Computational Cosmology and Gravitational Waves

The discovery of gravitational waves by the LIGO and Virgo collaborations opened a new window to the Universe. Space-based gravitational wave detectors, such as the LISA mission, enable an exciting opportunity: using gravitational waves from the very early Universe in the search for beyond-the-Standard-Model (BSM) particle physics.The LISA science case identifies first order phase transitions as the most promising source of cosmological gravitational waves. There are no phase transitions in the Standard Model, and observation of a phase transition would be revolutionary: a direct signal of BSM physics. It is absolutely necessary to have accurate and reliable theoretical control of the gravitational wave production in BSM phase transitions in order to fully realize the science potential of the observations.The overarching goal of CoCoS is to calculate, for a given BSM theory, the resulting gravitational wave power spectrum to 10--20% accuracy. This is more than an order of magnitude better than the current state of the art, where accuracy is limited by uncertainties inherent in standard perturbative approaches. In CoCoS these problems are avoided by using several novel and state-of-the-art simulation techniques.A first order phase transition in the early Universe proceeds through supercooling, critical bubble nucleation, and growth and collision of the bubbles. Bubbles cause pressure waves, shocks and turbulence, which remains long after the transition has completed and create gravitational waves. In CoCoS the stages of the phase transitions are studied with innovative computational methods: effective field theory approach, which optimally combines perturbation theory and lattice simulations, and state-of-the-art viscous relativistic hydrodynamics.The high-luminosity LHC and LISA will be operational at the same time, searching for complementary aspects of new physics. The accuracy reached in CoCoS is necessary to fully utilize this synergy.