Intuitive content of Loop Gravity--Rovelli's program

marcus

http://pirsa.org/12050050
Shape Dynamics and General Relativity
Speaker(s): Julian Barbour
Abstract: Shape Dynamics first arose as a theory of particle interactions formulated without any of Newton's absolute structures. Its fundamental arena is shape space, which is obtained by quotienting Newton's kinematic framework with respect to translations, rotations and dilatations. This leads to a universe defined purely intrinsically in relational terms. It is then postulated that a dynamical history is determined by the specification in shape space of an initial shape and an associated rate of change of shape. There is a very natural way to create a theory that meets such a requirement. It fully implements Mach's principle and shows how time and local inertial frames are determined by the universe as whole. If the same principles are applied to a spatially closed universe in which geometry is dynamical, they lead rather surprisingly to a theory that, modulo some caveats, is dynamically equivalent to general relativity but dual to it in that refoliation invariance is traded for three-dimensional conformal invariance. This shows that there is a hidden three-dimensional conformal symmetry within general relativity. It is in fact what underlies York's crucial method of solution of the initial-value problem in general relativity. It is also remarkable that, as in York's work, shape dynamics inescapably introduces a mathematically distinguished notion of absolute simultaneity, the desirability of which has been found in two currently popular approaches to quantum gravity: causal dynamical triangulations and Horava gravity. I aim to express the key ideas and techniques of shape dynamics as simply as possible.
Date: 09/05/2012 - 2:00 pm
Series: Colloquium
[My comment: a remarkable and convincing talk partly because of the clear personal perspective it gives on shape dynamics and its history. Best introduction to SD so far IMO.]

brief mention:
http://arxiv.org/abs/1205.2158
Massive gravity from bimetric gravity
Valentina Baccetti (Victoria University of Wellington), Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 10 May 2012)
We discuss the subtle relationship between so-called massive gravity (that is, gravity incorporating a non-zero graviton mass) and bimetric gravity, focussing particularly on the manner in which massive gravity may be viewed as a suitable limit of bimetric gravity. The limiting procedure is more delicate than currently appreciated, and in particular, in a cosmological context can lead to an interesting interplay between the "background" and "foreground" metrics...
25 pages

Blackforest

I like equation (19). Thanks for the reference.

marcus

http://pirsa.org/12050061
Conformal Gravity and Black Hole Complementarity
Speaker(s): Gerard t'Hooft
Date: 11/05/2012 - 4:40 pm
Collection: Conformal Nature of the Universe
[Fascinating talk, with possibly ground-breaking conceptual novelty. I would recommend anyone interested in quantum gravity watch at least the first 25-30 minutes. The audio needs a boost though, and requires close attention at some points.]

http://pirsa.org/12050072/
Two-dimensional Conformal Symmetry of Short-distance Spacetime
Speaker(s): Steve Carlip
Abstract: Evidence from several approaches to quantum gravity hints at the possibility that spacetime undergoes a "spontaneous dimensional reduction" at very short distances. If this is the case, the small scale universe might be described by a theory with two-dimensional conformal symmetry. I will summarize the evidence for dimensional reduction and indicate a tentative path towards using this conformal invariance to explore quantum gravity.
Date: 11/05/2012 - 9:00 am

http://arxiv.org/abs/1205.2953
Expressing entropy globally in terms of (4D) field-correlations
Rafael D. Sorkin
(Submitted on 14 May 2012)
We express the entropy of a scalar field φ directly in terms of its spacetime correlation function W(x,y)=<φ(x)φ(y)>, assuming that the higher correlators are of "Gaussian" form. The resulting formula associates an entropy S(R) to any spacetime region R; and when R is globally hyperbolic with Cauchy surface Σ, S(R) can be interpreted as the entropy of the reduced density-matrix belonging to Σ. One acquires in particular a new expression for the entropy of entanglement across an event-horizon. Thanks to its spacetime character, this expression makes sense in a causal set as well as in a continuum spacetime.
14 pages, 2 figures. To appear in proceedings of ICGC2011, held Goa, Journal of Physics Conference Series.

brief mention:
http://arxiv.org/abs/1205.2720
Why there is something rather than nothing: The finite, infinite and eternal
Peter Lynds

marcus

http://arxiv.org/abs/1205.3370
Lattice loop quantum cosmology: scalar perturbations
Edward Wilson-Ewing
(Submitted on 15 May 2012)
We study the scalar modes of linear perturbations in loop quantum cosmology. This is done on a lattice where each cell is taken to be homogeneous and isotropic and can be quantized via standard homogeneous loop quantum cosmology techniques. The appropriate interactions between nearby cells are included in the Hamiltonian in order to obtain the correct physics. It is shown that the quantum theory is anomaly-free: the scalar and diffeomorphism constraint operators weakly commute with the Hamiltonian. Finally, the effective theory encoding the leading order quantum gravity corrections is derived and is shown to give the same holonomy-corrected effective equations that have been obtained in previous studies.
32 pages

http://arxiv.org/abs/1205.3487
A New Term in the Microcanonical Entropy of Quantum Isolated Horizon
Abhishek Majhi
(Submitted on 15 May 2012)
The quantum geometric framework for Isolated Horizon has led to the Bekenstein-Hawking area law and the quantum logarithmic correction for the black hole entropy. The point to be noted here is that all the results have been derived in a model independent way and completely from within the quantum geometric framework where the quantum degrees of freedom are described by the states of the SU(2) Chern Simons theory on the Isolated Horizon. Here we show that a completely new term independent of the area of the Isolated Horizon appears in the microcanonical entropy. It has a coeffcient which is a function of the Barbero Immirzi parameter.
4 pages

http://arxiv.org/abs/1205.3291
The Unbearable Beingness of Light, Dressing and Undressing Photons in Black Hole Spacetimes
Timothy J. Hollowood, Graham M. Shore
(Submitted on 15 May 2012)
Gravitational tidal forces acting on the virtual e+ e- cloud surrounding a photon endow spacetime with a non-trivial refractive index. This has remarkable properties unique to gravitational theories including superluminal low-frequency propagation, in apparent violation of causality, and amplification of the renormalized photon field, in apparent violation of unitarity. Using the geometry of null congruences and the Penrose limit, we illustrate these phenomena and their resolution by tracing the history of a photon as it falls into the near-singularity region of a black hole.
8 pages, Essay awarded third prize in the Gravity Research Foundation essay competition 2012

marcus

http://arxiv.org/abs/1205.3636
Can we measure structures to a precision better than the Planck length?
Sabine Hossenfelder
(Submitted on 16 May 2012)
It was recently claimed that the Planck length is not a limit to the precision by which we can measure distances, but that instead it is merely the Planck volume that limits the precision by which we can measure volumes. Here, we investigate this claim and show that the argument does not support the conclusion.
9 pages, 1 figure

http://arxiv.org/abs/1205.3583
Running boundary actions, Asymptotic Safety, and black hole thermodynamics
D. Becker, M. Reuter
(Submitted on 16 May 2012)
Previous explorations of the Asymptotic Safety scenario in Quantum Einstein Gravity (QEG) by means of the effective average action and its associated functional renormalization group (RG) equation assumed spacetime manifolds which have no boundaries. Here we take a first step towards a generalization for non-trivial boundaries, restricting ourselves to action functionals which are at most of second order in the derivatives acting on the metric. We analyze two examples of truncated actions with running boundary terms: full fledged QEG within the single-metric Einstein-Hilbert (EH) truncation, augmented by a scale dependent Gibbons-Hawking (GH) surface term, and a bi-metric truncation for gravity coupled to scalar matter fields. The latter contains 17 running couplings, related to both bulk and boundary terms, whose beta-functions are computed in the induced gravity approximation. We find that the bulk and the boundary Newton constant, pertaining to the EH and GH term, respectively, show opposite RG running; proposing a scale dependent variant of the ADM mass we argue that the running of both couplings is consistent with gravitational anti-screening. We describe a simple device for counting the number of field modes integrated out between the infrared cutoff scale and the ultraviolet. This method makes it manifest that, in an asymptotically safe theory, there are effectively no field modes integrated out while the RG trajectory stays in the scaling regime of the underlying fixed point. As an application, we investigate how the semiclassical theory of Black Hole Thermodynamics gets modified by quantum gravity effects and compare the new picture to older work on `RG-improved black holes' which incorporated the running of the bulk Newton constant only. We find, for instance, that the black hole's entropy vanishes and its specific heat capacity turns positive at Planckian scales.
Comments: 65 pages, 2 figures

not loop quantum gravity but could alter foundations and hence prove of general interest:
http://arxiv.org/abs/1205.3707
Precedence and freedom in quantum physics
Lee Smolin
(Submitted on 16 May 2012)
A new interpretation of quantum mechanics is proposed according to which precedence, freedom and novelty play central roles. This is based on a modification of the postulates for quantum theory given by Masanes and Muller. We argue that quantum mechanics is uniquely characterized as the probabilistic theory in which individual systems have maximal freedom in their responses to experiment, given reasonable axioms for the behavior of probabilities in a physical theory. Thus, to the extent that quantum systems are free, in the sense of Conway and Kochen, there is a sense in which they are maximally free.
We also propose that laws of quantum evolution arise from a principle of precedence, according to which the outcome of a measurement on a quantum system is selected randomly from the ensemble of outcomes of previous instances of the same measurement on the same quantum system. This implies that dynamical laws for quantum systems can evolve as the universe evolves, because new precedents are generated by the formation of new entangled states.
12 pages

arivero

Quantum theory of weak gravitational fields by Matvei Bronstein is translated and republished free of charge (EDIT:hmm, no) here http://www.springerlink.com/content/...427080/?MUD=MP

marcus

Thanks for the Bronstein reference!

http://arxiv.org/abs/1205.3791
The transfer matrix in four-dimensional CDT
Jan Ambjorn, Jakub Gizbert-Studnicki, Andrzej Görlich, Jerzy Jurkiewicz
(Submitted on 16 May 2012)
The Causal Dynamical Triangulation model of quantum gravity (CDT) has a transfer matrix, relating spatial geometries at adjacent (discrete lattice) times. The transfer matrix uniquely determines the theory. We show that the measurements of the scale factor of the (CDT) universe are well described by an effective transfer matrix where the matrix elements are labeled only by the scale factor. Using computer simulations we determine the effective transfer matrix elements and show how they relate to an effective minisuperspace action at all scales.
32 pages, 19 figures

http://arxiv.org/abs/1205.3974
Statistical analysis of entropy correction from topological defects in Loop Black Holes
Kinjalk Lochan, Cenalo Vaz
(Submitted on 17 May 2012)
In this paper we discuss the entropy of quantum black holes in the LQG formalism when the number of punctures on the horizon is treated as a quantum hair, that is we compute the black hole entropy in the grand canonical (area) ensemble. The entropy is a function of both the average area and the average number of punctures and bears little resemblance to the Bekenstein-Hawking entropy. In the thermodynamic limit, both the "temperature" and the chemical potential can be shown to be functions only of the average area per puncture. At a fixed temperature, the average number of punctures becomes proportional to the average area and we recover the Bekenstein-Hawking area-entropy law to leading order provided that the Barbero-Immirzi parameter, γ, is appropriately fixed. This also relates the chemical potential to γ. We obtain a sub-leading correction, which differs in signature from that obtained in the microcanonical and canonical ensembles in its sign but agrees with earlier results in the grand canonical ensemble.
12 pages

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

http://arxiv.org/abs/1205.3807
Accelerated Expansion from Negative Λ
James B. Hartle, S. W. Hawking, Thomas Hertog
(Submitted on 16 May 2012)
Wave functions specifying a quantum state of the universe must satisfy the constraints of general relativity, in particular the Wheeler-DeWitt equation (WDWE). We show for a wide class of models with non-zero cosmological constant that solutions of the WDWE exhibit a universal semiclassical asymptotic structure for large spatial volumes. A consequence of this asymptotic structure is that a wave function in a gravitational theory with a negative cosmological constant can predict an ensemble of asymptotically classical histories which expand with a positive effective cosmological constant. This raises the possibility that even fundamental theories with a negative cosmological constant can be consistent with our low-energy observations of a classical, accelerating universe. We illustrate this general framework with the specific example of the no-boundary wave function in its holographic form. The implications of these results for model building in string cosmology are discussed.
28 pages

atyy

http://arxiv.org/abs/1205.4218
Limit cycles and quantum gravity
Daniel Litim, Alejandro Satz
(Submitted on 18 May 2012)
We study renormalization group equations of quantum gravity in four dimensions. We find an ultraviolet fixed point in accordance with the asymptotic safety conjecture, and infrared fixed points corresponding to general relativity with positive, vanishing or negative cosmological constant. In a minisuperspace approximation, we additionally find a renormalization group limit cycle shielding the ultraviolet from the infrared fixed points. We discuss implications of this pattern for asymptotically safe gravity in the continuum and on the lattice.

John86

http://arxiv.org/abs/1205.4107
Duality between a deterministic cellular automaton and a bosonic quantum field theory in 1+1 dimensions
Gerard 't Hooft
(Submitted on 18 May 2012)
Methods developed in a previous paper are employed to define an exact correspondence between the states of a deterministic cellular automaton in 1+1 dimensions and those of a bosonic quantum field theory. The result may be used to argue that quantum field theories may be much closer related to deterministic automata than what is usually thought possible.

marcus

http://arxiv.org/abs/1205.5161
Can effects of quantum gravity be observed in the cosmic microwave background?
Claus Kiefer, Manuel Kraemer
(Submitted on 23 May 2012)
We investigate the question whether small quantum-gravitational effects can be observed in the anisotropy spectrum of the cosmic microwave background radiation. An observation of such an effect is needed in order to discriminate between different approaches to quantum gravity. Using canonical quantum gravity with the Wheeler-DeWitt equation, we find a suppression of power at large scales. Current observations only lead to an upper bound on the energy scale of inflation, but the framework is general enough to study other situations in which such effects might indeed be seen.
5 pages, 1 figure, essay awarded first prize in the Gravity Research Foundation essay competition 2012
[my comment: in their conclusions K and K call attention to http://arxiv.org/abs/1101.5391 by Bojowald Calcagni Tsujikawa which derives a detectable prediction about CMB that can discriminate between theories]

brief mention:
http://arxiv.org/abs/1205.5251
Effective temperature, Hawking radiation and quasinormal modes
Christian Corda
(Submitted on 17 May 2012)
Parikh and Wilczek have shown that Hawking radiation's spectrum cannot be strictly thermal. Such a non-strictly thermal character implies that the spectrum is also not strictly continuous... In particular, the formula of the horizon's area quantization and the number of quanta of area are modified becoming functions of the quantum "overtone" number n. Consequently, Bekenstein-Hawking entropy, its sub-leading corrections and the number of microstates, i.e. quantities which are fundamental to realize unitary quantum gravity theory, are also modified. They become functions of the quantum overtone number too...
11 pages, founded on the research paper JHEP 1108, 101 (2011), http://arxiv.org/abs/1107.5334. This essay received an honorable mention in the 2012 Essay Competition of the Gravity Research Foundation.

http://pirsa.org/12050002/
New Probes of Initial State of Quantum Fluctuations During Inflation
Speaker(s): Eiichiro Komatsu
Abstract: ...When we calculate statistical properties of primordial fluctuations generated during inflation, we usually assume that the initial state of quantum fluctuations is in a preferred vacuum state called Bunch-Davies vacuum. While there is some motivation for choosing such a state, this is an assumption, and thus needs to be tested by observations. In this talk I will present new probes of initial state of quantum fluctuations during inflation: the 3-point function of the cosmic microwave background anisotropy, the 2-point function of galaxies, and a spectral distortion of the thermal spectrum of the cosmic microwave background.
Date: 22/05/2012 - 11:00 am

http://arxiv.org/abs/1205.5046
Diffusion in multi-fractional spacetimes
Gianluca Calcagni
(Submitted on 22 May 2012)
We study diffusion processes in anomalous spacetimes regarded as models of quantum geometry... The case of multi-scale (in particular, multi-fractal) spacetimes is then considered through a number of examples and the most general spectral-dimension profile of multi-fractional spaces is constructed.
43 pages, 5 figures

MTd2

http://arxiv.org/abs/1205.5431

Asymptotic Safety, Fractals, and Cosmology

Martin Reuter, Frank Saueressig
(Submitted on 24 May 2012)
These lecture notes introduce the basic ideas of the Asymptotic Safety approach to Quantum Einstein Gravity (QEG). In particular they provide the background for recent work on the possibly multifractal structure of the QEG space-times. Implications of Asymptotic Safety for the cosmology of the early Universe are also discussed.

marcus

http://arxiv.org/abs/1205.5325
Horizon energy as the boost boundary term in general relativity and loop gravity
Eugenio Bianchi, Wolfgang Wieland
(Submitted on 24 May 2012)
We show that the near-horizon energy introduced by Frodden, Ghosh and Perez arises from the action for general relativity as a horizon boundary term. Spin foam variables are used in the analysis. The result provides a derivation of the horizon boost Hamiltonian introduced by one of us to define the dynamics of the horizon degrees of freedom, and shows that loop gravity provides a realization of the horizon Schrodinger equation proposed by Carlip and Teitelboim.
3 pages, 1 figure

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin
(Submitted on 24 May 2012)
Building on recent significant results of Frodden, Ghosh and Perez (FGP) and Bianchi, I present a quantum version of Jacobson's argument that the Einstein equations emerge as the equation of state of a quantum gravitational system. I give three criteria a quantum theory of gravity must satisfy if it is to allow Jacobson's argument to be run. I then show that the results of FGP and Bianchi provide evidence that loop quantum gravity satisfies two of these criteria and argue that the third should also be satisfied in loop quantum gravity. I also show that the energy defined by FGP is the canonical energy associated with the boundary term of the Holst action.
9 pages, 3 figures

John86

Nice Rovelli talk at Edge.org !

http://www.edge.org/

marcus

thanks! I think a permanent link to that Edge conversation with Rovelli is:
http://edge.org/conversation/a-philosophy-of-physics

marcus

http://arxiv.org/abs/1205.5677
Generating Functions for Coherent Intertwiners
Valentin Bonzom, Etera R. Livine
(Submitted on 25 May 2012)
We study generating functions for the scalar products of SU(2) coherent intertwiners, which can be interpreted as coherent spin network evaluations on a 2-vertex graph. We show that these generating functions are exactly summable for different choices of combinatorial weights. Moreover, we identify one choice of weight distinguished thanks to its geometric interpretation. As an example of dynamics, we consider the simple case of SU(2) flatness and describe the corresponding Hamiltonian constraint whose quantization on coherent intertwiners leads to partial differential equations that we solve. Furthermore, we generalize explicitly these Wheeler-DeWitt equations for SU(2) flatness on coherent spin networks for arbitrary graphs.
31 pages

http://arxiv.org/abs/1205.5733
Entropy in the Classical and Quantum Polymer Black Hole Models
Etera R. Livine, Daniel R. Terno
(Submitted on 25 May 2012)
We investigate the entropy counting for black hole horizons in loop quantum gravity (LQG). We argue that the space of 3d closed polyhedra is the classical counterpart of the space of SU(2) intertwiners at the quantum level. Then computing the entropy for the boundary horizon amounts to calculating the density of polyhedra or the number of intertwiners at fixed total area. Following the previous work arXiv:1011.5628, we dub these the classical and quantum polymer models for isolated horizons in LQG. We provide exact micro-canonical calculations for both models and we show that the classical counting of polyhedra accounts for most of the features of the intertwiner counting (leading order entropy and log-correction), thus providing us with a simpler model to further investigate correlations and dynamics. To illustrate this, we also produce an exact formula for the dimension of the intertwiner space as a density of "almost-closed polyhedra".
24 pages

http://arxiv.org/abs/1205.5683
Equipartition energy, Noether energy and boundary term in gravitational action
T. Padmanabhan
(Submitted on 25 May 2012)
Padmanabhan indicates in his conclusions that his results are relevant to four recent Loop Gravity papers (references [10] and [11] by Frodden Ghosh Perez, by Bianchi, by Smolin, and by Bianchi Wieland:
==quote T.P. conclusions and references==
One motivation for writing this note stems from the recent interest in E_N = TS in a few papers [10] which do not mention the connection between E_N and the Noether charge, viz., that they are the same and E_N is not a physical entity unrelated to previously known expressions! The relationship between E_N and the boundary term of the gravitational action (which is essentially the relationship between the Noether charge and the boundary term of the action, a relationship that is probably of deeper significance) also seems to have gone unnoticed earlier. While this note was in the final stages of preparation, two papers appeared in the arXiv [11] which related E_N to spinfoam based models and their boundary action, etc. However, as pointed out above, the relationship is actually very simple. It holds for the standard general relativistic action and its boundary term and is physically transparent once the connection between the Noether charge and E_N is recognized.
...
...
[10] See for eg., E. Frodden, A. Ghosh, A. Perez, [arXiv:1110.4055]; E. Bianchi, [arXiv:1204.5122].
[11] L. Smolin, arXiv:1205.5529; E. Bianchi, W. Wieland, [arXiv:1205.5325].
==endquote==

brief mention:
http://arxiv.org/abs/1205.5768
Arguments Against a Finite N=8 Supergravity
T. Banks
(Submitted on 25 May 2012)

atyy

http://arxiv.org/abs/1205.6127
From the discrete to the continuous - towards a cylindrically consistent dynamics
Bianca Dittrich (Perimeter Institute for Theoretical Physics, Max Planck Institute for Gravitational Physics)
(Submitted on 28 May 2012)
Discrete models usually represent approximations to continuum physics. Cylindrical consistency provides a framework in which discretizations mirror exactly the continuum limit. Being a standard tool for the kinematics of loop quantum gravity we propose a coarse graining procedure that aims at constructing a cylindrically consistent dynamics in the form of transition amplitudes and Hamilton's principal functions. The coarse graining procedure, which is motivated by tensor network renormalization methods, provides a systematic approximation scheme towards this end. A crucial role in this coarse graining scheme is played by embedding maps that allow the interpretation of discrete boundary data as continuum configurations. These embedding maps should be selected according to the dynamics of the system, as a choice of embedding maps will determine a truncation of the renormalization flow.

http://arxiv.org/abs/1205.6166
On the space of generalized fluxes for loop quantum gravity
Bianca Dittrich, Carlos Guedes, Daniele Oriti
(Submitted on 28 May 2012)
We show that the space of generalized fluxes - momentum space - for loop quantum gravity cannot be constructed by Fourier transforming the projective limit construction of the space of generalized connections - position space - due to the non-abelianess of the gauge group SU(2). From the abelianization of SU(2), U(1)^3, we learn that the space of generalized fluxes turns out to be an inductive limit, and we determine the consistency conditions the fluxes should satisfy under coarse-graining of the underlying graphs. We comment on the applications to loop quantum cosmology, in particular, how the characterization of the Bohr compactification of the real line as a projective limit opens the way for a similar analysis for LQC.

MTd2

Is he a relative of the other Smoot?

http://arxiv.org/abs/1205.6389

An alternate derivation of Padmanabhan's differential bulk-surface relation in General Relativity

Dennis G. Smoot
(Submitted on 29 May 2012)
A differential bulk-surface relation of the lagrangian of General Relativity has been derived by Padmanabhan. This has relevance to gravitational information and degrees of freedom. An alternate derivation is given based on the differential form gauge theory formulation of gravity due to Gockeler and Schucker. Also an entropy functional of Padmanabhan and Paranjape can be rewritten as the Gockeler and Schucker lagrangian.