  First order phase transitions (FOPTs) constitute an active area of
contemporary research as a promising cosmological source of observable
gravitational waves. The spacetime dynamics of the background scalar field
undergoing the phase transition can also directly produce quanta of particles
that couple to the scalar, which has not been studied as extensively in the
literature. This paper provides the first careful examination of various
important aspects of this phenomenon. In particular, the contributions from
various stages of FOPTs (bubble nucleation, expansion, collision) are
disentangled. It is demonstrated that heavy particles primarily originate from
the relative motion of bubble walls at distances comparable to the Compton
wavelength of the particle rather than from the bubble collision itself.
Subtleties related to non-universality of particle interactions and masses in
different vacua are discussed, and a prescription to choose the correct vacuum
for the calculation is provided. The suppression of non-perturbative effects
such as tachyonic instability and parametric resonance due to the inhomegeneous
nature of the process is examined.
