Research

The chiral anomaly in the early Universe

Within the standard model, the chiral anomaly of fermions leads to various macroscopic transport phenomena in the presence of an asymmetry between left-handed and right-handed fermions. This includes, for example, the chiral magnetic effect (CME) and the chiral separation effect, which involve electric current and axial current, respectively, in the direction of an external magnetic field. These effects are most prominent in temperature scales $\gtrsim$ 10 MeV, when electrons and positrons are effectively massless.

Over the last three decades, such chiral effects have been shown to amplify helical magnetic fields in the early Universe, making them central to the origin and dynamics of cosmological magnetic fields. With Prof. Jennifer Schober in Bonn, we are investigating these chiral effects in the context of the early Universe. We are focusing on understanding the chiral vortical effect, which is an effect similar to the CME with the current parallel to the vorticity of the cosmic plasma instead of external magnetic fields.

Generation of magnetic fields during (axion) inflation

One of the central open questions in cosmology is the fundamental nature of the inflaton. Axion-like particles (ALPs) are among the best-motivated candidates, since they naturally provide a sufficiently flat potential. Their simplest symmetry-allowed coupling to gauge fields produces helical gauge fields, with striking phenomenology: parity-breaking signatures in the generated gravitational waves and in the helical magnetic fields.

Those magnetic fields also concern another open problem, the origin of cosmological magnetic fields. There is now evidence that large-scale fields may exist even in the voids of the large-scale structure, where an astrophysical origin is hard to sustain and a primordial one becomes attractive. Crucially, if the generated fields are helical, helicity conservation slows down their turbulent dissipation and transfers their energy toward ever-larger scales. This makes ALPs a very appealing candidate, naturally providing helical fields along with a flat potential and requiring a thorough investigation of their impact on the inflationary perturbations.

With Profs. Ruth Durrer and Stanislav Vilchinskii in Geneva and Prof. Kai Schmitz's working group in Münster, we are developing a semi-analytical formalism for the impact of magnetic-field production on the spectrum and bispectrum of the inflationary perturbations and on the background inflaton. We are developing the formalism for a general inflationary model and its general coupling with the gauge fields, and then apply it to the interesting case of axion inflation.

Stochastic gravitational waves during first-order phase transitions

The Universe passed through several phase transitions in its earliest moments. In the Standard Model these are smooth crossovers, but many of its extensions turn them into first-order transitions, which proceed through the nucleation and growth of bubbles of the new phase. When these bubbles collide, they stir violent perturbations in the cosmic plasma, sourcing gravitational waves that build up into a stochastic gravitational-wave background.

The LIGO/Virgo detections and the 2023 pulsar-timing-array evidence for a stochastic background have together opened gravitational-wave cosmology as a genuine window onto early-Universe physics. With Prof. Chiara Caprini, and Dr. Alberto Roper Pol in Geneva, we are studying how relativistic turbulence shapes the production of vorticity and gravitational waves during such transitions.