Two-particle binding energy of interacting Bose gases

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.