Speaker
Description
Cosmological first-order phase transitions are of great phenomenological interest: they can generate a detectable stochastic gravitational wave background, source the baryon asymmetry of the Universe, and seed primordial magnetic fields. A key parameter governing all of these phenomena is the bubble wall velocity $v_w$, which controls the efficiency of each process.
Computing $v_w$ from first principles, however, remains a difficult problem. Existing analytic approaches either assume local thermal equilibrium (LTE) or work in the ballistic limit, each providing only a bound on the true velocity. More complete methods based on solving the Boltzmann equation can capture the intermediate out-of-equilibrium regime, but are numerically involved and offer limited physical intuition.
In this talk, I present a new framework for computing $v_w$ based on the entropy produced as the bubble wall sweeps through the plasma. I show that the entropy production directly encodes the frictional force on the wall and can be used to interpolate systematically between the LTE and ballistic limits in a physically transparent way, without the complexity of a full Boltzmann treatment.