  The particle distribution in a Bose condensate under the trapping potential
and its time evolution after switching off the trapping potential suddenly are
calculated. We investigate the problem from the viewpoint of quantum field
theory,using a model of self-interacting neutral boson field. Within the
approximation of retainng the most dominant term in the Hamiltonian after
applying the Bogoliubov replacement, we can calculate analytically the particle
distribution as a function of space and time coordinates.
