Supervisor: Luigi Delle Rose, Ricardo Zambujal Ferreira
Abstract:
In this thesis, we have studied two different cases in which cosmological and astrophysical observables can be used to constrain extensions of the Standard Model. The first one was centered on the bubble wall dynamics at a First-Order Electroweak Phase Transition. The main achievements of this thesis regarding this topic have been the following: first, we developed a very efficient numerical method with which we could find, for the first time, the full solution of the linearized Boltzmann equation in the context of the mentioned cosmological transition. Second, we extended our method by including the equations describing the evolution of the background plasma and the bubble wall. By doing so we were able to find the value of the bubble wall velocity for a benchmark model. Third, we confirmed that the out-of-equilibrium perturbations have an important impact on the value of the bubble wall speed, so they need to be included in the analysis. The other case studied in this thesis involved the QCD axion, the particle proposed to solve the strong CP problem. In particular, we computed the luminosity resulting from the resonant conversion from axions to photons inside a superradiant cloud around a Black Hole. Regarding this project, the main achievements are the following ones: first, the method we used to compute the luminosity is analytical, is mathematically very transparent and allows to have clear quantitative and qualitative control over the assumptions. Second, we find that for Primordial Black Holes and axion masses around $10^{-5}\, M_\odot$, $10^{-7}$ eV respectively the resulting signal is so intense that it could be detected by the LOFAR telescope. Third, the mentioned values of PBH masses for which such detection could be possible match the mass range that has been used to explain some ultra-short microlensing events observed by OGLE.