Contact

**Name**

Gilles Buldgen

**Position**

PhD student

**Address**

Centre for Cosmology, Particle Physics and Phenomenology - CP3

Université catholique de Louvain

2, Chemin du Cyclotron - Box L7.01.05

B-1348 Louvain-la-Neuve

Belgium

Université catholique de Louvain

2, Chemin du Cyclotron - Box L7.01.05

B-1348 Louvain-la-Neuve

Belgium

**Office**

**UCL member card**

Research statement

I am interested in the particle physics of the early Universe and its application to cosmology, including formal aspects of finite temperature and nonequilibrium quantum field theory (QFT).

In my PhD project I study quantum and dissipative effects in the early Universe from first principles of nonequilibrium quantum field theory. A key application is the question whether “Warm Inflation” is a viable alternative to the standard inflationary paradigm in cosmology. While the universe is empty and cold in the standard scenario, Warm Inflation is characterised by a quasi-equilibrium between the particle production due to dissipation and the dilution due to the Hubble expansion, which keeps a thermal bath of radiation at constant temperature during inflation. In this scenario, thermal damping helps to maintain the slow rolling of the inflaton field, and thermal fluctuations contribute to cosmological perturbations. The question whether Warm Inflation is feasible is interesting from a particle physics viewpoint because it has been claimed that this scenario can alleviate some of the difficulties that one faces when trying to embed conventional models of inflation into theories of particle physics. It is even more interesting from a cosmological viewpoint because it would radically change our view on the origin of the small density perturbations that formed the seeds for galaxies formations: Those perturbations are purely quantum in conventional models of inflation, while they are both quantum and thermal in the Warm Inflation case. Also the transition from the inflationary period to the radiation era is rather different from the standard scenario because there is no distinct “reheating period” after inflation, the transition to the radiation domination occurs smoothly.

In my PhD project I study quantum and dissipative effects in the early Universe from first principles of nonequilibrium quantum field theory. A key application is the question whether “Warm Inflation” is a viable alternative to the standard inflationary paradigm in cosmology. While the universe is empty and cold in the standard scenario, Warm Inflation is characterised by a quasi-equilibrium between the particle production due to dissipation and the dilution due to the Hubble expansion, which keeps a thermal bath of radiation at constant temperature during inflation. In this scenario, thermal damping helps to maintain the slow rolling of the inflaton field, and thermal fluctuations contribute to cosmological perturbations. The question whether Warm Inflation is feasible is interesting from a particle physics viewpoint because it has been claimed that this scenario can alleviate some of the difficulties that one faces when trying to embed conventional models of inflation into theories of particle physics. It is even more interesting from a cosmological viewpoint because it would radically change our view on the origin of the small density perturbations that formed the seeds for galaxies formations: Those perturbations are purely quantum in conventional models of inflation, while they are both quantum and thermal in the Warm Inflation case. Also the transition from the inflationary period to the radiation era is rather different from the standard scenario because there is no distinct “reheating period” after inflation, the transition to the radiation domination occurs smoothly.

Projects

**Research directions:**

**Active projects**

**Neutrinos in the Early Universe**

*Gilles Buldgen, Marco Drewes*

We study the production and interactions of neutrinos in the primordial plasma from first principles of quantum field theory. This includes Standard Model computations such as QED corrections to Neff as well as constraints on new neutrinos species from cosmology and astrophysics.

**Scalar Fields in the Early Universe**

*Wenyuan Ai, Gilles Buldgen, Marco Drewes*

We study the nonequilibrium dynamics of scalar fields in the early universe in the framework of the Schwinger-Keldysh formalism. We are interested in several applications, including inflation, reheating, Dark Matter production and the fate of the Higgs field in the early universe.

Publications in CP3

All my publications on Inspire

Number of publications as CP3 member: 2 Download BibTeX

CP3-19-51:

Number of publications as CP3 member: 2 Download BibTeX

**2019**

**Towards a precision calculation of the effective number of neutrinos $N_{\rm eff}$ in the Standard Model I: The QED equation of state**