  We perform a coarse-graining analysis of the paradigmatic active matter
model, Active Brownian Particles, yielding a continuum description in terms of
balance laws for mass, linear and angular momentum, and energy. The derivation
of the balance of linear momentum reveals that the active force manifests
itself directly as a continuum-level body force proportional to an order
parameter-like director field, which therefore requires its own evolution
equation to complete the continuum description of the system. We derive this
equation, demonstrating in the process that bulk currents may be sustained in
homogeneous systems only in the presence of inter-particle aligning
interactions. Further, we perform a second coarse-graining of the balance of
linear momentum and derive the expression for active or swim pressure in the
case of mechanical equilibrium.
