Weak gravitational lensing of finite beams

The standard theory of weak gravitational lensing relies on the infinitesimal light beam approximation. In this context, images are distorted by convergence and shear, the respective sources of which unphysically depend on the resolution of the distribution of matter---the so-called Ricci-Weyl problem. In this letter, we propose a strong-lensing-inspired formalism to describe the lensing of finite beams. We address the Ricci-Weyl problem by showing explicitly that convergence is caused by the matter enclosed by the beam, regardless of its distribution. Furthermore, shear turns out to be systematically enhanced by the finiteness of the beam. This implies, in particular, that the Kaiser-Squires relation between shear and convergence is violated, which could have profound consequences on the interpretation of weak lensing surveys.Comment: 6 pages, 2 figures, v2: matches published version, some typos correcte

The theory of stochastic cosmological lensing

On the scale of the light beams subtended by small sources, e.g. supernovae, matter cannot be accurately described as a fluid, which questions the applicability of standard cosmic lensing to those cases. In this article, we propose a new formalism to deal with small-scale lensing as a diffusion process: the Sachs and Jacobi equations governing the propagation of narrow light beams are treated as Langevin equations. We derive the associated Fokker-Planck-Kolmogorov equations, and use them to deduce general analytical results on the mean and dispersion of the angular distance. This formalism is applied to random Einstein-Straus Swiss-cheese models, allowing us to: (1) show an explicit example of the involved calculations; (2) check the validity of the method against both ray-tracing simulations and direct numerical integrations of the Langevin equation. As a byproduct, we obtain a post-Kantowski-Dyer-Roeder approximation, accounting for the effect of tidal distortions on the angular distance, in excellent agreement with numerical results. Besides, the dispersion of the angular distance is correctly reproduced in some regimes.Comment: 37+13 pages, 8 figures. A few typos corrected. Matches published versio

Interpretation of the Hubble diagram in a nonhomogeneous universe

In the standard cosmological framework, the Hubble diagram is interpreted by assuming that the light emitted by standard candles propagates in a spatially homogeneous and isotropic spacetime. However, the light from "point sources"--such as supernovae--probes the Universe on scales where the homogeneity principle is no longer valid. Inhomogeneities are expected to induce a bias and a dispersion of the Hubble diagram. This is investigated by considering a Swiss-cheese cosmological model, which (1) is an exact solution of the Einstein field equations, (2) is strongly inhomogeneous on small scales, but (3) has the same expansion history as a strictly homogeneous and isotropic universe. By simulating Hubble diagrams in such models, we quantify the influence of inhomogeneities on the measurement of the cosmological parameters. Though significant in general, the effects reduce drastically for a universe dominated by the cosmological constant.Comment: 25 pages, 25 figures. Typos and Eqs. (2.15), (2.17), (4.16), (4.20), and (4.43) corrected. Fig. 25 updated. Matches published versio

Local organic food supply chains in Rhône-Alpes (France): form of resistance or vehicle for conventionalisation of organic agriculture?

For ten years, innovative forms of distribution of organic products emerged between long supply chain and short supply chain. We defined them as " localized agri-food supply chains". Our work is based on the analysis of 18 localized food supply chains in Rhône-Alpes (France). Firstly, it presents the diversity of these localized supply chains and the mechanisms implemented to address the limits of short supply chains, particularly an increasing number of intermediaries. Then we assess their performances with regard to the advantages commonly lent to short supply chains. We demonstrate that localized supply chains are in some cases a vehicle for conventionalisation of organic agriculture and in other a form of mitigation of the agribusiness model

A localised development of organic farming as a response to the problem of water quality: a new challenge

Initiatives aiming at developing organic farming (OF) to meet water quality stakes are increasing. This article is a comparative study of four such projects. It is based on semi-structured interviews. Two logics for public action are implemented to foster the development of OF in areas facing water quality problems. The first one aims at concentrating actions in water sensitive areas. The main tool that is then mobilized is the use of localized agri-environmental measures supporting conversion to organic farming. The second logic aims at implementing actions for organic development at a geographical scale considered as relevant for developing OF and differing from water sensitive areas. The main actions that are then activated are coordination, consciousness-raising about OF for agricultural stakeholders and structuring of processing and distribution chains for organic products. Our result showed that the actor networks differ according to local contexts, project leaders, and objectives. They tended to expand and become more complex over time. This explains why such projects are difficult and slow to put in place. Nevertheless, water conservation has now become an asset for the development of OF. This entails reconsidering the theoretical development models for OF, thus enriching them with a territorial dimension

Description of the Grover algorithm based on geometric considerations

This paper concerns the Grover algorithm that permits to make amplification of quantum states previously tagged by an Oracle. Grover's algorithm allows searches in an unstructure database of n entries finding a marked element with a quadratic speedup. The algorithm requires a predefined number of runs to succeed with probability close to one.This article provides a description of the amplitude amplification quantum algorithm mechanism in a very short computational way, based on tensor products and provides a geometric presentation of the successive system states. All the basis changes are fully described to provide an alternative to the wide spread Grover description based only on matrices and complex tensor computation. Our experiments encompass numerical evaluations of circuit using the Qiskit library of IBM that meet the theoretical consideration

A technical note for the 91-clauses SAT resolution with Indirect QAOA based approach

This paper addresses the resolution of the 3-SAT problem using a QAOA-like approach. The chosen principle involves modeling the solution ranks of the 3-SAT problem, which, in this particular case, directly represent a solution. This results in a highly compact circuit with few gates, enabling the modeling of large-sized 3-SAT problems. Numerical experimentation demonstrates that the approach can solve instances composed of 91 clauses and 20 variables with an implementation based on Qiskit

Landscape Maintenance and Farming in the Alps: From Family Firms Up-Keeping to Inter-Institutional Arrangements

This contribution discusses assets and limits of the local/territorial level as a core level to improve the sustainability of agriculture. The focus is on the issue of rural landscape maintenance through farming. Some possible institutional solutions to overcome the difficulties of family farms are examined. New institutional settings such as the "local group", constituting an interface for the negotiation among different local stakeholders, seem to effectively facilitate the stipulation of local contracts for landscape management. The participation to these contracts requires a change in farmers' identity and a more general re-conceptualization of agricultural and environmental problems as community problems.Land use, Participatory approach, Rural development, Community/Rural/Urban Development, Land Economics/Use,

On the stability and causality of scalar-vector theories

Various extensions of standard inflationary models have been proposed recently by adding vector fields. Because they are generally motivated by large-scale anomalies, and the possibility of statistical anisotropy of primordial fluctuations, such models require to introduce non-standard couplings between vector fields on the one hand, and either gravity or scalar fields on the other hand. In this article, we study models involving a vector field coupled to a scalar field. We derive restrictive necessary conditions for these models to be both stable (Hamiltonian bounded by below) and causal (hyperbolic equations of motion).Comment: 20 pages, references added, v2 matches published version in JCA

Localized organic agri-food supply chains in Rhône-Alpes (France): diversity of organisations and highlights of the notion of proximity

For ten years, innovative forms of distribution of organic products have emerged between long and short supply chains. They are often defined as "localized agrifood systems". We based this work on the analysis of 18 localized agrifood systems in the Rhône-Alpes region (France). First, it presents the dynamics of construction and organization of these agrifood systems: diversity of actors involved, relationships between actors and rules that have been gradually established. We used the concept of geographical and relational proximity to analyse these supply chains and to highlight the mechanisms implemented to address the limits of short supply chains, particularly an increasing number of intermediaries. Then we assessed their performances with regard to the advantages commonly given to short supply chains. Finally we demonstrated that localized supply chains are in some cases a way towards conventionalisation of organic agriculture and in others a form of mitigation of the agribusiness model