Inflation is a high-energy early phase of accelerated expansion that provides a causal mechanism for generating inhomogeneities on cosmological scales. In this setup, the deviations from homogeneity and isotropy result from the amplification of quantum fluctuations of the gravitational and matter fields that are stretched to distances of cosmological interest today. The quantum-to-classical transition of cosmological perturbations, including their quantum decoherence, will be studied using recently developed tools in quantum information theory. When this transition occurs, a stochastic formalism that effectively incorporates the effect of quantum backreaction in cosmology can be developed. A second task will be to generalise this approach to contracting and bouncing scenarios, as well as to black holes. Finally, observational consequences of these quantum effects on predictions for the cosmic microwave background and the abundance of primordial black holes will be investigated.
Inflation is the most favoured paradigm to describe the physics at play in the primordial Universe, about 10-35 second after the big bang. Its key prediction is the generation of cosmic inhomogeneities (i.e. density fluctuations and gravitational waves) that are extracted out of the quantum vacuum and stretched to astronomical scales. They later leave their imprints as anisotropies in the Cosmic Microwave Background (CMB), and they seed the formation of large-scale structures in the universe. Their statistical properties are now well constrained from the measurements of the CMB temperature and polarisation anisotropies, in particular thanks to the Planck satellite.
Although these measurements corroborate the inflationary scenario by confirming its main predictions, they are still far from singling out a unique model, they do not yet exclude all alternatives to inflation, and they leave several theoretical questions open.
Inflation is indeed one of the only places in physics where an effect based on general relativity and quantum mechanics leads to predictions that can be tested experimentally. It is thus an ideal playground to discuss various fundamental questions related to both these theories and how they behave when combined together.
In this context, an important task is to determine whether one can observationally prove (or disprove) the quantum origin of cosmic inhomogeneities. A first goal of this research program is therefore to study the quantum-to-classical transition of cosmological perturbations, building on recently developed tools in quantum information theory. This includes the investigation of scenarios of quantum decoherence.
When this transition takes place, the appearance of a cosmological horizon in inflating space-times allows one to develop a stochastic formalism, the so-called ``stochastic inflation’’ formalism, in order to model the effect of quantum fluctuations on space-time itself. A second task will be to generalise the use of such an approach to other situations in cosmology, such as contracting and bouncing cosmologies, as well as black holes, for which the space-time shares similar properties.
Finally, the observational consequences of these quantum effects will be investigated. In particular, if primordial black holes are formed in the early universe, there must be a phase during inflation where large cosmological fluctuations are produced. During such an epoch, quantum corrections to the space-time dynamics are expected to be large, which makes primordial black holes, and the associated stochastic gravitational wave background and imprint into dark matter, natural objects to investigate the presence of these quantum effects.
The upcoming data from gravitational wave detectors such as LIGO and LISA will therefore complement CMB measurements to bring new insight into these issues, and the PhD research program proposes to carry out the theoretical investigations that will allow us to take the full benefit of it.
The PhD will be conducted at the Astroparticles and Cosmology Laboratory (APC) of the University Paris Diderot (supervisor Vincent Vennin), and at the Institut d'Astrophysique Spatiale (IAS) in Orsay (co-supervisor Julien Grain).
APC provides a vibrant research environment, with 75 academic staff and more than 60 postdoctoral researchers and PhD students at present. The PhD student will have access to the state-of-the-art IN2P3 computing center (16496 cores), supported by a full-time supercomputer team. APC is strongly involved in many international collaborations in cosmology (Planck, Euclid, LiteBIRD), gravitational waves detection (LIGO, eLISA), astroparticles (Auger, Hess, Integral) and particle physics (Double Chooz for neutrino oscillations). APC also hosts the Paris Centre for Cosmological Physics, headed by the Nobel Prize winner George Smoot.
Beyond the goals of the research proposal, working at APC will also help the student to develop skills useful to her/his career, whether it develops in academia or not. APC teams up with many other CNRS laboratories and is part of the UnivEarthS labex, the groupement de recherche “gravitational waves”, the IDPASC international doctorate network, and an international research training network with Hamburg and Oxford universities. These connections will help the student to develop professional contacts and collaborations. APC has weekly colloquia, seminars and journal clubs, and several internal research discussion groups to which the student will be encouraged to contribute.
The PhD will be officially based at APC. However, she/he will be invited to spend at least one day per week at IAS. J. Grain It is also “chercheur associé” at APC and spends two days per week there. At IAS, she/he will be a full member of the COSM!X team, currently involved in LiteBIRD, EUCLID and the ERC Byopic . This will offer the PhD student a framework covering a wide spectrum of topics in cosmology, including theoretical aspects and scientific interpretation of data, and a very stimulating environment for carrying out this research program.