Cosmological inflation is a period of accelerated expansion that occurred at very high energy in the early Universe. During this epoch, vacuum quantum fluctuations were amplified to become large-scale cosmological perturbations that seeded the cosmic microwave background (CMB) anisotropies and the large-scale structure of our Universe.
In the range of scales accessible to CMB experiments, these perturbations are constrained to be small until they re-enter the Hubble radius during the radiation era. At smaller scales however, they may be sufficiently large so that when they re-enter the Hubble radius, they overcome the pressure forces and collapse to form primordial black holes (PBHs).
These large curvature perturbations can be produced during inflation from enhanced quantum fluctuations, or during the preheating epoch from resonant instabilities. PBHs thus give access to the physical processes that took place in the early Universe in a range of scales that is inaccessible by - and hence is complementary to - CMB observations.
These astrophysical objects have recently regained interest with the detection by advanced LIGO/VIRGO of the gravitational wave emission signals associated with black hole mergers, with progenitors that could be of primordial origin and that could play a role in the dark matter content of our Universe.
The emergence of gravitational-wave astronomy and the significant progresses it is expected to make in the next decades thus brings PBHs into the realm of observationally testable objects. This opens up a new observational window into the early Universe and the goal of this thesis project is to take advantage of it both at the theoretical and the observational level.
At the theoretical level, the goal will be to identify the properties of inflationary models that can give rise to PBHs, and to understand how one can constrain the details of the inflaton field and of its coupling with the standard model fields using PBHs. Since PBHs form in a regime of large quantum fluctuations, another goal will be to understand what effects quantum backreaction may have on the dynamics of fields during this epoch.
At the observational level, we will aim at connecting these large curvature perturbations at the end of inflation to mass spectra that LIGO is able to constrain. This requires to evolve primordial density fluctuations through the preheating epoch, their collapsing into black holes, their merging and evaporation. This will also allow us to bring in the analysis constraints from other astrophysical probes (big bang nucleosynthesis, the extragalactic photon background, CMB distorsion, gravitational lensing, LSS constraints, stochastic gravitational wave background, etc.).
At the end of the PhD, the student will have developed new theoretical ideas and assessed their range of validity. She or he will have related conceptual aspects of primordial cosmology to observational tests with current and forthcoming data. She or he will be trained with the main analytical and numerical methods in physical cosmology.
The PhD will be conducted at the Astroparticles and Cosmology Laboratory (APC) of the University Paris Diderot. 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), 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 help the student to develop skills that will be useful to her/his career, whether it takes place in academia or not. APC teams up with many other CNRS laboratories and is part of the labex “UnivEarthS”, the groupement de recherche “gravitational waves”, and the “International Doctorate Network in Particle Physics, Astrophysics and Cosmology” (IDPASC). 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.