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Intuitive content of Loop Gravity-Rovelli's program

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marcus
#1675
Feb12-12, 07:27 PM
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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
#1676
Feb13-12, 07:12 PM
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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
#1677
Feb15-12, 07:18 PM
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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
#1678
Feb16-12, 04:38 PM
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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 γm=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
#1679
Feb20-12, 08:10 PM
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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
marcus
#1680
Feb23-12, 08:22 PM
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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
marcus
#1681
Feb28-12, 07:26 PM
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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]
marcus
#1682
Mar3-12, 10:15 AM
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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)
marcus
#1683
Mar6-12, 07:49 PM
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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
marcus
#1684
Mar7-12, 07:43 PM
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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
marcus
#1685
Mar11-12, 07:27 PM
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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
marcus
#1686
Mar13-12, 08:22 PM
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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
marcus
#1687
Mar14-12, 07:31 PM
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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)
marcus
#1688
Mar15-12, 07:31 PM
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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
marcus
#1689
Mar18-12, 07:27 PM
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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
marcus
#1690
Mar19-12, 07:16 PM
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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(ΩmL, ΩLm), 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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#1691
Mar22-12, 07:49 PM
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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.
marcus
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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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