  In the present thesis, the author reviews the physics of cosmological
first-order phase transitions that may have occured shortly after the Big Bang.
Such transitions proceed via the nucleation and expansion of true vacuum
bubbles and give rise to a rich phenomenology, for instance the emission of a
stochastic gravitational-wave background caused by bubble collisions. The
author discusses, in depth, the formalism of the effective scalar potential and
its different contributions in the loop expansion, points out the necessary
ingredients for a first-order transition, and assesses the detectability of the
associated gravitational-wave spectrum via future space-based observatories and
pulsar timing arrays. He then applies the the developed phenomenological
toolbox to investigate the detection prospect for phase transitions in the
context of specific theories such as the Vev Flip-Flop (a dark matter
mechanism) and the Dark Photon Model.
