Gravitational waves from cosmic bubble collisions
  Cosmic bubbles are nucleated through the quantum tunneling process. After
nucleation they would expand and undergo collisions with each other. In this
paper, we focus in particular on collisions of two equal-sized bubbles and
compute gravitational waves emitted from the collisions. First, we study the
mechanism of the collisions by means of a real scalar field and its quartic
potential. Then, using this model, we compute gravitational waves from the
collisions in a straightforward manner. In the quadrupole approximation,
time-domain gravitational waveforms are directly obtained by integrating the
energy-momentum tensors over the volume of the wave sources, where the
energy-momentum tensors are expressed in terms of the scalar field, the local
geometry and the potential. We present gravitational waveforms emitted during
(i) the initial-to-intermediate stage of strong collisions and (ii) the final
stage of weak collisions: the former is obtained numerically, in \textit{full
General Relativity} and the latter analytically, in the flat spacetime
approximation. We gain qualitative insights into the time-domain gravitational
waveforms from bubble collisions: during (i), the waveforms show the
non-linearity of the collisions, characterized by a modulating frequency and
cusp-like bumps, whereas during (ii), the waveforms exhibit the linearity of
the collisions, featured by smooth monochromatic oscillations.
