The early universe is believed to have reached the state of a hot plasma close to thermal equilibrium, which could allow the formation of the atomic nuclei that build stars and galaxies. Such primordial hot plasma is thought to have been the outcome of a reheating process ending up with particles quickly interacting with each other, redistributing their energy and momentum and leading to a well-defined temperature and spatially smooth number densities. As the universe expanded, it cooled down, leading to smaller interaction rates and causing some particles to drop out of thermal equilibrium. The ensuing out-of-equilibrium dynamics, for example during a phase transition proceeding by the production of bubbles (pockets of the universe with different thermodynamic properties, which subsequently expand and collide) could have played a decisive role in shaping our current world. For example, non-equilibrium dynamics could provide a key to understand why there is hardly any antimatter around today (the so-called matter-antimatter asymmetry), or it could lead to a background of gravitational waves permeating the cosmos, amenable for detection in future experiments as illustrated in the following plotter co-developed with F. Muia and A. Ringwald:

In the group we study processes in the hot plasma by using the tools of quantum field theory at finite-temperature and out of equilibrium. We are also interested in the reheating process, which can be modelled with semiclassical numerical simulations in a discretized spacetime, and investigate connections with gravitational waves.

While many of the predictions for elementary particle processes using quantum field theory rely on perturbative expansions that can be implemented in terms of so-called Feynman diagrams, this approach can fail to capture relevant physical effects, such as vacuum tunneling, the violation of conservation laws associated with baryon and lepton number, or with parity. This is particularly the case for the strong interactions, as the large interaction strength invalidates perturbative expansions. The theoretical tools used in the group are mostly based on semiclassical expansions, such as instantons in QCD.