  Vacuum excitation by time-varying boundary conditions is not only of
fundamental importance but also has recently been confirmed in a laboratory
experiment. In this paper, we study the vacuum excitation of a scalar field by
the instantaneous appearance and disappearance of a both-sided Dirichlet wall
in the middle of a 1D cavity, as toy models of bifurcating and merging
spacetimes, respectively. It is shown that the energy flux emitted positively
diverges on the null lines emanating from the appearance and disappearance
events, which is analogous to the result of Anderson and DeWitt. This result
suggests that the semiclassical effect prevents the spacetime both from
bifurcating and merging. In addition, we argue that the diverging flux in the
disappearance case plays an interesting role to compensate for the lowness of
ambient energy density after the disappearance, which is lower than the
zero-point level.
