Loop-and-allied QG bibliography

[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]

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

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/f557746004427080/?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]

Nice Rovelli talk at Edge.org !

http://www.edge.org/

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.

 

[marcus]

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.

Here is Dennis Smoot's 2010 PhD thesis from which the 7-page "Alternate Derivation" paper was excerpted. His advisor at the University of Illinois at Chicago was Louis Kauffman, whom we know in connection with some Perimeter work on knot-matter.
http://dsmoot.net/Thesis.pdf
DS got PhD in 2011 and is now teaching physics at Prairie State College in Chicago Heights, Illinois.
Smoot is not an unusual name in the Usa as far as I know.

 

[marcus]

http://arxiv.org/abs/1205.6763
Quantum gravitational Kasner transitions in Bianchi-I spacetime
Brajesh Gupt, Parampreet Singh
(Submitted on 30 May 2012)
Due to non-perturbative quantum gravitational effects, the classical big bang singularity is replaced by a quantum big bounce of the mean scale factor in loop quantization of Bianchi-I spacetime. An important issue is to understand various differences in the physical properties of the spacetime across the bounce. We investigate this issue in the context of various geometrical structures, identified by Kasner exponents of the metric, which arise on approach to the singularity in the classical theory. Using effective spacetime description of Bianchi-I model in loop quantum cosmology with dust, radiation and stiff matter, we find that as in the classical theory, geometrical structures such as a cigar or a pancake form, but they are finite and non-singular. Depending on the initial conditions of matter and anisotropies, different geometric structures are possible in the pre- and post-bounce phases in physical evolution. Thus, quantum gravitational effects can cause a Kasner transition in Bianchi-I spacetime, which is not possible at the classical level. Interestingly, we find that not all transitions are allowed at the level of effective dynamics in loop quantum cosmology. We find the selection rules and underlying conditions for all allowed and forbidden transitions. Selection rules suggest that for a given set of initial conditions on anisotropies, occurrence of Kasner transitions follows a distinct pattern, and certain transitions are more favored than others.
26 pages, 10 figures

brief mention:
http://arxiv.org/abs/1205.6633

 

[marcus]

http://pirsa.org/12050053 [video and pdf]
Black Hole Entropy from Loop Quantum Gravity
Speaker(s): Eugenio Bianchi
Abstract: There is strong theoretical evidence that black holes have a finite thermodynamic entropy equal to one quarter the area A of the horizon. Providing a microscopic derivation of the entropy of the horizon is a major task for a candidate theory of quantum gravity. Loop quantum gravity has been shown to provide a geometric explanation of the finiteness of the entropy and of the proportionality to the area of the horizon. The microstates are quantum geometries of the horizon. What has been missing until recently is the identification of the near-horizon quantum dynamics and a derivation of the universal form of the Bekenstein-Hawking entropy with its 1/4 prefactor. I report recent progress in this direction. In particular, I discuss the covariant spin foam dynamics and and show that the entropy of the quantum horizon reproduces the Bekenstein-Hawking entropy S=A/4 with the proper one-fourth coefficient for all values of the Immirzi parameter.
Date: 30/05/2012 - 2:00 pm
Series: Colloquium

http://pirsa.org/12050084/ [video and pdf]
Pentahedral Volume, Chaos, and Quantum Gravity
Speaker(s): Hal Haggard
Abstract: The space of convex polyhedra can be given a dynamical structure. Exploiting this dynamics we have performed a Bohr-Sommerfeld quantization of the volume of a tetrahedral grain of space, which is in excellent agreement with loop gravity. Here we present investigations of the volume of a 5-faced convex polyhedron. We give for the first time a constructive method for finding these polyhedra given their face areas and normals to the faces and find an explicit formula for the volume. This results
in new information about cylindrical consistency in loop gravity and a couple of surprises about polyhedra. In particular, we are interested in discovering whether the evolution generated by this volume is chaotic or integrable as this will impact the interpretation of the spin network basis in loop gravity.
Date: 30/05/2012 - 4:00 pm
Series: Quantum Gravity

http://arxiv.org/abs/1205.7045
Pure Connection Formalism for Gravity: Linearized Theory
Gianluca Delfino, Kirill Krasnov, Carlos Scarinci
(Submitted on 31 May 2012)
We give a description of gravitons in terms of an SL(2,C) connection field. The gauge-theoretic Lagrangian for gravitons is simpler than the metric one. Moreover, all components of the connection field have the same sign in front of their kinetic term, unlike what happens in the metric formalism. The gauge-theoretic description is also more economic than the standard one because the Lagrangian only depends on 8 components of the field per spacetime point as compared to 10 in the Einstein-Hilbert case. Particular care is paid to the treatment of the reality conditions that guarantee that one is dealing with a system with a hermitian Hamiltonian. We give general arguments explaining why the connection cannot be taken to be real, and then describe a reality condition that relates the hermitian conjugate of the connection to its (second) derivative. This is quite analogous to the treatment of fermions where one describes them by a second-order in derivatives Klein-Gordon Lagrangian, with an additional first-order reality condition (Dirac equation) imposed. We find many other parallels with fermions, e.g. the fact that the action of parity on the connection is related to the hermitian conjugation. Our main result is the mode decomposition of the connection field, which is to be used in forthcoming works for computations of graviton scattering amplitudes.
32 pages

 

[John86]

http://arxiv.org/abs/1205.6750
Decoherence without Dissipation
W. G. Unruh
(Submitted on 30 May 2012)
That decoherence can take place in the presence of energy conservation seems to be a poorly known fact. That lack of knowledge has for example bedevilled the discussion of the "black hole information" problem. I present a simple model which illustrates such energy free decoherence. http://arxiv.org/abs/1205.6751
Irrotational, two-dimensional Surface waves in fluids
W. G. Unruh
(Submitted on 30 May 2012)
The equations for waves on the surface of an irrotational incompressible fluid are derived in the coordinates of the velocity potential/stream function. The low frequency shallow water approximation for these waves is derived for a varying bottom topography. Most importantly, the conserved norm for the surface waves is derived, important for quantisation of these waves and their use in analog models for black holes. http://arxiv.org/abs/1205.6814
Area products for black hole horizons
Matt Visser (Victoria University of Wellington)
(Submitted on 30 May 2012)
Area products for multi-horizon black holes often have intriguing properties, and are often independent of the mass of the black hole (depending only on various charges, angular momenta, and moduli). Such products are often formulated in terms of the areas of inner (Cauchy) horizons and event horizons, and often include the effects of unphysical "virtual'" horizons. For the Schwarzschild-de Sitter [Kottler] black hole in (3+1) dimensions it is shown by explicit exact calculation that the product of event horizon area and cosmological horizon area is not mass independent. (Including the effect of the third "virtual" horizon does not improve the situation.) Similarly, in the Reissner-Nordstrom-anti-de Sitter black hole in (3+1) dimensions the product of inner (Cauchy) horizon area and event horizon area is calculated (perturbatively), and is shown to be not mass independent. That is, the mass-independence of the product of physical horizon areas is not generic. In the generic situation, whenever the quasi-local mass m(r) is a Laurent polynomial in aerial radius, r=sqrt{A/4 pi}, there are more complicated mass-independent quantities, the elementary symmetric polynomials built up from the complete set of horizon radii (physical and virtual). Sometimes it is possible to eliminate the unphysical virtual horizons, constructing combinations of physical horizon areas that are mass independent, but they tend to be considerably more complicated than the simple products and related constructions currently mooted in the literature. http://arxiv.org/abs/1205.7072
On vacuum-energy decay from particle production
F. R. Klinkhamer
(Submitted on 31 May 2012)
An effective description of particle-production backreaction in de-Sitter spacetime is given.

 

[MTd2]

http://arxiv.org/abs/1206.0085

Spacetime Emergence and General Covariance Transmutation

Chiu Man Ho, Thomas W. Kephart, Djordje Minic, Y. Jack Ng
(Submitted on 1 Jun 2012)
Spacetime emergence refers to the notion that classical spacetime "emerges" as an approximate macroscopic entity from a non-spatio-temporal structure present in a more complete theory of interacting fundamental constituents. In this article, we propose a novel mechanism involving the "soldering" of internal and external spaces for the emergence of spacetime and the twin transmutation of general covariance. In the context of string theory, this mechanism points to a critical four dimensional spacetime background.

 

[MTd2]

http://arxiv.org/abs/1205.6988

Probable Entropic Nature of Gravity in Ultraviolet and Infrared Limits. I.Ultraviolet Case

A.E. Shalyt-Margolin
(Submitted on 30 May 2012)
This work presents a study of the possibility for extending the well-known results of E.Verlinde concerning the entropic nature of gravity to the ultraviolet region (Planck's energies) and also the derivation of quantum corrections to Einstein Equations

 

[marcus]

http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise
(Submitted on 4 Jun 2012)
Hamiltonian gravity, relying on arbitrary choices of "space," can obscure spacetime symmetries. We present an alternative, manifestly spacetime covariant formulation that nonetheless distinguishes between "spatial" and "temporal" variables. The key is viewing dynamical fields from the perspective of a field of observers -- a unit timelike vector field that also transforms under local Lorentz transformations. On one hand, all fields are spacetime fields, covariant under spacetime symmeties. On the other, when the observer field is normal to a spatial foliation, the fields automatically fall into Hamiltonian form, recovering the Ashtekar formulation. We argue this provides a bridge between Ashtekar variables and covariant phase space methods. We also outline a framework where the 'space of observers' is fundamental, and spacetime geometry itself may be observer-dependent.
8 pages; Essay written for the 2012 Gravity Research Foundation Awards for Essays on Gravitation

http://arxiv.org/abs/1206.0657
The R² phase-diagram of QEG and its spectral dimension
Stefan Rechenberger, Frank Saueressig
(Submitted on 4 Jun 2012)
Within the gravitational asymptotic safety program, the RG flow of the R² truncation in three and four spacetime dimensions is analyzed in detail. In particular, we construct RG trajectories which emanate from the non-Gaussian UV fixed point and possesses long classical regimes where the effective average action is well approximated by the classical Einstein-Hilbert action. As an application we study the spectral dimension of the effective QEG spacetimes resulting from these trajectories, establishing that the picture of a multi-fractal spacetime is robust under the extension of the truncated theory space. We demonstrate that regimes of constant spectral dimensions can either be attributed to universal features of RG fixed points or singular loci in the beta functions.
13 pages, 10 figures

brief mention:
http://arxiv.org/abs/1206.0306
Lambda-Perturbations of Keplerian Orbits
Yurii V. Dumin (Theoretical Department, IZMIRAN, Russian Academy of Sciences, Troitsk, Moscow reg., Russia)
(Submitted on 1 Jun 2012)
To estimate the influence of the dark energy on the planetary orbits, we solve the general relativistic equations of motion of a test particle in the field of a point-like mass embedded in the cosmological background formed by the Lambda-term...
4 pages, 2 figures

 

[MTd2]

http://arxiv.org/abs/1206.0657

The R^2 phase-diagram of QEG and its spectral dimension

Stefan Rechenberger, Frank Saueressig
(Submitted on 4 Jun 2012)
Within the gravitational asymptotic safety program, the RG flow of the R^2 truncation in three and four spacetime dimensions is analyzed in detail. In particular, we construct RG trajectories which emanate from the non-Gaussian UV fixed point and possesses long classical regimes where the effective average action is well approximated by the classical Einstein-Hilbert action. As an application we study the spectral dimension of the effective QEG spacetimes resulting from these trajectories, establishing that the picture of a multi-fractal spacetime is robust under the extension of the truncated theory space. We demonstrate that regimes of constant spectral dimensions can either be attributed to universal features of RG fixed points or singular

 

[atyy]

http://arxiv.org/abs/1206.0574
Renormalization group approach to matrix models via noncommutative space
Shoichi Kawamoto, Tsunehide Kuroki, Dan Tomino
(Submitted on 4 Jun 2012)
We develop a new renormalization group approach to the large-N limit of matrix models. It has been proposed that a procedure, in which a matrix model of size (N-1) \times (N-1) is obtained by integrating out one row and column of an N \times N matrix model, can be regarded as a renormalization group and that its fixed point reveals critical behavior in the large-N limit. We instead utilize the fuzzy sphere structure based on which we construct a new map (renormalization group) from N \times N matrix model to that of rank N-1. Our renormalization group has great advantage of being a nice analog of the standard renormalization group in field theory. It is naturally endowed with the concept of high/low energy, and consequently it is in a sense local and admits derivative expansions in the space of matrices. In construction we also find that our renormalization in general generates multi-trace operators, and that nonplanar diagrams yield a nonlocal operation on a matrix, whose action is to transport the matrix to the antipode on the sphere. Furthermore the noncommutativity of the fuzzy sphere is renormalized in our formalism. We then analyze our renormalization group equation, and Gaussian and nontrivial fixed points are found. We further clarify how to read off scaling dimensions from our renormalization group equation. Finally the critical exponent of the model of two-dimensional gravity based on our formalism is examined.

http://arxiv.org/abs/1206.0678
Notes on Emergent Gravity
Sunggeun Lee, Raju Roychowdhury, Hyun Seok Yang
(Submitted on 4 Jun 2012)
Emergent gravity is aimed at constructing a Riemannian geometry from U(1) gauge fields on a noncommutative spacetime. But this construction can be inverted to find corresponding U(1) gauge fields on a (generalized) Poisson manifold given a Riemannian metric (M, g). We examine this bottom-up approach with the LeBrun metric which is the most general scalar-flat Kahler metric with a U(1) isometry and contains the Gibbons-Hawking metric, the real heaven as well as the multi-blown up Burns metric which is a scalar-flat Kahler metric on C^2 with n points blown up. The bottom-up approach clarifies some important issues in emergent gravity.

 

[MTd2]

http://arxiv.org/abs/1206.1030

Einstein Equations and MOND Theory from Debye Entropic Gravity

K. Rezazadeh Sarab, A. Sheykhi
(Submitted on 4 Jun 2012)
Verlinde's proposal on the entropic origin of gravity is based strongly on the assumption that the equipartition law of energy holds on the holographic screen induced by the mass distribution of the system. However, from the theory of statistical mechanics we know that the equipartition law of energy does not hold in the limit of very low temperature. Inspired by the Debye model for the equipartition law of energy in statistical thermodynamics and adopting the viewpoint that gravitational systems can be regarded as a thermodynamical system, we modify Einstein field equations. We also perform the study for Poisson equation and modified Newtonian dynamics (MOND). Interestingly enough, we find that the origin of the MOND theory can be understood from Debye entropic gravity perspective. Thus our study may fill in the gap existing in the literature understanding the theoretical origin of MOND theory. In the limit of high temperature our results reduce to their respective standard gravitational equations.

http://arxiv.org/abs/1206.1030

Hawking radiation, chirality, and the principle of effective theory of gravity

Adamantia Zampeli, Douglas Singleton, Elias C. Vagenas
(Submitted on 5 Jun 2012)
In this paper we combine the chirality of field theories in near horizon regions with the principle of effective theory of gravity to define a new energy-momentum tensor for the theory. This new energy-momentum tensor has the correct radiation flux to account for Hawking radiation for space-times with horizons. This method is connected to the chiral anomaly cancellation method, but it works for space-times for which the chiral anomaly cancellation method fails. In particular the method presented here works for the non-asymptotically flat de Sitter space-time and its associated Hawking-Gibbons radiation, as well as Rindler space-time and its associated Unruh radiation. This indicates that it is the chiral nature of the field theory in the near horizon regions which is of primary importance rather than the chiral anomaly.

http://arxiv.org/abs/1206.0841

"Meta" relativity: Against special relativity?

Jakub Rembielinski, Marta Wlodarczyk
(Submitted on 5 Jun 2012)
We introduce a Lorentz-covariant description of tachyons, free of inconsistencies. Our approach is based on an appropriate extension of the special relativity beyond the light barrier, owing to the freedom of synchronization of distant clocks.

 

[marcus]

http://arxiv.org/abs/1206.1192
Quantum gravity in the sky
Aurelien Barrau, Julien Grain
(Submitted on 6 Jun 2012)
Quantum gravity is known to be mostly a kind of metaphysical speculation. In this brief essay, we try to argue that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". With the example of Loop Quantum Gravity, we detail some expected features.
7 pages. Originally written for the "Gravity Research Foundation", adapted to serve as introduction for a lecture on gravity.

http://arxiv.org/abs/1206.1128
Coherent State Functional Integral in Loop Quantum Cosmology: Alternative Dynamics
Li Qin, Yongge Ma
(Submitted on 6 Jun 2012)
Coherent state functional integral for the minisuperspace model of loop quantum cosmology is studied. By the well-established canonical theory, the transition amplitude in the path integral representation of loop quantum cosmology with alternative dynamics can be formulated through group averaging. The effective action and Hamiltonian with higher-order quantum corrections are thus obtained. It turns out that for a non-symmetric Hamiltonian constraint operator, the Moyal (star)-product emerges naturally in the effective Hamiltonian. For the corresponding symmetric Hamiltonian operator, the resulted effective theory implies a possible quantum cosmological effect in large scale limit in the alternative dynamical scenario, which coincides with the result in canonical approach. Moreover, the first-order modified Friedmann equation still contains the particular information of alternative dynamics and hence admits the possible phenomenological distinction between the different proposals of quantum dynamics.
10 pages.

http://arxiv.org/abs/1206.1131
Path Integrals and Alternative Effective Dynamics in Loop Quantum Cosmology
Li Qin, Guo Deng, Yongge Ma
(Submitted on 6 Jun 2012)
The alternative dynamics of loop quantum cosmology is examined by the path integral formulation. We consider the spatially flat FRW models with a massless scalar field, where the alternative quantization inherit more features from full loop quantum gravity. The path integrals can be formulated in both timeless and deparameterized frameworks. It turns out that the effective Hamiltonians derived from the two different viewpoints are equivalent to each other. Moreover, the first-order modified Friedmann equations are derived and predict quantum bounces for contracting universe, which coincide with those obtained in canonical theory.
8 pages

 

[atyy]

http://arxiv.org/abs/1206.1092
Modelling gravity on a hyper-cubic lattice
Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 5 Jun 2012)
We present an elegant and simple dynamical model of symmetric, non-degenerate (n x n) matrices of fixed signature defined on a n-dimensional hyper-cubic lattice with nearest-neighbor interactions. We show how this model is related to General Relativity, and discuss multiple ways in which it can be useful for studying gravity, both classical and quantum. In particular, we show that the dynamics of the model when all matrices are close to the identity corresponds exactly to a finite-difference discretization of weak-field gravity in harmonic gauge. We also show that the action which defines the full dynamics of the model corresponds to the Einstein-Hilbert action to leading order in the lattice spacing, and use this observation to define a lattice analogue of the Ricci scalar and Einstein tensor. Finally, we perform a mean-field analysis of the statistical mechanics of this model.

 

[marcus]

http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain
(Submitted on 7 Jun 2012)
The primordial Universe can be used as a laboratory to set constraints on quantum gravity. In the framework of Loop Quantum Cosmology, we show that such a proposal for quantum gravity not only solves for the big bang singularity issue but also naturally generates inflation. Thanks to a quantitative computation of the amount of gravity waves produced in the loopy early Universe, we show that future cosmological datas on the polarized anisotropies of the Cosmic Microwave Background can be used to probe LQC model of the Universe.
5 pages. Proceeding of the ICGC international conference, Goa, December 2011

brief mention:
http://arxiv.org/abs/1206.1320
Can a closed critical surface in a quark-gluon plasma serve as a model for the behavior of quantum gravity near to an event horizon?
George Chapline
(Submitted on 5 Jun 2012)
11 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1206.2107
Measurements and Information in Spin Foam Models
J. Manuel Garcia-Islas
(Submitted on 11 Jun 2012)
We present a problem relating measurements and information theory in spin foam models. In the three dimensional case of quantum gravity we can compute probabilities of spin network graphs and study the behaviour of the Shannon entropy associated to the corresponding information. We present a general definition, compute the Shannon entropy of some examples, and find some interesting inequalities.
12 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1206.2403
Problem of Time in Quantum Gravity
Edward Anderson
(Submitted on 11 Jun 2012)
The Problem of Time occurs because the `time' of GR and of ordinary Quantum Theory are mutually incompatible notions. This is problematic in trying to replace these two branches of physics with a single framework in situations in which the conditions of both apply, e.g. in black holes or in the very early universe. Emphasis in this Review is on the Problem of Time being multi-faceted and on the nature of each of the eight principal facets. Namely, the Frozen Formalism Problem, Configurational Relationalism Problem (formerly Sandwich Problem), Foliation Dependence Problem, Constraint Closure Problem (formerly Functional Evolution Problem), Multiple Choice Problem, Global Problem of Time, Problem of Beables (alias Problem of Observables) and Spacetime Reconstruction or Replacement Problem. Strategizing in this Review is not just centred about the Frozen Formalism Problem facet, but rather about each of the eight facets. Particular emphasis is placed upon A) relationalism as an underpinning of the facets and as a selector of particular strategies (especially a modification of Barbour relationalism, though with some consideration also of Rovelli relationalism). B) Classifying approaches by the full ordering in which they embrace constrain, quantize, find time/history and find observables, rather than only by partial orderings such as "Dirac-quantize". C) Foliation (in)dependence and Spacetime Reconstruction for a wide range of physical theories, strategizing centred about the Problem of Beables, the Patching Approach to the Global Problem of Time, and the role of the question-types considered in physics. D) The Halliwell- and Gambini-Porto-Pullin-type combined Strategies in the context of semiclassical quantum cosmology.
Invited Review: 25 pages including 2 Figures

brief mention:
http://arxiv.org/abs/1206.2397
Survey of analogue spacetimes
Matt Visser (Victoria University of Wellington)
(Submitted on 11 Jun 2012)
Analogue spacetimes, (and more boldly, analogue models both of and for gravity), have attracted significant and increasing attention over the last decade and a half. Perhaps the most straightforward physical example, which serves as a template for most of the others, is Bill Unruh's model for a dumb hole, (mute black hole, acoustic black hole), wherein sound is dragged along by a moving fluid --- and can even be trapped behind an acoustic horizon. This and related analogue models for curved spacetimes are useful in many ways: Analogue spacetimes provide general relativists with extremely concrete physical models to help focus their thinking, and conversely the techniques of curved spacetime can sometimes help improve our understanding of condensed matter and/or optical systems by providing an unexpected and countervailing viewpoint. In this introductory chapter, I shall provide a few simple examples of analogue spacetimes as general background for the rest of the contributions.
25 pages. Draft of a chapter submitted to the proceedings of the IX'th SIGRAV graduate school: Analogue Gravity, Lake Como, Italy, May 2011

http://arxiv.org/abs/1206.2605
Quantum gravity, space-time structure, and cosmology
Martin Bojowald
(Submitted on 12 Jun 2012)
A set of diverse but mutually consistent results obtained in different settings has spawned a new view of loop quantum gravity and its physical implications, based on the interplay of operator calculations and effective theory: Quantum corrections modify, but do not destroy, space-time and the notion of covariance. ...
8 pages, 7 figures, Plenary talk at CosGrav12, held at Indian Statistical Institute, Kolkata
[MY COMMENT: something unusual here. Over half of the 40 references are to papers by M.B. himself. An alternative version of Loop may be taking shape. It may be too early to assess this.]