Holographic description for beyond Standard Model scenarios

This Ph.D. thesis focuses on applications of the gauge/gravity duality to problems of phenomenological interest. In particular, we employ the Witten-Sakai-Sugimoto (WSS) model, the top-down holographic model that better resembles QCD at low-energy and in the planar regime. In the first part of the thesis, we generalize the WSS model in order to include the QCD axion. We derive the low-energy effective theory that governs the axion interactions and then study the model at finite temperature, extracting the temperature dependence of the axion mass from the topological susceptibility. In the second part, we apply the WSS model to the description of beyond Standard Model scenarios which predict cosmological first-order phase transitions that trigger the production of gravitational waves. We provide effective approaches for finding the bounce associated to the confinement and the chiral-symmetry phase transitions occurring in the WSS model. Next, we compute the quantities needed for estimating the gravitational wave spectrum. Finally, a comparison between the latter and the sensitivity curves of near-future experiments is performed.