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Person# Benjamin Audren

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Dark matter

Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the elect

Neutrino

A neutrino (njuːˈtriːnoʊ ; denoted by the Greek letter ν) is a fermion (an elementary particle with spin of 1 /2) that interacts only via the weak interaction and gravity.

Lambda-CDM model

The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant

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Benjamin Audren, Diego Blas Temino, Mikhail Ivanov, Julien Lesgourgues, Sergey Sibiryakov

We consider a scenario where local Lorentz invariance is violated by the existence of a preferred time direction at every space-time point. This scenario can arise in the context of quantum gravity and its description at low energies contains a unit time-like vector field which parameterizes the preferred direction. The particle physics tests of Lorentz invariance preclude a direct coupling of this vector to the fields of the Standard Model, but do not bear implications for dark matter. We discuss how the presence of this vector and its possible coupling to dark matter affect the evolution of the Universe. At the level of homogeneous cosmology the only effect of Lorentz invariance violation is a rescaling of the expansion rate. The physics is richer at the level of perturbations. We identify three effects crucial for observations: the rescaling of the matter contribution to the Poisson equation, the appearance of an extra contribution to the anisotropic stress and the scale-dependent enhancement of dark matter clustering. These effects result in distinctive features in the power spectra of the CMB and density fluctuations. Making use of the data from Planck and WiggleZ we obtain the most stringent cosmological constraints to date on departures from Lorentz symmetry. Our analysis provides the first direct bounds on deviations from Lorentz invariance in the dark matter sector.

We apply the principles of Bayesian statistics to the main probes of cosmology, in order to refine our knowledge of the Standard Model and possibly extend it. Notably, we investigate the basic elements of the model in detail in order to reinforce this basic foundation of the field, and lay down a systematic way of obtaining model-independent constraints on parts of the Standard Model. We further try to constrain some of the unknown properties of Dark Matter, namely its decay or annihilation rates, to help reducing the range of possibilities for model builders. By using recent cosmological probes and making as little assumptions as possible, we are able to meaningfully constrain these properties in the prospect of narrowing down a particle physics search. Eventually, we show how future experiments will be able to put strong bounds on the neutrino total mass, as long as the theoretical uncertainty is handled carefully. Despite being cautiously pessimistic, we prove how EUCLID will be able to detect even the lowest possible allowed neutrino mass, by simply using properly the linear scales. We also show the target precision for the theoretical prediction in order to make full use of the forthcoming wealth of data at mildly non-linear scales.

Benjamin Audren, Julien Lesgourgues

The existence of a cosmic neutrino background can be probed indirectly by CMB experiments, not only by measuring the background density of radiation in the universe, but also by searching for the typical signatures of the fluctuations of free-streaming species in the temperature and polarisation power spectrum. Previous studies have already proposed a rather generic parametrisation of these fluctuations, that could help to discriminate between the signature of ordinary free-streaming neutrinos, or of more exotic dark radiation models. Current data are compatible with standard values of these parameters, which seems to bring further evidence for the existence of a cosmic neutrino background. In this work, we investigate the robustness of this conclusion under various assumptions. We generalise the definition of an effective sound speed and viscosity speed to the case of massive neutrinos or other dark radiation components experiencing a non-relativistic transition. We show that current bounds on these effective parameters do not vary significantly when considering an arbitrary value of the particle mass, or extended cosmological models with a free effective neutrino number, dynamical dark energy or a running of the primordial spectrum tilt. We conclude that it is possible to make a robust statement about the detection of the cosmic neutrino background by CMB experiments.