Abstract: I will review the current status of bubble wall velocity calculations in cosmological first‑order phase transitions and introduce a simplified framework that bounds the velocity between two analytically tractable limits: local thermal equilibrium and ballistic transport. In each regime, the description is streamlined relative to conventional approaches, enabling closed‑form expressions for slow walls and transparent parameter dependence. I will illustrate the method with the QCD phase transition, which is made first‑order at large baryon chemical potential, and show that commonly assumed luminal wall speeds substantially overestimate the resulting gravitational wave amplitude by many orders of magnitude. These results provide practical guidance for modeling wall dynamics and for interpreting gravitational wave forecasts across a broad class of phase transition scenarios.

This seminar is part of the "Holography in the Gravitational Wave Era” (HOLOGW) project (with grant agreement number 101141909) funded by the European Union / ERC: