Tidal interactions in the expanding universe: the formation of prolate systems

The possibility is investigated that the tendency of galaxies and clusters of galaxies to be slowly rotating but aspherical could be due to early tidal distortion by neighbouring protostructures. Distortions of the right order could have been induced between recombination and the present epoch. It is argued that these distortions should not be associated with a great deal of angular momentum: during the linear stage of the development of protostructures, anisotropy energy is a growing first-order quantity, whereas angular momentum grows only in second order and, during the non-linear stage, the alignments of tidally-induced anisotropy and the torquing tidal field are unfavourable to the acquisition of angular momentum. (author)