Evolution of Thick Walls in Curved Spacetimes
  We generalize our previous thick shell formalism to incorporate any codimension-1 thick wall with a peculiar velocity and proper thickness bounded by arbitrary spacetimes. Within this new formulation we obtain the equation of motion of a spherically symmetric dust thick shell immersed in vacuum as well as in Friedmann-Robertson-Walker spacetimes. 
Shahram Khosravi, Samad Khakshournia, Reza Mansouri