Gravitational wave cosmology for LISA

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Recently, a new tool for exploring the early universe has been receiving a great deal of attention, namely the "ripples in spacetime" termed gravitational waves. In the past couple of years we have used them to directly observe mergers of black holes for the first time. Once produced, gravitational waves do not interact with anything else, so they can provide a window not only into astrophysical processes, but also into the physics of the early universe. With gravitational waves we can see back to a time when the universe was still optically opaque.There are therefore many exciting things that can be seen with gravitational wave detectors. My project in particular concerns cosmological sources of gravitational waves on longer wavelengths than we can detect on Earth. The European Space Agency will launch a space-based detector in 2034, termed LISA. Since the detector arms can be many times longer in space than on Earth, the LISA mission is well placed to see gravitational waves produced around the time the Higgs boson itself 'turned on'.During my Fellowship I will use computer simulations to predict what evidence LISA should see of the Higgs boson 'turning on'. I shall connect this to the particle physics that is more conventionally studied at colliders like the LHC, too - by taking simple models which add one or two extra particles, I can look for cases that the LHC cannot rule out but that are promising from a cosmological point of view.