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

Back to the features: assessing the discriminating power of future CMB missions on inflationary models

Future Cosmic Microwave Background (CMB) experiments will deliver extremely accurate measurements of the E-modes pattern of the CMB polarization field. Given the sharpness of the E-modes transfer functions, such surveys make for a powerful detector of high-frequency signals from primordial features that may be lurking in current data sets. With a handful of toy models that increase the fit to the latest Planck data, but are of marginal statistical significance, we use a state-of-the-art forecast pipeline to illustrate the promising prospects to test primordial features in the next decade. Not only will future experiments allow us to detect such features in data, but they will also be able to discriminate between models and narrow down the physical mechanism originating them with high statistical significance. On the other hand, if the anomalies in the currently measured CMB spectra are just statistical fluctuations, all the current feature best fit candidates will be ruled out. Either way, our results show that primordial features are a clear target of forthcoming CMB surveys beyond the detection of tensor modes.Comment: 36 pages, 15 figures; v2 matches version published in JCA

Primordial Stochastic Gravitational Wave Background Anisotropies: in-in Formalization and Applications

Primordial non-Gaussianities of the scalar(tensor)-tensor-tensor type supporting a non-trivial squeezed component are known to induce anisotropies in the stochastic gravitational wave background. We derive the explicit form of such anisotropies by making use, for the first time in this context, of the in-in formalism for cosmological correlation functions. After illustrating the general method and using it for the minimal single-field slow-roll case, we apply it to multi-field models, providing both a tree-level and a one-loop example. First, we make contact with previous results on anisotropies due to the presence of an extra spin-2 field during inflation. Secondly, we calculate the 1-loop scalar-tensor-tensor three-point function in the context of so-called supersolid inflation. The corresponding gravitational wave anisotropy is induced atop a gravitational signal that may be sufficiently large for detection.Comment: 33 pages, 5 figure

Multifield inflation beyond Nfield=2N_\mathrm{field}=2: non-Gaussianities and single-field effective theory

International audienceIn this article, we study in detail the linear dynamics and cubic interactions for any number Nfield of scalar fields during inflation, directly in terms of the observable curvature perturbation ζ and Nfield-1 entropic fluctuations, a choice that is more suitable for analytical works. In the linear equations of motion for the perturbations, we uncover rich geometrical effects beyond terms involving just the scalar curvature of the field space, and that come from the non-canonical kinetic structure of the scalar fields when the dimension of the field space is larger than two. Moreover, we show that a fast rotation of the local entropic basis can result in negative eigenvalues for the entropic mass matrix, potentially destabilising the background dynamics when Nfield⩾ 3. We also explain how to render manifest the sizes of cubic interactions between the adiabatic and the entropic fluctuations, extending a previous work of ours to any number of interacting fields. As a first analytical application of our generic formalism, we derive the effective single-field theory for perturbations up to cubic order when all entropic fluctuations are heavy enough to be integrated out. In a slow-varying limit, we recover the cubic action expected from the effective field theory of inflation, but with a prediction for the usual Wilson coefficients in terms of the multifield parameters, thus proposing a new interpretation of the bispectrum in this generic Nfield context

Multifield aspects in the early Universe : Inflation and Reheating

Cette thèse de doctorat traite des aspects théoriques et phénoménologiques de l'inflation cosmologique lorsqu'elle est provoquée par la présence de plusieurs champs scalaires. Après quelques rappels à propos du cadre relativiste dans lequel s’inscrit la cosmologie physique contemporaine, une introduction au paradigme inflationnaire au niveau homogène et linéaire est proposée, non seulement dans le cadre à un champ mais aussi multichamps. S’ensuit une étude approfondie des non-gaussianités primordiales. Il est montré que l’inflation multichamps laisse des traces caractéristiques dans ces déviations à la statistique gaussienne des fluctuations primordiales. Un intérêt particulier est porté aux effets géométriques dus à la courbure de l’espace interne de ces champs scalaires. Cette thèse montre aussi comment étendre le formalisme stochastique aux théories inflationnaires multichamps avec des termes cinétiques non canoniques. Elle dévoile une ambiguïté présente de manière générique dans le formalisme usuel et, en s'appuyant sur la nature fondamentalement quantique du système, en propose une résolution à la fois pratique et conceptuellement satisfaisante. Enfin, l’époque de réchauffement après l’inflation multichamps est étudiée. Il est expliqué comment coupler ces champs scalaires à des fluides cosmologiques. Ceci permet de suivre la dynamique de l’inflation, du (p)réchauffement et des ères de radiation et de matière au niveau homogène et linéaire, sans spécifier de manière ad hoc leurs transitions. Dans ces modèles à plusieurs espèces, une quantité importante de perturbations non-adiabatiques sont transmises du secteur scalaire aux fluides de radiation et de matière.This PhD thesis aims at studying theoretical and phenomenological aspects of cosmic inflation when it is driven by the presence of several scalar fields. After some reminders about the relativistic framework in which modern physical cosmology is embedded, one proposes an introduction to the inflationary paradigm, at the level of the homogeneous Universe but also of linear perturbations, both for single-field and multifield scenarios. Then, primordial non-Gaussianities are investigated. It is shown that multifield inflation results in specific patterns in these deviations of primordial fluctuations from Gaussian statistics. A particular emphasis is put on the observational signatures of the geometrical effects due to the curvature of the internal field space. This thesis also shows how to extend the stochastic formalism to multifield inflationary models with non-canonical kinetic terms. It reveals an ambiguity that is generically present in the usual formalism and, by considering the very quantum nature of the system, proposes a resolution that is both practical and conceptually satisfying. Last but not least, the period of reheating after multifield inflation is studied. It is explained how to couple those scalar fields to cosmological fluids. This enables to follow the dynamics of inflation, (p)reheating, and the eras of radiation and matter dominations, both at the homogeneous and linear levels, without having to specify by hand their transitions. In these multi-species models, an important quantity of non-adiabatic perturbations is transferred from the scalar sector to the fluids of radiation and matter

Aspects multichamps dans l'univers primordial : inflation et réchauffement

This PhD thesis aims at studying theoretical and phenomenological aspects of cosmic inflation when it is driven by the presence of several scalar fields. After some reminders about the relativistic framework in which modern physical cosmology is embedded, one proposes an introduction to the inflationary paradigm, at the level of the homogeneous Universe but also of linear perturbations, both for single-field and multifield scenarios. Then, primordial non-Gaussianities are investigated. It is shown that multifield inflation results in specific patterns in these deviations of primordial fluctuations from Gaussian statistics. A particular emphasis is put on the observational signatures of the geometrical effects due to the curvature of the internal field space. This thesis also shows how to extend the stochastic formalism to multifield inflationary models with non-canonical kinetic terms. It reveals an ambiguity that is generically present in the usual formalism and, by considering the very quantum nature of the system, proposes a resolution that is both practical and conceptually satisfying. Last but not least, the period of reheating after multifield inflation is studied. It is explained how to couple those scalar fields to cosmological fluids. This enables to follow the dynamics of inflation, (p)reheating, and the eras of radiation and matter dominations, both at the homogeneous and linear levels, without having to specify by hand their transitions. In these multi-species models, an important quantity of non-adiabatic perturbations is transferred from the scalar sector to the fluids of radiation and matter.Cette thèse de doctorat traite des aspects théoriques et phénoménologiques de l'inflation cosmologique lorsqu'elle est provoquée par la présence de plusieurs champs scalaires. Après quelques rappels à propos du cadre relativiste dans lequel s’inscrit la cosmologie physique contemporaine, une introduction au paradigme inflationnaire au niveau homogène et linéaire est proposée, non seulement dans le cadre à un champ mais aussi multichamps. S’ensuit une étude approfondie des non-gaussianités primordiales. Il est montré que l’inflation multichamps laisse des traces caractéristiques dans ces déviations à la statistique gaussienne des fluctuations primordiales. Un intérêt particulier est porté aux effets géométriques dus à la courbure de l’espace interne de ces champs scalaires. Cette thèse montre aussi comment étendre le formalisme stochastique aux théories inflationnaires multichamps avec des termes cinétiques non canoniques. Elle dévoile une ambiguïté présente de manière générique dans le formalisme usuel et, en s'appuyant sur la nature fondamentalement quantique du système, en propose une résolution à la fois pratique et conceptuellement satisfaisante. Enfin, l’époque de réchauffement après l’inflation multichamps est étudiée. Il est expliqué comment coupler ces champs scalaires à des fluides cosmologiques. Ceci permet de suivre la dynamique de l’inflation, du (p)réchauffement et des ères de radiation et de matière au niveau homogène et linéaire, sans spécifier de manière ad hoc leurs transitions. Dans ces modèles à plusieurs espèces, une quantité importante de perturbations non-adiabatiques sont transmises du secteur scalaire aux fluides de radiation et de matière