18. October 2018 (was 17. October 2018)

BREMEN-OLDENBURG RELATIVITY SEMINAR
University of Oldenburg

Abstract: The cosmic microwave background (CMB) radiation and its angular power anisotropy spectrum are well explored in the theoretical and experimental sense, e.g. by measurements of the Planck satellite. But apart from the cosmic photon background, the cosmic neutrino background (CNB), which is much more difficult to explore because of the weakly interacting behavior of neutrinos, fills our Universe. In general, it is difficult to obtain information about neutrino physics from measurement. However, experiments among other things proved that neutrinos are massive, although they are massless according to the standard model of particle physics. Furthermore, the CNB impacts cosmology and e.g. influences the expansion rate of the Universe, the Big Bang Nucleosynthesis and finally also the photon background. In my talk I am going to present the basic ideas about cosmological perturbation theory, which is capable of explaining how the structures observed in our Universe nowadays have formed and evolved as well as it is able to predict the angular power spectrum of the microwave background with high accuracy. After the presentation of the formalism of linear perturbation theory, I am applying the theory to neutrinos and derive their so-called Boltzmann hierarchy, which exist for all the different particle species of the Universe. In combination with Einstein's field equations these hierarchies yield the temporal evolution of perturbations in the corresponding particles. These temporal evolutions for all the components of the Universe will briefly be dealt with in the talk. The Boltzmann hierarchies of massive and massless neutrinos differ. They thus influence the angular power spectrum of the CMB in different ways. Non-instantaneous decoupling of neutrinos causes extra energy in the neutrino sector as compared to the standard scenario. This extra energy is degenerate with neutrino temperature and their phase-space distribution function, wherefore the so-called number of relativistic neutrino species N_eff has been introduced to parametrize the extra neutrino energy. The degeneracy between neutrino temperature and N_eff has been analyzed in my work regarding their influence on the CMB angular power spectrum for massless and massive neutrinos. The influence of the neutrino phase-space distribution function was another important aspect of the analysis. Whereas the Fermi-Dirac distribution function is usually assumed to describe how neutrinos are distributed in phase-space, we used a Gaussian distribution function and showed that, with an appropriate normalization, its influence on the CMB angular power spectrum is rather small, also for massive neutrinos.