Topological defects can act as local impurities that seed cosmological phase transitions. In this Letter, we study the case of domain walls and how they can affect the electroweak phase transition in the singlet-extended standard model with a $Z_2$-symmetric potential. When the transition occurs in two steps, the early breaking of the $Z_2$ symmetry implies the formation of domain walls which then act as nucleation sites for the second step. We develop a method based on a Kaluza-Klein decomposition to calculate the rate of the catalyzed phase transition within the 3D theory on the domain wall surface. By comparison with the standard homogeneous rate, we conclude that the seeded phase transition is generically faster and it ultimately determines the way the phase transition is completed. We finally comment on the phenomenological implications for gravitational waves.

• Received 15 June 2022
• Revised 23 September 2022
• Accepted 21 November 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.261303

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Published by the American Physical Society