The Holographic QCD Axion

I will present a holographic model of QCD composite axion. Having a gravity dual, based on the Sakai-Sugimoto model, it is calculable in the strongly coupled regime. Its basic properties will be discussed, including the low energy Lagrangian, from which the axion couplings to nucleons can be derived. In the deconfined phase, the temperature dependence of the axion mass can be extracted from the topological susceptibility. Finally, I will sketch some work in progress on the spectrum of gravitational waves produced in first order transitions in the model.