Imprint of DESI fiber assignment on the anisotropic power spectrum of emission line galaxies

The Dark Energy Spectroscopic Instrument (DESI), a multiplexed fiber-fed spectrograph, is a Stage-IV ground-based dark energy experiment aiming to measure redshifts for 29 million Emission-Line Galaxies (ELG), 4 million Luminous Red Galaxies (LRG), and 2 million Quasi-Stellar Objects (QSO). The survey design includes a pattern of tiling on the sky and the locations of the fiber positioners in the focal plane of the telescope, with the observation strategy determined by a fiber assignment algorithm that optimizes the allocation of fibers to targets. This strategy allows a given region to be covered on average five times for a five-year survey, but with coverage varying between zero and twelve, which imprints a spatially-dependent pattern on the galaxy clustering. We investigate the systematic effects of the fiber assignment coverage on the anisotropic galaxy clustering of ELGs and show that, in the absence of any corrections, it leads to discrepancies of order ten percent on large scales for the power spectrum multipoles. We introduce a method where objects in a random catalog are assigned a coverage, and the mean density is separately computed for each coverage factor. We show that this method reduces, but does not eliminate the effect. We next investigate the angular dependence of the contaminated signal, arguing that it is mostly localized to purely transverse modes. We demonstrate that the cleanest way to remove the contaminating signal is to perform an analysis of the anisotropic power spectrum $P(k,\mu)$ and remove the lowest $\mu$ bin, leaving $\mu>0$ modes accurate at the few-percent level. Here, $\mu$ is the cosine of the angle between the line-of-sight and the direction of $\vec{k}$. We also investigate two alternative definitions of the random catalog and show they are comparable but less effective than the coverage randoms method.Comment: Submitted to JCA

Machine learning cosmic inflation

We present a machine-learning approach, based on the genetic algorithms (GA), that can be used to reconstruct the inflationary potential directly from cosmological data. We create a pipeline consisting of the GA, a primordial code and a Boltzmann code used to calculate the theoretical predictions, and Cosmic Microwave Background (CMB) data. As a proof of concept, we apply our methodology to the Planck CMB data and explore the functional space of single-field inflationary potentials in a non-parametric, yet analytical way. We show that the algorithm easily improves upon the vanilla model of quadratic inflation and proposes slow-roll potentials better suited to the data, while we confirm the robustness of the Starobinsky inflation model (and other small-field models). Moreover, using unbinned CMB data, we perform a first concrete application of the GA by searching for oscillatory features in the potential in an agnostic way, and find very significant improvements upon the best featureless potentials, $\Delta \chi^2 < -20$. These encouraging preliminary results motivate the search for resonant features in the primordial power spectrum with a multimodal distribution of frequencies. We stress that our pipeline is modular and can easily be extended to other CMB data sets and inflationary scenarios, like multifield inflation or theories with higher-order derivatives.Comment: 14 pages, 10 figures, 2 tables. Comments welcom

Borel resummation of secular divergences in stochastic inflation

We make use of Borel resummation to extract the exact time dependence from the divergent series found in the context of stochastic inflation. Correlation functions of self-interacting scalar fields in de Sitter spacetime are known to develop secular IR divergences via loops, and the first terms of the divergent series have been consistently computed both with standard techniques for curved spacetime quantum field theory and within the framework of stochastic inflation. We show that Borel resummation can be used to interpret the divergent series and to correctly infer the time evolution of the correlation functions. In practice, we adopt a method called Borel--Pad\'{e} resummation where we approximate the Borel transformation by a Pad\'{e} approximant. We also discuss the singularity structures of Borel transformations and mention possible applications to cosmology.Comment: 40 pages, 12 figure

Naissances de pensée

International audienceOffrant aux cliniciens et chercheurs français de nouveaux moyens de travail grâce à la traduction de l'Introduction aux idées psychanalytiques de Bion, Didier Anzieu (1976, XII) rappelait que, « s'il y a pour Bion un impensant, il n'y a pas d'impensable ». Et il regroupait,- sous la notion de « pensée primaire », tout à la fois la première « activité de penser », celle qui « à l'origine se confond avec un processus destiné à décharger le psychisme de l'excès de stimuli » et les pensées, qui ne sont alors « rien de plus que des impressions sensorielles et des vivances émotionnelles très primitives » (Grinberg et al., 1972, 51). Cette « pensée-acte », « je suis tenté, écrivait Anzieu, de l'appe- ler l'impensante : la pensée-mort psychique » (XI). Voilà pour « l'impensant » : celui qui ne peut être transformé chez le nourrisson que grâce à la « capacité de rêverie » de la mère (au sens de « capacité d'imagination non appuyée sur le raisonnement », Grinberg et al., 1972, 54), et celui qui est offert au travail psychanalytique, lorsque celui-ci se développe dans l'interaction avec des patients psycho- tiques ou souffrant de failles narcissiques : travail psychique « à la frontière de l'impensant et du premier pensant symbolisable » (XIII). La naissance de la pensée trouve alors son origine dans un travail effectué, selon Anzieu, « à la place du patient » (ou du bébé), puisque l'un et l'autre sont « hors d'état d'y pourvoir » (XIII). Remarque qui insiste sur la nécessité de l'engagement d'un appareil psychique extérieur à celui qui ne peut naître à la pensée sans l'aide d'un autre psychisme (Meltzer, 1984)