Tufts/CfA/MIT Cosmology Seminar, at MIT:

Antonio Ferrera,  Tufts

Title: Bubble Collision and Defect Formation in a Damping Environment

Abstract: Within the context of a first-order phase transition in the 
early Universe, we study the collision process for true vacuum bubbles
expanding in a surrounding plasma. The effects of the plasma are simulated 
by introducing a damping term in the equations of motion for a $U(1)$ global
field. We find that Lorentz-contracted spherically symmetric domain walls
adequately describe the overdamped motion of the bubbles in the thin wall
approximation, and study the process of collision and phase equilibration
both numerically and analytically. With an analytical model for the phase
propagation in 1+1 dimensions, we prove that the phase waves generated in
the bubble merging are reflected by the walls of the true vacuum cavity,
giving rise to a long-lived oscillating state that delays the phase
equilibration. The existence of such a state in higher dimensions
is then confirmed by numerical simulations, and the consequences for the
formation of vortices in three-bubble collisions are considered. Lastly 
we study the impact of the new dynamical effects on the overall 
production of defects during the phase transition.