Page numbers refer to the GR20 book of abstracts
Loop quantum gravity, twisted geometries and twistors
Speziale S
Loop quantum gravity is a background-independent approach to the quantization of general relativity. While the theory is continuous at the fundamental level, it is often useful to consider a truncation thereof, defined on the lattice dual to a graph. This truncation captures a finite number of degrees of freedom, which have been shown to describe a certain generalization of Regge geometries, called twisted geometries. In this talk, I will give a brief overview of the theory and its geometric interpretation. Then, I will describe how these discrete geometries can be described in terms of a collection of twistors associated to the lattice, thus providing a new sense in which twistors can be seen as non-linear gravitons. Finally, I will briefly discuss how dynamical transition amplitudes can be represented as integrals in twistor space.(p.55)
Complete quantization of vacuum spherically symmetric gravity
Pullin J
We find a rescaling of the Hamiltonian constraint for vacuum spherically symmetric gravity that makes the constraint algebra a true Lie algebra. We can implement the Dirac quantization procedure finding in closed form the space of physical states. New observables without classical counterpart arise. The metric can be understood as an evolving constant of the motion defined as a quantum operator on the space of physical states. For it to be self adjoint its range needs to be restricted, which in turn implies that the singularity is eliminated. One is left with a region of high curvature that tunnels into another portion of space-time. The results may have implications for the current discussion of ”firewalls” in black hole evaporation.(p.220)
On the quasilocal first law for isolated horizon and its uses in the euclidean partition function
Frodden E, Pérez A
In this talk I will discuss a new quasilocal black hole energy proposal, it is based on near horizon observers and is suitable for loop quantum gravity statistical computations. I will also present a simple application of this energy notion in the context of Euclidean partition function.(p.220)
Black hole entropy and entanglement in spinfoam gravity
Bianchi E
I report recent progress on the study of entanglement entropy in spinfoam quantum gravity, and its rela- tion with the Bekenstein-Hawking area law. Based on arXiv:1212.5183 in collaboration with R.Myers.(p.220)
Quantum isotropy and dynamical quantum symmetry reduction
Engle J
We give a diffeomorphism and gauge covariant condition equivalent to homogeneity and isotropy which can be quantized, yielding a definition of a diffeomorphism-invariant, homogeneous isotropic sector of loop quantum gravity without fixing a graph. We then specialize this condition to Bianchi I cosmologies, in which case it becomes a condition for isotropy. We show how, by quantizing and imposing this condition in Bianchi I loop quantum cosmology, one exactly recovers isotropic loop quantum cosmology, including the usual ‘improved dynamics.’ We will also discuss how this reduction sheds light on which operator ordering to use when defining operators corresponding to directional Hubble rates, expansion, and shear––quantities relevant for discussing the resolution of the initial singularity.(p.223)
Loop quantum cosmology: fundamentals and phenomenology
Ashtekar A
We will review recent advances in our understanding of conceptual as well as observational issues related to the very early universe that have come from loop quantum cosmology. We will emphasize the interplay between the theory and observations has has the potential to enrich both areas.(p.224)
Duration of inflation as a prediction of effective lqc
Linsefors L, Barrau A
Loop quantum cosmology, together with a massive scalar field, has been shown to predict a high probability of sufficiently long enough inflation to fit observations. However these predictions were derived from setting initial conditions at the bounce. In this study, we take seriously the direction of causality from past to future, and therefore set initial conditions before the bounce. The phase of the scalar field is assumed to be a random variable with a flat probability distribution. A key point of this distribution is that it is not linked to a specific point in time. Our result is independent of how long before the bounce we set the initial conditions, given reasonable assumptions. In this framework, we can show that the number of e-folds of slow-roll inflation is peaked around N=145. This is one of the first clear theoretical prediction for the duration of inflation and it is also in agreement with observations. In addition, the fraction of potential energy at the bounce, usually taken as a free unknown parameter, driving many observable effects, can also be shown to be sharply peaked. Finally, we use those results to derive an original upper limit on the Barbero-Immirzi parameter : gamma < 11, which is two orders of magnitude better than the previous limit coming from cosmology.(p.224)
Quantum reduced loop gravity
Alesci E
We present a new framework to study symmetric sectors of loop quantum gravity: mimicking the spinfoam quantization procedure the reduction is imposed weakly on the full kinematical Hilbert space of the canonical theory. As a first application we study the inhomogeneous Bianchi I model and discuss the semiclassical limit of the theory.(p.225)
Radiative corrections in covariant loop quantum gravity
Rovelli C
I explain the problem of the radiative corrections is covariant quantum gravity and summarize the present status of the research. The control of these radiative corrections is the main open issue in the theory.(p.227)
Curvature constraints in spin foam models
Hellman F, Kaminski W
I will describe surprising constraints on the internal holonomies in the asymptotic limit of current spin foam models like EPRL. Our result concerns euclidean models but indicates that similar phenomena may occur also in their physical lorentzian counterpart.(p.227)
Hamiltonian spinfoam gravity
Wieland W
The talk presents a new Hamiltonian formulation of discretised gravity, based upon the twistorial frame-work of loop quantum gravity. Within this framework, I am able to derive a continuum action adapted to a simplicial decomposition of space-time. The action is a sum of the spinorial analogue of the topological ”BF” action and the reality conditions that guarantee the existence of a metric. The equations of motion admit a Hamiltonian formulation, that allows to perform the constraint analysis. I do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. With the action polynomial in the spinors, canonical quantisation is straightforward. Transition amplitudes reproduce the EPRL (Engle–Pereira–Rovelli–Livine) spinfoam model.(p.227)
The continuum limit of spin foams and spin nets
Dittrich B
Spin foam models are candidate models for quantum gravity, constructed via a quantum mechanical (not Wick rotated) path integral for discrete gravity. We aim to extract the behaviour of these models on scales large compared to the discretization scale.
To this end we employ recently introduced coarse graining techniques, known as tensor network renor- malization methods, that allow us to obtain a renormalization flow of these models. Fixed points of this flow correspond to the infinite refinement, that is continuum, limit.
These techniques are applied to dimensionally reduced models, which we coined spin net models. However important general mechanisms can be already studied for these reduced models and we will comment on these as well as on general strategies for renormalization in background independent systems.(p.240)
What happens when a freely-falling observer crosses an event horizon–semiclassically?
Smerlak M
What happens when a freely-falling observer crosses a black hole horizon? In spite of recent challenges by Almheiri, Marolf, Polchinski and Scully—the ”firewall” hypothesis—, the consensual answer to this question tends to remain ”nothing special”. In this talk, I will show that something rather special happens near the horizon, already at the semiclassical level: particle detectors record ingoing Hawking radiation at a temperature inversely proportional to their velocity relative to the horizon. To establish this result, I will (i) introduce an adiabatic expansion for Unruh-DeWitt response functions along non-stationary trajectories, and apply it to radial Schwarzschild geodesics, and (ii) compute the flux perceived by such infalling observers. I will close with a few comments on the role of spacetime curvature in this surprising effect.(p.249)
Death and resurrection of the zeroth principle of thermodynamics
Haggard H , Rovelli C
The zeroth principle of thermodynamics in the form ”temperature is uniform at equilibrium” is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.(p.250)
Quantum space-times and unitarity of bh evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.(p.218)
Many interesting abstracts were necessarily omitted from this small sample. To see more:
http://gr20-amaldi10.edu.pl/userfiles/book_05_07_2013.pdf