In these introductory lectures I will give a brief introduction of phase transitions in (thermal) field theories, and highlight some cosmological applications. As opposed to typical classical phase transitions, this refers to situations where the physical system transitions from a 'false' to the 'true' vacuum state. We will look into some key aspects of such transitions and, at least qualitatively, understand how temperature can act as an external parameter to trigger them. We will then learn about how these phase transitions can lead to the spontaneous nucleation of 'bubbles' or the formation of topological defects. If time allows, I will also mention cosmological consequences of these phenomena, like gravitational waves emitted due to the bubble dynamics, or the impact of cosmological strings on structure formation.
This event is part of the master lecture series organized by the Section for Theoretical Physics.