HEP Seminar - Cosmological perturbations of axion dark matter скачать в хорошем качестве

HEP Seminar - Cosmological perturbations of axion dark matter

HEP Seminar - Cosmological perturbations of axion dark matter Jiji Fan, Brown University Abstract: Cosmological data provides us two key constraints on dark matter: it must have a particular abundance, and it must have an adiabatic spectrum of density perturbations in the early universe. Many different cosmological scenarios have been proposed that establish the abundance of axion dark matter in qualitatively different ways. In this talk, I will discuss that despite this variety of backgrounds, the perturbations in axion dark matter can be understood from universal principles. How does a feebly interacting axion field acquire per- turbations proportional to those of photons? How do the isocurvature power spectrum and non-Gaussianity depend on the background evolution of the universe? I will show that one could answer these questions for a completely general choice of cosmological background and temperature-dependent axion potential. I will show that the most general solution to the axion field equation on super-horizon scales is entirely determined by the family of background solutions for different initial field values θini. This holds for both the component in the field perturbation solution contributing to the dark matter isocurvature perturbation (enhanced at late times by the sen- sitivity of the dark matter abundance to the initial condition, ∂Ωa/∂θini, which can be large for initial conditions near the hilltop), and the other component that contributes to the dark matter curvature perturbation. In particular, I will explain that an unperturbed axion field in the early universe evolving into one with nontrivial adiabatic perturbations is guaranteed by Weinberg’s theorem on adiabatic modes. These results have been derived before with various assumptions, such as a radiation dominated background or a quadratic potential. My aim is to give a clear, simple derivation that is manifestly independent of those assumptions, and thus can be applied to any cosmological axion scenario. I will also briefly discuss some on-going work on using the latest CMB data to constrain axion isocurvature perturbation.