LAGRANGIAN THEORY OF GRAVITATIONAL-INSTABILITY OF FRIEDMAN LEMAITRE COSMOLOGIES-2ND-ORDER APPROACH - AN IMPROVED MODEL FOR NONLINEAR CLUSTERING

A large class of solutions for second-order irrotational perturbations is derived in the framework of the Lagrangian theory of gravitational instability of a homogeneous and isotropic universe investigated in e arlier papers. The solutions are evaluated in detail for perturbations in a flat background universe. The form of the solutions is designed for use in studies of the formation of large-scale structure from gene ric initial conditions. Some general remarks on the properties of the solutions are made. The result is illustrated by a special case and di scussed. In particular, it is found that sheet-like structures stay co mpact after shell-crossing (as in the competing 'adhesion model'), and that the collapse of first objects occurs earlier (as expected from n umerical simulations) in the second-order approach. Both these propert ies compensate shortcomings of the 'Zel'dovich approximation'. In cont rast to the 'adhesion model', the nth-order Lagrangian perturbation so lutions also describe internal structures of self-gravitating pancakes (= 2n + 1 stream systems) in terms of the nth orbit crossings within pancakes.