Institut Universitaire de France

About: Institut Universitaire de France is a education organization based out in Paris, France. It is known for research contribution in the topics: Population & Catalysis. The organization has 1905 authors who have published 9042 publications receiving 309878 citations.

Temporary Jobs, Employment Protection and Labor Market Performance

Abstract: Many European labor markets are characterized by heavy employment protection taxes and the widespread use of fixed-duration contracts. The simultaneous use of these two policy instruments seems somewhat contradictory since the former primarily aims at limiting job destruction whereas the latter clearly intensifies it. In this paper, we use a simple matching model with endogenous job destructions to analyze the impact of a combination of the two policies. We find that the political support of insiders to firing restrictions does not necessarily worsen unemployment. In fact, it may be the willingness of a majority of workers to support the "combination" of two instruments with opposite effects on job destruction and job creation that increase unemployment and deteriorates efficiency. Moreover, we argue, that the concentration of firm ownership is likely to influence labor market regulations. We show that the preferred point of a majority of workers is a very flexible labor market, without any firing cost when profits are evenly distributed across the labor force (which portrays a situation where firm ownership is extremely dispersed), whereas a combination of job protection and temporary jobs is preferred by workers when the share of profits that they earn is zero (which corresponds to a situation where firm ownership is concentrated among a few shareholders not participating in the labor market).

Planck 2015 results: XIX. Constraints on primordial magnetic fields

Abstract: We predict and investigate four types of imprint of a stochastic background of primordial magnetic fields (PMFs) on the cosmic microwave background (CMB) anisotropies: the impact of PMFs on the CMB temperature and polarization spectra, related to their contribution to cosmological perturbations; the effect on CMB polarization induced by Faraday rotation; magnetically-induced non-Gaussianities and related non-zero bispectra; and the magnetically-induced breaking of statistical isotropy. We present constraints on the amplitude of PMFs derived from different combinations of Planck data products, depending on the specific effect that is analysed. Overall, Planck data constrain the amplitude of PMFs to less than a few nanogauss, with different bounds depending on the considered model. In particular, individual limits coming from the analysis of the CMB angular power spectra, using the Planck likelihood, are B1Mpc < 4:4 nG (where B1Mpc is the comoving field amplitude at a scale of 1 Mpc) at 95 % confidence level, assuming zero helicity, and B1Mpc < 5:6 nG when we consider a maximally helical field. For nearly scaleinvariant PMFs we obtain B1Mpc < 2:1 nG and B1Mpc < 0:7 nG if the impact of PMFs on the ionization history of the Universe is included in the analysis. From the analysis of magnetically-induced non-Gaussianity we obtain three different values, corresponding to three applied methods, all below 5 nG. The constraint from the magnetically-induced passive-tensor bispectrum is B1Mpc < 2:8 nG. A search for preferred directions in the magnetically-induced passive bispectrum yields B1Mpc < 4:5 nG, whereas the the compensated-scalar bispectrum gives B1Mpc < 3 nG. The analysis of the Faraday rotation of CMB polarization by PMFs uses the Planck power spectra in EE and BB at 70 GHz and gives B1Mpc < 1380 nG. In our final analysis, we consider the harmonic-space correlations produced by Alfv´ en waves, finding no significant evidence for the presence of these waves. Together, these results comprise a comprehensive set of constraints on possible PMFs with Planck data.

A change in the optical polarization associated with a γ-ray flare in the blazar 3C 279

Abstract: It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight1. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (γ)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 105 gravitational radii.

Planck 2013 results. XVII. Gravitational lensing by large-scale structure

Abstract: On the arcminute angular scales probed by Planck, the cosmic microwave background (CMB) anisotropies are gently perturbed by gravitational lensing. Here we present a detailed study of this e_ect, detecting lensing independently in the 100, 143, and 217 GHz frequency bands with an overall significance of greater than 25_.We use the temperature-gradient correlations induced by lensing to reconstruct a (noisy) map of the CMB lensing potential, which provides an integrated measure of the mass distribution back to the CMB last-scattering surface. Our lensing potential map is significantly correlated with other tracers of mass, a fact which we demonstrate using several representative tracers of large-scale structure. We estimate the power spectrum of the lensing potential, finding generally good agreement with expectations from the best-fitting _CDM model for the Planck temperature power spectrum, showing that this measurement at z = 1100 correctly predicts the properties of the lower-redshift, latertime structures which source the lensing potential. When combined with the temperature power spectrum, our measurement provides degeneracy breaking power for parameter constraints; it improves CMB-alone constraints on curvature by a factor of two and also partly breaks the degeneracy between the amplitude of the primordial perturbation power spectrum and the optical depth to reionization, allowing a measurement of the optical depth to reionization which is independent of large-scale polarization data. Discarding scale information, our measurement corresponds to a 4% constraint on the amplitude of the lensing potential power spectrum, or a 2% constraint on the root-mean-squared amplitude of matter fluctuations at z _ 2.