This is intended as a semiannual poll ( twice a year, covering recorded seminar talks from the previous 6 months or so.)
Not all the talks are listed with abstracts at their respective sites, so where possible I'll retrieve the relevant abstract from the main source paper the talk is based on.
ILQGS Sept. 20
http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf
The Feynman diagramatics for the spin foam models
Jacek Puchta, University of Warszaw
[paper's abstract: Feynman diagrammatic approach to spin foams--"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from
operator spin network diagrams we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.]
PIRSA Sept. 21
http://pirsa.org/11090125/
Loop Gravity as the Dynamics of Topological Defects
Eugenio Bianchi, Perimeter Institute
A charged particle can detect the presence of a magnetic field confined into a solenoid. The strength of the effect depends only on the phase shift experienced by the particle's wave function, as dictated by the Wilson loop of the Maxwell connection around the solenoid. In this seminar I'll show that Loop Gravity has a structure analogous to the one relevant in the Aharonov-Bohm effect described above: it is a quantum theory of connections with curvature vanishing everywhere, except on a 1d network of topological defects. Loop states measure the flux of the gravitational magnetic field through a defect line. A feature of this reformulation is that the space of states of Loop Gravity can be derived from an ordinary QFT quantization of a classical diffeomorphism-invariant theory defined on a manifold. I'll discuss the role quantum geometry operators play in this picture, and the perspective of formulating the Spin Foam dynamics as the local interaction of topological defects.
ILQGS Oct. 18
http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf
Inhomogeneous loop quantum cosmology
William Nelson, PennState
[abstract of related talk based on the same research (invited talk to be given by Nelson's co-author Ivan Agullo at the April meeting of the APS): Beyond the standard inflationary paradigm--The inflationary paradigm provides a compelling argument to account for the origin of the cosmic inhomogeneities that we observe in the CMB and galaxy distribution. In this talk we introduce a
completion of the inflationary paradigm from a (loop) quantum gravity point of view, by addressing gravitational issues that have been open both for the background geometry and perturbations. These include a quantum gravity treatment of the Planck regime from which inflation arises, and a clarification of what the trans-Planckian problems are and what they are not. In addition, this approach provides examples of effects that may have observational implications, that may provide a window to test the basic quantum gravity principles employed here.]
ILQGS Nov. 1
http://relativity.phys.lsu.edu/ilqgs/koslowski110111.pdf
Shape dynamics
Tim Koslowski, Perimeter Institute
[based on several papers including: Coupling Shape Dynamics to Matter Gives Spacetime--Shape Dynamics is a metric theory of pure gravity, equivalent to General Relativity, but formulated as a gauge theory of spatial diffeomorphisms and local spatial conformal transformations. In this paper we extend the construction of Shape Dynamics form pure gravity to gravity-matter systems and find that there is no obstruction for the coupling of gravity to standard matter. We use the matter gravity system to construct a clock and rod model for Shape Dynamics which allows us to recover a spacetime interpretation of Shape Dynamics trajectories.]
ILQGS Nov. 15
http://relativity.phys.lsu.edu/ilqgs/engle111511.pdf
Plebanski sectors of the new spin foam models
Jonathan Engle, Florida Atlantic University
[paper's abstract: A proposed proper EPRL vertex amplitude--As established in a prior work of the author, the linear simplicity constraints used in the construction of the so-called 'new' spin-foam models mix three of the five sectors of Plebanski theory, only one of which is gravity in the usual sense, and this is the reason for certain 'unwanted' terms in the asymptotics of the EPRL vertex amplitude as calculated by Barrett et al.
In the present paper, an explicit classical discrete condition is derived that isolates the desired gravitational sector, which we call (II+), following other authors. This condition is quantized and used to modify the vertex amplitude, yielding what we call the 'proper EPRL vertex amplitude.' This vertex still depends only on standard SU(2) spin-network data on the boundary, is SU(2) gauge invariant, and is linear in the boundary state, as required. In addition, the asymptotics now consist in the single desired term of the form e
iSRegge, and all degenerate configurations are exponentially suppressed.]
PIRSA Dec. 7
http://pirsa.org/11120050/
Canonical Time Evolution in Simplicial Gravity
Philipp Hoehn, Utrecht to Perimeter Institute
[paper's abstract: Canonical simplicial gravity--A general canonical formalism for discrete systems is developed which can handle varying phase space dimensions and constraints. The central ingredient is Hamilton's principle function which generates canonical time evolution and ensures that the canonical formalism reproduces the dynamics of the covariant formulation following directly from the action. We apply this formalism to simplicial gravity and (Euclidean) Regge calculus, in particular. A discrete forward/backward evolution is realized by gluing/removing single simplices step by step to/from a bulk triangulation and amounts to Pachner moves in the triangulated hypersurfaces. As a result, the hypersurfaces evolve in a discrete `multi-fingered' time through the full Regge solution. Pachner moves are an elementary and ergodic class of homeomorphisms and generically change the number of variables, but can be implemented as canonical transformations on naturally extended phase spaces. Some moves introduce a priori free data which, however, may become fixed a posteriori by constraints arising in subsequent moves. The end result is a general and fully consistent formulation of canonical Regge calculus, thereby removing a longstanding obstacle in connecting covariant simplicial gravity models to canonical frameworks. The present scheme is, therefore, interesting in view of many approaches to quantum gravity, but may also prove useful for numerical implementations.]
PIRSA Jan. 11
http://pirsa.org/12010131/
Group Field Theory and Simplicial Path Integrals
Aristide Baratin, AEI
[paper's abstract: Group field theory and simplicial gravity path integrals--In a recent work, a dual formulation of group field theories as non-commutative quantum field theories has been proposed, providing an exact duality between spin foam models and non-commutative simplicial path integrals for constrained BF theories. In light of this new framework, we define a model for 4d gravity which includes the Immirzi parameter gamma. It reproduces the Barrett-Crane amplitudes when gamma goes to infinity, but differs from existing models otherwise; in particular it does not require any rationality condition for gamma. We formulate the amplitudes both as BF simplicial path integrals with explicit non-commutative B variables, and in spin foam form in terms of Wigner 15j-symbols. Finally, we briefly discuss the correlation between neighboring simplices, often argued to be a problematic feature, for example, in the Barrett-Crane model.]
PIRSA Jan. 18
http://pirsa.org/12010115/
Scalar Perturbations in Loop Quantum Cosmology
Edward Wilson-Ewing, Marseille to LSU
We study the dynamics of the scalar modes of linear perturbations around a flat, homogeneous and isotropic background in loop quantum cosmology.
PIRSA Feb. 1
http://pirsa.org/12020096/
Continuous Formulation of the Loop Quantum Gravity Phase Space
Jonathan Ziprick, Perimeter Institute
We relate the discrete classical phase space of loop gravity to the continuous phase space of general relativity. Our construction shows that the flux variables do not label a unique geometry, but rather a class of gauge-equivalent geometries. We resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewise-flat geometry, showing that both geometries belong to the same equivalence class. We also establish a clear relationship between Regge geometries and the piecewise-flat spin foam geometries. All of this is based on arXiv:1110.4833.
ILQGS Feb. 14
http://relativity.phys.lsu.edu/ilqgs/perini021412.pdf
Classical limit of spin foams on arbitrary triangulations
Claudio Perini, PennState
[earlier paper's abstract: Emergence of gravity from spinfoams--We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma) and large spins (j), with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.]
PIRSA Feb. 15
http://pirsa.org/12020088/
Fractal Space-times Under the Microscope: a RG View on Monte Carlo Data
Frank Saueressig, Univ. Mainz
The emergence of fractal features in the microscopic structure of space-time is a common theme in many approaches to quantum gravity. In particular the spectral dimension, which measures the return probability of a fictitious diffusion process on space-time, provides a valuable probe which is easily accessible both in the continuum functional renormalization group and discrete Monte Carlo simulations of the gravitational action. In this talk, I will give a detailed exposition of the fractal properties associated with the effective space-times of asymptotically safe Quantum Einstein Gravity (QEG). Comparing these continuum results to three-dimensional Monte Carlo simulations, we demonstrate that the resulting spectral dimensions are in very good agreement. This comparison also provides a natural explanation for the apparent conflicts between the short distance behavior of the spectral dimension reported from Causal Dynamical Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic Safety.
ILQGS Feb. 28
http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf
Continuous formulation of the LQG phase space
Marc Geiller, Univ. Paris
[paper's abstract: Continuous formulation of the Loop Quantum Gravity phase space--
In this paper, we study the discrete classical phase space of loop gravity, which is expressed in terms of the holonomy-flux variables, and show how it is related to the continuous phase space of general relativity. In particular, we prove an isomorphism between the loop gravity discrete phase space and the symplectic reduction of the continuous phase space with respect to a flatness constraint. This gives for the first time a precise relationship between the continuum and holonomy-flux variables. Our construction shows that the fluxes depend on the three-geometry, but also explicitly on the connection, explaining their non commutativity. It also clearly shows that the flux variables do not label a unique geometry, but rather a class of gauge-equivalent geometries. This allows us to resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewise flat geometry, since we establish that both geometries belong to the same equivalence class. This finally gives us a clear understanding of the relationship between the piecewise flat spin foam geometries and Regge geometries, which are only piecewise-linear flat: While Regge geometry corresponds to metrics whose curvature is concentrated around straight edges, the loop gravity geometry correspond to metrics whose curvature is concentrated around not necessarily straight edges.]
PIRSA Feb. 29
http://pirsa.org/12020129/
Spinor Quantisation for Complex Ashtekar Variables
Wolfgang Wieland, Marseille
During the last couple of years Dupuis, Freidel, Livine, Speziale and Tambornino developed a twistorial formulation for loop quantum gravity.
Constructed from Ashtekar--Barbero variables, the formalism is restricted to SU(2) gauge transformations.
In this talk, I perform the generalisation to the full Lorentzian case, that is the group SL(2,C).
The phase space of SL(2,C) (i.e. complex or selfdual) Ashtekar variables on a spinnetwork graph is decomposed in terms of twistorial variables. To every link there are two twistors---one to each boundary point---attached. The formalism provides a clean derivation of the solution space of the reality conditions of loop quantum gravity.
Key features of the EPRL spinfoam model are perfectly recovered.
If there is still time, I'll sketch my current project concerning a twistorial path integral for spinfoam gravity as well.
ILQGS Mar. 13
http://relativity.phys.lsu.edu/ilqgs/diazpolo031312.pdf
Black hole evaporation in LQG
Jacobo Diaz-Polo, LSU
[paper's abstract:Probing Loop Quantum Gravity with Evaporating Black Holes--
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.]
ILQGS Mar. 27
http://relativity.phys.lsu.edu/ilqgs/perez032712.pdf
Black hole entropy in LQG: new insights from a local perspective
Alejandro Perez, Marseille
[paper's abstract: A local first law for black hole thermodynamics--We first show that stationary black holes satisfy an extremely simple local form of the first law ∂ E=κ(l) ∂ A/(8 π) where the thermodynamical energy E=A/(8π l) and (local) surface gravity κ(l)=1/l, where A is the horizon area and l is a proper length characterizing the distance to the horizon of a preferred family of local observers suitable for thermodynamical considerations. Our construction is extended to the more general framework of isolated horizons. The local surface gravity is universal. This has important implications for semiclassical considerations of black hole physics as well as for the fundamental quantum description arising in the context of loop quantum gravity.]
NOTE the first 6 minutes of the Perez audio are spoiled by noise so I would advise dragging the time button to around 6:00 or 6:30 when you start it. This corresponds to around the slide numbered (2). The first part of the talk is a review, so missing the first few minutes need not be critical.
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