Gravitational waves from cosmological phase transitions: using LISA to probe particle physics

In many extensions of the Standard Model, the electroweak phase transition is first order. Such a phase transition proceeds via the formation and collision of bubbles. The bubble collisions can source a stochastic gravitational wave background signal, with characteristic frequency right in the sensitivity band of LISA. We can thus use data from gravitational wave experiments to probe physics beyond the standard model.

In this talk, I will focus on the contribution to the gravitational wave signals from sound waves that get formed in the interactions between the plasma and the bubble walls. In the standard picture, the corresponding gravitational wave spectrum is described in terms of four parameters: 

the wall velocity and the strength, duration and temperature of the phase transition. I will argue that further model dependence enters via the speed of sound, and demonstrate how this affects the gravitational wave spectrum.

I will then demonstrate how the gravitational wave spectrum can be affected by primordial density perturbations and briefly discuss the implications for observability.