Loop-and-allied QG bibliography

[atyy]

http://arxiv.org/abs/0911.3793
Gravity-Yang-Mills-Higgs unification by enlarging the gauge group
Alexander Torres-Gomez, Kirill Krasnov (University of Nottingham)
We revisit an old idea that gravity can be unified with Yang-Mills theory by enlarging the gauge group of gravity formulated as gauge theory. Our starting point is an action that describes a generally covariant gauge theory for a group G. The Minkowski background breaks the gauge group by selecting in it a preferred gravitational SU(2) subgroup. We expand the action around this background and find the spectrum of linearized theory to consist of the usual gravitons plus Yang-Mills fields charged under the centralizer of the SU(2) in G. In addition, there is a set of Higgs fields that are charged both under the gravitational and Yang-Mills subgroups. These fields are generically massive and interact with both gravity and Yang-Mills sector in the standard way. The arising interaction of the Yang-Mills sector with gravity is also standard. Parameters such as the Yang-Mills coupling constant and Higgs mass arise from the potential function defining the theory. Both are realistic in the sense explained in the paper.

 

[MTd2]

http://arxiv.org/abs/0911.4208
New aspects of two-dimensional quantum gravity

J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
(Submitted on 21 Nov 2009)
Causal dynamical triangulations (CDT) can be used as a regularization of quantum gravity. In two dimensions the theory can be solved anlytically, even before the cut-off is removed and one can study in detail how to take the continuum limit. We show how the CDT theory is related to Euclidean 2d quantum gravity (Liouville quantum gravity), how it can be generalized and how this generalized CDT model has a string field theory representation as well as a matrix model representationof a new kind, and finally how it examplifies the possibility that time in quantum gravity might be the stochastic time related to the branching of space into baby

http://arxiv.org/abs/0911.4211
Proper time is stochastic time in 2d quantum gravity

J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
(Submitted on 21 Nov 2009)
We show that proper time, when defined in the quantum theory of 2d gravity, becomes identical to the stochastic time associated with the stochastic quantization of space. This observation was first made by Kawai and collaborators in the context of 2d Euclidean quantum gravity, but the relation is even simpler and more transparent in he context of 2d gravity formulated in the framework of CDT (causal dynamical triangulations).http://arxiv.org/abs/0911.4141
Lattice Loop Quantum Gravity

Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 20 Nov 2009)
We present a separable version of Loop Quantum Gravity (LQG) based on an inductive system of cubic lattices. We construct semi-classical states for which the LQG operators -- the flux, the area and the volume operators -- have the right classical limits. Also, we present the Hamilton and diffeomorphism constraints as operator constraints and show that they have the right classical limit. Finally, we speculate whether the continuum limit, which these semi-classical states probe, can be defined for the entire construction and thereby restore an action of the diffeomorphism group.

 

[John86]

I think this paper is an interesting one for you Fra.

http://arxiv.org/abs/0911.4307
Quantum Darwinism in non-ideal environments
Authors: Michael Zwolak, H. T. Quan, Wojciech H. Zurek
(Submitted on 22 Nov 2009)
Abstract: Quantum Darwinism provides an information-theoretic framework for the emergence of the objective, classical world from the quantum substrate. The key to this emergence is the proliferation of redundant information throughout the environment where observers can then intercept it. We study this process for a purely decohering interaction when the environment, E, is in a non-ideal (e.g., mixed) initial state. In the case of good decoherence, that is, after the pointer states have been unambiguously selected, the mutual information between the system, S, and an environment fragment, F, is given solely by F's entropy increase. This demonstrates that the environment's capacity for recording the state of S is directly related to its ability to increase its entropy. Environments that remain nearly invariant under the interaction with S, either because they have a large initial entropy or a misaligned initial state, therefore have a diminished ability to acquire information. To elucidate the concept of good decoherence, we show that - when decoherence is not complete - the deviation of the mutual information from F's entropy change is quantified by the quantum discord, i.e., the excess mutual information between S and F is information regarding the initial coherence between pointer states of S. In addition to illustrating these results with a single qubit system interacting with a multi-qubit environment, we find scaling relations for the redundancy of information acquired by the environment that display a universal behavior independent of the initial state of S. Our results demonstrate that Quantum Darwinism is robust with respect to non-ideal initial states of the environment: the environment almost always acquires redundant information about the system but its rate of acquisition can be reduced.

 

[John86]

"Last but not least" two new papers by Martin Kober i spotted his work last march.

http://arxiv.org/abs/0910.3303
About the Origin of the Division between Internal and External Symmetries in Quantum Field Theory
Authors: Martin Kober
(Submitted on 17 Oct 2009)
Abstract: It is made the attempt to explain why there exists a division between internal symmetries referring to quantum numbers and external symmetries referring to space-time within the description of relativistic quantum field theories. It is hold the attitude that the symmetries of quantum theory are the origin of both sorts of symmetries in nature. Since all quantum states can be represented as a tensor product of two dimensional quantum objects, called ur objects, which can be interpreted as quantum bits of information, described by spinors reflecting already the symmetry properties of space-time, it seems to be possible to justify such an attitude. According to this, space-time symmetries can be considered as a consequence of a representation of quantum states by quantum bits. Internal symmetries are assumed to refer to relations of such fundamental objects, which are contained within the state of one single particle, with respect to each other. In this sense the existence of space-time symmetries, the existence of internal symmetries and their division could obtain a derivation from quantum theory interpreted as a theory of information.

http://arxiv.org/abs/0905.0408
Copenhagen Interpretation of Quantum Theory and the Measurement Problem
Authors: Martin Kober
(Submitted on 4 May 2009 (v1), last revised 5 Nov 2009 (this version, v3))
Abstract: The Copenhagen interpretation of quantum theory is investigated from a philosophical point of view. It is justified the opinion that the philosophical attitude the Copenhagen interpretation is based on is in principle inevitable for a real comprehension of quantum theory. This attitude is mainly related to epistemological arguments. However, the measurement problem often seems not to be treated clearly enough within the interpretation. By referring to the property of the necessity to use macroscopic measurement instruments obeying classical concepts it is made the attempt to solve the measurement problem. According to this consideration the indeterministic character of quantum theory seems to have its origin in a lack of knowledge and thus it appears in a similar but more principle way than in statistical mechanics. It is emphasized the ontological character of the uncertainty relation and the related non locality of quantum theory suggesting that the existence of a position space is not as fundamental as the assumptions of general quantum theory.

 

[MTd2]

http://arxiv.org/abs/0911.4903

Effective metric Lagrangians from an underlying theory with two propagating degrees of freedom

Kirill Krasnov
(Submitted on 25 Nov 2009)
We describe an infinite-parametric class of effective metric Lagrangians that arise from an underlying theory with two propagating degrees of freedom. The Lagrangians start with the Einstein-Hilbert term, continue with the standard R^2, (Ricci)^2 terms, and in the next order contain (Riemann)^3 as well as on-shell vanishing terms. This is exactly the structure of the effective metric Lagrangian that renormalizes quantum gravity divergences at two-loops. This shows that the theory underlying the effective field theory of gravity may have no more degrees of freedom than is already contained in general relativity. We show that the reason why an effective metric theory may describe just two propagating degrees of freedom is that there exists a (non-local) field redefinition that maps an infinitely complicated effective metric Lagrangian to the usual Einstein-Hilbert one. We describe this map for our class of theories and, in particular, exhibit it explicitly for the (Riemann)^3 term.

 

[marcus]

http://arxiv.org/abs/0911.4950
Effective Constraints and Physical Coherent States in Quantum Cosmology: A Numerical Comparison
Martin Bojowald, Artur Tsobanjan
32 pages, 25 figures
(Submitted on 25 Nov 2009)
"A cosmological model with a cyclic interpretation is introduced, which is subject to quantum back-reaction and yet can be treated rather completely by physical coherent state as well as effective constraint techniques. By this comparison, the role of quantum back-reaction in quantum cosmology is unambiguously demonstrated. Also the complementary nature of strengths and weaknesses of the two procedures is illustrated. Finally, effective constraint techniques are applied to a more realistic model filled with radiation, where physical coherent states are not available."

 

[marcus]

http://arxiv.org/abs/0911.5156
A Theory of Neutrino Oscillations and Late Time Acceleration
Stephon H.S. Alexander
6 Pages, 1 figure
(Submitted on 26 Nov 2009)
"We provide a microphysical model that connects neutrino oscillations to dark energy, which has predictions of Lorentz and CPT violating neutrino oscillations. We argue that the DE is a BCS condensate of flavored neutrinos. As neutrinos propigate in their own condensate they naturally have oscillations proportional to the DE energy density. All that is assumed in this model is a covariant coupling of neutrinos to gravity and a finite number density of neutrinos in the cosmic rest frame; this situation yields an attractive channel for the formation of a spin zero neutrino condensate leading to late time acceleration self consistently. Moreover, the vacuum oscillation has two predictions: (1) CPT violating oscillations proportional to the DE density (2) A connection between the evolution of the equation of state of the DE condensate and the neutrino oscillations. These predictions can be probed independently with future Supernovae and Neutrino Telescopes."

http://arxiv.org/abs/0911.5331
Born--Oppenheimer decomposition for quantum fields on quantum spacetimes
Kristina Giesel, Johannes Tambornino, Thomas Thiemann
38 pages, 2 figures
(Submitted on 27 Nov 2009)
"Quantum Field Theory on Curved Spacetime (QFT on CS) is a well established theoretical framework which intuitively should be a an extremely effective description of the quantum nature of matter when propagating on a given background spacetime. If one wants to take care of backreaction effects, then a theory of quantum gravity is needed. It is now widely believed that such a theory should be formulated in a non-perturbative and therefore background independent fashion. Hence, it is a priori a puzzle how a background dependent QFT on CS should emerge as a semiclassical limit out of a background independent quantum gravity theory. In this article we point out that the Born-Oppenheimer decomposition (BOD) of the Hilbert space is ideally suited in order to establish such a link, provided that the Hilbert space representation of the gravitational field algebra satisfies an important condition. If the condition is satisfied, then the framework of QFT on CS can be, in a certain sense, embedded into a theory of quantum gravity. The unique representation of the holonomy-flux algebra underlying Loop Quantum Gravity (LQG) violates that condition. While it is conceivable that the condition on the representation can be relaxed, for convenience in this article we consider a new classical gravitational field algebra and a Hilbert space representation of its restriction to an algebraic graph for which the condition is satisfied. An important question that remains and for which we have only partial answers is how to construct eigenstates of the full gravity-matter Hamiltonian whose BOD is confined to a small neighbourhood of a physically interesting vacuum spacetime."

http://arxiv.org/abs/0911.5004
Thermodynamical Aspects of Gravity: New insights
T.Padmanabhan
81 pages; 4 figures
(Submitted on 26 Nov 2009)
"The fact that one can associate thermodynamic properties with horizons brings together principles of quantum theory, gravitation and thermodynamics and possibly offers a window to the nature of quantum geometry. This review discusses certain aspects of this topic concentrating on new insights gained from some recent work. After a brief introduction of the overall perspective, Sections 2 and 3 provide the pedagogical background on the geometrical features of bifurcation horizons, path integral derivation of horizon temperature, black hole evaporation, structure of Lanczos-Lovelock models, the concept of Noether charge and its relation to horizon entropy. Section 4 discusses several conceptual issues introduced by the existence of temperature and entropy of the horizons. In Section 5 we take up the connection between horizon thermodynamics and gravitational dynamics and describe several peculiar features which have no simple interpretation in the conventional approach. The next two sections describe the recent progress achieved in an alternative perspective of gravity. In Section 6 we provide a thermodynamic interpretation of the field equations of gravity in any diffeomorphism invariant theory and in Section 7 we obtain the field equations of gravity from an entropy maximization principle. The last section provides a summary."

http://arxiv.org/abs/0911.5012
Neutrino Condensate as Origin of Dark Energy
Jitesh R. Bhatt, Bipin R. Desai, Ernest Ma, G. Rajasekaran, Utpal Sarkar
5 pages
(Submitted on 26 Nov 2009)
"We propose a new solution to the origin of dark energy. We suggest that it was created dynamically from the condensate of a singlet neutrino at a late epoch of the early Universe through its effective self interaction. This singlet neutrino is also the Dirac partner of one of the three observed neutrinos, hence dark energy is related to neutrino mass. The onset of this condensate formation in the early Universe is also related to matter density and offers an explanation of the coincidence problem of why dark energy (70%) and total matter (30%) are comparable at the present time. We demonstrate this idea in a model of neutrino mass with (right-handed) singlet neutrinos and a singlet scalar."

http://arxiv.org/abs/0911.5075
A quantum Bose-Hubbard model with evolving graph as toy model for emergent spacetime
Alioscia Hamma, Fotini Markopoulou, Seth Lloyd, Francesco Caravelli, Simone Severini, Klas Markstrom
23 pages, 6 figures
(Submitted on 27 Nov 2009)
"We present a toy model for interacting matter and geometry that explores quantum dynamics in a spin system as a precursor to a quantum theory of gravity. The model has no a priori geometric properties, instead, locality is inferred from the more fundamental notion of interaction between the matter degrees of freedom. The interaction terms are themselves quantum degrees of freedom so that the structure of interactions and hence the resulting local and causal structures are dynamical. The system is a Hubbard model where the graph of the interactions is a set of quantum evolving variables. We show entanglement between spatial and matter degrees of freedom. We study numerically the quantum system and analyze its entanglement dynamics. We analyze the asymptotic behavior of the classical model. Finally, we discuss analogues of trapped surfaces and gravitational attraction in this simple model."

 

[marcus]

http://arxiv.org/abs/0912.0090
The Box-Problem in Deformed Special Relativity
S. Hossenfelder
20 pages, 3 figures
(Submitted on 1 Dec 2009)
"We examine the transformation of particle trajectories in models with deformations of Special Relativity that have an energy-dependent and observer-independent speed of light. These transformations necessarily imply that the notion of what constitutes the same space-time event becomes dependent on the observer's inertial frame. To preserve observer-independence, the such arising nonlocality should not be in conflict with our knowledge of particle interactions. This requirement allows us to derive strong bounds on deformations of Special Relativity and rule out a modification to first order in energy over the Planck mass."

http://arxiv.org/abs/0911.5464
Quasi-linear formulation of MOND
Mordehai Milgrom
23 pages.
(Submitted on 29 Nov 2009)
"A new formulation of MOND as a modified-potential theory of gravity is propounded. In effect, the theory dictates that the MOND potential phi produced by a mass distribution rho is a solution of the Poisson equation for the modified source density rho*=-(4 pi G)^-1divergence(g), where g=nu(|g^N|/a₀)g^N, and g^N is the Newtonian acceleration field of rho. This makes phi simply the scalar potential of the algebraic acceleration field g. The theory thus involves solving only linear differential equations, with one nonlinear, algebraic step. It is derivable from an action, satisfies all the usual conservation laws, and gives the correct center-of-mass acceleration to composite bodies. The theory is akin in some respects to the nonlinear Poisson formulation of Bekenstein and Milgrom, but it is different from it, and is obviously easier to apply. The two theories are shown to emerge as natural modifications of a Palatini-type formulation of Newtonian gravity, and are members in a larger class of bi-potential theories."

 

[John86]

http://arxiv.org/abs/0911.5729
Critical dynamics of decoherence
Authors: Bogdan Damski, H. T. Quan, Wojciech H. Zurek
(Submitted on 30 Nov 2009)
Abstract: The quantum-classical border Niels Bohr postulated to account for the definiteness of measurement outcomes is explained by decoherence. Decoherence, as a destroyer of quantum coherence and entanglement, is also a respected foe in novel applications of quantum physics (such as quantum computing or quantum metrology). So far, studies of decoherence focused on systems prepared typically in a Schroedinger cat-like superposition, and then instantaneously coupled to an otherwise static environment. We study decoherence induced by many-body dynamic environment undergoing a non-equilibrium (quantum) phase transition. As environment "monitors" the quantum system, its sensitivity -- and, consequently, efficiency of decoherence -- is amplified by a phase transition, as is often the case in the real world detectors (bubble chambers, photographic emulsions, or rhodopsin in our eyes). We show that decoherence happens almost exclusively as the critical point of the environment is traversed, and is significantly enhanced by its non-equilibrium phase transition dynamics. Our calculation yields a simple expression that relates decoherence to the universal critical exponents in a way that parallels theory of topological defect creation in non-equilibrium phase transitions.

 

[marcus]

http://arxiv.org/abs/0912.0208
Asymptotic safety of gravity and the Higgs boson mass
Mikhail Shaposhnikov, Christof Wetterich
12 pages
(Submitted on 1 Dec 2009)
"There are indications that gravity is asymptotically safe. The Standard Model (SM) plus gravity could be valid up to arbitrarily high energies. Supposing that this is indeed the case and assuming that there are no intermediate energy scales between the Fermi and Planck scales we address the question of whether the mass of the Higgs boson m_H can be predicted. For a positive gravity induced anomalous dimension A_\lambda>0 the running of the quartic scalar self interaction \lambda at scales beyond the Planck mass is determined by a fixed point at zero. This results in m_H=m_{\rm min}=126 GeV, with only a few GeV uncertainty. This prediction is independent of the details of the short distance running and holds for a wide class of extensions of the SM as well. For A_\lambda <0 one finds m_H in the interval m_{\rm min}< m_H < m_{\rm max}\simeq 174 GeV, now sensitive to A_\lambda and other properties of the short distance running. The case A_\lambda>0 is favored by explicit computations existing in the literature."

http://arxiv.org/abs/0912.0162
The LQC evolution operator of FRW universe with positive cosmological constant
Wojciech Kaminski, Tomasz Pawlowski
8 pages
(Submitted on 1 Dec 2009)
"The self-adjointness of an evolution operator \Theta_{\Lambda} corresponding to the model of flat FRW universe with massless scalar field and cosmological constant quantized in the framework of Loop Quantum Cosmology is studied in the case \Lambda>0. It is shown, that for \Lambda<\Lambda_c\approx 10.3 \l_{Pl}^{-2} the operator admits many self-adjoint extensions, each of the purely discrete spectrum. On the other hand for \Lambda\geq\Lambda_c the operator is essentially self-adjoint, however the physical Hilbert space of the model does not contain any physically interesting states.

http://arxiv.org/abs/0912.0220
The microstructure of a quantum universe
Leonardo Modesto, Piero Nicolini
4 pages, 1 figure
(Submitted on 1 Dec 2009)
"In this paper, we calculate in a transparent way the spectral dimension of a quantum spacetime, considering a diffusion process propagating on a fluctuating manifold. To describe the erratic path of the diffusion, we implement a minimal length by averaging the graininess of the quantum manifold in the flat space case. As a result we obtain that for large diffusion times, the quantum spacetime behaves like a smooth differential manifold of discrete dimension. On the other hand, for smaller diffusion times, the spacetime looks like a fractal surface with a reduced effective dimension. For the specific case in which the diffusion time has the size of the minimal length, the spacetime turns out to have a spectral dimension equal to two, suggesting a possible renormalizable character of gravity in this regime. For smaller diffusion times, the spectral dimension approaches zero, making less reliable any physical interpretation in this extreme regime. We extend our result to the presence of a background field and curvature. We show that in this case the spectral dimension has a more complicated relation with the diffusion time and conclusions about the renormalizable character of gravity become less straightforward with respect to what found with the flat space analysis."

 

[marcus]

http://arxiv.org/abs/0912.0540
The EPRL intertwiners and corrected partition function
Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
14 pages, 5 figures
(Submitted on 3 Dec 2009)
"Do the SU(2) intertwiners parametrize the space of the EPRL solutions to the simplicity constraint? What is a complete form of the partition function written in terms of this parametrization? We prove that the EPRL map is injective in the general n-valent vertex case for the Barbero-Immirzi parameter less then 1. We find, however, that the EPRL map is not isometric. In the consequence, in order to be written in a SU(2) amplitude form, the formula for the partition function has to be rederived. We do it and obtain a new, complete formula for the partition function. The result goes beyond the SU(2) spin-foam models framework."

 

[marcus]

http://arxiv.org/abs/0912.0554
Emergence of General Relativity from Loop Quantum Gravity
Chun-Yen Lin (University of California at Davis)
7 pages, 2 figures
(Submitted on 3 Dec 2009)
"I show that general relativity emerges from loop quantum gravity, in a relational description of gravitation field in terms of coordinates defined by matter. Local Dirac observables and coherent states are constructed for an explicit evaluation of the dynamics. The dynamics of large scales conforms with general relativity, up to the corrections near singularities."

Chun-Yen is a PhD student of Steve Carlip at UC-Davis. Carlip has another PhD student doing Causal Dynamical Triangulations. Carlip's was the lead talk at the July 2009 Planck Scale conference and one of the best talks---comparing spontaneous dimensional reduction in several different nonstring quantum gravity approaches and suggesting a semiclassical explanation for why it happens. Chun-Yen has a longer GR-from-LQG paper in preparation and probably a dissertation on this topic. Details like this make Carlip's group at Davis. interesting.

http://arxiv.org/abs/0912.0556
On Unitary Evolution in Quantum Field Theory in Curved Spacetime
Daniele Colosi (UNAM), Robert Oeckl (UNAM)
11 pages
(Submitted on 3 Dec 2009)
"We investigate the question of unitarity of evolution between hypersurfaces in quantum field theory in curved spacetime from the perspective of the general boundary formulation. Unitarity thus means unitarity of the quantum operator that maps the state space associated with one hypersurface to the state space associated with the other hypersurface. Working in Klein-Gordon theory, we find that such an evolution is generically unitary given a one-to-one correspondence between classical solutions in neighborhoods of the respective hypersurfaces. This covers the case of pairs of Cauchy hypersurfaces, but also certain cases where hypersurfaces are timelike. The tools we use are the Schroedinger representation and the Feynman path integral."

Both Oeckl and Colosi have track records of publication in LQG and LQG-related areas. This particular paper, while not explicitly treating quantum gravity, can be seen as relevant to the program--joining quantum field theory with the path-integral version of LQG. A strong QG group has been assembled at UNAM-Morelia (which hosted the Loops 2007 conference and will host the 2010 Americas QG school.)

 

[MTd2]

Marcus left this one for me:
http://arxiv.org/abs/0912.0540

The EPRL intertwiners and corrected partition function

Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 3 Dec 2009)
Do the SU(2) intertwiners parametrize the space of the EPRL solutions to the simplicity constraint? What is a complete form of the partition function written in terms of this parametrization? We prove that the EPRL map is injective in the general n-valent vertex case for the Barbero-Immirzi parameter less then 1. We find, however, that the EPRL map is not isometric. In the consequence, in order to be written in a SU(2) amplitude form, the formula for the partition function has to be rederived. We do it and obtain a new, complete formula for the partition function. The result goes beyond the SU(2) spin-foam models framework.

 

[marcus]

http://arxiv.org/abs/0912.0808
Time and Spacetime: The Crystallizing Block Universe
George F. R. Ellis, Tony Rothman
25 Pages. 3 figures
(Submitted on 4 Dec 2009)
"The nature of the future is completely different from the nature of the past. When quantum effects are significant, the future shows all the signs of quantum weirdness, including duality, uncertainty, and entanglement. With the passage of time, after the time-irreversible process of state-vector reduction has taken place, the past emerges, with the previous quantum uncertainty replaced by the classical certainty of definite particle identities and states. The present time is where this transition largely takes place, but the process does not take place uniformly: Evidence from delayed choice and related experiments shows that isolated patches of quantum indeterminacy remain, and that their transition from probability to certainty only takes place later. Thus, when quantum effects are significant, the picture of a classical Evolving Block Universe (`EBU') cedes place to one of a Crystallizing Block Universe (`CBU'), which reflects this quantum transition from indeterminacy to certainty, while nevertheless resembling the EBU on large enough scales."

George Ellis (who co-authored Hawking-Ellis Large Scale Structure of Space Time) is a pioneer in cosmology, and in the philosophy of cosmology. He just celebrated his 70th birthday and a lot of people prominent in quantum gravity and cosmology went to Capetown to give papers in his honor. The guy has an extra good mind and IMHO if he gives us a new idea of the universe, like here, we do well to take a careful look.

 

[marcus]

http://arxiv.org/abs/0912.1095
Supergravity as a constrained BF theory
R. Durka, J. Kowalski-Glikman, M. Szczachor
7 pages
(Submitted on 6 Dec 2009)
"In this paper we formulate N=1 supergravity as a constrained BF theory with OSp(4|1) gauge superalgebra. We derive the modified supergravity Lagrangian that, apart from the standard supergravity with negative cosmological constant, contains terms proportional to the (inverse of) Immirzi parameter. Although these terms do not change classical field equations, they might be relevant in quantum theory. We briefly discuss the perturbation theory around the supersymmetric topological vacuum."

 

[marcus]

http://arxiv.org/abs/0912.1546
Encoding simplicial quantum geometry in group field theories
Daniele Oriti, Tamer Tlas
15 pages, 2 figures
(Submitted on 8 Dec 2009)
"We show that a new symmetry requirement on the GFT field, in the context of an extended GFT formalism, involving both Lie algebra and group elements, leads, in 3d, to Feynman amplitudes with a simplicial path integral form based on the Regge action, to a proper relation between the discrete connection and the triad vectors appearing in it, and to a much more satisfactory and transparent encoding of simplicial geometry already at the level of the GFT action."

http://arxiv.org/abs/0912.1817
From covariant to canonical formulations of discrete gravity
Bianca Dittrich, Philipp A Hoehn
37 pages, 5 figures
(Submitted on 9 Dec 2009)
"Starting from an action for discretized gravity we derive a canonical formalism that exactly reproduces the dynamics and (broken) symmetries of the covariant formalism. For linearized Regge calculus on a flat background -- which exhibits exact gauge symmetries -- we derive local and first class constraints for arbitrary triangulated Cauchy surfaces. These constraints have a clear geometric interpretation and are a first step towards obtaining anomaly--free constraint algebras for canonical lattice gravity. Taking higher order dynamics into account the symmetries of the action are broken. This results in a consistency conditions on the background gauge parameters arising from the lowest non--linear equations of motion. In the canonical framework the constraints to quadratic order turn out to depend on the background gauge parameters and are therefore pseudo constraints. These considerations are important for connecting path integral and canonical quantizations of gravity, in particular if one attempts a perturbative expansion."

http://arxiv.org/abs/0912.1823
A model for non-singular black hole collapse and evaporation
Sabine Hossenfelder, Leonardo Modesto, Isabeau Premont-Schwarz
6 pages, 3 figures
(Submitted on 9 Dec 2009)
"We study the formation of a black hole and its subsequent evaporation in a model employing a minisuperspace approach to loop quantum gravity. In previous work the static solution was obtained and shown to be singularity-free. Here, we examine the more realistic dynamical case by generalizing the static case with help of the Vaidya metric. We track the formation and evolution of trapped surfaces during collapse and evaporation and examine the buildup of quantum gravitationally caused stress-energy preventing the formation of a singularity."

 

[John86]

http://arxiv.org/abs/0912.1748
Nonequilibrium Dynamics in Noncommutative Spacetime
Authors: Chong-Sun Chu, Chiu Man Ho
(Submitted on 9 Dec 2009)
Abstract: We study the effects of spacetime noncommutativity on the nonequilibrium dynamics of particles in a thermal bath. We show that the noncommutative thermal bath does not suffer from any further IR/UV mixing problem in the sense that all the finite-temperature non-planar quantities are free from infrared singularities. We also point out that the combined effect of finite temperature and noncommutative geometry has a distinct effect on the nonequilibrium dynamics of particles propagating in a thermal bath: depending on the momentum of the mode of concern, noncommutative geometry may switch on or switch off their decay and thermalization. This momentum dependent alternation of the decay and thermalization rates could have significant impacts on the nonequilibrium phenomena in the early universe at which spacetime noncommutativity may be present. Our results suggest a re-examination of some of the important processes in the early universe such as reheating after inflation, baryogenesis and the freeze-out of superheavy dark matter candidates.

http://arxiv.org/abs/0912.1798
Quantum key distribution and 1 Gbit/s data encryption over a single fibre
Authors: Patrick Eraerds, Nino Walenta, Matthieu Legre, Nicolas Gisin, Hugo Zbinden
(Submitted on 9 Dec 2009)
Abstract: We perform quantum key distribution (QKD) in the presence of 4 classical channels in a C-band dense wavelength division multiplexing (DWDM) configuration using a commercial QKD system. The classical channels are used for key distillation and 1 Gbps encrypted communication, rendering the entire system independent from any other communication channel than a single dedicated fibre. We successfully distil secret keys over fibre spans of up to 50 km. The separation between quantum channel and nearest classical channel is only 200 GHz, while the classical channels are all separated by 100 GHz. In addition to that we discuss possible improvements and alternative configurations, for instance whether it is advantageous to choose the quantum channel at 1310 nm or to opt for a pure C-band configuration.

http://arxiv.org/abs/0912.1475
Quantum nonlocality: How does Nature perform the trick?\cite{Bellprize}
Authors: N. Gisin
(Submitted on 8 Dec 2009)
Abstract: Since our early childhood we know in our bones that in order to interact with an object we have either to go to it or to throw something at it. Yet, contrary to all our daily experience, Nature is nonlocal: there are spatially separated systems that exhibit nonlocal correlations. In recent years this led to new experiments, deeper understanding of the tension between quantum physics and relativity and to proposals for disruptive technologies.

 

[marcus]

http://arxiv.org/abs/0912.2441
The group field theory approach to quantum gravity: some recent results
Daniele Oriti
15 pages, 5 figures, aipproc class, extended version of the proceedings of the XXV Max Born symposium: The Planck Scale, Wroclaw, 29th June-3rd July 2009
(Submitted on 12 Dec 2009)
"We introduce the key ideas behind the group field theory approach to quantum gravity, and the basic elements of its formalism. We also briefly report on some recent results obtained in this approach, concerning both the mathematical definition of these models, and possible avenues towards extracting interesting physics from them."

http://arxiv.org/abs/0912.2712
A bound on Planck-scale modifications of the energy-momentum composition rule from atomic interferometry
M. Arzano, J. Kowalski-Glikman, A. Walkus
5 pages
(Submitted on 14 Dec 2009)
"High sensitivity measurements in atomic spectroscopy were recently used in Amelino-Camelia et. al. to constraint the form of possible modifications of the energy-momentum dispersion relation resulting from Lorentz invariance violation (LIV). In this letter we show that the same data can be used successfully to set experimental bounds on deformations of the energy-momentum composition rule. Such modifications are natural in models of deformed Lorentz symmetry which are relevant in certain quantum gravity scenarios. We find the bound for the deformation parameter kappa to be a few orders of magnitude below the Planck scale and of the same magnitude as the next-to-leading order effect found in Amelino-Camelia et. al. We briefly discuss how it would be possible to distinguish between these two scenarios."

http://arxiv.org/abs/0912.2435
On Quantum Regge Calculus of Einstein-Cartan Theory
She-Sheng Xue
32 pages and 5 figures. Expanded from the brief letter "Quantum Regge Calculus of Einstein-Cartan theory" in Phys. Lett. B682 (2009) 300. Detailed calculation, references.
(Submitted on 12 Dec 2009)
"This article presents detailed discussions and calculations of the recent letter "Quantum Regge Calculus of Einstein-Cartan theory" in Phys. Lett. B682 (2009) 300 [arXiv:0902.3407]. The Euclidean space-time is discretized by a 4-simplices complex. We adopt basic tetrad and spin-connection fields to describe the 4-simplices complex. Introducing diffeomorphism and local Lorentz invariant holonomy fields, we study a regularized Einstein-Cartan theory for the quantum dynamics of the 4-simplices complex and fermions. This regularized Einstein-Cartan action is shown to properly approaches to its continuum counterpart in the continuum limit. Based on the local Lorentz invariance, we derive the dynamical equations satisfied by invariant holonomy fields. In the mean-field approximation, we show the averaged size of 4-simplex, the element of the 4-simplices complex, has to be larger than the Planck length. This formulation provides a theoretical framework for analytical calculations and numerical simulations to study the quantum Einstein-Cartan theory."

 

[marcus]

http://arxiv.org/abs/0912.2862
Groups of flux-like transformations in loop quantum gravity
J. M. Velhinho
3 pages. Proceedings of the 12th Marcel Grossmann Meeting, Paris, Jul 2009. Talk based on arXiv:0804.3765
(Submitted on 15 Dec 2009)
"We present a group of transformations in the quantum configuration space of loop quantum gravity that contains the set of all transformations generated by the flux variables."

http://arxiv.org/abs/0912.3142
Fractal universe and quantum gravity
Gianluca Calcagni
5 pages
(Submitted on 16 Dec 2009)
"We propose a field theory which lives in fractal spacetime and is argued to be Lorentz invariant, power-counting renormalizable, UV finite, and causal. The system flows from an ultraviolet fixed point, where spacetime has Hausdorff dimension 2, to an infrared limit coinciding with a standard four-dimensional field theory. Classically, the fractal-world where fields live dissipates energy-momentum in the bulk with integer topological dimension. However, the total energy-momentum is conserved. We consider the dynamics and the propagator of a scalar field. The spectrum has a mass gap between a massless mode and a continuum of massive modes. Implications for quantum gravity, cosmology, and the cosmological constant are discussed."

http://arxiv.org/abs/0912.3165
Equipartition of energy in the horizon degrees of freedom and the emergence of gravity
T. Padmanabhan
6 pages
(Submitted on 16 Dec 2009)
"It is possible to provide a physical interpretation for the field equations of gravity based on a thermodynamical perspective. The virtual degrees of freedom associated with the horizons perceived by the local Rindler observers, play a crucial role in this approach. In this context, the relation S=E/2T between the entropy (S), active gravitational mass (E) and temperature (T) - obtained previously in gr-qc/0308070 [CQG, 21, 4485 (2004)] - can be reinterpreted as the law of equipartition E = (1/2) nkT where n is the number (density) of microscopic horizon degrees of freedom. Conversely, one can use the equipartition argument to provide a thermodynamic interpretation of even non-relativistic gravity. These results emphasize the intrinsic quantum nature of all gravitational phenomena and diminishes the distinction between thermal phenomena associated with local Rindler horizons and the usual thermodynamics of macroscopic bodies in non-inertial frames. Just like the original thermodynamic interpretation, these results also hold for a wide class of gravitational theories like the Lanczos-Lovelock models."

Brief mention:

http://pirsa.org/09120114/
On the Relation between Operator Constraint, Master Constraint, Reduced Phase Space, and Path Integral Quantisation, with Application to Quantum Gravity
Video lecture 16 December 2009
Muxin Han

http://arxiv.org/abs/0912.3011
Higher curvature counter terms cause the bounce in loop cosmology
Robert C. Helling
(Submitted on 15 Dec 2009)

 

[John86]

http://arxiv.org/abs/0912.2935
Post-Minkowskian Gravity: Dark Matter as a Relativistic Inertial Effect?
Authors: Luca Lusanna (INFN, Firenze)
(Submitted on 15 Dec 2009)
Abstract: A review is given of the theory of non-inertial frames (with the associated inertial effects and the study of the non-relativistic limit) in Minkowski space-time, of parametrized Minkowski theories and of the rest-frame instant form of dynamics for isolated systems admitting a Lagrangian description. The relevance and gauge equivalence of the clock synchronization conventions for the identification of the instantaneous 3-spaces (Euclidean only in inertial frames) are described. Then this formalism is applied to tetrad gravity in globally hyperbolic, asymptotically Minkowskian space-times without super-translations, where the equivalence principle implies the absence of global inertial frames. The recently discovered York canonical basis, diagonalizing the York-Lichnerowicz approach, allows to identify the gauge variables (inertial effects in general relativity) and the tidal ones (the gravitational waves of the linearized theory) and to clarify the meaning of the Hamilton equations. The role of the gauge variable ${}^3K$, the trace of the extrinsic curvature of the non-Euclidean 3-space (the York time not existing in Newton theory), as a source of inertial effects is emphasized. After the presentation of preliminary results on the linearization of tetrad gravity in the family of non-harmonic 3-orthogonal gauges with a free value of ${}^3K$, we define post-Minkowskian gravitational waves (without post-Newtonian approximations on the matter sources) propagating in a non-Euclidean 3-space, emphasizing the non-graviton-like aspects of gravity. It is conjectured that dark matter may be explained as a relativistic inertial effect induced by ${}^3K$: it would simulate the need to choose a privileged gauge connected with the observational conventions for the description of matter.

http://arxiv.org/abs/0912.3491
Sugawara-type constraints in hyperbolic coset models
Authors: Thibault Damour, Axel Kleinschmidt, Hermann Nicolai
(Submitted on 17 Dec 2009)
Abstract: In the conjectured correspondence between supergravity and geodesic models on infinite-dimensional hyperbolic coset spaces, and E10/K(E10) in particular, the constraints play a central role. We present a Sugawara-type construction in terms of the E10 Noether charges that extends these constraints infinitely into the hyperbolic algebra, in contrast to the truncated expressions obtained in arXiv:0709.2691 that involved only finitely many generators. Our extended constraints are associated to an infinite set of roots which are all imaginary, and in fact fill the closed past light-cone of the Lorentzian root lattice. The construction makes crucial use of the E10 Weyl group and of the fact that the E10 model contains both D=11 supergravity and D=10 IIB supergravity. Our extended constraints appear to unite in a remarkable manner the different canonical constraints of these two theories. This construction may also shed new light on the issue of `open constraint algebras' in traditional canonical approaches to gravity.

http://arxiv.org/abs/0912.3021
Emergent Abelian Gauge Fields from Noncommutative Gravity
Authors: A. Stern
(Submitted on 15 Dec 2009)
Abstract: We construct exact solutions to noncommutative gravity following the formulation of Chamseddine and show that they are in general accompanied by Abelian gauge fields which are first order in the noncommutative scale. This provides a mechanism for generating cosmological electromagnetic fields from an expanding space-time background, and also leads to multipole-like fields surrounding black holes. Exact solutions to noncommutative Einstein-Maxwell theory can give rise to first order corrections to the metric tensor, as well as to the electromagnetic fields. This leads to first order shifts in the horizons of charged black holes.

http://arxiv.org/abs/0911.4817
Emergent Spacetime
Authors: Robert de Mello Koch, Jeff Murugan
(Submitted on 25 Nov 2009 (v1), last revised 16 Dec 2009 (this version, v2))
Abstract: We give an introductory account of the AdS/CFT correspondence in the 1/2-BPS sector of ${\cal N}=4$ super Yang-Mills theory.Six of the dimensions of the string theory are emergent in the Yang-Mills theory. In this article we suggest how these dimensions and local physics in these dimensions emerge. The discussion is aimed at non-experts.

http://arxiv.org/abs/0912.2678
New Physics at Low Accelerations (MOND): an Alternative to Dark Matter
Authors: Mordehai Milgrom (Weizmann Institute)
(Submitted on 14 Dec 2009)
Abstract: I describe the MOND paradigm, which posits a departure from standard physics below a certain acceleration scale. This acceleration as deduced from the dynamics in galaxies is found mysteriously to agree with the cosmic acceleration scales defined by the present day expansion rate and by the density of `dark energy'. I put special emphasis on phenomenology and on critical comparison with the competing paradigm based on classical dynamics plus cold dark matter. I also describe briefly nonrelativistic and relativistic MOND theories.

 

[marcus]

http://arxiv.org/abs/0912.4054
Coherent spin-networks
Eugenio Bianchi, Elena Magliaro, Claudio Perini
11 pages
(Submitted on 20 Dec 2009)
"In this paper we discuss a proposal of coherent states for Loop Quantum Gravity. These states are labeled by a point in the phase space of General Relativity as captured by a spin-network graph. They are defined as the gauge invariant projection of a product over links of Hall's heat-kernels for the cotangent bundle of SU(2). The labels of the state are written in terms of two unit-vectors, a spin and an angle for each link of the graph. The heat-kernel time is chosen to be a function of the spin. These labels are the ones used in the Spin Foam setting and admit a clear geometric interpretation. Moreover, the set of labels per link can be written as an element of SL(2,C). Therefore, these states coincide with Thiemann's coherent states with the area operator as complexifier. We study the properties of semiclassicality of these states and show that, for large spins, they reproduce a superposition over spins of spin-networks with nodes labeled by Livine-Speziale coherent intertwiners. Moreover, the weight associated to spins on links turns out to be given by a Gaussian times a phase as originally proposed by Rovelli."

http://arxiv.org/abs/0912.4093
Loop quantum cosmology and slow roll inflation
Abhay Ashtekar, David Sloan
8 pages, 1 table
(Submitted on 21 Dec 2009)
"In loop quantum cosmology the big bang is replaced by a quantum bounce which is followed by a robust phase of super-inflation. We show that this phase has an unforeseen implication: in presence of suitable inflationary potentials it funnels all dynamical trajectories to conditions which virtually guarantee a slow roll inflation with more than 68 e-foldings, without any input from the pre-big bang regime. This is in striking contrast to the situation in general relativity where it has been argued that the a priori probability of obtaining a slow roll inflation with N e-foldings is suppressed by a factor Exp(-3N)."

 

[marcus]

http://arxiv.org/abs/0912.4563
From time to timescape - Einstein's unfinished revolution
David L. Wiltshire
13 pages, 3 figures; A runner-up in the 2008 FQXi Essay Contest on the Nature of Time; Int. J. Mod. Phys. D 18
(Submitted on 23 Dec 2009)
"I argue that Einstein overlooked an important aspect of the relativity of time in never quite realizing his quest to embody Mach's principle in his theory of gravity. As a step towards that goal, I broaden the Strong Equivalence Principle to a new principle of physics, the Cosmological Equivalence Principle, to account for the role of the evolving average regional density of the universe in the synchronisation of clocks and the relative calibration of inertial frames. In a universe dominated by voids of the size observed in large-scale structure surveys, the density contrasts of expanding regions are strong enough that a relative deceleration of the background between voids and the environment of galaxies, typically of order 10^-10 m/s², must be accounted for. As a result one finds a universe whose present age varies by billions of years according to the position of the observer: a timescape. This model universe is observationally viable: it passes three critical independent tests, and makes additional predictions. Dark energy is revealed as a mis-identification of gravitational energy gradients and the resulting variance in clock rates. Understanding the biggest mystery in cosmology therefore involves a paradigm shift, but in an unexpected direction: the conceptual understanding of time and energy in Einstein's own theory is incomplete."

http://arxiv.org/abs/0912.4581
Five-dimensional metric f(R) gravity and the accelerated universe
Biao Huang, Song Li, Yongge Ma
14 pages, 9 figures
(Submitted on 23 Dec 2009)
"The metric f(R) theories of gravity are generalized to five-dimensional spacetimes. By assuming a hypersurface-orthogonal Killing vector field representing the compact fifth dimension, the five-dimensional theories are reduced to their four-dimensional formalism. Then we study the cosmology of a special class of f(R)=\alpha R^m models in a spatially flat FRW spacetime. It is shown that the parameter m can be constrained to a certain range by the current observed deceleration parameter, and its lower bound corresponds to the Kaluza-Klein theory. It turns out that both expansion and contraction of the extra dimension may prescribe the smooth transition from the deceleration era to the acceleration era in the recent past as well as an accelerated scenario for the present universe. Hence five-dimensional f(R) gravity can naturally account for the present accelerated expansion of the universe. Moreover, the models predict a transition from acceleration to deceleration in the future, followed by a cosmic recollapse within finite time. This differs from the prediction of the five-dimensional Brans-Dicke theory but is in consistent with a recent prediction based on loop quantum cosmology."

 

[MTd2]

http://arxiv.org/abs/0912.4907

Quantum gravity asymptotics from the SU(2) 15j symbol

John W. Barrett, Winston J. Fairbairn, Frank Hellmann
(Submitted on 24 Dec 2009)
The asymptotics of the SU(2) 15j symbol are obtained using coherent states for the boundary data. The geometry of all non-suppressed boundary data is given. For some boundary data, the resulting formula is interpreted in terms of the Regge action of the geometry of a 4-simplex in 4-dimensional Euclidean space. This asymptotic formula can be used to derive and extend the asymptotics of the spin foam amplitudes for quantum gravity models. The relation of the SU(2) Ooguri model to these quantum gravity models and their continuum Lagrangians is discussed.

 

[marcus]

Briefly noted:
http://arxiv.org/abs/0912.5234
Gravitational energy as dark energy: Towards concordance cosmology without Lambda
David L. Wiltshire
8 pages, 1 figure; in E. Pecontal, T. Buchert, Ph. Di Stefano and Y. Copin (eds), "Dark Energy and Dark Matter: Observations, Experiments and Theories", Proceedings, Lyon, 7-11 July, 2008; EAS Publ.Ser.36:91-98,2009
(Submitted on 29 Dec 2009)

http://arxiv.org/abs/0912.5236
Gravitational energy as dark energy: Average observational quantities
David L. Wiltshire
10 pages, 7 figures; submitted to the Proceedings of the Invisible Universe Conference, Paris, 29 June - 3 July, 2009; J.-M. Alimi (ed), AIP Conf. Proc., to appear
(Submitted on 29 Dec 2009)
"In the timescape scenario cosmic acceleration is understand as an apparent effect, due to gravitational energy gradients that grow when spatial curvature gradients become significant with the nonlinear growth of cosmic structure. ...I discuss recent work on defining observational tests for average geometric quantities which can distinguish the timescape model from a cosmological constant or other models of dark energy."

 

[MTd2]

You forgot this one! It is about LQG, although not explicit in the title neither in the abstract:

http://arxiv.org/abs/0912.5384

Asymptotics of the Wigner 9j symbolHal M. Haggard, Robert G. Littlejohn
(Submitted on 29 Dec 2009)
We present the asymptotic formula for the Wigner 9j-symbol, valid when all quantum numbers are large, in the classically allowed region. As in the Ponzano-Regge formula for the 6j-symbol, the action is expressed in terms of lengths of edges and dihedral angles of a geometrical figure, but the angles require care in definition. Rules are presented for converting spin networks into the associated geometrical figures. The amplitude is expressed as the determinant of a 2x2 matrix of Poisson brackets. The 9j-symbol possesses caustics associated with the fold and elliptic and hyperbolic umbilic catastrophes. The asymptotic formula obeys the exact symmetries of the 9j-symbol.

 

[marcus]

http://arxiv.org/abs/1001.0161
Phenomenology of loop quantum cosmology
Mairi Sakellariadou
16 pages, 3 figures; Invited talk in the First Mediterranean Conference on Classical and Quantum Gravity (Crete, Greece)
(Submitted on 31 Dec 2009)
"After introducing the basic ingredients of Loop Quantum Cosmology, I will briefly discuss some of its phenomenological aspects. Those can give some useful insight about the full Loop Quantum Gravity theory and provide an answer to some long-standing questions in early universe cosmology."

http://arxiv.org/abs/1001.0288
Dynamical Horizon Entropy Bound Conjecture in Loop Quantum Cosmology
Li-Fang Li, Jian-Yang Zhu
4 pages, 2 figures
(Submitted on 2 Jan 2010)
"The covariant entropy bound conjecture is an important hint for the quantum gravity, with several versions available in the literature. For cosmology, Ashtekar and Wilson-Ewing ever show the consistence between the loop gravity theory and one version of this conjecture. Recently, S. He and H. Zhang proposed a version for the dynamical horizon of the universe, which validates the entropy bound conjecture for the cosmology filled with perfect fluid in the classical scenario when the universe is far away from the big bang singularity. However, their conjecture breaks down near big bang region. We examine this conjecture in the context of the loop quantum cosmology. With the example of photon gas, this conjecture is protected by the quantum geometry effects as expected."

http://arxiv.org/abs/1001.0306
Thermodynamics properties of the dark energy in loop quantum cosmology
Kui Xiao, Jian-Yang Zhu
10 pages, 4 figures
(Submitted on 2 Jan 2010)
"Considering an arbitrary, varying equation of state parameter, the thermodynamic properties of the dark energy fluid in semiclassical loop quantum cosmology scenario, which we consider the inverse volume modification, is studied. The equation of state parameters are corrected as semiclassical one during considering the effective behavior. Assuming that the apparent horizon has Hawking temperature, the modified entropy-area relation is obtained. We find that this relation is different from the one which is obtained by considering the holonomy correction. Considering the dark energy is a thermal equilibrium fluid, we get the expressions for modified temperature, chemical potential and entropy. The temperature, chemical potential and entropy are well-defined in the semiclassical regions. But if one assume that the dark energy has the same temperature as the apparent horizon, dS\geq 0 will be invalid in some regions."

Brief notice:
http://arxiv.org/abs/1001.0316
Dark Matter: The evidence from astronomy, astrophysics and cosmology
Matts Roos
Review article of 25 pages, 18 figures
(Submitted on 4 Jan 2010)

 

[marcus]

http://arxiv.org/abs/1001.0571
Quantum field theory, gravity and cosmology in a fractal universe
Gianluca Calcagni
39 pages, 4 figures
(Submitted on 4 Jan 2010)
"We propose a model for a power-counting renormalizable field theory living in a fractal spacetime. The action is Lorentz covariant and equipped with a Stieltjes measure. The system flows, even in a classical sense, from an ultraviolet regime where spacetime has Hausdorff dimension 2 to an infrared limit coinciding with a standard D-dimensional field theory. We discuss the properties of a scalar field model at classical and quantum level. Classically, the field lives on a fractal which exchanges energy-momentum with the bulk of integer topological dimension D. Although an observer experiences dissipation, the total energy-momentum is conserved. The field spectrum is a continuum of massive modes. The gravitational sector and Einstein equations are discussed in detail, also on cosmological backgrounds. We find ultraviolet cosmological solutions and comment on their implications for the early universe."

 

[John86]

http://arxiv.org/abs/1001.0785

On the Origin of Gravity and the Laws of Newton
Authors: Erik P. Verlinde
(Submitted on 6 Jan 2010)
Abstract: Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.

 

[MTd2]

http://arxiv.org/abs/1001.1330

Introduction to Loop Quantum Gravity

Simone Mercuri
(Submitted on 8 Jan 2010)
The questions I have been asked during the 5th International School on Field Theory and Gravitation, have compelled me to give an account of the premises that I consider important for a beginner's approach to Loop Quantum Gravity. After a description of some general arguments and an introduction to the canonical theory of gravity, I review the background independent approach to quantum gravity, giving only a brief survey of Loop Quantum Gravity.

 

[marcus]

http://arxiv.org/abs/1001.1227
The cosmological perturbation theory in loop cosmology with holonomy corrections
Jian-Pin Wu, Yi Ling
18 pages
(Submitted on 8 Jan 2010)
"In this paper we investigate the scalar mode of first-order metric perturbations over spatially flat FRW spacetime when the holonomy correction is taken into account in the semi-classical framework of loop quantum cosmology. By means of the Hamiltonian derivation, the cosmological perturbation equations is obtained in longitudinal gauge. It turns out that in the presence of metric perturbation the holonomy effects influence both background and perturbations, and contribute a non-trivial sector S_h in the cosmological perturbation equations."