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the light from "point sources"--such as supernovae--probes the Universe on scales where the homogeneity principle is no longer valid.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

Generating a 3D Simulation of a Car Accident from a Written Description in Natural Language: the CarSim System

This paper describes a prototype system to visualize and animate 3D scenes from car accident reports, written in French. The problem of generating such a 3D simulation can be divided into two subtasks: the linguistic analysis and the virtual scene generation. As a means of communication between these two modules, we first designed a template formalism to represent a written accident report. The CarSim system first processes written reports, gathers relevant information, and converts it into a formal description. Then, it creates the corresponding 3D scene and animates the vehicles.Comment: 8 pages, ACL 2001, Workshop on Temporal and Spatial Information Processin

Variaciones sobre Vértigo

La película de Alfred Hitchcock data de 1958. Mi querido editor, Benoît Chantre, me ha pedido que resuma suintriga para los lectores que no la hayan visto. Es un doble crimen el que me pide que cometa, en contra demi voluntad. Primero, porque les voy a estropear a estos lectores el placer o mejor –como digo yo– el choquetraumático de la primera vez. Pero sobre todo porque –por las razones profundas que aparecerán cuando selea este capítulo–, Vértigo (como un objeto complejo) no se resume. Sin embargo voy a tratar de hacerlo. Scottie Ferguson (interpretado por James Stewart) es un antiguo inspector expulsado de la policía a causa de su tendencia al vértigo, resultado de una persecución por los techos de San Francisco, la cual terminó mal, pues se mostró incapaz de impedir la caída mortal de un policía que trataba de ayudarlo. Gavin Elste, un viejo amigo de Scottie, le ha encargado vigilar a su esposa, Madeleine (interpretada por Kim Novak), cuyo extraño comportamiento deja temer su suicidio

Soft Law and the International Law of the Environment

Because the existing body of international environmental law has, in part, emerged on the basis of soft norms, it provides a good field for observing the general sociological and juridical phenomenon termed soft law. The 1972 Stockholm Declaration adopted by the UN Conference on the Human Environment, for example, constitutes the normative program for the world community \u27in this field. Although, from a formal point of view, the Declaration is only a nonbinding resolution, many of its principles, particularly Principle 21, have been relied upon by governments to justify their legal rights and duties. The subsequent State practice has been, no doubt, influenced by such provisions. It is in this context and with the benefit of these introductory remarks that we shall briefly and successively examine the creation (I), the forms and content (II), and the legal effects (III) of soft law in the field of international environmental law

Can all cosmological observations be accurately interpreted with a unique geometry?

The recent analysis of the Planck results reveals a tension between the best fits for ({\Omega}m0, H0) derived from the cosmic microwave background or baryonic acoustic oscillations on the one hand, and the Hubble diagram on the other hand. These observations probe the universe on very different scales since they involve light beams of very different angular sizes; hence the tension between them may indicate that they should not be interpreted the same way. More precisely, this Letter questions the accuracy of using only the (perturbed) Friedmann-Lema\^itre geometry to interpret all the cosmological observations, regardless of their angular or spatial resolution. We show that using an inhomogeneous "Swiss-cheese" model to interpret the Hubble diagram allows to reconcile it with the Planck results. Such an approach does not require us to invoke new physics nor to violate the Copernican principle.Comment: 4 pages, 1 figure; typos corrected; v2 matches published versio

Bacterivory by benthic organisms in sediment: quantification using 15N enriched bacteria

International audienceThe fate of benthic bacterial biomass in benthic food webs is a topic of major importance but poorly described. This paper describes an alternative method for evaluation of bacterial grazing rate by meiofauna and macrofauna using bacteria pre-enriched with stable isotopes. Natural bacteria from the sediment of an intertidal mudflat were cultured in a liquid medium enriched with 15NH4Cl. Cultured bacteria contained 2.9 % of 15N and were enriched sufficiently to be used as tracers during grazing experiments. Cultured-bacteria presented a biovolume (0.21 µm3) and a percentage of actively respiring bacteria (10 %) similar to those found in natural communities. The number of Operational Taxon Units found in cultures fluctuated between 56 and 75 % of that found in natural sediment. Despite this change in community composition, the bacterial consortium used for grazing experiments exhibited characteristics of size, activity and diversity more representative of the natural community than usually noticed in many other grazing studies. The bacterial ingestion rates of three different grazers were in the range of literature values resulting from other methods: 1149 ngC ind-1 h-1 for the mud snail Hydrobia ulvae, 0.027 ngC ind-1 h-1 for the nematode community, and 0.067 ngC ind-1 h-1 for the foraminifera Ammonia tepida. The alternative method described in this paper overcomes some past limitations and it presents interesting advantages such as short time incubation and in situ potential utilisation