Friction pressure on relativistic bubble walls
  During a cosmological first-order phase transition, particles of the plasma
crossing the bubble walls can radiate a gauge boson. The resulting pressure
cannot be computed perturbatively for large coupling constant and/or large
supercooling. We resum the real and virtual emissions at all leading-log
orders, both analytically and numerically using a Monte-Carlo simulation. We
find that radiated bosons are dominantly soft and that the resulting retarding
pressure on relativistic bubble walls is linear both in the Lorentz boost and
in the order parameter, up to a log. We further quantitatively discuss IR
cut-offs, wall thickness effects, the impact of various approximations entering
the calculation, and comment on the fate of radiated bosons that are reflected.
