  This article addresses some issues related to statistical description of
gravitating systems in an expanding backgrounds. In particular, I describe (a)
how the non linear mode-mode coupling transfers power from one scale to another
in the Fourier space if the initial power spectrum is sharply peaked at a given
scale and (b) what are the asymptotic characteristics of gravitational
clustering that are independent of the initial conditions. The analysis uses a
new approach based on an integro-differential equation for the evolution of the
gravitational potential in the Fourier space. I show how this equation allows
one to understand several aspects of nonlinear gravitational clustering and
provides insight in to the transfer of power from one scale to another through
nonlinear mode coupling. Numerical simulations as well as analytic work shows
that power transfer leads to a universal power spectrum at late times, somewhat
reminiscent of the existence of Kolmogorov spectrum in fluid turbulence.
