Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1005.3808
Constraining Fundamental Physics with Future CMB Experiments
Silvia Galli, Matteo Martinelli, Alessandro Melchiorri, Luca Pagano, Blake D. Sherwin, David N. Spergel
11 pages, 14 figures
(Submitted on 20 May 2010)
"The Planck experiment will soon provide a very accurate measurement of Cosmic Microwave Background anisotropies. This will let cosmologists determine most of the cosmological parameters with unprecedented accuracy. Future experiments will improve and complement the Planck data with better angular resolution and better polarization sensitivity. This unexplored region of the CMB power spectrum contains information on many parameters of interest, including neutrino mass, the number of relativistic particles at recombination, the primordial Helium abundance and the injection of additional ionizing photons by dark matter self-annihilation. We review the imprint of each parameter on the CMB and forecast the constraints achievable by future experiments by performing a Monte Carlo analysis on synthetic realizations of simulated data. We find that next generation satellite missions such as CMBPol could provide valuable constraints with a precision close to that expected in current and near future laboratory experiments. Finally, we discuss the implications of this intersection between cosmology and fundamental physics."

http://arxiv.org/abs/1005.4276
Gravitational Waves in the Spectral Action of Noncommutative Geometry
William Nelson, Joseph Ochoa, Mairi Sakellariadou
15 pages, 3 figures
(Submitted on 24 May 2010)
The spectral triple approach to noncommutative geometry allows one to develop the entire standard model (and supersymmetric extensions) of particle physics from a purely geometry stand point and thus treats both gravity and particle physics on the same footing. The bosonic sector of the theory contains a modification to Einstein-Hilbert gravity, involving a nonconformal coupling of curvature to the Higgs field and conformal Weyl term (in addition to a nondynamical topological term). In this paper we derive the weak field limit of this gravitational theory and show that the production and dynamics of gravitational waves are significantly altered. In particular, we show that the graviton contains a massive mode that alters the energy lost to gravitational radiation, in systems with evolving quadrupole moment. We explicitly calculate the general solution and apply it to systems with periodically varying quadrupole moments, focusing in particular on the the well know energy loss formula for circular binaries.

http://arxiv.org/abs/1005.4279
Constraining the Noncommutative Spectral Action via Astrophysical Observations
William Nelson, Joseph Ochoa, Mairi Sakellariadou
5 pages
(Submitted on 24 May 2010)
The noncommutative spectral action extends our familiar notion of commutative spaces, using the data encoded in a spectral triple on an almost commutative space. Varying a rather simple action, one can derive all of the standard model of particle physics in this setting, in addition to a modified version of Einstein-Hilbert gravity. Thus, noncommutative geometry provides a geometric interpretation of particle physics coupled to curvature. In this letter we use observations of pulsar timings, assuming that no deviation from General Relativity has been observed, to constrain the gravitational sector of this theory. Thus, we directly constrain noncommutative geometry, a potential grand unified theory of physics, via astrophysical observations. Whilst the bounds on the coupling constants remain rather weak, they are comparable to existing bounds on deviations from General Relativity in other settings and are likely to be further constrained by future observations.

http://arxiv.org/abs/1005.4172
A Derivation of Special Relativity from Causal Sets
Kevin H. Knuth, Newshaw Bahrenyi
17 pages, 7 figures
(Submitted on 23 May 2010)
We present a novel derivation of special relativity based on the information physics of events comprising a causal set. We postulate that events are fundamental, and that some events have the potential to receive information about other events, but not vice versa. This leads to the concept of a partially-ordered set of events, which is called a causal set. Quantification proceeds by selecting two chains of coordinated events, each of which represents an observer, and assigning a valuation to each chain. Events can be projected onto each chain by identifying the earliest event on the chain that can be informed about the event. In this way, each event can be quantified by a pair of numbers, referred to a pair, that derives from the valuations on the chains. Pairs can be decomposed into a sum of symmetric and antisymmetric pairs, which correspond to time-like and space-like coordinates. From this pair, we derive a scalar measure and show that this is the Minkowski metric. The Lorentz transformations follow, as well as the fact that speed is a relevant quantity relating two inertial frames, and that there exists a maximal speed, which is invariant in all inertial frames. All results follow directly from the Event Postulate and the adopted quantification scheme.

 

[MTd2]

http://arxiv.org/abs/1005.5491

The Big Bang and the Quantum

Abhay Ashtekar
(Submitted on 29 May 2010)
This short review is addressed to cosmologists.
General relativity predicts that space-time comes to an end and physics comes to a halt at the big-bang. Recent developments in loop quantum cosmology have shown that these predictions cannot be trusted. Quantum geometry effects can resolve singularities, thereby opening new vistas. Examples are: The big bang is replaced by a quantum bounce; the `horizon problem' disappears; immediately after the big bounce, there is a super-inflationary phase with its own phenomenological ramifications; and, in presence of a standard inflaton potential, initial conditions are naturally set for a long, slow roll inflation independently of what happens in the pre-big bang branch.

http://arxiv.org/abs/1005.5565

Loop quantum cosmology of Bianchi type IX models

Edward Wilson-Ewing
(Submitted on 30 May 2010)
The loop quantum cosmology "improved dynamics" of the Bianchi type IX model are studied. The action of the Hamiltonian constraint operator is obtained via techniques developed for the Bianchi type I and type II models, no new input is required. It is shown that the big bang and big crunch singularities are resolved by quantum gravity effects. We also present the effective equations which provide modifications to the classical equations of motion due to quantum geometry effects.

http://arxiv.org/abs/1005.5589

Generalized Second Law of Thermodynamics on the Event Horizon for Interacting Dark Energy

Nairwita Mazumder, Subenoy Chakraborty
(Submitted on 31 May 2010)
Here we are trying to find the conditions for the validity of the generalized second law of thermodynamics (GSLT) assuming the first law of thermodynamics on the event horizon in both cases when the FRW universe is filled with interacting two fluid system- one in the form of cold dark matter and the other is either holographic dark energy or new age graphic dark energy. Using the recent observational data we have found that GSLT holds both in quintessence era as well as in phantom era for new age graphic model while for holographic dark energy GSLT is valid only in phantom era.

http://arxiv.org/abs/1005.5605

Charged rotating noncommutative black holes

Leonardo Modesto, Piero Nicolini
(Submitted on 31 May 2010)
In this paper we complete the program of the Noncomutative Geometry inspired black holes, providing the richest possible solution, endowed with mass, charge and angular momentum. After providing a prescription for employing the Newmann-Janis algorithm in case of nonvanishing stress tensors, we find regular axisymmetric charged black holes in the presence of a minimal length. We study also the new thermodynamics and we determine the corresponding higher-dimensional solutions. As a conclusion we make some consideration about possible applications.

http://arxiv.org/abs/1005.5654

Inhomogeneous Loop Quantum Cosmology: Hybrid Quantization of the Gowdy Model

L.J. Garay, M. Martín-Benito, G.A. Mena Marugán
(Submitted on 31 May 2010)
The Gowdy cosmologies provide a suitable arena to further develop Loop Quantum Cosmology, allowing the presence of inhomogeneities. For the particular case of Gowdy spacetimes with the spatial topology of a three-torus and a content of linearly polarized gravitational waves, we detail a hybrid quantum theory in which we combine a loop quantization of the degrees of freedom that parametrize the subfamily of homogeneous solutions, which represent Bianchi I spacetimes, and a Fock quantization of the inhomogeneities. Two different theories are constructed and compared, corresponding to two different schemes for the quantization of the Bianchi I model within the {\sl improved dynamics} formalism of Loop Quantum Cosmology. One of these schemes has been recently put forward by Ashtekar and Wilson-Ewing. We address several issues including the quantum resolution of the cosmological singularity, the structure of the superselection sectors in the quantum system, or the construction of the Hilbert space of physical states.

 

[MTd2]

http://arxiv.org/abs/1005.5460

Shape in an Atom of Space: Exploring quantum geometry phenomenology
Authors: Seth A. Major
(Submitted on 29 May 2010)

Abstract: A phenomenology for the deep spatial geometry of loop quantum gravity is introduced. In the context of a simple model, an atom of space, it is shown how purely combinatorial structures can affect observations. The angle operator is used to develop a model of angular corrections to local, continuum flat-space 3-geometries. The physical effects involve neither breaking of local Lorentz invariance nor Planck scale suppression, but rather reply on only the combinatorics of SU(2) recoupling. Bhabha scattering is discussed as an example of how the effects might be observationally accessible.

 

[marcus]

http://arxiv.org/abs/1006.0199
From twistors to twisted geometries
Laurent Freidel, Simone Speziale
9 pages
(Submitted on 1 Jun 2010)
"In a previous paper we showed that the phase space of loop quantum gravity on a fixed graph can be parametrized in terms of twisted geometries, quantities describing the intrinsic and extrinsic discrete geometry of a cellular decomposition dual to the graph. Here we unravel the origin of the phase space from a geometric interpretation of twistors."

http://arxiv.org/abs/1006.0192
Inflationary solutions in asymptotically safe f(R) gravity
Alfio Bonanno, Adriano Contillo, Roberto Percacci
31 pages, 8 figures
(Submitted on 1 Jun 2010)
"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."

http://arxiv.org/abs/1006.0099
Bimetric Renormalization Group Flows in Quantum Einstein Gravity
Elisa Manrique, Martin Reuter, Frank Saueressig
35 pages, 3 figures
(Submitted on 1 Jun 2010)
"The formulation of an exact functional renormalization group equation for Quantum Einstein Gravity necessitates that the underlying effective average action depends on two metrics, a dynamical metric giving the vacuum expectation value of the quantum field, and a background metric supplying the coarse graining scale. The central requirement of "background independence" is met by leaving the background metric completely arbitrary. This bimetric structure entails that the effective average action may contain three classes of interactions: those built from the dynamical metric only, terms which are purely background, and those involving a mixture of both metrics. This work initiates the first study of the full-fledged gravitational RG flow, which explicitly accounts for this bimetric structure, by considering an ansatz for the effective average action which includes all three classes of interactions. It is shown that the non-trivial gravitational RG fixed point central to the Asymptotic Safety program persists upon disentangling the dynamical and background terms. Moreover, upon including the mixed terms, a second non-trivial fixed point emerges, which may control the theory's IR behavior."

http://arxiv.org/abs/1006.0007
Testing quantum-spacetime relativity with gamma-ray telescopes
Giovanni Amelino-Camelia, Antonino Marciano, Marco Matassa, Giacomo Rosati
5 pages
(Submitted on 31 May 2010)
"Observations of gamma-ray bursts are being used to test for a momentum dependence of the speed of photons, partly motivated by preliminary results reported in analyses of some quantum-spacetime scenarios. The relationship between time of arrival, momentum of the photon and redshift of the source which is used for these purposes assumes a 'breakdown' of relativistic symmetries, meaning that it is a preferred-frame scenario which does not satisfy the Relativity Principle. The alternative hypothesis of a 'deformation' of relativistic symmetries, which preserves the Relativity Principle by adopting deformed laws of relativistic transformation between observers, could not so far be tested in gamma-ray-burst observations because it was not known how to formulate it in expanding spacetimes. We here provide such a formulation, and we find that also for the symmetry-deformation scenario the analysis of gamma-ray-burst data take us very close to the desired 'Planck-scale sensitivity'."

 

[marcus]

http://arxiv.org/abs/1006.0232
Spinning Loop Black Holes
Francesco Caravelli, Leonardo Modesto
18 pages, 18 figures
(Submitted on 1 Jun 2010)
"In this paper we construct four Kerr-like spacetimes starting from the loop black hole Schwarzschild solutions (LBH) and applying the Newman-Janis transformation. In previous papers the Schwarzschild LBH was obtained replacing the Ashtekar connection with holonomies on a particular graph in a minisuperspace approximation which describes the black hole interior. Starting from this solution, we use a Newman-Janis transformation and we specialize to two different and natural complexifications inspired from the complexifications of the Schwarzschild and Reissner-Nordstrom metrics. We show explicitly that the space-times obtained in this way are singularity free and thus there are no naked singularities. We show that the transformation move, if any, the causality violating regions of the Kerr metric far from r=0. We study the space-time structure with particular attention to the horizons shape. We conclude the paper with a discussion on a regular Reissner-Nordstrom black hole derived from the Schwarzschild LBH and then applying again the Newmann-Janis transformation."

http://arxiv.org/abs/1006.0388
On loop quantum gravity kinematics with non-degenerate spatial background
Hanno Sahlmann
13 pages, 2 figures
(Submitted on 2 Jun 2010)
"In a remarkable paper, T. Koslowski introduced kinematical representations for loop quantum gravity in which there is a non-degenerate spatial background metric present. He also considered their properties, and showed that Gauss and diffeomorphism constraints can be implemented. With the present article, we streamline and extend his treatment. In particular, we show that the standard regularization of the geometric operators leads to well defined operators in the new representations, and we work out their properties fully. We also give details on the implementation of the constraints. All of this is done in such a way as to show that the standard representation is a particular (and in some ways exceptional) case of the more general constructions. This does not mean that these new representations are as fundamental as the standard one. Rather, we believe they might be useful to find some form of effective theory of loop quantum gravity on large scales."

Sahlmann refers to a 2007 paper of Koslowski which for some reason was unfamiliar to me. Since I may have failed to notice it, I will post the abstract here along with Sahlmann's:

http://arxiv.org/abs/0709.3465
Dynamical Quantum Geometry (DQG Programme)
Tim A. Koslowski
19 pages, 3 figures
(Submitted on 21 Sep 2007)
"In this brief note (written as a lengthy letter), we describe the construction of a representation for the Weyl-algebra underlying Loop Quantum Geometry constructed from a diffeomorphism variant state, which corresponds to a 'condensate' of Loop Quantum Geometry, resembling a static spatial geometry. We present the kinematical GNS-representation and the gauge- and diffeomorphism invariant Hilbert space representation and show that the expectation values of the geometric operators take essentialy classical values plus quantum corrections, which is similar to a 'local condensate' of quantum geometry. We describe the idea for the construction of a scale dependent asymptotic map into a family of scale dependent lattice gauge theories, where scale separates the essential geometry and a low energy effective theory, which is described as degrees of freedom in the lattice gauge theory. If this idea can be implemented then it is likely to turn out that this Hilbert space contains in addition to gravity also gauge coupled 'extra degrees of freedom', which may not be dynamically irrelevant."

http://pirsa.org/10060000/
Gravity: A different Perspective [video]
Thanu Padmanabhan
2 June 2010
"Combining the principles of general relativity and quantum theory still remains as elusive as ever. Recent work, that concentrated on one of the points of contact (and conflict) between quantum theory and general relativity, suggests a new perspective on gravity. It appears that the gravitational dynamics in a wide class of theories - including, but not limited to, standard Einstein's theory - can be given a purely thermodynamic interpretation. In this approach gravity appears as an emergent phenomenon, like e.g., gas or fluid dynamics. I will describe the necessary background, key results and their implications as suggested by my recent work in this area."

 

[marcus]

http://arxiv.org/abs/1006.0634
Black hole entropy from an SU(2)-invariant formulation of Type I isolated horizons
Jonathan Engle, Karim Noui, Alejandro Perez, Daniele Pranzetti
30 pages, 1 figure
(Submitted on 3 Jun 2010)
"A detailed analysis of the spherically symmetric isolated horizon system is performed in terms of the connection formulation of general relativity. The system is shown to admit a manifestly SU(2) invariant formulation where the (effective) horizon degrees of freedom are described by an SU(2) Chern-Simons theory. This leads to a more transparent description of the quantum theory in the context of loop quantum gravity and modifications of the form of the horizon entropy."

http://arxiv.org/abs/1006.0700
On knottings in the physical Hilbert space of LQG as given by the EPRL model
Benjamin Bahr
22 pages, 14 figures
(Submitted on 3 Jun 2010)
"We consider the EPRL spin foam amplitude for arbitrary embedded two-complexes. Choosing a definition of the face- and edge amplitudes which lead to spin foam amplitudes invariant under trivial subdivisions, we investigate invariance properties of the amplitude under consistent deformations, which are deformations of the embedded two-complex where faces are allowed to pass through each other in a controlled way. Using this surprising invariance, we are able to show that in the physical Hilbert space as defined by the sum over all spin foams contains no knotting classes of graphs anymore."

http://arxiv.org/abs/1006.0714
Lost in Translation: Topological Singularities in Group Field Theory
Razvan Gurau
(Submitted on 3 Jun 2010)
"Random matrix models generalize to Group Field Theories (GFT) whose Feynman graphs are dual to gluings of higher dimensional simplices. It is generally assumed that GFT graphs are always dual to pseudo manifolds. In this paper we prove that already in dimension three (and in all higher dimensions), this is not true due to subtle differences between simplicial complexes and gluings dual to GFT graphs. We prove however that, fortunately, the recently introduced "colored" GFT models [1] do not suffer from this problem and only generate graphs dual to pseudo manifolds in any dimension."

http://arxiv.org/abs/1006.0718
Asymptotic Safety, Asymptotic Darkness, and the hoop conjecture in the extreme UV
Sayandeb Basu, David Mattingly
9 pages
(Submitted on 3 Jun 2010)
"Assuming the hoop conjecture in classical general relativity and quantum mechanics, any observer who attempts to perform an experiment in an arbitrarily small region will be stymied by the formation of a black hole within the spatial domain of the experiment. This behavior is often invoked in arguments for a fundamental minimum length. Extending a proof of the hoop conjecture for spherical symmetry to include higher curvature terms we investigate this minimum length argument when the gravitational couplings run with energy in the manner predicted by asymptotically safe gravity. We show that argument for the mandatory formation of a black hole within the domain of an experiment fails. Neither is there a proof that a black hole doesn't form. Instead, whether or not an observer can perform measurements in arbitrarily small regions depends on the specific numerical values of the couplings near the UV fixed point. We further argue that when an experiment is localized on a scale much smaller than the Planck length, at least one enshrouding horizon must form outside the domain of the experiment. This implies that while an observer may still be able to perform local experiments, communicating any information out to infinity is prevented by a large horizon surrounding it, and thus compatibility with general relativity can still be restored in the infrared limit."

 

[John86]

http://arxiv.org/abs/1006.0338
Closed timelike curves, superluminal signals, and "free will" in universal quantum mechanics
Authors: H. Nikolic
(Submitted on 2 Jun 2010)
Abstract: We explore some implications of the hypothesis that quantum mechanics (QM) is universal, i.e., that QM does not merely describe information accessible to observers, but that it also describes the observers themselves. From that point of view, "free will" (FW) - the ability of experimentalists to make free choices of initial conditions - is merely an illusion. As a consequence, by entangling a part of brain (responsible for the illusion of FW) with a distant particle, one may create nonlocal correlations that can be interpreted as superluminal signals. In addition, if FW is an illusion, then QM on a closed timelike curve can be made consistent even without the Deutch nonlinear consistency constraint.

an older paper, via FQXI

http://arxiv.org/abs/0910.4222
Quantum information: primitive notions and quantum correlations
Authors: Valerio Scarani
(Submitted on 22 Oct 2009)
Abstract: This series of introductory lectures consists of two parts. In the first part, I rapidly review the basic notions of quantum physics and many primitives of quantum information (i.e. notions that one must be somehow familiar with in the field, like cloning, teleportation, state estimation...). The second part is devoted to a detailed introduction to the topic of quantum correlations, covering the evidence for failure of alternative theories, some aspects of the formalism of no-signaling probability distributions and some hints towards some current research topics in the field.

http://arxiv.org/abs/1006.0723
Action in the Entropic Revolution of Newtonian Gravity
Authors: Joakim Munkhammar
(Submitted on 17 May 2010)
Abstract: The theory of gravity has undergone somewhat of a revolution lately. Gravity is no longer a fundamental force it seems, but rather an effect of holographic entropy. Building on the works by Jacobsson, Padmanabhan and Verlinde we review the concept of Newtonian gravity as an entropic force and discuss a possible general action approach to Verlinde's theory. We also discuss some open problems and future prospects of Verlinde's approach.

http://arxiv.org/abs/0912.4740
Foliable Operational Structures for General Probabilistic Theories
Authors: Lucien Hardy
(Submitted on 23 Dec 2009)
Abstract: In this chapter a general mathematical framework for probabilistic theories of operationally understood circuits is laid out. Circuits are comprised of operations and wires. An operation is one use of an apparatus and a wire is a diagrammatic device for showing how apertures on the apparatuses are placed next to each other. Mathematical objects are defined in terms of the circuit understood graphically. In particular, we do not think of the circuit as sitting in a background time. Circuits can be foliated by hypersurfaces comprised of sets of wires. Systems are defined to be associated with wires. A closable set of operations is defined to be one for which the probability associated with any circuit built from this set is independent both of choices on other circuits and of extra circuitry that may be added to outputs from this circuit. States can be associated with circuit fragments corresponding to preparations. These states evolve on passing through circuit fragments corresponding to transformations. The composition of transformations is treated. A number of theorems are proven including one which rules out quaternionic quantum theory. The case of locally tomographic theories (where local measurements on a systems components suffice to determine the global state) is considered. For such theories the probability can be calculated for a circuit from matrices pertaining the operations that comprise that circuit. Classical probability theory and quantum theory are exhibited as examples in this framework.

http://arxiv.org/abs/1005.5164
A formalism-local framework for general probabilistic theories including quantum theory
Authors: Lucien Hardy
(Submitted on 27 May 2010)
Abstract: In this paper we consider general probabilistic theories that pertain to circuits which satisfy two very natural assumptions. We provide a formalism that is local in the following very specific sense: calculations pertaining to any region of spacetime employ only mathematical objects associated with that region. We call this "formalism locality". It incorporates the idea that space and time should be treated on an equal footing. Formulations that use a foliation of spacetime to evolve a state do not have this property nor do histories-based approaches. An operation (see figure on left) has inputs and outputs (through which systems travel). A circuit is built by wiring together operations such that we have no open inputs or outputs left over. A fragment (see figure on right) is a part of a circuit and may have open inputs and outputs. We show how each operation is associated with a certain mathematical object which we call a "duotensor" (this is like a tensor but with a bit more structure). In the figure on the left we show how a duotensor is represented graphically. We can link duotensors together such that black and white dots match up to get the duotensor corresponding to any fragment. The figure on the right is the duotensor for the above fragment. Links represent summing over the corresponding indices. We can use such duotensors to make probabilistic statements pertaining to fragments. Since fragments are the circuit equivalent of arbitrary spacetime regions we have formalism locality. The probability for a circuit is given by the corresponding duotensorial calculation (which is a scalar since there are no indices left over). We show how to put classical probability theory and quantum theory into this framework. [Note: the abstract in the paper has pictures.]

 

[marcus]

http://arxiv.org/abs/1006.1294
Physical boundary Hilbert space and volume operator in the Lorentzian new spin-foam theory
You Ding, Carlo Rovelli
11 pages
(Submitted on 7 Jun 2010)
"A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In this paper we reconsider the implementation of the constraints that defines the model. We define in a simple way the boundary Hilbert space of the theory, introducing a slight modification of the embedding of the SU(2) representations into the SL(2,C) ones. We then show directly that all constraints vanish on this space in a weak sense. The vanishing is exact (and not just in the large quantum number limit.) We also generalize the definition of the volume operator in the spinfoam model to the Lorentzian signature, and show that it matches the one of loop quantum gravity, as does in the Euclidean case."

http://arxiv.org/abs/1006.1299
Warm inflationary model in loop quantum cosmology
Ramon Herrera
15 pages, 1 figure, Accepted for publication in Phys. Rev. D
(Submitted on 7 Jun 2010)
"A warm inflationary universe model in loop quantum cosmology is studied. In general we discuss the condition of inflation in this framework. By using a chaotic potential, $$V(\phi)\propto \phi^2$$, we develop a model where the dissipation coefficient $$\Gamma=\Gamma_0=$$ constant. We use recent astronomical observations for constraining the parameters appearing in our model."

http://arxiv.org/abs/1006.1248
Holography in the EPRL Model
Louis Crane
8 pages, research announcement
(Submitted on 7 Jun 2010)
"In this research announcement, we propose a new interpretation of the EPR quantization of the BC model using a functor we call the time functor, which is the first example of a CLa-ren functor. Under the hypothesis that the universe is in the Kodama state, we construct a holographic version of the model. Generalisations to other CLa-ren functors and connections to model category theory are considered."

 

[marcus]

http://arxiv.org/abs/1006.1763
Might black holes reveal their inner secrets?
Ted Jacobson, Thomas P. Sotiriou
Essay written for FQXi essay competition "What is Ultimately Possible in Physics", Third Prize Winner
(Submitted on 9 Jun 2010)
"Black holes harbor a spacetime singularity of infinite curvature, where classical spacetime physics breaks down, and current theory cannot predict what will happen. However, the singularity is invisible from the outside because strong gravity traps all signals, even light, behind an event horizon. In this essay we discuss whether it might be possible to destroy the horizon, if a body is tossed into the black hole so as to make it spin faster and/or have more charge than a certain limit. It turns out that one could expose a "naked" singularity if effects of the body's own gravity can be neglected. We suspect however that such neglect is unjustified." (based on Phys. Rev. Lett. 103, 141101 (2009), arXiv:0907.4146 [gr-qc])

http://arxiv.org/abs/1006.1764
Destroying black holes with test bodies
Ted Jacobson, Thomas P. Sotiriou
Contribution to the proceedings of the First Mediterranean Conference on Classical and Quantum Gravity (talk given by T. P. S.). Summarizes the results of Phys. Rev. Lett. 103, 141101 (2009), arXiv:0907.4146 and considers further examples
(Submitted on 9 Jun 2010)
"If a black hole can accrete a body whose spin or charge would send the black hole parameters over the extremal limit, then a naked singularity would presumably form, in violation of the cosmic censorship conjecture. We review some previous results on testing cosmic censorship in this way using the test body approximation, focusing mostly on the case of neutral black holes. Under certain conditions a black hole can indeed be over-spun or over-charged in this approximation, hence radiative and self-force effects must be taken into account to further test cosmic censorship."

http://arxiv.org/abs/1006.1814
Shortcomings of the Big Bounce derivation in Loop Quantum Cosmology
Francesco Cianfrani, Giovanni Montani
4 pages
(Submitted on 9 Jun 2010)
"A prescription to define in Loop Quantum Gravity the electric field operator related to the scale factor of an homogeneous and isotropic cosmological space-time is given. This procedure allows to link the fundamental theory with its cosmological implementation. In view of the conjugate relation existing between holonomies and fluxes, the edge length and the area of surfaces in the fiducial metric satisfy a duality condition. As a consequence, the area operator has a discrete spectrum also in Loop Quantum Cosmology. This feature leaves open the question about the super-Hamiltonian regularization."

 

[MTd2]

http://arxiv.org/abs/1006.2126

Taming nonlocality in theories with deformed Poincare symmetry

Giovanni Amelino-Camelia, Marco Matassa, Flavio Mercati, Giacomo Rosati
(Submitted on 10 Jun 2010)
We here advocate a perspective on recent research investigating possible Planck-scale deformations of relativistic symmetries, which is centered on Einstein's characterization of spacetime points, given exclusively in terms of physical events. We provide the first ever explicit construction of worldlines governed by a Planck-scale deformation of Poincar\'e symmetry. And we show that the emerging physical picture allows a description that is faithful to Einstein's program, but forces the renunciation of the idealization of the coincidence of events. We use this to expose the limitations of the pre-Einsteinian description of spacetime points adopted in some recent related studies. In particular we find that the estimate of nonlocal effects reported in the recent Physical Review Letters 104, 140402 (2010) is incorrect by 29 orders of magnitude.

 

[atyy]

http://arxiv.org/abs/1006.1902
Wilsonian Approach to Fluid/Gravity Duality
Irene Bredberg, Cynthia Keeler, Vyacheslav Lysov, Andrew Strominger
(Submitted on 9 Jun 2010)
The problem of gravitational fluctuations confined inside a finite cutoff at radius $r=r_c$ outside the horizon in a general class of black hole geometries is considered. Consistent boundary conditions at both the cutoff surface and the horizon are found and the resulting modes analyzed. For general cutoff $r_c$ the dispersion relation is shown at long wavelengths to be that of a linearized Navier-Stokes fluid living on the cutoff surface. A cutoff-dependent line-integral formula for the diffusion constant $D(r_c)$ is derived. The dependence on $r_c$ is interpreted as renormalization group (RG) flow in the fluid. Taking the cutoff to infinity in an asymptotically AdS context, the formula for $D(\infty)$ reproduces as a special case well-known results derived using AdS/CFT. Taking the cutoff to the horizon, the effective speed of sound goes to infinity, the fluid becomes incompressible and the Navier-Stokes dispersion relation becomes exact. The resulting universal formula for the diffusion constant $D(horizon)$ reproduces old results from the membrane paradigm. Hence the old membrane paradigm results and new AdS/CFT results are related by RG flow. RG flow-invariance of the viscosity to entropy ratio $\eta /s$ is shown to follow from the first law of thermodynamics together with isentropy of radial evolution in classical gravity. The ratio is expected to run when quantum gravitational corrections are included.

 

[marcus]

http://arxiv.org/abs/1006.2369
Hybrid Quantization: From Bianchi I to the Gowdy Model
Mercedes Martín-Benito, Guillermo A. Mena Marugán, Edward Wilson-Ewing
20 pages
(Submitted on 11 Jun 2010)
"The Gowdy cosmologies are vacuum solutions to the Einstein equations which possesses two space-like Killing vectors and whose spatial sections are compact. We consider the simplest of these cosmological models: the case where the spatial topology is that of a three-torus and the gravitational waves are linearly polarized. The subset of homogeneous solutions to this Gowdy model are vacuum Bianchi I spacetimes with a three-torus topology. We deepen the analysis of the loop quantization of these Bianchi I universes adopting the improved dynamics scheme put forward recently by Ashtekar and Wilson-Ewing. Then, we revisit the hybrid quantization of the Gowdy T³ cosmologies by combining this loop quantum cosmology description with a Fock quantization of the inhomogeneities over the homogeneous Bianchi I background. We show that, in vacuo, the Hamiltonian constraint of both the Bianchi I and the Gowdy models can be regarded as an evolution equation with respect to the volume of the Bianchi I universe. This evolution variable turns out to be discrete, with a strictly positive minimum. Furthermore, we argue that this evolution is well-defined inasmuch as the associated initial value problem is well posed: physical solutions are completely determined by the data on an initial section of constant Bianchi I volume. This fact allows us to carry out to completion the quantization of these two cosmological models."

Torsten and Helge have discussed their idea some with us here at PF Beyond. It is a radical and high-risk idea.
http://arxiv.org/abs/1006.2230
On the geometrization of matter by exotic smoothness
Torsten Asselmeyer-Maluga, Helge Rose
17 pages
(Submitted on 11 Jun 2010)
"Clifford's hypothesis is investigated: A particle is made up of nothing but a distinct type of a space manifold, differing from the surrounding manifold of empty space. It is shown that this distinct space manifold representing matter differs from the surrounding vacuum by the exotic smoothness of its spacetime. The smoothness structure of spacetime can be described by a tree-like subset -- the Casson handle -- consisting of immersed discs and connecting tubes between them. The Weierstrass representation shows that the immersed discs are represented by spinors fulfilling the Dirac equation and leading to a mass-less Dirac term in the Einstein-Hilbert action. The connecting tubes between the discs realize an action term of a gauge field. Both terms are purly geometrical and characterized by the mean curvature of the components of the Casson handle. This gives a good support to Clifford's conjecture that matter is nothing more but an exotic kind of space."

interesting conjecture quoted from right at the very end of the paper, on page 15:
"At the end we want to give another interpretation of the Casson handle. Connes [37] showed that by means of the non-commutative geometry the action of the standard model can be reproduced. His model is based on the space M×F where the additional space F is ad hoc and has no relation to the spacetime M.
In our model the space F could be interpreted as an expression of the Casson handle and so of the smoothness of spacetime establishing a deep relation between quantum matter and space."

 

[marcus]

http://arxiv.org/abs/1006.2451
Dynamics for a 2-vertex Quantum Gravity Model
Enrique F. Borja, Jacobo Diaz-Polo, Iñaki Garay, Etera R. Livine
28 pages
(Submitted on 12 Jun 2010)
"We use the recently introduced U(N) framework for loop quantum gravity to study the dynamics of spin network states on the simplest class of graphs: two vertices linked with an arbitrary number N of edges. Such graphs represent two regions, in and out, separated by a boundary surface. We study the algebraic structure of the Hilbert space of spin networks from the U(N) perspective. In particular, we describe the algebra of operators acting on that space and discuss their relation to the standard holonomy operator of loop quantum gravity. Furthermore, we show that it is possible to make the restriction to the isotropic/homogeneous sector of the model by imposing the invariance under a global U(N) symmetry. We then propose a U(N) invariant Hamiltonian operator and study the induced dynamics. Finally, we explore the analogies between this model and loop quantum cosmology and sketch some possible generalizations of it."

 

[marcus]

http://arxiv.org/abs/1006.3655
Gedanken experiments on nearly extremal black holes and the Third Law
Goffredo Chirco, Stefano Liberati, Thomas P. Sotiriou
5 pages
(Submitted on 18 Jun 2010)
"A gedanken experiment in which a black hole is pushed to spin at its maximal rate by tossing into it a test body is considered. After demonstrating that this is kinematically possible for a test body made of reasonable matter, we focus on its implications for black hole thermodynamics and the apparent violation of the third law (unattainability of the extremal black hole). We argue that this is not an actual violation, due to subtleties in the absorption process of the test body by the black hole, which are not captured by the purely kinematic considerations."

 

[marcus]

http://arxiv.org/abs/1006.3808
Low energy Quantum Gravity from the Effective Average Action
A.Satz, A. Codello, F.D. Mazzitelli
23 pages, 1 figure
(Submitted on 18 Jun 2010)
"Within the effective average action approach to quantum gravity, we recover the low energy effective action as derived in the effective field theory framework, by studying the flow of possibly non-local form factors that appear in the curvature expansion of the effective average action. We restrict to the one-loop flow where progress can be made with the aid of the non-local heat kernel expansion. We discuss the possible physical implications of the scale dependent low energy effective action through the analysis of the quantum corrections to the Newtonian potential."
(My comment: Codello has co-authored with Percacci on major papers in AsymSafe gravity. The effective average action is a primary tool in A.S. gravity.)

http://arxiv.org/abs/1006.4000
Boundary conditions of the RGE flow in noncommutative cosmology

Daniel Kolodrubetz, Matilde Marcolli
12 pages, 13 figures
(Submitted on 21 Jun 2010)
We investigate the effect of varying boundary conditions on the renormalization group flow in a noncommutative geometry model. Specifically, changing conditions at unification energy run down to the electroweak scale. Varying a value even slightly can be shown to have drastic effects on the running of many model parameters. The model used has several constraints at the unification scale. These restrictions arise out of the geometry of the model. Matching these constraints and adjusting other parameters, it is possible to get the renormalization flow to agree in order of magnitude with the predictions at the electroweak scale."

 

[MTd2]

http://arxiv.org/abs/1006.4164

Stability of self-dual black holes

Eric Brown, Robert Mann, Leonardo Modesto
(Submitted on 21 Jun 2010)
We study the stability properties of the Cauchy horizon for two different self-dual black hole solutions obtained in a model inspired by Loop Quantum Gravity. The self-dual spacetimes depend on a free dimensionless parameter called a polymeric parameter P. For the first metric the Cauchy horizon is stable for supermassive black holes only if this parameter is sufficiently small. For small black holes, however the stability is easily implemented. The second metric analyzed is not only self-dual but also "form-invariant" under the transformation r -> r*^2/r and r* = 2 m P. We find that this symmetry protects the Cauchy horizon for any value of the polymeric parameter.

http://arxiv.org/abs/1006.4295

Simplicity in simplicial phase space

Bianca Dittrich, James P. Ryan
(Submitted on 22 Jun 2010)
A key point in the spin foam approach to quantum gravity is the implementation of simplicity constraints in the partition functions of the models. Here, we discuss the imposition of these constraints in a phase space setting corresponding to simplicial geometries. On the one hand, this could serve as a starting point for a derivation of spin foam models by canonical quantisation. On the other, it elucidates the interpretation of the boundary Hilbert space that arises in spin foam models.
More precisely, we discuss different versions of the simplicity constraints, namely gauge-variant and gauge-invariant versions. In the gauge-variant version, the primary and secondary simplicity constraints take a similar form to the reality conditions known already in the context of (complex) Ashtekar variables. Subsequently, we describe the effect of these primary and secondary simplicity constraints on gauge-invariant variables. This allows us to illustrate their equivalence to the so-called diagonal, cross and edge simplicity constraints, which are the gauge-invariant versions of the simplicity constraints. In particular, we clarify how the so-called gluing conditions arise from the secondary simplicity constraints. Finally, we discuss the significance of degenerate configurations, and the ramifications of our work in a broader setting.

 

[John86]

http://arxiv.org/abs/1006.3837
Consistent Probabilities in Wheeler-DeWitt Quantum Cosmology
Authors: David A. Craig, Parampreet Singh
(Submitted on 19 Jun 2010)
Abstract: We give an explicit, rigorous framework for calculating quantum probabilities in a model theory of quantum gravity. Specifically, we construct the decoherence functional for the Wheeler-DeWitt quantization of a flat Friedmann-Robertson-Walker cosmology with a free, massless, minimally coupled scalar field, thus providing a complete decoherent histories formulation for this quantum cosmological model. The decoherence functional is applied to study predictions concerning the model's Dirac (relational) observables; the behavior of semiclassical states and superpositions of such states; and to study the singular behavior of quantum Wheeler-DeWitt universes. Within this framework, rigorous formulae are given for calculating the corresponding probabilities from the wave function when those probabilities may be consistently defined, thus replacing earlier heuristics for interpreting the wave function of the universe with explicit constructions. It is shown according to a rigorously formulated standard, and in a quantum-mechanically consistent way, that in this quantization these models are generically singular. Independent of the choice of state we show that the probability for these Wheeler-DeWitt quantum universes to ever encounter a singularity is unity. In addition, the relation between histories formulations of quantum theory and relational Dirac observables is clarified.

http://arxiv.org/abs/1006.4254
Fermions from classical statistics
Authors: C. Wetterich
(Submitted on 22 Jun 2010)
Abstract: We describe fermions in terms of a classical statistical ensemble. The states $\tau$ of this ensemble are characterized by a sequence of values one or zero or a corresponding set of two-level observables. The classical probability distribution for these states can describe a quantum theory for fermions. If the time evolution of the classical probabilities $p_\tau$ amounts to rotations of the wave function $q_\tau(t)=\pm \sqrt{p_\tau(t)}$, we infer the unitary time evolution of a quantum system of fermions according to a Schr\"odinger equation. We establish how such classical statistical ensembles can be mapped to Grassmann functional integrals. Quantum field theories for fermions arise for a suitable time evolution of classical probabilities.

http://arxiv.org/abs/1006.4150
Necessity of Acceleration-Induced Nonlocality
Authors: Bahram Mashhoon
(Submitted on 21 Jun 2010)
Abstract: The purpose of this paper is to explain clearly why nonlocality must be an essential part of the theory of relativity. In the standard local version of this theory, Lorentz invariance is extended to accelerated observers by assuming that they are pointwise inertial. This locality postulate is exact when dealing with phenomena involving classical point particles and rays of radiation, but breaks down for electromagnetic fields, as field properties in general cannot be measured instantaneously. The problem is corrected in nonlocal relativity by supplementing the locality postulate with a certain average over the past world line of the observer.

http://arxiv.org/abs/1006.3809
Cosmological fluctuation growth in bimetric MOND
Authors: Mordehai Milgrom
(Submitted on 18 Jun 2010)
Abstract: I look at the growth of weak density inhomogeneities of nonrelativistic matter, in bimetric-MOND (BIMOND) cosmology. I concentrate on matter-twin-matter-symmetric versions of BIMOND, and assume that, on average, the universe is symmetrically populated in the two sectors. MOND effects are absent in an exactly symmetric universe, apart from the appearance of a cosmological constant, Lambda~(a0/c)^2. MOND effects--local and cosmological--do enter when density inhomogeneities that differ in the two sectors appear and develop. MOND later takes its standard form in systems that are islands dominated by pure matter. I derive the nonrelativistic equations governing small-scale fluctuation growth. The equations split into two uncoupled systems, one for the sum, the other for the difference, of the fluctuations in the two sectors. The former is governed strictly by Newtonian dynamics. The latter is governed by MOND dynamics, which entails stronger gravity, and nonlinearity even for the smallest of perturbations. These cause the difference to grow faster than the sum, conducing to matter-twin-matter segregation. The nonlinearity also causes interaction between nested perturbations on different scales. Because matter and twin matter repel each other in the MOND regime, matter inhomogeneities grow not only by their own self gravity, but also through shepherding by flanking TM overdensitie. The relative importance of gravity and pressure in the MOND system depends also on the strength of the perturbation. The development of structure in the universe, in either sector, thus depends crucially on two initial fluctuation spectra: that of matter alone and that of the matter-TM difference. I also discuss the back reaction on cosmology of BIMOND effects that appear as ``phantom matter'' resulting from inhomogeneity differences between the two sectors. (abridged)

http://arxiv.org/abs/1006.4146
On plane gravitational waves in real connection variables
Authors: Franz Hinterleitner, Seth Major
(Submitted on 21 Jun 2010)
Abstract: We investigate using plane fronted gravitational wave space-times as model systems to study loop quantization techniques and dispersion relations. In this classical analysis, we start with planar symmetric space-times in the real connection formulation. We reduce via Dirac constraint analysis to a final form with one canonical pair and one constraint, equivalent to the metric and Einstein equations of plane-fronted with parallel rays waves. Due to the symmetries and use of special coordinates general covariance is broken. However, this allows us to simply express the constraints of the consistent system. A recursive construction of Dirac brackets results in non-local brackets, analogous to those of self-dual fields, for the triad variables chosen in this approach.

 

[marcus]

http://arxiv.org/abs/1006.4587
Reply to arXiv:1006.2126 by Giovanni Amelino-Camelia et al
Sabine Hossenfelder
(Submitted on 23 Jun 2010)
"It was previously shown that models with deformations of special relativity that have an energy-dependent yet observer-independent speed of light suffer from nonlocal effects that are in conflict with observation to very high precision. In a recent preprint it has been claimed that this conclusion is false. This claim was made by writing down expressions for modified Lorentz-transformations the use of which does not reproduce the result. I will show here that the failure to reproduce the result is not a consequence of a novel and improved calculation, but a consequence of repeating the same calculation but making an assumption that is in conflict with the assumptions made to produce the original scenario. I will here explain what the physical meaning of either assumption is and why the original assumption is the physically meaningful one. I will then further explain why even making the differing assumption does not remove but merely shift the problem and why the bound derived by Amelino-Camelia et al is wrong."

Brief mention, not obviously on topic for this thread but to keep an eye on where Rivasseau is going
http://arxiv.org/abs/1006.4617
How are Feynman graphs resummed by the Loop Vertex Expansion?
Vincent Rivasseau, Zhituo Wang

http://arxiv.org/abs/1006.4834
Primordial Density Perturbations and Reheating from Gravity
N. C. Tsamis (University of Crete), R. P. Woodard (University of Florida)
(Submitted on 24 Jun 2010)
We consider the presence and evolution of primordial density perturbations in a cosmological model based on a simple ansatz which captures -- by providing a set of effective gravitational field equations -- the strength of the enhanced quantum loop effects that can arise during inflation. After deriving the general equations that perturbations obey, we concentrate on scalar perturbations and show that their evolution is quite different than that of conventional inflationary models but still phenomenologically acceptable. The main reason for this novel evolution is the presence of an oscillating regime after the end of inflation which makes all super-horizon scalar modes oscillate. The same reason allows for a natural and very fast reheating mechanism for the universe.

 

[atyy]

http://arxiv.org/abs/1006.4749
2D quantum gravity from quantum entanglement
Ferdinando Gliozzi
(Submitted on 24 Jun 2010)
In quantum systems with many degrees of freedom the replica method is a useful tool to study the entanglement of arbitrary spatial regions. We apply it in a way which allows them to back-react. As a consequence, they become dynamical subsystems whose position, form and extension is determined by their interaction with the whole system. We analyze in particular quantum spin chains described at criticality by a conformal field theory (CFT). Its coupling to the Gibbs' ensemble of all possible subsystems is relevant and drives the system into a new fixed point which is argued to be that of the 2D quantum gravity coupled to this system. Numerical experiments on the critical Ising model show that the new critical exponents agree with those predicted by the formula of Knizhnik, Polyakov and Zamolodchikov.

 

[qsa]

http://arxiv.org/PS_cache/gr-qc/pdf/0006/0006061v3.pdf

Carlo Rovelli

I sketch the main lines of development of the research in quantum gravity,
from the first explorations in the early thirties to nowadays.

 

[jfy4]

http://arxiv.org/abs/1006.2369

Torsten and Helge have discussed their idea some with us here at PF Beyond. It is a radical and high-risk idea.
http://arxiv.org/abs/1006.2230
On the geometrization of matter by exotic smoothness
Torsten Asselmeyer-Maluga, Helge Rose
17 pages
(Submitted on 11 Jun 2010)
"Clifford's hypothesis is investigated: A particle is made up of nothing but a distinct type of a space manifold, differing from the surrounding manifold of empty space. It is shown that this distinct space manifold representing matter differs from the surrounding vacuum by the exotic smoothness of its spacetime. The smoothness structure of spacetime can be described by a tree-like subset -- the Casson handle -- consisting of immersed discs and connecting tubes between them. The Weierstrass representation shows that the immersed discs are represented by spinors fulfilling the Dirac equation and leading to a mass-less Dirac term in the Einstein-Hilbert action. The connecting tubes between the discs realize an action term of a gauge field. Both terms are purly geometrical and characterized by the mean curvature of the components of the Casson handle. This gives a good support to Clifford's conjecture that matter is nothing more but an exotic kind of space."

interesting conjecture quoted from right at the very end of the paper, on page 15:
"At the end we want to give another interpretation of the Casson handle. Connes [37] showed that by means of the non-commutative geometry the action of the standard model can be reproduced. His model is based on the space M×F where the additional space F is ad hoc and has no relation to the spacetime M.
In our model the space F could be interpreted as an expression of the Casson handle and so of the smoothness of spacetime establishing a deep relation between quantum matter and space."

Ahh! recently I have been considering to myself whether it would be possible to construct matter from space using a transformation of sorts. in fact it was 2 days ago that this idea hit me, and then yesterday I come to PF only to find a Paper on that very idea, and a long history of interest in the possibility forming matter out of geometry. My intuition told me that using a discrete set of space-time quanta, one could assemble a wave in much the same way a discrete transform takes quanta inputs and out-puts a continuous wave. A very alluring idea.

 

[marcus]

jfy4,
If we discussed and commented on papers in the bibliography thread it would get overcrowded. The practice here is to just list papers in this thread, with the paper's own abstract summary or a brief sample quote indicating what it is about. If you want to comment or discuss a particular paper please start a different thread for that purpose.
Since you are new and didn't know the custom, I will start a thread for you to comment on the Helge Torsten paper. I don't know but other people might want to comment, so a longer discussion could develop.

 

[marcus]

http://arxiv.org/abs/1006.4908
An Explicit Embedding of Gravity and the Standard Model in E8
A. Garrett Lisi
14 pages. For peer review and publication in the "Proceedings of the Conference on Representation Theory and Mathematical Physics."
(Submitted on 25 Jun 2010)
"The algebraic elements of gravitational and Standard Model gauge fields acting on a generation of fermions may be represented using real matrices. These elements match a subalgebra of spin(11,3) acting on a Majorana-Weyl spinor, consistent with GraviGUT unification. This entire structure embeds in the quaternionic real form of the largest exceptional Lie algebra, E8. These embeddings are presented explicitly and their implications discussed."

 

[marcus]

http://arxiv.org/abs/1006.5145
Functional Renormalization of Noncommutative Scalar Field Theory
Alessandro Sfondrini, Tim A. Koslowski
37 pages
(Submitted on 26 Jun 2010)
"In this paper we apply the Functional Renormalization Group Equation of Wetterich to the non-commutative scalar field theory proposed by Grosse and Wulkenhaar. We derive the flow equation in the matrix representation, and construct the theory space for the self-dual model. The features introduced by the external dimensionful scale provided by the non-commutativity parameter, originally pointed out in [1], are discussed in this new context. Using a technical assumption, but without resorting to a truncation, it is then shown that the theory is asymptotically safe for a suitable small value of the couplings, recovering the result of [2]. Finally, we show how the FRGE can be easily used to compute the one loop beta-functions of the duality covariant model."

http://arxiv.org/abs/1006.5666
Revisiting the Simplicity Constraints and Coherent Intertwiners
Maité Dupuis, Etera R. Livine
28 pages
(Submitted on 29 Jun 2010)
"In the context of loop quantum gravity and spinfoam models, the simplicity constraints are essential in that they allow to write general relativity as a constrained topological BF theory. In this work, we apply the recently developed U(N) framework for SU(2) intertwiners to the issue of imposing the simplicity constraints to spin network states. More particularly, we focus on solving them on individual intertwiners in the 4d Euclidean theory. We review the standard way of solving the simplicity constraints using coherent intertwiners and we explain how these fit within the U(N) framework. Then we show how these constraints can be written as a closed u(N) algebra and we propose a set of U(N) coherent states that solves all the simplicity constraints weakly for an arbitrary Immirzi parameter."

 

[marcus]

http://arxiv.org/abs/1007.0718
On limitations of the extent of inertial frames in non-commutative relativistic spacetimes
Lee Smolin
21 pages
(Submitted on 5 Jul 2010)
"We study the interplay of non-locality and Lorentz invariance in a version of deformed or doubly special relativity (DSR) based on kappa-Minkowski spacetime. We find that Einstein's procedure for an inertial observer to assign coordinates to distant events becomes ambiguous for sufficiently distant events. The accuracy to which two clocks can be synchronized turns out to depend on the distance between them. These are consequences of the non-commutativity of space and time coordinates or a dependence of the speed of light on energy in relativistic theories. These ambiguities grow with distance and only become relevant for real observations for the description of cosmologically distant events. They do not afflict the interpretation of the detection of gamma rays in stationary or moving frames near the detector. Consequently there is no disagreement between the principles of DSR and the observation that interactions in nature are local down to currently observable scales."

http://arxiv.org/abs/1007.0735
Unimodular Loop Quantum Cosmology
Dah-Wei Chiou, Marc Geiller
26 pages
(Submitted on 5 Jul 2010)
"Unimodular gravity is based on a modification of the usual Einstein-Hilbert action that allows one to recover general relativity with a dynamical cosmological constant. It also has the interesting property of providing, as the momentum conjugate to the cosmological constant, an emergent clock variable. In this paper we investigate the cosmological reduction of unimodular gravity, and its quantization within the framework of flat homogeneous and isotropic loop quantum cosmology. It is shown that the unimodular clock can be used to construct the physical state space, and that the fundamental features of the previous models featuring scalar field clocks are reproduced. In particular, the classical singularity is replaced by a quantum bounce, which takes place in the same condition as obtained previously. We also find that requirement of semi-classicality demands the expectation value of the cosmological constant to be small (in Planck units). The relation to spin foam models is also studied, and we show that the use of the unimodular time variable leads to a unique vertex expansion."

 

[marcus]

http://arXiv.org/abs/1007.0402
Introductory lectures to loop quantum gravity
Pietro Doná, Simone Speziale
Based on lectures given at the 3eme Ecole de Physique Theorique de Jijel, Algeria, 26 Sep -- 3 Oct, 2009. 52 pages, many figures. To be published in the proceedings
(Submitted on 2 Jul 2010)
"We give a standard introduction to loop quantum gravity, from the ADM variables to spin network states. We include a discussion on quantum geometry on a fixed graph and its relation to a discrete approximation of general relativity."

 

[marcus]

http://arxiv.org/abs/1007.1317
Black holes in an asymptotically safe gravity theory with higher derivatives
Yi-Fu Cai, Damien A. Easson
22 pages, 3 figures
(Submitted on 8 Jul 2010)
"We present a class of spherically symmetric vacuum solutions to an asymptotically safe theory of gravity containing high-derivative terms. We find quantum corrected Schwarzschild-(anti)-de Sitter solutions with running gravitational coupling parameters. The evolution of the couplings is determined by their corresponding renormalization group flow equations. These black holes exhibit properties of a classical Schwarzschild solution at large length scales. At the center, the metric factor remains smooth but the curvature singularity, while softened by the quantum corrections, persists. The solutions have an outer event horizon and an inner Cauchy horizon which equate when the physical mass decreases to a critical value. Super-extremal solutions with masses below the critical value correspond to naked singularities. The Hawking temperature of the black hole vanishes when the physical mass reaches the critical value. Hence, the black holes in the asymptotically safe gravitational theory never completely evaporate. For appropriate values of the parameters such stable black hole remnants make excellent dark matter candidates."

In case anyone missed it, Easson co-authored earlier this year with N-laureate George Smoot. There were a couple of Easson Frampton Smoot cosmology papers.

 

[MTd2]

http://arxiv.org/abs/1007.1437

Time Evolution of Horizons

Arundhati Dasgupta
(Submitted on 8 Jul 2010)
A density matrix is defined using coherent states for space-times with apparent horizons. Evolving the density matrix in time gives the origin of Hawking radiation.

 

[marcus]

http://arxiv.org/abs/1007.1563
New formulation of Horava-Lifgarbagez quantum gravity as a master constraint theory
Chopin Soo, Jinsong Yang, Hoi-Lai Yu
4 pages
(Submitted on 9 Jul 2010)
"Horava-Lifgarbagez theory of quantum gravity attempts to preserve unitarity by relinquishing space-time covariance, and improve renormalizability by including higher order (spatial) derivatives. For theories without full space-time covariance, departures of the constraint algebra from the Dirac algebra are to be expected. In the non-projectable version of Horava-Lifgarbagez gravity, the commutator of two local Hamiltonian constraints leads to severely restrictive secondary constraints and perplexing 'troubles'. On the other hand, the projectable version has an integrated Hamiltonian constraint and consistent constraint algebra. But an extra graviton mode which can be problematic is then allowed, whereas in Einstein's theory the spurious mode is eliminated precisely by the local Hamiltonian constraint. A new formulation of Horava-Lifgarbagez gravity, naturally realized as a representation of the master constraint algebra studied by loop quantum gravity researchers, is presented in this work. This reformulation yields a consistent canonical theory with 1st class constraints. It captures the essence of Horava-Lifgarbagez gravity in retaining only spatial diffeomorphisms (instead of full space-time covariance) as the physically relevant non-trivial gauge symmetry; at the same time the local Hamiltonian constraint which is needed to remove the spurious mode is equivalently enforced by the master constraint."

"Master constraint" is a version of canonical LQG developed by Thiemann.

 

[atyy]

http://arxiv.org/abs/1007.1724
Gravity, Two Times, Tractors, Weyl Invariance and Six Dimensional Quantum Mechanics
Roberto Bonezzi, Emanuele Latini, Andrew Waldron
(Submitted on 10 Jul 2010)
Fefferman and Graham showed some time ago that four dimensional conformal geometries could be analyzed in terms of six dimensional, ambient, Riemannian geometries admitting a closed homothety. Recently it was shown how conformal geometry provides a description of physics manifestly invariant under local choices of unit systems. Strikingly, Einstein's equations are then equivalent to the existence of a parallel scale tractor (a six component vector subject to a certain first order covariant constancy condition at every point in four dimensional spacetime). These results suggest a six dimensional description of four dimensional physics, a viewpoint promulgated by the two times physics program of Bars. The Fefferman--Graham construction relies on a triplet of operators corresponding, respectively to a curved six dimensional light cone, the dilation generator and the Laplacian. These form an sp(2) algebra which Bars employs as a first class algebra of constraints in a six-dimensional gauge theory. In this article four dimensional gravity is recast in terms of six dimensional quantum mechanics by melding the two times and tractor approaches. This "parent" formulation of gravity is built from an infinite set of six dimensional fields. Successively integrating out these fields yields various novel descriptions of gravity including a new four dimensional one built from a scalar doublet, a tractor vector multiplet and a conformal class of metrics.

http://arxiv.org/abs/1007.1795
Emergent Geometry and Quantum Gravity
Hyun Seok Yang
(Submitted on 11 Jul 2010)
We explain how quantum gravity can be defined by quantizing spacetime itself. A pinpoint is that the gravitational constant G = L_P^2 whose physical dimension is of (length)^2 in natural unit introduces a symplectic structure of spacetime which causes a noncommutative spacetime at the Planck scale L_P. The symplectic structure of spacetime $M$ leads to an isomorphism between symplectic geometry (M, \omega) and Riemannian geometry (M, g) where the deformations of symplectic structure \omega in terms of electromagnetic fields F=dA are transformed into those of Riemannian metric g. This approach for quantum gravity allows a background independent formulation where spacetime as well as matter fields is equally emergent from a universal vacuum of quantum gravity which is thus dubbed as the quantum equivalence principle.

http://arxiv.org/abs/1007.1813
Entanglement entropy for the n-sphere
H. Casini, M. Huerta
(Submitted on 12 Jul 2010)
We calculate the entanglement entropy for a sphere and a massless scalar field in any dimensions. The reduced density matrix is expressed in terms of the infinitesimal generator of conformal transformations keeping the sphere fixed. The problem is mapped to the one of a thermal gas in a hyperbolic space and solved by the heat kernel approach. The coefficient of the logarithmic term in the entropy for 2 and 4 spacetime dimensions are in accordance with previous numerical and analytical results. In particular, the four dimensional result, together with the one reported by Solodukhin, gives support to the Ryu-Takayanagi holographic anzats. We also find there is no logarithmic contribution to the entropy for odd space time dimensions.

 

[John86]

http://arxiv.org/abs/1006.1769
Eddington's theory of gravity and its progeny
Authors: Maximo Banados (PUC, Chile and Oxford), Pedro G. Ferreira (Oxford)
(Submitted on 9 Jun 2010)
Abstract: We resurrect Eddington's proposal for the gravitational action in the presence of a cosmological constant and extend it to include matter fields. We show that the Newton-Poisson equation is modified in the presence of sources and that charged black holes show great similarities with those arising in Born-Infeld electrodynamics coupled to gravity. When we consider homogeneous and isotropic space-times we find that there is a minimum length (and maximum density) at early times, clearly pointing to an alternative theory of the Big Bang. We thus argue that the modern formulation of Eddington's theory, Born-Infeld gravity, presents us with a novel, non-singular description of the Universe.

 

[marcus]

http://arxiv.org/abs/1007.2594
Coupling the inflaton to an expanding aether
William Donnelly, Ted Jacobson
13 pages
(Submitted on 15 Jul 2010)
"We consider a Lorentz-violating theory of inflation consisting of Einstein-aether theory with a scalar inflaton coupled bilinearly to the expansion of the aether. We determine the conditions for linearized stability, positive energy and vanishing of preferred-frame post-Newtonian parameters, and find that all these conditions can be met. In homogeneous and isotropic cosmology, the inflaton-aether expansion coupling leads to a driving force on the inflaton that is proportional to the Hubble parameter. This force affects the slow-roll dynamics, but still allows for a natural end to inflation."

http://arxiv.org/abs/1007.2560
CDT---an Entropic Theory of Quantum Gravity
J. Ambjorn, A. Goerlich, J.Jurkiewicz, R. Loll
49 pages, many figures. Lectures presented at the "School on Non-Perturbative Methods in Quantum Field Theory" and the "Workshop on Continuum and Lattice Approaches to Quantum Gravity", Sussex, September 15th-19th 2008 . To appear as part of a volume in the Springer Lecture Notes in Physics series.
(Submitted on 15 Jul 2010)
"In these lectures we describe how a theory of quantum gravity may be constructed in terms of a lattice formulation based on so-called causal dynamical triangulations (CDT). We discuss how the continuum limit can be obtained and how to define and measure diffeomorphism-invariant correlators. In four dimensions, which has our main interest, the lattice theory has an infrared limit which can be identified with de Sitter spacetime. We explain why this infrared property of the quantum spacetime is nontrivial and due to 'entropic' effects encoded in the nonperturbative path integral measure. This makes the appearance of the de Sitter universe an example of true emergence of classicality from microscopic quantum laws. We also discuss nontrivial aspects of the UV behaviour, and show how to investigate quantum fluctuations around the emergent background geometry. Finally, we consider the connection to the asymptotic safety scenario, and derive from it a new, conjectured scaling relation in CDT quantum gravity."

http://arxiv.org/abs/1007.2410
General Covariance in Quantum Gravity at a Lifgarbagez Point
Petr Horava, Charles M. Melby-Thompson
40 pages
(Submitted on 14 Jul 2010)
"In the minimal formulation of gravity with Lifgarbagez-type anisotropic scaling, the gauge symmetries of the system are foliation-preserving diffeomorphisms of spacetime. Consequently, compared to general relativity, the spectrum contains an extra scalar graviton polarization. Here we investigate the possibility of extending the gauge group by a local U(1) symmetry to 'nonrelativistic general covariance.' This extended gauge symmetry eliminates the scalar graviton, and forces the coupling constant lambda in the kinetic term of the minimal formulation to take its relativistic value, lambda=1. The resulting theory exhibits anisotropic scaling at short distances, and reproduces many features of general relativity at long distances."

 

[marcus]

http://arxiv.org/abs/1007.2768
Generic isolated horizons in loop quantum gravity
Christopher Beetle, Jonathan Engle
13 pages
(Submitted on 16 Jul 2010)
"Isolated horizons model equilibrium states of classical black holes. A detailed quantization, starting from a classical phase space restricted to spherically symmetric horizons, exists in the literature and has since been extended to axisymmetry. This paper extends the quantum theory to horizons of arbitrary shape. Surprisingly, the Hilbert space obtained by quantizing the full phase space of all generic horizons with a fixed area is identical to that originally found in spherical symmetry. The entropy of a large horizon remains one quarter its area, with the Barbero-Immirzi parameter retaining its value from symmetric analyses. These results suggest a reinterpretation of the intrinsic quantum geometry of the horizon surface."

 

[marcus]

http://arxiv.org/abs/1007.3150
Quantum Corrections in the Group Field Theory Formulation of the EPRL/FK Models
Thomas Krajewski, Jacques Magnen, Vincent Rivasseau, Adrian Tanasa, Patrizia Vitale
35 pages, 5 figures
(Submitted on 19 Jul 2010)
"We investigate the group field theory formulation of the EPRL/FK spin foam models. These models aim at a dynamical, i.e. non-topological formulation of 4D quantum gravity. We introduce a saddle point method for general group field theory amplitudes and compare it with existing results, in particular for a second order correction to the EPRL/FK propagator."

http://arxiv.org/abs/1007.3086
A New Solution of The Cosmological Constant Problems
John D. Barrow, Douglas J. Shaw
5 pages
(Submitted on 19 Jul 2010)
"We extend the usual gravitational action principle by promoting the bare cosmological constant (CC) from a parameter to a field which can take many possible values. Variation leads to a new integral constraint equation which determines the classical value of the effective CC that dominates the wave function of the universe. In a Friedmann background cosmology with observed matter and radiation content the expected value of the effective CC, is calculated from measurable quantities to be O(t_U^-2)~ 10^-122 (in natural units), as observed, where t_U is the present age of the universe. Any application of our model produces a falsifiable prediction for Lambda in terms of other measurable quantities. This leads to a specific prediction for the observed spatial curvature parameter of Omega_k0 = 5.2 x 10^-5, which is of the magnitude expected if inhomogeneities have an inflationary origin. This explanation of the CC requires no fine tunings, extra dark energy fields, or Bayesian selection in a multiverse."


Brief mention:
http://arxiv.org/abs/1007.2875
Unstable growth of curvature perturbation in non-singular bouncing cosmologies
BingKan Xue, Paul J. Steinhardt
4 pages, 2 figures
(Submitted on 16 Jul 2010)
"We consider non-singular bouncing cosmologies, such as the new ekpyrotic model,..."

http://arxiv.org/abs/1007.2971
Reasoning by analogy: attempts to solve the cosmological constant paradox
Rafael A. Porto, A. Zee
4 pages. To appear in Proceedings of the Conference in Honor of Murray Gell-Mann's 80th Birthday
(Submitted on 18 Jul 2010)

http://arxiv.org/abs/1007.2873
Is There Unification in the 21st Century?
Yuan K. Ha
Lecture delivered in Conference in Honor of Murray Gell-Mann's 80th Birthday, February 24 - 26, 2010.
(Submitted on 16 Jul 2010)

 

[MTd2]

http://arxiv.org/abs/1007.2976

Holograph in noncommutative geometry: Part 1

Jingbo Wang
(Submitted on 18 Jul 2010)
In this paper, we consider the holograph principle emergent from noncommutative geometry, based on the spectral action principle. We show that under some appropriate conditions, the gravity theory on a manifold with boundary could be equivalent to a gauge theory $SU(N)$ on the boundary. Then an expression for $N$ with the geometrical quantities of the manifold is given. Based on this result, we find that the volume of the manifold and the boundary have some discrete structure. Applying the result to the black hole, we get that the radium of the Schwarzschild black hole is quantized. We also find an explanation why the extremal RN-black hole has zero temperature but with finite entropy.

http://arxiv.org/abs/1007.2941

The modified wave function of test particle approaching holographic screen from entropy force-> This paper discusses the nature of Verlinde's entropy.

Bin Liu, Yun-Chuan Dai, Xian-Ru Hu, Jian-Bo Deng
(Submitted on 17 Jul 2010)
In this note we generalize entropy based on the quantum mechanical probability density distribution. Motivated by J. Munkhammar and the uncertainty of entropy we modified the origin wave function of the test particle. The corrected one acting on the quantum particle is subject to the uncertainty principle. Considering the uncertain relation, the corrected probability of the particle for measurement on holographic screen has been proposed. We also derivate the speed of information transfer described by wave function. Our quantum approach to entropy stress the information in a physical system is directly associated with its quantum