FROM PRIMORDIAL BLACK HOLES TO CEPHEIDS, A FOCUS ON THE EARLY AND LATE UNIVERSE

Abstract

This thesis studies two categories of objects that are relevant for studies of dark matter: primordial black holes (PBHs) and axions. (1) PBHs may be relevant to the origin of dark matter and the matter-antimatter asymmetry of the Universe. We study two different scenarios in which light PBHs disappear through Hawking evaporation, producing (i) dark matter and a beyond-Standard Model particle that decays to produce baryon asymmetry; and (ii) an ultra-heavy dark sector consisting of two or more species. These scenarios use a combination of first-order phase transitions and scalar curvature perturbations to produce PBHs. Both formation mechanisms can create a gravitational wave background in the (MHz-GHz) regime. (2) We also study axions, which can cause anomalous energy loss in Cepheid variables through their production in the stellar core and subsequent escape. We find that the elimination of the blue loop stage for Cepheids can provide bounds on the axion-photon coupling gaγγ. The bounds are found to be sensitive to the mass of the Cepheid and the modeling of the convection. The more massive the Cepheid, the stronger the resulting constraints on the axion-photon coupling. We also conduct a study of the effect of millicharged particles on the evolution of Cepheids, using the mass discrepancy problem to constrain the fractional charge of millicharged particles.