La cosmologie ne peut s’envisager qu’en faisant des hypothèses simplificatrices que l’on appelle des « principes cosmologiques ». Sans cet artifice, il faudrait en effet connaître les vitesses et les positions de toutes les particules dans l’espace, ce qui est tout simplement impossible. On distingue actuellement quatre grands principes :

Le principe cosmologique d'homogénéité et d'isotropie ;

Le principe cosmologique parfait (ou d'équivalence temporelle) ;

Le principe cosmologique global ;

Le principe cosmologique de l'Univers fractal.

Principe cosmologique d'homogénéité et d'isotropie Le principe cosmologique d'homogénéité et d'isotropie est l'hypothèse aujourd'hui généralement considérée comme vérifiée selon laquelle l'Univers est spatialement homogène, c'est-à-dire que son apparence générale ne dépend pas de la position de l'observateur. Il a pour conséquence que l'Homme n'occupe pas de position privilégiée dans l'Univers, en opposition avec la théorie géocen

Neutrino masses and cosmology with Lyman-alpha forest power spectrum

Arnaud Borde, Julien Lesgourgues

We present constraints on neutrino masses, the primordial fluctuation spectrum from inflation, and other parameters of the ACDM model, using the one-dimensional Ly alpha-forest power spectrum measured by [1] from the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey (SDSS-III), complemented by Planck 2015 cosmic microwave background (CMB) data and other cosmological probes. This paper improves on the previous analysis by [2] by using a more powerful set of calibrating hydrodynamical simulations that reduces uncertainties associated with resolution and box size, by adopting a more flexible set of nuisance parameters for describing the evolution of the intergalactic medium, by including additional freedom to account for systematic uncertainties, and by using Planck 2015 constraints in place of Planck 2013. Fitting Ly alpha data alone leads to cosmological parameters in excellent agreement with the values derived independently from CMB data, except for a weak tension on the scalar index n(s). Combining BOSS Ly alpha with Planck CMB constrains the sum of neutrino masses to Sigma m(nu) < 0.12 eV (95% C.L.) including all identified systematic uncertainties, tighter than our previous limit (0.15 eV) and more robust. Adding Ly alpha data to CMB data reduces the uncertainties on the optical depth to reionization tau, through the correlation of tau with sigma(8). Similarly, correlations between cosmological parameters help in constraining the tensor-toscalar ratio of primordial fluctuations r. The tension on n(s) can be accommodated by allowing for a running dn(s)/d In k. Allowing running as a free parameter in the fits does not change the limit on Sigma m(nu). We discuss possible interpretations of these results in the context of slow-roll inflation.

Iop Publishing Ltd2015

Neutrino Mass from Cosmology

Julien Lesgourgues

Neutrinos can play an important role in the evolution of the universe, modifying some of the cosmological observables. In this contribution we summarize the main aspects of cosmological relic neutrinos, and we describe how the precision of present cosmological data can be used to learn about neutrino properties, in particular their mass, providing complementary information to beta decay and neutrinoless double-beta decay experiments. We show how the analysis of current cosmological observations, such as the anisotropies of the cosmic microwave background or the distribution of large-scale structure, provides an upper bound on the sum of neutrino masses of order 1 eV or less, with very good perspectives from future cosmological measurements which are expected to be sensitive to neutrino masses well into the sub-eV range.

Hindawi Publishing Corporation2012

Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies

Richard Irving Anderson, Hsin-Yu Chen, Frédéric Courbin, Fabio Finelli, Mikhail Ivanov, Melissa Joseph, Suresh Kumar, Julien Lesgourgues, Florian Niedermann, Emre Ozulker

The standard Lambda Cold Dark Matter (Lambda CDM) cosmological model provides a good description of a wide range of astrophysical and cosmological data. However, there are a few big open questions that make the standard model look like an approximation to a more realistic scenario yet to be found. In this paper, we list a few important goals that need to be addressed in the next decade, taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant H-0, the sigma(8)-S-8 tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the 5.0 sigma tension between the Planck CMB estimate of the Hubble constant H-0 and the SH0ES collaboration measurements. After showing the H-0 evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the Planck CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density Omega(m), and the amplitude or rate of the growth of structure (sigma(8), f sigma(8)). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the H-0-S-8 tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals. Finally, we give an overview of upgraded experiments and next-generation space missions and facilities on Earth that will be of crucial importance to address all these open questions. (C) 2022 The Author(s). Published by Elsevier B.V.

ELSEVIER2022

Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies

Richard Irving Anderson, Hsin-Yu Chen, Frédéric Courbin, Fabio Finelli, Mikhail Ivanov, Melissa Joseph, Suresh Kumar, Julien Lesgourgues, Florian Niedermann, Emre Ozulker

ELSEVIER2022