Effect of density fluctuations on gravitational wave production in first-order phase transitions

Understand

We study the effect of density perturbations on the process of first-order phase transitions and gravitational wave production in the early Universe.

• We are mainly interested in how the distribution of nucleated bubbles is affected by fluctuations in the local temperature.
• We find that large-scale density fluctuations ($H_* < k_* < \beta$) result in a larger effective bubble size at the time of collision, enhancing the produced amplitude of gravitational waves.
• The amplitude of the density fluctuations necessary for this enhancement is ${\cal P}_\zeta (k_*) \gtrsim (\beta / H_*)^{-2}$, and therefore the gravitational wave signal from first-order phase transitions with relatively large $\beta / H_*$ can be significantly enhanced by this mechanism even for fluctuations with moderate amplitudes.