Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1101.5792
Particle Creation by Loop Black Holes
Emanuele Alesci, Leonardo Modesto
14 pages, 6 figures
(Submitted on 30 Jan 2011)
"We study the black hole particle production in a regular spacetime metric obtained in a minisuperspace approach to loop quantum gravity. In different previous papers the static solution was obtained and shown to be singularity-free and self-dual. In this paper expanding a previous study of the black hole dynamics we repeat the Hawking analysis which leads to a thermal flux of particles at the future infinity. The evaporation time is infinite and the unitarity is recovered due to the regularity of the spacetime and to the characteristic behavior of the surface gravity."

http://arxiv.org/abs/1101.6078
Induced standard model and unification
John W. Barrett
(Submitted on 31 Jan 2011)
"A proposal that the bosonic action of the standard model is induced from the fermionic action is investigated."

http://arxiv.org/abs/1101.6007
QED coupled to QEG
Ulrich Harst, Martin Reuter
25 pages, 3 figures
(Submitted on 31 Jan 2011)
"We discuss the non-perturbative renormalization group flow of Quantum Electrodynamics (QED) coupled to Quantum Einstein Gravity (QEG) and explore the possibilities for defining its continuum limit at a fixed point that would lead to a non-trivial, i.e. interacting field theory. We find two fixed points suitable for the Asymptotic Safety construction. In the first case, the fine-structure constant vanishes at the fixed point and its infrared ("renormalized") value is a free parameter not determined by the theory itself. In the second case, the fixed point value of the fine-structure constant is non-zero, and its infrared value is a computable prediction of the theory."

A notable paper included despite no direct explicit connection with QG
http://arxiv.org/abs/1101.5690
Division Algebras and Quantum Theory
John C. Baez
30 pages, 3 figures
(Submitted on 29 Jan 2011)
"Quantum theory may be formulated using Hilbert spaces over any of the three associative normed division algebras: the real numbers, the complex numbers and the quaternions. Indeed, these three choices appear naturally in a number of axiomatic approaches. However, there are internal problems with real or quaternionic quantum theory. Here we argue that these problems can be resolved if we treat real, complex and quaternionic quantum theory as part of a unified structure. Dyson called this structure the 'three-fold way'. It is perhaps easiest to see it in the study of irreducible unitary representations of groups on complex Hilbert spaces. These representations come in three kinds: those that are not isomorphic to their own dual (the truly 'complex' representations), those that are self-dual thanks to a symmetric bilinear pairing (which are 'real', in that they are the complexifications of representations on real Hilbert spaces), and those that are self-dual thanks to an antisymmetric bilinear pairing (which are 'quaternionic', in that they are the underlying complex representations of representations on quaternionic Hilbert spaces). This three-fold classification sheds light on the physics of time reversal symmetry, and it already plays an important role in particle physics. More generally, Hilbert spaces of anyone of the three kinds - real, complex and quaternionic - can be seen as Hilbert spaces of the other kinds, equipped with extra structure."

 

[atyy]

http://arxiv.org/abs/1102.0270
Topological Phases: An Expedition off Lattice
Michael H. Freedman, Lukas Gamper, Charlotte Gils, Sergei V. Isakov, Simon Trebst, Matthias Troyer
(Submitted on 1 Feb 2011)
Motivated by the goal to give the simplest possible microscopic foundation for a broad class of topological phases, we study quantum mechanical lattice models where the topology of the lattice is one of the dynamical variables. However, a fluctuating geometry can remove the separation between the system size and the range of local interactions, which is important for topological protection and ultimately the stability of a topological phase. In particular, it can open the door to a pathology, which has been studied in the context of quantum gravity and goes by the name of `baby universe', Here we discuss three distinct approaches to suppressing these pathological fluctuations. We complement this discussion by applying Cheeger's theory relating the geometry of manifolds to their vibrational modes to study the spectra of Hamiltonians. In particular, we present a detailed study of the statistical properties of loop gas and string net models on fluctuating lattices, both analytically and numerically.

 

[marcus]

http://arxiv.org/abs/1102.0954
The Holst Action by the Spectral Action Principle
Frank Pfaeffle, Christoph A. Stephan
11 pages
(Submitted on 4 Feb 2011)
"We investigate the Holst action for closed Riemannian 4-manifolds with orthogonal connections. For connections whose torsion has zero Cartan type component we show that the Holst action can be recovered from the heat asymptotics for the natural Dirac operator acting on left-handed spinor fields."

(If anyone is coming in new to this, Holst action is the action used in Spinfoam LQG and Spectral Action Principle means they are talking Connes NC geometry realization of standard model matter. The title indicates they want to put Connes-style standard model matter together with LQG. Read the paper to get more detail.)

In post #1410 I logged a paper by John Barrett, "Induced standard model and unification". In the course of revision this has been given a new title:
http://arxiv.org/abs/1101.6078
State sum models, induced gravity and the spectral action
John W. Barrett
13 pages
(Submitted on 31 Jan 2011)
"A proposal that the bosonic action of the standard model is induced from the fermionic action is investigated. It is suggested that this might occur naturally in state sum models."

http://arxiv.org/abs/1102.0789
The emergence of a universal limiting speed
Mohamed M. Anber, John F. Donoghue
13 pages, 3 figures
(Submitted on 3 Feb 2011)
"We display several examples of how fields with different limiting velocities (the "speed of light") at a high energy scale can nevertheless have a common limiting velocity at low energies due to the effects of interactions. We evaluate the interplay of the velocities through the self-energy diagrams and use the renormalization group to evolve the system to low energy. The differences normally vanish only logarithmically, so that an exponentially large energy trajectory is required in order to satisfy experimental constraints. However, we also display a model in which the running is power-law, which could be more phenomenologically useful. The largest velocity difference should be in system with the weakest interaction, which suggests that the study of the speed of gravitational waves would be the most stringent test of this phenomenon."

 

[marcus]

http://arxiv.org/abs/1102.1688
State Sums and Geometry
Frank Hellmann
PhD Thesis, 106 pages
(Submitted on 8 Feb 2011)
"In this thesis I review the definition of topological quantum field theories through state sums on triangulated manifolds. I describe the construction of state sum invariants of 3-manifolds from a graphical calculus and show how to evaluate the invariants as boundary amplitudes. I review how to define such a graphical calculus through SU(2) representation theory. I then review various geometricity results for the representation theory of SU(2), Spin(4) and SL(2,C), and define coherent boundary manifolds for state sums based on these representations. I derive the asymptotic geometry of the SU(2) based Ponzano-Regge invariant in three dimensions, and the SU(2) based Ooguri models amplitude in four dimensions. As a corollary to the latter results I derive the asymptotic behaviour of various recently proposed spin foam models motivated from the Plebanski formulation of general relativity. Finally the asymptotic geometry of the SL(2,C) based model is derived."

 

[marcus]

http://arxiv.org/abs/1102.1844
Comment on `Lost in Translation: Topological Singularities in Group Field Theory'
Matteo Smerlak
(Submitted on 9 Feb 2011)
"Gurau argued in [arXiv:1006.0714] that the gluing spaces arising as Feynman diagrams of three-dimensional group field theory are not all pseudo-manifolds. I dispute this conclusion: albeit not properly triangulated, these spaces are genuine pseudo-manifolds, viz. their singular locus is of codimension at most two."

 

[atyy]

http://arxiv.org/abs/1102.1592
Emergent of non-gravitational fields in dimensional reduction of 4d spin foam models
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 9 Feb 2011 (this version, v2))
We consider a Kaluza-Klein like approach for a 4d spin foam model. We apply this approach to a 4d TOCY model based on group field theory; and using the Peter-Weyl expansion of the gravitational field we find a mechanism for generation of matter and new dimensions from pure gravity.

 

[MTd2]

http://arxiv.org/abs/1102.2226

Discrete and continuum third quantization of Gravity

Steffen Gielen, Daniele Oriti
(Submitted on 10 Feb 2011)
We give a brief introduction to matrix models and the group field theory (GFT) formalism as realizations of the idea of a third quantization of gravity, and present in some more detail the idea and basic features of a continuum third quantization formalism in terms of a field theory on the space of connections, building up on the results of loop quantum gravity that allow to make the idea slightly more concrete. We explore to what extent one can rigorously define such a field theory. Concrete examples are given for the simple case of Riemannian GR in 3 spacetime dimensions. We discuss the relation between GFT and this formal continuum third quantized gravity, and what it can teach us about the continuum limit of GFTs.

 

[marcus]

Not directly about QG, but Baez is "family" and it could apply to Jacobson, Verlinde gravity conceivably

http://arxiv.org/abs/1102.2098
Renyi Entropy and Free Energy
John C. Baez
(Submitted on 10 Feb 2011)
"The Renyi entropy is a generalization of the usual concept of entropy which depends on a parameter beta. Any probability distribution can be seen as the Gibbs state for some Hamiltonian at temperature 1. Starting with this Hamiltonian, we can then compute the free energy at temperature T, and up to a certain factor this turns out to be the Renyi entropy where beta = 1/T. More precisely, the free energy is (1 - T) times the Renyi entropy. This is true not only classically but also quantum-mechanically."

 

[physikdan]

http://arxiv.org/abs/1102.1592
Emergent of non-gravitational fields in dimensional reduction of 4d spin foam models
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 9 Feb 2011 (this version, v2))
We consider a Kaluza-Klein like approach for a 4d spin foam model. We apply this approach to a 4d TOCY model based on group field theory; and using the Peter-Weyl expansion of the gravitational field we find a mechanism for generation of matter and new dimensions from pure gravity.

It is inersting that they do Kaluza-Klein strategy to a very different subject ! and ern some how similar results.

 

[marcus]

Please start a separate discussion thread about the paper. We don't discuss papers in this thread. This is a bibliography thread to help us keep track of current Loop-and-allied research. It overloads and makes it less useful to have discussion crowding in.
I would welcome a discussion thread about this paper by the two guys at Teheran (have already asked Atyy what he thinks of it.)

 

[marcus]

http://arxiv.org/abs/1102.2376
Local covariance and background independence
Klaus Fredenhagen, Katarzyna Rejzner
9 pages, submitted for the Proceedings of the conference "Quantum field theory and gravity", Regensburg (28 Sep - 1 Oct 2010)
(Submitted on 11 Feb 2011)
"One of the many conceptual difficulties in the development of quantum gravity is the role of a background geometry for the structure of quantum field theory. To some extent the problem can be solved by the principle of local covariance. The principle of local covariance was originally imposed in order to restrict the renormalization freedom for quantum field theories on generic spacetimes. It turned out that it can also be used to implement the request of background independence. Locally covariant fields then arise as background independent entities."

 

[marcus]

http://arxiv.org/abs/1102.2694
Scalar Perturbation in a Big Bounce of Loop Quantum Cosmology
Yu Li, Jian-Yang Zhu
6 pages, 5 figures
(Submitted on 14 Feb 2011)
"We study the behaviors of the scalar perturbation in the bounce phase of the effective loop quantum cosmology. Two models are discussed: one is the universe fulfilled by a messless scalar field; the other is a more realistic model which can translate to the universe dominated by radiation. We find that the behaviors of the Bardeen potential near both the bounce point and the transition point of null energy condition are good. This is different from the bounce models in pure general relativity, and we may conclude that the bounce in loop quantum cosmology is reasonable."

http://arxiv.org/abs/1102.2695
Stability analysis of an autonomous system in loop quantum cosmology
Kui Xiao, Jian-Yang Zhu
9 pages
(Submitted on 14 Feb 2011)
"We discuss the stability properties of an autonomous system in loop quantum cosmology (LQC) scenario. The system is described by a self-interacting scalar field phi with positive potential V coupled with a barotropic fluid in Universe. Considering Gamma = V V"/V'² as a function of \lambda=V'/V, the autonomous system is extended from 3-dimension to 4-dimension. We find that the dynamical behaviors of some fixed points suit for all potential, and some just suit for the concrete potential. Considering the higher-order derivatives of the potential, we get an infinite-dimensional autonomous system which can describe the dynamical behavior of scalar field with more general potential. We find that there is just a scalar-field-dominated scaling solution in LQC scenario."

http://arxiv.org/abs/1102.2720
Application of higher order holonomy corrections to perturbation theory of cosmology
Yu Li, Jian-Yang Zhu
7 pages, 1 figure
(Submitted on 14 Feb 2011)
"Applying the higher order holonomy corrections to the perturbation theory of cosmology, the lattice power law of Loop Quantum Cosmology, \tilde{\mu}\propto p^{\beta}, is analysed and the range of beta is decided to be [-1,0] which is different from the conventional range -0.1319>\beta\geq-5/2. At the same time, we find that there is a anomaly free condition in this theory, and we obtain this condition in the vector and tensor mode. We also find that the nonzero mass of gravitational wave essentially results from the quantum nature of Riemannian geometry of loop quantum gravity."

http://arxiv.org/abs/1102.2758
Interacting modified Chaplygin gas in loop quantum cosmology
Mubasher Jamil, Ujjal Debnath
7 pages, 7 figures, accepted for publication in 'Astrophysics and Space Science'
(Submitted on 14 Feb 2011)
"We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by loop quantum cosmology. We consider dark energy of the form modified Chaplygin gas. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. It henceforth resolves the famous cosmic coincidence problem in modern cosmology. The statefinder parameters are also calculated to classify this dark energy model."

http://arxiv.org/abs/1102.2613
Photon Gas Thermodynamics in Doubly Special Relativity
Xinyu Zhang, Lijing Shao, Bo-Qiang Ma
17 pages, 7 figures
(Submitted on 13 Feb 2011)
"Doubly special relativity (DSR), with both an invariant velocity and an invariant length scale, elegantly preserves the principle of relativity between moving observers, and appears as a promising candidate of the quantum theory of gravity. We study the modifications of photon gas thermodynamics in the framework of DSR with an invariant length |lambda|, after properly taking into account the effects of modified dispersion relation, upper bounded energy-momentum space, and deformed integration measure. We show that with a positive lambda, the grand partition function, the energy density, the specific heat, the entropy, and the pressure are smaller than those of special relativity (SR), while the velocity of photons and the ratio of pressure to energy are larger. In contrast, with a negative $\lambda$, the quantum gravity effects show up in the opposite direction. However, these effects only manifest themselves significantly when the temperature is larger than 10^-3 E_Planck. Thus, DSR can have considerable influence on the early universe in cosmological study."

http://arxiv.org/abs/1102.2784
New Limits on Planck Scale Lorentz Violation from Gamma-ray Burst Polarization
Floyd W. Stecker
3 pages, submitted to Phys. Rev. Letters
(Submitted on 14 Feb 2011)
"Constraints on possible Lorentz invariance violation to first order in E/M_Planck for photons in the framework of effective field theory are discussed. Using the detection of polarized soft gamma-ray emission from the gamma-ray burst GRB041219a that indicates the absence of vacuum birefringence, together with a method for estimating the redshift of the burst, we improve the previous constraints on the dimension 5 Lorentz violating modification to the Lagrangian of an effective local QFT for QED by 4 orders of magnitude."

http://arxiv.org/abs/1102.2855
Causality, Bell's theorem, and Ontic Definiteness
Joe Henson
40 pages (26 main text), 3 figures
(Submitted on 14 Feb 2011)
"Bell's theorem shows that the reasonable relativistic causal principle known as 'local causality' is not compatible with the predictions of quantum mechanics. It is not possible maintain a satisfying causal principle of this type while dropping any of the better-known assumptions of Bell's theorem. However, another assumption of Bell's theorem is the use of classical logic. One part of this assumption is the principle of 'ontic definiteness', that is, that it must in principle be possible to assign definite truth values to all propositions treated in the theory. Once the logical setting is clarified somewhat, it can be seen that rejecting this principle does not in any way undermine the type of causal principle used by Bell. Without ontic definiteness, the deterministic causal condition known as Einstein Locality succeeds in banning superluminal influence (including signalling) whilst allowing correlations that violate Bell's inequalities. Objections to altering logic, and the consequences for operational and realistic viewpoints, are also addressed."

 

[MTd2]

http://arxiv.org/abs/1102.3130

Physical Boundary State for the Quantum 4-Simplex

Maité Dupuis, Etera R. Livine
(Submitted on 15 Feb 2011)
In the spinfoam framework for quantum gravity, we investigate the conditions to have a physical quantum state for the Barrett-Crane model for the 4d quantum gravity path integral. More precisely, we look at the simplest case of a single 4-simplex boundary and show that the requirement of working with a physical boundary state fixes the width of the semi-classical Gaussian wave-packet for the boundary 3d geometry. This is directly relevant to the graviton propagator calculations done in this framework, since the Gaussian width enters the numerical factors in front of the graviton correlations in the large scale asymptotical limit. Finally, we discuss the application of our computations to the Barrett-Crane model beyond the first order (of a single 4-simplex in the bulk) and to the more recent EPRL-FK spinfoam model.

 

[marcus]

http://arxiv.org/abs/1102.2948
Energy equipartition and minimal radius in entropic gravity
Hanno Sahlmann
4 pages, 3 figures
(Submitted on 15 Feb 2011)
"In this article, we investigate the assumption of equipartition of energy in arguments for the entropic nature of gravity. It has already been pointed out by other authors that equipartition is not valid for low temperatures. Here we additionally point out that it is similarly not valid for systems with bounded energy. Many explanations for black hole entropy suggest that the microscopic systems responsible have a finite dimensional state space, and thus finite maximum energy. Assuming this to be the case leads to drastic corrections to Newton's law for high gravitational fields, and in particular to a singularity in acceleration at finite radius away from a point mass. This is suggestive of the physics at the Schwarzschild radius. We show, however, that the location of the singularity scales differently."

 

[marcus]

http://arxiv.org/abs/1102.3297
An action for higher spin gauge theory in four dimensions
Nima Doroud, Lee Smolin
28 pages
(Submitted on 16 Feb 2011)
"An action principle is presented for Vasiliev's Bosonic higher spin gauge theory in four spacetime dimensions. The action is of the form of a broken topological field theory, and arises by an extension of the MacDowell-Mansouri formulation of general relativity. In the latter theory the local degrees of freedom of general relativity arise by breaking the gauge invariance of a topological theory from sp(4) to the Lorentz algebra. In Vasiliev's theory the infinite number of degrees of freedom with higher spins similarly arise by the breaking of a topological theory with an infinite dimensional gauge symmetry extending sp(4) to the Lorentz algebra.
The Hamiltonian formulation of Vasilev's theory is then derived from our action, and it is shown that the Hamiltonian is a linear combination of constraints, as expected for a diffeomorphism invariant theory. The constraint algebra is computed and found to be first class."

Strictly speaking the next doesn't belong in this bibliography which focuses on non-string QG research papers: Loop-and-allied QG. But Torsten is PF Beyond forum "family", so in any event here is a new Torsten paper!

http://arxiv.org/abs/1102.3274
Quantum D-branes and exotic smooth R⁴
Torsten Asselmeyer-Maluga, Jerzy Krol
16 pages, see arXiv/1101.3169 for Part 1 This is part 2 of the work based on the talk "Small exotic smooth R⁴ and string theory'' given at the International Congress of Mathematicians, ICM2010, 19-28.08.2010, Hyderabad, India
(Submitted on 16 Feb 2011)
"In this paper, we present the idea that the formalism of string theory is connected with the dimension 4 in a new way, not covered by phenomenological or model-building approaches. The main connection is given by structures induced by small exotic smooth R⁴'s having intrinsic meaning for physics in dimension 4. We extend the notion of stable quantum D-branes in a separable noncommutative C* algebras over convolution algebras corresponding to the codimension-1 foliations of S³ which are mainly connected to small exotic R⁴. The tools of topological K-homology and K-theory as well KK-theory describe stable quantum branes in the C* algebras when naturally extended to algebras. In case of convolution algebras, small exotic smooth R⁴'s embedded in exotic R⁴ correspond to a generalized quantum branes on the algebras. These results extend the correspondence between exotic R⁴ and classical D and NS branes from our previous work."

 

[marcus]

http://arxiv.org/abs/1102.3660
Lectures on loop gravity
Carlo Rovelli
24 pages. 10 figures. I am still correcting and updating the lectures. Comments and corrections very welcome.
(Submitted on 17 Feb 2011)
"This is the first version of the introductory lectures on loop quantum gravity that I will give at the quantum gravity school in Zakopane. The theory is presented in self-contained form, without emphasis on its derivation from classical general relativity. Dynamics is given in the covariant form. The approximations needed to compute physical quantities are discussed. Some applications are described, including the recent derivation of de Sitter cosmology from full quantum gravity."


http://arxiv.org/abs/1102.3474
A Hamiltonian Formulation of the BKL Conjecture
Abhay Ashtekar, Adam Henderson, David Sloan
26 pages, 3 figures
(Submitted on 17 Feb 2011)
"The Belinskii, Khalatnikov and Lifgarbagez conjecture [bkl1] posits that on approach to a space-like singularity in general relativity the dynamics are well approximated by `ignoring spatial derivatives in favor of time derivatives.' In [ahs1] we examined this idea from within a Hamiltonian framework and provided a new formulation of the conjecture in terms of variables well suited to loop quantum gravity. We now present the details of the analytical part of that investigation. While our motivation came from quantum considerations, thanks to some of its new features, our formulation should be useful also for future analytical and numerical investigations within general relativity."

 

[marcus]

http://arxiv.org/abs/1102.3929
The Semiclassical Limit of Causal Dynamical Triangulations
J. Ambjorn, A. Gorlich, J. Jurkiewicz, R. Loll, J. Gizbert-Studnicki, T. Trzesniewski
30 pages, 10 figures
(Submitted on 18 Feb 2011)
"Previous work has shown that the macroscopic structure of the theory of quantum gravity defined by causal dynamical triangulations (CDT) is compatible with that of a de Sitter universe. After emphasizing the strictly nonperturbative nature of this semiclassical limit we present a detailed study of the three-volume data, which allows us to re-confirm the de Sitter structure, exhibit short-distance discretization effects, and make a first detailed investigation of the presence of higher-order curvature terms in the effective action for the scale factor. Technically, we make use of a novel way of fixing the total four-volume in the simulations."

Brief mention (unrelated but of wider interest):
http://arxiv.org/abs/1102.3926
Habitability of the Goldilocks Planet Gliese 581g: Results from Geodynamic Models
W. von Bloh, M. Cuntz, S. Franck, C. Bounama
(Submitted on 18 Feb 2011)

 

[MTd2]

http://arxiv.org/abs/1102.4109

Various Facets of Spacetime Foam

Y. Jack Ng
(Submitted on 20 Feb 2011)
Spacetime foam manifests itself in a variety of ways. It has some attributes of a turbulent fluid. It is the source of the holographic principle. Cosmologically it may play a role in explaining why the energy density has the critical value, why dark energy/matter exists, and why the effective dynamical cosmological constant has the value as observed. Astrophysically the physics of spacetime foam helps to elucidate why the critical acceleration in modified Newtonian dynamics has the observed value; and it provides a possible connection between global physics and local galactic dynamics involving the phenomenon of flat rotation curves of galaxies and the observed Tully-Fisher relation. Spacetime foam physics also sheds light on nonlocal gravitational dynamics.

 

[marcus]

http://arxiv.org/abs/1102.4755
Path Integral and Effective Hamiltonian in Loop Quantum Cosmology
Haiyun Huang, Yongge Ma, Li Qin
9 pages
(Submitted on 23 Feb 2011)
"We study the path integral formulation of Friedmann universe filled with a massless scalar field in loop quantum cosmology. All the isotropic models of k=0,+1,-1 are considered. Since the transition amplitude in the deparameterized framework can be expressed in terms of group averaging, the path integrals can be formulated for both deparameterized and timeless frameworks. It turns out that the effective Hamiltonian derived from the path integral in deparameterized framework is equivalent to the effective Hamiltonian constraint derived from the path integral in timeless framework, since they lead to same equations of motion. Moreover, the effective Hamiltonian constraints of above models derived in canonical theory are confirmed by the path integral formulation."

http://arxiv.org/abs/1102.4624
Renormalisation group and the Planck scale
Daniel F. Litim
14 pages, 6 figures, to appear in the Philosophical Transactions A of the Royal Society, London
(Submitted on 22 Feb 2011)
"I discuss the renormalisation group approach to gravity, its link to Steven Weinberg's asymptotic safety scenario, and give an overview of results with applications to particle physics and cosmology."

http://arxiv.org/abs/1102.4637
Speed of particles and a relativity of locality in kappa-Minkowski quantum spacetime
Giovanni Amelino-Camelia, Niccoló Loret, Giacomo Rosati
(Submitted on 22 Feb 2011)
"The last decade of research on kappa-Minkowski noncommutative spacetime has been strongly characterized by a controversy concerning the speed of propagation of massless particles. Most arguments suggested that this speed should depend on the momentum of the particle strongly enough to be of interest for some ongoing experimental studies. But the only explicit derivations of worldlines in kappa-Minkowski predicted no momentum dependence for the speed of massless particles. We return to this controversy equipped with the recent understanding (arXiv:1006.2126, arXiv:1007.0718, arXiv:1008.2962, arXiv:1101.0931) that in some quantum spacetimes coincidences of events assessed by an observer who is distant from the events can be artifactual, and particularly Smolin's thesis (arXiv:1007.0718) that kappa-Minkowski should be an example of such a spacetime. We therefore set up our investigation in such a way that we never rely on the assessment of coincidences of events by distant observers. This allows us to verify explicitly that in kappa-Minkowski simultaneously-emitted massless particles of different momentum are detected at different times, and establish a linear dependence of the detection times on momentum."

http://arxiv.org/abs/1102.4643
Analysis of the Semiclassical Solution of CDT
T. Trzesniewski
41 pages, 18 figures. Master Thesis, supervised by prof. dr hab. J. Jurkiewicz and defended in June 2010
(Submitted on 23 Feb 2011)
"Causal dynamical triangulations (CDT) constitute a background independent, nonperturbative approach to quantum gravity, in which the gravitational path integral is approximated by the weighted sum over causally well-behaving simplicial manifolds i.e. causal triangulations. This thesis is an analysis of the data from the Monte Carlo computer simulations of CDT in 3+1 dimensions. It is confirmed here that there exist the semiclassical limit of CDT for so-called (4,1) (or equivalent (1,4)) simplices, being a discrete version of the mini-superspace model. Next, the form of the corresponding discrete action is investigated. Furthermore, it is demonstrated that the effective, semiclassical solution works also after the inclusion of remaining (3,2) and (2,3) simplices, treated collectively. A specific form of the resulting extended discrete action is examined and a transition from the broader framework to the former narrower one is shown."

 

[marcus]

http://arxiv.org/abs/1102.5012
Asymptotically Safe Lorentzian Gravity
Elisa Manrique, Stefan Rechenberger, Frank Saueressig
4 pages, 2 figures
(Submitted on 24 Feb 2011)
"The gravitational asymptotic safety program strives for a consistent and predictive quantum theory of gravity based on a non-trivial ultraviolet fixed point of the renormalization group (RG) flow. We investigate this scenario by employing a novel functional renormalization group equation which takes the causal structure of space-time into account and connects the RG flows for Euclidean and Lorentzian signature by a Wick-rotation. Within the Einstein-Hilbert approximation, the beta-functions of both signatures exhibit ultraviolet fixed points in agreement with asymptotic safety. Surprisingly, the two fixed points have strikingly similar characteristics, suggesting that Euclidean and Lorentzian quantum gravity belong to the same universality class at high energies."

 

[MTd2]

http://arxiv.org/abs/1102.5084

Quantum Theory at Planck Scale, Limiting Values, Deformed Gravity and Dark Energy Problem

A.E. Shalyt-Margolin
(Submitted on 24 Feb 2011)
Within a theory of the existing fundamental length on the order of Planck's a high-energy deformation of the General Relativity for the space with horizon has been constructed. On this basis, Markov's work of the early eighties of the last century has been given a new interpretation to show that the heuristic model considered by him may be placed on a fundamental footing. The obtained results have been applied to solving of the dark energy problem, making it possible to frame the following hypothesis: a dynamic cosmological term is a measure of deviation from a thermodynamic identity (the first law of thermodynamics) of the high-energy (Planck's) deformation of Einstein equations for horizon spaces in their thermodynamic interpretation.

 

[MTd2]

Garrett Lisi presents today a talk at Institute for Astronomy, University of Hawaii.

http://www.ifa.hawaii.edu/specialevents/

Upcoming Events
Friday, February 25, 2011
Maikalani Community Lecture: Garrett Lisi, "A Geometry Theory of Everything," 6:30 p.m. at the Maikalani building in Pukalani. Free. Flier
Using pure geometry, Einstein described gravity as the warping of four-dimensional spacetime, with the measurements of rulers and clocks differing based on their position and motion. Similarly, the other forces and particles of nature are described as the twisting of geometric structures over spacetime, and around each other. Examining the patterns of twists, we’ll find that gravity and every elementary particle in our Universe may be facets of the most beautiful structure known to mathematics dancing over spacetime according to the laws of quantum physics.

The slides are available here:

www.deferentialgeometry.org , february 25th

 

[marcus]

http://arxiv.org/abs/1102.5759
The complete 1/N expansion of colored tensor models in arbitrary dimension
Razvan Gurau
(Submitted on 28 Feb 2011)
"In this paper we generalize the results of [1,2] and derive the full 1/N expansion of colored tensor models in arbitrary dimensions. We detail the expansion for the independent identically distributed model and the topological Boulatov Ooguri model."

http://arxiv.org/abs/1102.5439
Discreteness of the volume of space from Bohr-Sommerfeld quantization
Eugenio Bianchi, Hal M. Haggard
4 pages, 4 figures
(Submitted on 26 Feb 2011)
"A major challenge for any theory of quantum gravity is to quantize general relativity while retaining some part of its geometrical character. We present new evidence for the idea that this can be achieved by directly quantizing space itself. We compute the Bohr-Sommerfeld volume spectrum of a tetrahedron and show that it reproduces the quantization of a grain of space found in loop gravity."

 

[atyy]

http://arxiv.org/abs/1102.5524
Entanglement renormalization for quantum fields
Jutho Haegeman, Tobias J. Osborne, Henri Verschelde, Frank Verstraete
(Submitted on 27 Feb 2011)
p4, concluding paragraph: Looking further afield, the cMERA constitutes a realization of the holographic principle. It is tempting to speculate, building on [19], that cMERA are a natural candidate to establish a link between entanglement renormalization and the best known realization of the holographic principle, namely the AdS/CFT correspondence.

 

[atyy]

http://arxiv.org/abs/1103.0731
Geometric Aspects of Gauge and Spacetime Symmetries
Steffen Gielen
(Submitted on 3 Mar 2011)
We investigate several problems in relativity and particle physics where symmetries play a central role; in all cases geometric properties of Lie groups and their quotients are related to physical effects. The first part is concerned with symmetries in gravity. We apply the theory of Lie group deformations to isometry groups of exact solutions in general relativity, relating the algebraic properties of these groups to physical properties of the spacetimes. We then make group deformation local, generalising deformed special relativity (DSR) by describing gravity as a gauge theory of the de Sitter group. We find that in our construction Minkowski space has a connection with torsion; physical effects of torsion seem to rule out the proposed framework as a viable theory. A third chapter discusses a formulation of gravity as a topological BF theory with added linear constraints that reduce the symmetries of the topological theory to those of general relativity. We discretise our constructions and compare to a similar construction by Plebanski which uses quadratic constraints. In the second part we study CP violation in the electroweak sector of the standard model and certain extensions of it. We quantify fine-tuning in the observed magnitude of CP violation by determining a natural measure on the space of CKM matrices, a double quotient of SU(3), introducing different possible choices and comparing their predictions for CP violation. While one generically faces a fine-tuning problem, in the standard model the problem is removed by a measure that incorporates the observed quark masses, which suggests a close relation between a mass hierarchy and suppression of CP violation. Going beyond the standard model by adding a left-right symmetry spoils the result, leaving us to conclude that such additional symmetries appear less natural.

 

[marcus]

http://arxiv.org/abs/1103.1597
On the Asymptotics of Quantum Group Spinfoam Model
You Ding, Muxin Han
25 pages, 1 figure
(Submitted on 8 Mar 2011)
"Recently a quantum group deformation of EPRL spinfoam model was proposed in arXiv:1012.4216 by one of the authors, and in arXiv:1012.4784 by Fairbairn and Meusburger. It is interesting to study the high spin asymptotics of the quantum group spinfoam model, to see if it gives the discrete Einstein gravity with cosmological constant as its semiclassical limit. In this article we propose a new technique, which can simplify the analysis of the high spin asymptotics for quantum group spinfoam vertex amplitude. This technique can generalize the spinfoam asymptotic analysis developed by Barrett, et al to quantum group spinfoam. As a preparation of asymptotic analysis, we define and analyze the coherent states and coherent intertwiners for quantum group, which has certain 'factorization properties'. We show that in the high spin limit of quantum group spinfoam, many q-deformed noncommutative ingredients become classical and commutative. In particular, the squared norm of coherent intertwiner and the (Euclidean) vertex amplitude become integrals on classical group, while there are some additional terms (written in terms of classical group variables) make quantum group corrections to the usual (classical group) coherent intertwiner and vertex amplitude. These quantum group correction terms turn out to be proportional to the deformation parameter, which hopefully gives the cosmological term as its semiclassical limit."

http://arxiv.org/abs/1103.1428
Graviton propagator asymptotics and the classical limit of ELPR/FK spin foam models
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 8 Mar 2011)
"We study the classical limit of ELPR/FK spin foam models by computing the large-distance asymptotics of the spin foam graviton propagator. This is done by analyzing the large-spin asymptotics of the boundary spin-network wavefunction which corresponds to a flat space. By using the stationary phase method we determine the wavefunction asymptotics, which then determines the large-distance asymptotics of the corresponding graviton propagator. We show that the graviton propagator behaves for large distances as the inverse distance to the fourth power, which implies that general relativity is not the classical limit of the ELPR/FK spin foam models. Our result is a direct consequence of the large-spin asymptotics of the ELPR/FK spin-foam vertex amplitude and we show that the vertex amplitude can be modified such that the new amplitude has the desired asymptotics."

 

[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.