Thèse Univers Primordial et Systèmes Quantiques Ouverts Non-Linéaires H/F - 74

Établissement : Ecole normale supérieure - PSL
École doctorale : Physique en Ile de France
Laboratoire de recherche : Laboratoire de Physique de l'École normale supérieure
Direction de la thèse : Vincent VENNIN ORCID 0000000288514430
Début de la thèse : 2026-09-01
Date limite de candidature : 2026-04-15T23:59:59

Dans le paradigme inflationnaire de la cosmologie primordiale, les fluctuations quantiques à l'origine des structures cosmiques observées aujourd'hui interagissent avec des degrés de liberté inobservables, ce qui conduit à une dynamique effective non-unitaire encore mal comprise. Ce projet de thèse vise à en étudier les enjeux théoriques et conséquences phénoménologiques en combinant des méthodes issues de la physique hors équilibre et de la théorie quantique des champs relativistes. Il s'attachera en particulier à analyser la décohérence induite par les non-linéarités gravitationnelles, à développer des outils permettant de dépasser l'hypothèse usuelle de Markovianité, peu justifiée en cosmologie, et à caractériser les effets non unitaires associés aux modèles d'inflation multichamps.

Data probing the distribution of matter and energy in our universe are unveiling its anatomy on the largest observable
scales, with an unprecedented degree of accuracy. These include measurements of the Cosmic Microwave Background
(CMB) anisotropies, galaxy surveys, 21 cm tomography and gravitational-wave astronomy.

These observations are consistent with the standard big-bang model, in which density fluctuations originate from
an early phase of accelerated expansion known as inflation. During inflation [1,2], vacuum quantum fluctuations are
stretched to astrophysical scales by the expansion and amplified by gravitational instability, giving rise to primordial
cosmological perturbations. In the simplest models of inflation, these perturbations are predicted to be quasi-Gaussian,
phase coherent and scale invariant, which matches observations remarkably well. Further, their detailed statistics
help constrain the microphysics of inflation and its dynamics. Inflation has thus become a highly active research
field, as the energy scales involved during this early epoch are many orders of magnitude larger than those accessible
in terrestrial particle-physics experiments. This makes the early universe one of the most promising probes to
test far-beyond-standard-model physics. Moreover, the inflationary mechanism for the production of cosmological
perturbations makes explicit use of General Relativity and Quantum Mechanics, two theories that are notoriously
difficult to combine. Since this mechanism leads to theoretical predictions for the CMB anisotropies, inflation is one of
the only cases in physics where, given our present-day technological capabilities, a process based on gravitational and
quantum effects can be tested experimentally. This makes it an ideal playground to discuss fundamental questions
related to the interplay between these two theories.

Most physical setups that have been proposed to embed inflation contain a large number of additional degrees of
freedom. In particular, heavy fields are arguably ubiquitous when inflation is embedded in high-energy constructions,
both from a model-building perspective [3-14] and from an effective-field-theory (EFT) approach [15-20]. Even if
they provide negligible contributions to the dynamics of the universe's expansion, they may affect the emergence of
cosmic structures in various ways. For instance, they could lead to entropic fluctuations, or to deviations from Gaussian
statistics, that future cosmological surveys might be able to detect [21-23]. Additional degrees of freedom may also
alter the quantum state in which primordial density fluctuations are placed, in particular through the mechanism of
decoherence [24-35]. Decoherence [36-38] is usually associated with the erasure of genuine quantum signatures
and thus participates in the quantum-to-classical transition of cosmic structures [39, 40].

For those reasons, it has become of increasing importance to design reliable tools to model the presence of additional degrees of freedom in the early universe [7, 12, 14, 19, 41-53]. One such approach is the so-called master equation program (see for instance [54, 55]), where an effective equation of motion is obtained for the reduced density matrix of a system of interest, once the degrees of freedom contained in the environment have been traced out. One of its appealing advantages is its ability to resum late-time secular effects [56-64], hence to go beyond standard perturbation theory and implement non-perturbative resummation in cosmology.

However, master equations were primarily developed in the context of quantum optics and therefore rely on assumptions (e.g. that the environment consists of a large reservoir in thermal equilibrium) that are not necessarily satisfied in
cosmology. Since they were originally designed for non-relativistic systems, their application to quantum fields is also
not straightforward and raises several issues that need to be addressed. Moreover, it remains to be understood what the
observable consequences of decoherence in astronomical data are. The goal of this PhD project is therefore twofold:
- at the fundamental level, to develop an open-quantum-system framework for quantum fields in curved spacetime,
and to implement non-perturbative resummation techniques in cosmology;
- at the phenomenological level, to use these methods to refine the observational predictions of multiple-field
models of inflation and to maximise the scientific return of forthcoming cosmological surveys.