Life and death in an expanding cage and at the edge of a receding cliff

The survival probabilities of a particle diffusing within an expanding ‘‘cage’’ and near the edge of a receding ‘‘cliff,’’ with death occurring when the diffuser reaches a boundary of the system, are investigated. Especially interesting behavior arises when the position of the boundary recedes from the diffuser as √At. In this case, the recession matches the rms displacement √Dt with which diffusion tends to bring the diffuser to its demise. For both the cage and cliff problems, the survival probability S(t) exhibits a nonuniversal power-law decay in time, S(t)∼t−β, in which the value of β is dependent on the detailed properties of the boundary motion. Heuristic approaches are applied for the cases of ‘‘slow’’ (A/D≪1) and ‘‘fast’’ (A/D≫1) boundary motion which yield approximate expressions for β. An asymptotic analysis of the survival probability for the cage and cliff problems is also performed. The approximate expressions for β are in good agreement with the exact results for nearly the entire range of possible boundary motions.