abstract: We discuss the reconstruction problem for the early universe, following Peebles and Frisch and coauthors. The model is given by the pressure-less gravitational Euler Poisson system, with time dependent coefficients taking into account general relativity features. (This amounts to considering Newtonian gravation in an Einstein de Sitter background.) The standard least action principle is unable to take into account mass concentration effects. We show that a modification of the action is possible which allows dynamical concentrations. (detailed discussion available on HAL pre-print server.)