Loop-and-allied QG bibliography

[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."

 

[John86]

http://arxiv.org/abs/1001.1310

Acoustic geometry for general relativistic barotropic irrotational fluid flow
Authors: Matt Visser (Victoria University of Wellington), Carmen Molina-Paris (Leeds University)
(Submitted on 8 Jan 2010)
Abstract: "Acoustic spacetimes", in which techniques of differential geometry are used to investigate sound propagation in moving fluids, have attracted considerable attention over the last few decades. Most of the models currently considered in the literature are based on non-relativistic barotropic irrotational fluids, defined in a flat Newtonian background. The extension, first to special relativistic barotropic fluid flow, and then to general relativistic barotropic fluid flow in an arbitrary background, is less straightforward than it might at first appear. In this article we provide a pedagogical and simple derivation of the general relativistic "acoustic spacetime" in an arbitrary (d+1) dimensional curved-space background.

http://arxiv.org/abs/1001.1180

The quantum interest conjecture in (3+1)-dimensional Minkowski space
Authors: Gabriel Abreu (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 7 Jan 2010)
Abstract: The quantum inequalities, and the closely related quantum interest conjecture, impose restrictions on the distribution of the energy density measured by any time-like observer, potentially preventing the existence of exotic phenomena such as Alcubierre warp-drives or traversable wormholes. It has already been proved that both assertions can be reduced to statements concerning the existence or non-existence of bound states of a certain 1-dimensional quantum mechanical Hamiltonian. Using this approach, we will informally review a simple variational proof of one version of the Quantum Interest conjecture in (3+1)-dimensional Minkowski space.

 

[marcus]

http://arxiv.org/abs/1001.1758
Weak lensing, dark matter and dark energy
Dragan Huterer (University of Michigan)
Invited review article for the GRG special issue on gravitational lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). 17 pages, 8 figures
(Submitted on 11 Jan 2010)
"Weak gravitational lensing is rapidly becoming one of the principal probes of dark matter and dark energy in the universe. In this brief review we outline how weak lensing helps determine the structure of dark matter halos, measure the expansion rate of the universe, and distinguish between modified gravity and dark energy explanations for the acceleration of the universe. We also discuss requirements on the control of systematic errors so that the systematics do not appreciably degrade the power of weak lensing as a cosmological probe."

http://arxiv.org/abs/1001.1838
Spacetime geometry in (2+1)-gravity via measurements with returning lightrays
C. Meusburger
Talk given at the XXV Max Born Symposium: The Planck Scale (Wroclaw, June 29-July 3 2009) 10 pages, 4 figures
(Submitted on 12 Jan 2010)
"We consider an observer in a (2+1)-spacetime without matter and cosmological constant who measures spacetime geometry by emitting lightrays which return to him at a later time. We investigate several quantities associated with such lightrays: the return time, the directions into which light needs to be emitted to return and the frequency shift between the lightray at its emission and its return. We derive explicit expressions for these quantities as functions on the reduced phase space and show how they allow the observer to reconstruct the full geometry of the spacetime in finite eigentime. We comment on conceptual issues. In particular, we clarify the relation between these quantities and Dirac observables and show that Wilson loops arise naturally in these quantities."

http://arxiv.org/abs/1001.1842
Global Lorentzian geometry from lightlike geodesics: What does an observer in (2+1)-gravity see?
C. Meusburger
Talk given at the workshop Chern-Simons Gauge Theory: 20 years after, August 3-7 2009, Hausdorff Center for Mathematics, 18 pages, 9 figures
(Submitted on 12 Jan 2010)
"We show how an observer could measure the non-local holonomy variables that parametrise the flat Lorentzian 3d manifolds arising as spacetimes in (2+1)-gravity. We consider an observer who emits lightrays that return to him at a later time and performs several realistic measurements associated with such returning lightrays: the eigentime elapsed between the emission of the lightrays and their return, the directions into which the light is emitted and from which it returns and the frequency shift between the emitted and returning lightray. We show how the holonomy variables and hence the full geometry of these manifolds can be reconstructed from these measurements in finite eigentime."

 

[John86]

http://arxiv.org/abs/1001.2007

The Holographic Universe
Authors: Paul McFadden, Kostas Skenderis
(Submitted on 13 Jan 2010)
Abstract: We present a holographic description of four-dimensional single-scalar inflationary universes in terms of a three-dimensional quantum field theory. The holographic description correctly reproduces standard inflationary predictions in their regime of applicability. In the opposite case, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative QFT and present models capable of satisfying the current observational constraints while exhibiting a phenomenology distinct from standard inflation. This provides a qualitatively new method for generating a nearly scale-invariant spectrum of primordial cosmological perturbations.

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.

http://arxiv.org/abs/0912.0790

Bimetric MOND gravity
Authors: Mordehai Milgrom
(Submitted on 4 Dec 2009)
Abstract: A new relativistic formulation of MOND is advanced, involving two metrics as independent degrees of freedom: the MOND metric g_mn, to which alone matter couples, and an auxiliary metric g*_mn. The main idea hinges on the fact that we can form tensors from the difference, C^a_bc, of the Levi-Civita connections of the two metrics, and these act like gravitational accelerations. In the context of MOND we can form dimensionless `acceleration' scalars, and functions thereof, from contractions of C^a_bc/a0. I look at a class of bimetric MOND theories governed by an action with the gravitational Lagrangian density b sqrt(g)R+a sqrt(g*) R* -2(gg*)^{1/4}f(k)a0^2M(U/a0^2), and with matter actions I(g_mn,psi)+I*(g*_mn,chi), with U a scalar quadratic in the C^a_bc, k=(g/g*)^{1/4}, and allowing for the existence of twin matter, chi, that couples to g*_mn alone. In particular, I concentrate on one interesting and simple choice of the scalar U. This theory introduces only one new constant, a0; it tends simply to General Relativity in the limit a0 goes to 0, and to a phenomenologically valid MOND theory in the nonrelativistic limit. The theory naturally gives MOND and "dark energy" effects from the same term in the action, both controlled by the MOND constant a0. As regards gravitational lensing by nonrelativistic systems--a holy grail for relativistic MOND theories--the theory predicts that the same potential that controls massive-particle motion also dictates lensing in the same way as in GR. This last result can be modified with other choices of U, but lensing is still enhanced and MOND-like, with an effective logarithmic potential.

 

[MTd2]

http://arxiv.org/abs/1001.2748

Twisted geometries: A geometric parametrisation of SU(2) phase space

Laurent Freidel, Simone Speziale
(Submitted on 15 Jan 2010)
A cornerstone of the loop quantum gravity program is the fact that the phase space of general relativity on a fixed graph can be described by a product of SU(2) cotangent bundles per edge. In this paper we show how to parametrize this phase space in terms of quantities describing the intrinsic and extrinsic geometry of the triangulation dual to the graph. These are defined by the assignment to each triangle of its area, the two unit normals as seen from the two polyhedra sharing it, and an additional angle related to the extrinsic curvature. These quantities do not define a Regge geometry, since they include extrinsic data, but a looser notion of discrete geometry which is twisted in the sense that it is locally well-defined, but the local patches lack a consistent gluing among each other. We give the Poisson brackets among the new variables, and exhibit a symplectomorphism which maps them into the Poisson brackets of loop gravity. The new parametrization has the advantage of a simple description of the gauge-invariant reduced phase space, which is given by a product of phase spaces associated to edges and vertices, and it also provides an abelianisation of the SU(2) connection. The results are relevant for the construction of coherent states, and as a byproduct, contribute to clarify the connection between loop gravity and its subset corresponding to Regge geometries.

 

[marcus]

http://arxiv.org/abs/1001.2663
Cosmic recall and the scattering picture of Loop Quantum Cosmology
Wojciech Kaminski, Tomasz Pawlowski
19 pages, 3 figures
(Submitted on 15 Jan 2010)
"The global dynamics of a homogeneous universe in Loop Quantum Cosmology is viewed as a scattering process of its geometrodynamical equivalent. This picture is applied to build a flexible (easy to generalize) and not restricted just to exactly solvable models method of verifying the preservation of the semiclassicality through the bounce. The devised method is next applied to two simple examples: (i) the isotropic Friedman Robertson Walker universe, and (ii) the isotropic sector of the Bianchi I model. For both of them we show, that the dispersions in the logarithm of the volume ln(v) and scalar field momentum ln(p_phi) in the distant future and past are related via strong triangle inequalities. This implies in particular a strict preservation of the semiclassicality (in considered degrees of freedom) in both the cases (i) and (ii). Derived inequalities are general: valid for all the physical states within the considered models."

http://arxiv.org/abs/1001.2702
Emergent matter from 3d generalised group field theories
Alessandro Di Mare, Daniele Oriti
14 pages, 1 figure
(Submitted on 15 Jan 2010)
"We identify classical solutions of a generalised group field theory model in 3 dimensions, and study the corresponding perturbations, deriving their effective dynamics. We discuss their interpretation as emergent matter fields. This allows us, on the one hand to test the proposed mechanism for emergence of matter as a phase of group field theory, and on the other hand to expose some limitations of the generalised group field theory formalism."

http://arxiv.org/abs/1001.2699
Gravity in presence of fermions as a SU(2) gauge theory
F. Cianfrani, G. Montani
5 pages
(Submitted on 15 Jan 2010)
"The Hamiltonian formulation of the Holst action in presence of a massless fermion field with a non-minimal Lagrangian is performed without any restriction on the local Lorentz frame. It is outlined that the phase space structure does not resemble that one of a background independent Lorentz gauge theory, because some additional constraints are present. Proper phase space coordinates are introduced, such that SU(2) connections can be defined and the vanishing of conjugate momenta to boost variables is predicted. Finally, it is outlined that for a particular value of the non-minimal parameter the kinematics coincides with that one of a background independent SU(2) gauge theory and the Immirzi parameter becomes the coupling constant of such an interaction between fermions and the gravitational field."

 

[John86]

http://arxiv.org/abs/1001.2725

The Scalar Curvature of a Causal Set
Authors: Dionigi M.T. Benincasa, Fay Dowker
(Submitted on 15 Jan 2010 (v1), last revised 16 Jan 2010 (this version, v2))
Abstract: A one parameter family of retarded linear operators on scalar fields on causal sets is introduced. When the causal set is well-approximated by 4 dimensional Minkowski spacetime, the operators are Lorentz invariant but nonlocal, are parametrised by the scale of the nonlocality and approximate the continuum scalar D'Alembertian, $\Box$, when acting on fields that vary slowly on the nonlocality scale. The same operators can be applied to scalar fields on causal sets which are well-approximated by curved spacetimes in which case they approximate $\Box - {{1/2}}R$ where $R$ is the Ricci scalar curvature. This can used to define an approximately local action functional for causal sets.

http://arxiv.org/abs/1001.2758

Beyond the Quantum
Authors: Antony Valentini
(Submitted on 15 Jan 2010)
Abstract: At the 1927 Solvay conference, three different theories of quantum mechanics were presented; however, the physicists present failed to reach a consensus. Today, many fundamental questions about quantum physics remain unanswered. One of the theories presented at the conference was Louis de Broglie's pilot-wave dynamics. This work was subsequently neglected in historical accounts; however, recent studies of de Broglie's original idea have rediscovered a powerful and original theory. In de Broglie's theory, quantum theory emerges as a special subset of a wider physics, which allows non-local signals and violation of the uncertainty principle. Experimental evidence for this new physics might be found in the cosmological-microwave-background anisotropies and with the detection of relic particles with exotic new properties predicted by the theory.

 

[atyy]

http://arxiv.org/abs/1001.2706
Matrix Models, Emergent Spacetime and Symmetry Breaking
Harald Grosse, Fedele Lizzi, Harold Steinacker
(Submitted on 15 Jan 2010)
We discuss how a matrix model recently shown to describe emergent gravity may contain extra degrees of freedom which reproduce some characteristics of the standard model, in particular the breaking of symmetries and the correct quantum numbers of fermions.

http://arxiv.org/abs/1001.2703
Noncommutative gauge theory and symmetry breaking in matrix models
Harald Grosse, Fedele Lizzi, Harold Steinacker
(Submitted on 15 Jan 2010)
We show how the fields and particles of the standard model can be naturally realized in noncommutative gauge theory. Starting with a Yang-Mills matrix model in more than 4 dimensions, a SU(n) gauge theory on a Moyal-Weyl space arises with all matter and fields in the adjoint of the gauge group. We show how this gauge symmetry can be broken spontaneously down to SU(3)_c x SU(2)_L x U(1)_Q (resp. SU(3)_c x U(1)_Q), which couples appropriately to all fields in the standard model. An additional U(1)_B gauge group arises which is anomalous at low energies, while the trace-U(1) sector is understood in terms of emergent gravity. A number of additional fields arise which we assume to be massive, in a pattern that is reminiscent of supersymmetry. The symmetry breaking might arise via spontaneously generated fuzzy spheres, in which case the mechanism is similar to brane constructions in string theory.

 

[MTd2]

http://arxiv.org/abs/1001.2919

A Deformed Poincare Invariance for Group Field Theories

Florian Girelli, Etera R. Livine
(Submitted on 17 Jan 2010)
In the context of quantum gravity, group field theories are field theories that generate spinfoam amplitudes as Feynman diagrams. They can be understood as generalizations of the matrix models used for 2d quantum gravity. In particular Boulatov's theory reproduces the amplitudes of the Ponzano-Regge spinfoam model for 3d quantum gravity. Motivated by recent works on field theories on non-commutative flat spaces, we show that Boulatov's theory (and its colored version) is actually invariant under a global deformed Poincare symmetry. This allows to define a notion of flat/excited geometry states when considering scalar perturbations around classical solutions of the group field equations of motion. As a side-result, our analysis seems to point out that the notion of braiding of group field theories should be a key feature to study further in this context.

 

[MTd2]

http://arxiv.org/abs/1001.3282

Non-standard loop quantum cosmology

Wlodzimierz Piechocki
(Submitted on 19 Jan 2010)
We present results concerning the nature of the cosmological big bounce(BB) transition within the loop geometry underlying loop quantum cosmology (LQC). Our canonical quantization method is an alternative to the standard LQC. An evolution parameter we use has clear interpretation both at classical and quantum levels. The physical volume operator has discrete spectrum which is bounded from below. The minimum gap in the spectrum defines a quantum of the volume. The spectra of operators are parametrized by a free parameter to be determined.

 

[marcus]

http://arxiv.org/abs/1001.3292
On the regularization of the constraints algebra of Quantum Gravity in 2+1 dimensions with non-vanishing cosmological constant
Alejandro Perez, Daniele Pranzetti
(Submitted on 19 Jan 2010)
"We use the mathematical framework of loop quantum gravity (LQG) to study the quantization of three dimensional (Riemannian) gravity with positive cosmological constant (Lambda>0). We show that the usual regularization techniques (successful in the Lambda=0 case and widely applied in four dimensional LQG) lead to a deformation of the classical constraint algebra (or anomaly) proportional to the local strength of the curvature squared. We argue that this is an unavoidable consequence of the non-local nature of generalized connections."

http://arxiv.org/abs/1001.3380
Why Does the Universe Expand?
T. Padmanabhan
5 pages
(Submitted on 19 Jan 2010)
"The purpose of the paper is five-fold: (a) Argue that the question in the title can be presented in a meaningful manner and that it requires an answer. (b) Discuss the conventional answers and explain why they are unsatisfactory. (c) Suggest that a key ingredient in the answer could be the instability arising due to the 'wrong' sign in the Hilbert action for the kinetic energy term corresponding to expansion factor. (d) Describe how this idea connects up with another peculiar feature of our universe, viz. it spontaneously became more and more classical in the course of evolution. (e) Provide a speculative but plausible scenario, based on the thermodynamic perspective of gravity, in which one has the hope for relating the thermodynamic and cosmological arrows of time."

 

[qsa]

I think this guy is very ,very close to the final TOE

http://arxiv.org/PS_cache/arxiv/pdf/0711/0711.4232v5.pdf

Properties of the Curved Spaetime Dirac Equation
G. G. Nyambuya

Abstract. This paper is a continuation of the earlier paper (Nyambuya 2008) the
Curved Spacetime Dirac Equation has been derived. This equation has been developed
mainly to account in a natural way for the observed anomalous gyromagnetic ratio of
fermions and the suggestions is that particles including the Electron, which is thought
to be a point particle, do have a finite spatial size which is the reason for the observed
anomalous gyromagnetic ratio. Combining the idea Nyambuya 2008a and the
proposed Unified Field Theory (Nyambuya 2007), a total of 12 equations each with 16
sub-components are generated thus totaling 192 equations for the Curved Spacetime
Dirac Equation. Four symmetries of these equation are investigated, that is the Lorentz
symmetry, charge conjugation symmetry (C), time reversal symmetry (T) and Space
reversal (P). It is shown that these equations are Lorentz invariant, obey C invariance
symmetry and that some violate T and P symmetry while others do not and that they
all obey TP symmetry. These symmetries show that anti-particles have positive mass and
energy but a negative rest mass and the opposite sign in electronic charge. A suggestion is made that the rest mass of a particle must be related to the electronic charge of that particle.

 

[qsa]

here is a more recent paper

http://arxiv.org/PS_cache/arxiv/pdf/0807/0807.1754v3.pdf


Abstract. (Dated: May 5, 2009) The original Dirac Equation is modified in the simplest imaginable and
most trivial manner to include a universal 4-Vector Cosmological Field term in the space and
time dimensions. This cosmological field leads to a modified Dirac Equation capable of
explaining why the Universe appears to be made up chiefly of matter. It is seen that
this 4-Vector Cosmological Field is actually a particle field and this particle field can possibly
be identified with the darkmatter and darkenergy field. Further, this 4-Vector Cosmological
Field is seen to give spacetime the desired quantum mechanical properties of randomness.
Furthermore, it is seen that in the emergent Universe, the position coordinates of a particle
in space – contrary to the widely accepted belief that the position of a particle in space
has no physical significance, we see that that opposite is true – namely that the position
of a particle has physical significance. We further note that the 4-Vector Cosmological Field
modification to the Dirac Equation leads us to a vacuum model redolent but differrent
from that of Quantum Electrodynamics (QED). This new vacuum model is without virtual
particles but darkparticles. We dare to make the suggestion that these darkparticles may
possibly explain the current mystery of what really is darkmatter and darkenergy.

 

[gadzirai]

Have you found any thing with the paper:

http://arxiv.org/PS_cache/arxiv/pdf/...807.1754v3.pdf

 

[marcus]

http://arxiv.org/abs/1001.3505
Polymer quantization of the free scalar field and its classical limit
Alok Laddha, Madhavan Varadarajan
58 pages
(Submitted on 20 Jan 2010)
"Building on prior work, a generally covariant reformulation of free scalar field theory on the flat Lorentzian cylinder is quantized using Loop Quantum Gravity (LQG) type 'polymer' representations. This quantization of the continuum classical theory yields a quantum theory which lives on a discrete spacetime lattice. We explicitly construct a state in the polymer Hilbert space which reproduces the standard Fock vacuum- two point functions for long wavelength modes of the scalar field. Our construction indicates that the continuum classical theory emerges under coarse graining. All our considerations are free of the 'triangulation' ambiguities which plague attempts to define quantum dynamics in LQG. Our work constitutes the first complete LQG type quantization of a generally covariant field theory together with a semi-classical analysis of the true degrees of freedom and thus provides a perfect infinite dimensional toy model to study open issues in LQG, particularly those pertaining to the definition of quantum dynamics."


http://arxiv.org/abs/1001.3668
Newtonian gravity in loop quantum gravity
Lee Smolin
16 pages
(Submitted on 20 Jan 2010)
"We apply a recent argument of Verlinde to loop quantum gravity, to conclude that Newton's law of gravity emerges in an appropriate limit and setting. This is possible because the relationship between area and entropy is realized in loop quantum gravity when boundaries are imposed on a quantum spacetime."

 

[John86]

http://arxiv.org/abs/1001.3538

At the Frontier of Knowledge
Authors: Sabine Hossenfelder
(Submitted on 20 Jan 2010)
Abstract: At any time, there are areas of science where we are standing at the frontier of knowledge, and can wonder whether we have reached a fundamental limit to human understanding. What is ultimately possible in physics? I will argue here that it is ultimately impossible to answer this question. For this, I will first distinguish three different reasons why the possibility of progress is doubted and offer examples for these cases. Based on this, one can then identify three reasons for why progress might indeed be impossible, and finally conclude that it is impossible to decide which case we are facing.

http://arxiv.org/abs/1001.3391

Quantum theory, gravity, and the standard model of particle physics : using the hints of today to build the final theory of tomorrow
Authors: T. P. Singh
(Submitted on 19 Jan 2010)
Abstract: When a mountaineer is ascending one of the great peaks of the Himalayas she knows that an entirely new vista awaits her at the top, whose ramifications will be known only after she gets there. Her immediate goal though, is to tackle the obstacles on the way up, and reach the summit. In a similar vein, one of the immediate goals of contemporary theoretical physics is to build a quantum, unified description of general relativity and the standard model of particle physics. Once that peak has been reached, a new (yet unknown) vista will open up. In this essay I propose a novel approach towards this goal. One must address and resolve a fundamental unsolved problem in the presently known formulation of quantum theory : the unsatisfactory presence of an external classical time in the formulation. Solving this problem takes us to the very edge of theoretical physics as we know it today!

http://arxiv.org/abs/1001.3419

Quantum Darwinism of Scattering Radiation
Authors: C. Jess Riedel, Wojciech H. Zurek
(Submitted on 19 Jan 2010)
Abstract: We study Quantum Darwinism--the redundant recording of information about the preferred states of a decohering system by its environment--in the case of an object that is illuminated by photons. We calculate the linear quantum mutual information between the object and its photon environment for both monochromatic and thermal radiation. Then, we compare this to a semi-classical treatment of the monochromatic radiation and calculate the exact Shannon mutual information. We demonstrate that these realistic models exhibit fast and extensive proliferation of information about the object into the environment and result in redundancies orders of magnitude larger than the exactly soluble models considered to date.

 

[czes]

http://arxiv.org/abs/1001.3668
Newtonian gravity in loop quantum gravity
Lee Smolin
16 pages
(Submitted on 20 Jan 2010)
"We apply a recent argument of Verlinde to loop quantum gravity, to conclude that Newton's law of gravity emerges in an appropriate limit and setting. This is possible because the relationship between area and entropy is realized in loop quantum gravity when boundaries are imposed on a quantum spacetime."

Have you seen that I wrote the same 2 days earlier (18 Jan) but without high mathematic ?
They start to use Compton wave length but still do calculate by Shannon entropy using Boltzmann constant. Compton wave length does it much easier.

 

[marcus]

Czes,
This thread is primarily bibliography. I hope you continue this discussion, not here (where it interferes with the bibliography) but in a separate "Czes discussion thread" or wherever else is agreeable to you.

 

[marcus]

http://arxiv.org/abs/1001.3808
Notes Concerning "On the Origin of Gravity and the Laws of Newton" by E. Verlinde (arXiv:1001.0785)
Jarmo Mäkeä
6 pages, 1 figure
(Submitted on 21 Jan 2010)
"We point out that certain equations which, in a very recent paper written by E. Verlinde, are postulated as a starting point for a thermodynamical derivation of classical gravity, are actually consequences of a specific microscopic model of spacetime, which has been published earlier."

 

[marcus]

http://lanl.arxiv.org/abs/1001.3999
Observables for FRW model with cosmological constant in the framework of loop cosmology
Jakub Mielczarek, Wlodzimierz Piechocki
7 pages, 4 figures
(Submitted on 22 Jan 2010)
"We consider a flat cosmological model with a free massless scalar field and the cosmological constant Lambda in the framework of loop quantum cosmology. The scalar field plays the role of an intrinsic time. We apply the reduced phase space approach. The dynamics of the model is solved analytically. We identify elementary observables and their algebra. The compound physical observables like the volume and the energy density of matter field are analysed. Both compound observables are bounded and oscillate in the Lambda<0 case. The energy density is bounded and oscilltes in the Lambda>0 case. However, the volume is unbounded from above, but periodic."

http://lanl.arxiv.org/abs/1001.4041
Causal Sets: Quantum gravity from a fundamentally discrete spacetime
Petros Wallden
10 pages, review of causal sets based on talk given at the 1st MCCQG conference
(Submitted on 22 Jan 2010)
"In order to construct a quantum theory of gravity, we may have to abandon certain assumptions we were making. In particular, the concept of spacetime as a continuum substratum is questioned. Causal Sets is an attempt to construct a quantum theory of gravity starting with a fundamentally discrete spacetime. In this contribution we review the whole approach, focusing on some recent developments in the kinematics and dynamics of the approach."

http://lanl.arxiv.org/abs/1001.3876
Alternatives to dark matter: Modified gravity as an alternative to dark matter
Jacob D. Bekenstein
20 pages, 4 figures
(Submitted on 21 Jan 2010)
"The premier alternative to the dark matter paradigm is modified gravity. Following an introduction to the relevant phenomenology of galaxies, I review the MOND paradigm, an effective summary of the observations which any theory must reproduce. A simple nonlinear modified gravity theory does justice to MOND at the nonrelativistic level, but cannot be elevated to the relativistic level in a unique way. I go in detail into the covariant tensor-vector-theory (TeVeS) which not only recovers MOND but can also deal in detail with gravitational lensing and cosmology. Problems with MOND and TeVeS at the level of clusters of galaxies are given attention. I also summarize the status of TeVeS cosmology."

 

[MTd2]

http://arxiv.org/abs/1001.4188

Loop quantum gravity - a short review

Hanno Sahlmann
(Submitted on 23 Jan 2010)
In this article we review the foundations and the present status of loop quantum gravity. It is short and relatively non-technical, the emphasis is on the ideas, and the flavor of the techniques. In particular, we describe the kinematical quantization and the implementation of the Hamilton constraint, as well as the quantum theory of black hole horizons, semiclassical states, and matter propagation. Spin foam models and loop quantum cosmology are mentioned only in passing, as these will be covered in separate reviews to be published alongside this one.

http://arxiv.org/abs/1001.4364

Holographic actions from black hole entropy

Francesco Caravelli, Leonardo Modesto
(Submitted on 25 Jan 2010)
Using the Wald's relation between the Noether charge of diffeomorphisms and the entropy for a generic spacetime possessing a bifurcation surface, we introduce a method to obtain a family of higher order derivatives effective actions from the entropy of black holes. Our point of view is to consider fundamental the black hole entropy and the action an emerged object. We then specialize to a particular class of effective theories: the f(R) theories. We apply the idea, using a simple mind ansatz, to loop quantum gravity and to a general class of log-corrected entropy formulas.

http://arxiv.org/abs/1001.4364

Quantum Tetrahedra

Mauro Carfora, Annalisa Marzuoli, Mario Rasetti
(Submitted on 25 Jan 2010)
We discuss in details the role of Wigner 6j symbol as the basic building block unifying such different fields as state sum models for quantum geometry, topological quantum field theory, statistical lattice models and quantum computing. The apparent twofold nature of the 6j symbol displayed in quantum field theory and quantum computing -a quantum tetrahedron and a computational gate- is shown to merge together in a unified quantum-computational SU(2)-state sum framework.

 

[MTd2]

http://arxiv.org/abs/1001.4581

Geometry of the quantum universe

J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 26 Jan 2010)
A universe much like the (Euclidean) de Sitter space-time appears as background geometry in the causal dynamical triangulation (CDT) regularization of quantum gravity. We study the geometry of such universes which appear in the path integral as a function of the bare coupling constants of the theory.

 

[marcus]

http://arXiv.org/abs/1001.4538
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
E. Komatsu, K. M. Smith, J. Dunkley, C. L. Bennett, B. Gold, G. Hinshaw, N. Jarosik, D. Larson, M. R. Nolta, L. Page, D. N. Spergel, M. Halpern, R. S. Hill, A. Kogut, M. Limon, S. S. Meyer, N. Odegard, G. S. Tucker, J. L. Weiland, E. Wollack, E. L. Wright
48 pages, 19 figures. Submitted to Astrophysical Journal Supplement Series
(Submitted on 25 Jan 2010)
"(Abridged) The 7-year WMAP data and improved astrophysical data rigorously tests the standard cosmological model and its extensions. By combining WMAP with the latest distance measurements from the Baryon Acoustic Oscillations (BAO) and the Hubble constant (H0) measurement, we determine the parameters of the simplest LCDM model. The power-law index of the primordial power spectrum is n_s=0.963+-0.012, a measurement that excludes the scale-invariant spectrum by more than 3-sigma. The other parameters, including those beyond the minimal set, are also improved from the 5-year results. ...We report a significant detection of the SZ effect at the locations of known clusters, and show that the current simulations and analytical calculations overestimate the gas pressure, and do not reproduce the observed gas pressure in clusters of galaxies. This result is consistent with the lower-than-expected SZ power spectrum recently measured by the SPT collaboration."

For the whole series of 7-year WMAP reports see:
http://lambda.gsfc.nasa.gov/product/map/dr4/map_bibliography.cfm[/QUOTE]

http://arxiv.org/abs/1001.4758
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies?
C. L. Bennett (JHU), R. S. Hill (ADNET), G. Hinshaw (NASA/GSFC), D. Larson (JHU), K. M. Smith (Princeton), J. Dunkley (Oxford), B. Gold (JHU), M. Halpern (UBC), N. Jarosik (Princeton), A. Kogut (NASA/GSFC), E. Komatsu (U. Texas), M. Limon (Columbia), S. S. Meyer (U. Chicago), M. R. Nolta (CITA), N. Odegard (ADNET), L. Page (Princeton), D. N. Spergel (Princeton), G. S. Tucker (Brown), J. L. Weiland (ADNET), E. Wollack (NASA/GSFC), E. L. Wright (UCLA)
19 pages, 17 figures, also available with higher-res figures on this http URL; submitted to Astrophysical Journal Supplement Series
(Submitted on 26 Jan 2010)
"A simple six-parameter LCDM model provides a successful fit to WMAP data, both when the data are analyzed alone and in combination with other cosmological data. Even so, it is appropriate to search for any hints of deviations from the now standard model of cosmology, which includes inflation, dark energy, dark matter, baryons, and neutrinos. The cosmological community has subjected the WMAP data to extensive and varied analyses. While there is widespread agreement as to the overall success of the six-parameter LCDM model, various "anomalies" have been reported relative to that model. In this paper we examine potential anomalies and present analyses and assessments of their significance. In most cases we find that ...
...We examine several potential or previously claimed anomalies in the sky maps and power spectra, including cold spots, low quadrupole power, quadropole-octupole alignment, hemispherical or dipole power asymmetry, and quadrupole power asymmetry. We conclude that there is no compelling evidence for deviations from the LCDM model, which is generally an acceptable statistical fit to WMAP and other cosmological data."

Until now we have only had reports based on the 5-year WMAP data. These are the first I have seen based on 7-year data.

There has been a lot of fast reaction to the Verlinde paper. This is just a sample. The author has a decent publication track record---several Phys. Rev. D, and comparable:

http://arxiv.org/abs/1001.4585
Modified Entropic Force
Changjun Gao
3 pages, 2 figures
(Submitted on 26 Jan 2010 (v1), last revised 27 Jan 2010 (this version, v2))
The theory of thermodynamics tells us the equipartition law of energy does not hold in the limit of very low temperature. It is found the Debye model is very successful in explaining the experimental results. Motivated by this fact, we modify the entropic force formula which is proposed very recently. Since the Unruh temperature is proportional to the strength of gravitational field, so the modified entropic force formula is an extension of the Newtonian gravity to weak field. On the contrary, General Relativity extends Newtonian gravity to strong field case. Corresponding to Debye temperature, there exists a Debye acceleration $$g_D$$. It is found the Debye acceleration is $$g_D=10^{-14}\textrm{N}\cdot \textrm{{kg}}^{-1}$$. This acceleration is very much smaller than the gravitational acceleration $$10^{-4}\textrm{N}\cdot \textrm{{kg}}^{-1}$$ which is felt by the Neptune. Therefore, the modified entropic force can be very well approximated by the Newtonian gravity in solar system. With this Debye acceleration, we also find the current cosmic speeding up can be explained without invoking any kind of dark energy."

 

[marcus]

Renate Loll video lectures on quantum gravity
http://pirsa.org/10010094/
http://pirsa.org/10010095/
http://pirsa.org/10010096/
These are the first three lectures of the series, given Monday-Wednesday, the 25-27 of January 2010.

Planned as a 3-week series of daily one-hour lectures, part of the Perimeter Scholars program at Perimeter Institute. Loll will give some of the history of the subject and perspective on how the two main approaches to nonperturbative QG arose: the canonical and the path integral approaches.
She will cover both canonical LQG and her own triangulations path integral approaches as examples of the two main ways of doing nonperturbative QG.

http://arxiv.org/abs/1001.4823
Extended Horava gravity and Einstein-aether theory
Ted Jacobson
4 pages
(Submitted on 27 Jan 2010)
"Einstein-aether theory is general relativity coupled to a dynamical, unit timelike vector. If this vector is restricted in the action to be hypersurface orthogonal, the theory is identical to the IR limit of the extension of Horava gravity proposed by Blas, Pujolàs and Sibiryakov. Hypersurface orthogonal solutions of Einstein-aether theory are solutions to the IR limit of this theory, hence numerous results already obtained for Einstein-aether theory carry over."

http://arxiv.org/abs/1001.4929
Post-Inflationary Evolution via Gravitation
N. C. Tsamis (U. of Crete) R. P. Woodard (U. of Florida)
16 pages, 1 figure
(Submitted on 27 Jan 2010)
"We study a class of non-local, purely gravitational models which have the correct structure to reproduce the leading infrared logarithms of quantum gravitational back-reaction during the inflationary regime. These models end inflation in a distinctive phase of oscillations with slight and short violations of the weak energy condition and should, when coupled to matter, lead to rapid reheating. By elaborating this class of models we exhibit one that has the same behaviour during inflation, goes quiescent until the onset of matter domination, and induces a small, positive cosmological constant of about the right size thereafter."

http://arxiv.org/abs/1001.4960
On the impossibility of superluminal travel: the warp drive lesson
Carlos Barceló, Stefano Finazzi, Stefano Liberati
Second prize of the 2009 FQXi essay contest "What is Ultimately Possible in Physics?"
(Submitted on 27 Jan 2010)
"The question of whether it is possible or not to surpass the speed of light is already centennial. The special theory of relativity took the existence of a speed limit as a principle, the light postulate, which has proven to be enormously predictive. Here we discuss some of its twists and turns when general relativity and quantum mechanics come into play. In particular, we discuss one of the most interesting proposals for faster than light travel: warp drives. Even if one succeeded in creating such spacetime structures, it would be still necessary to check whether they would survive to the switching on of quantum matter effects. Here, we show that the quantum back-reaction to warp-drive geometries, created out of an initially flat spacetime, inevitably lead to their destabilization whenever superluminal speeds are attained. We close this investigation speculating the possible significance of this further success of the speed of light postulate."

 

[MTd2]

http://arxiv.org/abs/1001.4965

The Coulomb Force as an Entropic Force

Tower Wang
(Submitted on 27 Jan 2010)
Motivated by Verlinde's theory of entropic gravity, we give a tentative explain to the Coulomb's law with an entropic force. When trying to do this, we find the equipartition rule should be extended to charges and the concept of temperature should be reinterpreted. If one accepts the holographic principle as well as our generalizations and reinterpretations, then the coulomb's law, the Poisson's equation and the Maxwell's equations can be derived smoothly. Our attempt can be regarded as a new way to unify the electromagnetic force with gravity, from the entropic origin. Possibly some of our postulate are related to the D-brane picture of black hole thermodynamics.

 

[MTd2]

http://arxiv.org/abs/1001.5032

Ghost wave-function renormalization in Asymptotically Safe Quantum Gravity

Kai Groh, Frank Saueressig
(Submitted on 28 Jan 2010)
Motivated by Weinberg's asymptotic safety scenario, we investigate the gravitational renormalization group flow in the Einstein-Hilbert truncation supplemented by the wave-function renormalization of the ghost fields. The latter induces non-trivial corrections to the beta-functions for Newton's constant and the cosmological constant. The resulting ghost-improved phase diagram is investigated in detail. In particular, we find a non-trivial ultraviolet fixed point in agreement with the asymptotic safety conjecture, which also survives in the presence of extra dimensions. In four dimensions the ghost anomalous dimension at the fixed point is $\eta_c^* = -1.8$, supporting space-time being effectively two-dimensional at short distances.

http://arxiv.org/abs/1001.5033

Ghost anomalous dimension in asymptotically safe quantum gravity

Astrid Eichhorn, Holger Gies
(Submitted on 27 Jan 2010)
We compute the ghost anomalous dimension within the asymptotic-safety scenario for quantum gravity. For a class of covariant gauge fixings and using a functional RG scheme, the anomalous dimension $\eta_c$ is negative, implying an improved UV behavior of ghost fluctuations. At the non-Gaussian UV fixed point, we observe a maximum value of $\eta_c\simeq -0.78$ for the Landau-deWitt gauge within the given scheme and truncation. Most importantly, the backreaction of the ghost flow onto the Einstein-Hilbert sector preserves the non-Gaussian fixed point with only mild modifications of the fixed-point values for the gravitational coupling and cosmological constant and the associated critical exponents; also their gauge dependence is slightly reduced. Our results provide further evidence for the asymptotic-safety scenario of quantum gravity.

http://arxiv.org/abs/1001.5238

Friedmann equation of FRW universe in deformed Horava-Lifgarbagez gravity from entropic force

Shao-Wen Wei, Yu-Xiao Liu, Yong-Qiang Wang
(Submitted on 28 Jan 2010)
It is generally thought that there exists a logarithmic term in the entropy/area relation for the deformed Horava-Lifgarbagez gravity. With the entropy/area relation, we obtain the modified Friedmann equations from the first law of thermodynamics and the entropic force, respectively. Although the modified Friedmann equations derived from the two methods are different, they will reduce to the standard Friedmann equation in the case of small energy density $\rho$.

http://arxiv.org/abs/1001.5147

Casting Loop Quantum Cosmology in the Spin Foam Paradigm

Abhay Ashtekar, Miguel Campiglia, Adam Henderson
(Submitted on 28 Jan 2010)
The goal of spin foam models is to provide a viable path integral formulation of quantum gravity. Because of background independence, their underlying framework has certain novel features that are not shared by path integral formulations of familiar field theories in Minkowski space. As a simple viability test, these features were recently examined through the lens of loop quantum cosmology (LQC). Results of that analysis, reported in a brief communication [1], turned out to provide concrete arguments in support of the spin foam paradigm. We now present detailed proofs of those results. Since the quantum theory of LQC models is well understood, this analysis also serves to shed new light on some long standing issues in the spin foam and group field theory literature. In particular, it suggests an intriguing possibility for addressing the question of why the cosmological constant is positive and small.

 

[MTd2]

http://arxiv.org/abs/1002.0308

Projectable Horava-Lifgarbagez gravity in a nutshell

Silke Weinfurtner, Thomas P. Sotiriou, Matt Visser
(Submitted on 1 Feb 2010)
Approximately one year ago Horava proposed a power-counting renormalizable theory of gravity which abandons local Lorentz invariance. The proposal has been received with growing interest and resulted in various different versions of Horava-Lifgarbagez gravity theories, involving a colourful potpourri of new terminology. In this proceedings contribution we first motivate and briefly overview the various different approaches, clarifying their differences and similarities. We then focus on a model referred to as projectable Horava-Lifgarbagez gravity and summarize the key results regarding its viability.

http://arxiv.org/abs/1002.0260

Black holes and asymptotically safe gravity

Kevin Falls, Daniel F. Litim, Aarti Raghuraman
(Submitted on 1 Feb 2010)
Quantum gravitational corrections to black holes are studied in four and higher dimensions using a renormalisation group improvement of the metric. The quantum effects are worked out in detail for asymptotically safe gravity, where the short distance physics is characterized by a non-trivial fixed point of the gravitational coupling. We find that a weakening of gravity implies a decrease of the event horizon, and the existence of a Planck-size black hole remnant with vanishing temperature and vanishing heat capacity. The absence of curvature singularities is generic and discussed together with the conformal structure and the Penrose diagram of asymptotically safe black holes. The production cross section of mini-black holes in energetic particle collisions, such as those at the Large Hadron Collider, is analysed within low-scale quantum gravity models. Quantum gravity corrections imply that cross sections display a threshold, are suppressed in the Planckian, and reproduce the semi-classical result in the deep trans-Planckian region. Further implications are discussed.