Modelling the non-linear gravitational clustering in the expanding Universe

The gravitational clustering of collisionless particles in an expandinguniverse is modelled using some simple physical ideas. I show that it is indeedpossible to understand the nonlinear clustering in terms of three well definedregimes: (1) linear regime (2) quasilinear regime which is dominated byscale-invariant radial infall and (3) nonlinear regime dominated by nonradialmotions and mergers. Modelling each of these regimes separately I show how thenonlinear two point correlation function can be related to the linearcorrelation function in heirarchical models. This analysis leads to resultswhich are in good agreement with numerical simulations thereby providing anexplanation for numerical results. The ideas presented here will also serve asa powerful anlytical tool to investigate nonlinear clustering in differentmodels. Several implications of the result are discussed.Comment: LaTeX, 4 pages, 1 figure enclosed as a uuencoded postscript file, uses MN.sty and epsf.st