Space-time evolution of a condensate in the interaction representation

The space-time evolution of an inhomogeneous as well as homogeneous condensate is studied. We use the $\lambda \phi^4$ model and adopt the interaction representation in which the particle picture of the field $\phi$ with mass $m$ is definitely obtained. As the initial condition, a coherent state is assumed. In the case of the homogeneous condensate, the amplitude of it grows continuously with time and the perturbative calculation breaks down for large time. On the other hand, in the case of the inhomogeneous condensate with a finite initial size $\sim R$, the behavior of it is so different from that of the homogeneous one. For $0< t < m R^2/2$, the growth of the amplitude of the inhomogeneous condensate at the origin $({\boldsymbol x}={\bf 0})$ is slowed down compared with that of the homogeneous one, and the contribution of the interaction term $(\lambda /4) :\phi^4:$ is restricted to the small region around the origin of the condensate. For the late time, $t > m R^2/2$, the condensate spreads into space.