Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/0911.0386
Renormalization Group Flow in Scalar-Tensor Theories. I
Gaurav Narain, Roberto Percacci
18 pages, 10 figures
(Submitted on 2 Nov 2009)
"We study the renormalization group flow in a class of scalar-tensor theories involving at most two derivatives of the fields. We show in general that minimal coupling is self consistent, in the sense that when the scalar self couplings are switched off, their beta functions also vanish. Complete, explicit beta functions that could be applied to a variety of cosmological models are given in a five parameter truncation of the theory in d=4. In any dimension d>2 we find that the flow has only a 'Gaussian Matter' fixed point, where all scalar self interactions vanish but Newton's constant and the cosmological constant are nontrivial. The properties of these fixed points can be studied algebraically to some extent. In d=3 we also find a gravitationally dressed version of the Wilson-Fisher fixed point, but it seems to have unphysical properties. These findings are in accordance with the hypothesis that these theories are asymptotically safe."

http://arxiv.org/abs/0911.0394
Renormalization Group Flow in Scalar-Tensor Theories. II
Gaurav Narain, Christoph Rahmede
14 pages
(Submitted on 2 Nov 2009)
"We study the UV behaviour of actions including integer powers of scalar curvature and even powers of scalar fields with Functional Renormalization Group techniques. We find UV fixed points where the gravitational couplings have non-trivial values while the matter ones are Gaussian. We prove several properties of the linearized flow at such a fixed point in arbitrary dimensions in the one-loop approximation and find recursive relations among the critical exponents. We illustrate these results in explicit calculations in d=4 for actions including up to four powers of scalar curvature and two powers of the scalar field. In this setting we notice that the same recursive properties among the critical exponents, which were proven at one-loop order, still hold, in such a way that the UV critical surface is found to be five dimensional. We then search for the same type of fixed point in a scalar theory with minimal coupling to gravity in d=4 including up to eight powers of scalar curvature. Assuming that the recursive properties of the critical exponents still hold, one would conclude that the UV critical surface of these theories is five dimensional."

http://arxiv.org/abs/0911.0401
Spectral geometry as a probe of quantum spacetime
Dario Benedetti, Joe Henson
25 pages, 6 figures
(Submitted on 2 Nov 2009)
"Employing standard results from spectral geometry, we provide strong evidence that in the classical limit the ground state of three-dimensional causal dynamical triangulations is de Sitter spacetime. This result is obtained by measuring the expectation value of the spectral dimension on the ensemble of geometries defined by these models, and comparing its large scale behaviour to that of a sphere (Euclidean de Sitter). From the same measurement we are also able to confirm the phenomenon of dynamical dimensional reduction observed in this and other approaches to quantum gravity -- the first time this has been done for three-dimensional causal dynamical triangulations. In this case, the value for the short-scale limit of the spectral dimension that we find is approximately 2. We comment on the relevance of these results for the comparison to asymptotic safety and Horava-Lifgarbagez gravity, among other approaches to quantum gravity."

 

[marcus]

http://arxiv.org/abs/0911.0437
Fractal Space-Time from Spin-Foams
Elena Magliaro, Claudio Perini, Leonardo Modesto
5 pages, 2 figures
(Submitted on 2 Nov 2009)
"In this paper we perform the calculation of the spectral dimension of spacetime in 4d quantum gravity using the Barrett-Crane (BC) spinfoam model. We realize this considering a very simple decomposition of the 4d spacetime already used in the graviton propagator calculation and we introduce a boundary state which selects a classical geometry on the boundary. We obtain that the spectral dimension of the spacetime runs from ~ 2 to 4, across a ~1.5 phase, when the energy of a probe scalar field decreases from high E \lesssim E_P/25 to low energy. The spectral dimension at the Planck scale E \approx E_P depends on the areas spectrum used in the calculation. For three different spectra l_P^2 \sqrt{j(j+1)}, l_P^2 (2 j+1) and l_P^2 j we find respectively dimension ~ 2.31, 2.45 and 2.08."

http://arxiv.org/abs/0911.0543
The volume operator in covariant quantum gravity
You Ding, Carlo Rovelli
9 pages
(Submitted on 3 Nov 2009)
"A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In particular, the geometrical observable giving the area of a surface has been shown to be the same as the one in loop quantum gravity. Here we discuss the volume observable. We derive the volume operator in the covariant theory, and show that it matches the one of loop quantum gravity, as does the area. We also reconsider the implementation of the constraints that defines the model: we derive in a simple way the boundary Hilbert space of the theory from a suitable form of the classical constraints, and show directly that all constraints vanish weakly on this space."

 

[MTd2]

http://arxiv.org/abs/0911.0437

The volume operator in covariant quantum gravity

You Ding, Carlo Rovelli
(Submitted on 3 Nov 2009)
A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In particular, the geometrical observable giving the area of a surface has been shown to be the same as the one in loop quantum gravity. Here we discuss the volume observable. We derive the volume operator in the covariant theory, and show that it matches the one of loop quantum gravity, as does the area. We also reconsider the implementation of the constraints that defines the model: we derive in a simple way the boundary Hilbert space of the theory from a suitable form of the classical constraints, and show directly that all constraints vanish weakly on this space.

 

[John86]

http://arxiv.org/abs/0909.5421
In, Through and Beyond the Planck Scale
Authors: Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 29 Sep 2009 (v1), last revised 31 Oct 2009 (this version, v2))
Abstract: In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstr\"om-like metric is self-dual in the sense of T-duality: the form of the metric is invariant under the exchange r -> a0/r where a0 is proportional to the minimum area in LQG. Of particular interest, the symmetry imposes that if an observer at infinity sees a black hole of mass m an observer in the other asymptotic infinity beyond the horizon (near r=0) sees a dual mass proportional to m_P^2/m. We then show that small LQBHs are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately 10^(-5) m_P and emit radiation with a typical energy of about 10^(13) - 10^(14) eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.

http://arxiv.org/abs/0908.4224
Stochastic quantization and the role of time in quantum gravity
Authors: J. Ambjorn, R. Loll, W. Westra, S. Zohren
(Submitted on 28 Aug 2009 (v1), last revised 1 Nov 2009 (this version, v2))
Abstract: We show that the noncritical string field theory developed from two-dimensional quantum gravity in the framework of causal dynamical triangulations can be viewed as arising through a stochastic quantization. This requires that the proper time appearing in the string field theory be identified with the stochastic time of the stochastic formulation. The framework of stochastic quantization gives rise to a natural nonperturbative quantum Hamiltonian, which incorporates a sum over all spacetime topologies. We point out that the external character of stochastic time is a feature that pertains more generally to the proper time or distance appearing in nonperturbative correlation functions in quantum gravity.

http://arxiv.org/abs/0910.5733
Decoherence in an Interacting Quantum Field Theory: The Vacuum Case
Authors: Jurjen F. Koksma (Utrecht University), Tomislav Prokopec (Utrecht University), Michael G. Schmidt (Heidelberg University)
(Submitted on 29 Oct 2009)
Abstract: We apply the decoherence formalism to an interacting scalar field theory. In the spirit of the decoherence literature, we consider a "system field" and an "environment field" that interact via a cubic coupling. We solve for the propagator of the system field, where we include the self-energy corrections due to the interaction with the environment field. In this paper, we consider an environment in the vacuum state (T=0). We show that neglecting inaccessible non-Gaussian correlators increases the entropy of the system as perceived by the observer. Moreover, we consider the effect of a changing mass of the system field in the adiabatic regime, and we find that at late times no additional entropy has been generated.

 

[MTd2]

http://arxiv.org/abs/0911.1020

Constraining the energy-momentum dispersion relation with Planck-scale sensitivity using cold atoms

Giovanni Amelino-Camelia, Claus Laemmerzahl, Flavio Mercati, Guglielmo M. Tino
(Submitted on 5 Nov 2009)
We use the results of ultra-precise cold-atom-recoil experiments to constrain the form of the energy-momentum dispersion relation, a structure that is expected to be modified in several quantum-gravity approaches. Our strategy of analysis applies to the nonrelativistic (small speeds) limit of the dispersion relation, and is therefore complementary to an analogous ongoing effort of investigation of the dispersion relation in the ultrarelativistic regime using observations in astrophysics. For the leading correction in the nonrelativistic limit the exceptional sensitivity of cold-atom-recoil experiments remarkably allows us to set a limit within a single order of magnitude of the desired Planck-scale level, thereby providing the first example of Planck-scale sensitivity in the study of the dispersion relation in controlled laboratory experiments. For the next-to-leading term we obtain a limit which is a few orders of magnitude away from the Planck scale, but still amounts to the best limit on a class of Lorentz-symmetry test theories that has been extensively used to investigate the hypothesis of "deformation" (rather than breakdown) of spacetime symmetries.

 

[John86]

http://arxiv.org/abs/0911.1063
The cosmological constant and higher dimensional dilatation symmetry
Authors: C. Wetterich
(Submitted on 5 Nov 2009)
Abstract: We discuss the hypothesis of a fixed point for quantum gravity coupled to a scalar, in the limit where the scalar field goes to infinity, accompanied by a suitable scaling of the metric. We propose that no scalar potential is present for the dilatation symmetric quantum effective action at the fixed point. Dimensional reduction of such a higher dimensional effective action leads to solutions with a vanishing effective four-dimensional constant. Under rather general circumstances these are the only quasistatic stable solutions with finite four-dimensional gravitational constant. If cosmological runaway solutions approach the fixed point as time goes to infinity, the cosmological constant vanishes asymptotically. For our old Universe the fixed point is not yet reached completely, resulting in a tiny amount of dark energy, comparable to dark matter. We discuss explicitly higher dimensional geometries which realize such asymptotic solutions for $t\to\infty$. They include Ricci-flat spaces as well as warped spaces, potentially with singularities.

 

[MTd2]

http://arxiv.org/abs/0911.1132

Polynomial 6j-Symbols and States Sums

Nathan Geer, Bertrand Patureau-Mirand
(Submitted on 6 Nov 2009)
For q a root of unity of order 2r, we give explicit formulas of a family of 3-variable Laurent polynomials J_{i,j,k} with coefficients in Z[q] that encode the 6j-symbols associated with nilpotent representations of U_qsl_2. For a given abelian group G, we use them to produce a state sum invariant tau^r(M,L,h_1,h_2) of a quadruplet (compact 3-manifold M, link L inside M, homology class h_1\in H_1(M,Z), homology class h_2\in H_2(M,G)) with values in a ring R related to G. The formulas are established by a ``skein'' calculus as an application of the theory of modified dimensions introduced in [arXiv:0711.4229]. For an oriented 3-manifold M, the invariants are related to TV(M,L,f\in H^1(M,C^*)) defined in [arXiv:0910.1624] from the category of nilpotent representations of U_qsl_2. They refine them as TV(M,L,f)= Sum_h tau^r(M,L,h,f') where f' correspond to f with the isomorphism H_2(M,C^*) ~ H^1(M,C^*).

http://arxiv.org/abs/0911.1132

String-nets, single and double-stranded quantum loop gases for non-Abelian anyons

Andrea Velenich, Claudio Chamon, Xiao-Gang Wen
(Submitted on 5 Nov 2009)
String-net condensation can give rise to non-Abelian anyons whereas loop condensation usually gives rise to Abelian anyons. It has been proposed that generalized quantum loop gases with non-orthogonal inner products can produce non-Abelian anyons. We detail an exact mapping between the string-net and the generalized loop models and explain how the non-orthogonal products arise. We also introduce a loop model of double-stranded nets where quantum loops with an orthogonal inner product and local interactions supports non-Abelian Fibonacci anyons. Finally we emphasize the origin of the sign problem in such systems and its consequences on the complexity of their ground state wave functions.

 

[marcus]

http://arxiv.org/abs/0911.1719
Bosonic Colored Group Field Theory
Joseph Ben Geloun, Jacques Magnen, Vincent Rivasseau
ICMPA-MPA/2009/24, LPT-2009-93
(Submitted on 9 Nov 2009)
"Bosonic colored group field theory is considered. Focusing first on dimension four, namely the colored Ooguri group field model, the main properties of Feynman graphs are studied. This leads to a theorem on optimal perturbative bounds of Feynman amplitudes in the 'ultraspin' (large spin) limit. The results are generalized in any dimension. Finally integrating out two colors we write a new representation which could be useful for the constructive analysis of this type of models."

http://arxiv.org/abs/0911.1625
Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation
J. Grain
To be published in the AIP Proceedings of the 'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3, 2009; 9 pp., 4 Figs
(Submitted on 9 Nov 2009)
"Loop Quantum Gravity (L.Q.G.) is one of the two most promising tentative theory for a quantum description of gravity. When applied to the entire universe, the so-called Loop Quantum Cosmology (L.Q.C.) framework offers microscopical models of the very early stages of the cosmological history, potentially solving the initial singularity problem via bouncing solutions or setting the universe in the appropriate initial conditions for inflation to start, via a phase of super-inflation. More interestingly, L.Q.C. could leave a footprint on cosmological observables such as the Cosmic Microwave Background (CMB) anisotropies. Focusing on the modified dispersion relation when holonomy and inverse-volume corrections arising from the L.Q.C. framework are considered, it is shown that primordial gravity waves generated during inflation are affected by quantum corrections. Depending on the type of corrections, the primordial tensor power spectrum is either suppressed or boosted at large length scales, and strongly departs from the power-law behavior expected in the standard scenario."

http://arxiv.org/abs/0911.1403
A Physical Interpretation of Gravitational Field Equations
T. Padmanabhan
Based on the Plenary talk given at the International Conference on `Invisible Universe', 29 June- 3 July, 2009 Paris; to appear in the Proceedings; 2 figures; 16 pages
(Submitted on 7 Nov 2009)
"It is possible to provide a thermodynamic interpretation for the field equations in any diffeomorphism invariant theory of gravity. This insight, in turn, leads us to the possibility of deriving the gravitational field equations from another variational principle without using the metric as a dynamical variable. I review this approach and discuss its implications."

http://arxiv.org/abs/0911.1700
Four-Dimensional Spin Foam Perturbation Theory
Joao Faria Martins, Aleksandar Mikovic
23 pages, 6 figures
(Submitted on 9 Nov 2009)
"We formulate the four-dimensional spin-foam perturbation theory for a BF-theory with a B\wedge B potential term defined for a compact semi-simple Lie group G on a compact orientable 4-manifold M. This is done by using the formal spin foam perturbative series coming from the spin-foam generating functional. We then regularize the terms in the perturbative series by passing to the category of representations of the quantum group U_q (\mathfrak{g}) where \mathfrak{g} is the Lie algebra of G and q is a root of unity. The Chain-Mail formalism can be used to calculate the perturbative terms when the vector space of intertwiners \L\otimes \L \to A, where A is the adjoint representation of \mathfrak{g}, is 1-dimensional for each irrep L. We calculate the partition function Z in the dilute-gas limit for special classes of triangulations, which we conjecture to exist exist on any 4-manifold M. We prove that the first order perturbative contribution vanishes, so that we define a dilute-gas limit by using the second order contribution. Furthermore, we relate Z to the partition function for the F\wedge F theory."

 

[John86]

http://arxiv.org/abs/0911.1479
Is the spacetime metric Euclidean rather than Lorentzian?
Authors: Rafael D. Sorkin (Perimeter Institute and Syracuse University)
(Submitted on 8 Nov 2009)
Abstract: My answer to the question in the title is "No". In support of this point of view, we analyze some examples of saddle-point methods, especially as applied to quantum "tunneling" in nonrelativistic particle mechanics and in cosmology. Along the way we explore some of the interrelationships among different ways of thinking about path-integrals and saddle-point approximations to them

 

[marcus]

http://arxiv.org/abs/0911.1945
Topological Graph Polynomials in Colored Group Field Theory
Razvan Gurau
(Submitted on 10 Nov 2009)
"In this paper we analyze the open Feynman graphs of the Colored Group Field Theory introduced in [arXiv:0907.2582]. We define the boundary graph \mathfrak{G}_{\partial} of an open graph \mathfrak{G} and prove it is a cellular complex. Using this structure we generalize the topological (Bollobas-Riordan) Tutte polynomials associated to (ribbon) graphs to topological polynomials adapted to Colored Group Field Theory graphs in arbitrary dimension."

 

[MTd2]

http://arxiv.org/abs/0911.2204

Recurrence relations for spin foam vertices

Valentin Bonzom, Etera R. Livine, Simone Speziale
(Submitted on 11 Nov 2009)
We study recurrence relations for various Wigner 3nj-symbols and the non-topological 10j-symbol. For the 6j-symbol and the 15j-symbols which correspond to basic amplitudes of 3d and 4d topological spin foam models, recurrence relations are obtained from the invariance under Pachner moves and can be interpreted as quantizations of the constraints of the underlying classical field theories. We also derive recurrences from the action of holonomy operators on spin network functionals, making a more precise link between the topological Pachner moves and the classical constraints. Interestingly, our recurrence relations apply to any SU(2) invariant symbol, depending on the cycles of the corresponding spin network graph. Another method is used for non-topological objects such as the 10j-symbol and pseudo-isoceles 6j-symbols. The recurrence relations are also interpreted in terms of elementary geometric properties. Finally, we discuss the extension of the recurrences to take into account boundary states which leads to equations similar to Ward identities for correlation functions in the Barrett-Crane model.

http://arxiv.org/abs/0911.2135

Approaches To Quantum Gravity

Cecilia Flori
(Submitted on 11 Nov 2009)
One of the main challenges in theoretical physics over the last five decades has been to reconcile quantum mechanics with general relativity into a theory of quantum gravity. However, such a theory has been proved to be hard to attain due to i) conceptual difficulties present in both the component theories (General Relativity (GR) and Quantum Theory); ii) lack of experimental evidence, since the regimes at which quantum gravity is expected to be applicable are far beyond the range of conceivable experiments. Despite these difficulties, various approaches for a theory of Quantum Gravity have been developed.
In this thesis we focus on two such approaches: Loop Quantum Gravity and the Topos theoretic approach. The choice fell on these approaches because, although they both reject the Copenhagen interpretation of quantum theory, their underpinning philosophical approach to formulating a quantum theory of gravity are radically different. In particular LQG is a rather conservative scheme, inheriting all the formalism of both GR and Quantum Theory, as it tries to bring to its logical extreme consequences the possibility of combining the two. On the other hand, the Topos approach involves the idea that a radical change of perspective is needed in order to solve the problem of quantum gravity, especially in regard to the fundamental concepts of `space' and `time'. Given the partial successes of both approaches, the hope is that it might be possible to find a common ground in which each approach can enrich the other.

 

[marcus]

http://arxiv.org/abs/0911.2404
Emergent Dirac Hamiltonians in Quantum Gravity
Johannes Aastrup, Jesper M. Grimstrup, Mario Paschke
1 figure
(Submitted on 12 Nov 2009)
"We modify the construction of the spectral triple over an algebra of holonomy loops by introducing additional parameters in form of families of matrices. These matrices generalize the already constructed Euler-Dirac type operator over a space of connections. We show that these families of matrices can naturally be interpreted as parameterizing foliations of 4-manifolds. The corresponding Euler-Dirac type operators then induce Dirac Hamiltonians associated to the corresponding foliation, in the previously constructed semi-classical states."

Aastrup and Grimstrup have been pursuing the goal of merging the standard particle model in Connes NCG form, with a variant of canonical LQG. The "spectral triple" is the Connes NCG part---this has to be accommodated on a LQG space.
See their earlier papers for an overview of their project. This appears to be a minor contributory piece of the puzzle.

http://arxiv.org/abs/0911.2391
Wilson loops, geometric operators and fermions in 3d group field theory
R.J. Dowdall
13 pages, many figures
(Submitted on 12 Nov 2009)
"Group field theories whose Feynman diagrams describe 3d gravity with a varying configuration of Wilson loop observables and 3d gravity with volume observables at each vertex are defined. The volume observables are created by the usual spin network grasping operators which require the introduction of vector fields on the group. We then use this to define group field theories that give a previously defined spin foam model for fermion fields coupled to gravity, and the simpler quenched approximation, by using tensor fields on the group. The group field theory naturally includes the sum over fermionic loops at each order of the perturbation theory."

Richard Dowdall is at Nottingham, John Barrett's student and recent co-author. I guess this paper fits into the overall spin foam program, and contributes towards inclusion of matter.

 

[John86]

http://arxiv.org/abs/0911.2326
Superfluidity and Stationary Space-Times
Authors: George Chapline (LLNL), Pawel O. Mazur (USC Columbia)
(Submitted on 12 Nov 2009)
Abstract: A connection between superfluidity and gravitation is established for physical stationary gravitational fields. We show that the spinning cosmic string metric describes the gravitational field associated with the single vortex in a superfluid condensate model for space-time outside the vortex core. This metric differs significantly from the usual acoustic metric for the Onsager-Feynman vortex. We also consider the question of what happens when many vortices are present, and show that on large scales a G\"odel-like metric emerges. In both the single and multiple vortex cases the presence of closed time-like curves is attributed to the

 

[John86]

http://arxiv.org/abs/0911.2136
Noncommutative Solitons of Gravity
Authors: Tsuguhiko Asakawa, Shinpei Kobayashi
(Submitted on 11 Nov 2009)
Abstract: We investigate a three-dimensional gravitational theory on a noncommutative space which has a cosmological constant term only. We found various kinds of nontrivial solutions, by applying a similar technique which was used to seek noncommutative solitons in noncommutative scalar field theories. Some of those solutions correspond to bubbles of spacetimes, or represent dimensional reduction. The solution which interpolates $G_{\mu\nu}=0$ and Minkowski metric is also found. All solutions we obtained are non-perturbative in the noncommutative parameter $\theta$, therefore they are different from solutions found in other contexts of noncommutative theory of gravity and would have a close relation to quantum gravity.

 

[marcus]

http://arxiv.org/abs/0911.2653
Triangulated Loop Quantum Cosmology: Bianchi IX and inhomogenous perturbations
Marco Valerio Battisti, Antonino Marciano, Carlo Rovelli
21 pages
(Submitted on 13 Nov 2009)
"We develop the 'triangulated' version of loop quantum cosmology, recently introduced in the literature. We focus on the 'dipole' cosmology, where space is a three-sphere and the triangulation is formed by two tetrahedra. We show that the discrete fiducial connection has a simple and appealing geometrical interpretation and we correct the ansatz on the relation between the model variables and the Friedmann-Robertson-Walker scale factor. The modified ansatz leads to the convergence of the Hamiltonian constraint to the continuum one. We then ask which degrees of freedom are captured by this model. We show that the model is rich enough to describe the (anisotropic) Bianchi IX Universe, and give the explicit relation between the Bianchi IX variables and the variables of the model. We discuss the possibility of using this path in order to define the quantization of the Bianchi IX Universe. The model contains more degrees of freedom than Bianchi IX, and therefore captures some inhomogeneous degrees of freedom as well. Inhomogeneous degrees of freedom can be expanded in representations of the SU(2) Bianchi IX isometry group, and the dipole model captures the lowest integer representation of these, connected to hyper-spherical harmonic of angular momentum j=1."


http://arxiv.org/abs/0911.2698
A solution of the strong CP problem a la Peccei-Quinn through the Nieh-Yan modified gravity and cosmological implications
Massimiliano Lattanzi, Simone Mercuri
8 pages, 1 figure
(Submitted on 13 Nov 2009)
"By identifying the recently introduced Barbero-Immirzi field with the QCD axion, the strong CP problem can be solved through the Peccei-Quinn mechanism. A specific energy scale for the Peccei-Quinn symmetry breaking is naturally predicted by this model. This provides a complete dynamical setting to evaluate the contribution of such an axion to the cold dark matter content of the Universe, as function of the initial misalignment angle. Furthermore, a tight upper bound on the tensor-to-scalar ratio production of primordial gravitational waves can be fixed, representing a strong experimental test for this model."

My comment, see footnote [18].

 

[MTd2]

http://arxiv.org/abs/0911.3097

On the spinfoam expansion in cosmology

Carlo Rovelli, Francesca Vidotto
(Submitted on 16 Nov 2009)
We consider the technique introduced in a recent work by Ashtekar, Campiglia and Henderson, which generate a spinfoam-like sum from a Hamiltonian theory. We study the possibility of using it for finding the generalized projector of a constraint on physical states, without first deparametrising the system. We illustrate this technique in the context of a very simple example. We discuss the infinities that appear in the calculation, and argue that they can be appropriately controlled. We apply these ideas to write a spinfoam expansion for the "dipole cosmology".

http://arxiv.org/abs/0911.2761
Phenomenological Quantum Gravity

If the history of science has taught us anything, it's that persistence and creativity makes the once impossible possible. It has long been thought experimental tests of quantum gravity are impossible. But during the last decade, several different approaches have been proposed that allow us to test, if not the fundamental theory of quantum gravity itself, so at least characteristic features this theory can have. For the first time we can probe experimentally domains in which quantum physics and gravity cohabit, in spite of our failure so far to make a convincing marriage of them on a theoretical level.

 

[marcus]

http://arxiv.org/abs/0911.2727
Astrophysical implications of the Asymptotic Safety Scenario in Quantum Gravity
Alfio Bonanno
28 pages, 6 figures. Invited talk at Workshop on Continuum and Lattice Approaches to Quantum Gravity. Sept. 2008, Brighton UK. To appear in the Proceedings
(Submitted on 13 Nov 2009)
"In recent years it has emerged that the high energy behavior of gravity could be governed by an ultraviolet non-Gaussian fixed point of the (dimensionless) Newton's constant, whose behavior at high energy is thus antiscreened. This phenomenon has several astrophysical implications. In particular in this article recent works on renormalization group improved cosmologies based upon a renormalization group trajectory of Quantum Einstein Gravity with realistic parameter values will be reviewed. It will be argued that quantum effects can account for the entire entropy of the present Universe in the massless sector and give rise to a phase of inflationary expansion. Moreover the prediction for the final state of the black hole evaporation is a Planck size remnant which is formed in an infinite time."
If you choose to print this paper by Bonanno, limit the printing to pages 1-29
After page 29 there is an earlier, imperfect, copy of the paper, which someone neglected to erase when the finished copy was uploaded.


MTd2 has already spotted the next one, so I just make brief mention of it:
http://arxiv.org/abs/0911.2761
Phenomenological Quantum Gravity
Sabine Hossenfelder, Lee Smolin
3 pages. To appear in Physics in Canada

 

[MTd2]

http://arxiv.org/abs/0911.3165

Asymptotically Safe Inflation

Steven Weinberg
(Submitted on 16 Nov 2009)
Inflation is studied in the context of asymptotically safe theories of gravitation. It is found to be possible under several circumstances to have a long period of nearly exponential expansion that eventually comes to an end.

 

[marcus]

http://arxiv.org/abs/0911.3428
On the Relation between Operator Constraint --, Master Constraint --, Reduced Phase Space --, and Path Integral Quantisation
Muxin Han, Thomas Thiemann
43 pages
(Submitted on 17 Nov 2009)
"Path integral formulations for gauge theories must start from the canonical formulation in order to obtain the correct measure. A possible avenue to derive it is to start from the reduced phase space formulation. In this article we review this rather involved procedure in full generality. Moreover, we demonstrate that the reduced phase space path integral formulation formally agrees with the Dirac's operator constraint quantisation and, more specifically, with the Master constraint quantisation for first class constraints. For first class constraints with non trivial structure functions the equivalence can only be established by passing to Abelian(ised) constraints which is always possible locally in phase space. Generically, the correct configuration space path integral measure deviates from the exponential of the Lagrangian action. The corrections are especially severe if the theory suffers from second class secondary constraints. In a companion paper we compute these corrections for the Holst and Plebanski formulations of GR on which current spin foam models are based."

http://arxiv.org/abs/0911.3431
On the Relation between Rigging Inner Product and Master Constraint Direct Integral Decomposition
Muxin Han, Thomas Thiemann
25 pages
(Submitted on 17 Nov 2009)
"Canonical quantisation of constrained systems with first class constraints via Dirac's operator constraint method proceeds by the thory of Rigged Hilbert spaces, sometimes also called Refined Algebraic Quantisation (RAQ). This method can work when the constraints form a Lie algebra. When the constraints only close with nontrivial structure functions, the Rigging map can no longer be defined.
To overcome this obstacle, the Master Constraint Method has been proposed which replaces the individual constraints by a weighted sum of absolute squares of the constraints. Now the direct integral decomposition methods (DID), which are closely related to Rigged Hilbert spaces, become available and have been successfully tested in various situations.
It is relatively straightforward to relate the Rigging Inner Product to the path integral that one obtains via reduced phase space methods. However, for the Master Constraint this is not at all obvious. In this paper we find sufficient conditions under which such a relation can be established. Key to our analysis is the possibility to pass to equivalent, Abelian constraints, at least locally in phase space. Then the Master Constraint DID for those Abelian constraints can be directly related to the Rigging Map and therefore has a path integral formulation."

http://arxiv.org/abs/0911.3432
Path-integral for the Master Constraint of Loop Quantum Gravity
Muxin Han
19 pages
(Submitted on 17 Nov 2009)
"In the present paper, we start from the canonical theory of loop quantum gravity and the master constraint programme. The physical inner product is expressed by using the group averaging technique for a single self-adjoint master constraint operator. By the standard technique of skeletonization and the coherent state path-integral, we derive a path-integral formula from the group averaging for the master constraint operator. Our derivation in the present paper suggests there exists a direct link connecting the canonical Loop quantum gravity with a path-integral quantization or a spin-foam model of General Relativity."

http://arxiv.org/abs/0911.3433
Canonical path integral measures for Holst and Plebanski gravity. I. Reduced Phase Space Derivation
Jonathan Engle, Muxin Han, Thomas Thiemann
26 pages
(Submitted on 17 Nov 2009)
An important aspect in defining a path integral quantum theory is the determination of the correct measure. For interacting theories and theories with constraints, this is non-trivial, and is normally not the heuristic "Lebesgue measure" usually used. There have been many determinations of a measure for gravity in the literature, but none for the Palatini or Holst formulations of gravity. Furthermore, the relations between different resulting measures for different formulations of gravity are usually not discussed.
In this paper we use the reduced phase technique in order to derive the path-integral measure for the Palatini and Holst formulation of gravity, which is different from the Lebesgue measure up to local measure factors which depend on the spacetime volume element and spatial volume element.
From this path integral for the Holst formulation of GR we can also give a new derivation of the Plebanski path integral and discover a discrepancy with the result due to Buffenoir, Henneaux, Noui and Roche (BHNR) whose origin we resolve. This paper is the first in a series that aims at better understanding the relation between canonical LQG and the spin foam approach."

http://arxiv.org/abs/0911.3436
Canonical Path-Integral Measures for Holst and Plebanski Gravity. II. Gauge Invariance and Physical Inner Product
Muxin Han
34 pages
(Submitted on 17 Nov 2009)
"This article serves as a continuation for the discussion in arXiv:0911.3433, we analyze the invariance properties of the gravity path-integral measure derived from canonical framework, and discuss which path-integral formula may be employed in the concrete computation e.g. constructing a spin-foam model, so that the final model can be interpreted as a physical inner product in the canonical theory."

http://arxiv.org/abs/0911.3553
The Fine Structure of SU(2) Intertwiners from U(N) Representations
Laurent Freidel, Etera R. Livine
21 pages
(Submitted on 18 Nov 2009)
"In this work we study the Hilbert space space of N-valent SU(2) intertwiners with fixed total spin, which can be identified, at the classical level, with a space of convex polyhedra with N face and fixed total boundary area. We show that this Hilbert space provides, quite remarkably, an irreducible representation of the U(N) group. This gives us therefore a precise identification of U(N) as a group of area preserving diffeomorphism of polyhedral spheres. We use this results to get new closed formulae for the black hole entropy in loop quantum gravity."

 

[MTd2]

http://arxiv.org/abs/0911.3745

Loop quantum gravity and the CMB: toward pre-Big Bounce cosmology
Aurelien Barrau
Comments: Proceedings of the 12th Marcel Grossman Meeting on General Relativity. 3 pages, no figure
Subjects: General Relativity and Quantum Cosmology (gr-qc)
This brief article sums up the possible imprints of loop quantum gravity effects on the cosmological microwave background. We focus on semi-classical terms and show that "Big Bounce" corrections, together with the "pre Big Bounce" state, could modify the observed spectrum.

 

[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."