Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1212.2852
Primordial tensor power spectrum in holonomy corrected Omega-LQC
Linda Linsefors, Thomas Cailleteau, Aurelien Barrau, Julien Grain
(Submitted on 12 Dec 2012)
The holonomy correction is one of the main terms arising when implementing loop quantum gravity ideas at an effective level in cosmology. The recent construction of an anomaly free algebra has shown that the formalism used, up to now, to derive the primordial spectrum of fluctuations was not correct. This article aims at computing the tensor spectrum in a fully consistent way within this deformed and closed algebra.
5 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1212.3527
Asymptotic silence in loop quantum cosmology
Jakub Mielczarek
(Submitted on 14 Dec 2012)
The state of asymptotic silence, characterized by causal disconnection of the space points, emerges from various approaches aiming to describe gravitational phenomena in the limit of large curvatures. In particular, such behavior was anticipated by Belinsky, Khalatnikov and Lifgarbagez (BKL) in their famous conjecture put forward in the early seventies of the last century. While the BKL conjecture is based on purely classical considerations, one can expect that asymptotic silence should have its quantum counterpart at the level of a more fundamental theory of quantum gravity, which is the relevant description of gravitational phenomena in the limit of large energy densities. Here, we summarize some recent results which give support to such a possibility. More precisely, we discuss occurrence of the asymptotic silence due to polymerization of space at the Planck scale, in the framework of loop quantum cosmology. In the discussed model, the state of asymptotic silence is realized at the energy density ρ = ρ_c/2, where ρ_c is the maximal allowed energy density, being of the order of the Planck energy density. At energy densities ρ > ρ_c/2, the universe becomes 4D Euclidean space without causal structure. Therefore, the asymptotic silence appears to be an intermediate state of space between the Lorentzian and Euclidean phases.
4 pages, 3 figures, talk presented at the Multiverse and Fundamental Cosmology Conference, 10-14 September, 2012, Szczecin, Poland

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 17 Dec 2012)
In loop quantum gravity, the number N_Γ(a_H, γ) of microstates of a black hole for a given discrete geometry Γ depends on the so-called Barbero-Immirzi parameter γ. Using a suitable analytic continuation of γ to complex values, we show that the number N_Γ(a_H, ±i) of microstates behaves as exp(a_H/(4 l_p²)) for large area a_H in the large spin semiclassical limit. Such a correspondence with the semiclassical Bekenstein-Hawking entropy law points towards an unanticipated and remarkable feature of the original complex Ashtekar variables for quantum gravity.
5 pages

http://arxiv.org/abs/1212.3614
Chiral description of ghost-free massive gravity
Sergei Alexandrov, Kirill Krasnov, Simone Speziale
(Submitted on 14 Dec 2012)
We propose and study a new first order version of the ghost-free massive gravity. Instead of metrics or tetrads, it uses a connection together with Plebanski's chiral 2-forms as fundamental variables, rendering the phase space structure similar to that of SU(2) gauge theories. The chiral description simplifies computations of the constraint algebra, and allows us to perform the complete canonical analysis of the system. In particular, we explicitly compute the secondary constraint and carry out the stabilization procedure, thus proving that in general the theory propagates 7 degrees of freedom, consistently with previous claims. Finally, we point out that the description in terms of 2-forms opens the door to an infinite class of ghost-free massive bi-gravity actions.
25 pages

http://arxiv.org/abs/1211.3062
Bananaworld: Quantum Mechanics for Primates
Jeffrey Bub
(Submitted on 13 Nov 2012)
This is intended to be a serious paper, in spite of the title. The idea is that quantum mechanics is about probabilistic correlations, i.e., about the structure of information, insofar as a theory of information is essentially a theory of probabilistic correlations. To make this clear, it suffices to consider measurements of two binary-valued observables, x with outcomes a = 0 or 1, performed by Alice in a region A, and y with outcomes b = 0 or 1 performed by Bob in a separated region B — or, to emphasize the banality of the phenomena, two ways of peeling a banana, resulting in one of two tastes. The imagined bananas of Bananaworld are non-standard, with probabilistic correlations for peelings and tastes that lie outside the classical correlation polytope, which has the structure of a simplex. The 'no go' theorems tell us that we can't shoe-horn these correlations into the classical simplex by supposing that something has been left out of the story. The nonclassical features of quantum mechanics, including the irreducible information loss on measurement, are shown to be generic features of non-simplex theories. As far as the conceptual problems are concerned, we might as well talk about bananas.
23 pages, 4 figures

http://arxiv.org/abs/1212.3606
Interpreting Bananaworld: A response to Bub's Quantum Mechanics for Primates
Ulrich Mohrhoff
(Submitted on 16 Dec 2012)
The interpretative principle proposed by Bub in 1211.3062v1 [quant-ph] is justified only for all practical purposes (Bell's "FAPP trap"). An alternative interpretative principle is proposed. It brings to light those features of the quantum world because of which the fundamental theoretical framework of physics is a "mere" probability calculus, and it amply justifies Bohr's insistence that quantum-mechanical observables cannot be defined without reference to the experimental conditions in which they are measured. It implies that the spatial distinctions we make cannot be intrinsic to space, that regions "of space" must be realized by macroscopic objects, that the spatiotemporal differentiation of the physical world is incomplete, that the positions of macroscopic objects (suitably defined) are definite in a nonclassical sense, and that unconditional factuality can be consistently attributed to them.
16 pages, response to 1211.3062

 

[John86]

http://arxiv.org/abs/1212.4176
Empty Black Holes, Firewalls, and the Origin of Bekenstein-Hawking Entropy
Mehdi Saravani, Niayesh Afshordi, Robert B. Mann
(Submitted on 17 Dec 2012)
We propose a novel solution for the endpoint of gravitational collapse, in which spacetime ends (and is orbifolded) at a microscopic distance from black hole event horizons. This model is motivated by the emergence of singular event horizons in the gravitational aether theory, a semi-classical solution to the cosmological constant problem(s), and thus suggests a catastrophic breakdown of general relativity close to black hole event horizons. A similar picture emerges in fuzzball models of black holes in string theory, as well as the recent firewall proposal to resolve the information paradox. We then demonstrate that positing a surface fluid with vanishing energy density (but non-vanishing pressure) at the new boundary of spacetime, which is required by Israel junction conditions, yields a thermodynamic entropy that is identical to the Bekenstein-Hawking area law for charged rotating black holes. To our knowledge, this is the first derivation of black hole entropy which only employs local thermodynamics. Finally, a model for the microscopic degrees of freedom of the surface fluid (which constitute the micro-states of the black hole) is suggested, which has a finite, but Lorentz-violating, quantum field theory.

http://arxiv.org/abs/1212.4274
Asymptotic Safety and Black Hole Thermodynamics
D. Becker, M. Reuter
(Submitted on 18 Dec 2012)
We present recent results on the non-perturbative renormalization group flow of Quantum Einstein Gravity (QEG) on spacetime manifolds with boundaries. As an application, novel quantum gravity corrections to the thermodynamics of black holes are discussed.

http://arxiv.org/abs/1212.4325
On quantum gravity, Asymptotic Safety, and paramagnetic dominance
Andreas Nink, Martin Reuter
(Submitted on 18 Dec 2012)
We discuss the conceptual ideas underlying the Asymptotic Safety approach to the nonperturbative renormalization of gravity. By now numerous functional renormalization group studies predict the existence of a suitable nontrivial ultraviolet fixed point. We use an analogy to elementary magnetic systems to uncover the physical mechanism behind the emergence of this fixed point. It is seen to result from the dominance of certain paramagnetic-type interactions over diamagnetic ones. Furthermore, the spacetimes of Quantum Einstein Gravity behave like a polarizable medium with a "paramagnetic" response to external perturbations. Similarities with the vacuum state of Yang-Mills theory are pointed out.

http://arxiv.org/abs/1212.4473
Statistical Entropy of a BTZ Black Hole from Loop Quantum Gravity
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 18 Dec 2012)
We compute the statistical entropy of a BTZ black hole in the context of three-dimensional Euclidean loop quantum gravity with a cosmological constant $\Lambda$. As in the four-dimensional case, a quantum state of the black hole is characterized by a spin network state. Now however, the underlying colored graph $\Gamma$ lives in a two-dimensional spacelike surface $\Sigma$, and some of its links cross the black hole horizon, which is viewed as a circular boundary of $\Sigma$. Each link $\ell$ crossing the horizon is colored by a spin $j_\ell$ (at the kinematical level), and the length $L$ of the horizon is given by the sum $L=\sum_\ell L_\ell$ of the fundamental length contributions $L_\ell$ carried by the spins $j_\ell$ of the links $\ell$. We propose an estimation for the number $N^\text{BTZ}_\Gamma(L,\Lambda)$ of the Euclidean BTZ black hole microstates (defined on a fixed graph $\Gamma$) based on an analytic continuation from the case $\Lambda>0$ to the case $\Lambda<0$. In our model, we show that $N^\text{BTZ}_\Gamma(L,\Lambda)$ reproduces the Bekenstein-Hawking entropy in the classical limit. This asymptotic behavior is independent of the choice of the graph $\Gamma$ provided that the condition $L=\sum_\ell L_\ell$ is satisfied, as it should be in three-dimensional quantum gravity.

http://arxiv.org/abs/1212.3687
Time delays across saddles as a test of modified gravity
Joao Magueijo, Ali Mozaffari
(Submitted on 15 Dec 2012)
Modified gravity theories can produce strong signals in the vicinity of the saddles of the total gravitational potential. In a sub-class of these models this translates into diverging time-delays for echoes crossing the saddles. Such models arise from the possibility that gravity might be infrared divergent or confined, and if suitably designed they are very difficult to rule out. We show that Lunar Laser Ranging during an eclipse could probe the time-delay effect within meters of the saddle, thereby proving or excluding these models. Very Large Baseline Interferometry, instead, could target delays across the Jupiter-Sun saddle. Such experiments would shed light on the infrared behaviour of gravity and examine the puzzling possibility that there might be well-hidden regions of strong gravity and even singularities inside the solar system.

http://arxiv.org/abs/1212.3699
Scaling up the extrinsic curvature in asymptotically flat gravitational initial data: Generating trapped surfaces
Shan Bai, Niall Ó Murchadha
(Submitted on 15 Dec 2012)
The existence of the initial value constraints means that specifying initial data for the Einstein equations is non-trivial. The standard method of constructing initial data in the asymptotically flat case is to choose an asymptotically flat 3-metric and a transverse-tracefree (TT) tensor on it. One can find a conformal transformation that maps these data into solutions of the constraints. In particular, the TT tensor becomes the extrinsic curvature of the 3-slice. We wish to understand how the physical solution changes as the free data is changed. In this paper we investigate an especially simple change: we multiply the TT tensor by a large constant. One might assume that this corresponds to pumping up the extrinsic curvature in the physical initial data. Unexpectedly, we show that, while the conformal factor monotonically increases, the physical extrinsic curvature decreases. The increase in the conformal factor however means that the physical volume increases in such a way that the ADM mass become unboundedly large. In turn, the blow-up of the mass combined with the control we have on the extrinsic curvature allows us to show that trapped surfaces, i.e., surfaces that are simultaneously future and past trapped, appear in the physical initial data.

http://arxiv.org/abs/1212.3698
Scaling up the extrinsic curvature in gravitational initial data
Shan Bai, Niall Ó Murchadha
(Submitted on 15 Dec 2012)
Vacuum solutions to the Einstein equations can be viewed as the interplay between the geometry and the gravitational wave energy content. The constraints on initial data reflect this interaction. We assume we are looking at cosmological solutions to the Einstein equations so we assume that the 3-space is compact, without boundary. In this article we investigate, using both analytic and numerical techniques, what happens when the extrinsic curvature is increased while the background geometry is held fixed. This is equivalent to trying to magnify the local gravitational wave kinetic energy on an unchanged background. We find that the physical intrinsic curvature does not blow up. Rather the local volume of space expands to accommodate this attempt to increase the kinetic energy.

 

[John86]

http://arxiv.org/abs/1212.3710
Displacing entanglement back and forth between the micro and macro domains
Natalia Bruno, Anthony Martin, Pavel Sekatski, Nicolas Sangouard, Rob Thew, Nicolas Gisin
(Submitted on 15 Dec 2012)
Quantum theory is often presented as the theory describing the microscopic world, and admittedly, it has done this extremely well for decades. Nonetheless, the question of whether it applies at all scales and in particular at human scales remains open, despite considerable experimental effort. Here, we report on the displacement of quantum entanglement into the domain where it involves two macroscopically distinct states, i.e. two states characterised by a large enough number of photons to be seen, at least in principle, with our eyes and that could be distinguished using mere linear - coarse-grained - detectors with a high probability. Specifically, we start by the generation of entanglement between two spatially separated optical modes at the single photon level and subsequently displace one of these modes up to almost a thousand photons. To reliably check whether entanglement is preserved, the mode is re-displaced back to the single photon level and a well established entanglement measure, based on single photon detection, is performed. The ability to displace an entangled state from the micro to the macro domain and back again provides a fascinating tool to probe fundamental questions about quantum theory and holds potential for more applied problems such as quantum sensing.

 

[marcus]

http://arxiv.org/abs/1212.4773
Deformed General Relativity
Martin Bojowald, George M. Paily
(Submitted on 19 Dec 2012)
Deformed special relativity is embedded in deformed general relativity using the methods of canonical relativity and loop quantum gravity. Phase-space dependent deformations of symmetry algebras then appear, which in some regimes can be rewritten as non-linear Poincare algebras with momentum-dependent deformations of commutators between boosts and time translations. In contrast to deformed special relativity, the deformations are derived for generators with an unambiguous physical role, following from the relationship between canonical constraints of gravity with stress-energy components. The original deformation does not appear in momentum space and does not give rise to non-locality issues or problems with macroscopic objects. Contact with deformed special relativity may help to test loop quantum gravity or restrict its quantization ambiguities.
14 pages

 

[marcus]

http://arxiv.org/abs/1212.5166
Modeling black holes with angular momentum in loop quantum gravity
Ernesto Frodden, Alejandro Perez, Daniele Pranzetti, Christian Roeken
(Submitted on 20 Dec 2012)
We construct a SU(2) connection formulation of Kerr isolated horizons. As in the non-rotating case, the model is based on a SU(2) Chern-Simons theory describing the degrees of freedom on the horizon. The presence of a non-vanishing angular momentum modifies the admissibility conditions for spin network states. Physical states of the system are in correspondence with open intertwiners with total spin matching the angular momentum of the spacetime.
18 pages.

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers
(Submitted on 20 Dec 2012)
We propose entanglement entropy as a probe of the architecture of spacetime in quantum gravity. We argue that the leading contribution to this entropy satisfies an area law for any sufficiently large region in a smooth spacetime, which, in fact, is given by the Bekenstein-Hawking formula. This conjecture is supported by various lines of evidence from perturbative quantum gravity, simplified models of induced gravity and loop quantum gravity, as well as the AdS/CFT correspondence.
8 pages, 1 figure

http://arxiv.org/abs/1212.5246
Gravitational origin of the weak interaction's chirality
Stephon Alexander, Antonino Marciano, Lee Smolin
(Submitted on 20 Dec 2012)
We present a new unification of the electro-weak and gravitational interactions based on the joining the weak SU(2) gauge fields with the left handed part of the space-time connection, into a single gauge field valued in the complexification of the local Lorentz group. Hence, the weak interactions emerge as the right handed chiral half of the space-time connection, which explains the chirality of the weak interaction. This is possible, because, as shown by Plebanski, Ashtekar, and others, the other chiral half of the space-time connection is enough to code the dynamics of the gravitational degrees of freedom.
This unification is achieved within an extension of the Plebanski action previously proposed by one of us. The theory has two phases. A parity symmetric phase yields, as shown by Speziale, a bi-metric theory with eight degrees of freedom: the massless graviton, a massive spin two field and a scalar ghost. Because of the latter this phase is unstable. Parity is broken in a stable phase where the eight degrees of freedom arrange themselves as the massless graviton coupled to an SU(2) triplet of chirally coupled Yang-Mills fields. It is also shown that under this breaking a Dirac fermion expresses itself as a chiral neutrino paired with a scalar field with the quantum numbers of the Higgs.
21 pages

http://arxiv.org/abs/1212.4987
Does Gravity's Rainbow induce Inflation without an Inflaton?
Remo Garattini, Mairi Sakellariadou
(Submitted on 20 Dec 2012)
We study aspects of quantum cosmology in the presence of a modified space-time geometry. In particular, within the context of Gravity's Rainbow modified geometry, motivated from quantum gravity corrections at the Planck energy scale, we show that the distortion of the metric leads to a Wheeler-De Witt equation whose solution admits outgoing plane waves. Hence, a period of cosmological inflation may arise without the need for introducing an inflaton field.
13 pages

http://arxiv.org/abs/1212.5064
A note on the Holst action, the time gauge, and the Barbero-Immirzi parameter
Marc Geiller, Karim Noui
(Submitted on 20 Dec 2012)
In this note, we review the canonical analysis of the Holst action in the time gauge, with a special emphasis on the Hamiltonian equations of motion and the fixation of the Lagrange multipliers. This enables us to identify at the Hamiltonian level the various components of the covariant torsion tensor, which have to be vanishing in order for the classical theory not to depend upon the Barbero-Immirzi parameter. We also introduce a formulation of three-dimensional gravity with an explicit phase space dependency on the Barbero-Immirzi parameter as a potential way to investigate its fate and relevance in the quantum theory.
22 pages

http://arxiv.org/abs/1212.5150
A loop quantum multiverse?
Martin Bojowald
(Submitted on 20 Dec 2012)
Inhomogeneous space-times in loop quantum cosmology have come under better control with recent advances in effective methods. Even highly inhomogeneous situations, for which multiverse scenarios provide extreme examples, can now be considered at least qualitatively.
10 pages, 9 figures, based on a plenary talk given at Multicosmofun '12, Szeczin, Poland

http://arxiv.org/abs/1212.5233
Causal loop in the theory of Relative Locality
Lin-Qing Chen
(Submitted on 20 Dec 2012)
Relative locality is a proposal for describing the Planck scale modifications to relativistic dynamics resulting from non-trivial momentum space geometry. A simple construction of interaction processes shows that Relative Locality allows for existence of causal loops, which arises from the phase space structure of the theory. The general condition allowing such process to happen is studied. We showcase this when the geometry of momentum space is taken to be Kappa-Poincare momentum space.
5 pages, 3 figures

brief mention:
http://arxiv.org/abs/1212.5226
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
G. Hinshaw, D. Larson, E. Komatsu, D. N. Spergel, C. L. Bennett, J. Dunkley, M. R. Nolta, M. Halpern, R. S. Hill, N. Odegard, L. Page, K. M. Smith, J. L. Weiland, B. Gold, N. Jarosik, A. Kogut, M. Limon, S. S. Meyer, G. S. Tucker, E. Wollack, E. L. Wright
(Submitted on 20 Dec 2012)
We present cosmological parameter constraints based on the final nine-year WMAP data, in conjunction with additional cosmological data sets...
...
31 pages, 12 figures

 

[marcus]

http://arxiv.org/abs/1212.5571
A positive formalism for quantum theory in the general boundary formulation
Robert Oeckl (CCM-UNAM)
(Submitted on 21 Dec 2012)
We introduce a new "positive formalism" for encoding quantum theories in the general boundary formulation, somewhat analogous to the mixed state formalism of the standard formulation. This makes the probability interpretation more natural and elegant, eliminates operationally irrelevant structure and opens the general boundary formulation to quantum information theory.
28 pages

to clarify the relevance here is a quote from end of section 2 on page 4:
"...The time-evolution operator U ̃ restricted to self-adjoint operators produces self-adjoint operators. Moreover, it is positive, i.e., it maps positive operators to positive operators. It also conserves the trace so that it maps mixed states to mixed states. These considerations suggest that positivity and order structure should play a more prominent role at a foundational level than say the Hilbert space structure of H or the algebra structure of the operators on it from which they are usually derived.
Algebraic quantum field theory [7] is a great example of the fruitfulness of taking serious some of these issues. There, one abandons in fact the notion of Hilbert spaces in favor of more flexible structures built on C∗-algebras. Also, positivity plays a crucial role there in the concept of state."

http://arxiv.org/abs/1212.5572
On the no-gravity limit of gravity
J. Kowalski-Glikman, M. Szczachor
(Submitted on 21 Dec 2012)
We argue that Relative Locality may arise in the no gravity G→0 limit of gravity. In this limit gravity becomes a topological field theory of the BF type that, after coupling to particles, may effectively deform its dynamics. We briefly discuss another no gravity limit with a self dual ground state as well as the topological ultra strong G→∞ one.
3 pages. Based on a talk given at the 13th Marcel Grossmann Meeting

 

[John86]

http://arxiv.org/abs/1212.5630
Infinite Shannon entropy
Valentina Baccetti (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 21 Dec 2012)
Even if a probability distribution is properly normalizable, its associated Shannon (or von Neumann) entropy can easily be infinite. We carefully analyze conditions under which this phenomenon can occur. Roughly speaking, this happens when arbitrarily small amounts of probability are dispersed into an infinite number of states; we shall quantify this observation and make it precise. We develop several particularly simple, elementary, and useful bounds, and also provide some asymptotic estimates, leading to necessary and sufficient conditions for the occurrence of infinite Shannon entropy. We go to some effort to keep technical computations as simple and conceptually clear as possible. In particular, we shall see that large entropies cannot be localized in state space; large entropies can only be supported on an exponentially large number of states. We are for the time being interested in single-channel Shannon entropy in the information theoretic sense, not entropy in a stochastic field theory or QFT defined over some configuration space, on the grounds that this simple problem is a necessary precursor to understanding infinite entropy in a field theoretic context.

 

[marcus]

http://arxiv.org/abs/1212.6824
Black hole entanglement entropy and the renormalization group
Ted Jacobson, Alejandro Satz
(Submitted on 31 Dec 2012)
We investigate the contributions of quantum fields to black hole entropy by using a cutoff scale at which the theory is described with a Wilsonian effective action. For both free and interacting fields, the total black hole entropy can be partitioned into a contribution derived from the gravitational effective action and a contribution from quantum fluctuations below the cutoff scale. In general the latter includes a quantum contribution to the Noether charge. We analyze whether it is appropriate to identify the rest with horizon entanglement entropy, and find several complications for this interpretation, which are especially problematic for interacting fields.
29 pages

http://arxiv.org/abs/1212.6944
Boundary unitarity without firewalls
Ted Jacobson
(Submitted on 31 Dec 2012)
Both AdS/CFT duality and more general reasoning from quantum gravity point to a rich collection of boundary observables that always evolve unitarily. The physical quantum gravity states described by these observables must be solutions of the spatial diffeomorphism and Wheeler-deWitt constraints, which implies that the state space does not factorize into a tensor product of localized degrees of freedom. The recent "firewall" argument that unitarity of black hole S-matrix implies the presence of a highly excited quantum state near the horizon is based on such a factorization, hence is not applicable in quantum gravity.
Comments: 7 pages

http://arxiv.org/abs/1212.6821
Black holes and Hawking radiation in spacetime and its analogues
Ted Jacobson
(Submitted on 31 Dec 2012)
These notes introduce the fundamentals of black hole geometry, the thermality of the vacuum, and the Hawking effect, in spacetime and its analogues. Stimulated emission of Hawking radiation, the trans-Planckian question, short wavelength dispersion, and white hole radiation in the setting of analogue models are also discussed. No prior knowledge of differential geometry, general relativity, or quantum field theory in curved spacetime is assumed.
31 pages, 9 figures; to appear in the proceedings of the IX SIGRAV School on 'Analogue Gravity', Como (Italy), May 2011, eds. D. Faccio et. al. (Springer)

 

[John86]

http://arxiv.org/abs/1212.6967
Entropic Inference: some pitfalls and paradoxes we can avoid
Ariel Caticha
(Submitted on 31 Dec 2012)
The method of maximum entropy has been very successful but there are cases where it has either failed or led to paradoxes that have cast doubt on its general legitimacy. My more optimistic assessment is that such failures and paradoxes provide us with valuable learning opportunities to sharpen our skills in the proper way to deploy entropic methods. The central theme of this paper revolves around the different ways in which constraints are used to capture the information that is relevant to a problem. This leads us to focus on four epistemically different types of constraints. I propose that the failure to recognize the distinctions between them is a prime source of errors. I explicitly discuss two examples. One concerns the dangers involved in replacing expected values with sample averages. The other revolves around misunderstanding ignorance. I discuss the Friedman-Shimony paradox as it is manifested in the three-sided die problem and also in its original thermodynamic formulation.
Comments: 14 pages, 1 figure. Invited paper presented at MaxEnt 2012, The 32nd International Workshop on Bayesian Inference and Maximum Entropy Methods in Science and Engineering, (July 15--20, 2012, Garching, Germany)

http://arxiv.org/abs/1212.6946
The Entropic Dynamics of Relativistic Quantum Fields
Ariel Caticha
(Submitted on 31 Dec 2012)
The formulation of quantum mechanics within the framework of entropic dynamics is extended to the domain of relativistic quantum fields. The result is a non-dissipative relativistic diffusion in the infinite dimensional space of field configurations. On extending the notion of entropic time to the relativistic regime we find that the field fluctuations provide the clock that sets the scale of duration. We also find that the usual divergences that affect all quantum field theories do not refer to the real values of physical quantities but rather to epistemic quantities invariably associated to unphysical probability distributions such as variances and other measures of uncertainty.
Comments: 10 pages. Presented at MaxEnt 2012, The 32nd International Workshop on Bayesian Inference and Maximum Entropy Methods in Science and Engineering, (July 15-20, 2012, Garching, Germany)

 

[atyy]

http://arxiv.org/abs/1209.4779
On antiscreening in perturbative quantum gravity and resolving the Newtonian singularity
Anja Marunovic, Tomislav Prokopec
(Submitted on 21 Sep 2012)
We calculate the quantum corrections to the Newtonian potential induced by a massless, nonminimally coupled scalar field on Minkowski background. We make use of the graviton vacuum polarization calculated in our previous work and solve the equation of motion non-perturbatively. When written as the quantum-corrected gauge invariant Bardeen potentials, our results show that quantum effects generically antiscreen the Newtonian singularity 1/r. This result supports the point of view that gravity on (super-)Planckian scales is an asymptotically safe theory. In addition, we show that, in the presence of quantum fluctuations of a massless, (non)minimally coupled scalar field, dynamical gravitons propagate superluminally. The effect is, however, unbservably small and it is hence of academic interest only.

 

[marcus]

http://arxiv.org/abs/1301.1264
Inflation as a prediction of loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013)
Loop quantum cosmology is known to be closely linked with an inflationary phase. In this article, we study quantitatively the probability for a long enough stage of slow-roll inflation to occur, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "pre-bounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is indeed more than enough to solve all the standard cosmological problems. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture about inflation, super-inflation, deflation and super-deflation is also much clarified in the framework of bouncing cosmologies.
6 pages, 4 figures

http://arxiv.org/abs/1301.0879
On unimodular quantum gravity
Astrid Eichhorn
(Submitted on 5 Jan 2013)
Unimodular gravity is classically equivalent to standard Einstein gravity, but differs when it comes to the quantum theory: The conformal factor is non-dynamical, and the gauge symmetry consists of transverse diffeomorphisms only. Furthermore, the cosmological constant is not renormalized. Thus the quantum theory is distinct from a quantization of standard Einstein gravity. Here we show that within a truncation of the full Renormalization Group flow of unimodular quantum gravity, there is a non-trivial ultraviolet-attractive fixed point, yielding a UV completion for unimodular gravity. We discuss important differences to the standard asymptotic-safety scenario for gravity, and provide further evidence for this scenario by investigating a new form of the gauge-fixing and ghost sector.
10 pages, 1 figure

brief mention:
http://arxiv.org/abs/1301.1069
A Snapshot of Foundational Attitudes Toward Quantum Mechanics
Maximilian Schlosshauer, Johannes Kofler, Anton Zeilinger
(Submitted on 6 Jan 2013)
... Here, we present the results of a poll carried out among 33 participants of a conference on the foundations of quantum mechanics...
17 pages 3 figures

 

[MTd2]

http://arxiv.org/abs/1301.1538

Right about time?

Sean Gryb, Flavio Mercati
(Submitted on 8 Jan 2013)
Have our fundamental theories got time right? Does size really matter? Or is physics all in the eyes of the beholder? In this essay, we question the origin of time and scale by reevaluating the nature of measurement. We then argue for a radical scenario, supported by a suggestive calculation, where the flow of time is inseparable from the measurement process. Our scenario breaks the bond of time and space and builds a new one: the marriage of time and scale.

 

[marcus]

http://arxiv.org/abs/1301.1535
The 1/N expansion of multi-orientable random tensor models
S. Dartois, V. Rivasseau, A. Tanasa
(Submitted on 8 Jan 2013)
Multi-orientable group field theory (GFT) has been introduced in A. Tanasa, J. Phys. A 45 (2012) 165401, arXiv:1109.0694, as a quantum field theoretical simplification of GFT, which retains a larger class of tensor graphs than the colored one. In this paper we define the associated multi-orientable identically independent distributed multi-orientable tensor model and we derive its 1/N expansion. In order to obtain this result, a partial classification of general tensor graphs is performed and the combinatorial notion of jacket is extended to the multi-orientable graphs. We prove that the leading sector is given, as in the case of colored models, by the so-called melon graphs.
18 pages, 17 figures

 

[marcus]

http://arxiv.org/abs/1301.1949
Hamiltonian dynamics of a quantum of space: hidden symmetries and spectrum of the volume operator, and discrete orthogonal polynomials
Vincenzo Aquilanti, Dimitri Marinelli, Annalisa Marzuoli
(Submitted on 9 Jan 2013)
The action of the quantum mechanical volume operator, introduced in connection with a symmetric representation of the three-body problem and recently recognized to play a fundamental role in discretized quantum gravity models, can be given as a second order difference equation which, by a complex phase change, we turn into a discrete Schrödinger-like equation. The introduction of discrete potential-like functions reveals the surprising crucial role here of hidden symmetries, first discovered by Regge for the quantum mechanical 6j symbols; insight is provided into the underlying geometric features. The spectrum and wavefunctions of the volume operator are discussed from the viewpoint of the Hamiltonian evolution of an elementary "quantum of space", and a transparent asymptotic picture emerges of the semiclassical and classical regimes. The definition of coordinates adapted to Regge symmetry is exploited for the construction of a novel set of discrete orthogonal polynomials, characterizing the oscillatory components of torsion-like modes.
13 pages, 5 figures

http://arxiv.org/abs/1301.1933
Shape Dynamics
Tim Koslowski
(Submitted on 9 Jan 2013)
Barbour's formulation of Mach's principle requires a theory of gravity to implement local relativity of clocks, local relativity of rods and spatial covariance. It turns out that relativity of clocks and rods are mutually exclusive. General Relativity implements local relativity of clocks and spatial covariance, but not local relativity of rods. It is the purpose of this contribution to show how Shape Dynamics, a theory that is locally equivalent to General Relativity, implements local relativity of rods and spatial covariance and how a BRST formulation, which I call Doubly General Relativity, implements all of Barbour's principles.
8 pages, based on a talk given at Relativity and Gravitation 100 years after Einstein in Prague, June 2012

http://arxiv.org/abs/1301.1692
Contracted Lorentz Invariance for Gravity with a Preferred Foliation
Steffen Gielen
(Submitted on 8 Jan 2013)
In canonical gravity, the choice of a local time direction is not obviously compatible with local Lorentz invariance. One way to address this issue is to view gravity as a gauge theory on observer space, rather than spacetime. In a Lorentz covariant theory observer space is the space of unit future-directed timelike vectors; picking such a vector locally breaks the symmetry to a subgroup SO(D) of SO(D,1), so that on observer space the local symmetry group is SO(D).
Observer space geometries naturally describe any gravitational theory that only assumes local invariance under SO(D). Here we construct non-relativistic observer spaces for gravity with a fixed foliation, applicable to preferred foliation theories such as Horava-Lifgarbagez gravity. Different Horava-Lifgarbagez observers at a point are related by a change in the shift vector, leaving the preferred foliation invariant. Gravity is formulated on a non-relativistic frame bundle with structure group ISO(D); the shift vector acts as a symmetry breaking field breaking ISO(D) symmetry spontaneously to SO(D). A natural torsion-free connection is constructed, including the usual SO(D,1) connection of relativistic gravity plus terms depending on derivatives of the shift vector. This observer space geometry provides a novel geometric framework for the study of preferred foliation theories.
15 pages

brief mention (possibly useful reference/review):
http://arxiv.org/abs/1301.1836
Tomita-Takesaki Modular Theory vs. Quantum Information Theory
Lin Zhang, Junde Wu
(Submitted on 9 Jan 2013)

 

[marcus]

http://arxiv.org/abs/1301.2214
Corrigendum: The Plebanski sectors of the EPRL vertex
Jonathan Engle
(Submitted on 10 Jan 2013)
We correct what amounts to a sign error in the proof of part (i.) of theorem 3 in Class.Quant.Grav.28 225003 (arXiv:1107.0709). The Plebanski sectors isolated by the linear simplicity constraints do not change --- they are still the three sectors (deg), (II+), and (II-). What changes is the characterization of the continuum Plebanski two-form corresponding to the first two terms in the asymptotics of the EPRL vertex amplitude for Regge-like boundary data. These two terms do not correspond to Plebanski sectors (II+) and (II-), but rather to the two possible signs of the product of the sign of the sector --- +1 for (II+) and -1 for (II-) --- and the sign of the orientation ε_IJKLB^IJ ∧ B^KL determined by B^IJ. This is consistent with what one would expect, as this is exactly the sign which classically relates the BF action, in Plebanski sectors (II+) and (II-), to the Einstein-Hilbert action, whose discretization is the Regge action appearing in the asymptotics.
4 pages. Corrigendum to http://arxiv.org/abs/1107.0709

 

[John86]

http://arxiv.org/abs/1301.2245
Quantum-Reduced Loop Gravity: Cosmology
Emanuele Alesci, Francesco Cianfrani
(Submitted on 10 Jan 2013)
We introduce a new framework for loop quantum gravity: mimicking the spinfoam quantization procedure we propose to study the symmetric sectors of the theory imposing the reduction weakly on the full kinematical Hilbert space of the canonical theory. As a first application of Quantum-Reduced Loop Gravity we study the inhomogeneous Bianchi I model. The emerging quantum cosmological model represents a simplified arena on which the complete canonical quantization program can be tested. The achievements of this analysis could elucidate the relationship between Loop Quantum Cosmology and the full theory.

 

[marcus]

brief mention:
http://arxiv.org/abs/1301.2515
Quantum information and randomness
Johannes Kofler, Anton Zeilinger
(Submitted on 11 Jan 2013)
This is an article for a special edition of "European Review" with a focus on the topic "Causality".
Comments: 8 pages, 3 figures

http://arxiv.org/abs/1301.2586
On the Consistency of the Consistent Histories Approach to Quantum Mechanics
Elias Okon, Daniel Sudarsky
(Submitted on 11 Jan 2013)
The Consistent Histories (CH) formalism aims at a quantum mechanical framework for the universe as a whole. CH stresses the importance of histories for quantum mechanics, as opposed to measurements, and maintains that a satisfactory formulation of quantum mechanics allows one to assign probabilities to alternative histories of the universe. It further proposes that each realm, that is, each set of histories to which probabilities can be assigned, provides a valid quantum-mechanical account, but that different realms can be mutually incompatible. Finally, some of its proponents offer an "evolutionary" explanation of our existence in the universe and of our preference for quasiclassical descriptions of nature. The present work questions the validity of claims offered by CH proponents asserting that it solves many interpretational problems in quantum mechanics. In particular, we point out that the interpretation of the framework leaves vague two crucial points, namely, whether realms are fixed or chosen and the link between measurements and histories. Our claim is that by doing so, CH overlooks the main interpretational problems of quantum mechanics. Furthermore, we challenge the evolutionary explanation offered and we critically examine the proposed notion of a realm-dependent reality.
17 pages

http://arxiv.org/abs/1301.2563
Noncommutative spectral geometry and the deformed Hopf algebra structure of quantum field theory
Mairi Sakellariadou, Antonio Stabile, Giuseppe Vitiello
(Submitted on 11 Jan 2013)
We report the results obtained in the study of Alain Connes noncommutative spectral geometry construction focusing on its essential ingredient of the algebra doubling. We show that such a two-sheeted structure is related with the gauge structure of the theory, its dissipative character and carries in itself the seeds of quantization. From the algebraic point of view, the algebra doubling process has the same structure of the deformed Hops algebra structure which characterizes quantum field theory.
11 pages. Invited talk in the Sixth International Workshop DICE2012, Castello Pasquini/Castiglioncello (Tuscany), September 17-21, 2012

 

[John86]

http://arxiv.org/abs/1301.2334
Is there vacuum when there is mass? Vacuum and non-vacuum solutions for massive gravity
Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 10 Jan 2013)
Massive gravity is a theory which has a tremendous amount of freedom to describe different cosmologies; but at the same time the various solutions one encounters must fulfill some rather nontrivial constraints. Most of the freedom comes not from the Lagrangian, which contains only a small number of free parameters (typically 3 depending on counting conventions), but on the fact that one is in principle free to choose the background reference metric almost arbitrarily --- which effectively introduces a non-denumerable infinity of free parameters. In the current paper we stress that although changing the reference metric would lead to a different cosmological model, this does not mean that the dynamics of our own universe can be entirely divorced from its matter content. That is, while the choice of reference metric certainly influences the evolution of the physically observable foreground metric, the effect of matter cannot be neglected. Nevertheless, the relation between matter and geometry can be significantly changed in some specific models; effectively since the graviton would be able to curve the spacetime by itself, without the need of matter. Thus, even the set of vacuum solutions for massive gravity can have significant structure. On the other hand, in some cases the effect of the reference metric could be so strong that no conceivable material content would be able to drastically affect the cosmological evolution.

 

[marcus]

http://arxiv.org/abs/1301.3171
Black hole entropy and the renormalization group
Alejandro Satz, Ted Jacobson
(Submitted on 14 Jan 2013)
Four decades after its first postulation by Bekenstein, black hole entropy remains mysterious. It has long been suggested that the entanglement entropy of quantum fields on the black hole gravitational background should represent at least an important contribution to the total Bekenstein-Hawking entropy, and that the divergences in the entanglement entropy should be absorbed in the renormalization of the gravitational couplings. In this talk, we describe how an improved understanding of black hole entropy is obtained by combining these notions with the renormalization group. By introducing an RG flow scale, we investigate whether the total entropy of the black hole can be partitioned in a "gravitational" part related to the flowing gravitational action, and a "quantum" part related to the unintegrated degrees of freedom. We describe the realization of this idea for free fields, and the complications and qualifications arising for interacting fields.
3 pages. Conference proceedings article from talk given by A. Satz at the Thirteenth Marcel Grossmann Meeting, Stockholm, 2012

 

[marcus]

http://arxiv.org/abs/1301.3480
Gauge networks in noncommutative geometry
Matilde Marcolli, Walter D. van Suijlekom
(Submitted on 15 Jan 2013)
We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. The configuration space of quiver representations (modulo equivalence) in the category of finite spectral triples is studied; gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences.
Given a representation in the category of spectral triples of a quiver embedded in a spin manifold, we define a discretized Dirac operator on the quiver. We compute the spectral action of this Dirac operator on a four-dimensional lattice, and find that it reduces to the Wilson action for lattice gauge theories and a Higgs field lattice system. As such, in the continuum limit it reduces to the Yang-Mills-Higgs system. For the three-dimensional case, we relate the spectral action functional to the Kogut-Susskind Hamiltonian.
30 pages

http://arxiv.org/abs/1301.3113
New coherent states and modified heat equations
Antonio Pittelli, Lorenzo Sindoni
(Submitted on 14 Jan 2013)
We clarify the relations between certain new coherent states for loop quantum gravity and the analytically continued heat kernel coherent states, highlighting the underlying general construction, the presence of a modified heat equation as well as the way in which the properties of the heat kernels are automatically inherited by these new states.
7 pages

 

[tom.stoer]

Have you seen

http://arxiv.org/abs/1201.2187
http://arxiv.org/abs/1111.2865

http://arxiv.org/abs/1101.3294
http://arxiv.org/abs/1005.1866
http://arxiv.org/abs/1110.4694
http://arxiv.org/abs/1005.1866

http://arxiv.org/abs/1207.7263

 

[marcus]

If you wish, you could start a discussion thread, or two, about some of these.

==============================

EDIT to reply to the next post in sequence, post #1883: Thanks for the reminders or suggestions, Tom!

Since I can still edit, I will add the new paper of Torsten that just appeared today:

http://arxiv.org/abs/1301.3628
On the origin of inflation by using exotic smoothness
T. Asselmeyer-Maluga, J. Krol
(Submitted on 16 Jan 2013)
In this paper we discuss a spacetime having the topology of S³x R but with a different smoothness structure leading to a geometric model for inflation, called geometric inflation. In particular this spacetime is not globally hyperbolic and we obtain a time line with a spatial topology change from the 3-sphere to a homology 3-sphere and back. The topology of the spacetime remains invariant. Among the infinite possible smoothness structures of this spacetime, we choose a homology 3-sphere constructed from the knot 8₁₀ with hyperbolic geometry, i.e. admitting a homogenous metric of negative scalar curvature. We discuss the accelerated expansion for FLRW cosmology caused by the topology change. In contrast to other inflation models, this process stops after a finite time. Alternatively, the topology change can be also described by a SU(2)-valued scalar field. Then we calculate the expansion rate (having more than 60 e-folds) and the energy time scale. The coupling to matter is also interpreted geometrically and the reheating process (as well the supercooled expansion during inflation) is naturally obtained. The model depends only on a single parameter, a topological invariant of the homology 3-sphere, and assumes a Planck size universe of S³-topology. The dependence of the model on the initial state and the a geometric interpretation of quantum fluctuations are also discussed.
28 pages, 9 figures

 

[tom.stoer]

If you wish, you could start a discussion thread, or two, about some of these.

no discussion, simply a hint in you thread;-)

 

[marcus]

http://arxiv.org/abs/1301.4191
A bootstrap towards asymptotic safety
K. Falls, D. F. Litim, K. Nikolakopoulos, C. Rahmede
(Submitted on 17 Jan 2013)
A search strategy for asymptotic safety is put forward and tested for a simplified version of gravity in four dimensions using the renormalization group. Taking the action to be a high-order polynomial of the Ricci scalar, a self-consistent ultraviolet fixed point is found where curvature invariants become increasingly irrelevant with increasing mass dimension. Intriguingly, universal scaling exponents take near-Gaussian values despite the presence of residual interactions. Asymptotic safety of metric gravity would seem in reach if this pattern carries over to the full theory.
4 pages, 1 figure

 

[MTd2]

http://arxiv.org/abs/1301.4148

Renormalization Group approach to Gravity: the running of G and L inside galaxies and additional details on the elliptical NGC 4494

Davi C. Rodrigues, Oliver F. Piattella, Júlio C. Fabris, Ilya L. Shapiro
(Submitted on 17 Jan 2013)
We explore the phenomenology of nontrivial quantum effects on low-energy gravity. These effects come from the running of the gravitational coupling parameter L and the cosmological constant L in the Einstein-Hilbert action, as induced by the Renormalization Group (RG). The Renormalization Group corrected General Relativity (RGGR model) is used to parametrize these quantum effects, and it is assumed that the dominant dark matter-like effects inside galaxies is due to these nontrivial RG effects. Here we present additional details on the RGGR model application, in particular on the Poisson equation extension that defines the effective potential, also we re-analyse the ordinary elliptical galaxy NGC 4494 using a slightly different model for its baryonic contribution, and explicit solutions are presented for the running of G and L. The values of the NGC 4494 parameters as shown here have a better agreement with the general RGGR picture for galaxies, and suggest a larger radial anisotropy than the previously published result.

 

[marcus]

http://arxiv.org/abs/1301.4422
On the number of relevant operators in asymptotically safe gravity
Dario Benedetti
(Submitted on 18 Jan 2013)
In this short note we answer a long standing question about the asymptotic safety scenario for quantum gravity. The term asymptotic safety refers to the conjecture that (i) the quantum field theory of gravity admits a non-trivial ultraviolet fixed point, and that (ii) this has only a finite number of relevant perturbations, i.e. a finite number of UV-stable directions (or in other words, a finite number of free parameters to be fixed experimentally). Within the f(R) approximation of the functional renormalization group equation of gravity, we show that assuming the first half of the conjecture to be true, the remaining half follows from general arguments, that is, we show that assuming the existence of a non-trivial fixed point, the fact that the number of relevant directions is finite is a general consequence of the structure of the equations.
5 pages

http://arxiv.org/abs/1301.4322
Can quantum gravity be exposed in the laboratory?: A tabletop experiment to reveal the quantum foam
Jacob D. Bekenstein
(Submitted on 18 Jan 2013)
I propose an experiment that may be performed, with present low temperature and cryogenic technology, to reveal Wheeler's quantum foam. It involves coupling an optical photon's momentum to the center of mass motion of a macroscopic transparent block with parameters such that the latter is displaced in space by approximately a Planck length. I argue that such displacement is sensitive to quantum foam and will react back on the photon's probability of transiting the block. This might allow determination of the precise scale at which quantum fluctuations of space-time become large, and so differentiate between the brane-world and the traditional scenarios of spacetime.
10 pages, one figure: Keynote lecture at the international workshop "Horizon of quantum physics: from foundations to quantum enabled technologies", Taipei, October 16, 2012. To appear in the proceedings in Foundations of Physics

 

[marcus]

http://arxiv.org/abs/1301.4687
Noncommutative Spectral Geometry: A Short Review
Mairi Sakellariadou
(Submitted on 20 Jan 2013)
We review the noncommutative spectral geometry, a gravitational model that combines noncommutative geometry with the spectral action principle, in an attempt to unify General Relativity and the Standard Model of electroweak and strong interactions. Despite the phenomenological successes of the model, the discrepancy between the predicted Higgs mass and the current experimental data indicate that one may have to go beyond the simple model considered at first. We review the current status of the phenomenological consequences and their implications. Since this model lives by construction at high energy scales, namely at the Grand Unified Theories scale, it provides a natural framework to investigate early universe cosmology. We briefly review some of its cosmological consequences.
11 pages. Invited talk in the Sixth International Workshop DICE2012, Castello Pasquini/Castiglioncello (Tuscany), September 17-21, 2012

http://arxiv.org/abs/1301.4920
Massless scalar field in de Sitter spacetime: unitary quantum time evolution
Jeronimo Cortez, Daniel Martin-de Blas, Guillermo A. Mena Marugan, Jose M. Velhinho
(Submitted on 21 Jan 2013)
We prove that, under the standard conformal scaling, a massless field in de Sitter spacetime admits an O(4)-invariant Fock quantization such that time evolution is unitarily implemented. This result disproves previous claims in the literature. We discuss the relationship between this quantization with unitary dynamics and the family of O(4)-invariant Hadamard states given by Allen and Folacci, as well as with the Bunch-Davies vacuum.
Comments: 23 pages

http://arxiv.org/abs/1301.4553
Quantum Hairs and Isolated Horizon Entropy from Chern-Simons Theory
Abhishek Majhi, Parthasarathi Majumdar
(Submitted on 19 Jan 2013)
We articulate the fact that the loop quantum gravity description of the quantum states of black hole horizons, modeled as Quantum Isolated Horizons (QIHs), is completely characterized in terms of two independent integer-valued quantum 'hairs', viz,. the coupling constant of the quantum SU(2) Chern Simons theory describing QIH dynamics, and the number of punctures produced by the bulk spin network edges piercing the isolated horizon (which act as pointlike sources for the Chern Simons fields). We demonstrate that the microcanonical entropy of macroscopic (both parameters assuming very large values) QIHs can be obtained directly from the microstates of this Chern-Simons theory, using standard statistical mechanical methods, without having to additionally postulate the horizon as an ideal gas of punctures, or incorporate any additional classical or semi-classical input from general relativity vis-a-vis the functional dependence of the IH mass on its area, or indeed, without having to restrict to any special class of spins. Requiring the validity of the Bekenstein-Hawking area law relates these two parameters (as an equilibrium `equation of state') and consequently imposes restrictions on the allowed values of the Barbero-Immirzi parameter. The logarithmic correction to the area law obtained a decade ago by R. Kaul and one of us (P.M.), ensues straightforwardly, with precisely the coefficient -3/2, making it a signature of the loop quantum gravity approach to black hole entropy.

 

[marcus]

http://arxiv.org/abs/1301.5135
Einstein-Cartan gravity, Asymptotic Safety, and the running Immirzi parameter
Jan-Eric Daum, Martin Reuter
(Submitted on 22 Jan 2013)
In this paper we analyze the functional renormalization group flow of quantum gravity on the Einstein-Cartan theory space. The latter consists of all action functionals depending on the spin connection and the vielbein field (co-frame) which are invariant under both spacetime diffeomorphisms and local frame rotations. In the first part of the paper we develop a general methodology and corresponding calculational tools which can be used to analyze the flow equation for the pertinent effective average action for any truncation of this theory space. In the second part we apply it to a specific three-dimensional truncated theory space which is parametrized by Newton's constant, the cosmological constant, and the Immirzi parameter. A comprehensive analysis of their scale dependences is performed, and the possibility of defining an asymptotically safe theory on this hitherto unexplored theory space is investigated. In principle Asymptotic Safety of metric gravity (at least at the level of the effective average action) is neither necessary nor sufficient for Asymptotic Safety on the Einstein-Cartan theory space which might accommodate different "universality classes" of microscopic quantum gravity theories. Nevertheless, we do find evidence for the existence of at least one non-Gaussian renormalization group fixed point which seems suitable for the Asymptotic Safety construction in a setting where the spin connection and the vielbein are the fundamental field variables.
121 pages, 8 figures

http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=140
Proceedings of the 3rd Quantum Gravity and Quantum Geometry School, held at Zakopane in 2011, are now online. Included are lecture series providing students with an introduction to several related QG areas, and single lectures on specialized topics. Incidentally the main person responsible for editing the proceedings was Frank Hellmann, a PF member since February 2006. https://www.physicsforums.com/showthread.php?p=4162474#post4162474 The paper with Eugenio Bianchi mentioned in this post is http://arxiv.org/abs/1207.4596

http://arxiv.org/abs/1301.5110
On the Concept of Law in Physics
Claus Kiefer
(Submitted on 22 Jan 2013)
I discuss the main features of the concept of law in physics. Fundamental laws from Newtonian mechanics to general relativity are reviewed. I end with an outlook on the new form of laws in the emerging theory of quantum gravity.
10 pages, to appear in the Proceedings of the conference "The concept of law in science", Heidelberg, 4-5 June 2012

http://arxiv.org/abs/1301.5130
No Conformal Anomaly in Unimodular Gravity
Enrique Álvarez, Mario Herrero-Valea
(Submitted on 22 Jan 2013)
The conformal invariance of unimodular gravity survives quantum corrections, even in the presence of conformal matter. Unimodular gravity can actually be understood as a certain truncation of the full Einstein-Hilbert theory, where in the Einstein frame the metric tensor enjoys unit determinant. Our result is compatible with the idea that the corresponding restriction in the functional integral is consistent as well.
20 pages

recent cosmology parameters:
http://arxiv.org/abs/1212.5226
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
G. Hinshaw, D. Larson, E. Komatsu, D. N. Spergel, C. L. Bennett, J. Dunkley, M. R. Nolta, M. Halpern, R. S. Hill, N. Odegard, L. Page, K. M. Smith, J. L. Weiland, B. Gold, N. Jarosik, A. Kogut, M. Limon, S. S. Meyer, G. S. Tucker, E. Wollack, E. L. Wright
(see table 9, page 19---from WMAP+eCMB+BAO+H₀)
Ω_k = −0.0027^+0.0039_−0.0038
Ω_tot = 1.0027^+0.0038_−0.0039

http://arxiv.org/abs/1210.7231
A Measurement of the Cosmic Microwave Background Damping Tail from the 2500-square-degree SPT-SZ survey
K. T. Story et al.
(see equation 21, page 14)
'' The tightest constraint on the mean curvature that we consider comes from combining the CMB, H₀ , and BAO datasets:
Ω_k = −0.0059±0.0040. "

 

[marcus]

http://arxiv.org/abs/1301.5362
Polymer Bose--Einstein Condensates
E. Castellanos, G. Chacon-Acosta
(Submitted on 22 Jan 2013)
In this work we analyze a non--interacting one dimensional polymer Bose--Einstein condensate in an harmonic trap within the semiclassical approximation. We use an effective Hamiltonian coming from the polymer quantization that arises in loop quantum gravity. We calculate the number of particles in order to obtain the critical temperature. The Bose--Einstein functions are replaced by series, whose high order terms are related to powers of the polymer length. It is shown that the condensation temperature presents a shift respect to the standard case, for small values of the polymer scale. In typical experimental conditions, it is possible to establish a bound for λ² up to ≤ 10^-16 m². To improve this bound we should decrease the frequency of the trap and also decrease the number of particles.
6 pages.

http://arxiv.org/abs/1301.5481
Quantum Gravity: the view from particle physics
Hermann Nicolai
(Submitted on 23 Jan 2013)
This lecture reviews aspects of and prospects for progress towards a theory of quantum gravity from a particle physics perspective, also paying attention to recent findings of the LHC experiments at CERN.
20 pages. Invited Lecture at the conference "Relativity and Gravitation: 100 Years after Einstein in Prague", June 25 - 29, 2012, Prague, Czech Republic

http://arxiv.org/abs/1301.5495
The asymptotic safety scenario and scalar field inflation
Christoph Rahmede
(Submitted on 23 Jan 2013)
We study quantum gravity corrections to early universe cosmology as resulting within the asymptotic safety scenario. We analyse if it is possible to obtain accelerated expansion in the regime of the renormalisation group fixed point in a theory with Einstein-Hilbert gravity and a scalar field. We show how this phase impacts cosmological perturbations observed in the cosmic microwave background.
3 pages. Contribution to the proceedings of the Thirteenth Marcel Grossmann Meeting, Stockholm, 2012

http://arxiv.org/abs/1301.5496
Duality-invariant bimetric formulation of linearized gravity
Claudio Bunster, Marc Henneaux, Sergio Hörtner
(Submitted on 23 Jan 2013)
A formulation of linearized gravity which is manifestly invariant under electric-magnetic duality rotations in the internal space of the metric and its dual, and which contains both metrics as basic variables (rather than the corresponding prepotentials), is derived. In this bimetric formulation, the variables have a more immediate geometrical significance, but the action is non-local in space, contrary to what occurs in the prepotential formulation. More specifically, one finds that: (i) the kinetic term is non-local in space (but local in time); (ii) the Hamiltonian is local in space and in time; (iii) the variables are subject to two Hamiltonian constraints, one for each metric.
7 pages. Based in part on the talk "Gravitational electric-magnetic duality" given by one of us (MH) at the 8-th Workshop "Quantum Field Theory and Hamiltonian Systems" (QFTHS), 19-22 September 2012, Craiova, Romania

http://arxiv.org/abs/1301.5471
New theories of gravity
Christian G. Boehmer, Nicola Tamanini
(Submitted on 23 Jan 2013)
We propose a new point of view for interpreting Newton's and Einstein's theories of gravity. By taking inspiration from Continuum Mechanics and its treatment of anisotropies, we formulate new theories of gravity. These models are simple and natural generalisations with many interesting properties. Above all, their precise form can, in principle, be determined experimentally.
8 pages

 

[marcus]

http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. This comes as a surprise. In the continuum theory, the reality conditions are preserved in time, only if the torsionless condition (a secondary constraint) holds true. Studying an additional conservation law for each spinfoam vertex, we discuss the issue of torsion and argue that spinfoam gravity may indeed miss an additional constraint. Next, we canonically quantise. Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1301.5704
The coevent formulation of quantum theory
Petros Wallden
(Submitted on 24 Jan 2013)
Understanding quantum theory has been a subject of debate from its birth. Many different formulations and interpretations have been proposed. Here we examine a recent novel formulation, namely the coevents formulation. It is a histories formulation and has as starting point the Feynman path integral and the decoherence functional. The new ontology turns out to be that of a coarse-grained history. We start with a quantum measure defined on the space of histories, and the existence of zero covers rules out single-history as potential reality (the Kochen Specker theorem casted in histories form is a special case of a zero cover). We see that allowing coarse-grained histories as potential realities avoids the previous paradoxes, maintains deductive non-contextual logic (alas non-Boolean) and gives rise to a unique classical domain. Moreover, we can recover the probabilistic predictions of quantum theory with the use of the Cournot's principle. This formulation, being both a realist formulation and based on histories, is well suited conceptually for the purposes of quantum gravity and cosmology.
18 pages

 

[marcus]

http://arxiv.org/abs/1301.6210
Embedding loop quantum cosmology without piecewise linearity
Jonathan Engle
(Submitted on 26 Jan 2013)
An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, as well as permitting lessons from the reduced theory to guide further development in the full theory. The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. This motivates for the first time a definition of operators in LQC corresponding to holonomies along non-piecewise-linear paths, without changing the usual kinematics of LQC in any way. The new embedding intertwines all operators corresponding to such holonomies, and all elements in its image satisfy an operator equation which classically implies homogeneity and isotropy. The construction is made possible by a recent result proven by Fleischhack.
18 pages

http://arxiv.org/abs/1301.6173
Scale Anomaly as the Origin of Time
Julian Barbour, Matteo Lostaglio, Flavio Mercati
(Submitted on 25 Jan 2013)
We explore the problem of time in quantum gravity in a point-particle analogue model of scale-invariant gravity. If quantized after reduction to true degrees of freedom, it leads to a time-independent Schrödinger equation. As with the Wheeler--DeWitt equation, time disappears, and a frozen formalism that gives a static wavefunction on the space of possible shapes of the system is obtained. However, if one follows the Dirac procedure and quantizes by imposing constraints, the potential that ensures scale invariance gives rise to a conformal anomaly, and the scale invariance is broken. A behaviour closely analogous to renormalization-group (RG) flow results. The wavefunction acquires a dependence on the scale parameter of the RG flow. We interpret this as time evolution and obtain a novel solution of the problem of time in quantum gravity. We apply the general procedure to the three-body problem, showing how to fix a natural initial value condition, introducing the notion of complexity. We recover a time-dependent Schrödinger equation with a repulsive cosmological force in the `late-time' physics and we analyse the role of the scale invariant Planck constant. We suggest that several mechanisms presented in this model could be exploited in more general contexts.
31 pages, 5 figures

http://arxiv.org/abs/1301.6259
Inconsistencies from a Running Cosmological Constant
Herbert W. Hamber, Reiko Toriumi
(Submitted on 26 Jan 2013)
We examine the general issue of whether a scale dependent cosmological constant can be consistent with general covariance, a problem that arises naturally in the treatment of quantum gravitation where coupling constants generally run as a consequence of renormalization group effects. The issue is approached from several points of view, which include the manifestly covariant functional integral formulation, covariant continuum perturbation theory about two dimensions, the lattice formulation of gravity, and the non-local effective action and effective field equation methods. In all cases we find that the cosmological constant cannot run with scale, unless general covariance is explicitly broken by the regularization procedure. Our results are expected to have some bearing on current quantum gravity calculations, but more generally should apply to phenomenological approaches to the cosmological vacuum energy problem.
34 pages.

http://arxiv.org/abs/1301.6483
Coupling dimers to CDT - conceptual issues
Lisa Glaser
(Submitted on 28 Jan 2013)
Causal dynamical triangulations allows for a non perturbative approach to quantum gravity. In this article a solution for dimers coupled to CDT is presented and some of the conceptual problems that arise are reflected upon.
3 pages. To appear in the Proceedings of the 13th Marcel Grossmann Meeting on General Relativity

brief mention:
http://arxiv.org/abs/1301.6440
The Preon Sector of the SL_q(2) (Knot) Model
Robert J. Finkelstein
(Submitted on 28 Jan 2013)
We describe a Lagrangian defining the preon sector of the knot model. The preons are the elements of the fundamental representation of SL_q(2). They exactly agree with the preons conjectured by Harari and Shupe. The coupling constants and masses required by this Lagrangian are in principle experimentally measurable...
26 Pages

 

[marcus]

http://arxiv.org/abs/1301.6795
Inhomogenous loop quantum cosmology with matter
Daniel Martín-de Blas, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 28 Jan 2013)
The linearly polarized Gowdy T³ model with a massless scalar field with the same symmetries as the metric is quantized by applying a hybrid approach. The homogeneous geometry degrees of freedom are loop quantized, fact which leads to the resolution of the cosmological singularity, while a Fock quantization is employed for both matter and gravitational inhomogeneities. Owing to the inclusion of the massless scalar field this system allows us to modelize flat Friedmann-Robertson-Walker cosmologies filled with inhomogeneities propagating in one direction. It provides a perfect scenario to study the quantum back-reaction between the inhomogeneities and the polymeric homogeneous and isotropic background.
4 pages, for the proceedings of the Loops 11-Madrid conference.

http://arxiv.org/abs/1301.7466
Report on the session QG4 of the 13th Marcel Grossmann Meeting
Jorge Pullin, Parampreet Singh
(Submitted on 30 Jan 2013)
We summarize the talks presented at the QG4 session (loop quantum gravity: cosmology and black holes) of the 13th Marcel Grossmann Meeting held in Stockholm, Sweden.
5 pages, to appear in the proceedings

http://arxiv.org/abs/1301.7688
Shape Dynamics and Gauge-Gravity Duality
Henrique Gomes, Tim Koslowski
(Submitted on 31 Jan 2013)
The dynamics of gravity can be described by two different systems. The first is the familiar spacetime picture of General Relativity, the other is the conformal picture of Shape Dynamics. We argue that the bulk equivalence of General Relativity and Shape Dynamics is a natural setting to discuss familiar bulk/boundary dualities. We discuss consequences of the Shape Dynamics description of gravity as well as the issue why the bulk equivalence is not explicitly seen in the General Relativity description of gravity.
4 pages, contribution to the 13th Marcel Grossmann Meeting

brief mention:
http://arxiv.org/abs/1301.7750
Quantization maps, algebra representation and non-commutative Fourier transform for Lie groups
Carlos Guedes, Daniele Oriti, Matti Raasakka
(Submitted on 31 Jan 2013)

 

[marcus]

http://arxiv.org/abs/1302.0254
The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 1 Feb 2013)
Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. Specifically, for a large class of initial conditions at the bounce, we are led to a self consistent extension of the inflationary paradigm over the 11 orders of magnitude in density and curvature, from the big bounce to the onset of slow roll. In addition, for a narrow window of initial conditions, there are departures from the standard paradigm, with novel effects ---such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities--- which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
64 pages, 15 figures

http://arxiv.org/abs/1302.0168
Warm inflation in loop quantum cosmology: a model with a general dissipative coefficient
Xiao-Min Zhang, Jian-Yang Zhu
(Submitted on 1 Feb 2013)
A general form of warm inflation with the dissipative coefficient Γ = Γ₀ (φ/φ₀)ⁿ (T/τ₀)^m in loop quantum cosmology is studied. In this case, we obtain conditions for the existence of a warm inflationary attractor in the context of loop quantum cosmology by using the method of stability analysis. The two cases when the dissipative coefficient is independent (m=0) and dependent (m≠0) on temperature are analyzed specifically. In the latter case, we use the new power spectrum which should be used when considering temperature dependence in the dissipative coefficient. We find that the thermal effect is enhanced in the case m>0. As in the standard inflation in loop quantum cosmology, we also reach the conclusion that quantum effect leaves a tiny imprint on the cosmic microwave background (CMB) sky.
12 pages, accepted for publication in Phys. Rev. D

http://arxiv.org/abs/1212.5226
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
G. Hinshaw, D. Larson, E. Komatsu, D. N. Spergel, C. L. Bennett, J. Dunkley, M. R. Nolta, M. Halpern, R. S. Hill, N. Odegard, L. Page, K. M. Smith, J. L. Weiland, B. Gold, N. Jarosik, A. Kogut, M. Limon, S. S. Meyer, G. S. Tucker, E. Wollack, E. L. Wright
(Submitted on 20 Dec 2012 (v1), last revised 30 Jan 2013 (this version, v2))
We present cosmological parameter constraints based on the final nine-year WMAP data, in conjunction with additional cosmological data sets. The WMAP data alone, and in combination, continue to be remarkably well fit by a six-parameter LCDM model. When WMAP data are combined with measurements of the high-l CMB anisotropy, the BAO scale, and the Hubble constant, the densities, Ω_bh², Ω_ch², and Ω_Λ, are each determined to a precision of ~1.5%. The amplitude of the primordial spectrum is measured to within 3%, and there is now evidence for a tilt in the primordial spectrum at the 5σ level, confirming the first detection of tilt based on the five-year WMAP data. At the end of the WMAP mission, the nine-year data decrease the allowable volume of the six-dimensional LCDM parameter space by a factor of 68,000 relative to pre-WMAP measurements. We investigate a number of data combinations and show that their LCDM parameter fits are consistent. New limits on deviations from the six-parameter model are presented, for example: the fractional contribution of tensor modes is limited to r<0.13 (95% CL); the spatial curvature parameter is limited to -0.0027 (+0.0039/-0.0038); the summed mass of neutrinos is Ʃm_v< 0.44 eV (95% CL); and the number of relativistic species is found to be 3.84±0.40 when the full data are analyzed. The joint constraint on N_eff and the primordial helium abundance agrees with the prediction of standard Big Bang nucleosynthesis. We compare recent PLANCK measurements of the Sunyaev-Zel'dovich effect with our seven-year measurements, and show their mutual agreement. Our analysis of the polarization pattern around temperature extrema is updated. This confirms a fundamental prediction of the standard cosmological model and provides a striking illustration of acoustic oscillations and adiabatic initial conditions in the early universe.
31 pages, 12 figures
For enlightening comment on the latest WMAP estimates see http://resonaances.blogspot.com/2013/01/how-many-neutrinos-in-sky.html

 

[marcus]

http://arxiv.org/abs/1302.0724
Death and resurrection of the zeroth principle of thermodynamics
Hal M. Haggard, Carlo Rovelli
(Submitted on 4 Feb 2013)
The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
5 pages, 2 figures

brief mention:
http://arxiv.org/abs/1302.0451
Macroscopic superpositions and black hole unitarity
Stephen D.H. Hsu
(Submitted on 3 Feb 2013)
We discuss the black hole information problem, including the recent claim that unitarity requires a horizon firewall, emphasizing the role of decoherence and macroscopic superpositions. We consider the formation and evaporation of a large black hole as a quantum amplitude, and note that during intermediate stages (e.g., after the Page time), the amplitude is a superposition of macroscopically distinct (and decohered) spacetimes, with the black hole itself in different positions on different branches. Small but semiclassical observers (who are themselves part of the quantum amplitude) that fall into the hole on one branch will miss it entirely on other branches and instead reach future infinity. This observation can reconcile the subjective experience of an infalling observer with unitarity. We also discuss implications for the nice slice formulation of the information problem, and to complementarity.
3 pages, 1 figure.

 

[MTd2]

http://arxiv.org/abs/1302.1357

A consistent Horava gravity without extra modes and equivalent to general relativity at the linearized level

J. Bellorin, A. Restuccia, A. Sotomayor
(Submitted on 6 Feb 2013)
We consider a Horava theory that has a consistent structure of constraints and propagates two physical degrees of freedom. The Lagrangian includes the terms of Blas, Pujolas ans Sibiryakov. The theory can be obtained from the general Horava's formulation by setting lambda = 1/3. This value of lambda is protected in the quantum formulation of the theory by the presence of a constraint. The theory has two second-class constraints that are absent for other values of lambda. They remove the extra scalar mode. There is no strong-coupling problem in this theory since there is no extra mode. We perform explicit computations on a model that put together a z=1 term and the IR effective action. We also show that the lowest-order perturbative version of the IR effective theory has a dynamics identical to the one of linearized general relativity. Therefore, this theory is smoothly recovered at the deepest IR without discontinuities in the physical degrees of freedom.

 

[marcus]

http://arxiv.org/abs/1302.1245
Dynamical behaviors of FRW Universe containing a positive/negative potential scalar field in loop quantum cosmology
Xiao Liu, Kui Xiao, Jian-Yang Zhu
(Submitted on 6 Feb 2013)
The dynamical behaviors of FRW Universe containing a posivive/negative potential scalar field in loop quantum cosmology scenario are discussed. The method of the phase-plane analysis is used to investigate the stability of the Universe. It is found that the stability properties in this situation are quite different from the classical cosmology case. For a positive potential scalar field coupled with a barotropic fluid, the cosmological autonomous system has five fixed points and one of them is stable if the adiabatic index γ satisfies 0<γ<2. This leads to the fact that the universe just have one bounce point instead of the singularity which lies in the quantum dominated area and it is caused by the quantum geometry effect. There are four fixed points if one considers a scalar field with a negative potential, but none of them is stable. Therefore, the universe has two kinds of bounce points, one is caused by the quantum geometry effect and the other is caused by the negative potential, the Universe may enter a classical re-collapse after the quantum bounce. This hints that the spatially flat FRW Universe containing a negative potential scalar field is cyclic.
6 pages, 2 figures, accepted for publication in General Relativity and Gravitation

brief mention:
http://arxiv.org/abs/1302.1312
Fixed Functionals in Asymptotically Safe Gravity
Maximilian Demmel, Frank Saueressig, Omar Zanusso
(Submitted on 6 Feb 2013)
We summarize the status of constructing fixed functionals within the f(R)-truncation of Quantum Einstein Gravity in three spacetime dimensions. Focusing on curvatures much larger than the IR-cutoff scale, it is shown that the fixed point equation admits three different scaling regimes: for classical and quantum dominance the equation becomes linear and has power-law solutions, while the balanced case gives rise to a generalized homogeneous equation whose order is reduced by one and whose solutions are non-analytical.
4 pages, to appear in Proceedings of the Thirteenth Marcel Grossman Meeting on General Relativity

http://arxiv.org/abs/1302.1206
Thermality and Heat Content of horizons from infinitesimal coordinate transformations
Bibhas Ranjan Majhi, T. Padmanabhan
(Submitted on 5 Feb 2013)

http://arxiv.org/abs/1302.1498
"The Waters I am Entering No One yet Has Crossed": Alexander Friedman and the Origins of Modern Cosmology
Ari Belenkiy
(Submitted on 6 Feb 2013)
Ninety years ago, in 1922, Alexander Friedman (1888-1925) demonstrated for the first time that the General Relativity equations admit non-static solutions and thus the Universe may expand, contract, collapse, and even be born. The fundamental equations he derived still provide the basis for the current cosmological theories of the Big Bang and the Accelerating Universe. Later, in 1924, he was the first to realize that General Relativity allows the Universe to be infinite. Friedman's ideas initially met strong resistance from Einstein, yet from 1931 he became their staunchest supporter. This essay connects Friedman's cosmological ideas with the 1998-2004 results of the astronomical observations that led to the 2011 Nobel Prize in Physics. It also describes Friedman's little known topological ideas of how to check General Relativity in practice and compares his contributions to those of Georges Lemaitre. Recently discovered corpus of Friedman's writings in the Ehrenfest Archives at Leiden University sheds some new light on the circumstances surrounding his 1922 work and his relations with Paul Ehrenfest.
26 pages, 11 figures. Accepted for publication in the proceedings of the conference "Origins of the Expanding Universe: 1912-1932", M. J. Way & D. Hunter, eds., ASP Conf. Ser., Vol. 471 in press

 

[marcus]

http://arxiv.org/abs/1302.1781
Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
Aldo Riello
(Submitted on 7 Feb 2013)
We calculate the most divergent contribution to the self-energy (or "melonic") graph in the context of the Lorentzian EPRL-FK Spin Foam model of Quantum Gravity. We find that such a contribution is logarithmically divergent in the cut-off over the SU(2)-representation spins when one chooses the face amplitude guaranteeing the face-splitting invariance of the foam. We also find that the dependence on the boundary data is different from that of the bare propagator. This fact has its origin in the non-commutativity of the EPRL-FK Y-map with the projector onto SL(2,C)-invariant states. In the course of the paper, we discuss in detail the approximations used during the calculations, its geometrical interpretation as well as the physical consequences of our result.
55 pages, 8 figures

http://arxiv.org/abs/1302.1841
Cosmological Parameters from Pre-Planck CMB Measurements
Erminia Calabrese, Renée A. Hlozek, Nick Battaglia, Elia S. Battistelli, J. Richard Bond, Jens Chluba, Devin Crichton, Sudeep Das, Mark J. Devlin, Joanna Dunkley, Rolando Dünner, Marzieh Farhang, Megan B. Gralla, Amir Hajian, Mark Halpern, Matthew Hasselfield, Adam D. Hincks, Kent D. Irwin, Arthur Kosowsky, Thibaut Louis, Tobias A. Marriage, Kavilan Moodley, Laura Newburgh, Michael D. Niemack, Mike R. Nolta, Lyman A. Page, Neelima Sehgal, Blake D. Sherwin, Jonathan L. Sievers, Cristóbal Sifón, David N. Spergel, Suzanne T. Staggs, Eric R. Switzer, Ed Wollack
(Submitted on 7 Feb 2013)
Recent data from the WMAP, ACT and SPT experiments provide precise measurements of the cosmic microwave background temperature power spectrum over a wide range of angular scales. The combination of these observations is well fit by the standard, spatially flat LCDM cosmological model, constraining six free parameters to within a few percent. The scalar spectral index, n_s = 0.9678 ± 0.0088, is less than unity at the 3.6 sigma level, consistent with simple models of inflation. The damping tail of the power spectrum at high resolution, combined with the amplitude of gravitational lensing measured by ACT and SPT, constrains the effective number of relativistic species to be N_eff = 3.24 ± 0.39, in agreement with the standard model's three species of light neutrinos.
5 pages, 4 figures
There is a slight inconsistency with the range of N_eff given in a similar paper by some of the same people a couple of days ago. See post #1893 about WMAP9 paper http://arxiv.org/abs/1212.5226 . See page 17, and Table 7: N_eff = 3.84 ± 0.40 (with all relevant data sets combined).

brief mention:
http://arxiv.org/abs/1302.1617
What if Planck's Universe isn't flat?
Philip Bull, Marc Kamionkowski
(Submitted on 6 Feb 2013)

http://arxiv.org/abs/1302.1860
On cosmic hair and "de Sitter breaking" in linearized quantum gravity
Ian A. Morrison
(Submitted on 7 Feb 2013)

 

[marcus]

http://arxiv.org/abs/1302.2173
Quantum Gravity via Causal Dynamical Triangulations
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 8 Feb 2013)
"Causal Dynamical Triangulations" (CDT) represent a lattice regularization of the sum over spacetime histories, providing us with a non-perturbative formulation of quantum gravity. The ultraviolet fixed points of the lattice theory can be used to define a continuum quantum field theory, potentially making contact with quantum gravity defined via asymptotic safety. We describe the formalism of CDT, its phase diagram, and the quantum geometries emerging from it. We also argue that the formalism should be able to describe a more general class of quantum-gravitational models of Horava-Lifgarbagez type.
31 pages. To appear in "Handbook of Spacetime", Springer Verlag.

http://arxiv.org/abs/1302.2181
Quantum Spacetime, from a Practitioner's Point of View
J. Ambjorn, S. Jordan, J. Jurkiewicz, R. Loll
(Submitted on 9 Feb 2013)
We argue that theories of quantum gravity constructed with the help of (Causal) Dynamical Triangulations have given us the most informative, quantitative models to date of quantum spacetime. Most importantly, these are derived dynamically from nonperturbative and background-independent quantum theories of geometry. In the physically relevant case of four spacetime dimensions, the ansatz of Causal Dynamical Triangulations produces - from a fairly minimal set of quantum field-theoretic inputs - an emergent spacetime which macroscopically looks like a de Sitter universe, and on Planckian scales possesses unexpected quantum properties. Important in deriving these results are a regularized version of the theory, in which the quantum dynamics is well defined, can be studied with the help of numerical Monte Carlo methods and extrapolated to infinite lattice volumes.
7 pages, 5 figures, submission to Multicosmofun '12, Szczecin.

http://arxiv.org/abs/1302.2210
The transfer matrix method in four-dimensional causal dynamical triangulations
J. Ambjorn, J. Gizbert-Studnicki, A.T. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 9 Feb 2013)
The Causal Dynamical Triangulation model of quantum gravity (CDT) is a proposition to evaluate the path integral over space-time geometries using a lattice regularization with a discrete proper time and geometries realized as simplicial manifolds. The model admits a Wick rotation to imaginary time for each space-time configuration. Using computer simulations we determined the phase structure of the model and discovered that it predicts a de Sitter phase with a four-dimensional spherical semi-classical background geometry. The model has a transfer matrix, relating spatial geometries at adjacent (discrete lattice) times. The transfer matrix uniquely determines the theory. We show that the measurements of the scale factor of the (CDT) universe are well described by an effective transfer matrix where the matrix elements are labelled only by the scale factor. Using computer simulations we determine the effective transfer matrix elements and show how they relate to an effective minisuperspace action at all scales.
6 pages, 6 figures, contribution to the MULTIVERSE conference, Szczecin, Poland, September 2012

brief mention:
http://arxiv.org/abs/1302.2440
Universality of 2d causal dynamical triangulations
J. Ambjorn, A. Ipsen
(Submitted on 11 Feb 2013)
The formalism of Causal Dynamical Triangulations (CDT) attempts to provide a non-perturbative regularization of quantum gravity, viewed as an ordinary quantum field theory. In two dimensions one can solve the lattice theory analytically and the continuum limit is universal, not depending on the details of the lattice regularization.
11 pages

http://arxiv.org/abs/1302.2285
Quantum Gravity: Meaning and Measurement
John Stachel, Kaća Bradonjić
(Submitted on 10 Feb 2013)
A discussion of the meaning of a physical concept cannot be separated from discussion of the conditions for its ideal measurement. We assert that quantization is no more than the invocation of the quantum of action in the explanation of some process or phenomenon, and does not imply an assertion of the fundamental nature of such a process. This leads to an ecumenical approach to the problem of quantization of the gravitational field. There can be many valid approaches,.. We advocate an approach to general relativity based on the unimodular group, which emphasizes the physical significance and measurability of the conformal and projective structures. ...
24 pages; Submitted to Studies in the History and Philosophy of Modern Physics special Quantum Gravity issue

 

[John86]

http://arxiv.org/abs/1302.2151
Lanczos-Lovelock models of gravity
T. Padmanabhan, D. Kothawala
(Submitted on 8 Feb 2013)
Lanczos-Lovelock models of gravity represent a natural and elegant generalization of Einstein's theory of gravity to higher dimensions. They are characterized by the fact that the field equations only contain up to second derivatives of the metric even though the action functional can be a quadratic or higher degree polynomial in the curvature tensor. Because these models share several key properties of Einstein's theory they serve as a useful set of candidate models for testing the emergent paradigm for gravity. This review highlights several geometrical and thermodynamical aspects of Lanczos-Lovelock models which have attracted recent attention.

http://arxiv.org/abs/1302.2336
Constraints of NonCommutative Spectral Action from Gravity Probe B
Gaetano Lambiase, Mairi Sakellariadou, Antonio Stabile
(Submitted on 10 Feb 2013)
Noncommutative spectral geometry offers a purely geometric explanation for the standard model of particle physics, including a geometric explanation for the origin of the Higgs field. Within this framework, gravity together with the electroweak and the strong forces are all described as purely gravitational forces on a unified noncommutative spacetime. In this letter, we infer a constraint on the parameter characterising the coupling constants at unification, by linearising the field equations in the limit of weak gravitational fields generated by a rotating gravitational source and by making use of the recent experimental data obtained by Gravity Probe B. We find a lower bound on the Weyl term appearing in the noncommutative spectral action, namely \beta > 1/ (10^6 m), which is much stronger than any limit imposed so far to curvature squared terms.

http://arxiv.org/abs/1302.2383
Surface gravities for non-Killing horizons
Bethan Cropp (SISSA/INFN), Stefano Liberati (SISSA/INFN), Matt Visser (Victoria University of Wellington)
(Submitted on 11 Feb 2013)
There are many logically and computationally distinct characterizations of the surface gravity of a horizon, just as there are many logically rather distinct notions of horizon. Fortunately, in standard general relativity, for stationary horizons, most of these characterizations are degenerate. However, in modified gravity, or in analogue spacetimes, horizons may be non-Killing or even non-null, and hence these degeneracies can be lifted. We present a brief overview of the key issues, specifically focusing on horizons in analogue spacetimes and universal horizons in modified gravity.

http://arxiv.org/abs/1302.2613
Nonviolent information transfer from black holes: a field theory parameterization
Steven B. Giddings
(Submitted on 11 Feb 2013)
A candidate parameterization is introduced, in an effective field theory framework, for the quantum information transfer from a black hole that is necessary to restore unitarity. This in particular allows description of the effects of such information transfer in the black hole atmosphere, for example seen by infalling observers. In the presence of such information transfer, it is shown that infalling observers need not experience untoward violence. Moreover, the presence of general moderate-frequency couplings to field modes with high angular momenta offers a mechanism to enhance information transfer rates, commensurate with the increased energy flux, when a string is introduced to "mine" a black hole. Generic such models for nonviolent information transfer predict extra energy flux from a black hole, beyond that of Hawking.

 

[marcus]

http://arxiv.org/abs/1302.2810
Four-dimensional Causal Dynamical Triangulations and an effective transfer matrix
Andrzej Görlich
(Submitted on 12 Feb 2013)
Causal Dynamical Triangulations is a background independent approach to quantum gravity. We show that there exists an effective transfer matrix labeled by the scale factor which properly describes the evolution of the quantum universe. In this framework no degrees of freedom are frozen, but, the obtained effective action agrees with the minisuperspace model.
Comments: To appear in the Proceedings of the 13th Marcel Grossmann Meeting on General

 

[John86]

http://arxiv.org/abs/1302.2849
Disappearance and emergence of space and time in quantum gravity
Daniele Oriti
(Submitted on 12 Feb 2013)
We discuss the hints for the disappearance of continuum space and time at microscopic scale. These include arguments for a discrete nature of them or for a fundamental non-locality, in a quantum theory of gravity. We discuss how these ideas are realized in specific quantum gravity approaches. Turning then the problem around, we consider the emergence of continuum space and time from the collective behaviour of discrete, pre-geometric atoms of quantum space, and for understanding spacetime as a kind of "condensate", and we present the case for this emergence process being the result of a phase transition, dubbed "geometrogenesis". We discuss some conceptual issues of this scenario and of the idea of emergent spacetime in general. As a concrete example, we outline the GFT framework for quantum gravity, and illustrate a tentative procedure for the emergence of spacetime in this framework. Last, we re-examine the conceptual issues raised by the emergent spacetime scenario in light of this concrete example.

http://arxiv.org/abs/1302.2850
The universal path integral
Seth Lloyd, Olaf Dreyer
(Submitted on 12 Feb 2013)
Path integrals represent a powerful route to quantization: they calculate probabilities by summing over classical configurations of variables such as fields, assigning each configuration a phase equal to the action of that configuration. This paper defines a universal path integral, which sums over all computable structures. This path integral contains as sub-integrals all possible computable path integrals, including those of field theory, the standard model of elementary particles, discrete models of quantum gravity, string theory, etc. The universal path integral possesses a well-defined measure that guarantees its finiteness, together with a method for extracting probabilities for observable quantities. The universal path integral supports a quantum theory of the universe in which the world that we see around us arises out of the interference between all computable structures.

http://arxiv.org/abs/1302.2687
Massive gravity as a limit of bimetric gravity
Prado Martin-Moruno (Victoria University of Wellington), Valentina Baccetti (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 12 Feb 2013)
Massive gravity may be viewed as a suitable limit of bimetric gravity. The limiting procedure can lead to an interesting interplay between the "background" and "foreground" metrics in a cosmological context. The fact that in bimetric theories one always has two sets of metric equations of motion continues to have an effect even in the massive gravity limit. Thus, solutions of bimetric gravity in the limit of vanishing kinetic term are also solutions of massive gravity, but the contrary statement is not necessarily true.

http://arxiv.org/abs/1302.2731
Quantum correlations which imply causation
Joseph Fitzsimons, Jonathan Jones, Vlatko Vedral
(Submitted on 12 Feb 2013)
In ordinary, non-relativistic, quantum physics, time enters only as a parameter and not as an observable: a state of a physical system is specified at a given time and then evolved according to the prescribed dynamics. While the state can, and usually does, extend across all space, it is only defined at one instant of time, in conflict with special relativity where space and time are treated on an equal footing. Here we ask what would happen if we defined the notion of the quantum density matrix for multiple spatial and temporal measurements. We introduce the concept of a pseudo-density matrix which treats space and time indiscriminately. This matrix in general fails to be positive for timelike separated measurements, motivating us to define a measure of causality that discriminates between spacelike and timelike correlations. Important properties of this measure, such as monotonicity under local operations, are proved. Two qubit NMR experiments are presented that illustrate how a temporal pseudo-density matrix approaches a genuinely allowed density matrix as the amount of decoherence is increased between two consecutive measurements.

 

[John86]

http://arxiv.org/abs/1302.2928
Modulated Ground State of Gravity Theories with Stabilized Conformal Factor
Alfio Bonanno, Martin Reuter
(Submitted on 12 Feb 2013)
We discuss the stabilization of the conformal factor by higher derivative terms in a conformally reduced $R+R^2$ Euclidean gravity theory. The flat spacetime is unstable towards the condensation of modes with nonzero momentum, and they "condense" in a modulated phase above a critical value of the coupling $\beta$ of the $R^2$ term. By employing a combination of variational, numerical and lattice methods we show that in the semiclassical limit the corresponding functional integral is dominated by a single nonlinear plane wave of frequency $\approx 1/\sqrt{\beta} \lp$. We argue that the ground state of the theory is characterized by a spontaneous breaking of translational invariance at Planckian scales.

 

[marcus]

http://arxiv.org/abs/1302.3226
Solution to the cosmological constant problem
T. Padmanabhan, Hamsa Padmanabhan
(Submitted on 13 Feb 2013)
The current, accelerated, phase of expansion of our universe can be modeled in terms of a cosmological constant. A key issue in theoretical physics is to explain the extremely small value of the dimensionless parameter Λ L_P² ~ 3.4 x 10^-122, where L_P is the Planck length. We show that this value can be understood in terms of a new dimensionless parameter N, which counts the number of modes inside a Hubble volume crossing the Hubble radius, from the end of inflation until the beginning of the accelerating phase. Theoretical considerations suggest that N = 4π. On the other hand, N is related to ln(ΛL_P²) and two other parameters which will be determined by high energy particle physics: (a) the ratio between the number densities of photons and matter and (b) the energy scale of inflation. For realistic values of (n_γ/n_m) ~ 4.3 x 10¹⁰ and E_inf ~ 10¹⁵ GeV, our postulate N =4π leads to the observed value of the cosmological constant. This provides a unified picture of cosmic evolution relating the early inflationary phase to the late accelerating phase.
15 pages; 2 figures

 

[MTd2]

http://arxiv.org/abs/1302.3406

Spontaneous Lorentz Violation in Gauge Theories

A. P. Balachandran, S. Vaidya
(Submitted on 14 Feb 2013)
Frohlich, Morchio and Strocchi long ago proved that Lorentz invariance is spontaneously broken in QED because of infrared effects. We develop a simple model where consequences of this breakdown can be explicitly and easily calculated. For this purpose, the superselected U(1) charge group of QED is extended to a superselected "Sky" group containing direction-dependent gauge transformations at infinity. It is the analog of the Spi group of gravity. As Lorentz transformations do not commute with Sky, they are spontaneously broken. These abelian considerations and model are extended to non-Abelian gauge symmetries. Basic issues regarding the observability of twisted non-Abelian gauge symmetries and of the asymptotic ADM symmetries of quantum gravity are raised.

 

[marcus]

http://arxiv.org/abs/1302.3833
Loop Quantum Cosmology
Ivan Agullo, Alejandro Corichi
(Submitted on 15 Feb 2013)
This Chapter provides an up to date, pedagogical review of some of the most relevant advances in loop quantum cosmology. We review the quantization of homogeneous cosmological models, their singularity resolution and the formulation of effective equations that incorporate the main quantum corrections to the dynamics. We also summarize the theory of quantized metric perturbations propagating in those quantum backgrounds. Finally, we describe how this framework can be applied to obtain a self-consistent extension of the inflationary scenario to incorporate quantum aspects of gravity, and to explore possible phenomenological consequences.
52 pages, 5 figures. To appear as a Chapter of "The Springer Handbook of Spacetime," edited by A. Ashtekar and V. Petkov. (Springer-Verlag, at Press).

 

[John86]

http://arxiv.org/abs/1302.1496
Standard Model Higgs field and energy scale of gravity
F.R. Klinkhamer
(Submitted on 6 Feb 2013 (v1), last revised 14 Feb 2013 (this version, v3))
The effective potential of the Higgs scalar field in the Standard Model may have a second degenerate minimum at an ultrahigh vacuum expectation value. This second minimum then determines, by radiative corrections, the values of the top-quark and Higgs-boson masses at the standard minimum corresponding to the electroweak energy scale. An argument is presented that this ultrahigh vacuum expectation value is proportional to the energy scale of gravity, E_{Planck} \equiv \sqrt{\hbar c^5/G_N}, considered to be characteristic of a spacetime foam. In the context of a simple model, the existence of kink-type wormhole solutions places a lower bound on the ultrahigh vacuum expectation value and this lower bound is of the order of E_{Planck}.

http://arxiv.org/abs/1302.3680
Quantum Gravity on a Quantum Computer?
Achim Kempf
(Submitted on 15 Feb 2013)
EPR-type measurements on spatially separated entangled spin qubits allow one, in principle, to detect curvature. Also the entanglement of the vacuum state is affected by curvature. Here, we ask if the curvature of spacetime can be expressed entirely in terms of the spatial entanglement structure of the vacuum. This would open up the prospect that quantum gravity could be simulated on a quantum computer and that quantum information techniques could be fully employed in the study of quantum gravity.

http://arxiv.org/abs/1302.3648
Causality and non-equilibrium second-order phase transitions in inhomogeneous systems
A. del Campo, T. W. B. Kibble, W. H. Zurek
(Submitted on 14 Feb 2013)
When a second-order phase transition is crossed at fine rate, the evolution of the system stops being adiabatic as a result of the critical slowing down in the neighborhood of the critical point. In systems with a topologically nontrivial vacuum manifold, disparate local choices of the ground state lead to the formation of topological defects. The universality class of the transition imprints a signature on the resulting density of topological defects: It obeys a power law in the quench rate, with an exponent dictated by a combination of the critical exponents of the transition. In inhomogeneous systems the situation is more complicated, as the spontaneous symmetry breaking competes with bias caused by the influence of the nearby regions that already chose the new vacuum. As a result, the choice of the broken symmetry vacuum may be inherited from the neighboring regions that have already entered the new phase. This competition between the inherited and spontaneous symmetry breaking enhances the role of causality, as the defect formation is restricted to a fraction of the system where the front velocity surpasses the relevant sound velocity and phase transition remains effectively homogeneous. As a consequence, the overall number of topological defects can be substantially suppressed. When the fraction of the system is small, the resulting total number of defects is still given by a power law related to the universality class of the transition, but exhibits a more pronounced dependence on the quench rate. This enhanced dependence complicates the analysis but may also facilitate experimental test of defect formation theories.

 

[marcus]

http://arxiv.org/abs/1302.5265
The loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 21 Feb 2013)
We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can construct explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. New observables living in the bulk appear at the quantum level (analogous to spin in quantum mechanics) that are not present at the classical level and are associated with the discrete nature of the spin network states of loop quantum gravity. The resulting quantum space-times resolve the singularity present in the classical theory inside black holes. The new observables that arise suggest a possible resolution for the "firewall" problem of evaporating black holes.
Comments: 4 pages,

 

[MTd2]

http://arxiv.org/abs/1302.5273

There exist no 4-dimensional geodesically equivalent metrics with the same stress-energy tensor

Volodymir Kiosak, Vladimir S. Matveev
(Submitted on 21 Feb 2013)
We show that if two 4-dimensional metrics of arbitrary signature on one manifold are geodesically equivalent (i.e., have the same geodesics considered as unparameterized curves) and are solutions of the Einstein field equation with the same stress-energy tensor, then they are affinely equivalent or flat. Under the additional assumption that the metrics are complete or the manifold is closed, the result survives in all dimensions >2.

http://arxiv.org/abs/1302.5162

On CCC-predicted concentric low-variance circles in the CMB sky

V. G. Gurzadyan, R. Penrose
(Submitted on 21 Feb 2013)
A new analysis of the CMB, using WMAP data, supports earlier indications of non-Gaussian features of concentric circles of low temperature variance. Conformal cyclic cosmology (CCC) predicts such features from supermassive black-hole encounters in an aeon preceding our Big Bang. The significance of individual low-variance circles in the true data has been disputed; yet a recent independent analysis has confirmed CCC's expectation that CMB circles have a non-Gaussian temperature distribution. Here we examine concentric sets of low-variance circular rings in the WMAP data, finding a highly non-isotropic distribution. A new "sky-twist" procedure, directly analysing WMAP data, without appeal to simulations, shows that the prevalence of these concentric sets depends on the rings being circular, rather than even slightly elliptical, numbers dropping off dramatically with increasing ellipticity. This is consistent with CCC's expectations; so also is the crucial fact that whereas some of the rings' radii are found to reach around $15^\circ$, none exceed $20^\circ$. The non-isotropic distribution of the concentric sets may be linked to previously known anomalous and non-Gaussian CMB features.

 

[marcus]

http://arxiv.org/abs/1302.5695
Quantum matter in quantum space-time
Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, Casey Tomlin
(Submitted on 22 Feb 2013)
Quantum matter in quantum space-time is discussed using general properties of energy-conservation laws. As a rather radical conclusion, it is found that standard methods of differential geometry and quantum field theory on curved space-time are inapplicable in canonical quantum gravity, even at the level of effective equations.
16 pages

http://arxiv.org/abs/1302.5564
Spin-cube Models of Quantum Gravity
A. Mikovic
(Submitted on 22 Feb 2013)
We study the state-sum models of quantum gravity based on a representation 2-category of the Poincare 2-group. We call them spin-cube models, since they are categorical generalizations of spin-foam models. A spin-cube state sum can be considered as a path integral for a constrained 2-BF theory, and depending on how the constraints are imposed, a spin-cube state sum can be reduced to a path integral for the area-Regge model with the edge-length constraints, or to a path integral for the Regge model. We also show that the effective actions for these spin-cube models have the correct classical limit.
16 pages

brief mention (Shapo is always interesting):
http://arxiv.org/abs/1302.5619
Spontaneously Broken Conformal Symmetry: Dealing with the Trace Anomaly
Roberta Armillis, Alexander Monin, Mikhail Shaposhnikov
(Submitted on 22 Feb 2013)
The majority of renormalizable field theories possessing the scale invariance at the classical level exhibits the trace anomaly once quantum corrections are taken into account. This leads to the breaking of scale and conformal invariance. At the same time any realistic theory must contain gravity and is thus non-renormalizable. We show that discarding the renormalizability it is possible to construct viable models allowing to preserve the scale invariance at the quantum level. We present explicit one-loop computations for two toy models to demonstrate the main idea of the approach. Constructing the renormalized energy momentum tensor we show that it is traceless, meaning that the conformal invariance is also preserved.
20 pages, 5 figures