Magnification Bias Corrections to Galaxy-Lensing Cross-Correlations

Galaxy-galaxy or galaxy-quasar lensing can provide important information on the mass distribution in the Universe. It consists of correlating the lensing signal (either shear or magnification) of a background galaxy/quasar sample with the number density of a foreground galaxy sample. However, the foreground galaxy density is inevitably altered by the magnification bias due to the mass between the foreground and the observer, leading to a correction to the observed galaxy-lensing signal. The aim of this paper is to quantify this correction. The single most important determining factor is the foreground redshift z: the correction is small if the foreground galaxies are at low redshifts but can become non-negligible for sufficiently high redshifts. For instance, we find that for the multipole l=1000, the correction is above 1%*(5s-2)/b for z<0.37, and above 5%*(5s-2)/b for z<0.67, where s is the number count slope of the foreground sample, and b its galaxy bias. These considerations are particularly important for geometrical measures, such as the Jain and Taylor ratio or its generalization by Zhang et al. Assuming (5s-2)/b=1, we find that the foreground redshift should be limited to z<0.45 in order to avoid biasing the inferred dark energy equation of state w by more than 5%, and that even for a low foreground redshift (< 0.45), the background samples must be well separated from the foreground to avoid incurring a bias of similar magnitude. Lastly, we briefly comment on the possibility of obtaining these geometrical measures without using galaxy shapes, using instead magnification bias itself.Comment: 10 pages, 7 figures; v2: minor revisions, as accepted for publication in Physical Review

Recovering General Relativity from massive gravity

We obtain static, spherically symmetric, and asymptotically flat numerical solutions of massive gravity with a source. Those solutions show, for the first time explicitly, a recovery of the Schwarzschild solution of General Relativity via the so-called Vainshtein mechanism.Comment: 4 pages, 3 figures; v2: minor changes, matches published versio

Algorithmic Management in the Food Delivery Sector - A Contested Terrain? Evidence from a Firm Level Case Study on Algorithmic Management and Co-Determination

Forms of algorithmic management (AM) play an essential role in organizing food delivery work by deploying AI-based systems for coordinating driver routes. Given the risks of precarity and threats posed by AM that are typically related to (migrant) platform work, the question arises to what extent structures of co-determination are able to positively shape this type of work and the technologies involved. Based on an intense case-study in a large food delivery company, this paper is guided by three questions: (1) How is algorithm-based management and control used by the company? (2) How is it perceived by the couriers, also in relation to other aspects of their work? (3) What are the works council's priorities, strategies, and achievements regarding co-determination practices? Contrary to the prevalent perception in the literature on the subject of AM, our analysis shows that human agency is still pivotal when algorithm-based systems are used to manage work processes. While data- and AM-related issues do not represent a central area of conflict, we find that co-determination rights in this domain can translate into a powerful bargaining resource of the works council with regard to the companies' digital business model. Our study also shows that algorithmic management poses problems of non-transparency and information asymmetry, which calls for new forms and procedures of co-determination

The Vainshtein mechanism in the Decoupling Limit of massive gravity

We investigate static spherically symmetric solutions of nonlinear massive gravities. We first identify, in an ansatz appropriate to the study of those solutions, the analog of the decoupling limit (DL) that has been used in the Goldstone picture description. We show that the system of equations left over in the DL has regular solutions featuring a Vainshtein-like recovery of solutions of General Relativity (GR). Hence, the singularities found to arise integrating the full nonlinear system of equations are not present in the DL, despite the fact those singularities are usually thought to be due to a negative energy mode also seen in this limit. Moreover, we show that the scaling conjectured by Vainshtein at small radius is only a limiting case in an infinite family of non singular solutions each showing a Vainshtein recovery of GR solutions below the Vainshtein radius but a different common scaling at small distances. This new scaling is shown to be associated with a zero mode of the nonlinearities left over in the DL. We also show that, in the DL, this scaling allows for a recovery of GR solutions even for potentials where the original Vainshtein mechanism is not working. Our results imply either that the DL misses some important features of nonlinear massive gravities or that important features of the solutions of the full nonlinear theory have been overlooked. They could also have interesting outcomes for the DGP model and related proposals.Comment: 52 pages, 7 figures; v3: minor typos corrected, discussion of the validity of the Decoupling Limit extended; accepted for publication in JHE

Ray-tracing through the Millennium Simulation: Born corrections and lens-lens coupling in cosmic shear and galaxy-galaxy lensing

(abridged) We study the accuracy of various approximations to cosmic shear and weak galaxy-galaxy lensing and investigate effects of Born corrections and lens-lens coupling. We use ray-tracing through the Millennium Simulation to calculate various cosmic-shear and galaxy-galaxy-lensing statistics. We compare the results from ray-tracing to semi-analytic predictions. We find: (i) The linear approximation provides an excellent fit to cosmic-shear power spectra as long as the actual matter power spectrum is used as input. Common fitting formulae, however, strongly underestimate the cosmic-shear power spectra. Halo models provide a better fit to cosmic shear-power spectra, but there are still noticeable deviations. (ii) Cosmic-shear B-modes induced by Born corrections and lens-lens coupling are at least three orders of magnitude smaller than cosmic-shear E-modes. Semi-analytic extensions to the linear approximation predict the right order of magnitude for the B-mode. Compared to the ray-tracing results, however, the semi-analytic predictions may differ by a factor two on small scales and also show a different scale dependence. (iii) The linear approximation may under- or overestimate the galaxy-galaxy-lensing shear signal by several percent due to the neglect of magnification bias, which may lead to a correlation between the shear and the observed number density of lenses. We conclude: (i) Current semi-analytic models need to be improved in order to match the degree of statistical accuracy expected for future weak-lensing surveys. (ii) Shear B-modes induced by corrections to the linear approximation are not important for future cosmic-shear surveys. (iii) Magnification bias can be important for galaxy-galaxy-lensing surveys.Comment: version taking comments into accoun

How Far Are We from the Quantum Theory of Gravity?

I give a pedagogical explanation of what it is about quantization that makes general relativity go from being a nearly perfect classical theory to a very problematic quantum one. I also explain why some quantization of gravity is unavoidable, why quantum field theories have divergences, why the divergences of quantum general relativity are worse than those of the other forces, what physicists think this means and what they might do with a consistent theory of quantum gravity if they had one. Finally, I discuss the quantum gravitational data that have recently become available from cosmology.Comment: 106 page review article solicited by Reports on Progress in Physic

Valorisation d’un fonds d’ouvrages d’art à l’heure du numérique dans le contexte d’une agence de publicité : le cas du service documentation de BETC

This dissertation studies how to exploit the full potential of a collection of books on art in an advertising agency where digital technology dominates. The author carried out qualitative interviews among a sample group of the company’s creative workers. These are used to show that the search needs for images by creative people, who are the main audience for books on art, are met by web resources. In the face of this competition, the image bank in the Documentation Center needs become more functional through an increased use of information management techniques. The approach should be two-fold : the implementation of a cataloguing plan for hard-copy resources and an online catalogue. To better use its resources, the Documentation Center should withdraw from direct competition with the web and become an instrument for business intelligence and a place for relaxing. The author highlights the roles played by the layout of the space and staff in terms of management, facilitation and mediation of the Documentation Center to fight effectively against the loss of interest in the collection.Ce mémoire étudie les moyens de revaloriser un fonds de livres d’art au sein d’une entreprise de publicité où le numérique est partout présent. L’auteur s’est appuyé pour sa réflexion sur des entretiens qualitatifs menés auprès d’un échantillon de créatifs de l’entreprise.Il montre qu’il existe une adéquation certaine entre la recherche d’images par les créatifs, premiers destinataires du fonds de livres d’art et les ressources offertes par Internet.Pour faire face à cette concurrence, le fonds image doit devenir plus fonctionnel, en s’appuyant davantage sur les techniques documentaires et ce à deux niveaux : par la mise en place d’un plan de classement physique des ouvrages et la mise en ligne du catalogue.Pour être revalorisé le fonds devra également se redéployer, hors concurrence directe avec Internet, en tant qu’instrument de veille et de détente culturelle.L’auteur souligne également le rôle joué par l’aménagement de l’espace et le rôle du personnel en termes de gestion, de médiation et d’animation pour lutter efficacement contre la désaffection du fonds

Cosmologie et modifications à grandes distances de l'interaction gravitationnelle

In the framework of General Relativity, the observed current acceleration of the expansion of the Universe requires the presence of a “Dark Energy” component, whose nature is not well understood. In order to explain the acceleration of the Universe without introducing such a tantalizing source of energy, other gravitation theories have been designed. This thesis is devoted to the study of some of these modified gravity theories, as well as to the observation methods that could constrain them. The first part of this thesis presents a review of modified gravity theories and their motivations. The second part is devoted to the study of the massive gravity theories and of the so-called Vainshtein's mechanism, which allows some of the solutions of Massive Gravity to strongly differ from General Relativity at cosmological scales while satisfying the experimental constraints inside the solar system. For the first time, the validity of the Vainshtein's mechanism is demonstrated, through the study of specific spherically symmetric solutions. The third part deals with scalar modification of gravity; a new model of this sort is presented, inspired by the Vainshtein's mechanism in Massive Gravity. Finally, the fourth part discusses local, astrophysical and cosmological observations that might constrain modified gravity theories.Dans le cadre de la relativité générale, l'observation de la phase actuelle d'accélération de l'expansion de l'Univers soulève de nombreuses questions car elle semble indiquer l'existence d'une "énergie noire" dont on ne connaît pas la nature. Afin de pouvoir expliquer l'accélération de l'Univers sans énergie noire, d'autres théories de la gravité ont été proposées. Cette thèse est consacrée à l'étude de certaines de ces théories de gravité modifiée, ainsi qu'aux méthodes d'observation qui peuvent les contraindre. La première partie de cette thèse présente un panorama des théories de gravité modifiée ainsi que leurs motivations. La seconde partie analyse les théories de gravité massive et le mécanisme dit « de Vainshtein », qui permet à certaines solutions de la gravité massive de différer fortement de la relativité générale aux échelles cosmologiques tout en satisfaisant les contraintes expérimentales au sein du système solaire. La validité de ce mécanisme y est démontré pour la première fois, au travers de l'étude de certaines solutions à symétrie sphérique. La troisième partie traite des modifications scalaires de la gravité ; un nouveau modèle de gravité scalaire y est notamment proposé, inspiré du mécanisme de Vainshtein de la gravité massive. Enfin, la quatrième partie décrit les différentes observations locales, astrophysiques et cosmologiques, susceptibles de contraindre les théories de gravité modifiée

Cosmologie et modifications à grandes distances de l'interaction gravitationnelle

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