/1103.5993
Vacuum Fluctuations and the Small Scale Structure of Spacetime
S. Carlip, R. A. Mosna, J. P. M. Pitelli
4 pages
(Submitted on 30 Mar 2011)
"We show that vacuum fluctuations of the stress-energy tensor in two-dimensional dilaton gravity lead to a sharp focusing of light cones near the Planck scale, effectively breaking space up into a large number of causally disconnected regions. This phenomenon, called 'asymptotic silence' when it occurs in cosmology, might help explain several puzzling features of quantum gravity, including evidence of spontaneous dimensional reduction at short distances. While our analysis focuses on a simplified two-dimensional model, we argue that the qualitative features should still be present in four dimensions."

 

[marcus]

http://arxiv.org/abs/1103.6264
Spin foam models with finite groups
Benjamin Bahr, Bianca Dittrich, James P. Ryan
47 pages, 6 figures
(Submitted on 31 Mar 2011)
"Spin foam models, loop quantum gravity and group field theory are discussed as quantum gravity candidate theories and usually involve a continuous Lie group. We advocate here to consider quantum gravity inspired models with finite groups, firstly as a test bed for the full theory and secondly as a class of new lattice theories possibly featuring an analogue diffeomorphism symmetry. To make these notes accessible to readers outside the quantum gravity community we provide an introduction to some essential concepts in the loop quantum gravity, spin foam and group field theory approach and point out the many connections to lattice field theory and condensed matter systems."

brief mention:
http://arxiv.org/abs/1103.6272
Directions in Causal Set Quantum Gravity
Sumati Surya
31 pages, 7 figures, To appear in Recent Research in Quantum Gravity, edited by A. Dasgupta (Nova Science Publishers NY)
(Submitted on 31 Mar 2011)
"... Recent highlights include a causal set expression for the Einstein-Hilbert action and the construction of a scalar field Feynman propagator on a fixed causal set. The aim of the present article is to give a broad overview of the results in causal set theory while pointing out directions for future investigations."

 

[atyy]

http://arxiv.org/abs/1104.0561
Modeling of Time with Metamaterials
Igor I. Smolyaninov, Yu-Ju Hung
(Submitted on 4 Apr 2011)
Metamaterials have been already used to model various exotic "optical spaces". Here we demonstrate that mapping of monochromatic extraordinary light distribution in a hyperbolic metamaterial along some spatial direction may model the "flow of time". This idea is demonstrated in experiments performed with plasmonic hyperbolic metamaterials. Appearance of the "statistical arrow of time" is examined in an experimental scenario which emulates a Big Bang-like event.

 

[marcus]

http://arxiv.org/abs/1104.0723
A phenomenology analysis of the tachyon warm inflation in loop quantum cosmology
Kui Xiao, Jian-Yang Zhu
7 pages,accepted for publication in Physics Letters B
(Submitted on 5 Apr 2011)
"We investigate the warm inflation condition in loop quantum cosmology. In our consideration, the system is described by a tachyon field interacted with radiation. The exponential potential function, $$V(\phi)=V_0 e^{-\alpha\phi}\label{exp-p}$$, with the same order parameters $$V_0$$ and $$\alpha$$, is taken as an example of this tachyon warm inflation model. We find that, for the strong dissipative regime, the total number of e-folds is less than the one in the classical scenario, and for the weak dissipative regime, the beginning time of the warm inflation will be later than the tachyon (cool) inflation."

 

[John86]

http://arxiv.org/abs/1104.0997
Toward a "fundamental theorem of quantal measure theory"
Authors: Rafael D. Sorkin (Perimeter Institute and Syracuse University)
(Submitted on 6 Apr 2011)

Abstract: We address the extension problem for quantal measures of path-integral type, concentrating on two cases: sequential growth of causal sets, and a particle moving on the finite lattice Z_n. In both cases the dynamics can be coded into a vector-valued measure mu on Omega, the space of all histories. Initially mu is defined only on special subsets of Omega called cylinder-events, and one would like to extend it to a larger family of subsets (events) in analogy to the way this is done in the classical theory of stochastic processes. Since quantally mu is generally not of bounded variation, a new method is required. We propose a method that defines the measure of an event by means of a sequence of simpler events which in a suitable sense converges to the event whose measure one is seeking to define. To this end, we introduce canonical sequences approximating certain events, and we propose a measure-based criterion for the convergence of such sequences. Applying the method, we encounter a simple event whose measure is zero classically but non-zero quantally.

 

[marcus]

http://arxiv.org/abs/1104.1384
Effective action and semiclassical limit of spin foam models
A. Mikovic, M. Vojinovic
15 pages
(Submitted on 7 Apr 2011)
"We define an effective action for spin foam models of quantum gravity by adapting the background field method from quantum field theory. We show that the Regge action is the leading term in the semi-classical expansion of the spin foam effective action if the vertex amplitude has the large-spin asymptotics which is proportional to an exponential function of the vertex Regge action. In the case of the known three-dimensional and four-dimensional spin foam models this amounts to modifying the vertex amplitude such that the exponential asymptotics is obtained. In particular, we show that the ELPR/FK model vertex amplitude can be modified such that the new model is finite and has the Einstein-Hilbert action as its classical limit. We also calculate the first-order and some of the second-order quantum corrections in the semi-classical expansion of the effective action."

 

[marcus]

http://arxiv.org/abs/1104.1800
Inflationary tensor fluctuations, as viewed by Ashtekar variables and their imaginary friends
Laura Bethke, Joao Magueijo
17pages
(Submitted on 10 Apr 2011)
"We investigate tensor modes in infllationary scenarios from the point of view of Ashtekar variables and their generalizations labelled by Immirzi parameter gamma, which we'll assume imaginary. By defining the classical perturbed Hamiltonian, we reproduce, on-shell, the usual expression found in cosmological perturbation theory. However the quantum Hamiltonian displays significant differences, namely in the vacuum energy and fluctuations of the various modes. Graviton states are represented by combinations of metric and connection variables. It turns out that half of these modes have negative energy but after defining the inner product we conclude that they are non-physical and should be selected out. We are left with the usual graviton modes but with a chiral asymmetry in the the vacuum energy and fluctuations. The latter depends on gamma and on the ordering prescription (namely in the Hamiltonian constraint). Such an effect would leave a distinctive imprint in the polarization of the cosmic microwave background, thus finally engaging quantum gravity in meaningful experimental test."

http://arxiv.org/abs/1104.2019
Relative locality and the soccer ball problem
Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin
4 pages
(Submitted on 11 Apr 2011)
"We consider the behavior of macroscopic bodies within the framework of relative locality, which is a recent proposal for Planck scale modifications of the relativistic dynamics of particles which are described as arising from deformations in the geometry of momentum space. These lead to the addition of non-linear terms to the energy-momentum relations and conservation laws, which are suppressed by powers of ratio between the energy E of the particles involved and the Planck mass M_P. We consider and resolve a common objection against such proposals, which is that, even if the corrections are small for elementary particles in current experiments, they are huge when applied to composite systems such as soccer balls, planets and stars, with energies E_macro much larger than M_P. We show that this "soccer-ball problem" does not arise within the framework of relative locality, because the non-linear effects for the dynamics of a composite system with N elementary particles appear at most of order E_macro/ N M_P."

brief mention:
http://arxiv.org/abs/1104.1733
Arrows of Time in the Bouncing Universes of the No-boundary Quantum State
James Hartle, Thomas Hertog
13 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1104.2066
Reformulating and Reconstructing Quantum Theory
Lucien Hardy
159 pages. Many pictures
(Submitted on 11 Apr 2011)
"We provide a reformulation of finite dimensional quantum theory in the circuit framework in terms of mathematical axioms, and a reconstruction of quantum theory from operational postulates. The mathematical axioms for quantum theory are the following:
[Axiom 1] Operations correspond to operators.
[Axiom 2] Every complete set of positive operators corresponds to a complete set of operations.

The following operational postulates are shown to be equivalent to these mathematical axioms:
[P1] Definiteness. Associated with any given pure state is a unique maximal effect giving probability equal to one. This maximal effect does not give probability equal to one for any other pure state.
[P2] Information locality. A maximal measurement on a composite system is effected if we perform maximal measurements on each of the components.
[P3] Tomographic locality. The state of a composite system can be determined from the statistics collected by making measurements on the components.
[P4] Compound permutatability. There exists a compound reversible transformation on any system effecting any given permutation of any given maximal set of distinguishable states for that system.
[P5] Preparability. Filters are non-mixing and non-flattening.

Hence, from these postulates we can reconstruct all the usual features of quantum theory: States are represented by positive operators, transformations by completely positive trace non-increasing maps, and effects by positive operators. The Born rule (i.e. the trace rule) for calculating probabilitieso follows. A more detailed abstract is provided in the paper."

 

[marcus]

http://arxiv.org/abs/1104.2765
Spin Foam Models for Quantum Gravity and semi-classical limit
Maité Dupuis
PhD Thesis; Ecole Normale Supérieure de Lyon. 192pages, many figures
(Submitted on 14 Apr 2011)
"The spinfoam framework is a proposal for a regularized path integral for quantum gravity. Spinfoams define quantum space-time structures describing the evolution in time of the spin network states for quantum geometry derived from Loop Quantum Gravity (LQG). The construction of this covariant approach is based on the formulation of General Relativity as a topological theory plus the so-called simplicity constraints which introduce local degrees of freedom. The simplicity constraints are essential in turning the non-physical topological theory into 4d gravity.

In this PhD manuscript, an original way to impose the simplicity constraints in 4d Euclidean gravity using harmonic oscillators is proposed and new coherent states, solutions of the constraints, are given. Moreover, a consistent spinfoam model for quantum gravity has to be connected to LQG and must have the right semi-classical limit. An explicit map between the spin network states of LQG and the boundary states of spinfoam models is given connecting the canonical and the covariant approaches. Finally, new techniques to compute semiclassical asymptotic expressions for the transition amplitudes of 3d quantum gravity and to extract semi-classical information from a spinfoam model are introduced. Explicit computations based on approximation methods and on the use of recurrence relations on spinfoam amplitudes have been performed. The results are relevant to derive quantum corrections to the dynamics of the gravitational field."

Maité is the PhD student of Etera Livine.

http://arxiv.org/abs/1104.2822
A real ensemble interpretation of quantum mechanics
Lee Smolin
14 pages
(Submitted on 14 Apr 2011)
"A new ensemble interpretation of quantum mechanics is proposed according to which the ensemble associated to a quantum state really exists: it is the ensemble of all the systems in the same quantum state in the universe. Individual systems within the ensemble have microscopic states, described by beables. The probabilities of quantum theory turn out to be just ordinary relative frequencies probabilities in these ensembles. Laws for the evolution of the beables of individual systems are given such that their ensemble relative frequencies evolve in a way that reproduces the predictions of quantum mechanics. These laws are highly non-local and involve a new kind of interaction between the members of an ensemble that define a quantum state. These include a stochastic process by which individual systems copy the beables of other systems in the ensembles of which they are a member. The probabilities for these copy processes do not depend on where the systems are in space, but do depend on the distribution of beables in the ensemble. Macroscopic systems then are distinguished by being large and complex enough that they have no copies in the universe. They then cannot evolve by the copy law, and hence do not evolve stochastically according to quantum dynamics. This implies novel departures from quantum mechanics for systems in quantum states that can be expected to have few copies in the universe. At the same time, we are able to argue that the centre of masses of large macroscopic systems do satisfy Newton's laws."

 

[MTd2]

http://arxiv.org/abs/1104.2889

Black holes in Einstein-aether and Horava-Lifgarbagez gravity

Enrico Barausse, Ted Jacobson, Thomas P. Sotiriou
(Submitted on 14 Apr 2011)
We study spherical black-hole solutions in Einstein-aether theory, a Lorentz-violating gravitational theory consisting of General Relativity with a dynamical unit timelike vector (the "aether") that defines a preferred timelike direction. These are also solutions to the infrared limit of Horava-Lifgarbagez gravity. We explore parameter values of the two theories where all presently know experimental constraints are satisfied, and find that spherical black-hole solutions of the type expected to form by gravitational collapse exist for all those parameters. Outside the metric horizon, the deviations away from the Schwarzschild metric are typically no more than a few percent for most of the explored parameter regions, which makes them difficult to observe with electromagnetic probes, but in principle within reach of future gravitational-wave detectors. Remarkably, we find that the solutions possesses a universal horizon, not far inside the metric horizon, that traps waves of any speed relative to the aether. A notion of black hole thus persists in these theories, even in the presence of arbitrarily high propagation speeds.

 

[marcus]

http://arxiv.org/abs/1104.3126
Mass Inflation in the Loop Black Hole
Eric G. Brown, Robert B. Mann, Leonardo Modesto
20 pages, 7 figures
(Submitted on 15 Apr 2011)
"In classical general relativity the Cauchy horizon within a two-horizon black hole is unstable via a phenomenon known as mass inflation, in which the mass parameter (and the spacetime curvature) of the black hole diverges at the Cauchy horizon. Here we study this effect for loop black holes -- quantum gravitationally corrected black holes from loop quantum gravity -- whose construction alleviates the r=0 singularity present in their classical counterparts. We use a simplified model of mass inflation, which makes use of the generalized DTR[*] relation, to conclude that the Cauchy horizon of loop black holes indeed results in a curvature singularity similar to that found in classical black holes. The DTR relation is of particular utility in the loop black hole because it does not directly rely upon Einstein's field equations. We elucidate some of the interesting and counterintuitive properties of the loop black hole, and corroborate our results using an alternate model of mass inflation due to Ori."

[*By DTR they refer to work by Dray 't Hooft, and Redmount.]

brief mention:
http://arxiv.org/abs/1104.2910
The Relation Between Geometry and Matter in classical and Quantum Gravity and Cosmology
Daniele Regoli
PhD thesis
(Submitted on 14 Apr 2011)
"The present thesis is divided into two main research areas: Classical Cosmology and (Loop) Quantum Gravity. The first part concerns cosmological models... The second part concerns the spinfoam approach to (Loop) Quantum Gravity,..."

 

[marcus]

http://arxiv.org/abs/1104.3683
Holomorphic Simplicity Constraints for 4d Spinfoam Models
Maité Dupuis, Etera R. Livine
27 pages
(Submitted on 19 Apr 2011)
"Within the framework of spinfoam models, we revisit the simplicity constraints reducing topological BF theory to 4d Riemannian gravity. We use the reformulation of SU(2) intertwiners and spin networks in term of spinors, which has come out from both the recently developed U(N) framework for SU(2) intertwiners and the twisted geometry approach to spin networks and spinfoam boundary states. Using these tools, we are able to perform a holomorphic/anti-holomorphic splitting of the simplicity constraints and define a new set of holomorphic simplicity constraints, which are equivalent to the standard ones at the classical level and which can be imposed strongly on intertwiners at the quantum level. We then show how to solve these new holomorphic simplicity constraints using coherent intertwiner states. We further define the corresponding coherent spin network functionals and introduce a new spinfoam model for 4d Riemannian gravity based on these holomorphic simplicity constraints and whose amplitudes are defined from the evaluation of the new coherent spin networks."

http://arxiv.org/abs/1104.3688
Pre-Big-Bang Cosmology and Circles in the Cosmic Microwave Background
William Nelson, Edward Wilson-Ewing
21 pages, 3 figures
(Submitted on 19 Apr 2011)
"We examine the possibility that circles in the cosmic microwave background could be formed by the interaction of a gravitational wave pulse emitted in some pre-big-bang phase of the universe with the last scattering surface. We derive the expected size distribution of such circles, as well as their typical width and (for concentric circles) angular separation. We apply these results in particular to conformal cyclic cosmology, ekpyrotic cosmology as well as loop quantum cosmology with and without inflation in order to determine how the predicted geometric properties of these circles would vary from one model to the other, and thus, if detected, could allow us to differentiate between various pre-big-bang cosmological models. We also show that the angular width and the sine of the angular radius of such circles are inversely proportional. This relation can be used in order to determine whether or not circles observed in the cosmic microwave background are due to energetic pre-big-bang events."

 

[John86]

http://arxiv.org/abs/1104.3443
Constructive Renormalization for $\Phi^{4}_2$ Theory with Loop Vertex Expansion
Vincent Rivasseau, Zhituo Wang
(Submitted on 18 Apr 2011)
In this paper we construct the 2 dimensional Euclidean $\phi^4$ quantum field theory using the method of loop vertex expansion. We reproduce the results of standard constructive theory, for example the Borel summability of the Schwinger functions in the coupling constant. Our method should be also suitable for the future construction of Grosse-Wulkenhaar models on non-commutative space-time.

 

[marcus]

http://arxiv.org/abs/1104.4028
Perturbative quantum gravity with the Immirzi parameter
Dario Benedetti, Simone Speziale
(Submitted on 20 Apr 2011)
"We study perturbative quantum gravity in the first-order tetrad formalism. The lowest order action corresponds to Einstein-Cartan plus a parity-odd term, and is known in the literature as the Holst action. The coupling constant of the parity-odd term can be identified with the Immirzi parameter of loop quantum gravity. We compute the quantum effective action in the one-loop expansion. As in the metric second-order formulation, we find that in the case of pure gravity the theory is on-shell finite, and the running of Newton's constant and the Immirzi parameter is inessential. In the presence of fermions, the situation changes in two fundamental aspects. First, non-renormalizable logarithmic divergences appear, as usual. Second, the Immirzi parameter becomes a priori observable, and we find that it is renormalized by a four-fermion interaction generated by radiative corrections. We compute its beta function and discuss possible implications. The sign of the beta function depends on whether the Immirzi parameter is larger or smaller than one in absolute value, and the values plus or minus one are UV fixed-points (we work in Euclidean signature). Finally, we find that the Holst action is stable with respect to radiative corrections in the case of minimal coupling, up to higher order non-renormalizable interactions."

 

[jason11]

http://arxiv.org/abs/1104.1499"

Semiclassical Analysis of the Wigner $9J$-Symbol with Small and Large Angular Momenta
Robert G. Littlejohn, Liang Yu
(Submitted on 8 Apr 2011 (v1), last revised 13 Apr 2011 (this version, v2))
We derive a new asymptotic formula for the Wigner $9j$-symbol, in the limit of one small and eight large angular momenta, using a novel gauge-invariant factorization for the asymptotic solution of a set of coupled wave equations. Our factorization eliminates the geometric phases completely, using gauge-invariant non-canonical coordinates, parallel transports of spinors, and quantum rotation matrices. Our derivation generalizes to higher $3nj$-symbols. We display without proof some asymptotic formulas for the $12j$-symbol and the $15j$-symbol in the appendices. This work contributes a new asymptotic formula of the Wigner $9j$-symbol to the quantum theory of angular momentum, and serves as an example of a new general method for deriving asymptotic formulas for $3nj$-symbols.



http://arxiv.org/abs/1104.3275"

Semiclassical Analysis of the Wigner $12J$-Symbol with One Small Angular Momentum: Part I
Liang Yu
(Submitted on 17 Apr 2011)
We derive a new asymptotic formula for the Wigner $12j$-symbol, in the limit of one small and eleven large angular momenta. There are two kinds of asymptotic formulas for the $12j$-symbol with one small angular momentum. We present the first kind in this paper. Our derivation relies on the techniques developed in the semiclassical analysis of the Wigner $9j$-symbol, where we used a gauge-invariant form of the multicomponent WKB wavefunctions to derive new asymptotic formulas for the $9j$-symbol with small and large angular momenta. When applying the same technique to the $12j$-symbol in this paper, we find that the spinor is diagonalized in the direction of an intermediate angular momentum. In addition, we find that the geometry of the new asymptotic formula for the $12j$-symbol is expressed in terms of the vector diagram for a $9j$-symbol. This illustrates a general geometric connection between asymptotic limits of the various $3nj$-symbols. This work contributes the first known asymptotic formula for the $12j$-symbol to the quantum theory of angular momentum, and serves as a basis for finding asymptotic formulas for the Wigner $15j$-symbol with two small angular momenta.



http://arxiv.org/abs/1104.3641"

Asymptotic Limits of the Wigner $15J$-Symbol with Small Quantum Numbers
Liang Yu
(Submitted on 19 Apr 2011)
"We present new asymptotic formulas for the Wigner $15j$-symbol with two, three, or four small quantum numbers, and provide numerical evidence of their validity. These formulas are of the WKB form and are of a similar nature as the Ponzano-Regge formula for the Wigner $6j$-symbol. They are expressed in terms of edge lengths and angles of geometrical figures associated with angular momentum vectors. In particular, the formulas for the $15j$-symbol with two, three, and four small quantum numbers are based on the geometric figures of the $9j$-, $6j$-, and $3j$-symbols, respectively, The geometric nature of these new asymptotic formulas pave the way for further analysis of the semiclassical limits of vertex amplitudes in loop quantum gravity models."

 

[marcus]

http://arxiv.org/abs/1104.4691
Black hole horizons from within loop quantum gravity
Hanno Sahlmann
13 pages, 10 figures
(Submitted on 25 Apr 2011)
"In general relativity, the fields on a black hole horizon are obtained from those in the bulk by pullback and restriction. Similarly, in quantum gravity, the quantized horizon degrees of freedom should result from restricting, or pulling-back, the quantized bulk degrees of freedom. This is not yet fully realized in the - otherwise very successful - quantization of isolated horizons in loop quantum gravity. In this work we outline a setting in which the quantum horizon degrees of freedom are simply components of the quantized bulk degrees of freedom. There is no need to quantize them separately. We present evidence that for a horizon of sphere topology, the resulting horizon theory is remarkably similar to what has been found before."

http://arxiv.org/abs/1104.4546
Implications of the gauge-fixing in Loop Quantum Cosmology
Francesco Cianfrani, Giovanni Montani
12 pages
(Submitted on 23 Apr 2011)
"The restriction to invariant connections in a Friedmann-Robertson-Walker space-time is discussed via the analysis of the Dirac brackets associated with the corresponding gauge fixing. This analysis allows us to infer the proper relation between fluxes and reduced phase-space variables. In this respect, it is outlined how the holonomy-flux algebra coincides with the one of Loop Quantum Gravity if edges are parallel to simplicial vectors and the quantization of the model is performed via standard techniques by restricting admissible paths. Within this scheme, the discretization of the area spectrum is emphasized, while the fundamental path underlying the continuous picture is described by a cubical lattice. Finally, the perspectives for a consistent dynamical treatment are discussed."

brief mention (not Loop-and-allied QG, but possibly of general interest)
http://arxiv.org/abs/1104.4543
A class of elementary particle models without any adjustable real parameters
Gerard 't Hooft
28 pages
(Submitted on 23 Apr 2011)
"Conventional particle theories such as the Standard Model have a number of freely adjustable coupling constants and mass parameters, depending on the symmetry algebra of the local gauge group and the representations chosen for the spinor and scalar fields. There seems to be no physical principle to determine these parameters as long as they stay within certain domains dictated by the renormalization group. Here however, reasons are given to demand that, when gravity is coupled to the system, local conformal invariance should be a spontaneously broken exact symmetry. The argument has to do with the requirement that black holes obey a complementarity principle relating ingoing observers to outside observers, or equivalently, initial states to final states. This condition fixes all parameters, including masses and the cosmological constant. We suspect that only examples can be found where these are all of order one in Planck units, but the values depend on the algebra chosen. This paper combines findings reported in two previous preprints, and puts these in a clearer perspective by shifting the emphasis towards the implications for particle models."

 

[marcus]

http://relativity.phys.lsu.edu/ilqgs/gurau042611.pdf
The 1/N expansion in colored tensor models
Razvan Gurau, Perimeter Institute
ILQGS talk Tuesday, Apr 26th
I expect the audio to be posted within a day or so.
Gurau is a frequent collaborator with Vincent Rivasseau, central in the development of a certain group-field-theory approach applicable to QG including spinfoam LQG.
Assuming the audio for this seminar lecture is posted soon, it will be found here:
http://relativity.phys.lsu.edu/ilqgs/
In the meantime I will look for related technical papers to give more of an idea what he is talking about.

http://arxiv.org/abs/1011.2726
The 1/N expansion of colored tensor models
http://arxiv.org/abs/1101.4182
The 1/N expansion of colored tensor models in arbitrary dimension
http://arxiv.org/abs/1102.5759
The complete 1/N expansion of colored tensor models in arbitrary dimension

brief mention (not directly involved with Loop-and-allied QG, but possibly of wider interest)
http://arxiv.org/abs/1104.4849
Noncommutative Geometry Inspired Entropic Inflation
Kourosh Nozari, Siamak Akhshabi
14 pages, 5 figuers, Accepted for Publication in Phys. Lett. B
(Submitted on 26 Apr 2011)
"Recently Verlinde proposed that gravity can be described as an emergent phenomena arising from changes in the information associated with the positions of material bodies. By using noncommutative geometry as a way to describe the microscopic, microstructure of quantum spacetime, we derive modified Friedmann equation in this setup and study the entropic force modifications to the inflationary dynamics of early universe."

 

[marcus]

http://arxiv.org/abs/1104.5158
Bounding bubbles: the vertex representation of 3d Group Field Theory and the suppression of pseudo-manifolds
Sylvain Carrozza, Daniele Oriti
28 pages, 17 figures
(Submitted on 27 Apr 2011)
"Based on recent work on simplicial diffeomorphisms in colored group field theories, we develop a representation of the colored Boulatov model, in which the GFT fields depend on variables associated to vertices of the associated simplicial complex, as opposed to edges. On top of simplifying the action of diffeomorphisms, the main advantage of this representation is that the GFT Feynman graphs have a different stranded structure, which allows a direct identification of subgraphs associated to bubbles, and their evaluation is simplified drastically. As a first important application of this formulation, we derive new scaling bounds for the regularized amplitudes, organized in terms of the genera of the bubbles, and show how the pseudo-manifolds configurations appearing in the perturbative expansion are suppressed as compared to manifolds. Moreover, these bounds are proved to be optimal."

 

[marcus]

http://arxiv.org/abs/1104.5471
Tensor models and embedded Riemann surfaces
James P. Ryan
9 pages, 7 figures
(Submitted on 28 Apr 2011)
"Tensor models and, more generally, group field theories are candidates for higher-dimensional quantum gravity, just as matrix models are in the 2d setting. With the recent advent of a 1/N-expansion for coloured tensor models, more focus has been given to the study of the topological aspects of their Feynman graphs. Crucial to the aforementioned analysis were certain subgraphs known as bubbles and jackets. We demonstrate in the 3d case that these graphs are generated by matrix models embedded inside the tensor theory. Moreover, we show that the jacket graphs represent (Heegaard) splitting surfaces for the triangulation dual to the Feynman graph. With this in hand, we are able to re-express the Boulatov model as a quantum field theory on these Riemann surfaces."
Ryan is at AEI Potsdam, in Dittrich's group. he just gave a talk on 12 April at ILQGS
http://relativity.phys.lsu.edu/ilqgs/

The audio for Razvan Gurau's ILQGS seminar talk is now available online. When I posted about the talk two days ago only the slides PDF was available.http://arxiv.org/abs/1104.5486
Examination of the nature of the Bianchi type cosmological singularities
Piotr Dzierzak
PhD Thesis, about 80 pages
(Submitted on 28 Apr 2011)
"We present quantum (and classical) Bianchi I model, with free massless scalar field, of the Universe. Our model may be treated as the simplest prototype of the quantum BKL (Belinskii-Khalatnikov-Lifgarbagez) scenario. The quantization is done by making use of the nonstandard Loop Quantum Cosmology (LQC). Since the method is quite new, we present in details its motivation and the formalism. To make the nonstandard method easily understandable, we include its application to the FRW model. In the nonstandard LQC, we first solve the Hamiltonian constraint of the theory at the classical level and find elementary observables. Physical compound observables are defined in terms of elementary ones. We find that classical Big Bang singularity is replaced by quantum Big Bounce transition due to modification of classical theory by holonomy around a loop with finite size. The energy density of matter fields at the Big Bounce depends on a free parameter lambda, which value is expected to be determined from future cosmological observations. The phase space is divided it into two distinct regions: Kasner-like and Kasner-unlike. We use the elementary observables to quantize volume and directional volume operators in both cases. Spectra of these operators are bounded from below and discrete, and depend on lambda. The discreteness may imply a foamy structure of spacetime at semiclassical level. At the quantum level an evolution of the model is generated by the so-called true Hamiltonian. This enables introducing a time parameter valued in the set of all real numbers."
Dzierzak is at Warsaw. Jerzy Lewandowski heads the LQG group there.

 

[marcus]

http://arxiv.org/abs/1104.5509
Effective Hamiltonian Constraint from Group Field Theory
Etera R. Livine, Daniele Oriti, James P. Ryan
14 pages
(Submitted on 28 Apr 2011)
"Spinfoam models provide a covariant formulation of the dynamics of loop quantum gravity. They are non-perturbatively defined in the group field theory (GFT) framework: the GFT partition function defines the sum of spinfoam transition amplitudes over all possible (discretized) geometries and topologies. The issue remains, however, of explicitly relating the specific form of the group field theory action and the canonical Hamiltonian constraint. Here, we suggest an avenue for addressing this issue. Our strategy is to expand group field theories around non-trivial classical solutions and to interpret the induced quadratic kinematical term as defining a Hamiltonian constraint on the group field and thus on spin network wave functions. We apply our procedure to Boulatov group field theory for 3d Riemannian gravity. Finally, we discuss the relevance of understanding the spectrum of this Hamiltonian operator for the renormalization of group field theories."

http://arxiv.org/abs/1104.5527
Affine holomorphic quantization
Robert Oeckl (UNAM)
38 pages
(Submitted on 28 Apr 2011)
"We present a rigorous and functorial quantization scheme for affine field theories, i.e., field theories where local spaces of solutions are affine spaces. The target framework for the quantization is the general boundary formulation, allowing to implement manifest locality without the necessity for metric or causal background structures. The quantization combines the holomorphic version of geometric quantization for state spaces with the Feynman path integral quantization for amplitudes. We also develop an adapted notion of coherent states, discuss vacuum states, and consider observables and their Berezin-Toeplitz quantization. Moreover, we derive a factorization identity for the amplitude in the special case of a linear field theory modified by a source-like term and comment on its use as a generating functional for a generalized S-matrix."

We should get familiar with the acronym UNAM. It is where Corichi is based (although he also seems to be at Penn State with Ashtekar group sometimes as well.) The Loops 2007 conference was held at UNAM. I guess it means University National Autonomous Mexico. Or else the M stands for Morelia, the beautiful old city where the university is located and where they had the conference.

 

[marcus]

http://arxiv.org/abs/1105.0216
Regge gravity from spinfoams
Elena Magliaro, Claudio Perini
8 pages
(Submitted on 1 May 2011)
We consider spinfoam quantum gravity in the double scaling limit $$\gamma\rightarrow 0$$, $$j\rightarrow\infty$$ with $$\gamma j$$ constant, where $$\gamma$$ is the Immirzi parameter, j is the spin and $$\gamma j$$ gives the physical area in Planck units. We show how in this regime the partition function for a 2-complex takes the form of a path integral over continuous Regge metrics and enforces Einstein equations in the semiclassical regime. The Immirzi parameter must be considered as dynamical in the sense that it runs towards zero when the small wavelengths are integrated out. In addition to quantum corrections which vanish for $$\hbar\rightarrow 0$$, we find new corrections due to the discreteness of geometric spectra which is controlled by $$\gamma$$.


http://arxiv.org/abs/1105.0194
From Quantum Gravity to Quantum Field Theory via Noncommutative Geometry
Johannes Aastrup, Jesper M. Grimstrup
34 pages, 3 figures
(Submitted on 1 May 2011)
"A link between canonical quantum gravity and fermionic quantum field theory is established in this paper. From a spectral triple construction which encodes the kinematics of quantum gravity semi-classical states are constructed which, in a semi-classical limit, give a system of interacting fermions in an ambient gravitational field. The interaction involves flux tubes of the gravitational field. In the additional limit where all gravitational degrees of freedom are turned off, a free fermionic quantum field theory emerges."

http://arxiv.org/abs/1105.0183
Shape Dynamics. An Introduction
Julian Barbour
48 pages, 8 figures,46 references. To be published in the refereed proceedings of the conference Quantum Field Theory and Gravity (Regensburg 2010)
(Submitted on 1 May 2011)
"Shape dynamics is a completely background-independent universal framework of dynamical theories from which all absolute elements have been eliminated. For particles, only the variables that describe the shapes of the instantaneous particle configurations are dynamical. In the case of Riemannian three-geometries, the only dynamical variables are the parts of the metric that determine angles. The local scale factor plays no role. This leads to a shape-dynamic theory of gravity in which the four-dimensional diffeomorphism invariance of general relativity is replaced by three-dimensional diffeomorphism invariance and three-dimensional conformal invariance. Despite this difference of symmetry groups, it is remarkable that the predictions of the two theories -- shape dynamics and general relativity -- agree on spacetime foliations by hypersurfaces of constant mean extrinsic curvature. However, the two theories are distinct, with shape dynamics having a much more restrictive set of solutions. There are indications that the symmetry group of shape dynamics makes it more amenable to quantization and thus to the creation of quantum gravity. This introduction presents in simple terms the arguments for shape dynamics, its implementation techniques, and a survey of existing results."