Gravitational Waves from Cosmic Bubble Collisions

Cosmic bubbles nucleated through the quantum tunneling process would expand and undergo collisions with each other. We focus on collisions of two equal-sized bubbles and compute gravitational waves emitted from the collisions. The mechanism of the collisions can be modeled by means of a real scalar field and its quartic potential. Out of this model we can compute gravitational waves from the collisions by integrating the energy-momentum tensors over the volume of the wave sources; in the quadrupole approximation. Our computational results show that the waveforms are characterized by (i) cusp-like bumps with frequency modulation during the initial-to-intermediate stage of strong collisions and (ii) smooth monochromatic oscillations during the final stage of weak collisions.