Stochastic formalism of cosmic inflation, with applications to the physics of primordial black holes. La soutenance HDR Vincent Vennin

Dear friends and colleagues,

It is my pleasure to invite you to my habilitation defense, which will take place on the 30th of June at 3pm. 

I will present my work on the stochastic formalism of cosmic inflation, with applications to the physics of primordial black holes (a short description follows below).

Due to the current covid-19 restrictions, unfortunately, only members of the jury will be allowed to be physically present. The defense will however be broadcasted online, via the BlackBoard Collaborate platform:

(please make sure you turn off your microphone upon joining). 

Under other circumstances, I would have ended this message by mentioning the cocktail following the defense, but I am sure we will have other occasions to celebrate in the future!

Please feel free to forward this email to anyone who might be interested.

Best regards,

Vincent Vennin.


Inflation is a phase of accelerated expansion that occurs at extremely high energy in the very early universe. During this epoch, vacuum quantum fluctuations are amplified and stretched to astrophysical distances. They give rise to fluctuations in the cosmic microwave background temperature and polarisation, and to large-scale structures in our universe.

They can also trigger the formation of primordial black holes. Such objects could provide the progenitors of the black-hole mergers recently detected through their gravitational-wave emission, and constitute part or all of the dark matter. Their observation would give invaluable access to parts of the inflationary sector that are unconstrained by the cosmic microwave background, at energy scales far beyond those accessible in particle physics experiments.

Since primordial black holes require large inhomogeneities to form, they are produced in scenarios where quantum fluctuations substantially modify the large-scale dynamics of the universe. In the present habilitation thesis, this ``backreaction'' effect is investigated by means of the stochastic-inflation formalism, an effective theory for the long-wavelengths of quantum fields during inflation, which can be described in a classical but stochastic way once the small wavelengths have been integrated out. It describes an inflating background that gets randomly and constantly corrected by the vacuum quantum fluctuations as they get stretched to large distances.

After reviewing salient aspects of the stochastic inflation formalism, I will explain how it can be combined with standard techniques of cosmological perturbation theory to provide a framework in which the full statistics of curvature perturbations can be computed in the presence of non-perturbative quantum diffusion (the so-called "stochastic-delta N formalism"). I will then apply these results to the calculation of primordial black holes in various setups, and show that quantum diffusion can change the expected abundance and mass distributions by several orders of magnitude, opening up new windows into the physics of primordial black holes.

The habilitation manuscript can be found here:

The jury will be composed of:

Clifford P. Burgess, Perimeter Institute (Waterloo, Canada)
Julien Grain, Institut d’Astrophysique Spatiale (Orsay)
David Langlois, Laboratoire Astroparticules et Cosmologie (Paris)
Patrick Peter, Institut d’Astrophysique de Paris
David Polarski, Laboratoire Charles Coulomb (Montpellier)
Christophe Ringeval, Université de Louvain la Neuve (Belgique)


Tuesday, 30 June, 2020 - 15:00 to 18:00

Localisation / Location: 

  • Autre
Soutenance HDR

Equipe(s) organisatrice(s) / Organizing team(s): 

  • Théorie