This monograph delves into the subjects of braneworld cosmology and string theory phenomenology. Various supergravity inflationary models are discussed in the 5-dimensional braneworld scenario, and their predictions are compared to observations. Special emphasis is put on the gauge dependence of scalar-induced gravitational waves (SIGWs), focusing on the radiation domination (RD) and matter domination (MD) eras.In the second part, we present our modern understanding of gravity and the standard model of particle physics in the context of string theory phenomenology. Specifically, a...