Cosmological bubble friction in local equilibrium
  In first-order cosmological phase transitions, the asymptotic velocity of
expanding bubbles is of crucial relevance for predicting observables like the
spectrum of stochastic gravitational waves, or for establishing the viability
of mechanisms explaining fundamental properties of the universe such as the
observed baryon asymmetry. In these dynamic phase transitions, it is generally
accepted that subluminal bubble expansion requires out-of-equilibrium
interactions with the plasma which are captured by friction terms in the
equations of motion for the scalar field. This has been disputed in works
pointing out subluminal velocities in local equilibrium arising either from
hydrodynamic effects in deflagrations or from the entropy change across the
bubble wall in general situations. We argue that both effects are related and
can be understood from the conservation of the entropy of the degrees of
freedom in local equilibrium, leading to subluminal speeds for both
deflagrations and detonations. The friction effect arises from the background
field dependence of the entropy density in the plasma, and can be accounted for
by simply imposing local conservation of stress-energy and including field
dependent thermal contributions to the effective potential. We illustrate this
with explicit calculations of dynamic and static bubbles for a first-order
electroweak transition in a Standard Model extension with additional scalar
fields.
