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.