Cosmic inhomogeneities with large amplitudes

par Julien Grain (Institut d'Astrophysique Spatiale (IAS) /Université Paris Saclay)

jeudi 5 nov. 2026, 14:00 → 15:00 Europe/Paris

Amphi G.Besse (Subatech IMT Atlantique)

In the standard model of cosmology, all large-scale structures emerge from tiny density fluctuations produced during inflation. In this setup, cosmic inhomogeneities of primordial origin are extracted out of the quantum fluctuations of the matter and gravitational fields. Their production is usually treated by means of perturbation theory and quantum fields theory on curved spaces. Yet, primordial fluctuations with large amplitudes are also expected to form. They are rare but strongly impacted by non-linearities, and they drastically modify the emergence of structures as dark matter halos and cosmic voids. Stochastic inflation is the main approach to deal with the large-amplitude regime. It is an open effective theory expected to capture non-linear evolution at large scale in a non-perturbative manner. It relies on the separate-universe approximation and the quantum-to-classical transition of cosmological perturbations that I will discuss. As a result, the statistics of cosmic inhomogeneities is obtained averaging over independent patches of the universe, as each evolves as a random walk. Using large-deviation theory, I will show strong departure from gaussianity for over- and underdensities with large amplitudes, and highlight their impact on the abundances of cosmic structures. It also means that our Hubble patch results from a random process. It leads to extra-variability of cosmological parameters that I will illustrate considering large-scale anisotropy. If time permits, I will discuss in more details the quantum-to-classical transition in the light of decoherence and open quantum systems.