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We review the theoretical arguments which show how the macroscopic wave function in a Bose liquid leads to an anomalous (condensate-dependent) contribution to the momentum distribution n(p) of excited atoms. We believe this is the source of the non-Gaussian behaviour observed in superfluid 4He at both low and high values of p. Confirmation of this anomalous contribution 6n(p) is as important as the detection of the elusive condensate peak. Moreover, it is crucial to understand 8n(p) if one is to estimate the fraction of atoms in the condensate with any reliability.

Using functional differentiation to obtain the two-particle Green's functions, we give a systemmatic discussion of the collective modes in an electron-hole gas at low temperatures. Our analysis is based on the well known pairing approximation for the single particle self-energies and corresponds to the generalized RPA. The excitons with finite momentum appear as fluctuations of the electron-hole condensate and have a linear dispersion relation (Keldysh and Kozlov).