### (1) Presentation(s)

 Ven. 02/10/2020 14:00 Grande Ourse, Bâtiment 13, Etage 1 (à confirmer)Séminaire MOREAU Gabriel (L2C, Montpellier) Nonperturbative dynamics of quantum fields in de Sitter spacetime (Interactions fondamentales, Astroparticules et Cosmologie) Sommaire: The study of quantum field theory in de Sitter spacetime is notably motivated by the success of inflation, and the possibility to test fundamental physics through cosmological experiments. Going beyond the usual linearized description, one has to use nonperturbative methods to circumvent the infrared and secular divergences arising from the gravitational amplification of superhorizon modes.We will first focus on the backreaction of an $O(N)$ quantum scalar theory on the background geometry, related to the cosmological constant problem, as such divergences may signal an instability of de Sitter space towards a Minkowski spacetime. Our study uses the functional renormalization group (FRG) and shows a finite backreaction in the presence of interactions, which saturates as a result of the nonperturbative generation of a mass.The second topic of the talk is the effect of selfinteractions on scalar field correlators at separate spacetime points, in the infrared limit. The spacetime dependence gives access to several interesting physical quantities such as power spectra, relaxation time, or decoherence time. The stochastic formalism is considered, and we formulate the $1/N$ expansion of its Fokker-Planck formulation. This can be phrased as an equivalent, Schrödinger-like, eigenvalue problem. The corresponding spectrum is entirely computed at NLO, giving access to the spacetime dependence of any correlator.Pour plus d'informations, merci de contacter Frigerio M.