  The pole of the two-particle T-matrix including the influence of the
surrounding medium is analyzed for an interacting Bose gas. The phase diagram
of the Bose -Einstein condensation (BEC) depending on the temperature, density,
scattering length, and momentum is derived from this pole. The critical
momentum for the occurrence of superfluidity is obtained in this way. As a new
observation a two- particle binding energy is reported intimately connected
with the occurrence of the BEC. It is suggested that this might have
cosmological consequences on the dark energy problem.
