Gravitational waves from bubble collisions and fluid motion in strongly supercooled phase transitions

We estimate the gravitational wave spectra generated in strongly supercooled phase transitions by bubble collisions and fluid motion. We derive analytically in the thin-wall approximation the efficiency factor that determines the share of the energy released in the transition between the scalar field and the fluid. We perform numerical simulations including the efficiency factor as a function of bubble radius separately for all points on the bubble surfaces to take into account their different collision times. We find that the efficiency factor does not significantly change the gravitational wave spectra and show that the result can be approximated by multiplying the spectrum obtained without the efficiency factor by its value at the radius R-eff ? 5/beta, where )5 is the approximate inverse duration of the transition. We also pro-vide updated fits for the gravitational wave spectra produced in strongly supercooled transitions from both bubble colli-sions and fluid motion depending on the behaviour of the sources after the collision.

Automated summary

This study estimates the gravitational wave spectra generated in strongly supercooled phase transitions through bubble collisions and fluid motion. An efficiency factor is derived analytically in the thin-wall approximation to determine the energy distribution between the scalar field and the fluid. Numerical simulations are conducted, considering the efficiency factor as a function of bubble radius and taking into account different collision times on the bubble surfaces. The study finds that the efficiency factor does not significantly impact the gravitational wave spectra and proposes an approximate method to obtain the result. Updated fits for the gravitational wave spectra in strongly supercooled transitions are also provided, considering both bubble collisions and fluid motion depending on the behavior of the sources after the collision.