Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1201.4151
Group field cosmology: a cosmological field theory of quantum geometry
Gianluca Calcagni, Steffen Gielen, Daniele Oriti
(Submitted on 19 Jan 2012)
Following the idea of a field quantization of gravity as realized in group field theory, we construct a minisuperspace model where the wavefunction of canonical quantum cosmology (either Wheeler-DeWitt or loop quantum cosmology) is promoted to a field, the coordinates are minisuperspace variables, the kinetic operator is the Hamiltonian constraint operator, and the action features a nonlinear and possibly nonlocal interaction term. We discuss free-field classical solutions, the quantum propagator, and a mean-field approximation linearizing the equation of motion and augmenting the Hamiltonian constraint by an effective term mixing gravitational and matter variables. Depending on the choice of interaction, this can reproduce, for example, a cosmological constant, a scalar-field potential, or a curvature contribution.
14 pages, 2 figures

http://arxiv.org/abs/1201.3969
Breaking the uniqueness of the Shape Dynamics Hamiltonian
Henrique Gomes
(Submitted on 19 Jan 2012)
In Gomes 2011, a linear method of solving a particular set of Lichnerowicz-type equations through the implicit function theorem was sketched in order to implicitly construct Shape Dynamics' global Hamiltonian and eliminate second class constraints. This method was completely laid out, and in Gomes 2011 it was used for extending Shape Dynamics (SD) to the non-vacuum case, showing how other fields are coupled to the theory. In the latter paper it was noticed that, unlike the vacuum case, the use of such methods yielded puzzling bounds on the density of some types of fields. Here we show that the original SD cannot be extended beyond such bounds, but that a slight modification of the original can withstand any type of coupling. When the bound is broken, the theory does not come equipped with a single Hamiltonian as in vacuum SD, but with a finite set of weakly commuting Hamiltonians, which we describe.
14 pages

http://arxiv.org/abs/1110.6350
Evolution of Primordial Black Holes in Loop Quantum Gravity
Debabrata Dwivedee, Bibekananda Nayak, Mubasher Jamil, Lambodar Prasad Singh
(Submitted on 28 Oct 2011 (v1), last revised 24 Nov 2011 (this version, v2))
In this work, we study the evolution of Primordial Black Holes within the context of Loop Quantum Gravity. First we calculate the scale factor and energy density of the universe for different cosmic era and then taking these as inputs we study evolution of primordial black holes. From our estimation it is found that accretion of radiation does not affect evolution of primordial black holes in loop quantum gravity even though a larger number of primordial black holes may form in early universe in comparison with Einstein's or scalar-tensor theories.
8 pages, 1 figure

brief mention:
http://arxiv.org/abs/1201.4147
Are OPERA neutrinos faster than light because of non-inertial reference frames?
Claudio Germana
(Submitted on 19 Jan 2012)
Recent results from the OPERA experiment reported a neutrino beam traveling faster than light. The experiment measured the neutrino time of flight (TOF) over a baseline from the CERN to the Gran Sasso site. The neutrino beam arrives 60 ns earlier than a light ray would do. Because the result has an enormous impact on science, it might be worth double-checking the time definitions with respect to the non-inertial system in which the neutrino travel time was measured. Potential problems in the OPERA data analysis connected with the definition of the reference frame and time synchronization are emphasized. We aim to investigate the synchronization of non-inertial clocks on Earth by relating this time to the proper time of an inertial observer at Solar System Barycenter(SSB). The Tempo2 software was used to time-stamp events observed on the geoid with respect to the SSB inertial observer time. Neutrino results from OPERA might carry the fingerprint of non-inertial effects. The CERN-Gran Sasso clock synchronization is accomplished by applying corrections that depend on special and general relativistic time dilation effects at the clocks, depending on the position of the clocks in the solar system gravitational well. As a consequence, TOF distributions are centered on values shorter by tens of ns than expected, integrating over a period from April to December, longer if otherwise. It is worth remarking that the OPERA runs have always been carried out from April/May to November. If the analysis by Tempo2 holds for the OPERA experiment, the excellent measurement by the OPERA collaboration will turn into a proof of the General Relativity theory in a weak field approximation. The analysis presented here is falsifiable because it predicts that performing the experiment from January to March/April, the neutrino beam will be detected to arrive 50 ns later than light.
5 pages, 4 figures, accepted for publication in Astronomy and Astrophysics Letters

 

[marcus]

http://arxiv.org/abs/1201.4247
On the relations between gravity and BF theories
Laurent Freidel, Simone Speziale
(Submitted on 20 Jan 2012)
We review, in the light of recent developments, the existing relations between gravity and topological BF theories at the classical level. We include the Plebanski action in both self-dual and non-chiral formulations, their generalizations, and the MacDowell-Mansouri action.
16 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

http://arxiv.org/abs/1201.4229
Revisiting the quantum scalar field in spherically symmetric quantum gravity
Enrique F. Borja, Iñaki Garay, Eckhard Strobel
(Submitted on 20 Jan 2012)
We extend previous results in spherically symmetric gravitational systems coupled with a massless scalar field within the loop quantum gravity framework. As starting point, we take the Schwarzschild spacetime. The results presented here rely on the uniform discretization method. We are able to minimize the associated discrete master constraint using a variational method. The trial state for the vacuum consists of a direct product of a Fock vacuum for the matter part and a Gaussian centered around the classical Schwarzschild solution. This paper follows the line of research presented by Gambini, Pullin and Rastgoo and a comparison between their result and the one given in this work is made.
16 pages

http://arxiv.org/abs/1201.4226
Gravitational four-fermion interaction on the Planck scale
I.B. Khriplovich
(Submitted on 20 Jan 2012)
The four-fermion gravitational interaction is induced by torsion, and gets essential on the Planck scale. On this scale, the axial-axial contribution dominates strongly in the discussed interaction. The energy-momentum tensor, generated by this contribution, is analyzed, as well as stability of the problem with respect to compression. The trace of this energy-momentum tensor can be negative.
4 pages

 

[marcus]

http://arxiv.org/abs/1201.4598
Introduction to Loop Quantum Gravity
Abhay Ashtekar
(Submitted on 22 Jan 2012)
This article is based on the opening lecture at the third quantum geometry and quantum gravity school sponsored by the European Science Foundation and held at Zakopane, Poland in March 2011. The goal of the lecture was to present a broad perspective on loop quantum gravity for young researchers. The first part is addressed to beginning students and the second to young researchers who are already working in quantum gravity.
30 pages, 2 figures.

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya
(Submitted on 23 Jan 2012)
We numerically investigate the solutions to the effective equations of the Bianchi II model within the "improved" Loop Quantum Cosmology (LQC) dynamics. The matter source is a massless scalar field. We perform a systematic study of the space of solutions, and focus on the behavior of several geometrical observables. We show that the big-bang singularity is replaced by a bounce and the point-like singularities do not saturate the energy density bound. There are up to five directional bounces in the scale factors, one global bounce in the expansion, the shear presents up to four local maxima and can be zero at the bounce. This allows for solutions with density larger than the maximal density for the isotropic and Bianchi I cases. The asymptotic behavior is shown to behave like that of a Bianchi I model, and the effective solutions connect anisotropic solutions even when the shear is zero at the bounce. All known facts of Bianchi I are reproduced. In the "vacuum limit", solutions are such that almost all the dynamics is due to the anisotropies. Since Bianchi II plays an important role in the Bianchi IX model and the the Belinskii, Khalatnikov, Lifgarbagez (BKL) conjecture, our results can provide an intuitive understanding of the behavior in the vicinity of general space-like singularities.
23 pages, 26 figures

http://arxiv.org/abs/1201.4543
Aspects of Quantum Gravity in Cosmology
Massimiliano Rinaldi
(Submitted on 22 Jan 2012)
We review some aspects of quantum gravity in the context of cosmology. In particular, we focus on models with a phenomenology accessible to current and near-future observations, as the early Universe might be our only chance to peep through the quantum gravity realm.
15 pages, 1 figure. Invited review for Modern Physics Letter A

 

[marcus]

http://arxiv.org/abs/1201.4874
Divergences in spinfoam quantum gravity
Matteo Smerlak
(Submitted on 23 Jan 2012)
In this thesis we study the flat model, the main buidling block for the spinfoam approach to quantum gravity, with an emphasis on its divergences. Besides a personal introduction to the problem of quantum gravity, the manuscript consists in two part. In the first one, we establish an exact powercounting formula for the bubble divergences of the flat model, using tools from discrete gauge theory and twisted cohomology. In the second one, we address the issue of spinfoam continuum limit, both from the lattice field theory and the group field theory perspectives. In particular, we put forward a new proof of the Borel summability of the Boulatov-Freidel-Louapre model, with an improved control over the large-spin scaling behaviour. We conclude with an outlook of the renormalization program in spinfoam quantum gravity.
113 pages. PhD thesis, introduction and conclusion in French, main text in English.

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak
(Submitted on 24 Jan 2012)
The spinfoam approach to quantum gravity rests on a "quantization" of BF theory using 2-complexes and group representations. We explain why, in dimension three and higher, this "spinfoam quantization" must be amended to be made consistent with the gauge symmetries of discrete BF theory. We discuss a suitable generalization, called "cellular quantization", which (1) is finite, (2) produces a topological invariant, (3) matches with the properties of the continuum BF theory, (4) corresponds to its loop quantization. These results significantly clarify the foundations - and limitations - of the spinfoam formalism, and open the path to understanding, in a discrete setting, the symmetry-breaking which reduces BF theory to gravity.
6 pages

 

[marcus]

http://arxiv.org/abs/1201.5423
Dirac fields and Barbero-Immirzi parameter in Cosmology
G. de Berredo-Peixoto, L. Freidel, I.L. Shapiro, C.A. de Souza
(Submitted on 26 Jan 2012)
We consider cosmological solution for Einstein gravity with massive fermions with a four-fermion coupling, which emerges from the Holst action and is related to the Barbero-Immirzi (BI) parameter. This gravitational action is an important object of investigation in a non-perturbative formalism of quantum gravity. We study the equation of motion for for the Dirac field within the standard Friedman-Robertson-Walker (FRW) metric. Finally, we show the theory with BI parameter and minimally coupling Dirac field, in the zero mass limit, is equivalent to an additional term which looks like a perfect fluid with the equation of state p = wρ, with w = 1 which is independent of the BI parameter. The existence of mass imposes a variable w, which creates either an inflationary phase with w=-1, or assumes an ultra hard equation of states w = 1 for very early universe. Both phases relax to a pressureless fluid w = 0 for late universe (corresponding to the limit m→∞).
16 pages

http://arxiv.org/abs/1201.5470
New tools for Loop Quantum Gravity with applications to a simple model
Enrique F. Borja, Jacobo Díaz-Polo, Laurent Freidel, Iñaki Garay, Etera R. Livine
(Submitted on 26 Jan 2012)
Loop Quantum Gravity is now a well established approach to quantum gravity. One of the main challenges still faced by the theory is constructing a consistent dynamics which would lead back to the standard dynamics of the gravitational field at large scales. Here we will review the recent U(N) framework for Loop Quantum Gravity and the new spinor representation (that provides a classical setting for the U(N) framework). Then, we will apply these techniques to a simple model in order to propose a dynamics for a symmetry reduced sector of the theory. Furthermore, we will explore certain analogies of this model with Loop Quantum Cosmology.
4 pages, to appear in Proceedings of Spanish Relativity Meeting 2011 (ERE 2011) held in Madrid, Spain

 

[marcus]

http://arxiv.org/abs/1201.6102
Entropy of quantum black holes
Romesh K. Kaul
(Submitted on 30 Jan 2012)
In the Loop Quantum Gravity, black holes (or even more general Isolated Horizons) are described by a SU(2) Chern-Simons theory. There is an equivalent formulation of the horizon degrees of freedom in terms of a U(1) gauge theory which is just a gauged fixed version of the SU(2) theory. These developments will be surveyed here. Quantum theory based on either formulation can be used to count the horizon micro-states associated with quantum geometry fluctuations and from this the micro-canonical entropy can be obtained. We shall review the computation in SU(2) formulation. Leading term in the entropy is proportional to horizon area with a coefficient depending on the Barbero-Immirzi parameter which is fixed by matching this result with the Bekenstein-Hawking formula. Remarkably there are corrections beyond the area term, the leading one is logarithm of the horizon area with a definite coefficient -3/2, a result which is more than a decade old now. How the same results are obtained in the equivalent U(1) framework will also be indicated. Over years, this entropy formula has also been arrived at from a variety of other perspectives. In particular, entropy of BTZ black holes in three dimensional gravity exhibits the same logarithmic correction. Even in the String Theory, many black hole models are known to possesses such properties. This may suggests a possible universal nature of this logarithmic correction.
42 pages, 1 figure. Invited article for the special issue on Loop Quantum Gravity and Cosmology of SIGMA

brief mention:
http://arxiv.org/abs/1201.6164
Cosmological inflation
K. Enqvist (Univ. Helsinki and Helsinki Inst. Phys.)
(Submitted on 30 Jan 2012)
The very basics of cosmological inflation are discussed. We derive the equations of motion for the inflaton field, introduce the slow-roll parameters, and present the computation of the inflationary perturbations and their connection to the temperature fluctuations of the cosmic microwave background.
9 pages

http://arxiv.org/abs/1201.6094
Gerbert of Aurillac: astronomy and geometry in tenth century Europe
Costantino Sigismondi
5 pages

 

[atyy]

http://arxiv.org/abs/1201.6212
Quantum fermions and quantum field theory from classical statistics
C. Wetterich
(Submitted on 30 Jan 2012)
An Ising-type classical statistical ensemble can describe the quantum physics of fermions if one chooses a particular law for the time evolution of the probability distribution. It accounts for the time evolution of a quantum field theory for Dirac particles in an external electromagnetic field. This yields in the non-relativistic one-particle limit the Schrödinger equation for a quantum particle in a potential. Interference or tunneling arise from classical probabilities.

 

[marcus]

http://arxiv.org/abs/1201.6505
Geometry and symmetries in lattice spinor gravity
C.Wetterich
(Submitted on 31 Jan 2012)
Lattice spinor gravity is a proposal for regularized quantum gravity based on fermionic degrees of freedom. In our lattice model the local Lorentz symmetry is generalized to complex transformation parameters. The difference between space and time is not put in a priori, and the euclidean and Minkowski quantum field theory are unified in one functional integral. The metric and its signature arise as a result of the dynamics, corresponding to a given ground state or cosmological solution. Geometrical objects as the vierbein, spin connection or the metric are expectation values of collective fields built from an even number of fermions. The quantum effective action for the metric is invariant under general coordinate transformations in the continuum limit. The action of our model is found to be also invariant under gauge transformations. We observe a "geometrical entanglement" of gauge- and Lorentz-transformations due to geometrical objects transforming non-trivially under both types of symmetry transformations.
38 pages

My comment: (He refers to Rovelli's http://arxiv.org/abs/1102.3660 the Zakopane Lectures on Loop Gravity, and also to papers on CDT and Regge QG, as part of clarifying why his approach is different.)

 

[marcus]

http://arxiv.org/abs/1202.0008
Super-renormalizable Higher-Derivative Quantum Gravity
Leonardo Modesto
(Submitted on 31 Jan 2012)
In this paper we study perturbatively an extension of the Stelle higher derivative gravity involving an infinite number of derivative terms. We know that the usual quadratic action is renormalizable but is not unitary because of the presence of a ghost in the theory (pole with negative residue in the propagator). The new theory is instead ghost-free since an entire function (or form factor) is introduced in the model without involving new poles in the propagator. The local high derivative theory is recovered expanding the entire functions to the lowest order in the mass scale of the theory. Any truncation of the entire function gives rise to unitarity violation. The theory is divergent at one loop and finite from two loops upwards: the theory is then super-renormalizable. Using the modified graviton propagator, we demonstrate the regularity of the gravitational potential in r=0.
5 pages, 2 figures, text overlap with arXiv:1107.2403

 

[marcus]

http://arxiv.org/abs/1202.0412
Emission spectra of self-dual black holes
Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 2 Feb 2012)
We calculate the particle spectra of evaporating self-dual black holes that are potential dark matter candidates. We first estimate the relevant mass and temperature range and find that the masses are below the Planck mass, and the temperature of the black holes is small compared to their mass. In this limit, we then derive the number-density of the primary emission particles, and, by studying the wave-equation of a scalar field in the background metric of the black hole, show that we can use the low energy approximation for the greybody factors. We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of self-dual black holes.
15 pages, 6 figures

This excerpt from page 1 helps clarify what the paper is about:

One approach to quantum gravity, Loop Quantum Gravity (LQG) [1–4], has given rise to models that allow to describe the very early universe. Simplified frameworks of LQG using a minisuperspace approximation has been shown to resolve the initial singularity problem [5, 6]. In the present work we will study the properties of black holes in such a minisuperspace model. The metric of black holes in this model was previously derived in [7], where it was shown in particular that the singularity is removed by a self-duality of the metric that replaces the black hole’s usually singular inside by another asymptotically flat region. The thermodynamical properties of these self-dual black holes have been examined in [8], and in [9] the dynamical aspects of the collapse and evaporation were studied...​

http://arxiv.org/abs/1202.0526
Coherent states for quantum gravity: towards collective variables
Daniele Oriti, Roberto Pereira, Lorenzo Sindoni
(Submitted on 2 Feb 2012)
We investigate the construction of coherent states for quantum theories of connections based on graphs embedded in a spatial manifold, as in loop quantum gravity. We discuss the many subtleties of the construction, mainly related to the diffeomorphism invariance of the theory. Aiming at approximating a continuum geometry in terms of discrete, graph-based data, we focus on coherent states for collective observables characterizing both the intrinsic and extrinsic geometry of the hypersurface, and we argue that one needs to revise accordingly the more local definitions of coherent states considered in the literature so far. In order to clarify the concepts introduced, we work through a concrete example that we hope will be useful to applying coherent state techniques to cosmology.
25 pages, 1 figure

Increasing tendency for LQG papers to simply say "quantum gravity" in the title or not even say it in the title and have "quantum gravity" as a keyword tag for the search engine. So unless it is in the abstract you may have to look at the first page and the leading references cited at the end to see it is Loop.

 

[marcus]

The FGZ paper seems to be turning out to be important. (Freidel Geiller Ziprick).
http://arxiv.org/abs/1110.4833.
Jonathan Z. just gave an excellent PIRSA video presentation, which can aid us in understanding the paper:
http://pirsa.org/12020096/
The title of the video talk is Continuous Formulation of the Loop Quantum Gravity Phase Space
He also gave a more introductory parallel session talk at Loops 2011 Madrid last May:
http://161.111.24.32/loops11/Archives/Parallel-Sessions/Jonathan-Ziprick_Geometry-of-Loop-Gravity.pdf
The talk was titled Geometry of Loop Gravity
The slides PDF is clear and helpful. It augments what one gets from the February 2012 video.

The first idea in this slide set, from which the rest takes off, is
LOOP CLASSICAL GRAVITY and the diagram of this idea is

GR —discretization→ LCG —quantization→ LQGZiprick is a PhD student at Waterloo/PI, Freidel we know, Marc Geiller is at Paris-Diderot.

This same FGZ paper will be presented by Geiller on 28 February at the ILQGS
http://relativity.phys.lsu.edu/ilqgs/schedulesp12.html
http://relativity.phys.lsu.edu/ilqgs/
Audio and slides PDF are normally available online.

This PIRSA presentation by Ziprick is interesting for several reasons, one is that his presentation went smoothly and was over in 25 minutes but was then followed by 40 minutes of discussion! The people in the audience doing the discussing included Laurent Freidel, Lee Smolin, and Bianca Dittrich.
So the total video is 65 minutes.
But if all you want is the straight presentation you just need to watch the first 25 minutes.
It gets extra interesting around 20 minutes where he is summarizing and drawing conclusions.

 

[marcus]

brief mention, not primarily QG but potentially of interest:
http://arxiv.org/abs/1202.1101
On classicalization in nonlinear sigma models
Roberto Percacci, Leslaw Rachwal
(Submitted on 6 Feb 2012)
We consider the phenomenon of classicalization in nonlinear sigma models with both positive and negative target space curvature and with any number of derivatives. We find that the theories with only two derivatives exhibit a weak form of classicalization, and that the quantitative results depend on the sign of the curvature. Nonlinear sigma models with higher derivatives show a strong form of the phenomenon which is independent of the sign of curvature. We argue that weak classicalization may actually be equivalent to asymptotic safety, whereas strong classicalization seems to be a genuinely different phenomenon. We also discuss possible ambiguities in the definition of the classical limit.
14 pages

 

[marcus]

I think this paper will influence how we view the AsymSafe QG approach. It would be reallyi nice to have an explanation for inflation. Easson has been a co-author of Nobelist Smoot.
http://arxiv.org/abs/1202.1285
Higgs Boson in RG running Inflationary Cosmology
Yi-Fu Cai, Damien A. Easson
(Submitted on 6 Feb 2012)
An intriguing hypothesis is that gravity may be non-perturbatively renormalizable via the notion of asymptotic safety. We show that the Higgs sector of the SM minimally coupled to asymptotically safe gravity can generate the observed near scale-invariant spectrum of the Cosmic Microwave Background through the curvaton mechanism. The resulting primordial power spectrum places an upper bound on the Higgs mass, which for canonical values of the curvaton parameters, is compatible with the recently released Large Hadron Collider data.
5 pages
I put this out for discussion in a separate related thread in case anyone wants to ask or comment about it https://www.physicsforums.com/showthread.php?p=3751800#post3751800

If curious about "curvaton" see:
http://en.wikipedia.org/wiki/Curvaton
"The curvaton is a hypothetical elementary particle which mediates a scalar field in early universe cosmology. It can generate fluctuations during inflation, but does not itself drive inflation, instead it generates curvature perturbations at late times after the inflaton field has decayed and the decay products have redshifted away, when the curvaton is the dominant component of the energy density.
The model was proposed by David Wands and David H. Lyth in 2001."

 

[marcus]

brief mention (though not QG may still be of use in QG):

http://arxiv.org/abs/1202.1818
Measure and Probability in Cosmology
Joshua S. Schiffrin, Robert M. Wald
(Submitted on 8 Feb 2012)
General relativity has a Hamiltonian formulation, which formally provides a canonical (Liouville) measure on the space of solutions. In ordinary statistical physics, the Liouville measure is used to compute probabilities of macrostates, and it would seem natural to use the similar measure arising in general relativity to compute probabilities in cosmology, such as the probability that the universe underwent an era of inflation. Indeed, a number of authors have used the restriction of this measure to the space of homogeneous and isotropic universes with scalar field matter (minisuperspace)---namely, the Gibbons-Hawking-Stewart measure---to make arguments about the likelihood of inflation. We argue here that there are at least four major difficulties with using the measure of general relativity to make probability arguments in cosmology: (1) Equilibration does not occur on cosmological length scales. (2) Even in the minisuperspace case, the measure of phase space is infinite and the computation of probabilities depends very strongly on how the infinity is regulated. (3) The inhomogeneous degrees of freedom must be taken into account (we illustrate how) even if one is interested only in universes that are very nearly homogeneous. The measure depends upon how the infinite number of degrees of freedom are truncated, and how one defines "nearly homogeneous." (4) In a universe where the second law of thermodynamics holds, one cannot make use of our knowledge of the present state of the universe to "retrodict" the likelihood of past conditions.
43 pages, 2 figures

http://arxiv.org/abs/1202.1793
The presence of Primordial Gravitational Waves in the Cosmic Microwave Background
Wytler Cordeiro dos Santos
(Submitted on 8 Feb 2012)
The General Relativity affirms that any field is a source of gravitational field, thus one should affirm that the energy of Cosmic Microwave Background (CMB) generated primordial gravitational waves. The present article shows that a gravitational wave with dimensionless amplitude ~ 10^-5 and large wave length ~ 10 megaparsecs shifts temperature of CMB radiation about of a part in 10⁵
7 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1202.1846
Effective constrained polymeric theories and their continuum limit
Alejandro Corichi, Tatjana Vukasinac
(Submitted on 8 Feb 2012)
The classical limit of polymer quantum theories yields a one parameter family of 'effective' theories labeled by λ. Here we consider such families for constrained theories and pose the problem of taking the 'continuum limit', λ→0. We put forward criteria for such question to be well posed, and propose a concrete strategy based in the definition of appropriately constructed Dirac observables. We analyze two models in detail, namely a constrained oscillator and a cosmological model arising from loop quantum cosmology. For both these models we show that the program can indeed be completed, provided one makes a particular choice of λ-dependent internal time with respect to which the dynamics is described and compared. We show that the limiting theories exist and discuss the corresponding limit. These results might shed some light in the problem of defining a renormalization group approach, and its associated continuum limit, for quantum constrained systems.
19 pages, 5 figures

http://arxiv.org/abs/1202.1824
The Hidden Quantum Groups Symmetry of Super-renormalizable Gravity
Stephon Alexander, Antonino Marciano, Leonardo Modesto
(Submitted on 8 Feb 2012)
In this paper we consider the relation between the super-renormalizable theories of quantum gravity (SRQG) studied in [arXiv:1110.5249v2, arXiv:1202.0008] and an underlying non-commutativity of spacetime. For one particular super-renormalizable theory we show that at linear level (quadratic in the Lagrangian) the propagator of the theory is the same we obtain starting from a theory of gravity endowed with θ-Poincaré quantum groups of symmetry. Such a theory is over the so called θ-Minkowski non-commuative spacetime. We shed new light on this link and show that among the theories considered in [arXiv:1110.5249v2, arXiv:1202.0008], there exist only one non-local and Lorentz invariant super-renormalizable theory of quantum gravity that can be described in terms of a quantum group symmetry structure. We also emphasize contact with pre-existent works in the literature and discuss preservation of the equivalence principle in our framework.
10 pages

 

[marcus]

http://arxiv.org/abs/1202.2274
Quantum Einstein Gravity
Martin Reuter, Frank Saueressig
(Submitted on 10 Feb 2012)
We give a pedagogical introduction to the basic ideas and concepts of the Asymptotic Safety program in Quantum Einstein Gravity. Using the continuum approach based upon the effective average action, we summarize the state of the art of the field with a particular focus on the evidence supporting the existence of the non-trivial renormalization group fixed point at the heart of the construction. As an application, the multifractal structure of the emerging space-times is discussed in detail. In particular, we compare the continuum prediction for their spectral dimension with Monte Carlo data from the Causal Dynamical Triangulation approach.
87 pages, 13 figures, review article prepared for the New Journal of Physics focus issue on Quantum Einstein Gravity

http://arxiv.org/abs/1202.2301
Canonical Partition function of Loop Black Holes
Kinjalk Lochan, Cenalo Vaz
(Submitted on 10 Feb 2012)
We compute the canonical partition for quantum black holes in the approach of Loop Quantum Gravity (LQG). We argue that any quantum theory of gravity in which the horizon area is built of non-interacting constituents cannot yield qualitative corrections to the Bekenstein-Hawking (B-H) area law, but corrections to the area law can arise as a consequence additional constraints inducing interactions between the constituents. In LQG this is implemented by requiring spherical horizons. The canonical approach for LQG favours a logarithmic correction to the B-H law with a coefficient of -1/2, independently of the area spectrum. Our initial calculation of the partition function uses certain approximations that, we show, do not qualitatively affect the expression for the black hole entropy. We later discuss the quantitative corrections to these results when the simplifying approximations are relaxed and the full LQG spectrum is dealt with. We show how these corrections can be recovered to all orders in perturbation theory. However, the convergence properties of the perturbative series remains unknown.
16 pages, 1 figure

brief mention (a sweet bit of history, possibly of general interest)
http://arxiv.org/abs/1202.2347
A Sommerfeld Explanation
Jeremy Bernstein
(Submitted on 10 Feb 2012)
Sommerfeld shows that the Wien displacement formula implies the existence of Planck's constant.

 

[marcus]

http://arxiv.org/abs/1202.2710
Dynamical dimensional reduction in toy models of 4D causal quantum gravity
Georgios Giasemidis, John F. Wheater, Stefan Zohren
(Submitted on 13 Feb 2012)
In recent years several approaches to quantum gravity have found evidence for a scale dependent spectral dimension of space-time varying from four at large scales to two at small scales of order of the Planck length. The first evidence came from numerical results of four-dimensional causal dynamical triangulations (CDT) [Ambj{\o}rn et al., Phys. Rev. Lett. 95 (2005) 171]. Since then little progress has been made in analytically understanding the numerical results coming from the CDT approach and showing that they remain valid when taking the continuum limit. In this letter we propose a new toy model of "radially reduced" four-dimensional CDT in which we can take the continuum limit analytically and obtain a scale dependent spectral dimension varying from four to two with scale. Furthermore, the functional behaviour of the spectral dimension is exactly of the form which was conjectured on the basis of the numerical results.
5 pages, 2 figures

brief mention:
http://arxiv.org/abs/1202.2539
Quantum Time Crystals
Frank Wilczek
(Submitted on 12 Feb 2012)
Difficulties around the idea of spontaneous breaking of time translation symmetry in a closed quantum mechanical system are identified, and then overcome in a simple model. The possibility of ordering in imaginary time is also discussed.
6 pages,

 

[marcus]

brief mention (may be of general interest though not directly QG-related):
http://arxiv.org/abs/1202.3324
On the inertia of heat
Matteo Smerlak
(Submitted on 15 Feb 2012)
Does heat have inertia? This question is at the core of a long-standing controversy on Eckart's dissipative relativistic hydrodynamics. Here I show that the troublesome inertial term in Eckart's heat flux arises only if one insists on defining thermal diffusivity as a spacetime constant. I argue that this is the most natural definition, and that all confusion disappears if one considers instead the space-dependent comoving diffusivity, in line with the fact that, in the presence of gravity, space is an inhomogeneous medium.
3 pages

 

[marcus]

Great talk by Frank Saueressig on Asym Safe QG. Perimeter video here:
http://pirsa.org/12020088/
Part of the talk explains the basics--a clear introduction. Part goes into advanced topics.
He's an excellent presenter IMO. He's worked with Martin Reuter for around 10 years and is now at Mainz as junior faculty, so about as authoritative as anyone concerning AS.

http://arxiv.org/abs/1202.3637
Random tensor models in the large N limit: Uncoloring the colored tensor models
Valentin Bonzom, Razvan Gurau, Vincent Rivasseau
(Submitted on 16 Feb 2012)
Tensor models generalize random matrix models in yielding a theory of dynamical triangulations in arbitrary dimensions. Colored tensor models have been shown to admit a 1/N expansion and a continuum limit accessible analytically. In this paper we prove that these results extend to the most general tensor model for a single generic, i.e. non-symmetric, complex tensor. Colors appear in this setting as a canonical book-keeping device and not as a fundamental feature. In the large N limit, we exhibit a set of Virasoro constraints satisfied by the free energy and an infinite family of multicritical behaviors with entropy exponents γ_m=1-1/m.
15 pages

extremely brief mention (a reminder about Ingemar Bengtsson):
http://arxiv.org/abs/1202.3559
http://arxiv.org/abs/1202.3561

 

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http://arxiv.org/abs/1202.4066
Comment on arXiv:1104.2019, "Relative locality and the soccer ball problem," by Amelino-Camelia et al
Sabine Hossenfelder
(Submitted on 18 Feb 2012)
It is explained why the argument in arXiv:1104.2019 does not answer the question how to describe multi-particle states in models with a deformed Lorentz-symmetry in momentum space.
3 pages

brief mention:
http://arxiv.org/abs/1202.4435
New multicritical matrix models and multicritical 2d CDT
Jan Ambjorn, Lisa Glaser, Andrzej Gorlich, Yuki Sato
(Submitted on 20 Feb 2012)
We define multicritical CDT models of 2d quantum gravity and show that they are a special case of multicritical generalized CDT models obtained from the new scaling limit, the so-called "classical" scaling limit, of matrix models. The multicritical behavior agrees with the multicritical behavior of the so-called branched polymers.
16 pages, 4 figures

http://arxiv.org/abs/1202.4322
An Analytical Analysis of CDT Coupled to Dimer-like Matter
Max R. Atkin, Stefan Zohren
(Submitted on 20 Feb 2012)
We consider a model of restricted dimers coupled to two-dimensional causal dynamical triangulations (CDT), where the dimer configurations are restricted in the sense that they do not include dimers in regions of high curvature. It is shown how the model can be solved analytically using bijections with decorated trees. At a negative critical value for the dimer fugacity the model undergoes a phase transition at which the critical exponent associated to the geometry changes. This represents the first account of an analytical study of a matter model with two-dimensional interactions coupled to CDT.

http://arxiv.org/abs/1202.4186
http://arxiv.org/abs/1202.4187
Edward Anderson

 

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http://arxiv.org/abs/1202.5039
Degenerate Plebanski Sector and its Spin Foam Quantization
Sergei Alexandrov
(Submitted on 22 Feb 2012)
We show that the degenerate sector of Spin(4) Plebanski formulation of four-dimensional gravity is exactly solvable and describes covariantly embedded SU(2) BF theory. This fact provides its spin foam quantization and allows to test various approaches of imposing the simplicity constraints. Our analysis suggests a unique method of imposing the constraints which leads to a consistent and well defined spin foam model.
34 pages

 

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http://arxiv.org/abs/1202.6183
A Gauge Theoretic Approach to Gravity
Kirill Krasnov
(Submitted on 28 Feb 2012)
Einstein's General Relativity (GR) is a dynamical theory of the spacetime metric. We describe an approach in which GR becomes an SU(2) gauge theory. We start at the linearised level and show how a gauge theoretic Lagrangian for non-interacting massless spin two particles (gravitons) takes a much more simple and compact form than in the standard metric description. Moreover, in contrast to the GR situation, the gauge theory Lagrangian is convex. We then proceed with a formulation of the full non-linear theory. The equivalence to the metric-based GR holds only at the level of solutions of the field equations, that is, on-shell. The gauge-theoretic approach also makes it clear that GR is not the only interacting theory of massless spin two particles, in spite of the GR uniqueness theorems available in the metric description. Thus, there is an inifnite-parameter class of gravity theories all describing just two propagating polarisations of the graviton. We describe how matter can be coupled to gravity in this formulation and, in particular, how both the gravity and Yang-Mills arise as sectors of a general diffeomorphism invariant gauge theory. We finish by outlining a possible scenario of the UV completion of quantum gravity within this approach.
40 pages; invited review to appear in Proceedings of the Royal Society A
[my comment: see page 34, section 4.3, and the next section Conclusions. conjecture about RG flow of theory, tie-in with asym-safe approach]

http://arxiv.org/abs/1202.6322
Multigraph models for causal quantum gravity and scale dependent spectral dimension
Georgios Giasemidis, John F Wheater, Stefan Zohren
(Submitted on 28 Feb 2012)
We study random walks on ensembles of a specific class of random multigraph graphs associated with theories of causal quantum gravity. In particular, we investigate the spectral dimension of the graph ensemble for recurrent as well as transient walks. We investigate the circumstances in which the spectral dimension and Hausdorff dimension are equal and show that this occurs when rho, the exponent for anomalous behaviour of the resistance to infinity, is zero. The concept of scale dependent spectral dimension in these models is introduced. We apply this notion to a multigraph ensemble with a measure induced by a size biased critical Galton-Watson process which has a scale dependent spectral dimension of two at large scales and one at small scales. We conclude by discussing a specific model related to four dimensional quantum gravity which has a spectral dimension of four at large scales and two at small scales.
30 pages, 3 figures
[my comment: most references are to "causal dynamical triangulations" and related qg. they call it "causal QG" but it's a close relative of CDT, more about the interesting running of dimensionality with scale]

 

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http://arxiv.org/abs/1203.0174
Quantum isolated horizons and black hole entropy
J. Fernando Barbero G., Jerzy Lewandowski, Eduardo J. S. Villaseñor
(Submitted on 1 Mar 2012)
We give a short introduction to the approaches currently used to describe black holes in loop quantum gravity. We will concentrate on the classical issues related to the modeling of black holes as isolated horizons, give a short discussion of their canonical quantization by using loop quantum gravity techniques, and a description of the combinatorial methods necessary to solve the counting problems involved in the computation of the entropy.
28 pages. Contribution to the Proceedings of the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

 

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http://arxiv.org/abs/1203.1040
Cleaning up the cosmological constant
Ian Kimpton, Antonio Padilla
(Submitted on 5 Mar 2012)
We present a novel idea for screening the vacuum energy contribution to the overall value of the cosmological constant, thereby enabling us to choose the bare value of the vacuum curvature empirically, without any need to worry about the zero-point energy contributions of each particle. The trick is to couple matter to a metric that is really a composite of other fields, with the property that the square-root of its determinant is the integrand of a topological invariant, and/or a total derivative. This ensures that the vacuum energy contribution to the Lagrangian is non-dynamical. We then give an explicit example of a theory with this property that is free from Ostrogradski ghosts, and is consistent with solar system physics and cosmological tests.
4 pages
[my comment: depends on prior work http://arxiv.org/abs/1106.2000 published in Physical Review Letters in 2012]

http://arxiv.org/abs/1203.1173
Cosmological particle creation in the lab?
Ralf Schützhold, William G. Unruh
(Submitted on 6 Mar 2012)
One of the most striking examples for the production of particles out of the quantum vacuum due to external conditions is cosmological particle creation, which is caused by the expansion or contraction of the Universe. Already in 1939, Schrödinger understood that the cosmic evolution could lead to a mixing of positive and negative frequencies and that this "would mean production or annihilation of matter, merely by the expansion". Later this phenomenon was derived via more modern techniques of quantum field theory in curved space-times by Parker (who apparently was not aware of Schrödinger's work) and subsequently has been studied in numerous publications. Even though cosmological particle creation typically occurs on extremely large length scales, it is one of the very few examples for such fundamental effects where we actually may have observational evidence: According to the inflationary model of cosmology, the seeds for the anisotropies in the cosmic microwave background (CMB) and basically all large scale structures stem from this effect. In this Chapter, we shall provide a brief discussion of this phenomenon and sketch a possibility for an experimental realization via an analogue in the laboratory.
13 pages

 

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http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

 

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http://arxiv.org/abs/1203.2161
Highlights of Noncommutative Spectral Geometry
Mairi Sakellariadou
(Submitted on 9 Mar 2012)
A summary of noncommutative spectral geometry as an approach to unification is presented. The role of the doubling of the algebra, the seeds of quantization and some cosmological implications are briefly discussed.
4 pages, to be published in the Journal of Physics Conference Series under the title "Vishwa Mimansa - An Interpretative Exposition of the Universe"; talk given at the 7th International Conference on Gravitation and Cosmology, 14-19 December 2011, Goa, India

http://arxiv.org/abs/1203.1934
Loop Quantization of the Supersymmetric Two-Dimensional BF Model
Clisthenis P. Constantinidis, Ruan Couto, Ivan Morales, Olivier Piguet
(Submitted on 8 Mar 2012)
In this paper we consider the quantization of the 2d BF model coupled to topological matter. Guided by the rigid supersymmetry this system can be viewed as a super-BF model, where the field content is expressed in terms of superfields. A canonical analysis is done and the constraints are then implemented at the quantum level in order to construct the Hilbert space of the theory under the perspective of Loop Quantum Gravity methods.
17 pages

http://arxiv.org/abs/1203.1962
An effective action for asymptotically safe gravity
Alfio Bonanno
(Submitted on 8 Mar 2012)
Asymptotically safe theories of gravitation have received great attention in recent times. In this framework an effective action embodying the basic features of the renormalized flow around the non-gaussian fixed point is derived and its implications for the early universe are discussed. In particular, a "landscape" of a countably infinite number of cosmological inflationary solutions characterized by an unstable de Sitter phase lasting for a large enough number of e-folds is found.
5 pages, to appear as a Rapid Communication in Physical Review D

http://arxiv.org/abs/1203.2158
The "tetrad only" theory space: Nonperturbative renormalization flow and Asymptotic Safety
Ulrich Harst, Martin Reuter
(Submitted on 9 Mar 2012)
We set up a nonperturbative gravitational coarse graining flow and the corresponding functional renormalization group equation on the as to yet unexplored "tetrad only" theory space. It comprises action functionals which depend on the tetrad field (along with the related background and ghost fields) and are invariant under the semi-direct product of spacetime diffeomorphisms and local Lorentz transformations. This theory space differs from that of Quantum Einstein Gravity (QEG) in that the tetrad rather than the metric constitutes the fundamental variable and because of the additional symmetry requirement of local Lorentz invariance. It also differs from "Quantum Einstein Cartan Gravity" (QECG) investigated recently since the spin connection is not an independent field variable now. We explicitly compute the renormalization group flow on this theory space within the tetrad version of the Einstein-Hilbert truncation. A detailed comparison with analog results in QEG and QECG is performed in order to assess the impact the choice of a fundamental field variable has on the renormalization behavior of the gravitational average action, and the possibility of an asymptotically safe infinite cutoff limit is investigated. Implications for nonperturbative studies of fermionic matter coupled to quantum gravity are also discussed. It turns out that, in the context of functional flow equations, the "hybrid calculations" proposed in the literature (using the tetrad for fermionic diagrams only, and the metric in all others) are unlikely to be quantitatively reliable. Moreover we find that, unlike in perturbation theory, the non-propagating Faddeev-Popov ghosts related to the local Lorentz transformations may not be discarded but rather contribute quite significantly to the beta functions of Newton's constant and the cosmological constant.
45 pages, 10 figures

brief mention (not QG but possibly of general interest):
http://arxiv.org/abs/1203.2035
A Noether Theorem for Markov Processes
John C. Baez, Brendan Fong
(Submitted on 9 Mar 2012)
Noether's theorem links the symmetries of a quantum system with its conserved quantities, and is a cornerstone of quantum mechanics. Here we prove a version of Noether's theorem for Markov processes. In quantum mechanics, an observable commutes with the Hamiltonian if and only if its expected value remains constant in time for every state. For Markov processes that no longer holds, but an observable commutes with the Hamiltonian if and only if both its expected value and standard deviation are constant in time for every state.
9 pages

 

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http://arxiv.org/abs/1203.2733
From Classical To Quantum Gravity: Introduction to Loop Quantum Gravity
Kristina Giesel, Hanno Sahlmann
(Submitted on 13 Mar 2012)
We present an introduction to the canonical quantization of gravity performed in loop quantum gravity, based on lectures held at the 3rd quantum geometry and quantum gravity school in Zakopane in 2011. A special feature of this introduction is the inclusion of new proposals for coupling matter to gravity that can be used to deparametrize the theory, thus making its dynamics more tractable. The classical and quantum aspects of these new proposals are explained alongside the standard quantization of vacuum general relativity in loop quantum gravity.
55 pages. Contribution to the Proceedings of the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

brief mention (not directly Loop-related but possibly of general interest):
http://arxiv.org/abs/1203.2622
The Optimal Cosmic Epoch for Precision Cosmology
Abraham Loeb (Harvard)
(Submitted on 12 Mar 2012)
The statistical uncertainty in measuring the primordial density perturbations on a given comoving scale is dictated by the number of independent regions of that scale that are accessible to an observer. This number varies with cosmic time and diminishes per Hubble volume in the distant past or future of the standard cosmological model. We show that the best constraints on the initial power spectrum of linear density perturbations are accessible (e.g. through 21-cm intensity mapping) at redshifts z~10-50, and that the ability to constrain the cosmological initial conditions will deteriorate quickly in our cosmic future.
4 pages, 4 figures

http://arxiv.org/abs/1203.2642
Very special relativity as particle in a gauge field and two-time physics

Juan M. Romero, Eric Escobar, Etelberto Vazquez
(Submitted on 12 Mar 2012)
The action for a particle in very special relativity is studied. It is shown that this system is equivalent to a relativistic particle in a gauge field. A new symmetry for this system is found. A general action with restored Lorentz symmetry is proposed for this system. It is shown that this new action contain very special relativity and two-time physics.
12 pages

http://arxiv.org/abs/1203.2679
Noncommutative Mixmaster Cosmologies
Christopher Estrada, Matilde Marcolli
(Submitted on 13 Mar 2012)
In this paper we investigate a variant of the classical mixmaster universe model of anisotropic cosmology, where the spatial sections are noncommutative 3-tori. We consider ways in which the discrete dynamical system describing the mixmaster dynamics can be extended to act on the noncommutative torus moduli, and how the resulting dynamics differs from the classical one, for example, in the appearance of exotic smooth structures. We discuss properties of the spectral action, focussing on how the slow-roll inflation potential determined by the spectral action affects the mixmaster dynamics. We relate the model to other recent results on spectral action computation and we identify other physical contexts in which this model may be relevant.
24 pages

http://arxiv.org/abs/1203.2641
Internal Relativity
Olaf Dreyer
(Submitted on 12 Mar 2012)
General relativity differs from other forces in nature in that it can be made to disappear locally. This is the essence of the equivalence principle. In general relativity the equivalence principle is implemented using differential geometry. The connection that comes from a metric is used to glue together the different gravity-free Minkowski spaces. In this article we argue that there is another way to implement the equivalence principle. In this new way it is not different Minkowski spaces that are connected but different vacua of an underlying solid-state like model. One advantage of this approach to gravity is that one can start with a quantum mechanical model so that the question of how to arrive at a quantum theory of gravity does not arise. We show how the gravitational constant can be calculated in this setup.
9 pages

 

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A new ILQGS talk (given 13 March) by Diaz-Polo
Black Hole Evaporation in Loop Quantum Gravity
Slides PDF:
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.pdf
Audio:
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.wav
or
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.aif
based on this paper:
http://arxiv.org/abs/1109.4239
Probing Loop Quantum Gravity with Evaporating Black Holes
Aurelien Barrau, Xiangyu Cao, Jacobo Diaz-Polo, Julien Grain, Thomas Cailleteau
(Submitted on 20 Sep 2011)
This letter aims at showing that the observation of evaporating black holes should allow distinguishing between the usual Hawking behavior and Loop Quantum Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.
5 pages, 3 figures. Published in Physical Review Letters 107, 251301 (2011)

 

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http://arxiv.org/abs/1203.3449
Non-singular Power-law and Assisted inflation in Loop Quantum Cosmology
Evan Ranken, Parampreet Singh
(Submitted on 15 Mar 2012)
We investigate the dynamics of single and multiple scalar fields with exponential potentials, leading to power-law and assisted inflation, in loop quantum cosmology. Unlike in the classical theory, dynamical trajectories in loop quantum cosmology are generically non-singular, with a big bounce replacing classical big bang in the Planck regime. Post bounce, after a phase of super-inflation, dynamical trajectories evolve towards the classical attractor in the inflationary scenarios. Depending on the initial conditions, bounce is shown to occur in kinetic as well as potential dominated regimes. We analyze the number of e-foldings resulting from the phase of super-inflation, and find the dependence of the maximum possible number of e-foldings on the equation of state at the bounce and on the steepness of the potential. We find that if the potential is not steep, this phase can lead to large number of e-foldings in power-law inflation. For the assisted inflation scenario, an increase in the number of fields can yield a significant increase in the number of e-foldings during super-inflation.
21 pages, 15 figures

 

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http://arxiv.org/abs/1203.3591
Nonperturbative Quantum Gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 15 Mar 2012)
Asymptotic safety describes a scenario in which general relativity can be quantized as a conventional field theory, despite being nonrenormalizable when expanding it around a fixed background geometry. It is formulated in the framework of the Wilsonian renormalization group and relies crucially on the existence of an ultraviolet fixed point, for which evidence has been found using renormalization group equations in the continuum.
"Causal Dynamical Triangulations" (CDT) is a concrete research program to obtain a nonperturbative quantum field theory of gravity via a lattice regularization, and represented as a sum over spacetime histories. In the Wilsonian spirit one can use this formulation to try to locate fixed points of the lattice theory and thereby provide independent, nonperturbative evidence for the existence of a UV fixed point.
We describe the formalism of CDT, its phase diagram, possible fixed points and the "quantum geometries" which emerge in the different phases. We also argue that the formalism may be able to describe a more general class of Horava-Lifgarbagez gravitational models.
146 pages, many figures

 

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http://arxiv.org/abs/1203.4105
Lorentz breaking Effective Field Theory and observational tests
Stefano Liberati
(Submitted on 19 Mar 2012)
Analogue models of gravity have provided an experimentally realizable test field for our ideas on quantum field theory in curved spacetimes but they have also inspired the investigation of possible departures from exact Lorentz invariance at microscopic scales. In this role they have joined, and sometime anticipated, several quantum gravity models characterized by Lorentz breaking phenomenology. A crucial difference between these speculations and other ones associated to quantum gravity scenarios, is the possibility to carry out observational and experimental tests which have nowadays led to a broad range of constraints on departures from Lorentz invariance. We shall review here the effective field theory approach to Lorentz breaking in the matter sector, present the constraints provided by the available observations and finally discuss the implications of the persisting uncertainty on the composition of the ultra high energy cosmic rays for the constraints on the higher order, analogue gravity inspired, Lorentz violations.
47 pages, 4 figures. Lecture Notes for the IX SIGRAV School on "Analogue Gravity", Como (Italy), May 2011

http://arxiv.org/abs/1203.4207
The phase diagram of quantum gravity from diffeomorphism-invariant RG-flows
Ivan Donkin, Jan M. Pawlowski
(Submitted on 19 Mar 2012)
We evaluate the phase diagram of quantum gravity within a fully diffeomorphism-invariant renormalisation group approach. The construction is based on the geometrical or Vilkovisky-DeWitt effective action. We also resolve the difference between the fluctuation metric and the background metric. This allows for fully background-independent flows in gravity. The results provide further evidence for the ultraviolet fixed point scenario in quantum gravity with quantitative changes for the fixed point physics. We also find a stable infrared fixed point related to classical Einstein gravity. Implications and possible extensions are discussed.
23 pages, 13 figures

brief mention:
http://arxiv.org/abs/1203.4197
Is the Cosmological Coincidence a Problem?
Navin Sivanandam
(Submitted on 19 Mar 2012)
The matching of our epoch of existence with the approximate equality of dark energy and dark matter energy densities is an apparent further fine-tuning, beyond the already troubling 120 orders of magnitude that separate dark energy from the Planck scale. In this paper I will argue that the coincidence is not a fine-tuning problem, but instead an artifact of anthropic selection. Rather than assuming measurements are equally likely in all epochs, one should insist that measurements of a quantity be typical amongst all such measurements. As a consequence, particular observations will reflect the epoch in which they are most easily made. In the specific case of cosmology, most measurements of dark energy and dark matter will done during an epoch when large numbers of linear modes are available to observers, so we should not be surprised at living at such a time. This is made precise in a particular model for the probability distribution for r=min(Ω_m/Ω_L, Ω_L/Ω_m), where it is shown that if p(r) ~ [N(r)]^b (where N(r) is the number of linear modes, and b is some arbitrary positive power), the probability that r is greater than its observed value of 0.4, is close to 1. Thus the cosmological coincidence is no longer problematic.
10 pages, 5 figures

http://arxiv.org/abs/1203.3827
Where will Einstein fail? Lessons for gravity and cosmology
Niayesh Afshordi (U-Waterloo/Perimeter Institute)
(Submitted on 16 Mar 2012)
Einstein's theory of General Relativity is the benchmark example for empirical success and mathematical elegance in theoretical physics. However, in spite of being the most successfully tested theory in physics, there are strong theoretical and observational arguments for why General Relativity should fail. It is not a question of if, but rather a question of where and when! I start by recounting the tremendous success in observational cosmology over the past three decades, that has led to the era of precision cosmology. I will then summarize the pathologies in Einstein's theory of gravity, as the cornerstone of standard cosmological model. Attempts to address these pathologies are either inspired by mathematical elegance, or empirical falsifiability. Here, I provide different arguments for why a falsifiable solution should violate Lorentz symmetry, or revive "gravitational aether". Deviations from Einstein's gravity are then expected in: 1) cosmological matter-radiation transition, 2) neutron stars, 3) gravitomagnetic effect, 4) astrophysical black holes, and their potential connection to dark energy, and 5) early Universe, where the predictions are ranked by their degree of robustness and falsifiability.
20 pages, 3 figures, Based on the Professor M.K. Vainu Bappu gold medal award (2008) lecture given at IUCAA, Pune on 2011 October 15, To appear in the 2012 March issue of the Bulletin of the Astronomical Society of India

 

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http://arxiv.org/abs/1203.5082
Bubbles and jackets: new scaling bounds in topological group field theories
Sylvain Carrozza, Daniele Oriti
(Submitted on 22 Mar 2012)
We use a reformulation of topological group field theories in 3 and 4 dimensions in terms of variables associated to vertices, in 3d, and edges, in 4d, to obtain new scaling bounds for their Feynman amplitudes. In both 3 and 4 dimensions, we obtain a bubble bound proving the suppression of singular topologies with respect to the first terms in the perturbative expansion (in the cut-off). We also prove a new, stronger jacket bound than the one currently available in the literature. We expect these results to be relevant for other tensorial field theories of this type, as well as for group field theory models for 4d quantum gravity.
32 pages

brief mention:
http://arxiv.org/abs/1203.4995
Emergence of time in quantum gravity: is time necessarily flowing ?
Pierre Martinetti
(Submitted on 22 Mar 2012)
We discuss the emergence of time in quantum gravity, and ask whether time is always "something that flows"'. We first recall that this is indeed the case in both relativity and quantum mechanics, although in very different manners: time flows geometrically in relativity (i.e. as a flow of proper time in the four dimensional space-time), time flows abstractly in quantum mechanics (i.e. as a flow in the space of observables of the system). We then ask the same question in quantum gravity, in the light of the thermal time hypothesis of Connes and Rovelli. The latter proposes to answer the question of time in quantum gravity (or at least one of its many aspects), by postulating that time is a state dependent notion. ...
...
12 pages. Contribution to the Workshop "Temps et Emergence", Ecole Normale Supérieure, Paris 14-15 october 2011.

 

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http://arxiv.org/abs/1203.5214
Universality of geometry
C. Wetterich
(Submitted on 23 Mar 2012)
In models of emergent gravity the metric arises as the expectation value of some collective field. Usually, many different collective fields with appropriate tensor properties are candidates for a metric. Which collective field describes the "physical geometry"? We resolve this "metric ambiguity" by an investigation of the most general form of the quantum effective action for several metrics. In the long-distance limit the physical metric is universal and accounts for a massless graviton. Other degrees of freedom contained in the various metric candidates describe very massive scalars and symmetric second rank tensors. They only play a role at microscopic distances, typically around the Planck length. The universality of geometry at long distances extends to the vierbein and the connection. On the other hand, for distances and time intervals of Planck size geometry loses its universal meaning. Time is born with the big bang.
6 pages

brief mention (not QG but of possible interest):
http://arxiv.org/abs/1203.5153
Self-healing of unitarity in effective field theories and the onset of new physics
Ufuk Aydemir, Mohamed M. Anber, John F. Donoghue
(Submitted on 23 Mar 2012)
In effective field theories it is common to identify the onset of new physics with the violation of tree-level unitarity. However, we show that this is parametrically incorrect in the case of chiral perturbation theory, and is probably theoretically incorrect in general. In the chiral theory, we explore perturbative unitarity violation as a function of the number of colors and the number of flavors, holding the scale of the "new physics" (i.e. QCD) fixed. This demonstrates that the onset of new physics is parametrically uncorrelated with tree-unitarity violation. When the latter scale is lower than that of new physics, the effective theory must heal its unitarity violation itself, which is expected because the field theory satisfies the requirements of unitarity. In the chiral theory, the self-healing results in a resonant structure with scalar quantum numbers. In the electroweak variant of this argument, the structure must have the properties of the Higgs and must couple proportional to the mass in both gauge boson and fermion scattering. A similar example can be seen in the case of general relativity coupled to multiple matter fields, where iteration of the vacuum polarization diagram restores unitarity. We present arguments that suggest the correct identification should be connected to the onset of inelasticity rather than unitarity violation. We describe how the onset of inelasticity can occur in the effective theory, although it does not appear possible to predict the onset reliably.
11 pages, 5 figures

http://arxiv.org/abs/1203.5238
The small-scale structure of quantum spacetime
Christopher D. Burton
(Submitted on 23 Mar 2012)
Planck-scale quantum spacetime undergoes probabilistic local curvature fluctuations whose distributions cannot explicitly depend on position otherwise vacuum's small-scale quantum structure would fail to be statistically homogeneous. Since the collection of fluctuations is a many-body system, the natural explanation for their position-independent statistics is that they are in equilibrium with each other and distributed at maximum entropy. Consequently, their probability distributions obey the laws of statistical physics which enforces small-scale smoothness, prevents the homogeneity-violating diffusion found in any free quantum system, and maintains decoherence. Their entropy, calculated using the explicitly-constructed phase space of the Riemann whose statistics are derived using a background-independent graviton exchange ensemble, is proportional to the Einstein-Hilbert action evaluated on the macroscopic expected geometry and includes a small, positive cosmological constant. Entropy maximization yields quantum spacetime's Ehrenfest equations of motion which are identical to Einstein's expectation-valued field equations. This background-independent dynamical formulation reveals curvature fluctuation entropy as a source of expansion and raises the possibility that matter's zero-point energy problem, which is action-based and not energy shift invariant, may not be a problem after all.
16 pages

 

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http://arxiv.org/abs/1203.5425
CODATA Recommended Values of the Fundamental Physical Constants: 2010
Peter J. Mohr, Barry N. Taylor, David B. Newell
(Submitted on 24 Mar 2012)
This paper gives the 2010 self-consistent set of values of the basic constants and conversion factors of physics and chemistry recommended by the Committee on Data for Science and Technology (CODATA) for international use. The 2010 adjustment takes into account the data considered in the 2006 adjustment as well as the data that became available from 1 January 2007, after the closing date of that adjustment, until 31 December 2010, the closing date of the new adjustment. Further, it describes in detail the adjustment of the values of the constants, including the selection of the final set of input data based on the results of least-squares analyses. The 2010 set replaces the previously recommended 2006 CODATA set and may also be found on the World Wide Web at physics.nist.gov/constants.
94 pages, 8 figures, 48 tables

strange:
http://arxiv.org/abs/1203.5557
Quantum Theory without Planck's Constant
John P. Ralston
(Submitted on 25 Mar 2012)
Planck's constant was introduced as a fundamental scale in the early history of quantum mechanics. We find a modern approach where Planck's constant is absent: it is unobservable except as a constant of human convention. Despite long reference to experiment, review shows that Planck's constant cannot be obtained from the data of Ryberg, Davisson and Germer, Compton, or that used by Planck himself. In the new approach Planck's constant is tied to macroscopic conventions of Newtonian origin, which are dispensable. The precision of other fundamental constants is substantially improved by eliminating Planck's constant...
42 pages, 9 figures

brief mention:
http://arxiv.org/abs/1203.5367
Domain structures in quantum graphity
James Q. Quach, Chun-Hsu Su, Andrew M. Martin, Andrew D. Greentree
(Submitted on 23 Mar 2012)
Quantum graphity offers the intriguing notion that space emerges in the low energy states of the spatial degrees of freedom of a dynamical lattice. Here we investigate metastable domain structures which are likely to exists in the low energy phase of lattice evolution. Through a simulated annealing process we explore the formation of metastable defects at domain boundaries and the effects of domain structures on the propagation of bosons. We show that these structures should have observable consequences including scattering, double imaging, and gravitational lensing-like effects.
10 pages, 11 figures

 

[marcus]

http://arxiv.org/abs/1203.5875
Big bounce from gravitational four-fermion interaction
I.B. Khriplovich
(Submitted on 27 Mar 2012)
The four-fermion gravitational interaction is induced by torsion, and gets dominating on the Planck scale. The regular, axial-axial part of this interaction by itself does not stop the gravitational compression. However, the anomalous, vector-vector interaction results in a natural way in big bounce.
4 pages.

http://arxiv.org/abs/1203.6191
Minimal Length Scale Scenarios for Quantum Gravity
Sabine Hossenfelder
(Submitted on 28 Mar 2012)
We review the question whether the fundamental laws of nature limit our ability to probe arbitrarily short distances. First, we examine what insights can be gained from thought experiments for probes of shortest distances, and summarize what can be learned from different approaches to a theory of quantum gravity. Then we discuss some models that have been developed to implement a minimal length scale into quantum mechanics and quantum field theory. These models have entered the literature under the names of generalized uncertainty principle or modified dispersion relation, and have allowed to study the effects of a minimal length scale in quantum mechanics, quantum electrodynamics, thermodynamics, black hole physics and cosmology. Finally, we touch upon the question if there are ways to circumvent the manifestation of a minimal length scale in short-distance physics.
86 pages, prepared for Living Reviews in Relativity

http://arxiv.org/abs/1203.6164
Intersecting Quantum Gravity with Noncommutative Geometry - a Review
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 28 Mar 2012)
We review applications of noncommutative geometry in canonical quantum gravity. First, we show that the framework of loop quantum gravity includes natural noncommutative structures which have, hitherto, not been explored. Next, we present the construction of a spectral triple over an algebra of holonomy loops. The spectral triple, which encodes the kinematics of quantum gravity, gives rise to a natural class of semiclassical states which entail emerging fermionic degrees of freedom. In the particular semiclassical approximation where all gravitational degrees of freedom are turned off, a free fermionic quantum field theory emerges. We end the paper with an extended outlook section.

 

[marcus]

http://arxiv.org/abs/1203.6525
Loop quantum gravity without the Hamiltonian constraint
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn
(Submitted on 29 Mar 2012)
We show that under certain technical assumptions, including a generalisation of CMC foliability and strict positivity of the scalar field, general relativity conformally coupled to a scalar field can be quantised on a partially reduced phase space, meaning reduced only with respect to the Hamiltonian constraint and a proper gauge fixing. More precisely, we introduce, in close analogy to shape dynamics, the generator of a local conformal transformation acting on both, the metric and the scalar field. A new metric, which is invariant under this transformation, is constructed and used to define connection variables which can be quantised by standard loop quantum gravity methods. Since this connection is invariant under the local conformal transformation, the generator of which is shown to be a good gauge fixing for the Hamiltonian constraint, the Dirac bracket associated with implementing these constraints coincides with the Poisson bracket for the connection. Thus, the well developed kinematical quantisation techniques for loop quantum gravity are available, while the Hamiltonian constraint has been solved (more precisely, gauge fixed) classically. The physical interpretation of this system is that of general relativity on a fixed spatial slice, the associated "time" of which is given by the value of the generator of local conformal transformations. While it is hard to address dynamical problems in this framework (due to the complicated "time" function), it seems, due to good accessibility properties of the gauge in certain situations, to be well suited for problems such as the computation of black hole entropy, where actual physical states can be counted and the dynamics is only of indirect importance. Also, the interpretation of the geometric operators gets an interesting twist, which exemplifies the deep relationship between observables and the choice of a time function.
5 pages

http://arxiv.org/abs/1203.6526
On a partially reduced phase space quantisation of general relativity conformally coupled to a scalar field
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn
(Submitted on 29 Mar 2012)
The purpose of this paper is twofold: On the one hand, after a thorough review of the matter free case, we supplement the derivations in our companion paper on "loop quantum gravity without the Hamiltonian constraint" with calculational details and extend the results to standard model matter, a cosmological constant, and non-compact spatial slices. On the other hand, we provide a discussion on the role of observables, focussed on the situation of a symmetry exchange, which is key to our derivation. Furthermore, we comment on the relation of our model to reduced phase space quantisations based on deparametrisation.
51 pages, 5 figures

 

[atyy]

http://arxiv.org/abs/1203.6564
Infrared fixed point in quantum Einstein gravity
S. Nagy, J. Krizsan, K. Sailer
(Submitted on 29 Mar 2012)
We performed the renormalization group analysis of the quantum Einstein gravity in the deep infrared regime for different types of extensions of the model. It is shown that an attractive infrared point exists in the broken symmetric phase of the model. It is also shown that due to the Gaussian fixed point the IR critical exponent $\nu$ of the correlation length is 1/2. However, there exists a certain extension of the model which gives finite correlation length in the broken symmetric phase. It typically appears in case of models possessing a first order phase transitions as is demonstrated on the example of the scalar field theory with a Coleman-Weinberg potential.

 

[marcus]

http://arxiv.org/abs/1203.6688
Observable Equivalence between General Relativity and Shape Dynamics
Tim Koslowski
(Submitted on 29 Mar 2012)
In this conceptual paper we construct a local version of Shape Dynamics that is equivalent to General Relativity in the sense that the algebras of Dirac observables weakly coincide. This allows us to identify Shape Dynamics observables with General Relativity observables, whose observables can now be interpreted as particular representative functions of observables of a conformal theory at fixed York time. An application of the observable equivalence of General Relativity and Shape Dynamics is to define the quantization of General Relativity through quantizing Shape Dynamics and using observable equivalence. We investigate this proposal explicitly for gravity in 2+1 dimensions.
16 pages,

 

[marcus]

http://arxiv.org/abs/1204.0211
Constraint algebra in LQG reloaded : Toy model of a U(1)³ Gauge Theory I
Adam Henderson, Alok Laddha, Casey Tomlin
(Submitted on 1 Apr 2012)
We analyze the issue of anomaly-free representations of the constraint algebra in Loop Quantum Gravity (LQG) in the context of a diffeomorphism-invariant gauge theory in three spacetime dimensions. We construct a Hamiltonian constraint operator whose commutator matches with a quantization of the classical Poisson bracket involving structure functions. Our quantization scheme is based on a geometric interpretation of the Hamiltonian constraint as a generator of phase space-dependent diffeomorphisms. The resulting Hamiltonian constraint at finite triangulation has a conceptual similarity with the "mu-bar"-scheme in loop quantum cosmology and highly intricate action on the spin-network states of the theory. We construct a subspace of non-normalizable states (distributions) on which the continuum Hamiltonian constraint is defined which leads to an anomaly-free representation of the Poisson bracket of two Hamiltonian constraints in loop quantized framework.
60 pages, 6 figures

brief mention:
http://arxiv.org/abs/1204.0492
Non-detection of the Tooth Fairy at Optical Wavelengths

 

[marcus]

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito
(Submitted on 2 Apr 2012)
We achieve a group theoretical quantization of the flat Friedmann-Robertson-Walker model coupled to a massless scalar field adopting the improved dynamics of loop quantum cosmology. Deparemeterizing the system using the scalar field as internal time, we first identify a complete set of phase space observables whose Poisson algebra is isomorphic to the su(1,1) Lie algebra. It is generated by the volume observable and the Hamiltonian. These observables describe faithfully the regularized phase space underlying the loop quantization: they account for the polymerization of the variable conjugate to the volume and for the existence of a kinematical non-vanishing minimum volume. Since the Hamiltonian is an element in the su(1,1) Lie algebra, the dynamics is now implemented as SU(1,1) transformations. At the quantum level, the system is quantized as a time-like irreducible representation of the group SU(1,1). These representations are labeled by a half-integer spin, which gives the minimal volume. They provide superselection sectors without quantization anomalies and no factor ordering ambiguity arises when representing the Hamiltonian. We then explicitly construct SU(1,1) coherent states to study the quantum evolution. They not only provide semiclassical states but truly dynamical coherent states. Their use further clarifies the nature of the bounce that resolves the big bang singularity.
33 pages

http://arxiv.org/abs/1204.0683
Shape dynamics and Mach's principles: Gravity from conformal geometrodynamics
Sean Gryb
(Submitted on 3 Apr 2012)
In this PhD thesis, we develop a new approach to classical gravity starting from Mach's principles and the idea that the local shape of spatial configurations is fundamental. This new theory, "shape dynamics", is equivalent to general relativity but differs in an important respect: shape dynamics is a theory of dynamic conformal 3-geometry, not a theory of spacetime. Equivalence is achieved by trading foliation invariance for local conformal invariance (up to a global scale). After the trading, what is left is a gauge theory invariant under 3d diffeomorphisms and conformal transformations that preserve the volume of space. The local canonical constraints are linear and the constraint algebra closes with structure constants. Shape dynamics, thus, provides a novel new starting point for quantum gravity.
The procedure for the trading of symmetries was inspired by a technique called "best matching". We explain best matching and its relation to Mach's principles. The key features of best matching are illustrated through finite dimensional toy models. A general picture is then established where relational theories are treated as gauge theories on configuration space. Shape dynamics is then constructed by applying best matching to conformal geometry. We then study shape dynamics in more detail by computing its Hamiltonian and Hamilton-Jacobi functional perturbatively.
This thesis is intended as a pedagogical but complete introduction to shape dynamics and the Machian ideas that led to its discovery. The reader is encouraged to start with the introduction, which gives a conceptual outline and links to the relevant sections in the text for a more rigorous exposition. When full rigor is lacking, references to the literature are given. It is hoped that this thesis may provide a starting point for anyone interested in learning about shape dynamics.
117 pages, 2 tables, 10 figures, PhD thesis

http://arxiv.org/abs/1204.0702
Radiation from quantum weakly dynamical horizons in LQG
Daniele Pranzetti
(Submitted on 3 Apr 2012)
Using the recent thermodynamical study of isolated horizons by Ghosh and Perez, we provide a statistical mechanical analysis of isolated horizons near equilibrium in the grand canonical ensemble. By matching the description of the dynamical phase in terms of weakly dynamical horizons with this local statistical framework, we introduce a notion of temperature in terms of the local surface gravity. This provides further support to the recovering of the semiclassical area law just by means of thermodynamical considerations. Moreover, it allows us to study the radiation process generated by the LQG dynamics near the horizon, providing a quantum gravity description of the horizon evaporation. For large black holes, the spectrum we derive presents a discrete structure which could be potentially observable and might be preserved even after the inclusion of all the relevant transition lines.
9 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 5 Apr 2012)
The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB.
4 pages, ICGC (2011) Goa Conference proceedings

http://arxiv.org/abs/1204.0965
Quantum-gravity-induced matter self-interactions in the asymptotic-safety scenario
Astrid Eichhorn
(Submitted on 4 Apr 2012)
We investigate the high-energy properties of matter theories coupled to quantum gravity. Specifically, we show that quantum gravity fluctuations generically induce matter self-interactions in a scalar theory. Our calculations apply within asymptotically safe quantum gravity, where our results indicate that the UV is dominated by an interacting fixed point, with non-vanishing gravitational as well as matter couplings. We show that the number of relevant directions of the fixed point depends on the inclusion of these quantum-gravity induced matter self-interactions. Furthermore we point out that terms of this type can have observable consequences in the context of scalar-field driven inflation, where they can induce potentially observable non-Gaussianities in the CMB.
15 pages, 9 figures

Rovelli's introductory QG series "Explorations in quantum gravity" http://pirsa.org/C12012
Lecture 1: http://pirsa.org/12040019/
Lecture 2: http://pirsa.org/12040020/
Lecture 3: http://pirsa.org/12040021/
Lecture 4: http://pirsa.org/12040022/

 

[marcus]

http://arxiv.org/abs/1204.2550
Diffusion in quantum gravity
Gianluca Calcagni
(Submitted on 11 Apr 2012)
The change of the effective dimension of spacetime with the probed scale is a universal phenomenon shared by independent models of quantum gravity. Using tools of probability theory and multifractal geometry, we show how dimensional flow is controlled by a multiscale fractional diffusion equation, and physically interpreted as a composite stochastic process. The simplest example is a fractional telegraph process, describing quantum spacetimes with a spectral dimension equal to 2 in the ultraviolet and monotonically rising to 4 towards the infrared.
4 pages, 1 figure

http://arxiv.org/abs/1204.1530
Hawking radiation from dynamical horizons
Ayan Chatterjee, Bhramar Chatterjee, Amit Ghosh
(Submitted on 6 Apr 2012)
In completely local settings, we establish that a dynamically evolving black hole horizon can be assigned a Hawking temperature. Moreover, we calculate the Hawking flux and show that the radius of the horizon shrinks.
5 Pages

brief mention:
http://arxiv.org/abs/1204.2520
Quantum Black Holes from Cosmic Rays
Xavier Calmet, Lauretiu Ioan Caramete, Octavian Micu
(Submitted on 11 Apr 2012)
We investigate the possibility for cosmic ray experiments to discover non-thermal small black holes with masses in the TeV range. Such black holes would result due to the impact between ultra high energy cosmic rays or neutrinos with nuclei from the upper atmosphere and decay instantaneously. They could be produced copiously if the Planck scale is in the few TeV region. As their masses are close to the Planck scale, these holes would typically decay into two particles emitted back-to-back. Depending on the angles between the emitted particles with respect to the center of mass direction of motion, it is possible for the simultaneous showers to be measured by the detectors.
4 pages, 1 figure

 

[atyy]

http://arxiv.org/abs/1112.0302
Spacetime as a topological insulator: Mechanism for the origin of the fermion generations
David B. Kaplan, Sichun Sun
(Submitted on 1 Dec 2011 (v1), last revised 13 Apr 2012 (this version, v3))
We suggest a mechanism whereby the three generations of quarks and leptons correspond to surface modes in a five-dimensional theory. These modes arise from a nonlinear fermion dispersion relation in the extra dimension, much in the same manner as fermion surface modes in a topological insulator or lattice implementation of domain wall fermions. We also show that the topological properties can persist in a deconstructed version of the model in four dimensions.

 

[marcus]

http://arxiv.org/abs/1204.3039
The analogue cosmological constant in Bose-Einstein condensates: a lesson for quantum gravity
Stefano Finazzi, Stefano Liberati, Lorenzo Sindoni
(Submitted on 13 Apr 2012)
For almost a century, the cosmological constant has been a mysterious object, in relation to both its origin and its very small value. By using a Bose-Einstein condensate analogue model for gravitational dynamics, we address here the cosmological constant issue from an analogue gravity standpoint. Starting from the fundamental equations describing a system of condensed bosons, we highlight the presence of a vacuum source term for the analogue gravitational field, playing the role of a cosmological constant. In this simple system it is possible to compute from scratch the value of this constant, to compare it with other characteristic energy scales and hence address the problem of its magnitude within this framework, suggesting a different path for the solution of this longstanding puzzle. We find that, even though this constant term is related with quantum vacuum effects, it is not immediately related to the ground state energy of the condensate. On the gravity side this result suggests that the interpretation and computation of the cosmological term as a form of renormalized vacuum energy might be misleading, its origin being related to the mechanism that instead produces spacetime from its pregeometric progenitor, shedding a different light on the subject and at the same time suggesting a potentially relevant role of analogue models in the understanding of quantum gravity.
24 pages, 1 figure, Proceedings of the II Amazonian Symposium on Physics

http://arxiv.org/abs/1204.3505
Conserved quantities in isotropic loop quantum cosmology
Daniel Cartin
(Submitted on 16 Apr 2012)
We develop an action principle for those models arising from isotropic loop quantum cosmology, and show that there is a natural conserved quantity Q for the discrete difference equation arising from the Hamiltonian constraint. This quantity Q relates the semi-classical limit of the wavefunction at large values of the spatial volume, but opposite triad orientations. Moreover, there is a similar quantity for generic difference equations of one parameter arising from a self-adjoint operator.
6 pages, to be published in Europhysics Letters

http://arxiv.org/abs/1204.3541
The local potential approximation in quantum gravity
Dario Benedetti, Francesco Caravelli
(Submitted on 16 Apr 2012)
Within the context of the functional renormalization group flow of gravity, we suggest that a generic f(R) ansatz (i.e. not truncated to any specific form, polynomial or not) for the effective action plays a role analogous to the local potential approximation (LPA) in scalar field theory. In the same spirit of the LPA, we derive and study an ordinary differential equation for f(R) to be satisfied by a fixed point of the renormalization group flow. As a first step in trying to assess the existence of global solutions (i.e. true fixed point) for such equation, we investigate here the properties of its solutions by a comparison of various series expansions and numerical integrations. In particular, we study the analyticity conditions required because of the presence of fixed singularities in the equation, and we develop an expansion of the solutions for large R up to order N=29. Studying the convergence of the fixed points of the truncated solutions with respect to N, we find a characteristic pattern for the location of the fixed points in the complex plane, with one point stemming out for its stability. Finally, we establish that if a non-Gaussian fixed point exists within the full f(R) approximation, it corresponds to an R² theory.
31 pages, 7 figures

http://arxiv.org/abs/1204.3531
Interacting Generalised Cosmic Chaplygin gas in Loop quantum cosmology: A singularity free universe
Ratul Chowdhury (Jadavpur U.), Prabir Rudra (Bengal Engin. Sci. U., Howrah)
(Submitted on 13 Apr 2012)
In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by Loop quantum cosmology. Dark energy in the form of Generalised Cosmic Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is taken into account in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence problem is taken. The statefinder parameters are also calculated to classify the dark energy model. Graphs and phase diagrams are drawn to study the variations of these parameters. It is seen that the background dynamics of Generalised Cosmic Chaplygin gas is completely consistent with the notion of an accelerated expansion in the late universe. From the graphs, generalised cosmic Chaplygin gas is identified as a dark fluid with a lesser negative pressure compared to Modified Chaplygin gas, thus supporting a 'No Big Rip' cosmology. It has also been shown that in this model the universe follows the power law form of expansion around the critical point, which is consistent with the known results. Future singularities that may be formed in this model as an ultimate fate of the universe has been studied in detail. It was found that the model is completely free from any types of future singularities.
10 pages, 10 figures

brief mention (of general interest though not directly LoopQG-related)
http://arxiv.org/abs/1204.3138
Quantization of area for event and Cauchy horizons of the Kerr-Newman black hole
Matt Visser (Victoria University of Wellington)
(Submitted on 14 Apr 2012)
Based on various string theoretic constructions, there have been repeated suggestions that the areas of black hole event horizons should be quantized in a quite specific manner, involving linear combinations of square roots of natural numbers. It is important to realize the significant physical limitations of such proposals when one attempts to extend them outside their original framework. Specifically, in their most natural and direct physical interpretations, these specific proposals for horizon areas fail for the ordinary Kerr-Newman black holes in (3+1) dimensions, essentially because the fine structure constant is not an integer. A more baroque interpretation involves asserting the fine structure constant is the square root of a rational number; but such a proposal has its own problems. Insofar as one takes (3+1) general relativity (plus the usual quantization of angular momentum and electric charge) as being paramount, the known explicitly calculable spectra of horizon areas for the physically compelling Kerr-Newman spacetimes do not resemble those of currently available string theoretic constructions.
15 pages

 

[marcus]

http://arxiv.org/abs/1204.4345
The spacetime in the neighborhood of a general isolated black hole
Badri Krishnan
(Submitted on 19 Apr 2012)
We construct the spacetime in the vicinity of a general isolated, rotating, charged black hole. The black hole is modeled as a weakly isolated horizon, and we use the characteristic initial value formulation of the Einstein equations with the horizon as an inner boundary. The spacetime metric and other geometric fields are expanded in a power series in a radial coordinate away from the horizon by solving the characteristic field equations in the Newman-Penrose formalism. This is the first in a series of papers which investigate the near horizon geometry and its physical applications using the isolated horizon framework.
23 pages, 1 figure

http://arxiv.org/abs/1204.4344
Reply to the comment on "Black hole entropy and isolated horizons thermodynamics"
Amit Ghosh, Alejandro Perez
(Submitted on 19 Apr 2012)
The comment http://arxiv.org/abs/1204.2729v1 is completely wrong. The author makes serious mistakes in calculations and judgement...

brief mention:
http://arxiv.org/abs/1204.4339
Teleparallel Gravity as a Higher Gauge Theory
John C. Baez, Derek K. Wise
(Submitted on 19 Apr 2012)
We show that general relativity can be viewed as a higher gauge theory involving a categorical group, or 2-group, called the teleparallel 2-group. On any semi-Riemannian manifold M, we first construct a principal 2-bundle with the Poincaré 2-group as its structure 2-group. Any flat metric-preserving connection on M gives a flat 2-connection on this 2-bundle, and the key ingredient of this 2-connection is the torsion. Conversely, every flat strict 2-connection on this 2-bundle arises in this way if M is simply connected and has vanishing 2nd deRham cohomology. Extending from the Poincaré 2-group to the teleparallel 2-group, a 2-connection includes an additional piece: a coframe field. Taking advantage of the teleparallel reformulation of general relativity, in which a coframe field, a flat connection and its torsion are the key ingredients, this let's us rewrite general relativity as a theory with a 2-connection for the teleparallel 2-group as its only field.
35 pages

 

[marcus]

Unfortunately not available online, a talk given today at Princeton Institute for Advanced Studies:
Monday, April 23, 2012
High Energy Theory Seminar
“Loop Quantum Gravity: Recent Results and Open Problems”
Location: Bloomberg Lecture Hall
Time: 2:30 PM
Speaker(s): Carlo Rovelli, Centre de Physique Théorique de Luminy, Aix-Marseille University, France
Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit.
http://www.princeton.edu/physics/events/viewevent.xml?id=347

http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi
(Submitted on 23 Apr 2012)
We compute the entropy of non-extremal black holes using the quantum dynamics of Loop Gravity. The horizon entropy is finite, scales linearly with the area A, and reproduces the Bekenstein-Hawking expression S = A/4 with the one-fourth coefficient for all values of the Immirzi parameter. The near-horizon geometry of a non-extremal black hole - as seen by a stationary observer - is described by a Rindler horizon. We introduce the notion of a quantum Rindler horizon in the framework of Loop Gravity. The system is described by a quantum surface and the dynamics is generated by the boost Hamiltonion of Lorentzian Spinfoams. We show that the expectation value of the boost Hamiltonian reproduces the local horizon energy of Frodden, Ghosh and Perez. We study the coupling of the geometry of the quantum horizon to a two-level system and show that it thermalizes to the local Unruh temperature. The derived values of the energy and the temperature allow one to compute the thermodynamic entropy of the quantum horizon. The relation with the Spinfoam partition function is discussed.
6 pages, 1 figure

brief mention--not Loop-and-allied QG, but possibly of general interest:

http://arxiv.org/abs/1204.4926
Relating the quantum mechanics of discrete systems to standard canonical quantum mechanics
Gerard 't Hooft
(Submitted on 22 Apr 2012)
Discrete quantum mechanics is here defined to be a quantum theory of wave functions defined on integers P_i and Q_i, while canonical quantum mechanics is assumed to be based on wave functions on the real numbers, R^n. We study reversible mappings from the position operators q_i and their quantum canonical operators p_i of a canonical theory, onto the discrete, commuting operators Q_i and P_i. In this paper we are particularly interested in harmonic oscillators. In the discrete system, these turn into deterministic models, which is our motivation for this study. We regard the procedure worked out here as a "canonical formalism" for discrete dynamics, and as a stepping stone to handling discrete deterministic systems in a quantum formalism.
20 pages (incl. title page), 2 figures

http://arxiv.org/abs/1204.4683
A Long View of Particle Physics
Frank Wilczek
(Submitted on 20 Apr 2012)
2011 marked the hundredth anniversary both of the famous Solvay conferences, and of the Geiger-Marsden experiment that launched the modern understanding of subatomic structure. I was asked to survey the status and prospects of particle physics for the anniversary Solvay conference, with appropriate perspective. This is my attempt.
8 pages, no figures. Rapporteur talk at the 25th Solvay Conference on Physics, "Theory of the Quantum World", October 2011. To be published in the Proceedings

 

[negru]

I was at the talk at IAS, everyone was pretty confused by what he was doing. One point brought up was that there exist other models, like YM in 5d i think, whose discrete version has the correct classical limit and is uv and ir finite, but does not make sense quantum mechanically. And there was no concrete argument for why lqg would be a better example. Some numerical checks are needed, and he said they are very hard to do but people are working on them.

 

[MTd2]

This thread is for bibliographical help. You might wish to open a thread to tell your experience.

 

[marcus]

http://arxiv.org/abs/1204.5394
Discrete gravity models and Loop Quantum Gravity: a short review
Maite Dupuis, James P. Ryan, Simone Speziale
(Submitted on 24 Apr 2012)
We review the relation between loop quantum gravity on a fixed graph and discrete models of gravity. We compare Regge and twisted geometries, and discuss discrete actions based on twisted geometries and on the discretization of the Plebanski action. We discuss the role of discrete geometries in the spin foam formalism, with particular attention to the definition of the simplicity constraints.
32 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

brief mention:
http://arxiv.org/abs/1204.5455
Quantization of Lorentzian 3d Gravity by Partial Gauge Fixing
Rodrigo M S Barbosa, Clisthenis P Constantinidis, Zui Oporto, Olivier Piguet
(Submitted on 24 Apr 2012)
D = 2+1 gravity with a cosmological constant has been shown by Bonzom and Livine to present a Barbero-Immirzi like ambiguity depending on a parameter. We make use of this fact to show that, for positive cosmological constant, the Lorentzian theory can be partially gauge fixed and reduced to an SU(2) Chern-Simons theory...
19 pages

 

[marcus]

http://arxiv.org/abs/1204.6349
Gravitation and vacuum entanglement entropy
Ted Jacobson
(Submitted on 28 Apr 2012)
The vacuum of quantum fields contains correlated fluctuations. When restricted to one side of a surface these have a huge entropy of entanglement that scales with the surface area. If UV physics renders this entropy finite, then a thermodynamic argument implies the existence of gravity. That is, the causal structure of spacetime must be dynamical and governed by the Einstein equation with Newton's constant inversely proportional to the entropy density. Conversely, the existence of gravity makes the entanglement entropy finite. This thermodynamic reasoning is powerful despite the lack of a detailed description of the dynamics at the cutoff scale, but it has its limitations. In particular, we should not expect to understand corrections to Einstein gravity in this way.
9 pages; Essay written for the Gravity Research Foundation 2012 Awards for Essays on Gravitation