  First order phase transitions could play a major role in the early universe,
providing important phenomenological consequences, such as the production of
gravitational waves and the generation of baryon asymmetry. An important aspect
that determines the properties of the phase transition is the dynamics of the
true-vacuum bubbles, which is controlled by the density perturbations in the
hot plasma. We study this aspect presenting, for the first time, the full
solution of the linearized Boltzmann equation for the top quark species coupled
to the Higgs field during a first-order electroweak phase transition. Our
approach, differently from the traditional one based on the fluid
approximation, does not rely on any ansatz and can fully capture the density
perturbations in the plasma. We find that our results significantly differ from
the ones obtained in the fluid approximation (including its extensions and
modifications), both at the qualitative and quantitative level. In particular
sizable differences are found for the friction acting on the bubble wall.
