Abstract

Cosmological phase transitions are fascinating events taking place in the first instants of the Universe. First order transitions proceeding via the nucleation of bubbles are of particular interest as they can lead to a strong departure from thermal equilibrium, with important implications for the generation of the observed matter-antimatter asymmetry as well as for the production of primordial gravitational waves. These properties strongly depend on the dynamics controlling bubble nucleation: in a homogenous Universe, critical bubbles formed out of thermal or quantum fluctuations are expected to be spherical in shape, with a nucleation probability that is the same everywhere. This picture, however, can dramatically change if the Universe contains impurities, or seeds, that can exponentially enhance the nucleation rate in their vicinity. In this talk, I will discuss the general idea behind seeded phase transitions, and provide few natural examples where topological defects, such as strings and domain walls, can play the role of impurities in the early Universe.