We study the gravitational wave ( GW ) production in extremely strong first order phase transitions where the latent heat density dominates the plasma energy density , \alpha \gtrsim 1 .
In such transitions , bubbles develop extremely thin and relativistic fluid configurations , resulting in strong shock waves after collisions .
We first propose a strategy to understand the GW production in such a system by separating the problem into the propagation part and the collision part .
Focusing on the former , we next develop an effective theory for the propagation of the relativistic fluid shells .
Using this effective theory , we finally calculate the expected duration of the relativistic fluid configurations and discuss its implications to the GW production .