Intuitive content of Loop Gravity--Rovelli's program

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/Archive...op-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→ LQG


Ziprick 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 http://physicsforums.com/showthread....00#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

marcus

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

marcus

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

marcus

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]

marcus

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)

marcus

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