  It is well known that the hydrodynamics of a zero-temperature superfluid can
be formulated in field-theoretic terms, relating for example the superfluid
four-velocity to the gradient of the phase of a Bose-condensed scalar field. At
nonzero temperatures, where the phenomenology of a superfluid is usually
described within a two-fluid picture, this relationship is less obvious. For
the case of a uniform, dissipationless superfluid at small temperatures and
weak coupling we discuss this relationship within a phi^4 model. For instance,
we compute the entrainment coefficient, which describes the interaction between
the superfluid and the normal-fluid components, and the velocities of first and
second sound in the presence of a superflow. Our study is very general, but can
also be seen as a step towards understanding the superfluid properties of
various phases of dense nuclear and quark matter in the interior of compact
stars.
