I am based at Cardiff University, I study black holes and gravitational waves. Black holes are the most extreme objects in the universe (that we know of!), and gravitational waves are the opposite: they are so weak that they can only be detected with the most sensitive instruments that humans have ever built; when they were measured for the first time in 2015, it was one hundred years after Einstein first predicted them. Those signals were produced by black holes colliding with each other. My research has focussed on understanding and modelling the gravitational-wave signals from just such events, and the models we develop are used to measure the properties of those events — how massive were the black holes, how fast were they spinning, and where were they in the universe? These measurements fill in details in our understanding of how black holes form, and, in turn, about the past, present, and future of our universe.