RESEARCH OVERVIEW

Generation of magnetic fields during (axion) inflation

One of the main open questions in cosmology concerns the fundamental nature of the inflaton with axion-like particles being one of the best candidates because of their ability to provide a sufficiently flat potential. Their simplest possible coupling with gauge fields, satisfying all the requisite symmetries, produces helical gauge fields which lead to interesting phenomenological consequences because of the possibility of parity-breaking signatures on the generated gravitational waves and the helical magnetic fields (MFs). These helical MFs also present an opportunity to address another open problem, that concerning the origin of cosmological MFs (for a review, see here). There is now evidence that large scale MFs could be present even in the voids of the Large Scale Structure of the Universe (LSS) and the intracluster medium. If confirmed, the origin of these fields would be difficult to explain astrophysically given a lack of sufficient plasma processes in the voids, and, thus, a cosmological origin seems very attractive, especially since most of the cosmological models require physics beyond the standard model, including the aforementioned axion inflation, which makes these primordial MFs an invaluable window into the early Universe as well as an attractive probe into the extensions of the standard model. These cosmological MFs could also help answer some of the other open problems like the origin of the seed fields for galactic MFs and Hubble tension.

While inflationary magnetogenesis models naturally provide MFs at large length scales, their strength is usually much weaker than what would explain the observations, especially once the turbulent dissipation in later stages of the Universe is taken into account. However, if the generated fields are helical, it turns out that the helicity conservation protects these MFs from dissipating and, in fact, transfers energy from small length scales to larger length scales, making the MFs even more attractive to fill the voids of the LSS. 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. This has been the subject of our ongoing investigation (with Profs. Ruth Durrer and Stanislav Vilchinskii in Geneva and Prof. Kai Schmitz's working group in Münster). In a semi-analytical approach, we are investigating the impact of the production of magnetic fields on the spectrum and the bispectrum of the inflationary perturbations as well as on the dynamics of the background inflaton field. 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

While the standard model has been remarkably successful in the last half a century of experiments, there are many open cosmological questions that have prompted its extensions. It is well known that the Universe went through multiple phase transitions in its early stages, with these phase transitions being smooth crossovers in the standard model but first-order transitions in the various extensions. A first-order phase transition (FOPT) proceeds through the nucleation and the growth of bubbles of the new phase. When these bubbles merge, their collisions produce violent perturbations in the cosmic plasma, leading to gravitational waves (GWs) that form a stochastic gravitational wave background (SGWB). The recent detection of GWs by LIGO/Virgo over the last decade and the detection of SGWB through pulsar arrays in 2023 has marked the advent of GW astronomy and GW cosmology, which can provide us with an unprecedented access to the physics in the early Universe. This requires a thorough understanding of the various processes that could contribute to these GW signals, including the contribution of the FOPTs to the SGWB, offering invaluable insights into high-energy physics and the conditions of the early Universe. A lot of work is being currently done on understanding the properties of GWs produced during cosmological FOPTs from different vantage points. With Prof. Chiara Caprini, Dr. Simona Procacci and Dr. Alberto Roper Pol in Geneva, we are investigating the impact of relativistic turbulence on the production of vorticity and GWs during such FOPTs in the early Universe.

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 scalesi≳i10iMeV , when the electrons and positrons can be considered almost massless. In the last three decades, these chiral effects have been shown to be relevant to the early Universe, where they could amplify helical magnetic fields. This makes them crucial for understanding the origin and the 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.