From twistors to twisted geometries

In a previous paper we showed that the phase space of loop quantum gravity on a fixed graph can be parametrized in terms of twisted geometries, quantities describing the intrinsic and extrinsic discrete geometry of a cellular decomposition dual to the graph. Here we unravel the origin of the phase space from a geometric interpretation of twistors.Comment: 9 page

Incidental sun exposure in North Queensland: an application of the prototype willingness (PW) model

Objective: The prevalence of skin cancer in North Queensland is among the highest in the world (Buettner & Rasch, 1998), yet comparably few individuals report deliberately sunbathing to get a tan. This might suggest that skin damage in this region is due to incidental exposure that is not planned or deliberate. While the theoretical frameworks which have been adopted in past research can account for planned behaviours such as deliberate tanning, such models may not be suited to predicting unplanned behaviour such as incidental sun exposure. Therefore, alternative frameworks need to be explored. The prototype willingness (PW) model is a dual-process model which has been useful in the prediction of behaviours that are considered unplanned or spontaneous such as binge drinking and drug abuse. The current study aims to explore whether the dual pathways of the PW model differentially predict deliberate and incidental sun exposing behaviours. Methods: Community participants (n=218) from the North Queensland region completed a survey about their sun-related behaviours. Participants also responded to items that assessed standing on the PW model variables. Predictive modelling techniques were used to determine which of the model's proximal antecedents in each pathway was a better predictor of each type of exposure behaviour. Results: When controlling for age, skin sensitivity and gender; behavioural intention was the sole predictor of deliberate sun exposure (p<.05). Incidental sun exposure was predicted by both behavioural intention and behavioural willingness (p's<.05). This indicates that while engaging in deliberate sun exposure may be a deliberative process, there may be a number of cognitive mechanisms influencing incidental sun exposure. Conclusions: The current research provides support for the use of the PW model to explain variance across a range of sun-related behaviours, particularly incidental sun exposure. To date, skin cancer prevention efforts have focused on decreasing deliberate sun exposure. In North Queensland though, where the ambient ultra-violet radiation is high to extreme all year, incidental sun exposure may need to be targeted in order to reduce the prevalence of skin cancer. The current findings suggest that the PW model may be able to provide possible avenues for future health promotion activities to reduce incidental sun exposure in high risk regions

Radiative observables for linearized gravity on asymptotically flat spacetimes and their boundary induced states

We discuss the quantization of linearized gravity on globally hyperbolic, asymptotically flat, vacuum spacetimes and the construction of distinguished states which are both of Hadamard form and invariant under the action of all bulk isometries. The procedure, we follow, consists of looking for a realization of the observables of the theory as a sub-algebra of an auxiliary, non-dynamical algebra constructed on future null infinity $\Im^+$. The applicability of this scheme is tantamount to proving that a solution of the equations of motion for linearized gravity can be extended smoothly to $\Im^+$. This has been claimed to be possible provided that a suitable gauge fixing condition, first written by Geroch and Xanthopoulos, is imposed. We review its definition critically showing that there exists a previously unnoticed obstruction in its implementation leading us to introducing the concept of radiative observables. These constitute an algebra for which a Hadamard state induced from null infinity and invariant under the action of all spacetime isometries exists and it is explicitly constructed.Comment: 31 pages, added reference

Towards the graviton from spinfoams: higher order corrections in the 3d toy model

We consider the recent calculation gr-qc/0508124 of the graviton propagator in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102, we test how the spinfoam formalism can be used to construct the perturbative expansion of graviton amplitudes. Although the 3d graviton is a pure gauge, one can choose to work in a gauge where it is not zero and thus reproduce the structure of the 4d perturbative calculations. We compute explicitly the next to leading and next to next to leading orders, corresponding to one-loop and two-loop corrections. We show that while the first arises entirely from the expansion of the Regge action around the flat background, the latter receives contributions from the microscopic, non Regge-like, quantum geometry. Surprisingly, this new contribution reduces the magnitude of the next to next to leading order. It thus appears that the spinfoam formalism is likely to substantially modify the conventional perturbative expansion at higher orders. This result supports the interest in this approach. We then address a number of open issues in the rest of the paper. First, we discuss the boundary state ansatz, which is a key ingredient in the whole construction. We propose a way to enhance the ansatz in order to make the edge lengths and dihedral angles conjugate variables in a mathematically well-defined way. Second, we show that the leading order is stable against different choices of the face weights of the spinfoam model; the next to leading order, on the other hand, is changed in a simple way, and we show that the topological face weight minimizes it. Finally, we extend the leading order result to the case of a regular, but not equilateral, tetrahedron.Comment: 24 pages, many figure

Concurrent constraint programming with process mobility

We propose an extension of concurrent constraint programming with primitives for process migration within a hierarchical network, and we study its semantics. To this purpose, we first investigate a &quot;pure &quot; paradigm for process migration, namely a paradigm where the only actions are those dealing with transmissions of processes. Our goal is to give a structural definition of the semantics of migration; namely, we want to describe the behaviour of the system, during the transmission of a process, in terms of the behaviour of the components. We achieve this goal by using a labeled transition system where the effects of sending a process, and requesting a process, are modeled by symmetric rules (similar to handshaking-rules for synchronous communication) between the two partner nodes in the network. Next, we extend our paradigm with the primitives of concurrent constraint programming, and we show how to enrich the semantics to cope with the notions of environment and constraint store. Finally, we show how the operational semantics can be used to define an interpreter for the basic calculus.

Estimates of the total gravitation radiation in the head-on black hole collision

We report on calculations of the total gravitational energy radiated in the head-on black hole collision, where we use the geometry of the Robinson-Trautman metrics.Comment: 10 pages, 2 figures, LaTeX2

Hardware prototyping and validation of a W-ΔDOR digital signal processor

Microwave tracking, usually performed by on ground processing of the signals coming from a spacecraft, represents a crucial aspect in every deep-space mission. Various noise sources, including receiver noise, affect these signals, limiting the accuracy of the radiometric measurements obtained from the radio link. There are several methods used for spacecraft tracking, including the Delta-Differential One-Way Ranging (ΔDOR) technique. In the past years, European Space Agency (ESA) missions relied on a narrowband ΔDOR system for navigation in the cruise phase. To limit the adverse effect of nonlinearities in the receiving chain, an innovative wideband approach to ΔDOR measurements has recently been proposed. This work presents the hardware implementation of a new version of the ESA X/Ka Deep Space Transponder based on the new tracking technique named Wideband ΔDOR (W-ΔDOR). The architecture of the new transponder guarantees backward compatibility with narrowband ΔDOR

Potential applications of natural origin polymer-based systems in soft tissue regeneration

Despite the many advances in tissue engineering approaches, scientists still face significant challenges in trying to repair and replace soft tissues. Nature-inspired routes involving the creation of polymer-based systems of natural origins constitute an interesting alternative route to produce novel materials. The interest in these materials comes from the possibility of constructing multi-component systems that can be manipulated by composition allowing one to mimic the tissue environment required for the cellular regeneration of soft tissues. For this purpose, factors such as the design, choice, and compatibility of the polymers are considered to be key factors for successful strategies in soft tissue regeneration. More recently, polysaccharide-protein based systems have being increasingly studied and proposed for the treatment of soft tissues. The characteristics, properties, and compatibility of the resulting materials investigated in the last 10 years, as well as commercially available matrices or those currently under investigation are the subject matter of this review.S.S. Silva would like to acknowledge the Portuguese Foundation for Science and Technology (FCT) for a post doctoral fellowship (SFRH/BD/45307/2008). This work was partially supported by the European-Union-funded FP7 Project: Find and Bind (NMP4-SL-2009-229292) and STREP project HIPPOCRATES (NMP3-CT-2003-505758) and was carried out under the scope of the European NoE EXPERTISSUES (NMP3-CT-2004-500283)