Gravitational waves from defect-driven phase transitions: domain walls

Abstract

We discuss the gravitational wave spectrum produced by first-order phase transitions seeded by domain wall networks. This setup is important for many two-step phase transitions as seen for example in the singlet extension of the standard model. Whenever the correlation length of the domain wall network is larger than the typical bubble size, this setup leads to a gravitational wave signal that is shifted to lower frequencies and with an enhanced amplitude compared to homogeneous phase transitions without domain walls. We discuss our results in light of the recent PTA hints for gravitational waves.

Cosmological defects

“…1 right, and eventually expand as large planar bubbles until collision with the neighbor walls. The effective bubble size is therefore increased, and the corresponding GW signal is enhanced and shifted to lower frequencies [17].…”